87 research outputs found
Generation of Rabi frequency radiation using exciton-polaritons
We study the use of exciton-polaritons in semiconductor microcavities to
generate radiation spanning the infrared to terahertz regions of the spectrum
by exploiting transitions between upper and lower polariton branches. The
process, which is analogous to difference-frequency generation (DFG), relies on
the use of semiconductors with a nonvanishing second-order susceptibility. For
an organic microcavity composed of a nonlinear optical polymer, we predict a
DFG irradiance enhancement of , as compared to a bare nonlinear
polymer film, when triple resonance with the fundamental cavity mode is
satisfied. In the case of an inorganic microcavity composed of (111) GaAs, an
enhancement of is found, as compared to a bare GaAs slab. Both
structures show high wavelength tunability and relaxed design constraints due
to the high modal overlap of polariton modes.Comment: 8 pages, 7 figure
Spatial coherence and stability in a disordered organic polariton condensate
Although only a handful of organic materials have shown polariton
condensation, their study is rapidly becoming more accessible. The spontaneous
appearance of long-range spatial coherence is often recognized as a defining
feature of such condensates. In this work, we study the emergence of spatial
coherence in an organic microcavity and demonstrate a number of unique features
stemming from the peculiarities of this material set. Despite its disordered
nature, we find that correlations extend over the entire spot size and we
measure values of nearly unity at short distances and of 50%
for points separated by nearly 10 m. We show that for large spots, strong
shot to shot fluctuations emerge as varying phase gradients and defects,
including the spontaneous formation of vortices. These are consistent with the
presence of modulation instabilities. Furthermore, we find that measurements
with flat-top spots are significantly influenced by disorder and can, in some
cases, lead to the formation of mutually incoherent localized condensates.Comment: Revised versio
Organic Photodiodes with an Extended Responsivity using Ultrastrong Light-Matter Coupling
In organic photodiodes (OPDs) light is absorbed by excitons, which dissociate
to generate photocurrent. Here, we demonstrate a novel type of OPD in which
light is absorbed by polaritons, hybrid light-matter states. We demonstrate
polariton OPDs operating in the ultra-strong coupling regime at visible and
infrared wavelengths. These devices can be engineered to show narrow
responsivity with a very weak angle-dependence. More importantly, they can be
tuned to operate in a spectral range outside that of the bare exciton
absorption. Remarkably, we show that the responsivity of a polariton OPD can be
pushed to near infrared wavelengths, where few organic absorbers are available,
with external quantum efficiencies exceeding those of a control OPD
Inverting Singlet and Triplet Excited States using Strong Light-Matter Coupling
In organic microcavities, hybrid light-matter states can form with energies
that differ from the bare molecular excitation energies by nearly 1 eV. A
timely question, given recent advances in the development of thermally
activated delayed fluorescence materials, is whether strong light-matter
coupling can be used to invert the ordering of singlet and triplet states and,
in addition, enhance reverse intersystem crossing (RISC) rates. Here, we
demonstrate a complete inversion of the singlet lower polariton and triplet
excited states. We also unambiguously measure the RISC rate in strongly-coupled
organic microcavities and find that, regardless of the large energy level
shifts, it is unchanged compared to films of the bare molecules. This
observation is a consequence of slow RISC to the lower polariton due to the
delocalized nature of the state across many molecules and an inability to
compete with RISC to the dark exciton reservoir, which occurs at a rate
comparable to that in bare molecules
Triplet harvesting in the polaritonic regime: a variational polaron approach
We explore the electroluminescence efficiency for a quantum mechanical model
of a large number of molecular emitters embedded in an optical microcavity. We
characterize the circumstances under which a microcavity enhances harvesting of
triplet excitons via reverse intersystem-crossing (R-ISC) into singlet
populations that can emit light. For that end, we develop a time-local master
equation in a variationally optimized frame which allows for the exploration of
the population dynamics of chemically relevant species in different regimes of
emitter coupling to the condensed phase vibrational bath and to the microcavity
photonic mode. For a vibrational bath that equilibrates faster than R-ISC (in
emitters with weak singlet-triplet mixing), our results reveal that significant
improvements in efficiencies with respect to the cavity-free counterpart can be
obtained for strong coupling of the singlet exciton to a photonic mode, as long
as the singlet to triplet exciton transition is within the inverted Marcus
regime; under these circumstances, we show the possibility to overcome the
detrimental delocalization of the polariton states across a macroscopic number
of molecules. On the other hand, for a vibrational bath that equilibrates
slower than R-ISC (i.e., emitters with strong singlet-triplet mixing), we find
that while enhancemnents in photoluminiscence can be obtained via vibrational
relaxation into polaritons, this only occurs for small number of emitters
coupled to the photon mode, with delocalization of the polaritons across many
emitters eventually being detrimental to electroluminescence efficiency. These
findings provide insight on the tunability of optoelectronic processes in
molecular materials due to weak and strong light-matter coupling
Time-resolved imaging of non-diffusive carrier transport in long-lifetime halide perovskite thin films
Owing to their exceptional semiconducting properties, hybrid
inorganic-organic perovskites show great promise as photovoltaic absorbers. In
these materials, long-range diffusion of charge carriers allows for most of the
photogenerated carriers to contribute to the photovoltaic efficiency. Here,
time-resolved photoluminescence (PL) microscopy is used to directly probe
ambipolar carrier diffusion and recombination kinetics in hybrid perovskites.
