# Enhancement of molecular coherent anti-Stokes Raman scattering with silicon
nano-antennas

Shamsul Abedin1, Yong Li2, Abid Anjum Sifat3, Khokan Roy2, Eric O. Potma2,3
<EMAIL_ADDRESS>2Department of Chemical and Biomolecular Engineering,
University of California, Irvine  2Department of Chemistry, University of
California, Irvine  3Department of Electrical Engineering and Computer
Science, University of California, Irvine

###### Abstract

Surface-enhanced coherent anti-Stokes Raman scattering (SE-CARS) takes
advantage of surface plasmon resonances supported on metallic nanostructures
to amplify the coherent Raman response of target molecules. While these
metallic antennas have found significant success in SE-CARS studies, photo-
induced morphological changes to the nanoantenna under ultrafast excitation
introduce significant hurdles in terms of stability and reproducilibty. These
hurdles need to be overcome in order to establish SE-CARS as a reliable tool
for rapid biomolecular sensing. Here we address this challenge by performing
molecular CARS measurements enhanced by nanoantennas made from high-index
dielectric particles with more favorable thermal properties. We present the
first experimental demonstration of enhanced molecular CARS signals observed
at Si nano-antennas, which offer much improved thermal stability compared to
their metallic counterparts.

##  I Introduction

Surface-enhanced Raman scattering (SERS) is a popular tool for molecular
sensing at ultralow analyte concentrations. In the conventional implementation
of this technique, antennas composed of metallic nano-structures are used to
efficiently capture and confine light to nanoscopic regions that contain the
molecular species of interest McCreery (2005); Fast and Potma (2019). The
molecule is efficiently driven by the confined optical fields in the
electromagnetic “hotspot” and the nanoantenna then efficiently outcouples the
induced near-field Raman polarization to far-field radiation, which is
subsequently captured with a photodetector Moskovits (1985); Willets and Van
Duyne (2007); Zhang  _et al._ (2013). Ever since the inception of SERS in 1973
Fleischmann  _et al._ (1974), numerous SERS enhancement structures have been
devised and the technique has been successfully extended to capture Raman
spectra from single molecules Nie and Emory (1997); Le Ru and Etchegoin
(2012); Dieringer  _et al._ (2007). The mechanisms behind optical field
localization and enhancement of the signal in SERS have been intensely studied
and are generally well understood. While SERS allows enormous enhancement of
the feeble Raman signal, acquisition rates beyond one to few tens of Hz are
difficult to achieve in the single or few-molecule limit Kneipp  _et al._
(1998); Chen  _et al._ (2018). DNA sequencing, rapid molecular screening and
other high-throughput applications necessitate faster readout rates that SERS
alone may not be able to provide.

Surface-enhanced analogues of coherent Raman scattering (CRS) methods are
promising candidates to probe molecular vibrations at much faster rates
compared to SERS. Surface-enhanced CRS was first demonstrated in 1979 where
the coherent anti-Stokes Raman scattering (CARS) signal from liquid benzene
was amplified with the help of surface plasmon polaritons supported on a flat
Ag film antenna Chen  _et al._ (1979). Subsequent progress on surface-enhanced
CRS has focused largely on engineering metallic nanoantennas based on Au and
Ag to increase and/or optimize the sensitivity of detection Kumar  _et al._
(2020); Fabelinsky  _et al._ (2018); Hayazawa  _et al._ (2004); Ichimura  _et
al._ (2004); Liang  _et al._ (1994); Voronine  _et al._ (2012); Steuwe  _et
al._ (2011); Addison  _et al._ (2009); Zhang and Song (2017); Fast  _et al._
(2016); Kenison  _et al._ (2017); Zong  _et al._ (2022, 2021); Frontiera  _et
al._ (2011); Schlücker  _et al._ (2011). The quest has seen considerable
success and single-molecule sensitivity has been demonstrated by multiple
groups Koo  _et al._ (2005); Yampolsky  _et al._ (2014); Zhang  _et al._
(2014); Zong  _et al._ (2019). However, significant local heating at metallic
junctions has proved to be a major obstacle, affecting the reproducibility of
the SE-CARS measurements. When subjecting metallic nanoantennas to ultrafast
irradiation, the local peak intensities can reach $\sim 10^{12}-10^{13}$
W/cm2, a range in which photo-induced ionization of the molecule can happen
due to the bending of the potential followed by electron tunneling Keldysh
_et al._ (1965); Sun  _et al._ (1993); Groeneveld  _et al._ (1995); Elsayed-
Ali  _et al._ (1991); Bouhelier  _et al._ (2005); Li  _et al._ (2013);
Brongersma  _et al._ (2015); Alabastri  _et al._ (2015); Dey  _et al._ (2016);
Crampton  _et al._ (2016). The morphological changes to the antenna due to
intense local heating and plasmon-induced chemistry make it notoriously
difficult to perform SE-CARS measurement in a reliable and reproducible manner
Fast and Potma (2019).

