contents=  Submitted to the Proceedings of the US Community Study on the
Future of Particle Physics (Snowmass 2021) ,
scale=1,placement=top,opacity=1,color=black,position=3.25in,1.2in

# Photon counting from the vacuum ultraviolet to the short wavelength infrared
using semiconductor and superconducting technologies

Jonathan Asaadi  The University of Texas at Arlington, Texas, USA  Dan Baxter
Fermi National Accelerator Laboratory, Illinois, USA  Karl K. Berggren
Massachusetts Institute of Technology, Massachusetts, USA  Davide Braga  Fermi
National Accelerator Laboratory, Illinois, USA  Serge A. Charlebois
Université de Sherbrooke, Québec, Canada  Clarence Chang  Argonne National
Laboratory, Illinois, USA  Centro Atomico Bariloche, Argentina  Angelo Dragone
SLAC National Accelerator Laboratory, California, USA  Alex Drlica-Wagner
Fermi National Accelerator Laboratory, Illinois, USA  Centro Atomico
Bariloche, Argentina  Carlos O. Escobar  Fermi National Accelerator
Laboratory, Illinois, USA  Juan Estrada  Fermi National Accelerator
Laboratory, Illinois, USA  Farah Fahim  Fermi National Accelerator Laboratory,
Illinois, USA  Michael Febbraro  Oak Ridge National Laboratory, Tennessee, USA
Guillermo Fernandez Moroni  Fermi National Accelerator Laboratory, Illinois,
USA  Stephen Holland  Lawrence Berkeley National Laboratory , California, USA
Todd Hossbach  Pacific Northwest National Laboratory, Washington, USA  Stewart
Koppell  Stanford University, California, USA  Christopher Leitz  MIT Lincoln
Labs, Massachusetts, USA  Agustina Magnoni  Universidad de Buenos Aires,
Argentina  University of Chicago, Illinois, USA  Benjamin A. Mazin  University
of California Santa Barbara, California, USA  Jean-François Pratte  Université
de Sherbrooke, Québec, Canada  Bernie Rauscher  NASA Goddard Space Flight
Center, Maryland, USA  Dario Rodrigues  Universidad de Buenos Aires, Argentina
Lingjia Shen  SLAC National Accelerator Laboratory, California, USA  Miguel
Sofo-Haro  Centro Atomico Bariloche, Argentina  Javier Tiffenberg  Fermi
National Accelerator Laboratory, Illinois, USA  Joshua Turner  SLAC National
Accelerator Laboratory, California, USA  Lorenzo Rota  SLAC National
Accelerator Laboratory, California, USA  Christopher J. Kenney  SLAC National
Accelerator Laboratory, California, USA  Frédéric Vachon  Université de
Sherbrooke, Québec, Canada  Gensheng Wang  Argonne National Laboratory,
Illinois, USA

###### Abstract

In the last decade, several photon counting technologies have been developed
opening a new window for experiments in the low photon number regime. Several
ongoing and future projects in HEP benefit from these developments, which will
also have a large impact outside HEP. During the next decade there is a clear
technological opportunity to fully develop these sensors and produce a large
impact in HEP. In this white paper we discuss the need for photon counting
technologies in future projects, and present some technological opportunities
to address those needs.

###### Contents

  1. 1 Needs for Future Projects
    1. 1.1 Observational Cosmology
      1. 1.1.1 Ground-Based Spectroscopy
      2. 1.1.2 Special Considerations for Space
    2. 1.2 Direct Dark Matter search
    3. 1.3 Neutrino detection
    4. 1.4 The Material Challenge : Replacing Silicon for Short-Wave Infrared Detection
    5. 1.5 Outside HEP
      1. 1.5.1 Quantum Sensing
      2. 1.5.2 Basic Energy Science
      3. 1.5.3 Applied Radiation Detection
  2. 2 Instrumentation Opportunities
    1. 2.1 MKIDs
    2. 2.2 SNSPDs
    3. 2.3 Germanium Detectors
    4. 2.4 Skipper-CCDs
    5. 2.5 Skipper-CMOS
    6. 2.6 Single Photon Avalanche Diode Array Based Detectors
      1. 2.6.1 Enhancing VUV sensitivity
      2. 2.6.2 Near Infrared Enhanced Silicon Photomultipliers
      3. 2.6.3 Photon-to-Digital Converters
    7. 2.7 Photon Counting with TES
    8. 2.8 Novel Photoconductors for VUV photon detection
  3. 3 Summary

##  1 Needs for Future Projects

In this section we describe future projects that will benefit from photon
counting detectors in the vacuum ultraviolet, visible, and near-infrared
(near-IR), what they need for pixel count, efficiency, timing, etc.

###  1.1 Observational Cosmology

####  1.1.1 Ground-Based Spectroscopy

The Cosmic Frontier explores the nature of dark energy, dark matter, and
cosmic inflation through the observations of faint stars, galaxies, and
residual microwave photons from the Big Bang. Several novel cosmological
facilities for wide-field multi-object spectroscopy were proposed for the
Astro2020 decadal review, and are being considered as part of the Snowmass
process [e.g., 186, 203, 140, 62, 48]. Ground-based spectroscopic observations
of faint astronomical sources in the low-signal, low-background regime are
currently limited by detector readout noise. In particular, medium- to high-
resolution spectroscopy at shorter wavelengths has low sky-background levels
and significant gains can be achieved through reductions in readout noise
(${\sim}0.5$ e- rms/pix) [72]. Sub-electron noise would result in a $\sim
20\%$ increase in survey speed, i.e., allowing a five year survey to achieve
its goals in four years. Multi-object spectroscopic facilities are required to
observe objects of widely varying brightness with the same fixed exposure
time. Thus, the ability to control readout noise dynamically would allow
photon counting when needed for faint sources, but will not waste time on
bright sources that are shot-noise dominated. Another application for single-
photon counting comes from observing campaigns (imaging or spectroscopy) that
consist of many short-duration exposures become dominated by the readout noise
of each exposure [172, 209]. For example, high-cadence observations searching
for short duration transients (e.g., fast radio bursts, gravitational wave
events, etc.) could benefit from reduced detector readout noise.

Scientific CCDs are the detector technology of choice for current ground-based
observatories. CCDs provide many desirable properties for astronomical
observations including stability, linear response, large dynamic range, high
quantum efficiency over a wide wavelength range, and low dark count rates.
However, current scientific CCDs have a noise floor at $\sim 2$ e- rms/pix.
Ideally, a future photon counting technology for ground-based astronomical
observations would retain many of the beneficial properties of conventional
CCDs. In addition, detectors with lower band gap (i.e., germanium CCDs) would
enable observations of higher-redshift galaxies. However, fast readout time is
essential for any new technology, since every second spent in readout is a
second lost in exposure time.

####  1.1.2 Special Considerations for Space

Space affords certain unique observational advantages. Among these are the
complete absence of atmospheric “windows” and “seeing”, no electromagnetic
interference except that from the observatory itself, and generally superb
thermal/mechanical stability. However, to take advantage of these benefits,
detectors are required to operate in the harsh space radiation environment.

The Decadal Survey 2020 [156] has established a large space telescope as the
highest ranked large project for the next decades. The missions concepts
developed in this area [147, 161] have identified photon counting arrays for
visible and near-IR imaging and spectroscopy as key enabling technologies.
Some of the photon counting technologies discussed here are a good match to
these missions.

As an example, the James Webb Space Telescope (JWST) has a nominal mission
lifetime of five years, with all expendables provisioned for ten. Because
launch was so successful, we can now expect many more than five years of JWST
science in part because everything is radiation tolerant. The detectors
themselves were tested to withstand a lifetime dose of 5-10 krad-Si. The
SIDECAR application specific integrated circuits (ASIC) that control them were
tested to far higher levels still.

Beyond survival, the detectors themselves must still meet demanding
performance requirements even though the cosmic ray rate is far higher than on
the ground. The integrated ionizing particle rate at L2 is about
$5-10~{}\textrm{ions}~{}\textrm{cm}^{-2}\textrm{s}^{-1}$. For imaging
detectors including CCDs and IR arrays, this places an upper limit on the
useful exposure time. Figure 1 shows the fraction of pixels that cosmic rays
would corrupt in a hypothetical p-channel CCD at L2. Total exposure time
including readout must be kept shorter than about 1 minute to avoid corrupting
more than 10% of the pixels.

Figure 1: Panel a) shows a Monte Carlo simulation of a 250 $\mu$m thick
Skipper CCD used at L2. The tracks are long because thick silicon is used to
achieve excellent QE in the near-IR. We assumed that a cosmic ray disturbed
any pixel that it passed within about 9 $\mu$m of (orthogonal distance) and
the 4 nearet-neighbors. Panel b) shows that exposure times should be kept to
less than about 1 minute to disturb no more than about 10% of pixels.

###  1.2 Direct Dark Matter search

Among the most promising detector technologies for the construction of a large
multi-kg experiment for probing electron recoils from sub-GeV DM are new
generation of silicon Charged Coupled Devices with an ultralow readout noise,
so-called “skipper-CCDs”. These sensors are a new type of photon counters
discussed in detail in Section 2.4. Skipper-CCDs were designed by the Lawrence
Berkeley National Laboratory (LBL) Micro Systems Lab in close collaboration
with Fermilab. In a technological breakthrough in 2017, the SENSEI (“Sub-
Electron Noise Skipper-CCD Experimental Instrument”) Collaboration
demonstrated the ability to measure precisely the number of free electrons in
each of the million pixels across the CCD [207]. SENSEI packaged a small
prototype skipper-CCD sensor and took data at Fermilab on the surface and
underground, setting world-leading constraints on DM-electron interactions for
DM masses in the range of 500 keV to 5 MeV [58, 7, 25]. Using new, science-
grade skipper-CCDs, pathfinder experiments (already funded) based on this
technology are planned for the coming years. Specifically, SENSEI-100 plans to
install a $\sim$100 g detector at SNOLAB during 2021, while DAMIC-M plans to
install a $\sim$1 kg detector at Modane during 2023.

