Polariton Devices Based on Wide Bandgap Semiconductor Microcavities

Group III-nitrides have been considered a promising choice for the realization of optoelectronic devices since 1970. Since the first demonstration of the high-brightness blue light-emitting diodes (LEDs) by Shuji Nakamura and coworkers, the fabrication of highly efficient white LEDs has passed successful developments. A serious physical issue still remained, which prevents their use for high power and highly efficient LEDs: the drop of external quantum efficiency (EQE) of III-nitride LEDs when increasing the driving current, the so-called ''efficiency droop'' problem. In order to have a fast expansion to the lighting market, the cost-per-lumen of packaged LEDs must rapidly decrease. This indeed demands for having LED chips operating with high EQE under the high current operation. Besides the industrial interest of III-nitrides, owing to their large direct bandgap, they feature some interesting optical properties such as large exciton binding energy and large oscillator strength of excitons. However, when the carrier density raised in a semiconductor, a transition should occur from an insulating state consisting of a gas of excitons to a conductive electron-hole plasma, that is called the Mott transition. This crossover can drastically affect the optical and electrical characteristics of semiconductors and may, for instance, drive the transition from a polariton laser to a vertical cavity surface-emitting laser. More interestingly, even if biexcitons are frequently seen to dominate the emission of III-nitride heterostructures when the density is raised, no clear experimental report is available on the role of biexcitons in the Mott transition in a two-dimensional (2D) nanostructure. In the first part, the emission properties of high-quality GaN/AlGaN single quantum wells (QWs) at high-carrier densities are examined. They are of crucial importance to provide a deeper insight into the operating conditions of III-nitride based lasers and LEDs, as well as the transition from strong to weak coupling regime of exciton-polaritons in semiconductor microcavities. Employing the same technique then to investigate some InGaN/GaN QWs, the droop signature was investigated comprehensively in both polar and non-polar QWs. Having an accurate estimation of the carrier density in the QWs, the contribution of several non-radiative processes were quantitatively examined. These experiments can indeed provide a deeper insight on the physical phenomena responsible for the efficiency droop in III-nitride LEDs, and can stimulate several theoretical and experimental on this subject. The second part focuses on the transport mechanisms of excitons in ZnO and III-nitride based nano-structures. The transverse movement of donor-bound excitons in a purely bent ZnO microwire as a function of temperature have been investigated, owing to the high spatial, spectral and temporal resolutions of our original time resolved cathodoluminescence system. The movement mechanism was modeled by a hopping process of excitons. Our results pave the way to new experiments allowing to reveal the physics at play at the nanoscale in different materials. However, in case of highly disordered systems, one should take into account more complex considerations, as in the case of InGaN core-shell QW structures studied in the last chapter, the large energy fluctuations prevent excitons to move along the energy gradient at low temperatures.

ZnO and SnO2 are wide bandgap semiconductors, which have great potential for UV detectors, light emitting diodes, and other optoelectronic devices. The performance of these devices is significantly affected by exciton binding energies of oxides. Although bandgaps of ZnO and SnO2 are similar, their exciton binding energies differ by a large amount, because it was reported experimentally that the exciton binding energy of SnO2 is 130 meV, which is higher than that of ZnO with 60 meV. On the other hand, because ZnO has higher ionicity and a large overlap of electrons and hole wavefunctions, it is expected that it should have larger exciton binding energy than that of SnO2 [Dvorak et al., Phys. Rev. Lett. 110, 016402 (2013)]. Therefore, there is a controversy for relative magnitudes of the exciton binding energy between SnO2 and ZnO. Here, based on accurate GW + Bethe–Salpeter equation calculations, we clarify that the exciton binding energy of SnO2 is less than that of ZnO, following the general rule; therefore, more experimental study should be carried out to verify our predictions. We expect that this fundamental understanding of the relative magnitude of exciton binding energies will be useful for prospective applications of ZnO and SnO2 in optoelectronic devices.