The reproducibility challenges associated with SE-CARS measurements based on
plasmonic nanoantennas originate from photo-induced heating effects that are
intrinsic to metals. To make SE-CARS systems more reliable, moving beyond
these lossy and fragile metallic substrates is necessary. High-index
dielectric particles such as Si, Ge and GaP are known to exhibit favorable
electromagnetic and thermal properties at the optical and near-infrared
frequencies Caldarola  _et al._ (2015); Albella  _et al._ (2014, 2013);
Alessandri and Lombardi (2016); Barreda  _et al._ (2019). These properties
make them particularly suitable for surface-enhanced spectroscopy
applications, where heating of the antenna is a limiting factor. In
nanoantennas based on these materials, localization of the incident optical
fields occur as strong displacement currents are generated inside the
material. These currents produce electromagnetic fields both inside and
outside the material, which lead to confinement of the incident
electromagnetic fields when nanocavities are formed between these dielectric
particles. The potential of dielectric nano-structures to efficiently confine
the electromagentic field has been known for a while and has been implemented
in some SERS studies Alessandri and Lombardi (2016); Bontempi  _et al._
(2014); Wang  _et al._ (2011); Khorasaninejad  _et al._ (2012a, b); Huang  _et
al._ (2011). We have recently shown that the use of dielectric materials in
antenna designs can help mitigate photo-induced instabilities in SE-CARS
measurementsAbedin  _et al._ (2022). That work focused on antennas formed
between a Si nanoparticle and a flat Au film, which afforded reproducible SE-
CARS signals, but suffered from a strong electronic two-photon excited
luminescence (TPEL) background originating from the gold film.

In our current work, we present the first demonstration of SE-CARS
measurements without the involvement of surface plasmons supported by metallic
nanostructures. Instead, we use a Si-based all-dielectric nano-antenna with
favorable thermal properties for stabilizing the CARS signal as well as to
substantially reduce the TPEL background. Using all-dielectric nanojunctions,
we show that the stability of the CARS signal is dramatically improved
relative to the SE-CARS signal observed from Au nano-dumbbell based antennas.
This work paves the way for future surface-enhanced coherent Raman sensors
that offer the reproducibility needed for reliable high-speed biomolecular
screening applications.

##  II Methods

The silicon-based nano-antennas in this study are assembled from 150 nm
diameter, pyridine-functionalized Si nanoparticles (Meliorum Technologies),
which are used without further purification. The nano-particles are suspended
in water, sonicated and spin-cast on a BK7 coverslip prior to measurement.
Nanojunctions are formed by spontaneous congregation of the particles during
the spin-casting process. We use gold-based nano-antennas as a reference
system for comparison. This antenna system has previously been used for SE-
CARS experiments.Yampolsky  _et al._ (2014); Crampton  _et al._ (2016) The
system consist of dimers of $\sim 90$ nm diameter Au nanospheres, covered with
trans 1,2-bis(4- pyridyl) ethylene (BPE), obtained from Cabot Security
Materials. The nano-dumbbells are encapsulated in a silica shell of thickness
40-70 nm. The nano-dumbbells are suspended in water, sonicated and spin-cast
onto BK7 glass coverslips. The SE-CARS spectrum of the BPE-functionalized Au
dumbbell antenna shows two clear peaks in the 1560-1650 $\rm{cm}^{-1}$ range,
attributed to the pyridyl ring stretching mode and the ethylene C=C stretching
vibration (see Supporting Information).