The Oscura project has the goal of performing a dark matter search using
silicon Charge Coupled Devices with a threshold of two electrons and no
background events in a total exposure of 30 kg-yr. The R&D effort, started in
FY20, has focused on the three highest risk technical aspects of the
experiment: sensor fabrication, readout electronics, and background reduction.
The effort is now moving into a design phase with the plan of being ready to
start construction in FY24.

Superconducting nanowire single photon detectors (SNSPDs) (see Sec.2.2) are
also a promising technology for the detection of light dark matter, thanks to
their inherent low energy threshold, experimentally demonstrated to be 125 meV
[216], which can enable sensitivity to masses lower than 0.1 MeV when dark
matter produces an electron recoil leading to scintillation in a cryogenic
semiconductor, such as GaAs [66, 214]. The rate of background dark counts in
SNSPDs has been demonstrated to as low as one event per day in a (300
$\mathrm{\mu m)^{2}}$ pixel, many orders of magnitude lower than in the
photosensors used in previous generations of dark matter searches. A key
milestone towards a functional dark matter experiment scalable to target
masses up to 1 kg is the realization of highly-pixelated SNSPD sensors
covering active areas as large as several $\mathrm{cm}^{2}$, while maintaining
high time resolution and low noise. Other applications of SNSPDs for the
detection of light dark matter include the realization of optical haloscopes
based on dielectric cavities [27], searches for dark photons and axions with
dish antennas [130] and the direct detection of electron scattering from dark
matter in superconductors [100].

###  1.3 Neutrino detection

Current observation of low energy neutrino are limited by the high energy
threshold of available detector technologies. Single photon counting sensors
have gained importance for this application due to their ability to access to
energy depositions below a few hundred eV’s. The CEvNS interaction [85]
(Coherent Elastic neutrino Nucleus Scattering ) recently observed [10]
provides a new mechanism to observe low energy neutrinos (with energy below 50
MeV). Several low energy neutrino sources are available: nuclear reactor
(which are the most incense source of neutrino in the earth), spallation
neutron source, the sun, radiation sources, etc. This neutrino interaction
channel is a good candidate to test the standard model physics at low momentum
transfer [77] and possible deviations from it [75] since it can only interact
through the weak force with less backgrounds associated with other forces when
compared to other elementary particles. The deposited energy from CEvNS is
less than a few keV. Only part of this energy is converted into detectable
signal in the sensor (ionization, phonons, etc.) and therefore low threshold
technologies are needed. Semiconductor and superconducting technologies with
eV and sub-eV energy resolution for photon counting capability in the visible
and near-IR are natural candidates to reach the necessary resolution for this
application.

A large number of outstanding questions remain to the fundemental nature of
the neutrino, which can be probed through the use of higher energy (
$\mathcal{O}$(MeV) $<E<\mathcal{O}$(GeV)) neutrino sources (e.g. accelerator
and atmospheric neutrinos). The nature of these remaining puzzles break into
the distance over which the neutrinos are allowed to propogate before being
detected. Thus the future class of experiements are classified as “short-
baseline” and “long-baseline” experiments.

The next generation long-baseline neutrino experiments aim to answer the
questions of the exact ordering of the neutrino mass states, known as the mass
hierarchy, as well as the size of the CP-violating phase $\delta$. These, as
yet unknown quantities, remain one of the last major pieces of the Standard
Model of particle physics and offer the opportunity to answer such fundamental
questions as “what is the origin of the matter/antimatter asymmetry in the
universe?” and “do we understand the fundamental symmetries of the universe?”.
By measuring the asymmetry between appearance of electron neutrinos from a
beam of muon neutrinos ($P(\nu_{\mu}\rightarrow\nu_{e}$)) compared to the
appearance of electron antineutrinos from a beam of muon antineutrinos and
$P(\bar{\nu}_{\mu}\rightarrow\bar{\nu}_{e}$)) as well as the precise
measurement of the $\nu_{e}$ energy spectrum measured at the far detector,
both the CP violating phase ($\delta_{CP}$) and the mass hierarchy can be
measured in the same experiment.

The Short-Baseline Neutrino (SBN) program aims to address the anomalous
neutrino results seen by the LSND and MiniBooNE which suggest the possible
existence of a eV mass-scale sterile neutrino. However, the experimental
landscape is perplexing since a number of other experiments utilizing a range
of different neutrino sources which should have been sensitive to such a
sterile neutrino have observed only the standard three neutrino oscillations.
In this landscape, the conclusive assessment of the experimental hints of
sterile neutrinos becomes a very high priority for the field of neutrino
physics.

To address both of these areas of neutrino research, large scale noble element
time projection chambers (TPC’s) [223, 179] play a central role and offer an
opportunity to perform discovery level measurements through the enhancement of
their capabilities. In a noble element TPC, particles interact with the medium
and deposit their energy into three main channels: heat, ionization, and
scintillation light. Depending on the physics of interest, noble element
detectors attempt to exploit one or more of these signal components. Liquid
Noble TPC’s produce ionization electrons and scintillation photons as charged
particles traverse the bulk material. An external electric field allows the
ionization electrons to drift towards the anode of the detector and be
collected on charge sensitive readout or transform energy carried by the
charge into a secondary pulse of scintillation light.

Scintillation light from the interaction produces ultraviolet (UV) photons and
is typically detected by separate and independent photosensors located away
from the charge sensitive plane and provides the $t_{0}$. Detecting these UV
photons is traditionally accomplished using wavelength shifters to shift the
light to the visible wavelength for detection by conventional light detectors
like PMTs or SiPMs. This detection technique offers moderate overall detection
efficiencies with various levels of complications associated with the use of
wavelength shifters and/or UV sensitive devices.

The combined measurement of the scintillation light and the ionization charge
gives the TPC the capability of being a fully active 5D detector with 3D
tracking, calorimetric reconstruction capabilities and timing measurements.
However, the realization of the full multi-dimensional readout over the full
spectrum of produced signals of noble element TPCs remains one of the key
instrumentation challenges.

To realize this challenge and unlock the full potential of noble element TPC
requires multiple pieces including large area, high UV efficiency to
scintillation light, expanded sensitivity to the full spectrum of light
produced, and ideally these capabilities integrated into a single sensor
capable of detecting both light and charge. Research into novel photodetection
techniques utilizing various new materials is one of the paths which needs to
be explored to realize the full capabilites of these experiments.

###  1.4 The Material Challenge : Replacing Silicon for Short-Wave Infrared
Detection

As is well known noble elements emit light abundantly in the vacuum
ultraviolet (VUV) part of the spectrum [17], making it natural that the focus
of R&D in the last couple of decades has been on increasing the light
collection efficiency of this VUV scintillation using wavelength shifters plus
light guides or light traps. Section 2.8 discusses further developments on the
direct detection of VUV scintillation light in noble element detectors but now
it would be interesting to call attention to the non-VUV part of the spectrum
and its potential use as the light signal in such detectors, an application
that could bring some advantages in terms of abandoning the use of wavelength
shifters as well as bypassing the technological challenges of VUV sensitive
photon detectors, not ignoring the fact that going to longer wavelengths has
its own technological challenges, as made explicit in the following.

It has been known since the late 80s that noble gases present atomic emission
in the near-infrared (NIR) [127, 35] that would be quenched by collisions in
the condensed phases thus raising the question of NIR emission in liquid and
solid arising from excimers. After inconclusive results coming mostly from
Bressi, Borghesani, Carugno and collaborators [33], a revival of the subject
was brought forward independently by the Munich group of A. Ulrich and co-
workers [96] and the Novosibirsk group [32] with the discovery of NIR
scintillation around and beyond 900 nm in liquid argon thus urging the quest
for single photon counting devices at wavelenghts larger than 900 nm well into
the 1,000 nm silicon brick wall, with the finding of a strong emission in this
region when liquid argon is doped with parts per million of xenon [157].

Though silicon can absorb photons up to 1110 nm, using silicon above 800 nm
requires thick structures with extended and uniform electric field. Silicon
photodiodes therefore come in a variety of configurations optimized to have
either frontside photon incidence (i.e. entrance on the high field junction
side) or backside incidence (i.e. entrance from the substrate). These
configurations all share the goal of favoring avalanche triggered by electrons
[61]. In all cases, a compromise must be reached between higher sensitivity at
the cost of higher dark current which in photon counting detectors translates
to higher dark counts rates (larger device volume). Such devices also have
longer drift lengths and thus greater time jitter.

Practical sensitivity in the short-wave infrared (1 to 3 µm) thus requires
using other semiconductors such as germanium or small bandgap semiconductor
elements. But dark count rates are greatly increased by the exponential
dependence of pair generation rate across that small bandgap. Competing with
silicon with respect to dark count rate is a huge challenge for many reasons.
Firstly, silicon being an indirect bandgap semiconductor sees its band-to-band
transition generation rate greatly suppressed (5 orders of magnitude).
Therefore the main generation mechanism is through mid-gap impurity states
which depends on the density of those states, not on the doping of the
material [196, Sec. 1.5.4]. Secondly, silicon homojunction have the best
crystalline structure with minimal defect density.

