Giant Rabi Splitting of Whispering Gallery Polaritons in GaN/InGaN Core-Shell Wire

A near-resonant excitation strategy to achieve ultra-low threshold GaN polariton lasing

A near-resonant excitation strategy is proposed and implemented in a 4-µm-thick GaN microcavity to realize an exciton-polariton condensate/lasing with low threshold. Strong exciton-photon coupling is demonstrated, and polariton lasing is realized with an ultra-low threshold excitation power density of about 13.3 W/cm2 at room temperature. Such an ultra-low threshold is ascribed to the implementation of the near-resonant optical excitation strategy, which enables acceleration of the exciton and polariton relaxation and suppression of the heat generation in the cavity, thereby reducing the energy loss and enhance the cavity excitation efficiency.

Ultra-Confined Phonon Polaritons and Strongly Coupled Microcavity Exciton Polaritons in Monolayer MoSi2N4 and WSi2N4

The 2D semiconductors are an ideal platform for exploration of bosonic fluids composed of coupled photons and collective excitations of atoms or excitons, primarily due to large excitonic binding energies and strong light-matter interaction. Based on first-principles calculations, it is demonstrated that the phonon polaritons formed by two infrared-active phonon modes in monolayer MoSi2N4 and WSi2N4 possess ultra-high confinement factors of around ≈105 and 103, surpassing those of conventional polaritonic thin-film materials by two orders of magnitude. It is observed that the first bright exciton possesses a substantial binding energies of 750 and 740 meV in these two monolayers, with the radiative recombination lifetimes as long as 25 and 188 ns, and the Rabi splitting of the formed cavity-exciton polaritons reaching 373 and 321 meV, respectively. The effective masses of the cavity exciton polaritons are approximately 10-5me, providing the potential for high-temperature quantum condensation. The ultra-confined and ultra-low-loss phonon polaritons, as well as strongly-coupled cavity exciton polaritons with ultra-small polaritonic effective masses in these two monolayers, offering the flexible control of light at the nanoscale, probably leading to practical applications in nanophotonics, meta-optics, and quantum materials.

Whispering-Gallery Quantum-Well Exciton Polaritons in an Indium Gallium Arsenide Microdisk Cavity

Despite appealing high-symmetry properties that enable strong spatial confinement and ultrahigh-Q, optical whispering-gallery modes of spherical and circular resonators have been absent from the field of quantum-well exciton polaritons. Here we observe whispering-gallery exciton polaritons in a gallium arsenide microdisk cavity filled with indium gallium arsenide quantum wells, the test bed materials of polaritonics. Strong coupling is evidenced in photoluminescence and resonant spectroscopy accessed through concomitant confocal microscopy and near-field optical techniques. Excitonic and optical resonances are tuned by varying temperature and disk radius, revealing Rabi splittings between 5 and 10 meV. A dedicated analytical quantum model for such circular whispering-gallery polaritons is developed, which reproduces the measured values. At high power, lasing is observed and accompanied by a blueshift of the emission consistent with the regime of polariton lasing. With experimental methods and theory now established, whispering-gallery-mode polaritons in round dielectric resonators appear as a new viable platform toward low loss polaritonics.

Direct Observation of Self-Hybridized Exciton-Polaritons and Their Valley Polarizations in a Bare WS2 Layer

The strong excitonic properties of transition metal dichalcogenides (TMD) have led to the successful demonstration of exciton-polaritons (EPs) in various optical cavity structures. Recently, self-hybridized EPs have been discovered in a bare TMD layer, but experimental investigation is still lacking because of their nonradiative nature. Herein, the direct observation of self-hybridized EPs in a bare multilayer WS₂ via the evanescent field coupling technique is reported. Because of the thickness-dependent Rabi splitting energy, the dispersion curves of the EPs change sensitively with sample thickness. Moreover, continuous tuning of EP dispersion curves is demonstrated by controlling the excitation laser power. Lastly, it is observed that guided EPs retain valley polarization up to 0.2 at room temperature, representing a valley-preserved strong coupling regime. It is believed that the high tunability and valley polarization properties of the guided EPs in bare TMD layers can facilitate new nanophotonic and valleytronic applications.