Figure 1: Schematic of the SE-CARS microscope. GS, galvanometric scanner; M,
mirror; DM, dichroic mirror; OL, objective lens; CL, condenser lens; PMT,
photomultiplier tube

A schematic of the SE-CARS microscope system is shown in Figure 1. The SE-CARS
system is based on a laser-scanning optical microscope (Fluoview 300, Olympus)
with an Olympus IX71 inverted microscope frame. A modelocked fiber laser
(aeroPULSE, NKT Photonics) with $\sim$2 ps long pulses and 80 MHz repetition
rate provides the wavelength-fixed Stokes beam (1031 nm) for the SE-CARS
experiments. A portion of this beam is frequency-doubled and the generated
515.5 nm beam synchronously pumps an optical parametric oscillator
(Picoemerald, APE) to generate the tunable (700-960 nm) pump beam. The beams
are spatially and temporally overlapped and subsequently conditioned to
produce linearly polarized beams with clean transverse Gaussian profiles. The
collinear beams are then focused using a water immersion 1.15 NA 40x objective
lense (Olympus). The generated anti-Stokes signal is collimated using a
condenser, filtered (775 $\pm$ 25 nm, Semrock) and detected using a photo-
multiplier tube (R3896, Hamamatsu). In the experiments involving Si-particles,
the average power for each beam is varied between 500 $\mu$W - 2 mW at the
sample plane. A galvanometric scanner allows raster scanning over an 117
$\mu$m $\times$ 117 $\mu$m area of the sample, with a pixel dwell time of 4.27
$\mu$s per pixel. This corresponds to 1.12 frames/second acquisition rate for
a $512\times 512$ frame. For SE-CARS measurements on Au nano-dumbbells, the
power for each of the incident beams is varied between 10 $\mu$W - 500 $\mu$W
at the sample plane, while using the same dwell time of 4.27 $\mu$s per pixel.

Lumerical FDTD is used to perform local electromagnetic field calculations
using the finite-difference time-domain (FDTD) method. Data for the dielectric
function of all the materials are taken from the Palik library Edward and
Palik (1985). In the simulations, we consider nanojunctions formed between two
Si nanoparticles, forming a dimer as the exemplar nanoantenna system. Fine
meshing of 0.1 nm is used near the Si-dimer junction. The same geometry is
used to calculate the absorption spectrum of the antenna system.
Electromagnetic field enhancement data for the Au nano-dumbbell system are
taken from Crampton et al. Crampton  _et al._ (2016).

COMSOL Multiphysics is used to calculate the thermal response of both
nanoantenna systems due to exposure to electromagnetic radiation. Water is
chosen as the surrounding media in order to replicate actual experimental
conditions. The Palik library is used for the thermal simulations as well.

##  III Results

###  III.1 SE-CARS with Si nanoantenna

In this work, we move away from metal-based nanoantenna systems and use
dielectric antenna formed from clustered Si-nanoparticles to enhance the
coherent Raman response of the molecule. In order to examine the capability of
this dielectric antenna to efficiently capture and confine incident optical
fields, we use Lumerical FDTD to calculate the expected absorption spectrum of
the antenna and the local fields inside the nanojunction. Figure 2a shows the
calculated extinction spectrum of an antenna system from two Si-nanoparticles
that form the exemplar dimer, along with the pump, Stokes and anti-Stokes
frequencies involved in the experiment. The spectrum of the dielectric antenna
is composed of a magnetic response in the optical range in addition to an
electric resonance. This response is due to the displacement currents
originating from the oscillation of bound electrons. The electric field
calculations (shown in Figure 2b) reveal an effective field enhancement factor
$\beta_{p}$ of $\sim$14 for the pump wavelength of 885 nm. Similar
calculations are performed for the Stokes and anti-Stokes wavelengths
resulting in $\beta_{s}=13$ ($\lambda_{s}=1031~{}\rm{nm}$) and $\beta_{as}=15$
($\lambda_{as}=775~{}\rm{nm}$). As such, the expected overall CARS enhancement
factor is $\beta^{4}_{p}\beta^{2}_{s}\beta^{2}_{as}=1.46\times 10^{9}$. The
diameter of the electromagnetic hotspot is estimated as $\sim$6 nm. Assuming a
surface coverage of 6 molecules/nm2 and both particles have monolayers of
pyridine, we are exciting $\sim$340 molecules in each junction.