Infrared avalanche photodiodes have been demonstrated using a variety of small
bandgap semiconductors that can achieve single-photon sensitivity. Of central
interest to photon counting are single-photon avalanche diodes (SPAD) based
detectors such as silicon photomultipliers (SiPM) and photon-to-digital
converters (PDC), also known as digital SiPM***See section 2.6.3. All these
detectors operate in a metastable regime above the breakdown voltage of the
junctions called ”Geiger mode” [57].

In recent years, germanium-on-silicon single-photon avalanche diodes have been
demonstrated and used in various applications [134, 220, 108, 131, 118, 141,
217]. Although challenging because of the 4.2% crystal lattice mismatch, being
relevant to transistor and optoelectronic technologies, high quality growth of
germanium on silicon wafer up to 200 mm is now commercially possible. This is
a relevant issue if one seeks for large sensitive surfaces (SiPM) and
integration with advanced electronics (digital SiPM, PDC, CCD, and other
imagers). While germanium also benefits from reduced band-to-band generation
rate because of its indirect bandgap (E${}_{g}=$ 0.65 eV), the trap-assisted
(Shockley-Read-Hall) generation rate in a depletion region is 3 orders of
magnitude larger than for silicon. Also the $\Gamma$ direct bandgap being
rather small (0.75 eV) by comparison to that of silicon (3.4 eV), band-to-band
tunneling is strongly enhanced thus increasing dark count rates. To reduce
this further, most Ge-on-Si SPADs are designed to separate the photon
absorption region (in Ge) from the multiplication region (in Si) [205]. Note
that band discontinuities in Ge-on-Si heterostructure (type-II, [196, Fig.
34]) do not limit the photocarriers from drifting from the absorber to the
multiplication region in comparison to InGaAs/InP SPAD structures [205]. In
Ge-on-Si SPAD, the field in Ge is reduced limiting both generation and
tunneling rate in that region but still allowing for photoelectrons to drift
to the multiplication region. The multiplication region being in Si suffers
only from the lower thermal generation rate of that material. There exists
many variations of separated absorption-multiplication structures (SAM, SAA,
SACM, SAGCM…). Even with optimized structures, drastically reducing the
operating temperature of Ge devices†††This has the unfortunate consequence of
drastically reducing absorption at the bandgap edge for the infrared
communication C-band (centered around 1550 nm) [217]. is unavoidable to reduce
dark count rates.

Nevertheless, Ge-on-Si SPADs with 100 $\mu$m diameter where shown to provide
up to 38% of single-photon detection efficiency at a wavelength of 1310 nm, an
excess bias of 5.5% (breakdown voltage $\sim$40 V) and a temperature of 125 K
[217]. Smaller area Ge-on-Si SPADs (26 $\mu$m dia.) achieved a dark count rate
on the order of 104 cps at similar operating bias and temperature [132]. See
Thorburn et al. for a broader review of performances [205].

As is the case for APDs, III-V based SPADs all have a separated absorption-
multiplication structure. The InGaAsP semiconductor family offers the
possibility to grow small gap semiconductors on a larger bandgap binary member
(InP) of the same family and with a well matched crystal lattice [196, Fig.
32]. Most notable are In7Ga5As64P56/InP (quaternary compound) and
In53Ga47As/InP (ternary compound). Because they are direct bandgap
semiconductors, considerations similar to those stated above apply to thermal
and trap-assisted generation rate. In InP, which is used for the
multiplication region, holes have the largest ionization coefficient [228].
Therefore, the multiplication region should have its n-type side facing the
InGaAs absorber region for holes to be dominant in the triggering of the
avalanche in Geiger mode. In these heterostructures, the large band
discontinuities oppose the transition of the photocarriers (holes) from the
absorption to the multiplication region. The device’s conduction and valence
bands must be properly engineered to effectively lower the discontinuities and
mitigate their detrimental effects.

Among the first published works on InGaAsP/InP based SPADs are those of Levine
et al. at AT&T Bell Laboratories [126], Zappa et al. at Polytechnic University
of Milan in collaboration with EG&G Canada [227] and also McIntosh et al. at
MIT’s Lincoln Laboratory [145]. To our knowledge, the MIT’s LL group was the
first to report the digital readout of an array of InGaAsP/InP SPADs [146,
215, 107]. Since then, many have reported on single SPAD devices optimization
[192]. Notable works on recent SPAD arrays are those by Hamamatsu [24] and
MIT’s LL [23].

Major challenges pave the path to SPAD array for large sensitive area devices
and to integration with advanced electronic readouts as needed for imagers and
PDCs. Growth of III-V on InP is now reaching the 100 mm wafer size.

Primary sources of dark count in InGaAs/InP SPAD are tunneling through defect
levels in the InP avalanche region and thermal generation in the InGaAsP
absorber region [69]. Small diameter state-of-the-art InGaAs/InP SPADs were
reported with dark count rates ranging from 102 cps to 104 cps at excess
voltage between 2 V and 7 V and temperatures reachable with thermoelectric
cooling (225 K - 253 K) [192]. For similar excess voltage and temperature,
InGaAs/InP SPADs have shown photon detection efficiency around 40 % in the
range of 950 nm to 1550 nm with timing jitter close to 80 ps FWHM [211, 67].
However, a major limitation to III-V SPAD detectors is the long-lived
afterpulsing events affecting the maximum attainable count rate. The impact of
afterpulsing can be reduced by operating the detectors in a gated mode where
the SPAD is turned on synchronously with the optical signal and turned off
during long period of time in order for afterpulsing events to fade away
[184].

Although improvements in device structures and material growth contribute to
continuously increasing detector performances with respect to sensitivity and
dark count rate, small bandgap semiconductor infrared detectors will likely
always suffer from dark count rate much higher than their silicon counterpart,
even while operated at low temperatures. Nevertheless, they can provide
significant sensitivity between 900 and 1600 nm, wavelengths at which silicon
cannot. To further increase performances, one must also consider the readout
techniques. Of particular interest for large area detectors are photon-to-
digital converters (a.k.a. digital SiPM) with one-to-one coupling between a
SPAD and a quenching and readout circuit. This allows an increased control on
the SPAD to perform afterpulse mitigation and gating techniques that
effectively reduce dark count rates. This is discussed further in section
2.6.3.

###  1.5 Outside HEP

####  1.5.1 Quantum Sensing

The uncertainty when estimating the absorption/transmission of a sample
–compared to a measurement using classical light– is given by the combination
of random fluctuations inherent in the optical probe beam, and by the
stochastic nature of the interaction between light and matter within the
sampled object. To improve the precision of such measurement is of utmost
importance in Quantum Imaging.

Recently, much attention has been paid to the field of quantum metrology and
its applications in biological sciences [91, 199], and particularly to the
utilization of quantum light as a resource for surpassing the classical limits
of precision per unit intensity [31, 149, 36]. Schemes for estimating the
transmission of a sample generally consist in measuring the intensity
attenuation of a light beam that propagates through it, which can be done
using a single light beam as the source (direct measurement), or twin-beams:
one arm as reference and the other as probe (differential measurement).

The best performance achievable with classical light is obtained using a beam
with Poissonian statistics in the number of photons. The ultimate precision
for such direct measurement is usually called the Shot-Noise Limit (SNL).
Through literature, twin-beams and difference-based estimators have been used
for spatially resolved implementations [37] including the realization of the
first sub-shot-noise wide field microscope in 2017 [182]. The performance of
this technique depends upon the spatial resolution and reaches out a factor of
improvement in precision over the SNL of approximately 1.30. Recently, a
complete theoretical and experimental study of the performance of different
estimators using twin-beams was presented [133], achieving a maximum
improvement factor of 1.51.

State-of-the-art studies in general use conventional CCD cameras for
detection, with high efficiencies $(>90\%)$ and a readout noise usually
between 2 and 5 e-. These specifications impose the intensity regime in which
it is possible to work: scenarios of a few photons $(\approx 50)$ per pixel
cannot be explored in order to obtain an acceptable signal-to-noise ratio.

Using sensors that enables sub-electron noise in the readout of the pixel
charge, number-resolving photons in the optical and near-infrared wavelengths,
would open the possibility of exploring ultra-low light intensity regimes of a
few photons per pixel in transmission/absorption measurements – which is
particularly important for measuring biological samples [200].

The above-mentioned applications do not require time resolution and account
with the potential to produce an unprecedented impact in this field, for
instance, in Quantum Microscopy [182, 138]. Nevertheless, when time resolution
is available, another wide universe of opportunities arises, as using
coincidence, stray light reduction and background rejection becomes
straightforward.

####  1.5.2 Basic Energy Science

The photon energies of 20-500 eV offer many exciting opportunities in the
basic energy sciences, but the challenge of single photon detection has
hindered progress. With the Skipper-CCD, the ability to detect single photons
in this unique wavelength range can offer many opportunities for new
directions in this area of research. One area of interest is centered around
controlling and exploiting fluctuations in quantum matter for the design of
bulk materials with novel functionality. Novel x-ray methods including
resonant inelastic x-ray scattering (RIXS) [14] and x-ray photon fluctuation
spectroscopy (XPFS) [191] have been developed to tackle these challenges. Both
techniques are photon hungry, making single photon counting critical for
extracting out the fluctuations in the system being studied. This has been
demonstrated at the Linac Coherent Light Source X-ray free electron laser at
SLAC [189, 148], but remains in unexplored area in the energy range below 500
eV. The ability to detect single photons at lower energies will open numerous
scientific applications, and could act as a catalyst to broaden further an
already active area of research by enabling spontaneous fluctuation studies in
new systems yet to be investigated.

