Cr2+-doped II-VI materials for lasers and nonlinear optics

The paper reviews material, spectroscopic, laser and nonlinear optical properties of wide-band Cr2+-doped II-VI materials. The strong revival of research interest in these materials is explained by the announcement of the extremely efficient (up to >70% slope efficiency) laser operation at room-temperature. With up to 11 W of average output power, one can achieve super broad tunability (up to similar to1100 nm between 2 and 3.1 mum) in narrow-line continuous-wave operation and 4 ps pulses at 400 mW in mode-locked regime. Directly diode-pumping and lasers based on ceramic active media have been demonstrated, allowing development of cost-efficient compact tunable and mode-locked lasers, with a possibility to generate few-optical cycle pulses. In this wavelength range the Cr2+-doped lasers proved to be viable competitors to the conventional semiconductor lasers or more complex laser systems, based on frequency conversion techniques in such applications as medicine, trace gas monitoring, remote sensing, spectroscopy, metrology, optical radars, optical communications, and all-optical switching. In contrast to conventional dielectric laser materials the Cr2+-doped II-VI compounds combine properties of semiconductors with that of the traditionally used dielectric active media. The semiconductor nature determines strong nonlinear optical response of these materials, giving rise to charge transport and photorefractive-like phenomena, harmonic generation and parametric processes, and self-focusing effects of various origins. This calls for a considerable modification of the mode-locking techniques and reconsideration of the existing theories, which should finally enable generation of few-optical cycle pulses directly from the laser oscillator in the mid-infrared. In this connection a number of important new aspects are being discussed, such as contribution of cascaded second-order nonlinearity and Raman processes to the third-order nonlinearity, its dispersion and anisotropy, and others. (C) 2004 Elsevier B.V. All rights reserved.