Figure 2: a) Calculated absorption spectrum of Si dimer nanoantenna system.
The three colors represent the anti-Stokes, pump and Stokes wavelengths (from
shorter to longer wavelengths). b) local electric field enhancement near the
Si-Si nanojunction under 885 nm illumination. The local field enhancement
factor is denoted by $\beta(\omega)=|E_{loc}/E_{0}$—

We target the ring stretching modes near 1580 cm-1 for our SE-CARS experiment
based on the CARS spectrum for 99% pure pyridine, as seen in Figure 3(a). The
spectral measurements are taken at 6 cm-1 intervals, ranging from 1552 cm-1 to
1630 cm-1. This corresponds to scanning the pump wavelength between 882.7 nm
and 888.8 nm. The spontaneous Raman spectrum of pyridine shown in Figure 3(b)
reveals that the CARS signature derives from several Raman modes. The pyridine
modes in this range are substantially weaker than the ring breathing modes
near 1000 cm-1 Benassi and Fan (2021), but the selected modes are targeted
here because of better laser performance for the CARS process in this spectral
range. Note that in addition to the resonant signature in the CARS spectrum, a
nonresonant background from pyridine itself is present.

Figure 3: (a) CARS and (b) spontaneous Raman spectra of 99% pyridine in the
1540-1630 $\rm{cm}^{-1}$ range.

In the dielectric nanoantenna system, consisting of clusters of Si particles,
the measurements consistently produce vibrationally resonant signal from the
target molecule. Background signal is acquired by temporally separating the
pump and Stokes beams by 8 ps where no overlap is present. This background is
subtracted to obtain the vibrationally resonant SE-CARS image. Figure 4a shows
an SE-CARS image of clustered Si particles. The clusters appear as bright
spots that are close to the diffraction-limited resolution of the SE-CARS
microscope. Given the size of the spots, we estimate that these locations may
each contain one to six 150 nm Si particles. Two sample locations (labeled 1
and 2 in Figure 4a) show the expected SE-CARS response from pyridine,
suggesting the successful formation of a nanojunction, as evidenced by a
distinct and reproducible peak in the 1580-1600 cm-1 region. Figure 4b shows
the mean SE-CARS spectrum with one standard deviation of error, obtained from
50 individual particle clusters. The spectrum shows clear vibrational
sensitivity near the 1580-1600 cm-1 spectral region. To the best of our
knowledge, this is the first demonstration of surface-enhanced coherent Raman
scattering with non-metallic antenna.

The CARS vibrational feature observed in the imaging experiments appears blue-
shifted from the CARS signature seen in pure pyridine. Density functional
theory calculations show that the pyridyl ring stretching vibration is
expected to blue-shift upon bonding of the nitrogen to silicon (see Supporting
Information). The blue-shift observed in the SE-CARS measurements thus
supports the notion that the vibrational signature in the 1580-1600 cm-1 range
can be attributed to silicon-bonded pyridine molecules.

We note that there is a persistent background in most images taken in this
configuration. The background signal is vibrationally non-resonant, depends on
the spatial and temporal overlap of both beams and radiates at the
($2\omega_{p}-\omega_{s}$) frequency. Based on this observation, the origin of
this background is determined to be four-wave mixing (FWM) due to the strong
$\chi^{(3)}$ response of Si, which is found to contribute on average $\sim$24%
of the total signal on resonance. The contribution from broadband TPEL is on
average $<$4%.

Figure 4: SE-CARS with Si nanoantenna. (a) SE-CARS image of pyridine
functionalized Si nanoantenna system at 1582 cm-1. Two representative spectra
taken from locations 1 and 2. Scale bar is 10 $\mu$m. (b) Average of 50 SE-
CARS spectra obtained from different locations. The line connecting the dots
is used as a visual guide.

##  IV Thermal stability of dielectric and metal nanoantenna

Figure 5a shows an FEM calculation (using COMSOL Multiphysics) of
electromagnetic heating comparing the Si-dimer nanoantenna with a gold-based
one. Both systems are immersed in water. The calculation demonstrates that
under the exact same conditions and for the same geometrical dimension,
heating of the Au antenna is $\sim$13 times higher (37.1 K) compared to
heating of the Si (2.89 K) antenna. This simulated result is mirrored in
experimental findings as well. We acquire SE-CARS images from ensembles of
either Au or Si antenna systems for 291 seconds with 1.12 seconds per frame
(4.27 $\mu$s/pixel). The incident power levels for both pump and Stokes beams
were 2 mW at the sample plane for the Si antenna and 500 $\mu$W for Au
antenna. While signal from the Au nano-dumbbell system decays rapidly and
loses $\sim$60% signal within 120 seconds (Figure 5,top), the signal from the
Si nanoantenna system remains robust with no noticeable decay in signal even
after the end of the time series (Figure 5,bottom). The numerical calculation
and experimental observation of signal strength over time strongly supports
our hypothesis that Si-based nonplasmonic nanoantennas can provide a new way
to perform surface-enhanced coherent Raman scattering measurements with
improved stability, reliability and reproducibility.