For instance, one of the most interesting topics at present in condensed
matter is that of twisted graphene and ‘twistronics’ – which could form a
unique type of electronic devices [46, 45]. This is based on the dramatic
change of the properties of a material composed of two sheets of atomic layers
by twisting each layer with respect to each other in a precise way (See Fig.
2). Detectors which deliver single photon sensitivity at the carbon K-edge,
would allow low-energy RIXS and XPFS studies of these types of systems to look
at fluctuations related to the superconductivity, i.e. resistance-free current
flow that can harvest energy storage, and how this phenomenon differs from the
more well-known type of superconductors. Analogous to the high-temperature
superconductors based on copper [112], these studies would offer an
unprecedented opportunity to investigate the many features in the phase
diagram of this family of materials related to twisted graphene and could add
fresh insight into an already burgeoning field.

Figure 2: Illustration of a Moiré pattern that emerges upon stacking and
rotating two sheets of bilayer graphene. New physics can emerge in twisted
double bilayer graphene over a small range of twist angles, and can be tuned
with an electric field [95] (Image taken from:
https://phys.org/news/2020-10-spontaneous-symmetry-bilayer-graphene.html).

Another area in the field of materials is that of topological matter. Here the
mathematics field of topology has found an intersection with the study of
materials and is shedding new insights into quantum mechanics. For instance
skyrmions, particles named after the British physicist Tony Skyrme who
hypothesized their existence in nuclear physics [193], have been found to
occur in magnetic systems and form the basis for new types of technology [79].
One outstanding question is how the topological nature of these particles can
affect their fluctuations? This question is critical to understanding the
noise of skyrmion-based devices. With the Skipper-CCDs, single photon
detection at the M-edges of 3d transition metal elements, known to have strong
dichroic effects in magnetic materials, would be possible candidates to study.
This could offer the potential to use two-color capabilities at XFEL
facilities, such as the FERMI XUV free-electron lasers in Trieste, Italy [78],
to study fluctuations of different elements simultaneously. This could lead to
an understanding of how topology will modify ultrafast dynamics of these
magnetic textures.

A third area, and one important for global alternative energy solutions, is
the Nobel-prize-winning science behind Li-batteries [92]. One of the biggest
challenges in this area is realizing a high Li+ cation conductivity in an
electrolyte and across the electrode/electrolyte interface. This requires
careful material engineering and screening processes. In theory, this
diffusion process can be directly probed by XPFS by tuning specifically to the
Li ions for direct measurement. However, an element-specific screening is not
possible so far, due to the lack of a detector with single-photon sensitivity
that operates at the Li K-edge (54.7 eV). Skipper-CCDs would also have impact
in this broader field of battery research and technology.

Even traditional devices such as Si-based semiconductors will significantly
benefit from the availability of such detectors. Charge carriers in
semiconductors constitute the foundation for many key technologies at present,
including computers, lasers and light emitting devices. The ability to probe
their dynamics at the meV energy level, or equivalently, in the femto-to-pico
second time window, is in high demand. Performing RIXS and XPFS measurements
at different Si L-edges holds potential for measuring the orbital-selective
carrier dynamics in these devices as well.

####  1.5.3 Applied Radiation Detection

Photon counting detectors (CCDs) have been demonstrated as a powerful tool to
measure low levels of isotopic contamination in materials as part of their use
in direct dark matter detection [58]. Some ideas are now being considered for
implementing this detector technology for the monitoring of contamination of
isotopes with low energy signals ($\sim$ 1 keV). Further developing of photon
counting technologies like skipper-CCDs would benefit these applications.

##  2 Instrumentation Opportunities

In this section we we describe how the new technologies are addressing the
needs described in the previous section, with an attempt to estimate the
timescale of the developments.

###  2.1 MKIDs

Figure 3: The basic operation of an MKID, from [64]. (a) Photons with energy
$h\nu$ are absorbed in a superconducting film, producing a number of
excitations, called quasiparticles. (b) To sensitively measure these
quasiparticles, the film is placed in a high frequency planar resonant
circuit. The amplitude (c) and phase (d) of a microwave excitation signal sent
through the resonator. The change in the surface impedance of the film
following a photon absorption event pushes the resonance to lower frequency
and changes its amplitude. If the detector (resonator) is excited with a
constant on-resonance microwave signal, the energy of the absorbed photon can
be determined by measuring the degree of phase and amplitude shift.

MKIDs (microwave kinetic inductance detector) work on the principle that
incident photons change the surface impedance of a superconductor through the
kinetic inductance effect [142]. The kinetic inductance effect occurs because
energy can be stored in the supercurrent (the flow of Cooper Pairs) of a
superconductor. Reversing the direction of the supercurrent requires
extracting the kinetic energy stored in it, which yields an extra inductance
term in addition to the familiar geometric inductance. The magnitude of the
change in surface impedance depends on the number of Cooper Pairs broken by
incident photons, and hence is proportional to the amount of energy deposited
in the superconductor. This change can be accurately measured by placing a
superconducting inductor in a lithographed resonator, as shown in Figure 3. A
microwave probe signal is tuned to the resonant frequency of the resonator,
and any photons which are absorbed in the inductor will imprint their
signature as changes in phase and amplitude of this probe signal. Since the
quality factor $Q$ of the resonators is high and their microwave transmission
off resonance is nearly perfect, multiplexing can be accomplished by tuning
each pixel to a different resonant frequency with lithography during device
fabrication. A comb of probe signals can be sent into the device, and room
temperature electronics can recover the changes in amplitude and phase without
significant cross talk [144].

After a decade of development UCSB is currently producing high quality PtSi
and Hf MKID arrays. These are the best optical to infrared (OIR) MKID arrays
ever produced with up to 20,440 pixels, ${\sim}$80% of the pixels functional,
R=E/$\Delta$E$\sim$9.5 at 980 nm, and a quantum efficiency of ${\sim}$35%.
These state-of-the-art MKID arrays are discussed in detail in Optics Express
[197] and in the instrument paper of MEC, a 20 kpix MKID camera permanently
deployed at the Subaru Telescope on Maunakea [218].

Recent breakthroughs in the understanding of MKID noise, in conjunction with a
quantum-limited parametric amplifier [231], have dramatically improved MKID
spectral resolution, as shown in Figure 4. We expect this to improve further
in the next several years as we optimize designs that eliminate the primary
noise source, athermal phonon escape, and approach the Fano limit.

Figure 4: Top: The combined spectra for the Hf/In bilayer device at seven
laser frequencies. Bottom: The noise decomposition for this device. The filled
in green area represents the R achievable by reducing the phonon loss but
keeping the same noise spectrum.

###  2.2 SNSPDs

A Superconducting Nanowire Single Photon Detector (SNSPD) is a superconducting
film patterned into a wire with nanometer scale dimensions (although recently
devices with micrometer-scale widths have been shown to be single-photon
sensitive [116]). A typical SNSPD is made from a film roughly 5 nm thick and
patterned into a meandering wire roughly 100 nm. Arrays of thousands of SNSPDs
have been fabricated with 1 mm2 total active area [224]. A bias current,
$I_{B}$ is applied to the wire that is slightly below the nanowire’s critical
current, $I_{C}$ –the current that will drive the nanowire from the
superconducting to resistive state. Due to the presence of this bias current,
the energy of a single photon is sufficient to drive the superconducting
nanowire into its resistive state. Depending on the readout configuration, the
bias current is either shunted to a bias resistor, or drops due to the high
resistance of the wire. With the reduced bias current, the nanowire returns to
its superconducting states and is ready to detect another photon. These
devices offer a wide-wavelength detection range and can be tailored to
specific optical detection needs. Their lack of a shielding requirement, off-
the-shelf microwave amplifier readout, and high-temperature (T$>$1K, i.e.
relative to other superconducting detector technologies) operation make them
straightforward to implement for a wide variety of applications.

SNSPDs have been reported with single photon sensitivity for wavelengths out
to several microns, timing jitter as low as a few ps [117], dark count rates
(DCR) down to $6\times 10^{-6}$ Hz [52], and detection efficiency (DE) of 0.98
[171]. They have also been have been shown to function in magnetic fields of
up to 6T [121].

These properties are determined by several factors, both intrinsic to the
detector (e.g., material choice and device design) and from external operating
settings (e.g., bias current, and operating temperature). Often these
parameters present a trade-off when optimizing the detector performance.
Detection efficiency is primarily influenced by the optical coupling scheme
(if a photon is absorbed, it can often be detected), but benefits from a lower
energy gap for a given operating temperature and limited constrictions—from
either material grain size or detector geometry—which can limit the applied
bias current. The dark count rate is optimized by limiting sources of
fluctuations—either thermal or bias current—and benefits from a larger
critical temperature ($T_{C}$), energy gap, and critical current density –
conditions opposed to those benefiting detection efficiency. Thus the dark-
count rate is often optimized through operating parameters such as a lower
operating temperature and bias current. Timing jitter performance seems to
benefit from materials with a high $T_{C}$ (related to larger $I_{C}$ which
allows a larger $I_{B}$) and normal resistance—a larger normal resistance
results in larger signal to noise which enables reduced jitter—and a low
inductance which allows for a faster rise time. Jitter performance also
benefits from the use of low noise readout electronics, such as low noise
cryogenic amplifiers.

