Kazumasa Mitsunaga

Capture of photoexcited carriers in a single quantum well with different confinement structures

The authors report experimental and theoretical studies of photoexcited carrier trapping in a GaAs/Al/sub x/Ga/sub 1-x/As single quantum well with three different typical confinement layer structures: a separate-confinement heterostructure (SCH) and a graded SCH (GRIN-SCH), a linearly graded SCH (LGRIN-SCH), and a parabolically-graded SCH (PGRIN-SCH). The capture is studied by means of time-resolved luminescence with 600-fs resolution. Measurements, performed from 20 to 300 K, show an appreciably shorter trapping time in the GRIN-SCH (1 ps at 80 K to 8 ps at 300 K) than in the SCH (between 22 and 14 s in the same temperature range). Experimental results are fitted by a one-dimensional drift-diffusion model, and the mobility of holes is deduced from the calculations. A short but finite capture time at the edge of the well is demonstrated. >

Continuous wave operation of a surface‐emitting AlGaAs/GaAs multiquantum well distributed Bragg reflector laser

The first cw operation of an AlGaAs/GaAs distributed Bragg reflector laser was achieved at room temperature with a threshold current as low as 38 mA. Surface emission exceeding 4 mW was obtained with an external differential quantum efficiency of 9% and a beam divergence of 0.17°×10°. A two‐dimensional laser array with 3×4 gratings was also fabricated and an output power exceeding 500 mW was obtained under pulsed condition.

GaAs/AlGaAs Optical synaptic Interconnection Device for Neural Networks

A GaAs/AlGaAs optical synaptic interconnection device for neural networks is reported for the first time to our knowledge. This device consists of a light-emitting-diode array, an interconnection matrix, and a photodiode array, which are integrated into a hybrid-layered structure on a GaAs substrate. The device structure and characteristics are reported in detail. The fabricated device can simulate a 32-neuron system. Experimental results of the Hopfield associative memory with three stored vectors are also described.

Calculation of two‐dimensional quantum‐confined structures using the finite element method

The finite element method was used for calculating the wave functions and energy levels of electrons in arbitrarily shaped two‐dimensional quantum‐confined structures. The calculated results indicate the possibility of quasi‐quantum wires by simply growing single quantum wells on corrugated substrates.

Capture of photoexcited carriers by a laser structure

The capture of photoexcited carriers in different laser structures is investigated by subpicosecond luminescence spectroscopy. The capture time at 20 and 80 K is monitored by the decay of the luminescence of the confinement layers. It is found to range between 2 and 3 ps for both linear and parabolic grading, and to be much longer in nongraded structures. The improvement of the capture in the case of graded structures is due to the quasi‐electric field experienced by the carriers, and might allow the devices to run at much higher frequencies.

Edge‐ and surface‐emitting distributed Bragg reflector laser with multiquantum well active/passive waveguides

A novel AlGaAs/GaAs distributed Bragg reflector laser utilizing a multiquantum well structure both as an active waveguide and as a low loss passive waveguide was fabricated. The threshold current was 102 mA at room temperature, and the differential quantum efficiency for the edge‐ and surface‐emitted light was 16% and 3.7%, respectively. No mode hopping was observed in a temperature range of 35 K. A 0.17°×17° far‐field pattern was obtained for the surface‐emitted light.

Fabrication and characterization of quantum well wires grown on corrugated GaAs substrates by molecular beam epitaxy

Single quantum wells were grown on GaAs triangular gratings by molecular beam epitaxy. Cross‐sectional transmission electron microscopy revealed that the quantum wells had a triangular shape above the ridge of the grating. The photoluminescence spectra at 77 K showed strong anisotropy, indicating that the electrons are confined two dimensionally. The calculation confirmed that the electrons tend to be localized at the corner of the bent quantum well wires, and the effective width of the quantum well wire was estimated to be 30 nm.

AlGaAs/GaAs pnpn differential optical switch operable with 400 fJ optical input energy

Differential optical switching operation with an optical input of as low as 400 fJ (0.07 fJ/μm2) was realized by using parallel‐connected AlGaAs/GaAs pnpn optical switches. An improvement in the sensitivity of more than two orders of magnitude has been attained by introduction of a new operating concept. The electrical transient time was 10 ns, almost independent of optical input levels.

cw surface‐emitting grating‐coupled GaAs/AlGaAs distributed feedback laser with very narrow beam divergence

Room‐temperature cw operation of a GaAs/AlGaAs surface‐emitting grating‐coupled distributed feedback laser is reported. By using a transverse junction stripe scheme which provides a window structure for the surface‐emitted light, cw surface emission having a very narrow beam divergence angle of 0.13° and an output power of more than 3 mW was observed.

AlGaAs-GaAs pnpn differential optical switch

A differential optical switch based on parallel-connected AlGaAs-GaAs pnpn structures is presented. Continuous input light makes it possible to discriminate between two optical inputs with a power difference of as low as 0.1 nW. The minimum operating energy with pulse inputs was 400 fJ. With the introduction of a storage operation of pnpn structures, the optical S-R flip-flop operation is also demonstrated with a 7.2-pJ optical input energy. The expansibility of the 2-D array is attractive in optical parallel processing systems, especially in optical neural networks. >