14 - Polaritons and polariton condensates in perovskites

Excitons, bound electron–hole pairs in a 2D plane, dominate the optical properties of monolayer transition metal dichalcogenides (TMDs). A large exciton binding energy on the order of 0.5 eV was theoretically predicted and experimentally determined recently. These ultrastable excitons thus open an avenue to explore the exciton physics such as Bose–Einstein condensation and superfluidity at room temperature (Kasprzak et al 2006 Nature 443 409; Plumhof et al 2014 Nat. Mater. 13 247; Fogler et al 2014 Nat. Commun. 5 4555; Jiang and John 2014 Sci. Rep. 4 7432). Recent experiments further demonstrated the concept of Coulomb engineering via dielectric environments based on either solutions or few-layer graphene. However, the conducting nature of these dielectrics can lead to quenching of optical transitions. Thus, in order to utilize ‘dielectric tuning’ of the exciton binding energy and quasiparticle band gaps, one must use insulating dielectrics. Here, we investigate the impact of insulating dielectric environments on the exciton binding energy of monolayer WS2 and WSe2 by exciton Rydberg spectroscopy, in which the dielectric environment is systematically varied from κ = 1.49 to 3.82. We found that, with increasing κ value, the exciton binding energy and quasiparticle bandgap exhibit significant reductions. Quantitatively, our result follows the prediction of nonlocally-screened Keldysh potential very well. The fitted 2D polarizability, , agrees rather well with previous density function theory calculations. Their close agreement validates the nonlocally screened Keldysh model which can be used to quantitatively predict the exciton binding energy for monolayer TMDs (and possibly other 2D materials) in different dielectric environments. Such a predictive model will play an important role for the design of van der Waals heterostructures and TMD-based optoelectronic devices.

Quasiparticle band structures and optical properties of monolayer ZrNX (X=Cl, Br, I) under exciton effect

Quasi-2D halide perovskites are chemically synthesized realizations of quantum well stacks with giant exciton oscillator strengths, tunable emission spectra, and very large exciton binding energies. While these features render quasi-2D halide perovskites a promising platform for room-temperature polaritonics, bosonic condensation and polariton lasing in quasi-2D perovskites have so far remained elusive at ambient conditions. Here, we demonstrate room-temperature cavity exciton-polariton condensation in mechanically exfoliated crystals of the quasi-2D Ruddlesden-Popper iodide perovskite (BA)2(MA)2Pb3I10 in an open optical microcavity. We observe a polariton condensation threshold of 0.41 µJ cm−2 per pulse and detect a strong non-linear response. Interferometric measurements confirm the spontaneous emergence of spatial coherence across the condensate with an associated first-order autocorrelation reaching 0.6 with 1 ps coherence time and an effective de Broglie wavelength of 13 µm. Our results lay the foundation for a new class of room-temperature polariton lasers based on quasi-2D halide perovskites with great potential for hetero-integration with other van-der-Waals materials and combination with photonic crystals or waveguides.

With edge-emitting GaN-based lasers in commercial systems, attention is shifting to more demanding and rewarding emitters. These encompass microcavity (MC)-based vertical cavity surface-emitting lasers (VCSELs) and polariton lasers. The impetus centers on applications such as high-speed/high-resolution laser printing/scanning technology, lighting, and new types of coherent but nearly thresholdless optical sources. Room-temperature operations of GaN-based VCSELs by electrical injection have been recently reported, and the research on GaN-based VCSELs is segueing into new opportunities such as polariton-based lasers. While still in its infancy, polariton lasing in GaN-based MCs at room temperature has been observed. Observation of spontaneous emission buildup in polariton lasing emission is attributed to a Bose-Einstein condensate of cavity polaritons. However, the realization of a polariton laser by electrical injection is still being pursued. In this paper, we discuss the recent progress in wide-bandgap semiconductor-based VCSELs and GaN-/ZnO-based polariton lasers.

Investigation of light–matter interactions in two-dimensional (2D) semiconductors is essential to understand many-body effects and explore their applications in photonics. 2D semiconductor microcavities can be formed by the integration of 2D semiconductor active media with planar Fabry–Perot resonant cavities. The emerging exciton–photon interactions between the strongly bound excitons in 2D semiconductors and the cavity photons allow exploring on the wealthy photonic and polaritonic physics of bosonic quasiparticles in the 2D limit. This Perspective focuses on recent advances in exciton–photon interactions of 2D semiconductor microcavities and their inspiring applications. First, we picture the research scope of 2D semiconductor microcavities, sort out the historical development from conventional semiconductor microcavities to the emerged 2D semiconductor microcavities, and illustrate mostly employed device structures. Second, we classify exciton–photon interactions in 2D semiconductor microcavities according to their coupling strengths. In the weak coupling regime, control of spontaneous emission and photon lasing are discussed together with the Purcell effect. In the strong coupling regime, we summarize experimental observations and theoretical predictions on Rabi splitting, Bose–Einstein condensation, polariton lasing, superfluidity, and superconductivity. Third, four types of leading-edge applications are outlined, including on-chip coherent light sources, microcavity-enhanced single-photon sources, topological photonics, and other nonlinear optics. Finally, we highlight the remaining challenges and future opportunities for fundamental physics of 2D semiconductor microcavities and their technological applications.