Figure 5: Thermal response of Si and Au nanoantenna systems. (a) FEM
calculation of steady-state temperature of Si-dimer and Au nano-dumbbell
antennas under 1 mW, 885 nm cw illumination. (b) Time evolution of
experimental signal strength over 291 seconds for Au dumbbell (top) and Si-
particle cluster (bottom) nanoantenna at their respective molecular resonance
(1600 cm-1 for Au dumbbell and 1588 cm-1 for Si antenna system). The data
represents averaged signal from 100 individual locations for each system.

##  V Discussion

In this work, we present the first experimental demonstration of surface-
enhanced coherent Raman scattering of molecules with dielectric nanoantennas.
Our recent work Abedin  _et al._ (2022), where we introduced metal-dielectric
heterojunctions to improve thermal stability of the SE-CARS signal, motivated
us to explore the use of non-metallic nanoantennas for SE-CARS measurements.
The results discussed herein shows that it is possible to move beyond lossy
and thermally unstable metallic systems and use all-dielectric nanoantennas
for surface-enhanced coherent Raman scattering experiments instead.

We have used spatially and temporally overlapped pump and Stokes beams in a
laser-scanning microscope for the SE-CARS experiments. We successfully obtain
stable and reproducible coherent Raman signal from our target molecule. In
most of our measurements, we observe a persistent FWM background accompanying
the molecular signal, due to the strong $\chi^{(3)}$ response of Si. The non-
luminescent nature of the dielectric Si nanoparticles results in a much lower
luminescent background compared to metal-based SE-CARS systems. The fields
inside the dielectric dimer junctions are rather modest ($\beta\sim 14$)
compared to what can be achieved using metallic nanoantennas. However, the
overall CARS enhancement is substantial ($\sim 10^{9}$) due the
$\sim\beta^{8}$ dependence of the SE-CARS signal. Unlike previous SE-CARS
antenna systems where morphological distortion of the structure due to photo-
induced heating Crampton  _et al._ (2016); Yampolsky  _et al._ (2014) or
intense TPEL background Abedin  _et al._ (2022) limits the allowable
illumination dosage, this work enables us to apply higher field intensities to
enhance the molecular signal even further. The increased incident intensity
can compensate for the lower values of $\beta$, generating comparable signals
as previously observed with metal-based antenna systems.

Compared to SERS measurements, which scale as $\beta^{4}$, the SE-CARS
approach benefits from a much larger overall enhancement effect, which makes
dielectric antenna systems especially attractive for nonlinear Raman
measurements. In this context, we note that we have performed regular SERS
micro-spectroscopy measurements on the same Si-particle clusters. Using 532 nm
or 785 nm excitation light of up to 10 mW, focused on a single cluster of
pyridine-coated Si-particles, we have observed no vibrational SERS signatures
of pyridine with integration times up to 10 s. We surmise, therefore, that the
use of dielectric antennas may have a larger impact on the SE-CARS field
compared to their impact on SERS applications.

We have chosen Si as our antenna material because of its high refractive index
in the visible and near-infrared frequencies. As a result, Si based
nanoantennas can efficiently capture and confine incident optical fields to
nanoscopic regions. Furthermore, Si suffers less from the ohmic losses and
adverse thermal effects that are intrinsic to metallic nanoantennas. This
favorable thermal response is due to the higher specific heat of Si (5x higher
compared to Au Perry  _et al._ (1997)). The improved thermal stability is
evidenced in experiments as well. In metallic systems, blinking-type behavior
of the signal and the decay of signal over a short period of time is common.
However, when we use dielectric antennas, such behavior is not observed.

