D. S. McCallum

Magnitude, origin, and evolution of piezoelectric optical nonlinearities in strained [111]B InGaAs/GaAs quantum wells

The magnitudes of excitonic nonlinearities were compared at 12 K in InGaAs/GaAs multiple quantum well structures with growth directions oriented along the [100] and [111] crystal axes by measuring both the steady‐state and time‐resolved differential transmission spectra. As expected, the spectra for the [100] sample are indicative of excitonic bleaching at all times and for all excitation levels, and a carrier recombination time of 0.8 ns and a nonlinear cross section (change in absorption coefficient per carrier pair) of ∼8×10−14 cm2 are extracted for the [100] sample. By comparison, for low excitation levels, the spectra for the [111] sample are consistent with a blueshift of the exciton, indicating a screening of the strain‐induced piezoelectric field. At higher excitation levels, the spectra are dominated by excitonic bleaching. Under identical 1 ps pulsed excitation conditions, the magnitudes of the changes in the absorption coefficient caused by screening in the [111] sample are comparable to those ...

Room‐temperature optical nonlinearities in strained (InAs)2(GaAs)5 superlattice quantum wells

We report room‐temperature measurements of the nonlinear absorption cross section, σeh, and the nonlinear refraction coefficient, neh, associated with saturation of excitonic absorption and bandfilling, for high‐quality multiple quantum well (MQW) structures in which each well consists of a highly strained, all binary (InAs)2(GaAs)5 short‐period superlattice. The three samples studied, which have effective well thicknesses of 10.7, 14.8, and 18.8 nm, respectively, each display clearly resolved excitonic resonances at room temperature. Picosecond nonlinear transmission and transient grating measurements were performed on each sample in the spectral vicinity of the n = 1 heavy‐hole excitonic resonance. The peak values of neh and σeh extracted from these measurements are comparable to those measured in high‐quality GaAs/AlGaAs and unstrained InGaAs/InP MQWs. We determine the dependence of σeh and neh on the width of the quantum wells, and we discuss the fluence dependence of the nonlinearities. Consistency b...

Linear optical properties of quantum wells composed of all‐binary InAs/GaAs short‐period strained‐layer superlattices

The linear optical properties of a variety of (InAs/GaAs)‐GaAs multiple quantum well structures, where each well consists of a highly strained InAs/GaAs short‐period superlattice, have been investigated in detail. The results attest to the improvement in material quality over previously reported structures of this type. Clearly resolved excitonic absorption peaks have been observed at room temperature in all samples. Photoluminescence and excitonic absorption linewidths at 15 K are less than 10 meV in each case, with the photoluminescence Stokes shifted by less than 1 meV. Temporally resolved photoluminescence measurements at 15 K indicate carrier lifetimes of 1.4–1.8 ns. Dramatic strain‐related differences are observed when compared to random alloy InGaAs/GaAs quantum wells with an equivalent average indium mole fraction.

Polarization rotation modulator in a strained [110]‐oriented multiple quantum well

The inherent optical anisotropy of a strained [110]‐oriented multiple quantum well structure is used to make an optically addressed light modulator based on polarization rotation. The anisotropy of the multiple quantum well absorption coefficient for orthogonal polarizations causes rotation of the plane of polarization of light passing through the structure. By partially bleaching the quantum well exciton, the amount of polarization rotation is changed, and therefore modulation of the amplitude of the transmitted beam is achieved.The inherent optical anisotropy of a strained [110]‐oriented multiple quantum well structure is used to make an optically addressed light modulator based on polarization rotation. The anisotropy of the multiple quantum well absorption coefficient for orthogonal polarizations causes rotation of the plane of polarization of light passing through the structure. By partially bleaching the quantum well exciton, the amount of polarization rotation is changed, and therefore modulation of the amplitude of the transmitted beam is achieved.

Nonlinear optical response, screening, and distribution of strain in piezoelectric multiple quantum wells

By embedding piezoelectric InGaAs/GaAs multiple quantum wells (MQWs) in specifically designed p‐i‐n structures, we demonstrate that the nonlinear optical response can be used to identify the dominant screening mechanisms and simultaneously to determine the strain distribution. Furthermore, we show that a knowledge of the screening mechanisms and spatial band structure, in turn, can be used to control the nonlinear optical response. For this demonstration, we fabricate two p‐i(MQW)‐n samples on [111]‐oriented GaAs substrates. The samples are designed such that, if the dominant screening is associated with photogenerated carriers that remain in the wells, a blue shift of the exciton would be expected in each. By contrast, if the screening is associated with carriers that have escaped the wells and moved to screen the entire MQW, one will shift to the blue and the other to the red if the lattice is mechanically clamped, but both will shift to the red if the lattice is mechanically free. The observation of a ...