Two-dimensional metal halide phases, commonly known as 2D perovskites, have emerged as promising materials for exciton-polaritons, particularly for polariton condensation. This process entails the spontaneous accumulation of population in the polariton ground state and relies on efficient energy relaxation. In this class of materials, this relaxation is mediated by exciton reservoir emission, which pumps polariton states through radiative pumping. To achieve strong light-matter coupling and sustain a high polariton density, the material must possess excitations with large oscillator strength and high exciton binding energy. While 2D perovskites exhibit these desirable characteristics, there are no reports of room-temperature polariton condensation and only one successful demonstration at cryogenic temperatures. In this work, we systematically explore the role of energy alignment between the exciton reservoir emission and the lower polariton branch in populating the polariton ground state via radiative pumping. Through cavity detuning, we shift the lower polariton energy minimum to overlap with the emission of the exciton reservoir at different energies. We identify that the multiple radiative pathways of 2D perovskites lead to inefficient radiative pumping of the lower polariton branch at the lowest-energy state, ultimately posing challenges for polariton condensation in this class of materials.

Zinc oxide (ZnO), as a wide band gap (3.37 eV) semiconductor, are useful materials for wide range of applications in thin-film transistor (TFT), photovoltaics, photocatalysts, sensors, and so on. Large exciton binding energy (60 meV) at room temperature (RT) makes ZnO a promising luminescent material especially in optoelectronic devices such as UV lasers and light emitting diodes (LED). Among them, ZnO nanocrystals (quantum dots) has attracted increasing attention because their optical properties can be tuned by varying the crystal size. However, its nanoscale crystallites tend to aggregate to undergo Ostwald ripening due to their high surface energy. To stabilize them, surface modification are usually necessary. At the same time, the whole processes for nanocrystal synthesis and film fabrication become complicated.Here, we report a simple novel photochemical gel conversion approach for one-step preparation of ZnO thin films with an average nanocrystals in about 2-3 nm size (inset in Fig. 1a), by employing vacuum ultraviolet (VUV) light irradiation in dry N2. Systematic characterization is performed to understand the surface chemistry and the optical properties of ZnO nanocrystals. Thus obtained ZnO films show unique absorption peak at 294 nm (4.2 eV) (Fig. 1a) and strong tunable photoluminescence in the range of 386-496 nm when expose to UV light, from violet, blue to bluish-green upon increasing time of VUV irradiation (Fig. 1b). Clearly blue-shifted absorption peaks of the VUV treated samples are indicative of quantum-confinement effect, as contrasted to the broad character of the annealed film, which originates from band structure of bulky ZnO. The extension of VUV time only results in increase of absorbance, with no change of the peak wavelength, indicating population increase but no crystal growth by Ostwald ripening during the VUV irradiation. Therefore, the color change of the emitted light should be associated with the change of defect state distribution, rather than the size effect. However, such photoluminescence from defects was fairly strong to be clearly recognized by naked eyes, so that the university logo could be patterned by photomasking VUV to be seen under black light was demonstrated (inset in Fig. 1b). Figure 1

