M. S. Skolnick

Strong exciton-photon coupling in an organic semiconductor microcavity

The modification and control of exciton–photon interactions in semiconductors is of both fundamental and practical interest, being of direct relevance to the design of improved light-emitting diodes, photodetectors and lasers. In a semiconductor microcavity, the confined electromagnetic field modifies the optical transitions of the material. Two distinct types of interaction are possible: weak and strong coupling. In the former perturbative regime, the spectral and spatial distribution of the emission is modified but exciton dynamics are little altered. In the latter case, however, mixing of exciton and photon states occurs leading to strongly modified dynamics. Both types of effect have been observed in planar microcavity structures in inorganic semiconductor quantum wells and bulk layers. But organic semiconductor microcavities have been studied only in the weak-coupling regime. Here we report an organic semiconductor microcavity that operates in the strong-coupling regime. We see characteristic mixing of the exciton and photon modes (anti-crossing), and a room-temperature vacuum Rabi splitting (an indicator of interaction strength) that is an order of magnitude larger than the previously reported highest values for inorganic semiconductors. Our results may lead to new structures and device concepts incorporating hybrid states of organic and inorganic excitons, and suggest that polariton lasing may be possible.

Strong coupling phenomena in quantum microcavity structures

The physics of strong coupling phenomena in semiconductor quantum microcavities is reviewed. This is a relatively new field with most important developments having occurred in the last 5 years. We describe how such microcavities enable both electronic and photonic properties of semiconductors, and the interaction between them, to be controlled in the same structure. The resulting coupled exciton-photon eigenstates, cavity polaritons, have many interesting properties including very low mass for small in-plane wavevectors, and can be studied easily and directly in optical experiments, unlike exciton-polaritons in bulk semiconductors. A wealth of new optical phenomena has been reported in this field in the last few years. This review describes the most important of these phenomena. We discuss the reasons why polaritons have fundamentally different properties in microcavities as compared with those in bulk materials or quantum wells. We explain the factors which control the strength of the exciton-photon coupling and how the resulting optical spectra can be modelled. We then emphasize, in the main body of the review, the particularly important results of reflectivity experiments at both normal and oblique angles of incidence, the effects of external electric and magnetic fields, the results of coherent Raman scattering experiments, the effects of disorder on microcavity spectra, including the observation of motional narrowing over the exciton disorder potential, studies of coupled microcavities, and photoluminescence and time-resolved phenomena.

Improved performance of 1.3μm multilayer InAs quantum-dot lasers using a high-growth-temperature GaAs spacer layer

The use of a high-growth-temperature GaAs spacer layer is demonstrated to significantly improve the performance of 1.3μm multilayer self-assembled InAs∕InGaAs dot-in-a-well lasers. The high-growth-temperature spacer layer inhibits threading dislocation formation, resulting in enhanced electrical and optical characteristics. Incorporation of these spacer layers allows the fabrication of multilayer quantum-dot devices emitting above 1.3μm, with extremely low room-temperature threshold current densities and with operation up to 105°C.

Collective fluid dynamics of a polariton condensate in a semiconductor microcavity

Semiconductor microcavities offer unique systems in which to investigate the physics of weakly interacting bosons. Their elementary excitations, polaritons—mixtures of excitons and photons—can accumulate in macroscopically degenerate states to form various types of condensate in a wide range of experimental configurations, under either incoherent or coherent excitation. Condensates of polaritons have been put forward as candidates for superfluidity, and the formation of vortices as well as elementary excitations with linear dispersion are actively sought as evidence to support this. Here, using a coherent excitation triggered by a short optical pulse, we have created and set in motion a macroscopically degenerate state of polaritons that can be made to collide with a variety of defects present in the microcavity. Our experiments show striking manifestations of a coherent light–matter packet, travelling at high speed (of the order of one per cent of the speed of light) and displaying collective dynamics consistent with superfluidity, although one of a highly unusual character as it involves an out-of-equilibrium dissipative system. Our main results are the observation of a linear polariton dispersion accompanied by diffusionless motion; flow without resistance when crossing an obstacle; suppression of Rayleigh scattering; and splitting into two fluids when the size of the obstacle is comparable to the size of the wave packet. This work opens the way to the investigation of new phenomenology of out-of-equilibrium condensates.eprints@whiterose.ac.uk https://eprints.whiterose.ac.uk/ Reuse Unless indicated otherwise, fulltext items are protected by copyright with all rights reserved. The copyright exception in section 29 of the Copyright, Designs and Patents Act 1988 allows the making of a single copy solely for the purpose of non-commercial research or private study within the limits of fair dealing. The publisher or other rights-holder may allow further reproduction and re-use of this version refer to the White Rose Research Online record for this item. Where records identify the publisher as the copyright holder, users can verify any specific terms of use on the publisher’s website.

