# ‘Maser-in-a-Shoebox’: a portable plug-and-play maser device at room-
temperature and zero magnetic-field

Wern Ng  Department of Materials, Imperial College London, South Kensington
SW7 2AZ, London, United Kingdom<EMAIL_ADDRESS>Yongqiang Wen
Department of Materials, Imperial College London, South Kensington SW7 2AZ,
London, United Kingdom  Max Attwood  Department of Materials, Imperial College
London, South Kensington SW7 2AZ, London, United Kingdom  Daniel C Jones
Gemological Institute of America, New York, 10036, USA  Mark Oxborrow
Department of Materials, Imperial College London, South Kensington SW7 2AZ,
London, United Kingdom  Neil McN. Alford  Department of Materials, Imperial
College London, South Kensington SW7 2AZ, London, United Kingdom  Daan M.
Arroo  Department of Materials, Imperial College London, South Kensington SW7
2AZ, London, United Kingdom

###### Abstract

Masers, the microwave analogues of lasers, have seen a renaissance owing to
the discovery of gain media that mase at room-temperature and zero-applied
magnetic field. However, despite the ease with which the devices can be
demonstrated under ambient conditions, achieving the ubiquity and portability
which lasers enjoy has to date remained challenging. We present a maser device
with a miniaturized maser cavity, gain material and laser pump source that
fits within the size of a shoebox. The gain medium used is pentacene-doped in
para-terphenyl and it is shown to give a strong masing signal with a peak
power of -5 dBm even within a smaller form factor. The device is also shown to
mase at different frequencies within a small range of 1.5 MHz away from the
resonant frequency. The portability and simplicity of the device, which weighs
under 5 kg, paves the way for demonstrators particularly in the areas of low-
noise amplifiers, quantum sensors, cavity quantum electrodynamics and long-
range communications.

††preprint: AIP/123-QED

Few devices can match the exceptionally low noise temperatures of masers[1],
the microwave analogue of lasers. Following the discovery of the first solid-
state material that could mase at room-temperature, pentacene-doped para-
terphenyl (PcPTP)[2], there has been a surge in research exploring further
such room-temperature maser materials and coherent microwave sources[3, 4, 5,
6, 7], studying their use in ultrasensitive sensing and communication[8, 9],
performance in cavity quantum electrodynamics (cQED)[10, 11, 12, 13], use as
polarizers for triplet dynamic nuclear polarization[14, 15] and as microwave
mode coolers[16, 17, 18]. Masers have the potential to revolutionize medical
imaging and long-range communications through their ultra-low noise
amplification[19], but even though room-temperature gain media have immensely
simplified the maser device by removing the need for cryogenics and vacuum,
the contrast between masers and their laser descendants is stark; the latter
can be purchased from vendors straight to a home tabletop, while the former
has remained in the domain of specialized laboratories, having not yet
transcended to a more portable form. Conventional masers still weigh 100 kg
and require large footprints (60 $\times$ 80 $\times$ 91 cm3)[20]. Previous
room-temperature maser devices in literature still required huge 100 kg
electromagnets[4] or massive pump sources such as bulky xenon flash lamps[21]
or pulsed optoparametric-oscillators[10] which all had dimensions larger than
100 cm and weighed beyond 50 kg.

Here, we present a fully portable maser device with pumping optics and
microwave cavity all contained within an enclosure the size of a shoebox (33
$\times$ 23 $\times$ 11 cm3), weighing less than 5 kg. The gain medium used is
a cylindrical single crystal (OD 3.5 mm, height 8.7 mm) of 0.1% PcPTP grown
through the Bridgman method[22, 23]; PTP (Sigma-Aldrich, $\geq$99.5%) was zone
refined prior to use and mixed with pentacene at a concentration of 0.1%
(mol/mol) by grinding in a pestle and mortar. The ground powder was then
loaded into a 3.5 mm OD borosilicate NMR tube and sealed under argon. Bridgman
growth was performed at 4 mm/hr over 3 days at 218 $\degree$C to yield the
pink PcPTP crystal.

Figure 1: (a) Photo of the portable maser with an output masing pulse captured
by direct detection on the oscilloscope. An ND-filter is seen covering the
output port of the laser, but this was removed during masing experiments. (b)
Example S11 measured from the cavity coupling loop (c) Close-up photo of the
inside of the aluminum cavity housing the PcPTP gain medium crystal, with
various components labelled. (d) Polar plot of the S11 plot in (b), with plot
radius equal to one and showing undercoupling to the cavity.