The number of studies on Zinc oxide (ZnO) have increased tremendously over the last ten years due to its widespread applicability as optoelectronic device, sensor, catalyst, transparent conductive oxide layer (TCO) in solar cells and piezoelectric device. The potential compliance of ZnO in those various field is due to its unique properties which exhibit direct wide band gap (∼ 3.37 eV) semiconductor, large exciton binding energy (60 meV), possess good piezoelectric characteristics, chemical stability as well as biocompatibility. At nanometer scale, ZnO nanostructures such as nanorod, nanoneedle, nanobelt and nanoparticles have shown to exhibit different properties from the bulk. Likewise, they demonstrated significance enhancement of ZnO performance due to high surface to volume ratio characteristics. Among the various structures, ZnO nanoparticles offers great extent of applications ranging from chemical sensor, catalyst, anti-bacterial activities, transparent UV protection film, bio-imaging to bio medical application. However, the overall cost of producing these devices is relatively high compared to Si due to high cost of producing ZnO nanoparticles. Realizing of the advance functional device that ZnO nanoparticle is capable of, it is crucial to devise a method to synthesize ZnO nanoparticles for mass production. Up to now, a variety of techniques have been employed for the synthesis of ZnO nanoparticles via physical vapor deposition (PVD) and solution based method. PVD techniques, such as spray pyrolysis, sputter deposition, template assisted growth and chemical vapor deposition requires very complicated equipment, expensive raw materials, high temperature, high pressure and long deposition times which suggest that they are not suitable for large scale production at relatively low cost. In contrast to PVD, solution based techniques such as sol-gel, hydrothermal, sonochemical, microemulsion and precipitation are more feasible to be industrialized as they are low cost and produce high output volume. Comparing both physical and chemical techniques, the latter method is favored in this study. In solution technique, the selection of reactants is crucial as they are involved in the construction and formation of the particles. Most of the reagents employed for synthesizing ZnO nanoparticles are in zinc salt substance which will react with basic solution to provide a medium for nucleation and growth. Zinc salt commonly used are zinc acetate, zinc nitrate, zinc sulphate, and zinc chloride whereas for basic solution; sodium hydroxide, ammonium hydroxide, ammonium carbonate are the reagents that commonly employed. Only a few works that used elementary elements as the reactant in ZnO synthesis have been reported. For example, C. Wang and co-workers have synthesized ZnO nanoparticles using Zn and I 2 which they have discovered that I 2 plays some part in the nucleation of ZnO nanoparticles. However, the role of I 2 in ZnO formation is not thoroughly analyzed and discussed [1]. In this work, we present the study of I 2 role and improvement in ZnO nanoparticles synthesis by introducing Diethanolamine (DEA) as initiator and capping agent as well as O 2 diffusion as catalyst during nanoparticles formation. Moreover, we employed post-heat treatment to removes the byproduct. ZnO nanoparticles properties (structural, morphology and optical) produced under different calcination temperatures at 700, 850 and 1150 °C were characterized using X-ray powder diffractometer (XRD), Scanning electron microscope (SEM) and room temperature Photoluminescence (PL) spectroscopy. Based on the SEM results, uniformly spherical particles are observed for all the samples prepared in the presence of I 2 whereas microsphere with honey comb like structure were observed for samples without the presence of I 2 . Samples heated at 850°C prepared in the I 2 existence shows high intensity of blue-violet emission at 411 nm whereas sample without I 2 exhibit two emission peak centered at 380 nm (UV emission) and 523nm (green emission). Possible reason of the red shifted peak for samples prepared with I 2 with respect to UV may be attributed to zinc interstitial or zinc vacancy defects created via interaction with I 2 during nanoparticles formation. Weight % calculated for samples synthesized with I 2 is ∼ 1.795% whereas without I 2 is ∼ 0.9% indicating enhancement in terms of output volume.

Methylammonium lead iodide (CH3NH3PbI3) hybrid perovskite in the tetragonal and orthorhombic phases have different exciton binding energies and demonstrate different excitation kinetics. Here, we explore the role that crystal structure plays in the kinetics via fluence dependent transient absorption spectroscopy. We observe stronger saturation of the free carrier concentration under high pump energy density in the orthorhombic phase relative to the tetragonal phase. We attribute this phenomenon to small dielectric constant, large exciton binding energy, and weak Coulomb screening, which results in difficult exciton dissociation under high light intensity in the orthorhombic phase. At higher excitation intensities, we observe a coherent phonon with an oscillation frequency of 23.4 cm(-1) at 77 K, whose amplitude tracks the increase of the first-order lifetime.

