Bose-Einstein condensation (BEC) has been observed in a dilute gas of sodium atoms. A Bose-Einstein condensate consists of a macroscopic population of the ground state of the system, and is a coherent state of matter. In an ideal gas, this phase transition is purely quantum-statistical. The study of BEC in weakly interacting systems which can be controlled and observed with precision holds the promise of revealing new macroscopic quantum phenomena that can be understood from first principles.

Bose-Einstein condensation in a gas of sodium atoms

Microcavity polaritons—the bosonic quasiparticles that result from strong light–matter coupling—are observed for the first time in a dielectric cavity containing a monolayer of molybdenum disulphide at room temperature.

Strong light-matter coupling in two-dimensional atomic crystals

Polaritons are quasiparticles that form in semiconductors when an elementary excitation such as an exciton or a phonon interacts sufficiently strongly with light. In particular, exciton-polaritons have attracted tremendous attention for their unique properties, spanning from an ability to undergo ultra-efficient four-wave mixing to superfluidity in the condensed state. These quasiparticles possess strong intrinsic nonlinearities, while keeping most characteristics of the underlying photons. Here we review the most important features of exciton-polaritons in microcavities, with a particular emphasis on the emerging technological applications, the use of new materials for room-temperature operation, and the possibility of exploiting polaritons for quantum computation and simulation.

The road towards polaritonic devices

Layered materials can be assembled vertically to fabricate a new class of van der Waals heterostructures a few atomic layers thick, compatible with a wide range of substrates and optoelectronic device geometries, enabling new strategies for control of light-matter coupling. Here, we incorporate molybdenum diselenide/hexagonal boron nitride (MoSe2/hBN) quantum wells in a tunable optical microcavity. Part-light-part-matter polariton eigenstates are observed as a result of the strong coupling between MoSe2 excitons and cavity photons, evidenced from a clear anticrossing between the neutral exciton and the cavity modes with a splitting of 20 meV for a single MoSe2 monolayer, enhanced to 29 meV in MoSe2/hBN/MoSe2 double-quantum wells. The splitting at resonance provides an estimate of the exciton radiative lifetime of 0.4 ps. Our results pave the way for room-temperature polaritonic devices based on multiple-quantum-well van der Waals heterostructures, where polariton condensation and electrical polariton injection through the incorporation of graphene contacts may be realized.

/pdf/exciton-polaritons-in-van-der-waals-heterostructures-852cex81ct.pdf

Exciton-polaritons in van der Waals heterostructures embedded in tunable microcavities.

Cavity polaritons have been extensively studied in inorganic materials. An organic polariton condensate is now demonstrated to occur in the strongly interacting regime, at room temperature, in a cavity containing an organic polymer.

/pdf/nonlinear-interactions-in-an-organic-polariton-condensate-3oy2ui4jjf.pdf

Nonlinear interactions in an organic polariton condensate.

We report on the room temperature polariton lasing and photon lasing in a ZnO-based hybrid microcavity under optical pumping. A series of experimental studies of the polariton lasing (exciton-photon detunings of δ = -119 meV) in the strong-coupling regime are discussed and compared to a photon lasing (δ = -45 meV) in the weak-coupling regime obtained in the same structure. The measured threshold power density (31.8 kW/cm2) of polariton lasing is one order of magnitude lower than that of the photon lasing (318.2 kW/cm2). In addition, the comparison between polariton lasing and photon lasing is done in terms of the linewidth broadening, blue-shift of the emission peak, and polarization.

https://ir.nctu.edu.tw:443/bitstream/11536/15617/1/000301053200073.pdf

Room temperature polariton lasing vs. photon lasing in a ZnO-based hybrid microcavity.

The strong lightmatter interaction within a semiconductor high-Qmicrocavity hasbeen used to produce half-matter/half-light quasiparticles, exciton-polaritons. The exciton-polaritons have very small effective mass and controllable energy-momentum dispersion relation. These unique properties of polaritons provide the possibility to investigate the fundamental physics including solid-state cavity quantum electro- dynamics, and dynamical BoseEinstein condensates (BECs). Thus far the polariton BEC has been demonstrated using optical excitation. However, from a practical viewpoint, the current injection polariton devices operating at room temperature would be most desirable. Here we report the first realization of a current injection microcavity GaN exciton-polariton light emitting diode (LED) operating under room temperature. The exciton-polariton emission from the LED at photon energy 3.02 eV under strong coupling condition is confirmed through temperature-dependent and angle-resolved electroluminescence spectra.

https://ir.nctu.edu.tw:443/bitstream/11536/21914/1/000292849400039.pdf

Room temperature current injection polariton light emitting diode with a hybrid microcavity.

Here we report the first realization of a current injection microcavity GaN exciton-polariton light emitting diode (LED)
operating under room temperature (RT). The hybrid microcavity structure consists of InGaN/GaN quantum wells
sandwiched between bottom epitaxial DBR and top dielectric DBR. The anti-crossing behavior of polariton LED
denotes a clear signature of the strong interaction between excitons and cavity photons.

https://ir.nctu.edu.tw:443/bitstream/11536/15419/1/000301055700010.pdf

GaN-based microcavity polariton light emitting diodes

The characteristics of exciton-polaritons in ZnO-based microcavities (MCs) are demonstrated with a large vacuum Rabi
splitting due to large exciton binding energy and oscillator strength. The lower polariton branches (LPBs) can be clearly
observed. For low temperature and large negative detuning conditions, a clear polariton relaxation bottleneck in bulk
ZnO-based MCs has been observed in angle-resolved photoluminescence measurements from 100 to 353 K at different
cavity-exciton detunings. The bottleneck is strongly suppressed with increasing the temperature and pumping power and
reducing detuning. This observed results supposed to be due to more efficient phonon-assisted relaxation and a longer
radiative lifetime of the polaritons. In addition, the linewidth broadening, blue-shift of the emission peak, and
polarization of polariton lasing from below threshold to up threshold are also discussed.

https://ir.nctu.edu.tw:443/bitstream/11536/15896/1/000302993700031.pdf

Si Wei Huang

Papers

Room temperature polariton lasing vs. photon lasing in a ZnO-based hybrid microcavity.

Room temperature current injection polariton light emitting diode with a hybrid microcavity.

GaN-based microcavity polariton light emitting diodes

Exciton-polaritons study in ZnO-based hybrid microcavities