While this work provides a solution to a longstanding limitation of
traditional surface-enhanced CRS sensors, there are steps that can be taken to
improve it even further. Fabricating dielectric nanostructures with tailored
electric and magnetic properties can make the nanoantenna more efficient and
improve the reliability and repeatability of the signal. In addition,
immobilized periodic enhancement structures can be employed for high
throughput biomolecular screening purposes, an application where SE-CARS can
provide much faster signal acquisition than possible with SERS based screening
systems. Finally, red-shifting the pump and Stokes beams can reduce optical
absorption to even lower levels. The lower absorption can help with the
thermal response of the material, as well as reduce the electronic background.

##  VI Conclusion

We have presented an experimental demonstration of surface-enhanced coherent
anti-Stokes Raman scattering (SE-CARS) of molecules with a dielectric Si
nanoantenna. We have found that the thermal stability of the SE-CARS signal
from such dielectric antennas is significantly better compared to
corresponding signals from Au nano-dumbell antennas. This work shows that the
inclusion of a dielectric nanoantenna can overcome the longstanding limitation
of traditional metallic SE-CARS systems where unfavorable thermal properties
adversely affect the stability and reproducibility of the measurements.

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5Instituto de Astrofísica de Canarias, Calle Vía Láctea, s/n, 38205. La
Laguna, Santa Cruz de Tenerife, Spain
6Departamento de Astrofísica, Universidad de La Laguna, 38206, La Laguna,
Tenerife, Spain
7Centre for Astrophysics Research, University of Hertfordshire, Hatfield,
Hertfordshire AL10 9AB, U.K.
8Department of Physics and Astronomy, University College London, Gower Street,
London WC1E 6BT, U.K.
9Department of Physics, University of Warwick, Gibbet Hill Road, Coventry CV4
7AL, U.K.
10Université Côte d’Azur, Observatoire de la Côte d’Azur, CNRS, Laboratoire
Lagrange, Bd de l’Observatoire, CS 34229, 06304 Nice Cedex 4, France
11INAF – Osservatorio Astronomico di Brera, Via Brera, 28, 20121 Milano, Italy
12Aix Marseille Univ, CNRS, CNES, LAM, Marseille, France
13INAF – Osservatorio Astronomico di Padova, Vicolo Osservatorio 5, 35122
Padova, Italy
14Isaac Newton Group of Telescopes, Apartado 321, 38700 Santa Cruz de la
Palma, Tenerife, Spain
15Dipartimento di Fisica e Astrofisica, Univerisità degli Studi di Firenze,
via G. Sansone 1, I-50019 Sesto Fiorentino, Italy
16INAF – Osservatorio Astrofisico di Arcetri, Largo E. Fermi, 5, 20125
Firenze, Italy
17Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge
CB3 0HA, U.K.
18Institut de Ciències del Cosmos (ICCUB), Universitat de Barcelona (UB),
Martí Franquès 1, E-08028 Barcelona, Spain
19Departament de Física Quàntica i Astrofísica (FQA), Universitat de Barcelona
(UB), Martí i Franquès 1, E-08028 Barcelona, Spain
20Institut d’Estudis Espacials de Catalunya (IEEC), c. Gran Capità, 2-4, 08034
Barcelona, Spain
21School of Physics and Astronomy, University of Nottingham, University Park,
Nottingham NG7 2RD, U.K.
22Departamento de Física Teórica, Universidad Autónoma de Madrid, E-28049
Madrid, Spain
23Univ. Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France
24GEPI, Observatoire de Paris, Université PSL, CNRS: 5, place Jules Janssen,
92195 Meudon, France
25Lund Observatory, Department of Astronomy and Theoretical Physics, Box 43,
SE-221 00 Lund, Sweden
26INAF – Osservatorio di Astrofisica e Scienza dello Spazio, via P. Gobetti
93/3, 40129 Bologna, Italy