SNSPDs have been made from a wide range of materials with NbN and WSi two of
the most common. NbN films were used in the very first nanowires and have been
widely used ever since. NbN provides a high $T_{C}$ \- around 15 K for thick
films, but reduced to 7-8 K for the very thin films used for nanowires. WSi
films can have $T_{C}$ as high as 5 K and offer tunable $T_{C}$, resistivity,
and inductance based upon Si content. A key difference between the two films
is crystalline structure. NbN films offer a polycrystalline film and often
require a high temperature growth process to yield larger grain sizes, and the
lower resistances desired for good jitter performance. WSi films are
amorphous, and with a smaller energy gap, have yielded some of the highest
detection efficiency results. However, their corresponding higher resistance
and inductance generally results in a higher jitter and longer reset time than
NbN. Other materials (e.g. NbSi or MoSi) are being investigated for their
amorphous structure and may offer lower inductance than WSi. Some efforts have
been made to realize SNSPDs with still higher $T_{C}$ by using MgB2[50]
($T_{C}>30K$ in thin films) and YBCO[18] ($T_{C}>70K$ in thin films).

SNSPDs are widely implemented across a broad spectrum of Quantum Information
Science and Technology research including quantum communication and quantum
sensing. SNSPDs also have direct applications in HEP [167]. They are used as
detectors for foundational experiments utilizing quantum teleportation and
communication protocols and recently, they are being explored as novel
detectors for dark matter searches[52, 100].

The scalability of row/column readout scheme, such as those employed by the
current kilopixel arrays [225], is limited by a corresponding reduction in
maximum occupancy, signal amplitude, and signal-to-noise [12]. As the number
of pixels scale up toward the megapixel mark, cryogenic readout becomes
essential to limit the complexity of the system.

As part of one of DOE’s recent Microelectronics Co-Design Research programs,
Fermilab is currently leading a collaborative effort to enable the scaling of
large SNSPD arrays, coupled to cryogenic custom readout, both superconducting
and cryoCMOS. The resulting custom ASICs and superconducting readout will
handle fast multi-channel timing and sensor multiplexing directly at cryogenic
temperatures, which should improve the signal to noise of the system and
preserve the timing performance of the sensor. Other benefits include the
increase in maximum rates through the use of active quenching techniques
[170], and the potential to achieve multi-photon resolution, for example via
time-over-threshold measurement, since the risetime has been shown to be
inversely proportional to the square root of the simultaneous detection events
[41]. Through such developments, as well as recent work demonstrating SNSPDs
with wide wire geometries that are amenable to standard optical lithography,
very large SNSPD pixels and large arrays of SNSPDs will continue to open new
applications for HEP.

###  2.3 Germanium Detectors

Silicon CCDs are commonly utilized for scientific imaging applications in the
visible and near infrared. These devices offer numerous advantages described
previously, while the skipper CCD [206] adds to these capabilities by enabling
multiple samples during readout to reduce read noise to negligible levels [9,
58]. CCDs built on bulk germanium offer all of the advantages of silicon CCDs
while covering an even broader spectral range (Fig. 5). Furthermore, while
this technology is still under development, additional features such as
skipper readouts or orthogonal transfer (to correct for distortions due to
atmospheric turbulence) could be added to future generations of these devices.

Figure 5: Representative quantum efficiency curves for silicon and germanium
detectors [152, 168].

A front-illuminated germanium CCD was first demonstrated in the 1970s [187],
and later further explored in the 1990s [1], but these devices suffered from a
variety of limitations such as high dark-current and poor charge-transfer
efficiency (CTE). However, recent improvements to germanium material quality
and advances in surface passivation now enable germanium CCDs with performance
attributes that match silicon CCDs. High-quality bulk germanium wafers are now
commercially available in 150- and 200-mm diameters, enabling device
fabrication in the same facilities used to process silicon detectors.
Furthermore, gate dielectrics with low surface state density can now be grown
on germanium, generally through growth of thin layers of GeO2 via thermal
oxidation and subsequent encapsulation and protection during processing to
avoid etching or decomposition of this critical film. This enables high
charge-transfer efficiency and low surface dark current, both critical for
operation of scientific CCDs.

Germanium CCDs have been under development at both MIT Lincoln Laboratory
(MITLL) and Lawrence Berkeley National Laboratory (LBNL). MITLL has realized
kpixel-class front- and back-illuminated devices [123, 124, 125] fabricated on
200-mm wafers (Fig. 6 a). The current effort at MITLL is focused on
incorporating design and process improvements aimed at demonstrating low read
noise, charge-transfer efficiency suitable for scientific detectors, and
improved yield, with the goal of eventually matching the capabilities of
silicon CCDs on these key metrics. LBNL has worked with PHDS Corp., a
commercial company that produces gamma-ray detector systems, on the
development of 150-mm diameter high-purity Ge (HPGe) wafers. PHDS Corp. has
the capability to grow large diameter, high-purity crystals, and through a
Small-Business Innovation Research grant 150-mm wafers were produced at
Umicore from the PHDS Corp. cystals. Figure 6 b) compares the quantum
efficiency of 1 cm2 photodiodes fabricated on 150-mm diameter Ge wafers at
LBNL. The 10 ohm-cm devices have a depletion depth of about 10 $\mu$m, while
the 650 $\mu$m-thick HPGe diode is fully depleted at 6V substrate bias
voltage. The near-infrared quantum efficiency extends to longer wavelengths
for the HPGe device as expected.

Finally, germanium can also be utilized in hybrid active-pixel sensors. In
this context, germanium offers an important potential advantage over compound
semiconductors: transfer gates and/or amplifiers can be incorporated onto the
detector tier. This enables separation of charge collection and readout, as is
the case in hybrid silicon scientific active-pixel sensors, which should
enable read noise of a few electrons in a radiation-tolerant device with 100%
fill factor.

Figure 6: a) Image of CCDs fabricated at MITLL on 200-mm diameter germanium
wafers [125] . b) Measured quantum efficiency on 1 cm2 photodiodes fabricated
on 150-mm diameter germanium wafers at Lawrence Berkeley National Laboratory.
The depletion depths are about 10 $\mu$m for the 10 ohm-cm Ge, and 650 $\mu$m
(fully depleted) for the HPGe wafer fabricated from a PHDS Corp. crystal and
operated at a substrate bias voltage of 6V. The operating temperature was
100K.

###  2.4 Skipper-CCDs

Skipper-CCDs have an output readout stage that allows multiple non-destructive
sampling of the charge packet in each pixel of the array thanks to its
floating gate output sense node (shown in Figure 7(a)). The charge of each
pixel is moved to the sense node (SN) where it is measured by the output
transistor. Figure 7(b) shows its operation. After the charge is measured, it
is moved back to the summing-well gate (SG) while the voltage of the SN is
reset using the reset transistor (MR). At this time the SN is ready to take a
new sample of the same charge packet. This process can be repeated many times.
All charge measurements have an independent readout noise contribution, and
therefore its final pixel value is calculated as the average of the available
samples [76, 208]. Figure 8(a) shows the reduction of the standard deviation
of the noise error in the pixels as a function of the number of samples per
pixel. The different colors correspond to different time periods (ti) spent to
read each sample. The reduction factor is the square root of the number of
samples. Figure 8(b) shows the histogram of pixels with a charge from 206 to
212 carriers with deep sub-electron noise where the number of carriers can be
discretized with no ambiguity. The charge counting capability is similar in
the entire dynamic range of the sensor from zero to the maximum full well
capacity of the pixels.

The Skipper CCD fabricated on high resistivity silicon [102] has also
demonstrated an extremely low production of dark counts. Recent measurements
in [6] demonstrated production rates close to $10^{-4}$ e- per pixel per day
with a pixel size of 15 $\mu$m by 15 $\mu$m and a thickness of 675 $\mu$m.

Since the single counting capability was proven [206], it has motivated to
build a new generation of Dark Matter [2, 6] and neutrino experiments [155,
75] that will be at the forefront of exploring physics beyond the Standard
Model. The Sub-Electron Noise Skipper-CCD Experimental Instrument (SENSEI),
has produced world-leading constraints on low-mass dark matter searches [6].
Skipper-CCDs have also been identified as a powerful tool for optical
applications such as quantum information imaging giving access to entangled
measurements in momentum and spatial variables for single photons, and
astronomical applications where it provides an attractive approach to reduce
readout noise while keeping the beneficial characteristics of conventional
CCDs.

Since the noise reduction in the sensor involves spending extra time on each
pixel, the readout speed is currently a limiting factor for some applications.
There are several ongoing efforts to improve this aspect including
parallelization of the array readout through multiple single-amplifier output
stages; multiple readout amplifiers on a single readout stage, frame-shifting
architectures (where readout and exposure can be done at the same time), or
dynamic skipper readout, where photon counting is only targeted over a subset
of the detector pixels [51].

Figure 7: (a) Microscope view of the non-destructive readout output stage of
the Skipper CCD. (b) Typical charge transfer operation of the Skipper CCD in
the output stage. Figures were taken from [76]. Figure 8: (a) noise reduction
as a function of the number of samples per pixel. The different colors are
measurements for different integration time per sample. The solid line is the
best fit of the theoretical model to the points. (b) Pixel histogram of an
image taken with deep sub-electron noise. Figures were taken from [44].

