Aerospace & Defense Technology - September 2022 - 48

Tech Briefs
can be stored by the hundreds in a
Navy surface ship's weapon magazine.
* Cost exchange ratio. An SSL can be fired
for a marginal cost of less than one dollar
per shot (which is the cost of the fuel
needed to generate the electricity used
in the shot), while the EMRG's projectile/HVP
has an estimated unit procurement
cost of about $25,000.
This work was performed by Ronald
O'Rourke for the Congressional
Research Service. For more information,
download the Technical Support
Package (free white paper)
at mobilityengineeringtech.com/tsp
under the Optics, Photonics & Lasers
category. CRS-0001
Design, Demonstration, and Simulation of the Diode-Pumped
Thulium/Holmium (Tm/Ho) Composite Fiber 2.1-µm Laser
How using a designable hybrid photonic crystal cavity (HPCC) polariton system can contribute to
advancing the fundamental understanding and technological innovation in semiconductor photonics.
Army Research Laboratory, Adelphi, Maryland
C
oherence properties are what distinguish
a laser from other sources of
light and make it useful. This research
demonstrates for the first time a polariton
laser with coherence reaching the intrinsic
limit of single-mode matter-wave
lasers. It further demonstrates intensity
coherence at the shot-noise limit
expected of an ideal coherent state (Figure
1, left), and phase coherence revealing
interactions within the polariton
condensate due to its matter-wave
nature (Figure 1, right).
This research also demonstrates a new
mechanism of frequency comb generation
using coupled polariton condensates.
It results from the dynamic interplay
between on-site nonlinearity and
inter-site couplings. This is distinct
from other systems, such as microtoroidal
resonators and quantum cascade
lasers that are based on cascaded fourwave
mixing process. A unique feature
of this form of frequency comb generation
is that the comb spacing is not
given by the cavity transverse mode
spacing and can be engineered from
GHz to THz without having to drastically
change the physical dimension of the
system. Furthermore, since it is based
on polariton condensation, the polariton
comb allows a very low threshold
and an incoherent pump, including
electrical injection.
The matter-wave frequency comb is
created using the designable HPCC cavity
with a suspended high-contrast grating
mirror, which allows the creation of
two coupled condensate, as well as engineering
their interaction and coupling
48
60
50
40
30
n
20
10
500
1000
n
Figure 1. Left, Second-order auto-coherence function gg(2)
1500
0.5
0.25
γ
500
1000
n
1500
(0) vs. the normalized excitation power. It measures
the intensity noise of the polariton laser, which shows a sharp decrease above the threshold to the
shot-noise limit of 1. Large fluctuations around the threshold reflects relaxation oscillation. The blue (red)
data correspond to pulsed (continuous wave) excitation. The inset is the corrected for the time-resolution
of the photo detectors and the blue line is a theoretical fit. The grayed area corresponds to weak-coupling
regime. Right: Phase coherence time vs. the polariton number in the lasing mode. It is obtained from
first-order coherence measurements. A sharp increase of the coherent time was seen near threshold as
expected in typical lasers. The decrease of the coherence time at large polariton number, while maintaining
shot-noise limited intensity noise, results from interactions among polaritons in the lasing mode.
via deformation of the suspended mirrors.
Extensive experimental and theoretical
evidence was obtained to support
the results, including spectroscopy and
interferometric measurements showing
equidistant frequency lines and characteristic
phase relations between the two
coupled condensates (Figure 2).
This work paves the way for future
development and optimization of low
threshold optical frequency comb sources
and will open doors to other novel phenomena
and device concepts based on
dissipatively-coupled nonlinear cavity
systems.
This research also demonstrated the
first polariton laser in the Bardeen-Cooper-Schrieffer
(BCS) regime, which is
mobilityengineeringtech.com
also the first demonstration of the particle-hole
type BCS state as well as matter-light
hybrid BCS state. It validates
and pushes forward many decades of
theoretical efforts on this subject.
Both the BEC and BCS states were postulated
in excitonic systems half a century
ago. Research on exciton- and especially
polariton-BEC has exploded recently,
with a large part of the community
accepting spectral features as experimental
evidence of a polariton BEC despite
persistent skepticism. BCS has been considered
much more difficult to achieve,
and no experimentally accessible hallmark
signatures have been identified.
In the HPCC polariton system, both
the weakly and strongly cavity-couAerospace
& Defense Technology, September 2022
Coherence time (ps)
U (1/ps)

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Aerospace & Defense Technology - September 2022

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