This technique is applied to thin films of methylammonium lead tri-iodide
MAPbI obtained with two different fabrication routes, methylammonium lead
tribromide (MAPbBr), and an alloy of formamidinium lead tri-iodide
(FAPbI) and methylammonium lead bromide
FAMAPb(IBr_). Average diffusion
coefficients in the films leading to the highest device efficiencies and
longest lifetimes, i.e., in FAMAPb(IBr)
and acetonitrile-processed MAPbI, are found to be several orders of
magnitude lower than in the other films. Further examination of the
time-dependence shows strong evidence for non-diffusive transport. In
particular, acetonitrile-processed MAPbI shows distinct diffusion regimes
on short and long timescales with an effective diffusion constant varying over
2 orders of magnitude. Our results also highlight the fact that increases in
carrier lifetime in this class of materials are not necessarily concomitant
with increased diffusion lengths and that the PL quantum efficiency under solar
cell operating conditions is a greater indication of material, and ultimately
device, quality
Photonic Gap Antennas Based on High Index-Contrast Slot-Waveguides
Optical antennas made of low-loss dielectrics have several advantages over
plasmonic antennas, including high radiative quantum efficiency, negligible
heating and excellent photostability. However, due to weak spatial confinement,
conventional dielectric antennas fail to offer light-matter interaction
strengths on par with those of plasmonic antennas. We propose here an
all-dielectric antenna configuration that can support strongly confined modes
() while maintaining unity antenna quantum
efficiency. This configuration consists of a high-index pillar structure with a
transverse gap that is filled with a low-index material, where the contrast of
indices induces a strong enhancement of the electric field perpendicular to the
gap. We provide a detailed explanation of the operation principle of such
Photonic Gap Antennas (PGAs) based on the dispersion relation of symmetric and
asymmetric horizontal slot-waveguides. To discuss the properties of PGAs, we
consider silicon pillars with air or CYTOP as the gap-material. We show by
full-wave simulations that PGAs with an emitter embedded in the gap can enhance
the spontaneous emission rate by a factor of 1000 for air gaps and
400 for CYTOP gaps over a spectral bandwidth of
nm at \textmu m. Furthermore, the PGAs can be designed to
provide unidirectional out-of-plane radiation across a substantial portion of
their spectral bandwidth. This is achieved by setting the position of the gap
at an optimized off-centered position of the pillar so as to properly break the
vertical symmetry of the structure. We also demonstrate that, when acting as
receivers, PGAs can lead to a near-field intensity enhancement by a factor of
3000 for air gaps and 1200 for CYTOP gaps
Polariton-assisted Singlet Fission in Acene Aggregates
Singlet fission is an important candidate to increase energy conversion
efficiency in organic photovoltaics by providing a pathway to increase the
quantum yield of excitons per photon absorbed in select materials. We
investigate the dependence of exciton quantum yield for acenes in the strong
light-matter interaction (polariton) regime, where the materials are embedded
in optical microcavities. Starting from an open-quantum-systems approach, we
build a kinetic model for time-evolution of species of interest in the presence
of quenchers and show that polaritons can decrease or increase exciton quantum
yields compared to the cavity-free case. In particular, we find that hexacene,
a typically poor singlet-fission candidate, can feature a higher yield than
cavity-free pentacene when assisted by polaritonic effects. Similarly, we show
that pentacene yield can be increased when assisted by polariton states.
Finally, we address how various relaxation processes between bright and dark
states in lossy microcavities affect polariton photochemistry. Our results also
provide insights on how to choose microcavities to enhance similarly related
chemical processes.Comment: 12 pages, 4 figure
Hybrid Epsilon-Near-Zero Modes of Photonic Gap Antennas
We demonstrate that in photonic gap antennas composed of an epsilon-near-zero
(ENZ) layer embedded within a high-index dielectric, hybrid modes emerge from
the strong coupling between the ENZ thin film and the photonic modes of the
dielectric antenna. These hybrid modes show giant electric field enhancements,
large enhancements of the far-field spontaneous emission rate and a
unidirectional radiation response. We analyze both parent and hybrid modes
using quasinormal mode theory and find that the hybridization can be well
understood using a coupled oscillator model. Under plane wave illumination,
hybrid ENZ antennas can concentrate light with an electric field amplitude
100 times higher than that of the incident wave, which places them on par
with the best plasmonic antennas. In addition, the far-field spontaneous
emission rate of a dipole embedded at the antenna hotspot reaches up to
2300 that in free space, with nearly perfect unidirectional emission.Comment: 5 figures, 6 page
Halide perovskites enable polaritonic \u3ci\u3eXY\u3c/i\u3e spin Hamiltonian at room temperature
Exciton polaritons, the part-light and part-matter quasiparticles in semiconductor optical cavities, are promising for exploring Bose–Einstein condensation, non-equilibrium many-body physics and analogue simulation at elevated temperatures. However, a room-temperature polaritonic platform on par with the GaAs quantum wells grown by molecular beam epitaxy at low temperatures remains elusive. The operation of such a platform calls for long-lifetime, strongly interacting excitons in a stringent material system with large yet nanoscale-thin geometry and homogeneous properties. Here, we address this challenge by adopting a method based on the solution synthesis of excitonic halide perovskites grown under nanoconfinement. Such nanoconfinement growth facilitates the synthesis of smooth and homogeneous single-crystalline large crystals enabling the demonstration of XY Hamiltonian lattices with sizes up to 10 × 10. With this demonstration, we further establish perovskites as a promising platform for room temperature polaritonic physics and pave the way for the realization of robust mode-disorder-free polaritonic devices at room temperature
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