Recent advances in ZnO materials and devices

By placing an organic semiconductor material having a relatively narrow electronic transition into an optical microcavity, it is possible to 'mix' excitons with confined cavity photons, forming states termed 'cavity polaritons'. Here, I discuss the formation of polariton states in microcavities containing J-aggregates of two different molecular dyes (cyanine dyes), whose J-band electronic transitions are both coupled to the same cavity photon mode. Under such conditions, three polariton branches are formed, with the "middle" polariton branch composed of an admixture of the cavity photon and the two different exciton states. I show using both photoluminescence excitation spectroscopy and photoluminescence emission measurements that such "hybrid" polariton states effectively act as an energy transfer pathway, allowing energy to be transferred between the different exciton states. A model is presented that describes exciton scattering into polariton states, and the subsequent decay and energetic relaxation of polaritons. I argue that the transfer of middle-branch polaritons to the lower-lying excitonic states is an efficient process, that occurs in time-scale of less than 10 fs. I then discuss structures in which a single J-aggregated cyanine dye is placed into a microcavity in which the extended cavity path-length results in the formation of a series of closely spaced cavity photon modes. It is shown that excitons in the cavity can simultaneously undergo strong coupling with at least four cavity photon-modes, effectively forming a ladder of polariton states, with a significant polariton population found in 3 adjacent polariton branches.

In this review, we discuss first some of the recent works to reveal properties of conventional ZnO/ZnMgO QWs grown c-axis oriented. This will include the properties of the quantum confined Stark effect (QCSE) that results from the internal electric field in the unit cell. We will then discuss various unconventional QW growths, including non-polar ZnO QWs, graded barrier QWs and double QWs. We finish with a review of current progress towards light emitting devices based on ZnO/ZnMgO QWs. ZnO has been a material of interest for over 50 years; however, the ability to grow high-quality epilayers of ZnO and ZnO-based ternary systems[1–4 has led to renewed interest over the past decade in ZnO for device applications. The demand for optoelectronic devices in the blue-UV region of the electromagnetic spectrum has been well established and ZnO possesses several properties that are superior to GaN for many applications [5, 6]. The large exciton binding energy of ∼ 60 meV suggests excitonic emission that is very efficient above room temperature, leading to great potential for light emitting devices. The large piezoelectric and pyroelectric coefficients suggest potential for applications as piezoelectric sensors or actuators. Other advantages of ZnO are its comparatively low growth temperatures [5], low optical power threshold for lasing [7], radiation hardness [8, 9] and biocompatibility [10]. ZnO may also find application as a transparent conductive oxide [11] to replace indium-tin-oxide (ITO) in photovoltaic applications because it remains transparent even when doped above the level of degeneracy [12]. This ability to grow high-quality epilayers and thin films rapidly led to the development of ZnO-based quantum wells (QWs). The two main attractions of developing QWs are the tunability offered for the transition energy, and the increased exciton binding energy, which typically leads to increased oscillator strength and greater efficiency for light emitting devices. The two most common types of ZnO-based QWs are Zno/ZnMgo and ZnO/ZnCdO-based systems, where alloying with Mg leads to an increase in the band gap and with Cd leads to a reduction in the band gap. In this chapter, we will focus on the more common ZnO/ZnMgO QWs. The band gap of Znx − 1Mg x O is given by 3. 37 + 2. 51x eV [13], which together with a conduction to valence band offset ratio of approximately 70:30 leads to type I confinement in \({\mathrm{ZnO/Zn}}_{x-1}{\mathrm{Mg}}_{x}\mathrm{O}\) QWs. The value of x is limited to less than 0.43 [13] in these systems as above this value phase separation tends to occur. Nevertheless, even with values significantly less than 0.43, strong confinement is obtained. In such ZnO/ZnMgO QWs, exciton binding energies up to 120 meV have been reported [14]. The biexciton binding energy is also enhanced in QWs going from 15 meV in bulk ZnO to ∼ 25 meV in ZnO/ZnMgO QWs [15–17]. With biexciton binding energies greater than the thermal energy, it is possible that biexcitons may play a major role in the optical properties at room temperature. Since the first ZnO epitaxial layers were grown, high-quality ZnO quantum wells have been grown by several different methods, including molecular beam epitaxy (MBE), metal-organic vapour phase epitaxy (MOVPE) and pulsed laser deposition (PLD). The growth in almost all cases is c-axis oriented despite growth on a range of single crystal substrates, including sapphire, ZnO and Si oriented along various crystal planes.KeywordsTransition EnergyPulse Laser DepositionExciton Binding EnergyPotential ProfileInternal Electric FieldThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.