The critical components to miniaturize have been the microwave resonator and
the optical pump for the maser. The resonator that houses the PcPTP crystal
consists of an aluminum cavity enclosing a dielectric ring resonator made of a
single crystal of strontium titanate (STO). The STO ring has an OD 12.2 mm, ID
4.1 mm, and height of 8.7 mm, allowing it to achieve a large Purcell factor
through reducing the mode volume. This is important to reach the masing
threshold. The STO supports a TE01δ mode with a frequency $f_{\rm{mode}}$ of
1.4495 GHz, which could be tuned through adjusting the height of the inner
ceiling of the aluminum cavity. The aluminum cavity has an ID of 22 mm and the
maximum height the ceiling can be adjusted to is 20 mm.

The laser is a diode-pumped pulsed 532 nm Nd:YAG laser (Montfort Laser GmbH,
M-NANO-GREEN model PR190) with a pulse duration of 6 ns and controlled by a
computer via USB. The total measured pulse energy was 30 mJ using a Thorlabs
ES245C sensor, which includes both the 532 nm pulse and a residual 1064 nm
laser pulse that passes along the same axis. We note that this is well above
the threshold laser power required for PcPTP to mase for 532 nm light, which
we estimate to be less than 7 mJ (for a 6 ns pulse) based on previously
reported results[13]. The PcPTP does not suffer any noticeable degradation due
to the residual 1064 nm radiation.

Figure 1(a) shows the ‘maser-in-a-shoebox’ outputting a maser pulse into an
oscilloscope. The maser cavity and pulsed laser pump source are mounted within
an aluminum enclosure, which has an SMA output port installed for emitting the
maser pulse into a load or measurement device of choice. Figure 1(c) shows the
PcPTP inserted into the STO ring (with the main body of the aluminum cavity
removed to reveal the interior). The STO and PcPTP sit on a stand made of
3D-printed plastic (polylactic acid, or PLA) which supports the STO to a
height of 4.5 mm above the floor of the cavity. A loop of wire on a coaxial
cable acts as the coupling loop for receiving the maser pulses after the PcPTP
crystal is optically pumped. The device is designed to be as ‘plug-and-play’
as possible, where it is simply plugged into mains electricity and controlled
with a computer to output maser pulses at a certain repetition rate (governed
by the laser pumping repetition rate).

The laser and maser cavity are arranged as shown in Figure 2(a), where the
laser output is directed into the aluminum cavity through an aperture in the
cavity, and passes through the STO dielectric (which has been polished to be
transparent) to excite the PcPTP within. The maser operates as follows; the
repetition rate of the laser is set between 0.5 to 10 Hz (or single shot).
After the laser pulse hits the PcPTP, the maser burst is picked up by the
coupling loop and transmitted through a coaxial cable to the output port. The
$f_{\rm{mode}}$ of the resonator can be checked using the dip in the S11
reading from a vector network analyzer (VNA) and tuned to the maser frequency
$f_{\rm{res}}=1.4495$ GHz, prior to laser excitation (Figure 1(b)). The loaded
quality factor ($Q_{\rm{L}}$) of the resonator with coupling loop inserted can
be estimated using the polar plot of the S11 as explained in previous
work[13]. From Figure 1(d), the frequencies corresponding to the -3 dB
bandwidth were $f_{1}=1.44915$ GHz and $f_{2}=1.44986$ GHz respectively,
giving $Q_{\rm{L}}\approx 1.4495/(f_{2}-f_{1})=2042$, with possible
undercoupling as the polar plot does not intersect the center. Further
explanations of how to operate masers such as PcPTP are available from
previous literature[13, 4, 21].

Figure 2: (a) View of the inside of the enclosure housing the aluminium maser
cavity and laser pump. (b) Masing signal at 1.44969 GHz detected from the
portable maser. The inset shows a 10 ns slice of the oscillations of the
signal at the peak voltage region. The entire maser signal is also modulated
by Rabi oscillations.

Figure 2(b) shows a single shot time-domain signal from the maser with the
cavity tuned as close as possible to $f_{\rm{res}}$. The frequency power
spectrum for this signal showed that its actual central frequency (or carrier
frequency), and likely the cavity frequency, was 1.44969 GHz, which was very
close to $f_{\rm{res}}$. The signal was detected directly on an oscilloscope
(Keysight InfiniiVision DSOX6002A, 6 GHz bandwidth). The time at which the
laser pulse triggered the oscilloscope measurement was at 0 µs (detected using
a Thorlabs DET10A2 photodiode). The maser pulse reaches a peak voltage of 0.13
V when measured through the oscilloscope channel impedance of 50 $\Omega$,
which gives a peak power of 0.33 mW, or -5 dBm, slightly surpassing the
strongest maser peak power recorded for PcPTP[10]. Hence, our portable maser
maintains a similar output power compared to previous implementations[2, 10].

Rabi oscillations are seen in the masing signal at an estimated frequency of
0.8 MHz. This is similar to what has been observed previously in
literature[10, 13], where the Rabi oscillations indicate the strong coupling
regime in cQED.

