Study of interwell carrier transport by terahertz four-wave mixing in an optical amplifier with tensile and compressively strained quantum wells

Abstract

Interwell carrier transport in a semiconductor optical amplifier having a structure of alternating tensile and compressively strained quantum wells was studied by four-wave mixing at detuning frequencies up to 1 THz. A calculation of transbarrier transport efficiency is also presented to qualitatively explain the measured signal spectra. © 1994 American Institute of Physics.Interwell carrier transport in semiconductor quantum well ͑QW͒ lasers has been shown to affect both the static and dynamic performance characteristics of these devices, including their maximum modulation bandwidth.1-7 Since Rideout et al.1 first proposed a well-barrier hole burning model, a number of theoretical analyses 2-7 and experimental investigations, including modulation response measurements, 5,6 picosecond pump-probe, 8,9 and intermodulation distortion measurements 10 have studied these processes.Four-wave mixing ͑FWM͒ in semiconductor travelingwave amplifiers ͑TWAs͒ has recently emerged as an important technique for study of carrier dynamics in semiconductor active layers. [10][11][12][13] In this paper, we report on the first investigation of interwell transport in an MQW TWA using a novel FWM technique that enables selective excitation and probing of adjacent quantum wells.The semiconductor optical amplifier used in these measurements was an InGaAs/InGaAsP MQW TWA operating at 1.5 m.14 The active layer contains six alternating-strain ͑tensile and compressive͒ InGaAs QWs. The tensile QWs provide predominantly TM gain, while the compressive QWs provide TE gain but have vanishing TM gain. This device therefore enables selective excitation and probing of wells according to strain. We performed FWM experiments on the device using, first, a probe wave having a polarization orthogonal to the pump waves ͑cross-polarization FWM͒, and then having the same polarization as the pump waves ͑copo-larization FWM͒. The experimental arrangement for this study was similar to what we have described in Ref. 13 except here the input beams were combined using a beam splitter to allow independent polarization control. The FWM signal was measured using a high sensitivity optical heterodyne detection system. Three, single-frequency, tunable, Er-doped fiber ring lasers 15 were used as pump, probe, and local oscillators in the measurements.In a typical measurement, as illustrated in Fig. 1, input beam 1, having optical frequency f 1 and a given polarization ͑either TE or TM͒, and input beam 2, having optical frequency f 2 and a polarization at an angle of 45°with respect to beam 1, were coupled into the TWA. For the configuration shown in Fig. 1, TM-polarized beam 1 and the TM component of beam 2 serve as pumps to excite carriers in the tensile wells through interband transitions. Carrier dynamics in the compressive QWs were then probed using the TE-polarized component of beam 2 ͑cross-polarization FWM͒, while carrier dynamics in the tensile QWs were probed using the TMpolarized component of beam 2 ͑copolarization FWM͒. The cross-polarized ͑TE͒ and copolariz...

Measurement of the interwell carrier transport lifetime in multiquantum-well optical amplifiers by polarization-resolved four-wave mixing

“…This measurement is based on polarization-resolved four-wave mixing ͑FWM͒ spectroscopy in an InGaAsP alternatingstrain semiconductor optical amplifier ͑SOA͒, consisting of three pairs of tensile and compressively strained quantum wells. 9 In a recent article 7 it was shown how this technique enables selective excitation and probing of adjacent quantum wells, and the effect of interwell transport was qualitatively observed. Here we apply the results of a subsequent detailed study of the polarization properties of FWM 10 to also obtain quantitative information.…”

“…Of particular interest is the finite rate at which the carrier densities in neighboring wells reach equilibrium with one another and with the distribution of unconfined carriers in the barrier region. This process has received considerable attention in recent years [1][2][3][4][5][6][7][8] since it has been associated with nonlinear gain compression, and hence the maximum modulation bandwidth of quantum-well lasers.…”

Femtosecond dynamics of the nonlinear anisotropy in polarization insensitive semiconductor optical amplifiers

“…Some of these issues have been partially addressed in calculations of the cw saturation characteristics of these devices 5 and frequency-domain four-wave mixing measurements. 6 In this letter, we investigate PI-SOAs using a heterodyne pump-probe technique 7 to allow copolarized as well as cross-polarized time-domain measurement with femtosecond resolution. Pump-induced gain changes as well as pump-induced index changes are easily measured using this system.…”

Polarization properties of four-wave mixing in strained semiconductor optical amplifiers

“…We attribute this discrepancy to the strong coupling between neighboring, oppositely strained wells (recently observed in a similar SOA [7]), mediated by several mechanisms such as carrier diffusion, drift and phonon assisted tunneling. Inclusion of interwell transport in the above analysis leads to additional terms to ikfiikk of the form (x;Lc + x 2 L T ) & k k , where for instance xEhc describes a FWM process in which the gratings are formed in a compressive well and transferred to a tensile well where the pump is then scattered.…”