Enhanced ambipolar in-plane transport in an InAs/GaAs hetero-n-i-p-i-

In‐plane transport in an InAs/GaAs semiconductor hetero‐n‐i‐p‐i has been investigated using picosecond transient grating techniques and an order‐of‐magnitude enhancement of the ambipolar transport relative to that measured in a similar undoped sample has been demonstrated. Both the magnitude and the density dependence of this enhanced transport are consistent with an additional driving force that is associated with an in‐plane modulation of the screened n‐i‐p‐i field. This modulation is the result of the spatial separation by perpendicular transport of electrons and holes that also have an in‐plane density modulation.

Picosecond optical nonlinearities in a strained InAs/GaAs hetero n‐i‐p‐i structure

The nonlinear optical properties of a n‐i‐p‐i structure containing strained superlattice quantum wells in the intrinsic regions are studied using picosecond pump and probe pulses of the same photon energy. For pump fluences as low as 1.1 μJ/cm2, a blue shift of the excitonic resonance, caused by the screening of the built‐in space‐charge field and the accompanying reduction in the quantum‐confined Stark effect, is clearly observed. At higher fluences, the onset of bleaching of the excitonic absorption is observed. The nonlinearities associated with the quantum confined Stark effect in the hetero n‐i‐p‐i are directly compared to those arising from excitonic bleaching in identical strained superlattice quantum wells under flatband conditions. The picosecond time resolution allows a more accurate estimation of the carrier density in the hetero n‐i‐p‐i by ignoring the density‐dependent recombination and a quantitative comparison between the strength of the nonlinearities in the two structures. Although such c...

Optical nonlinearities and ultrafast charge transport in all‐binary InAs/GaAs strained hetero n‐i‐p‐i’s

We use picosecond differential spectroscopy to temporally and spectrally resolve the formation and decay of nonlinearities and space‐charge fields in a hetero n‐i‐p‐i that contains quantum wells in the intrinsic regions that are composed of all‐binary InAs/GaAs short‐period strained‐layer superlattices. The evolution of the optical response is determined by competition between excitonic bleaching and the excitonic shift caused by screening of the built‐in electric field of the n‐i‐p‐i. The relative contributions of the two resulting optical nonlinearities are complicated functions of fluence, time, and wavelength, with the detailed dynamics determined by thermionic emission from the wells, picosecond charge transport over nanometer dimensions, screening, and recombination. At low fluences, excitonic bleaching is the source of an ultrafast nonlinear response that can be turned on and off in <10 ps. This initial excitonic bleaching gives way to a blue shift of the exciton as the carriers escape the wells in...

Carrier and screening dynamics in strained [111]‐oriented multiple quantum wells

We have time‐resolved the piezoelectric optical nonlinearities of a strained, [111]‐oriented InGaAs/GaAs multiple quantum well p‐i‐n structure. By monitoring the temporal evolution of the excitonic absorption spectrum, we follow the photogenerated carriers as they drift to form the space‐charge field responsible for screening the piezoelectric field. From the carrier dynamics, we can separate the contribution of in‐well screening from that of the long‐range screening associated with carriers that have escaped the wells. We have determined that in narrow quantum wells, in‐well screening is smaller than long‐range screening and that in‐well screening is obscured by the concurrent onset of strong bleaching.

Per‐carrier nonlinear optical response of [111]‐oriented piezoelectric InGaAs/GaAs multiple quantum wells

We measure the per‐carrier nonlinear responses caused by photoexcited carriers in two strained [111]‐oriented InGaAs/GaAs multiple‐quantum‐well structures, and we compare them to that measured in a [100]‐oriented structure. Without an external bias, we find that the absorption coefficient changes per photogenerated carrier for the [111]‐oriented piezoelectric materials are smaller than for the [100]‐oriented materials, not larger, as originally suggested. Subsequently, measurements of the per‐carrier nonlinear responses as a function of reverse bias voltage demonstrate that the smaller per‐carrier nonlinearities measured for the [111] structures are partially the result of a broadening of the excitons by the huge in‐well fields, but can be primarily attributed to the quality of the [111]‐grown materials. When corrected for differing in‐well fields and for differing excitonic linewidths, the per‐carrier responses are similar in magnitude, suggesting that the [111] response may eventually approach that of [...