###  2.5 Skipper-CMOS

The extremely low readout noise of Skipper-CCDs allows the detection of single
photons in the optical and near-infrared range. Unlike other silicon detectors
with an avalanche gain, with Skipper-CCDs it is possible to count the exact
number of electrons per pixel and therefore the number of photons that
interacted on each pixel, being only limited by the Fano noise [110] [176].
Skipper-CCDs has been identified as a powerful tool for quantum information
science, giving access to entangled measurements in momentum and spatial
variables for single photons, with the initial demonstration recently
completed at Fermilab.

Skipper-CCDs are fabricated in a dedicated facility using a customized process
for scientific CCDs [202]. This process is required to produce the overlapping
of the gates structures needed to achieve high charge transfer efficiency
between pixels. Due to the very low demand of scientific CCDs, compared to
commercial CMOS imagers, the number of facilities dedicated to scientific CCDs
has been reduced to only a few in the world today, and this number is expected
to continue dropping [63]. On the other hand, imagers fabricated in CMOS
process have dominated the market of high-demand consumer cameras, and
therefore several fabrication facilities with many processing options are
available. Moreover, previous works have successfully implemented CCDs in
different single-poly CMOS fabrication technologies achieving high charge
transfer efficiencies [34][59][139][80], and the possibility to implement
Skipper-CCDs in CMOS have been recently analyzed in [195].

The scaling of CMOS technology of at least 100 nm has allowed the
implementation of pixels with a very low capacity, and therefore, high
sensitivity and low noise (1-2 $\rm e^{-}$) at room temperature and high frame
rates (50-100 fps) [135][84]. There are two new developments on CMOS
technologies that have achieved noises low enough to be considered photon-
resolving technologies. On 2021 Hamamatsu announced their first commercial
camera using what they call “quantitative CMOS” (qCMOS). This sensor, with a
proprietary processing, is able to get a readout noise of 0.27 $\rm e^{-}$ at
5 fps, and 0.43 $\rm e^{-}$ at 120 fps, with a 9.6 Mpix frame and a pixel size
of 4.6 $\mu m$[54]. The camera electronics has an active role, by calibrating
each pixel in real time in order to correct for sensitivity non-uniformity’s
and keeping a very stable temperature[55]. The second device is a Quanta Image
sensor(QIS), by Gigajot, a spin-off from Dartmouth college, where the QIS
(based on binary pixels called “jots”) was developed[136][82]. Their sensors
achieve 0.19 $\rm e^{-}$ at 30 fps with 16 Mpix frames and a pixel size of 1.1
$\mu m$. Their process consisted on suppressing temporal noise from the pixel
source followers and also increasing considerably the conversion gain[137].

The aim of the Skipper-CMOS effort is to demonstrate the non-destructive
charge readout of Skipper-CCD technology in CMOS technology and benefits from
the mainstream commercial integrated circuits developments. This will bring
the possibility to achieve an imager composed of pixels with extremely low
readout noise, that allows single-photon detection and precise counting in a
wide dynamic range. Also, this effort address directly the current challenge
in the fabrication of scientific Skipper-CCDs. It will have the additional
advantage of allowing in-chip integration of a video processing stage (on-chip
ADC), with the potential of converting the Skipper-CCD into a fully digital
device. Figure 9 shows the pixel concept of a CMOS imager with non-destructive
charge readout like Skipper-CCD detectors. The collection area is performed by
an pinned-photodiode (PPD), which provides low dark-current and large
collection area with all the optical characteristics of the CMOS process [83].
In a similar way to 4T CMOS imagers, the charge is transfer from the PPD to
the sensing stage through a gate, called transfer gate in figure 9. Unlike
CMOS imagers, the floating diffusion in the sensing stage has been replaced by
the output stage of an Skipper-CCD. In similar fashion to Skipper-CCD, the
output stage is composed by a buried channel CCD with four gates, where one is
the floating gate (FG) used for sensing of the charge packet non-
destructively. The MOSFET that shares the gate with the floating gate, is in a
source follower configuration for charge to voltage conversion, and the other
MOSFET is used for resetting the floating gate. The operation of the output
stage, is similar to the readout of Skipper-CCDs [76]. The output stage can be
covered by metal layers to shield it from the photon source. As was previously
mention, with CMOS process it is possible to achieve a low readout noise, and
therefore few skipper samples would be required to reach the sub-electron
readout noise regime. Moreover, the high parallelization of the imager can
allow image acquisition at high speeds. This development can provide the
necessary detectors for future astronomy, quantum imaging and basic energy
science that require increase the readout speed of the actual available
Skipper-CCDs, and also could be applied in dark-matter and neutrino
experiments.

Figure 9: Pixel concept of the Skipper-CMOS imager.

###  2.6 Single Photon Avalanche Diode Array Based Detectors

The single photon avalanche diode (SPAD) is an avalanche photodiode biased
above its breakdown in a metastable state awaiting an avalanche to be
triggered by a photoelectron or a thermally excited carrier. This operation is
often termed as Geiger-mode in reference to the metastable operation of Geiger
counters. Once triggered, a resistor or an electronic circuit quenches the
current flowing through the photodiode, thus reducing the photodiode bias
below the breakdown voltage. A SPAD is a non-linear photodetector by nature
because its response is identical whether it was triggered by one or many
photons. The SPAD’s response is Boolean: it is either waiting for a carrier in
the metastable state above breakdown or it is triggered with a current flowing
until quenched.

The development of SPADs started in the mid 70s [16] including discrete
quenching electronics with continuous improvement of performance through
device understanding and better electronics [210, 60, 120]. In the 90s, many
groups moved to integrate many SPADs into a single large-area photon counting
device leading to the first SiPMs, with resistively quenched SPADs in a large
parallel array in the early 2000s [183, 229, 113, 115, 114, 175, 173, 174,
158, 185].

####  2.6.1 Enhancing VUV sensitivity

Silicon SPADs and SiPMs have excellent PDE in the visible spectrum. However,
sensitivity falls dramatically for wavelengths below 350 nm. This is due first
by the commonly used passivation materials which absorb strongly at these
wavelengths. But foremost is the very short penetration length of those
photons (few nanometers) causing the photogenerated carriers to be trapped by
surface fields and recombined through surface defects. As mentioned before,
liquid argon detectors often rely on wavelength shifting materials to
circumvent this problem. Through much internal electric field and passivation
layer optimization,
Hamamatsu‡‡‡https://www.hamamatsu.com/us/en/product/optical-sensors/mppc.html
and FBK§§§https://sd.fbk.eu/en/projects/detail/sipm-evolution/ now offer SiPMs
reaching 10% to 20% PDE at the liquid xenon scintillation wavelength (175 nm).
These are contenders for nEXO and DARKSIDE experiments [153, 109, 89, 3].

Yet in the 1990s, Jet Propulsion Laboratory showed that CCD sensitivity can be
increased closed to the reflection-limited quantum efficiency of silicon down
[101]. This was done by blocking the surface fields and traps through the
epitaxial growth of a strongly doped very thin silicon layer (delta-doping).
Quantum efficiency exceeding 50% were demonstrated in CCDs down to 125 nm
wavelength [159]. The method was demonstrated efficient on backside
illuminated SPAD based detectors by Schuette in 2011[188]. Other methods to
address the surface fields and traps issues were also demonstrated [154]. Work
is being done at Caltech (D. Hitlin) to enhance SiPMs for the detection of the
fast scintillation component of BaF2 [99]. An extensive study of the delta-
doping approach to enhance VUV sensitivity in frontside illuminated SPAD based
detectors was done by Vachon [213].

Enhancing PDE above 30% would bring drastic performance improvements to noble
liquid TPC based experiments. Notably, it would allow direct detection of
liquid argon scintillation light.

####  2.6.2 Near Infrared Enhanced Silicon Photomultipliers

The single-photon detection nature of SPAD makes it an ideal candidate for
applications requiring high detection sensitivity and low temporal jitter such
as LIDAR, biophotonics and quantum key distribution. SPADs made of silicon can
in theory absorb photons up to $\sim$1110 nm (E${}_{g}=$ 1.11 eV) but
practically is limited to wavelength below 900 nm. Nonetheless silicon SPADs
are being used in the infrared wavelength range [198, 38, 94, 166]. However,
the typical p+n structure of SPADs is not suitable for near-infrared light.
Indeed, the pn junction is usually found at depths of 100 nm to 1 µm while NIR
photons are mainly absorbed beyond 10 µm. Nevertheless, the exceptional
qualities of silicon and its compatibility with CMOS technologies have
justified manufacturers to adapt them enhancing their NIR sensitivity.

The most frequent approach is to use an n+p junction with a thick p-type
epitaxial layer in order to favor the collection of deep photogenerated
electrons. Increasing the detection volume increases the SPAD photon detection
efficiency but at the expense of an increase in dark count rate and temporal
jitter. It also impacts manufacturing techniques and, for example, calls for
higher aspect ratio trenches to isolate SPADs when put together in an array.
Collecting NIR photons favors backside illuminated structures where photons
are incident on the thick p-type layer side (or substrate) of the device. In
this case, a structure with separate absorption and multiplication regions is
generally used [165, 11, 122]. A thick depletion region benefits the NIR-
enhanced SiPMs [93]. It also reduces capacitance and thus decreases
correlated-type events such as afterpulsing and optical crosstalk [8].

