Phase coherent photorefractive effect in II-VI semiconductor quantum wells and its application for optical coherence imaging

The phase coherent photorefractive (PCP) effect in different ZnSe quantum well structures and its dependence on various extrinsic and intrinsic parameters have been investigated using 90 fs laser pulse in a two-beam four-wave-mixing (FWM) configuration. At low excitation intensities the signal is dominated by the PCP effect (which is attributed to a long living electron grating formed in the QW due to coherent QW excitons) and pulse overlap (PO) effect while at high excitation intensities it is governed by Χ⁽³⁾ FWM processes and the PO effect. With increasing excitation intensity the signal dip at pulse overlap (τ ≈ 0) which is characteristic for the destructive interference between the PO and PCP effect shifts to positive delay times τ > 0. The higher PCP diffraction efficiency value of ~1.5 x10^-3 in QW B (Zn0.92Mg0.08Se/ZnSe) as compared to the value of ~3.5 x10^-4 in QW A (Zn_0.94Mg_0.06Se/ZnSe) at 55 K is attributed to an increased Mg concentration in the barrier of QW B leading to a higher captured equilibrium electron density n_e. Repetition rate dependent measurements on QW B show a drop of the diffraction efficiency for repetition times larger than 1.25 µs which is attributed to the reduction of the electron grating amplitude due to thermally activated electron tunneling.

FWM experiments on two 10 nm ZnSe QWs with different barrier thicknesses of 20 (QW1) and 50 nm (QW2) between the QW and substrate show a redshift of the exciton line and an increased exciton dephasing rate due to increasing E-field induced tilt of the QW structure indicating an increased density of captured electrons n_e. At temperatures below 35 K and laser excitation close to the exciton energy the creation of trions significantly compensates the formation of the spatially modulated electron density grating. At lower excitation energies increasing space-charge-fields significantly tilt the QW which reduces the trion binding energy leading to an enhanced thermal ionization of trions resulting in a strong PCP effect at even low temperature. Because of the thermal dissociation of trions at temperatures above 40 K a significant PCP effect exists even at nearly exciton resonant excitation. Model calculations of the signal traces which are based on the optical Bloch equations considering E-field induced inhomogeneous broadening of exciton energies are in good agreement to the experimental exciton traces observed at different excitation condition.

Using the time-gating capability of PCP QWs we have demonstrated “single-shot” three-dimensional optical coherence imaging (OCI) in which the depth of an object is determined from the brightness profile of its holographic image. We present real-time and depth-resolved OCI of moving glass beads of ~100 micrometer size in solution. We have also performed “contrast-enhanced” OCI experiment which enables the detection of both reflecting and absorbing objects providing real-size images within an unlimited field-of-depth. Improved PCP QW with higher diffraction efficiencies and a combined recording of objects in both OCI modes bear tremendous potential for monitoring dynamical processes in biological systems and for particle detection.