B. P. McGinnis

Degenerate four‐wave mixing from layered semiconductor clusters in the quantum size regime

We report the first measurement of the third‐order nonlinear susceptibility χ(3) in layered semiconductor clusters exhibiting pronounced quantum size effects at room temperature. BiI3 clusters prepared in colloidal form in acetonitrile had a thickness of ≂7 A and lateral dimensions between 60 and 90 A. Using degenerate four‐wave mixing, we observed that the conjugate pulses from the small and the large gratings had comparable intensities, verifying the electronic origin of the nonlinearity. The nonlinear susceptibility was found to be 2.3×10−11 esu for a colloid with a cluster volume fraction of 10−5.

Subpicosecond switching in a current injected GaAs/AlGaAs multiple‐quantum‐well nonlinear directional coupler

We have demonstrated ultrafast switching behavior in a current injected GaAs/AlGaAs multiple‐quantum‐well nonlinear directional coupler at room temperature. The results show low crossover pulse energy (10 pJ) and full recovery within 1 ps.

All‐optical, high contrast GaAlInAs multiple quantum well asymmetric reflection modulator at 1.3 μm

A high contrast, low intensity GaAlInAs/AlInAs multiple quantum well asymmetric Fabry–Perot reflection modulator for operation at 1.3 μm has been demonstrated. The reflection modulator takes advantage of the large absorptive and refractive nonlinearities associated with saturating the heavy‐hole exciton resonance. We achieve an on/off contrast ratio in excess of 1000:1 (30 dB) and an insertion loss of 2.2 dB at a pump intensity of 30 kW/cm2, corresponding to a carrier density of 4.5×1017 cm−3. The modulator was demonstrated to have a large operating bandwidth, achieving an on/off contrast ratio of greater than 100:1 over a 5 nm optical band. The operating speed of the modulator was measured and found to approach 1 GHz.

Formation of transverse spatial ring structures in increasing-absorption optical bistability

We report the observation of transverse spatial ring structures in the transmitted beam profile using a thin CdS platelet that exhibits increasing-absorption optical bistability. The observed ring structure is in good agreement with our theoretical predictions.

Temperature‐ and carrier‐density‐dependent electron tunneling kinetics in (Ga,In)As/(Al,In)As asymmetric double quantum wells

We present a detailed investigation of electron tunneling in (Ga,In)As/(Al,In)As asymmetric double quantum wells as a function of different excitation and temperature conditions. We show that tunneling dynamics depend strongly on the initial carrier temperature and momentum. For example, electron and hole tunneling out of the narrow well is complete at low temperature. However at room temperature carriers do not exhibit any tunneling kinetics. We propose a simple kinetic model which describes the observed population dynamics at different carrier densities, temperatures, and excitation conditions.

Room‐temperature optical nonlinearities of GaInAs/AlInAs and GaAlInAs/AlInAs multiple quantum wells and integrated‐mirror etalons at 1.3 μm

The room‐temperature nonlinear absorption spectra of a 40 A GaInAs/AlInAs and a 90 A GaAlInAs/AlInAs multiple quantum well (MQW) were measured near 1.3 μm using a pump and probe technique. Saturation carrier densities at the heavy‐hole exciton peak were determined to be 1.2×1018 and 1.0×1018 cm−3 with carrier recovery times of ≂5 ns and ≂750 ps for the two samples, respectively. These measured saturation carrier densities are close to the reported values for GaAs/AlGaAs MQWs. Fabry–Perot etalons with integrated mirrors grown by molecular beam epitaxy with GaInAs/AlInAs MQWs as spacer layers were also fabricated as optical logic gate devices. We demonstrate a 125‐ps recovery time for the etalon switching device at room temperature.

Hole-State Mixing and Nonlinear Optical Properties of Semiconductor Quantum Dots

Our recent experimental and theoretical results on optical nonlinearities of semiconductor microcrystallites in glass will be reviewed. One-photon and two-photon absorption spectroscopy are used to show the importance of confinement-induced valence-band mixing for small semiconductor particles. Deviations from the parabolic band approximation are demonstrated.

Four-Wave Mixing And Scanning Tunneling Microscopy Of Semiconductor Clusters

Semiconductor structures in lower dimensions, dubbed quantum dots, exhibit novel properties which result from size quantization of their charge carriers, as well as from their large surface-to-volume ratio. Optical measurements, combined with scanning tunneling microscopy, can provide the detailed information required to model the nonlinear optical response of these clusters.

Electroabsorption and carrier dynamics in (Ga,In)As/(Al,In)As asymmetric double quantum wells

We report enhanced electroabsorption in selectively doped (Ga,In)As/(Al,In)As Asymmetric Double Quantum Wells (ADQWs) by the use of real space electron transfer. The electron concentration in both the wide and narrow wells is investigated by field dependent absorption and photoluminescence spectroscopy. Additionally, a study of carrier dynamics in these ADQW structures indicates that electrons tunnel between the coupled wells on picosecond time-scales.

Ultrafast hole tunneling in asymmetric double quantum wells

We present the results of an experimental study of tunneling in Asymmetric Double Quantum Well (ADQW) structures for which holes were found to tunnel from the narrow well to the wide well on sub-picosecond time-scales. These times are as fast, or faster than electron tunneling times despite the absence of resonances between hole states. Valence band structure calculations for our ADQW structures indicate that ultrafast hole tunneling can be attributed spin-dependent delocalization of the hole wavefunctions with a concomitant singularity (in principle) in the density of final wide well states.