27Instituto Nacional de Astrofísica, Óptica y Electrónica, Luis Enrique Erro
1, Tonantzintla, Puebla, C.P. 72840, Mexico
28Institute for Astronomy, Royal Observatory, Blackford Hill, Edinburgh EH9
3HJ, U.K.
29Université de Strasbourg, CNRS, Observatoire astronomique de Strasbourg, UMR
7550, F-67000 Strasbourg, France
29Instituto de Astronomia y Ciencias Planetarias, Universidad de Atacama,
Copayapu 485, Copiapó, Chile
30UK Astronomy Technology Centre, Royal Observatory Edinburgh, Blackford Hill,
Edinburgh EH9 3HJ, U.K.
31Instituto de Astrofísica de Andalucía, CSIC, Glorieta de la Astronomía s/n,
18008 Granada, Spain
32ASTRON, Netherlands Institute for Radio Astronomy, Oude Hoogeveensedijk 4,
7991 PD Dwingeloo, the Netherlands
33Astrophysics Research Institute, Liverpool John Moores University, 146
Brownlow Hill, L3 5RF, Merseyside U.K.
34Department of Physics, McWilliams Center for Cosmology, Carnegie Mellon
University, 5000 Forbes Avenue, Pittsburgh, PA 15213, U.S.A.
35Institut d’Astrophysique de Paris, 98bis Bd Arago, 75014 Paris, France
36Technical University of Munich Institute for Advanced Studies
Lichtenbergstr. 2a, D-85748 Garching, Germany
37INAF – Osservatorio Astronomico di Capodimonte, Salita Moiariello, 16,
80131, Napoli, Italy
38Department of Astronomy, University of Illinois at Urbana-Champaign, Urbana,
IL 61801, U.S.A.
39Instituto de Astronomía y Ciencias Planetarias, Universidad de Atacama,
Copayapu 485, Copiapó, Chile
40Instituto Universitario Carlos I de Física Teórica y Computacional,
Universidad de Granada, E-18071 Granada, Spain
41Institut de Física d’Altes Energies (IFAE), The Barcelona Institute of
Science and Technology, Campus UAB, 08193 Bellaterra Barcelona, Spain
42Universitat Politècnica de Catalunya, Carrer de Jordi Girona, 31, 08034
Barcelona, Spain
43Mullard Space Science Laboratory, University College London, Holmbury St
Mary, Dorking RH5 6NT, U.K.
44Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD
21218, U.S.A.
45INAF – Osservatorio Astrofisico di Catania, Via S. Sofia 78, 95123 Catania,
Italy
46Department of Physics, Lancaster University, Bailrigg, Lancaster LA1 4YB,
U.K.
47Dpto. de Física Aplicada, Facultad de Ciencias, Universidad de Alicante,
Cta. de San Vicente s/n, E-03690 San Vicente del Raspeig, Alicante, Spain
48Astrophysics Group, Keele University, Keele ST5 5BG, U.K.
49INAF – Osservatorio Astronomico di Brera, via Bianchi 46, 23087 Merate (LC),
Italy
50Donostia International Physics Center, Paseo Manuel de Lardizabal, 4. 20018
San Sebastián, Spain
51IKERBASQUE, Basque Foundation for Science, Bilbao, Spain
52Leiden Observatory, Leiden University, Niels Bohrweg 2, 2333 CA Leiden, the
Netherlands
53Leibniz-Institut für Astrophysik Potsdam, An der Sternwarte 16, 14482
Potsdam, Germany
54Laboratoire d’astrophysique de Bordeaux, Université de Bordeaux, CNRS, B18N
Allée Geoffroy Saint-Hilaire, F-33615 Pessac, France
55Departamento de Física Teórica, Universidad Autónoma de Madrid (UAM), Campus
de Cantoblanco, Madrid 28049, Spain
56Australian Astronomical Optics, Macquarie University, 105 Delhi Rd, North
Ryde, NSW 2113, Australia
57ARC Centre of Excellence for All Sky Astrophysics in 3 Dimensions (ASTRO 3D)
58Centro de Astrobiología (CAB/CSIC-INTA), Ctra. de Ajalvir km 4, Torrejón de
Ardoz, E-28850, Madrid, Spain
59Institute for Computational Cosmology, Durham University, Durham DH1 3LE,
U.K.
60Centre for Extragalactic Astronomy, Durham University, Durham DH1 3LE, U.K.
61Departamento de Física Teórica y del Cosmos, Universidad de Granada, Campus
de Fuentenueva, E-18071 Granada, Spain
62Departamento de Física, Ingeniería de Sistemas y Teoría de la Seiñal,
