This article reviews recent theoretical and experimental advances in the fundamental understanding and active control of quantum fluids of light in nonlinear optical systems. In the presence of effective photon-photon interactions induced by the optical nonlinearity of the medium, a many-photon system can behave collectively as a quantum fluid with a number of novel features stemming from its intrinsically nonequilibrium nature. A rich variety of recently observed photon hydrodynamical effects is presented, from the superfluid flow around a defect at low speeds, to the appearance of a Mach-Cherenkov cone in a supersonic flow, to the hydrodynamic formation of topological excitations such as quantized vortices and dark solitons at the surface of large impenetrable obstacles. While the review is mostly focused on a specific class of semiconductor systems that have been extensively studied in recent years (planar semiconductor microcavities in the strong light-matter coupling regime having cavity polaritons as elementary excitations), the very concept of quantum fluids of light applies to a broad spectrum of systems, ranging from bulk nonlinear crystals, to atomic clouds embedded in optical fibers and cavities, to photonic crystal cavities, to superconducting quantum circuits based on Josephson junctions. The conclusive part of the article is devoted to a review of the future perspectives in the direction of strongly correlated photon gases and of artificial gauge fields for photons. In particular, several mechanisms to obtain efficient photon blockade are presented, together with their application to the generation of novel quantum phases.

Quantum fluids of light

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

Solid-state cavity quantum electrodynamics is a rapidly advancing field, which explores the frontiers of light–matter coupling. Metal-based approaches are of particular interest in this field, as they carry the potential to squeeze optical modes to spaces significantly below the diffraction limit. Transition metal dichalcogenides are ideally suited as the active material in cavity quantum electrodynamics, as they interact strongly with light at the ultimate monolayer limit. Here, we implement a Tamm-plasmon-polariton structure and study the coupling to a monolayer of WSe2, hosting highly stable excitons. Exciton-polariton formation at room temperature is manifested in the characteristic energy–momentum dispersion relation studied in photoluminescence, featuring an anti-crossing between the exciton and photon modes with a Rabi-splitting of 23.5 meV. Creating polaritonic quasiparticles in monolithic, compact architectures with atomic monolayers under ambient conditions is a crucial step towards the exploration of nonlinearities, macroscopic coherence and advanced spinor physics with novel, low-mass bosons. Thanks to their strong light-matter interaction, atomically thin transition metal dichalcogenides are ideal active materials for cavity quantum electrodynamics. Here, the authors embed a WSe2monolayer within a Tamm-plasmon-polariton cavity, and observe exciton-polariton formation at room temperature.

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Room-temperature Tamm-plasmon exciton-polaritons with a WSe2 monolayer.

GalliumNitridebasedhighelectronmobilitytransistors(HEMTs)areattractiveforuseinhighpowerandhighfrequencyapplications, with higher breakdown voltages and two dimensional electron gas (2DEG) density compared to their GaAs counterparts. Specific applications for nitride HEMTs include air, land and satellite based communications and phased array radar. Highly efficient GaNbased blue light emitting diodes (LEDs) employ AlGaN and InGaN alloys with different compositions integrated into heterojunctions and quantum wells. The realization of these blue LEDs has led to white light sources, in which a blue LED is used to excite a phosphor material; light is then emitted in the yellow spectral range, which, combined with the blue light, appears as white. Alternatively, multiple LEDs of red, green and blue can be used together. Both of these technologies are used in high-efficiency white electroluminescent light sources. These light sources are efficient and long-lived and are therefore replacing incandescent and fluorescent lamps for general lighting purposes. Since lighting represents 20‐30% of electrical energy consumption, and because GaN white light LEDs require ten times less energy than ordinary light bulbs, the use of efficient blue LEDs leads to significant energy savings. GaN-based devices are more radiation hard than their Si and GaAs counterparts due to the high bond strength in III-nitride materials. The response of GaN to radiation damage is a function of radiation type, dose and energy, as well as the carrier density, impurity content and dislocation density in the GaN. The latter can act as sinks for created defects and parameters such as the carrier removal rate due to trapping of carriers into radiation-induced defects depends on the crystal growth method used to grow the GaN layers. The growth method has a clear effect on radiation response beyond the carrier type and radiation source. We review data on the radiation resistance of AlGaN/GaN and InAlN/GaN HEMTs and GaN‐based LEDs to different types of ionizing radiation, and discuss ion stopping mechanisms. The primary energy levels introduced by different forms of radiation, carrier removal rates and role of existing defects in GaN are discussed. The carrier removal rates are a function of initial carrier concentration and dose but not of dose rate or hydrogen concentration in the nitride material grown by Metal Organic Chemical Vapor Deposition. Proton and electron irradiation damage in HEMTs creates positive threshold voltage shifts due to a decrease in the two dimensional electron gas concentration resulting from electron trapping at defect sites, as well as a decrease in carrier mobility and degradation of drain current and transconductance. State-of-art simulators now provide accurate predictions for the observed changes in radiation-damaged HEMT performance. Neutron irradiation creates more extended damage regions and at high doses leads to Fermi level pinning while 60 Co γ-ray irradiation leads to much smaller changes in HEMT drain current relative to the other forms of radiation. In InGaN/GaN blue LEDs irradiated with protons at fluences near 10 14 cm −2 or electrons at fluences near 10 16 cm −2 , both current-voltage and light output-current characteristics are degraded with increasing proton dose. The optical performance of the LEDs is more sensitive to the proton or electron irradiation than that of the corresponding electrical performances. © The Author(s) 2015. Published by ECS. This is an open access article distributed under the terms of the Creative Commons Attribution 4.0 License (CC BY, http://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse of the work in any