Figure 3: (a)-(d) Rows displaying the maser signal recorded when the cavity
was tuned to different frequencies, with the grey dotted lines showing the
time of the laser trigger. The right column displays the normalized frequency
power spectrum of the corresponding signal in the left column, with the Rabi
frequency in kHz estimated as the splitting between the two peak frequencies
in each spectrum (indicated by dashed lines). The on-resonance maser signal at
1.44969 GHz (b, colored differently from the other plots) gives the greatest
amplitude.

The cavity was then detuned away from $f_{\rm{res}}$ (Figure 3) in order to
observe the effect of detuning on the maser output power. The signal in Figure
3(b) is identical to that in Figure 2(b), and as expected is highest in
amplitude due to 1.44969 GHz being closest to $f_{\rm{res}}$. To resolve the
closely-spaced peaks associated with Rabi splitting of the maser mode around
the centre frequency, power spectra were obtained from the time-domain signals
using the maximum entropy method[24], which does not suffer from artifacts
associated with arbitrary window functions used in FFT-based spectral
estimation. The conversions were carried out using an implementation of Burg’s
method for maximum entropy spectral estimation in the memspectrum Python
package [25].

Predictably, the maser power output decreases when the cavity is tuned away
from $f_{\rm{res}}$. As shown in the right column of Figure 3, the power
spectra peaks will shift with the cavity frequency when it is tuned away from
$f_{\rm{res}}$. Through monitoring the VNA, the cavity was tuned in steps of
approximately 0.5 MHz and the signal was recorded. The central frequency of
the power spectra of each signal corresponds to the resulting maser frequency
measured.

The delay between laser excitation (at 0 µs) and the first peak of the maser
signal is increased the further away one is from $f_{\rm{res}}$, which is due
to the stimulated transition rate decreasing when one is off-resonance. The
frequency of the Rabi oscillations decreases as one moves away from
$f_{\rm{res}}$, resulting in narrower splittings in the power spectra. In the
case of Figure 3(d), the Rabi oscillation from being off-resonance is so slow
that there is only time for two cycles before spin-lattice decay has totally
quenched the masing signal. This is due to the relationship between the Rabi
frequency and the microwave photon number; for very large numbers of microwave
photons (which is the case for masers), the Rabi frequency is proportional to
the number of microwave photons in the cavity $n_{\rm{cavity}}$[12]. As the
maser is tuned away from the cavity resonance the maser amplitude decreases,
and so $n_{\rm{cavity}}$ decreases as well, resulting in the Rabi frequency
decreasing.

These results help to establish certain operation restrictions for the PcPTP
maser; if one desires stronger coupling for cavity quantum electrodynamics,
and the shortest delay to peak maser power, it is best to operate exactly on
resonance where the Rabi splitting is shown to be greatest and the delay to
peak power is shortest. However, if one desires a maser signal with a slightly
different frequency from resonance, then this can be achieved with a small
tuning window away from $f_{\rm{res}}$, corresponding to at least 1.5 MHz
above or below $f_{\rm{res}}$. A maser signal would still be visible and will
be at the frequency one has tuned to, but the amplitude will suffer.

In conclusion, we have demonstrated a conveniently portable maser device. Our
setup also demonstrates that cavities made from metals of lower electrical
conductivity, such as aluminum which is more easily machinable, are viable as
maser cavities. Due to the lower conductivity of aluminum compared with
copper, $Q_{\rm{L}}$ is decreased and stronger light pumping is required to
compensate for this. The use of PLA plastic for the stand and tape for
attaching the STO onto the stand are both lossy and decrease $Q_{\rm{L}}$;
better alternatives for stands would be teflon or sapphire. However, the fact
that strong masing is possible even under an unoptimized setup is a testament
to the robust masing capability of our device. Our ‘shoebox maser’ can also
have its gain medium swapped for other materials, so long as they use 532 nm
excitation wavelengths; the recently discovered 6,13-diazapentacene doped in
para-terphenyl maser would be compatible with this wavelength while offering
its faster masing startup time[13]. We further anticipate that miniaturized
room-temperature masers will find applications in quantum sensing[8, 9],
quantum information processing[26, 27], and serve as educational tools for
demonstrating cQED phenomena just on the tabletop, all at room-temperature.

###### Acknowledgements.

The authors dedicate this Letter to the memory of Michael Lennon, who made the
cavity used for this work and whose skill in manufacturing high-Q cavities has
been an important part of the story of room-temperature masers. We further
thank Ben Gaskell of Gaskell Quartz Ltd (London) for making the STO ring used
and Dr Tim Moorsom for the photographs of the maser setup. This work was
supported by the U.K. Engineering and Physical Sciences Research Council
through grant EP/V001914/1

## Author Declarations

### Conflict of Interest

The authors have no conflicts to disclose.

### Data Availability Statement

The data that support the findings of this study are available from the
corresponding author upon reasonable request.

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