LIDAR systems in the automotive market has prompted several companies to
produce analog NIR-enhanced SiPMs. They reach PDEs on the order of 10 to 20%
for wavelengths between 800 and 900 nm [105, 162, 56, 143, 29, 163]. Single
SPAD modules with a very large diameter and thickness reached PDEs up to 40%
at 905 nm and 55% at 850 nm [201]. Such high PDE is not yet achieved in SiPMs.

####  2.6.3 Photon-to-Digital Converters

Single photon avalanche diodes (SPAD) are the building block of both analog
SiPM and photon-to-digital converters (PDC). An analog SiPM sums the charges
of individual SPADs, passively quenched by a resistor and all connected in
parallel to the output node. In a PDC, each SPAD is coupled to its own
electronic quenching circuit.¶¶¶Note that digitally quenched SPADs were used
before the advent of SiPMs [16]. This one-to-one coupling provides control on
individual SPADs and signals each detected avalanche as a digital signal to a
signal processing unit within the PDC. Hence, PDCs provide a direct photon to
digital conversion considering that intrinsically a SPAD is a Boolean detector
by design. On the contrary, the summed current signal of an analog SiPM
requires a low noise high power preamplifier, current amplifier or
transimpedance amplifier followed by a shaping amplifier and analog-to-digital
converter to obtain a digital information. All this to get the Boolean
information that was readily available at the sensor level. Moreover, as the
charge of each avalanche varies from SPAD to SPAD and from event to event,
current fluctuations blur the signal for the same amount of photons detected
and limit single photon resolution to the low count range.

Analog SiPMs have large output capacitance coming from all SPADs connected in
parallel. This needs to be considered in details while designing the front-end
electronics, in particular with respect to power consumption. In large area
systems, performance compromises must be made between low power and fast time
response. This is not the case for PDCs in which the SPAD quenching can be
made with very low power consumption (tens of µW). Their output being digital,
PDCs with various wavelength sensitivity (or other properties) could be
assembled more easily in a system. The direct conversion also eliminates
signal variations due to SPAD-to-SPAD and event-to-event fluctuations. A PDC
therefore offers single photon resolution up to its full dynamic range (i.e.
number of SPADs in the array). The individual control on each SPAD allows to
disable noisy cells either due to fabrication or degradation. In a PDC one can
therefore mitigate SPAD degradation, be it due to either normal ware or
radiation damage, and maintain the noise floor to its minimum at the cost of
marginal dynamic range loss (number of disabled SPADs). Using an active
circuit to control and readout the SPADs allows to configure and optimize the
hold-off delay and recharge time according to specific applications. In
particular by properly tuning the hold-off delay, the impact of afterpulsing
events can be very efficiently reduced[212], a relevant feature in cryogenic
detectors. Lastly, depending on the application’s needs, complex digital
signal processing can be embedded in the PDC. This can include dark count rate
filters, time-to-digital converters (TDC), data compression, etc.

Digital SiPMs were first reported in 1998 [21] by the MIT Lincoln Lab and many
contributions followed [21, 22]. A major step came with microelectronics
integration to fabricate both the SPAD and readout quenching circuit in a
single commercial process [229, 113, 115, 114, 175, 173]. These innovations
led to the first multi-pixel digitally read SPAD arrays [174, 158]. Such 2D
digital SPAD arrays have progressed tremendously [see 185, and references
therein]. When implemented in two dimensional integration (i.e. the SPAD along
side the CMOS functions on the same chip), a compromise must be made between
the area devoted to the SPADs and that devoted to the CMOS functions. An
efficient way to overcome this is to stack the SPAD array onto the CMOS
readout in what is called 3D integration. Many large players of the industry
have followed this path for imagers [e.g., 194]. 3D integration allows for
larger sensitive area, and more uniform connections to the SPAD in order to
reach ultimate performances in timing resolution and/or power dissipation. A
review and at length discussion on the subject was published in 2021 by the
authors [169]. We are designing PDC based systems for various applications
including noble liquid detectors (in particular LXe for nEXO), scintillation
neutron detectors, positron emission tomography (PET), and QKD. Besides QKD in
the NIR, these applications use the high sensitivity of silicon in the visible
spectrum and include enhancing work to enhance their VUV sensitivity.

###  2.7 Photon Counting with TES

A superconducting Transition-Edge Sensor (TES) photon detector, which utilizes
a patterned superconducting film with a sharp superconducting-to-resistive
transition profile as a thermometer, is a thermal detector with a well
developed theoretical understanding. When a visible or infrared photon is
absorbed by a TES, the tiny electromagnetic energy of the photon increases the
temperature of the TES and therefore changes its resistance. See Figure 10(b)
for the temperature dependence of a TES resistance. The change of the TES
resistance is measured with a sensitive SQUID current amplifier in a voltage
biased circuit as shown in Figure 10(a), which provides linear operation and
fast response because of a negative electro-thermal feedback [106] to the
detector. TES photon detectors have been developed to measure single photons
at a high efficiency for quantum communication [177, 128, 88, 87] and for
axion-like particle searches in the shining light through the wall experiments
[28, 71]. Applications also include large area photon measurement in direct
detection of dark matter particles [15, 178] for an enhanced background
discrimination, calorimetric measurement of scintillation light and triplet
excimers of superfluid helium [98, 47] for searching light dark matter
particles, and high-precision astrophysical observations in the wavelengths
between ultraviolet and infrared [39]. Moreover, TES detectors can be
multiplexed enabling arrays of large channel counts [30, 70, 68]. Multiplexers
for detector arrays using 16,000 TESs have already been successfully
implemented [30], and new developments exploiting microwave resonance
techniques [68] have the potential to increase the multiplexing capacity by
another factor of 10.

Figure 10: (a) Schematic of operation of a TES detector with a resistance $R$.
The TES is in series with an inductor $L$ coupling to a readout SQUID current
amplifier. A bias current $I_{b}$ together with a shunt resistor $R_{sh}$
provides a voltage bias across the TES. (b) Resistance and temperature
relation of an Ir/Pt bilayer TES fabricated and measured at Argonne National
Laboratory.

A TES photon detector has an unparalleled energy resolution, which is an
unique figure-of-merit advancing scientific discoveries. Operated under a
negative electro-thermal feedback, the expected energy resolution [106] of a
TES detector is

$\Delta E\approx
2.35\sqrt{\frac{4k_{B}T_{c}^{2}C}{\alpha}\sqrt{\frac{n}{2}}},$ (1)

where $k_{B}$ is the Boltzmann constant, $T_{c}$ is the TES transition
temperature, $C$ is the TES heat capacity, and $n$ is an index around five
determined by electron-phonon decoupling [90]. Here
$\alpha\approx(T/R)(dR/dT)$ is a parameter characterizing the TES transition
profile, where $R$ is the temperature-dependent resistance of the TES in the
transition. Therefore, a TES detector favors a low-$T_{c}$, a sharp
superconducting-to-resistive transition with a large $\alpha$ and a small heat
mass (through $C$). A low-$T_{c}$ TES, which can be made of a W film [5, 81]
or an Ir-based bilayer [97], is an effective way to improve energy detection
resolution. Reducing the heat capacity of a TES by minimizing its size can
also effectively enhance its resolution [221, 164]. An more advanced
technology to reduce the heat capacity of a TES for a high resolution is to
use Quasiparticle-trap-assisted Electro-thermal-feedback Transition-edge-
sensor (QET) [40, 180]. By utilizing a tailored TES with a small volume in
intimate metallic contact with a large area superconducting photon absorber, a
TES detector measures photons with a high resolution and with a large photon
collection area. As an enabling photon detection technology, the well-
understood TES detector is a natural choice to count photons in many
applications. There are several major R&D and instrumentation opportunities
summarized below:

  * •

TES materials and fabrication techniques. The researches include precise
$T_{c}$ tuning in the range from sub-Kelvin down to 10 mK, QET technologies
[40, 180] for a large area photon detector with a high quantum efficiency,
nanoscale TES detectors [221, 164], and technologies to increase TES detector
fabrication throughput.

  * •

Large area pixelated TES photon detector array for direct detection of dark
matter particles. Each detector can be a patterned TES or a more advanced QET
[40, 180] with an enhanced photon collection area. The detector has a high
energy resolution, therefore a low energy threshold covering the wavelengths
from ultraviolet to infrared. This large area high resolution photon detector
is on demand in Sub-GeV dark matter searches. The TESSERACT project, which is
a R&D project funded under DOE Dark Matter Small Projects New Initiatives
[160], requires such detectors to measure scintillation light and other
excitations from detection targets including superfluid helium [98], gallium
arsenide crystal [65] and sapphire crystal [13]. A large area high resolution
photon detector is also required in new proposals searching for sub-Gev dark
matter particles, such as high efficiency spectroscopic measurements of
infrared photons from molecular vibration excitations of gases [74] and
hydrogen-rich crystals [219] (which are water ice and hydrocarbons that have
O-H and C-H bonds) for spin-dependent nuclear scatterings, and high efficiency
and low threshold measurements of the fluorescence photons when a bosonic dark
matter particle is absorbed by a molecule [19]. Moreover, spectroscopic
measurements with high resolution photon detectors help to identify a signal
in searching for dark matter made out of axion-like particles or hidden
photons. These include a dielectric haloscope [26] which converts axion-like
particles and dark photons into photons in periodic photonic materials, and a
spherical reflective surface dish antenna which converts axion-like particles
and dark photons into photons emitted perpendicular to the surface [103].

