Room-Temperature Optical Nonlinearities and Bistability in CdHgTe and CdTe

Abstract

The alloy semiconductor, Cd_xHg_1−xTe offers a variation of band gap energy with composition from 0 to 1.5 eV. This wide range makes available energy gaps resonant with a number of important laser frequencies and in particular, it is one of the few semiconductors suitable for the study of band gap resonant optical nonlinearities in the CO₂ laser output band around 10 µm. In recent publications, we have reported very large nonlinear refractive phenomena at 10 µm in low band gap CdHgTe under a variety of different conditions [1]. These nonlinearities result in selfdefocussing [2] and nonlinear etalon [3] behaviour leading to optical switching[4] and bistability [5]. The most sensitive effects have been apparent under band gap resonant conditions at low temperature [2], with CdHgTe possessing an effective nonlinear coefficient, X⁽³⁾, exceeding that of InSb at low powers. A thermal non-linearity acting in the same direction as the electronically induced nonlinear refraction was found to produce nonlinear etalon behaviour different to other semiconductors [3]. Room-temperature optical bistability was achieved in CdHgTe through two-photon excitation of a sample of appropriate band gap energy using laser pulses which maintain steady state conditions [5]. Employing shorter pulses, rapid dynamical switching behaviour was observed and analysed [6]. The benefits of studying an alloy semiconductor are further extended in the work reported here by making use of samples with a wide range of band gap energies to assess the variation of room-temperature nonlinear response at 10.6 µm wavelength. Optical bistability is also reported in CdTe at 850 nm, this material being at the large gap extreme of the alloy range.