Room-temperature polariton lasing in GaN microrods with large Rabi splitting

Room-temperature polariton lasing is achieved in GaN microrods grown by metal-organic vapor phase epitaxy. We demonstrate a large Rabi splitting (Ω = 2g₀) up to 162 meV, exceeding the results from both the state-of-the-art nitride-based planar microcavities and previously reported GaN microrods. An ultra-low threshold of 1.8 kW/cm² is observed by power-dependent photoluminescence spectra, with the linewidth down to 1.31 meV and the blue shift up to 17.8 meV. This large Rabi splitting distinguishes our coherent light emission from a conventional photon lasing, which strongly supports the preparation of coherent light sources in integrated optical circuits and the study of exciting phenomena in macroscopic quantum states.

Two-round quasi-whispering gallery mode exciton polaritons with large Rabi splitting in a GaN microrod

We investigate the exciton polaritons and their corresponding optical modes in a hexagonal GaN microrod at room temperature. The dispersion curves are measured by the angle-resolved micro-photoluminescence spectrometer, and two types of exciton polaritons are identified with the help of the finite-difference time-domain simulation. By changing the pump position, the photon part of the exciton polaritons is found to switch between the quasi-whispering gallery modes and the two-round quasi-whispering gallery modes. The exciton polaritons formed by the latter are observed and distinguished for the first time, with a giant Rabi splitting as large as 2Ω = 230.3 meV.

An electrically driven whispering gallery polariton microlaser

Near-infrared micro/nanolaser devices utilizing low-dimensional semiconductors can provide essential building blocks to achieve integrated optoelectronic devices and circuitry for advanced functionalities and are compatible with on-chip technologies. Although significant progress has been made through using narrow-band semiconductor micro/nanostructures to realize near-infrared stimulated radiation at room temperature, severe challenges still remain involving much lower quantum efficiencies and higher auger recombination. Herein, we report an experimental realization of a current-injection semiconductor polariton device made of a ZnO microwire via Ga-doping (ZnO:Ga MW) and p-type GaAs template. The device can emit polaritonic illumination directly from sharp edges of the hexagonal MW. The experimental results of angle-resolved electroluminescence measurements reveal a typical anticrossing feature between excitons and cavity modes, unambiguous evidence of the strong exciton-polariton coupling, with corresponding Rabi splitting energy extracted to be about 195 meV. As the applied bias goes above a certain value, electrically driven whispering gallery lasing action was achieved in the near-infrared spectrum, and the lasing features can be assigned to the exciton-polariton effect. The results not only can afford insights into the development of low-threshold coherent light sources via the exciton-polariton effect, but also can expand the fabrication of low-dimensional, near-infrared microlaser devices.

Point Defects in InGaN/GaN Core–Shell Nanorods: Role of the Regrowth Interface

Core-shell nanorod based light-emitting diodes (LEDs) with their exposed non-polar surfaces have the potential to overcome the limitations of planar LEDs by circumventing the quantum confined stark effect. In this experiment, InGaN/GaN core-shell nanorods were fabricated by a combination of top-down etching and bottom-up regrowth using metal-organic vapour phase epitaxy. When viewing the nanorods along their long axis, monochromatic cathodoluminescence maps taken at the GaN near-band-edge emission energy (3.39 eV) reveal a ring-like region of lower emission intensity. The diameter of this ring is found to be 530 (±20)nm corresponding to the ∼510 nm diameter nickel etch masks used to produce the initial GaN nanopillars. Thus, the dark ring corresponds to the regrowth interface. To understand the origin of the ring, scanning transmission electron microscopy (STEM) and cathodoluminescence (CL) hyperspectral mapping at 10K were performed. STEM imaging reveals the absence of extended defects in the nanorods and indeed near the regrowth interface. Monochromatic CL maps recorded at 10K show that the ring remains dark for monochromatic maps taken at the GaN near-band-edge emission energy (3.47 eV) but is bright when considering the donor-acceptor pair emission energy (3.27 eV). This peculiar anticorrelation indicates that the dark ring originates from an agglomeration of point defects associated with donor-acceptor pair emission. The point defects are incorporated and buried at the GaN regrowth interface from the chemical and/or physical damage induced by etching and lower the radiative recombination rate; limiting the radiative efficiency close to the regrowth interface.

