Optical second-harmonic imaging of PbxCd1−xTe ternary alloys

Abstract

We employ femtosecond laser pulses (80 fs, 1.59 eV, and 80 MHz) to study the optical second-harmonic (SH) response of PbxCd1−xTe ternary alloys (x about 0.2) grown by the vertical Bridgman method. The alloy segregates into a Pb-rich and a Cd-rich phase, the latter dominating the SH response of the ternary alloy by at least two orders of magnitude. Several sample regions show a regular layer-by-layer accommodation of the Pb-rich and Cd-rich phases as seen by a periodic alternation of the alloy’s SH response on a ∼10-μm length scale. Furthermore, we employ polarization-resolved SH imaging as well as SH imaging at different azimuthal angles to obtain spatially resolved mappings of the sample, which are sensitive to the composition as well as the growth orientation of the PbxCd1−xTe material system. We observe an azimuthal phase shift of approximately 30° between coherent macroscopic regions (several mm2) in the Cd-rich phase of the ternary alloy. We interpret these regions as large area crystalline grains of (111) and (411) crystal orientations and approximately equal composition. Hence, SH imaging is shown to spatially resolve regions of different growth directions within the PbxCd1−xTe sample.