T. A. Fisher

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.

Scattering mechanisms limiting two-dimensional electron gas mobility in Al0.25Ga0.75N/GaN modulation-doped field-effect transistors

In order to characterize the electron transport properties of the two-dimensional electron gas (2DEG) in AlGaN/GaN modulation-doped field-effect transistors, channel magnetoresistance has been measured in the magnetic field range of 0–12 T, the temperature range of 25–300 K, and gate bias range of +0.5 to −2.0 V. By assuming that the 2DEG provides the dominant contribution to the total conductivity, a one-carrier fitting procedure has been applied to extract the electron mobility and carrier sheet density at each particular value of temperature and gate bias. Consequently, the electron mobility versus 2DEG sheet density has been obtained for each measurement temperature. Theoretical analysis of these results suggests that for 2DEG densities below 7×1012u200acm−2, the electron mobility in these devices is limited by interface charge, whereas for densities above this level, electron mobility is dominated by scattering associated with the AlGaN/GaN interface roughness.

Wide optical bandwidth asymmetric Fabry–Pérot reflection modulator using the quantum confined Stark effect

A normally on, high-performance quantum confined Stark effect asymmetric Fabry–Perot reflection modulator with enhanced optical bandwidth is reported. The wide optical bandwidth is achieved by utilizing the variation in refractive index in the vicinity of the heavy-hole exciton. The nominal operating wavelength is set in the region where the on-state refractive index starts to increase and allows the Fabry–Perot resonance condition to be maintained over a wide wavelength range. An optical bandwidth of 5 nm is achieved for an operating voltage of 7 V, insertion loss 55% and contrast ratio >15 dB, and 7 nm if the contrast ratio is relaxed to >10 dB. These values are twice as wide as previously reported for a quantum confined Stark effect modulator structure, and correspond to an operating temperature range of 25u2009°C compared to 10u2009°C for conventional structures.

Simulation of mid-infrared HgTe/CdTe quantum-well vertical-cavity surface-emitting lasers

We theoretically show the feasibility of optically and electrically pumped Hg-based vertical-cavity surface-emitting lasers (VCSELs) that emit at midwave-infrared wavelengths up to thermoelectric cooler temperatures. The maximum operating temperature is significantly enhanced by employing a multiple quantum-well active region with very thin (20–30 A) HgTe wells engineered to yield a strong suppression of both Auger recombination and intervalence free-carrier absorption. Hg0.65Cd0.35Te/Hg0.1Cd0.9Te distributed Bragg reflectors are employed for one or both of the mirrors defining the optical cavity. Detailed numerical simulations of VCSELs emitting at λ≈4.3u2009μm predict that for optical pumping at 1.06 μm, a maximum operating temperature of 220 K should be achievable for pulsed operation and 160 K in cw mode, with a cw power output of up to 2.6 mW per array element at 100 K. Injection VCSELs are predicted to operate up to 200 K for pulsed operation and 105 K for quasi-cw with a 10% duty cycle.

Mechanochemical synthesis and characterization of GaN nanocrystals

A solid-state displacement reaction of Ga2O3 with Mg3N2 has been used to synthesize GaN nanocrystals by mechanochemical processing. X-ray diffraction, transmission electron microscopy (TEM) and selected area electron diffraction (SAED) measurements indicated that the nanocrystals had a hexagonal structure and sizes ranging from 4 to 20u2009nm. Optical absorption and transmission measurement showed the bandgap of the nanocrystals was consistent with that of bulk GaN samples (3.43u2009eV). This study demonstrates that mechanochemical processing has significant potential for the synthesis of GaN nanocrystals in a simple and efficient way.

Excitons and Polaritons in Semiconductor Microcavities

We review various aspects of the behaviour of semiconductor microcavities, focusing on the important role that polaritons play in determining their optical properties. The review covers angular dependent measurements of cavity polariton dispersions, the effects of motional narrowing on the inhomogeneous line widths, and the possibilities for observing consequences of Bose-Einstein statistics in microcavities.

Fermi sea shake-up in quantum well luminescence spectra

Abstract Evidence for many body shake-up in the magneto-luminescence spectra of quantum wells is presented. Results for both lattice matched InGaAs-InP and strained layer AlGaAs-InGaAs-GaAs structures, covering a wide range of carrier densities, are presented. In all the structures coupling between shake-up and LO phonon excitations is observed. Strong enhancement of the shake-up satellite of the N e = 1 Landau level (LL) recombination, due to resonant coupling with the N e = 0 LL is presented.

Enhancement of optical bandwidth using high barrier multiquantum well structures

We demonstrate the enhancement of optical bandwidth for modulators based on the quantum confined Stark effect using high barrier multiquantum well structures. This is achieved by carefully placing the Fabry-Perot mode in the wavelength region where the absorbing layer exhibits anomalous refractive index dispersion in the vicinity of the exciton peaks. The enhancement in both the exciton oscillator strength and the separation between the heavy hole and light hole in high barrier multiquantum well structures ultimately results in the enhancement of optical bandwidth. A simulation example shows an increase of 2.5 nm in optical bandwidth at the cost of a factor of three increase in electric field to obtain the same contrast ratio characteristics as a modulator fabricated with low barrier material.

Synthesis of nanocrystal GaN powders by mechanochemical reaction

The synthesis of nanocrystal GaN by mechanochemical reaction was studied by X-ray diffraction, transmission electron microscopy, selected area electron diffraction and optical absorption. The solid state displacement reaction of Ga/sub 2/O/sub 3/+Mg/sub 3/N/sub 2//spl rarr/2GaN+3MgO was induced by mechanical milling. Nanocrystal hexagonal GaN with an average size of 11.4 nm was obtained through subsequent annealing and removal of the by-product through washing. Optical absorption showed the bandgap of the sample is consistent with that of bulk GaN sample.

Photo-induced lifting of the degeneracy of excitonic states in coupled quantum microcavities

Abstract Optically induced coupling of spatially separated quantum well excitonic states is reported from angular-dependent studies of coupled quantum microcavity structures. The coupling is observed as a lifting of the degeneracy of quantum well exciton states separated by macroscopic distances of over 2xa0μm. We demonstrate that the coupling arises from the exciton–photon interaction between the symmetric and antisymmetric photon modes of the coupled cavity and the symmetric and antisymmetric combinations of the degenerate excitonic states.