A single microwire near-infrared exciton-polariton light-emitting diode

Exciton-polaritons, which originate from the strong coupling between photon modes of microresonators and excitons in semiconductor micro-/nanostructures, have drawn much attention due to their significance for fabricating coherent light sources which possess considerably lower emission thresholds. In this study, an exciton-polariton light-emitting diode (LED), made from a Ga-doped ZnO microwire (ZnO:Ga MW) and a p-GaAs template serving as the hole supplier, is fabricated. The n-ZnO:Ga MW/p-GaAs heterojunction device can emit light with a near-infrared wavelength of 880 nm and a narrow line width of about 60 nm. Due to the high quality whispering gallery mode (WGM) microcavities which are naturally self-constructed by the hexagon-shaped MW, the electroluminescence (EL) spectrum resolves into a series of resonance peaks which can be assigned to exciton-polariton features, leading to the strong coupling of the exciton and the WGM photon in the as-fabricated LED. The strong exciton-photon coupling is clearly evidenced via angle-resolved EL measurements, with the Rabi splitting energy extracted as 160 meV. Furthermore, by adjusting the size of the WGM microcavity structure naturally formed by the hexagonal MWs, particularly by adjusting the diameter of the wires, the exciton-polariton coupling strength in the single MW based LEDs can be tuned, with the as-extracted Rabi splitting energy varying in the range of 92-294 meV. The realization of a single MW based LED, which shows exciton-polariton behavior from a built-in optical microresonator, can enable a promising route for the future fabrication of polariton emitters, where the device performance no longer suffers from obstacles including the need for additional optical resonators, large lattice mismatch, and template availability.

Photonic Nanojets and Whispering Gallery Modes in Smooth and Corrugated Micro-Cylinders under Point-Source Illumination

We numerically investigate the generation of photonic nanojets (PNJs) and the excitation of whispering gallery modes (WGMs) supported by both smooth and corrugated dielectric micro-cylinders under point-source illumination. Results show that the location of the point-source defines the location and properties of PNJs, whereas stability of WGMs exists in smooth micro-cylinders but vanishes in corrugated ones. It is shown that the location of the point-source acts as an additional degree of freedom for controlling the characteristics of the generated PNJs for both smooth and corrugated dielectric micro-cylinders. Furthermore, the influence of the point-source location on the stability of the excited WGMs was diminished for the smooth micro-cylinders, while being fully pronounced for their corrugated counterparts.

Enhanced Optical Absorption and Slowed Light of Reduced-Dimensional CsPbBr3 Nanowire Crystal by Exciton-Polariton

Metallic halide perovskites are promising for low-cost, low-consumption, flexible optoelectronic devices. However, research is lacking on light propagation and dielectric behaviors as fundamental properties for optoelectronic perovskite applications, particularly the mechanism supporting a strong light-matter interaction and the different properties of low-dimensional structures from their bulk counterparts. We use spatially resolved photoluminescence (SRPL) spectroscopy to explore light propagation and measure the refractive index of CsPbBr₃ nanowires (NWs). Owing to strong exciton-photon interactions, light is guided as an exciton-polariton inside the NWs at room temperature. Remarkable spatial dispersion is confirmed, in which both the real and imaginary parts of the refractive index increase dramatically approaching exciton resonance, thus slowing light and enhancing absorption, respectively. Reducing the NWs dimension increases exciton-photon coupling and the exciton fraction, increasing the light absorption coefficient and group index 5- and 3-fold, respectively, relative to those of bulk films and slowing the light group velocity by ∼74%. Furthermore, dispersive absorption induces an energy redshift to the propagating PL at 4.1-5.5 meV μm^-1 until the bottleneck region. These findings clarify light-matter interaction in confined perovskite structures to improve their optoelectronic device performance.

Room temperature polariton lasing in quantum heterostructure nanocavities

Ultralow-threshold coherent light emitters can be achieved through lasing from exciton-polariton condensates, but this generally requires sophisticated device structures and cryogenic temperatures. Polaritonic nanolasers operating at room temperature lie on the crucial path of related research, not only for the exploration of polariton physics at the nanoscale but also for potential applications in quantum information systems, all-optical logic gates, and ultralow-threshold lasers. However, at present, progress toward room temperature polariton nanolasers has been limited by the thermal instability of excitons and the inherently low quality factors of nanocavities. Here, we demonstrate room temperature polaritonic nanolasers by designing wide-gap semiconductor heterostructure nanocavities to produce thermally stable excitons coupled with nanocavity photons. The resulting mixed states of exciton polaritons with Rabi frequencies of approximately 370 meV enable persistent polariton lasing up to room temperature, facilitating the realization of miniaturized and integrated polariton systems.

