C. Lei

Spontaneous emission from planar microstructures

Abstract The alteration of spontaneous emission characteristics in terms of the spontaneous lifetime and spectral emission characteristics are discussed for dipoles in the presence of nearby planar reflecting interfaces and cavities, specifically for the case of semiconductors. For dipoles closely spaced to absorbing metal mirrors, significant lifetime change is possible. Analysis and experimental data are presented for light emitting diodes. For dielectric Fabry-Perot microcavities, the expected lifetime change is small, but significant modification in the radiation pattern of the emitted light occurs. It is shown that the spectral characteristics of emission have a sensitive dependence on the dipole location in the cavity. Comparison is made between a classical against a quantum treatment of the spontaneous emission modification due to the cavity.

Low-voltage high-gain resonant-cavity avalanche photodiode

For p-i-n photodiodes and avalanche photodiodes (APDs) in the low-gain regime, there is a performance tradeoff between the transit-time contribution to the bandwidth and the quantum efficiency. A new photodetector structure is demonstrated that alleviates limitations imposed by this tradeoff. This structure utilizes a thin ( approximately=900 AA) depleted absorbing layer to reduce the transit time and achieve avalanche gain at low bias voltage (V/sub b/ approximately=9 V). The external quantum efficiency has been enhanced ( eta /sub e/>49%) by incorporating the structure into a resonant cavity.<<ETX>>

Spontaneous emission from a dipole in a semiconductor microcavity

The effect of a semiconductor microcavity on the radiative spontaneous recombination of an electron‐hole pair strategically placed (by virtue of a quantum well) in the microcavity is considered. First‐order perturbation theory is used in the quantum mechanical calculation of the spatially anisotropic radiation rate and shows a strong influence of the cavity, and dipole position in the cavity, on the spontaneous photon emission process. Calculations are compared with previous experiments [T. J. Rogers, D. G. Deppe, and B. G. Streetman, Appl. Phys. Lett. 57, 1858 (1990)].

Role of waveguide light emission in planar microcavities

The role of light emission into waveguide modes of planar microcavities is elucidated through the calculation of the emission characteristics in various structures. Discrepancies which exist in the literature are discussed in some detail for the most popular forms of half-wavelength and full-wavelength planar microcavities which make use of dielectric Bragg reflectors. Particular emphasis is paid to the semiconductor AlGaAs/GaAs microcavities. Recent experiments on spontaneous lifetime changes in thin film semiconductor structures are also discussed. In addition, a SiO/sub 2//Si structure containing active atomic emitters is considered. >

ZnSe/CaF2 quarter-wave Bragg reflector for the vertical-cavity surface-emitting laser

Data are presented on an electron‐beam evaporated ZnSe/CaF2 distributed Bragg reflector for use on a vertical‐cavity surface‐emitting laser operating at a wavelength ∼0.98 μm. Mirror characteristics are measured using optical transmission and reflectivity for quarter‐wave structures with varying numbers of pairs from one to five. The optical characteristics of the ZnSe/CaF2 quarter‐wave stack is compared to similar structures of electron‐beam evaporated Si/SiO2 reflectors. The ZnSe/CaF2 mirror is found to be superior to the Si/SiO2 mirror in terms of both higher reflectivity and lower optical loss for all structures investigated. Comparison is also made between ZnSe/CaF2 and Si/SiO2 mirrors in the continuous‐wave performance of AlAs‐GaAs‐InGaAs quantum‐well vertical‐cavity surface‐emitting lasers. Superior laser performance is achieved with the ZnSe/CaF2 mirror in terms of threshold current and lasing efficiency.

InGaAs-GaAs Quantum Well Vertical-Cavity Surface-Emitting Laser Using Molecular Beam Epitaxial Regrowth

Data are presented demonstrating a design and fabrication process for the realization of high‐efficiency, low‐threshold vertical‐cavity InGaAs‐GaAs quantum well lasers with light emission through the top (epitaxial) surface. Crystal growth is performed using a two‐step molecular beam epitaxial growth process to utilize lateral current injection into the device active region. The device structure allows the top surface (emission side) reflector to be optimized (for either high efficiency or low threshold) after crystal growth through the deposition of electron beam evaporated dielectric layers. Maximum continuous‐wave output power in excess of 1.2 mW at 300 K, and differential quantum efficiency greater than 25% (3.9 mA threshold) are demonstrated. Low‐threshold values of 2.3 mA are measured on devices with increased mirror reflectivity (through the addition of dielectric layers).

Controlled spontaneous emission in room‐temperature semiconductor microcavities

Data are presented demonstrating controlled spontaneous emission in room‐temperature AlGaAs‐GaAs Fabry–Perot microcavities, which utilize high contrast Bragg reflectors. The reflector materials are a CaF2/ZnSe combination. A GaAs quantum well contained in the microcavities is excited using a low power He‐Ne laser, and the spontaneous emission characteristics are measured in terms of spectral characteristics and radiation patterns. The measured data are compared with calculations which predict controlled spontaneous emission in such structures. We find that the dominant effects on spontaneous emission in these thin layer structures are due to cavity controlled emission into allowed optical modes and stress induced dipole orientation in the GaAs quantum well.

Bistability in an AlAs‐GaAs‐InGaAs vertical‐cavity surface‐emitting laser

Data are presented demonstrating bistability in the current versus voltage and light versus current characteristics of a quantum well vertical‐cavity surface‐emitting laser. The laser structures are grown using molecular beam epitaxy, and use an AlAs/GaAs Bragg reflector for the n‐side mirror, and a combination of AlAs/GaAs and either ZnSe/CaF2 or Si/SiO2 quarter‐wave dielectric layers for the p‐side mirror. Regrowth of molecular beam epitaxial layers is used for current funneling into the device active region. Light emission is measured from the epitaxial side of the device, and threshold currents range from 2 to 4 mA. The bistability stems from switching in a parasitic pnpn structure triggered by lasing in the vertical‐cavity laser, with the observed hysteresis width influenced by leakage current around the device active region.

Spontaneous emission and optical gain in a Fabry–Perot microcavity

The Fabry–Perot microcavity is analyzed in terms of its modification of the spontaneous emission and stimulated emission from a thin gain medium, which is contained between distributed Bragg reflectors. The structures correspond to the vertical‐cavity surface‐emitting semiconductor laser. Gain enhancement due to the cavity is calculated and compared to the case of a large Fabry–Perot cavity. The critical parameters in determining the degree of gain enhancement are the (large cavity) coherence length of the spontaneously emitted wave packet, and reflector design.

Spectral and intensity dependence on dipole localization in Fabry-Perot cavities

Data is presented on the characteristics of light emitted from localized dipoles contained in Fabry–Perot cavities. The cavities consist of AlGaAs semiconductor with the dipole localization achieved using GaAs quantum wells. Experimental data shows that the standard assumption of the spectral width of a cavity mode as having some fixed relationship to the photon lifetime in the cavity is an approximation which only becomes valid for dipoles throughout the cavity. Also, intensity differences are measured out either side of a cavity even when symmetrical mirrors are used. The intensity difference depends on the precise dipole position. Both the spectral and intensity differences can be derived from theory using a model which accounts for interference between coherent spontaneous wavepackets emitted in opposite directions from individual emission events.