Spectral and thermal dependence of phase coherent photorefractivity in ZnSe quantum wells

Abstract

We investigate the spectral and thermal dependence of an exciton resonant all-optical and phase coherent photorefractive effect in a ZnSe single quantum well by four-wave mixing using 90 fs pulses. The observed phase coherence is attributed to an exciton-induced formation of an electron grating within the quantum well. The diffracted signal exhibits an exponential decay that is equal to the exciton dephasing rate and it shows a pronounced suppression at the temporal overlap of incident pulses. With decreasing photon energy the diffracted signal traces reveal faster decay and the exciton transition is shifted towards lower energies indicating an increasing electron concentration within the quantum well. In temperature dependent measurements we observe a filling of the signal dip at pulse overlap and a transition from the photorefractive effect to a ͑3͒ four-wave-mixing process below 35 K. Additional experiments using a cw pump laser are consistent with the proposed mechanism of the observed photorefractive effect. The phase coherent photorefractive signal is reproduced by a phenomenological model that is based on the optical Bloch equations for a five-level system.

Holographic imaging using the phase coherent photorefractive effect in ZnSe quantum wells

“…Previous investigations 12,13 also revealed that the decay time of the exciton induced electron grating in the PCP QW is in the ϳ10 s range. 12 and 13͒ using ultrashort light pulses at a temperature of ϳ50 K. Compared to current PR QW devices these all-optical PCP QWs operate without electrical contacts at a spectral efficiency of Ϸ 5 ϫ 10 −4 .…”

“…The maximum frame rate is limited by the decay time of the electron grating in the PCP QW, which is in the ϳ10 s range. 12,13 Within an accuracy of Ϯ10 m, we determine a step height of ϳ65 m, which is somewhat less than the 80 m obtained from the micrograph. However, due to the coherence of excitons, the entire depth profile of a reflecting object is encoded by its brightness in the holographic image.…”

“…It consists of a 10 nm wide ZnSe single QW sandwiched between two 30 nm Zn 0.94 Mg 0.06 Se barriers, with a 20 nm thick ZnSe buffer layer between the barrier and the substrate. 12,13 This signal dip is used to determine the zero delay ͑ =0͒ between the image and reference pulses, the latter being reflected from an adjustable height on the object. The laser energy was resonantly tuned to the ZnSe QW exciton transition at ϳ2.81 eV.…”

“…12,13 In the experiments described above the delay stage was changed to provide the depth information of an object using the dark line in the image for "zero delay" calibration. Due to the high diffraction efficiency, no computational reconstruction was necessary to view the images.…”

Electron transfer and capture dynamics in ZnSe quantum wells grown on GaAs

“…Recently, we discovered an all-optical phasecoherent photorefractive (PCP) effect in a ZnSe single quantum well (QW) grown on (001) GaAs using ultrashort light pulses in a two-beam four-wavemixing (FWM) configuration [1,2]. The exciton resonant light pulses create both a coherent exciton polarization in the QW and highly excited electronhole (e-h) pairs in the GaAs substrate.…”