Constructing a Green Light Photodetector on Inorganic/Organic Semiconductor Homogeneous Hybrid Nanowire Arrays with Remarkably Enhanced Photoelectric Response

We demonstrate that a novel photodetector is constructed by CdS/poly( p-phenylene vinylene) (PPV) homogeneous hybrid nanowire arrays via a simple template-assisted electrochemical codeposition approach. Owing to the well-matched energy levels between CdS and PPV, the recombination of photogenerated electrons and holes in CdS/PPV hybrid nanowire arrays is greatly inhibited. It is found that the homogeneous hybrid nanowire arrays exhibit remarkably enhanced photoelectric response and the ON/OFF ratio by 17 times compared to the individual CdS component. More importantly, the CdS/PPV hybrid nanowire arrays are observed with significant spectral selectivity especially for green light under 545 nm. In addition, a straight linear relationship is obtained between the ON/OFF ratios and the illumination intensities, implying that the quantitative detection of illumination intensity can be achieved. The new as-prepared homogeneous hybrid organic/inorganic semiconductor nanowire arrays have a bright prospect for applications in high-sensitivity and high-speed green photodetectors.

Curved Nanomembrane-Based Concentric Ring Cavities for Supermode Hybridization

We report the mode interactions and resonant hybridization in nanomembrane-formed concentric dual ring cavities supporting whispering gallery mode resonances. Utilizing a rolled-up nanomembrane with subwavelength thickness as an interlayer, dual concentric microring cavities are formed by coating high-index nanomembranes on the inner and outer surfaces of the rolled-up dielectric nanomembrane. In such a hybrid cavity system, the conventional fundamental mode resonating along a single ring orbit splits into symmetric and antisymmetric modes confined by concentric dual ring orbits. Detuning of the coupled supermodes is realized by spatially resolved measurements along the cavity axial direction. A spectral anticrossing feature is observed as a clear evidence of strong coupling. Upon strong coupling, the resonant orbits of symmetric and antisymmetric modes cross over each other in the form of superwaves oscillating between the concentric rings with opposite phase. Notably, the present system provides high flexibilities in controlling the coupling strength by varying the thickness of the spacer layer and thus enables switching between strong and weak coupling regimes. Our work offers a compact and robust scheme using curved nanomembranes to realize novel cavity mode interactions for both fundamental and applied studies.

Non-equilibrium anisotropic colloidal single crystal growth with DNA

Anisotropic colloidal crystals are materials with novel optical and electronic properties. However, experimental observations of colloidal single crystals have been limited to relatively isotropic habits. Here, we show DNA-mediated crystallization of two types of nanoparticles with different hydrodynamic radii that form highly anisotropic, hexagonal prism microcrystals with AB₂ crystallographic symmetry. The DNA directs the nanoparticles to assemble into a non-equilibrium crystal shape that is enclosed by the highest surface energy facets (AB₂(10\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\overline 1$$\end{document}1¯0) and AB₂(0001)). Simulations and theoretical arguments show that this observation is a consequence of large energy barriers between different terminations of the AB₂(10\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\overline 1$$\end{document}1¯0) facet, which results in a significant deceleration of the (10\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\overline 1$$\end{document}1¯0) facet growth rate. In addition to reporting a hexagonal colloidal crystal habit, this work introduces a potentially general plane multiplicity mechanism for growing non-equilibrium crystal shapes, an advance that will be useful for designing colloidal crystal habits with important applications in both optics and photocatalysis.

Colloidal crystal engineering with DNA can be used to synthesize highly anisotropic hexagonal prismatic microcrystals. This manuscript introduces a plane multiplicity mechanism that can be used to deliberately design non-equilibrium Wulff shapes, a capability important in many areas, including optics and photocatalysis.