https://iopscience.iop.org/article/10.1149/2.0251602jss/pdf

Review—Ionizing Radiation Damage Effects on GaN Devices

We demonstrate that confined Tamm plasmon modes can be advantageously exploited for the realization of new kind of metal/semiconductor lasers. Laser emission is demonstrated for Tamm structures with various diameters of the metallic disks which provide the confinement. A reduction of the threshold with the size is observed. The competition between the acceleration of the spontaneous emission and the increase of the losses leads to an optimal size, which is in good agreement with calculations.

Confined Tamm Plasmon Lasers

InAs quantum-dot (QD) laser structures are grown on (113)B-oriented InP substrate by gas-source molecular-beam epitaxy. Following an optimized growth procedure, a high density of 1.1×1011cm−2 of uniformly sized QDs is achieved. Broad-area lasers containing three stacked QD layers have been realized and tested. Laser emission on the ground-state transition (λ=1.59μm) is obtained at room temperature (RT), at a threshold current density as low as 190A∕cm2. Ground-state modal gain and transparency current density is measured to be 7cm−1 and 23A∕cm2 per dot layer. Ground-state laser emission is also demonstrated from low temperature (100 K, Jth=33A∕cm2) to high temperature (350 K), exhibiting an insensitive threshold in the [100, 170] K range, and a 55 K characteristic temperature at RT.

High-gain and low-threshold InAs quantum-dot lasers on InP

We report on the observation of the strong coupling regime occurring between a Tamm plasmon (TP) mode and an exciton from inorganic quantum wells (QWs). The sample is formed by a silver thin film deposited onto an AlAs/GaAlAs Bragg reflector containing InGaAs QWs located in the high refractive index layers. Angular resolved reflectometry experiments evidence a clear anticrossing in the dispersion relations, a signature of the strong coupling regime. The Rabi splitting energy is 11.5 meV. The experimental data are in very good agreement with simple transfer matrix calculations. The emission from low and high energy TP/exciton polaritons is also demonstrated.

Emission of Tamm plasmon/exciton polaritons

We demonstrate that lasing can be obtained with the Tamm plasmon modes appearing at the interface between a multilayered dielectric structure and a metal. The sample consists in InGaAs/GaAs quantum wells embedded in an AlAs/AlGaAs Bragg reflector, on top of which a silver film was evaporated. A superlinear increase of the emission is observed under optical pumping as well as a drastic concentration of the emission pattern around the vertical direction, evidencing a lasing effect. Due to the particular features of these surface modes, Tamm lasers open interesting perspectives for the realization of integrated microlasers.

Lasing in a hybrid GaAs/silver Tamm structure

We demonstrate that an extraction enhancement by a factor of 2.8 can be obtained for a GaN quantum well structure using metallic nanostructures, compared to a flat semiconductor. The InGaN/GaN quantum well is inserted into a dielectric waveguide, naturally formed in the structure, and a silver grating is deposited on the surface and covered with a polymer film. The polymer layer greatly improves the extraction compared to a single metallic grating. The comparison of the experiments with simulations gives strong indications on the key role of weakly guided modes in the polymer layer diffracted by the grating.

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Enhanced light extraction from InGaN/GaN quantum wells with silver gratings

This article reports the improvement of broad area lasers epitaxially grown on InP(311)B substrate. Thanks to optimized growth techniques, a high density of uniformly sized InAs quantum dots (QDs) up to 1011 cm-2 is obtained. The device, which contains only two stacks of QDs, exhibits a ground state laser emission at 1.54 µm at room temperature associated with a threshold current density as low as 170 A/cm2. Experimental results also demonstrate a modal gain greater than 8 cm-1 per QD plane.

https://iopscience.iop.org/article/10.1143/JJAP.46.6903/pdf

Estelle Homeyer

Papers

High-gain and low-threshold InAs quantum-dot lasers on InP

Emission of Tamm plasmon/exciton polaritons

Lasing in a hybrid GaAs/silver Tamm structure

Enhanced light extraction from InGaN/GaN quantum wells with silver gratings

Demonstration of a Low Threshold Current in 1.54 µm InAs/InP(311)B Quantum Dot Laser with Reduced Quantum Dot Stacks