Optimizing the growth of 1.3 μm InAs/InGaAs dots-in-a-well structure

The structural and optical properties of GaAs-based 1.3 μm InAs/InGaAs dots-in-a-well (DWELL) structures have been optimized in terms of different InGaAs and GaAs growth rates, the amount of InAs deposited, and In composition of the InGaAs quantum well (QW). An improvement in the optical efficiency is obtained by increasing the growth rate of the InGaAs and GaAs layers. A transition from small quantum dots (QDs), with a high density (∼5.3×1010 cm−2) and broad size distribution, to larger quantum dots with a low dot density (∼3.6×1010 cm−2) and narrow size distribution, occurs as the InAs coverage is increased from 2.6 to 2.9 monolayers. The room-temperature optical properties also improve with increased InAs coverage. A strong dependence of the QD density and the QD emission wavelength on the In composition of InGaAs well has been observed. By investigating the dependence of the dot density and the high-to-width ratio of InAs islands on the matrix of InGaAs strained buffer layer (SBL), we show that the in...

Emission spectra and mode structure of InAs/GaAs self-organized quantum dot lasers

A study of the emission spectra and mode structure of InAs/GaAs self-organized quantum dot lasers is presented. In contrast to conventional bulk or quantum well lasers, the number of lasing modes increases above threshold. This behavior is shown to be consistent with carriers localized in noninteracting dots and a resultant inhomogeneously broadened gain spectrum. The lasing spectra are found to have a complicated form with groups of longitudinal modes separated by nonlasing spectral regions and lasing occurring via a number of different lateral modes. These characteristics are discussed in terms of the spatially discrete nature of the quantum dots.

Long lifetimes of quantum-dot intersublevel transitions in the terahertz range

Carrier relaxation is a key issue in determining the efficiency of semiconductor optoelectronic device operation. Devices incorporating semiconductor quantum dots have the potential to overcome many of the limitations of quantum-well-based devices because of the predicted long quantum-dot excited-state lifetimes. For example, the population inversion required for terahertz laser operation in quantum-well-based devices (quantum-cascade lasers) is fundamentally limited by efficient scattering between the laser levels, which form a continuum in the plane of the quantum well. In this context, semiconductor quantum dots are a highly attractive alternative for terahertz devices, because of their intrinsic discrete energy levels. Here, we present the first measurements, and theoretical description, of the intersublevel carrier relaxation in quantum dots for transition energies in the few terahertz range. Long intradot relaxation times (1.5 ns) are found for level separations of 14 meV (3.4 THz), decreasing very strongly to approximately 2 ps at 30 meV (7 THz), in very good agreement with our microscopic theory of the carrier relaxation process. Our studies pave the way for quantum-dot terahertz device development, providing the fundamental knowledge of carrier relaxation times required for optimum device design.

New physics and devices based on self-assembled semiconductor quantum dots

Self-assembled semiconductor quantum dots (QDs) exhibit fully quantized electronic states and high radiative efficiencies. This makes them highly suitable both for fundamental physics studies of zero-dimensionality, atomic-like semiconductor systems and applications in a range of novel electro-optical devices. This review discusses recent important advances in the study and application of semiconductor QDs. Using a wide range of optical spectroscopy techniques, it is possible to obtain a detailed understanding of the electronic structure and dynamical carrier processes. Such an understanding is required for the implementation of a wide range of QD-based devices.

High-performance three-layer 1.3-/spl mu/m InAs-GaAs quantum-dot lasers with very low continuous-wave room-temperature threshold currents

The combination of high-growth-temperature GaAs spacer layers and high-reflectivity (HR)-coated facets has been utilized to obtain low threshold currents and threshold current densities for 1.3-/spl mu/m multilayer InAs-GaAs quantum-dot lasers. A very low continuous-wave (CW) room-temperature threshold current of 1.5 mA and a threshold current density of 18.8 A/cm/sup 2/ are achieved for a three-layer device with a 1-mm HR/HR cavity. For a 2-mm cavity, the CW threshold current density is as low as 17 A/cm/sup 2/ for an HR/HR device. An output power as high as 100 mW is obtained for a device with HR/cleaved facets.

Exciton–polaritons in van der Waals heterostructures embedded in tunable microcavities

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.