Room-temperature polarized spin-photon interface based on a semiconductor nanodisk-in-nanopillar structure driven by few defects

Owing to their superior optical properties, semiconductor nanopillars/nanowires in one-dimensional (1D) geometry are building blocks for nano-photonics. They also hold potential for efficient polarized spin-light conversion in future spin nano-photonics. Unfortunately, spin generation in 1D systems so far remains inefficient at room temperature. Here we propose an approach that can significantly enhance the radiative efficiency of the electrons with the desired spin while suppressing that with the unwanted spin, which simultaneously ensures strong spin and light polarization. We demonstrate high optical polarization of 20%, inferring high electron spin polarization up to 60% at room temperature in a 1D system based on a GaNAs nanodisk-in-GaAs nanopillar structure, facilitated by spin-dependent recombination via merely 2–3 defects in each nanodisk. Our approach points to a promising direction for realization of an interface for efficient spin-photon quantum information transfer at room temperature—a key element for future spin-photonic applications.

Room-temperature spin-generation in 1D systems like semiconductor nanopillars is typically inefficient. Here, the authors demonstrate an approach to achieve efficient spin polarization, even in the absence of a magnetic field, by selectively enhancing the radiative efficiency of one spin direction.

Multi-wavelength emission from a single InGaN/GaN nanorod analyzed by cathodoluminescence hyperspectral imaging

Multiple luminescence peaks emitted by a single InGaN/GaN quantum-well(QW) nanorod, extending from the blue to the red, were analysed by a combination of electron microscope based imaging techniques. Utilizing the capability of cathodoluminescence hyperspectral imaging it was possible to investigate spatial variations in the luminescence properties on a nanoscale. The high optical quality of a single GaN nanorod was demonstrated, evidenced by a narrow band-edge peak and the absence of any luminescence associated with the yellow defect band. Additionally two spatially confined broad luminescence bands were observed, consisting of multiple peaks ranging from 395 nm to 480 nm and 490 nm to 650 nm. The lower energy band originates from broad c-plane QWs located at the apex of the nanorod and the higher energy band from the semipolar QWs on the pyramidal nanorod tip. Comparing the experimentally observed peak positions with peak positions obtained from plane wave modelling and 3D finite difference time domain(FDTD) modelling shows modulation of the nanorod luminescence by cavity modes. By studying the influence of these modes we demonstrate that this can be exploited as an additional parameter in engineering the emission profile of LEDs.

Understanding GaN/InGaN core-shell growth towards high quality factor whispering gallery modes from non-polar InGaN quantum wells on GaN rods

GaN microrods are used as a basis for subsequent InGaN quantum well (QW) and quantum dot deposition by metal-organic vapor phase epitaxy. The coverage of the shell along the sidewall of rods is dependent on the rod growth time and a complete coverage is obtained for shorter rod growth times. Transmission electron microscopy measurements are performed to reveal the structural properties of the InGaN layer on the sidewall facet and on the top facet. The presence of layers in the microrod and on the microrod surface will be discussed with respect to GaN and InGaN growth. A detailed model will be presented explaining the formation of multiple SiN layers and the partial and full coverage of the shell around the core. Cathodoluminescence measurements are performed to analyze the InGaN emission properties along the microrod and to study the microresonator properties of such hexagonal core-shell structures. High quality factor whispering gallery modes with [Formula: see text] are reported for the first time in a GaN microrod/InGaN non-polar QW core-shell geometry. The GaN/InGaN core-shell microrods are expected to be promising building blocks for low-threshold laser diodes and ultra-sensitive optical sensors.

Unidirectional Emission of a Site-Controlled Single Quantum Dot from a Pyramidal Structure

Emission control of a quantum emitter made of semiconductor materials is of significance in various optical applications. Specifically, the realization of efficient quantum emitters is important because typical semiconductor quantum dots are associated with low extraction efficiency levels due to their high refractive index contrast. Here, we report bright and unidirectional emission from a site-controlled InGaN quantum dot formed on the apex of a silver-coated GaN nanopyramidal structure. We show that the majority of the extracted light from the quantum dot is guided toward the bottom of the pyramid with high directionality. We also demonstrate that nanopyramid structures can be detached from a substrate, thus demonstrating great potential of this structure in various applications. To clarify the directional radiation, the far-field radiation pattern is measured using Fourier microscopy. This scheme will pave the way toward the realization of a bright and unidirectional quantum emitter along with easy fabrication and large-area reproducibility.