  * •

Ultra-sensitive TES photon detectors with a single sub-terahertz photon
sensitivity [221, 164] for detection of dark matter axions at a large mass.
The detectors are required to detect dark matter axions using a dielectric
haloscope which consist of dielectric disks placed in a magnetic field to
convert axions into photons [42], or a haloscope that consists of a
cylindrical metal barrel in magnetic field to convert axions into photons and
a novel parabolic reflector to focus the photons onto a photon detector [129].

  * •

TES infrared photon detectors with very high energy resolving power in
astrophysics and cosmology. Such detectors are required not only in
measurements of infrared radiations from ions, atoms, molecules, dust, water
vapor and ice, but also astrophysical observations that trace our cosmic
history from the formation of the first galaxies and the rise of metals to the
development of habitable worlds and present-day life. The infrared detector
researches are engineering challenges in TES sensor design, fabrication, and
multiplexing readout of the large arrays [150, 151].

  * •

A TES detector with a large dynamic range and high quantum efficiency for
quantum sensing. The photon number-resolving capability of the detector can
allow a bit error probability that is unconditionally better than the standard
quantum limit (SQL) [204].

###  2.8 Novel Photoconductors for VUV photon detection

Noble element TPCs produce a broad wavelength range of light from the vacuum
ultraviolet (VUV) scintillation photons of ($<190$ nm), to “blue” Cherenkov
photons (300nm - 600nm), and near infrared (NIR) light (600 - 2500nm). In a
conventional noble element TPC the light detection devices utilize the semi-
transparent charge readout planes (wires) and reside behind them. Although the
noble elements themselves are highly transparent at these wavelength, the
majority of commercially available optical detectors such as SiPMs and
photomultiplier tubes are not sensitive to the VUV spectral range. While some
devices now exist with VUV sensitivity [73, 104, 226], this problem has
traditionally been solved in large-scale systems by employing a wavelength
shifting coatings to convert VUV light into a visible range where it can be
detected by conventional sensors. One of the most commonly used fluors is the
organic compound tetraphenyl butadiene (TPB). However, the long term stability
and behavior of TPB and other wavelength shifters is still largely unknown
with some instabilities and undesirable effects already being noticed [20].

#### Amorphous Selenium

Amorphous Selenium (a-Se) has long been identified as a useful photoconductor,
first being widely used in photocopiers in the 1970’s [222] and later
garnering widespread commercial use in direct conversion active matrix flay
panel imagers in the fields of digital mammography [111] and digital breast
tomosynthesis [230]. The nature of amorphous materials means it easy to
deposit over large areas (e.g. through thermal evaporation or similar
techniques) without the use of expensive bonding processes. Moreover, a-Se
detectors have a high quantum efficiency (QE), defined as
$QE(E_{\gamma})=1-e^{-\alpha d}$ where $\alpha$ is the absorption coefficent
for a given energy of photon ($E_{\gamma}$) and $d$ is the thickness of the
a-Se, in the X-ray regime where they have been typically used. The charge
released ($Q_{\gamma}$) due to a photon interaction in the a-Se is
characterized by $Q_{\gamma}=E_{\gamma}/W_{\pm}$ where $W_{\pm}$ is the
conversion gain (in keV/electron-hole pair) and depends on a number of
application specific parameters such as bias field, sensor thickness, and
$E_{\gamma}$. For the energies in the X-Ray regime, a-Se based photon
detectors have a very high QE and a good charge yield, which have made them
very popular photon integrating imagers.

The usefulness of a-Se based detectors as photon counting devices has not been
met with as much interest due to the relatively low charge gain when compared
to other materials (such as silicon), low charge carrier mobility, and poor
time resolution. This has held true until relatively recently, where
amplification from avalanche gain due to impact ionization at high external
fields allows a-Se based detectors to achieve charge conversion similar to
crystalline semiconductors. These high-gain avalanche rushing photoconductors
(HARPs) have been commercialized and used in the broadcast industry.
Additionally, detector designs using multi-well solid state detectors have
vastly improved the temporal response while simultaneously being able to
achieve avalanche gain and through the introduction of various dopants (e.g.
As, Cl, and CeO2) the charge mobility has been improved while keeping the
intrinsic dark current low ($\mathcal{O}$(pA/mm2).

A series of recent results have peaked the interest of the groups involved in
this research enough to begin to explore the feasibility of using a-Se as a
photoconductor in noble element TPC’s. These results include an a-Se p-n
junction device sensitive to UV light [181], microfabricaiton of a UV
sensitive a-Se detector [4], the production of a hybrid a-Se CMOS photon
counting sensor [43], and timing resolutions less than 1 nanosecond for a-Se
avalanche detectors [119]. Taken together, it seems that the synthesis of
these approaches along with integration into a noble element TPC would provide
a potentially game changing method for direct UV photon detection with the
possibility to expand to a wider range of wavelengths.

Recent experimental work has shown the viability of aSe in a cryogenic
environment to respond to VUV light at low applied electric field ($E\leq
5$V/$\mu$m). This technique utilizes a non-standard geometry for aSe detectors
dubbed a “horizontal” configuration in order to circumvent the fact that the
typical electrodes used (e.g. ITO, gold, copper, etc) will result in a large
fraction of all the UV light being absorbed. A “horizontal geometry” can be
constructed from a bare printed circuit board (PCB) with interdigitated
electrodes, as shown in left of Figure 11. The selenium is thermally
evaporated directly onto the board and thus can be exposed directly to the UV
source. The right of Figure 11 shows an example of the response of such a aSe
detector to VUV light across various temperatures. While the size of the
signal does decrease as a function of temperature, the detector does continue
to respond all the way down to $\sim 77$K.

Figure 11: Left: Schematic the horizontal geometry under development for
absorption of UV light by an aSe based detector. Right: Early results showing
the response of an aSe based photon detector as a function of temperature
ranging between 290 K to 77 K.

Additional work is ongoing to adapt this concept to allow for higher applied
electric fields $\mathcal{O}$(80 V/$\mu$m) in order to allow charge gain and
to explore the design considerations needed for single photon sensitivity.

#### Organic Semiconductors

The field of organic semiconductor devices has seen rapid growth in recent
years. Their use can be found in a wide range of devices from QLED displays
used in cell phones to quantum-dot based televisions, to wearable tech, and in
the form of photovoltaics for solar energy. They are relatively simple and
cheap to produce, easily adaptable for use with modern additive manufacturing
technologies, and are highly tunable with regards to their optical bandgap and
chemical structure. Recent work on organic photodiodes has demonstrated their
low noise can be on the level of state-of-the-art silicon photodiodes [86].
These properties offer exciting possibilities for their use high energy
physics experiments. Possible applications include, large-area pixeled
photosensors which could be additivity manufactured on rigid or flexible
substrates. The use of VUV transparent conductive electrodes would allow for
direct detection of VUV scintillation light in liquid noble gas detectors. Due
to their highly tunable optical bandgaps, a stack of organic photodiodes can
be used as a spectroscopic or color sensitive photosensor. This could provide
for example, separation of Cherenkov and scintillation signals within the same
device without loss of photo coverage. This type of stack geometry has already
been demonstrated for red-green-blue sensitive optical sensors [190].

While offering numerous advantages there are still significant R&D technical
challenges which need to be addressed. The nature of charge transport within
$\pi$-conjugated materials results in a slower response time compared to
silicon photosensors [53]. While high photoconductor gain has been
demonstrated for CW light [49], gain has yet to be demonstrated for faint
pulsed light characteristic of scintillators. Their performance in extreme
environments such as cryogenic temperature or high radiation fields.
Investigations into possible VUV transparent electrodes are warranted for use
in direct detection of liquid noble gas detector VUV scintillation light.

##  3 Summary

In Section 1 we discussed the needs for photon counting developments in the
next generation of HEP experiments. The next generation of spectroscopic
cosmic surveys will become more efficient with photon counting sensors and
improved sensitivity in the IR (see Sec. 1.1). These technologies could also
play an important role in future space telescope missions. Photon counting
technologies have also demonstrated world leading performance in the direct
search for dark matter (Sec. 1.2) and low energy neutrino experiments (Sec.
1.3). Furthermore, unlocking the full potential of noble elements TPCs
requires large area, high UV efficiency sensors capable of detecting both
light and charge. These developments needed for HEP will also impact quantum
sensing, research in basic energy science and low energy radiation detection
applications as discussed in Sec. 1.5.

In Section 2 we discussed the opportunity to address these needs with
technology currently being developed as part of the HEP program. Photon
counting in the visible can be achieved now with semiconductor technologies
with large number of pixels in skipper-CCDs and CMOS sensors (Sections 2.4 and
2.5 ), the capabilities of these sensors are being extended into lower
energies with Ge as discussed in Sec. 2.3. SiPMs allow single photon
resolution in the visible range and will be assembled into very large area
photon detection systems, in particular for noble liquid detectors. Research
is pursued to enhance their sensitivity both at NIR and VUV wavelengths (Sec.
2.6.1 and 2.6.2). PDCs perform the direction conversion of photon counts to
digital signal and offer low power consumption, low time jitter capability,
and embedded signal processing (Sec. 2.6.3). Photon counting with higher time
resolution and for lower energy photons is possible in superconducting
detectors TES (Sec. 2.7), MKIDs (Sec. 2.1) and SNSPDs (Sec. 2.2), with
significant effort ongoing to make larger arrays of these cryogenic detectors.
Novel photon detectors for VUV are being developed for noble elements TPCs as
discussed in Sec. 2.8.

The photon counting technologies presented here are maturing fast. The further
development of these promising photon counting technologies as part of HEP is
expected to have a large impact on the field in the coming decade.

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