Universidad de Alicante, Cta. de San Vicente s/n, E-03690 San Vicente del
Raspeig, Alicante, Spain
63Dipartimento di Fisica ‘G. Occhialini’, Università degli Studi di Milano
Bicocca, Piazza della Scienza 3, I-20126 Milano, Italy
64Department of Physics, University of Surrey, Guildford, Surrey, GU2 7XH,
U.K.
65INAF – Osservatorio Astronomico di Trieste, Via Giambattista Tiepolo, 11,
34131 Trieste TS, Italy
66Department of Chemistry and Physics, Saint Mary’s College, Notre Dame, IN
46556, U.S.A.
67Research School of Astronomy & Astrophysics, Mount Stromlo Observatory,
Cotter Road, Weston Creek, ACT 2611, Australia
68Institute of Physics, University of Graz, Universitätsplatz 5, 8010 Graz,
Austria
69INAF – Osservatorio Astronomico di Palermo, Piazza del Parlamento 1, 90134,
Palermo, Italy
70INAF – Fundación Galileo Galilei, Rambla José Ana Fernández Pérez 7, 38712
Breñ Baja, Tenerife, Spain
71Kavli Institute for Cosmology, University of Cambridge, Madingley Road,
Cambridge, CB3 0HA, U.K.
72Department of Physics & Astronomy, University of the Western Cape, Private
Bag X17, Bellville, Cape Town 7535, South Africa
73Université Lyon 1, ENS de Lyon, CNRS, Centre de Recherche Astrophysique de
Lyon UMR5574, Saint-Genis-Laval, France
74Department of Astrophysics/IMAPP, Radboud University, P.O. Box 9010, 6500 GL
Nijmegen, the Netherlands
75Astronomisches Rechen-Institut, Zentrum für Astronomie der Universität
Heidelberg, Mönchhofstr. 12-14, D-69120 Heidelberg, Germany
76ESA – ESTEC, MMO, Keplerlaan 1, PO Box 299, 2200AG Noordwijk, the
Netherlands
77Department of Astronomy, Stockholm University, AlbaNova University Centre,
SE-106 91 Stockholm, Sweden
78Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
78Department of Physics and Astronomy, Uppsala University, Box 516, SE-751 20
Uppsala, Sweden
79Department of Astronomy and Astrophysics, University of Chicago, 5640 South
Ellis Avenue, Chicago, IL 60637, U.S.A.
80Centro de Astrobiología (CAB/CSIC-INTA). Campus ESAC, Camino bajo del
castillo s/n. E-28 692 Villanueva de la Cañada, Madrid, Spain
81Institució Catalana de Recerca i Estudis Avançats, Passeig Lluís Companys
23, E-08010 Barcelona, Spain
82INAF – Osservatorio Astronomico di Cagliari, Via della Scienza 5, 09047
Selargius CA, Italy
83School of Physics, University of Exeter, Stocker Road, Exeter EX4 4QL, U.K.
84Facultad de física, Universidad de Sevilla, Avda. Reina Mercedes s/n. Campus
Reina Mercedes, E-41012, Seville, Spain
85National Astronomical Observatory of Japan, Mitaka-shi, Tokyo 181-8588,
Japan
86NOVA Optical-Infrared Instrumentation Group at ASTRON, PO Box 2, 7990 AA,
Dwingeloo, the Netherlands
87Département de Physique Théorique, Université de Genève, 24 quai Ernest
Ansermet, 1211 Genève 4, Switzerland
88Dipartimento di Fisica “E.R. Caianiello”, Università degli Studi di Salerno,
Via Giovanni Paolo II 132, I-84084 Fisciano (SA), Italy
89Armagh Observatory and Planetarium, College Hill, Armagh BT61 9DG, Northern
Ireland, U.K.
90Joint ALMA Observatory, Alonso de Córdova 3107, Vitacura, Santiago 763-0355,
Chile
91National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA
22903, U.S.A.
92School of Physics and Astronomy, Sir William Henry Bragg Building,
University of Leeds, Leeds LS2 9JT, U.K.
93School of Physics and Astronomy, University of St Andrews, North Haugh, St
Andrews KY16 9SS, U.K.
94Zentrum für Astronomie der Universität Heidelberg, Landessternwarte,
Königstuhl 12, D-69117 Heidelberg, Germany
95School of Architecture – Universidad Europea de Canarias, La Orotava,
Tenerife, Spain
96Université de Liège, Allée du 6 Aoüt 19c, B-4000 Sart Tilman, Liège, Belgium

⋆E-mail<EMAIL_ADDRESS>
†Deceased
‡Anna Boyksen fellow