Cr 4+-doped crystals: their use as lasers and passive Q-switches

Abstract

This review provides an overview on the subject of Cr 4+-doped solids and their numerous applications in laser physics, technology, and industry. The transition metal ions have spectral properties which make them favorable candidates for active lasing ions. In particular, the Cr 4+-doped crystal has the unique characteristics of being both a saturable absorber for the 1.06 μm laser emission of Nd:YAG as well as being a tunable mid-IR laser in the spectral range of ≈1.3 to ≈1.6 μm with significant practical implications. The review covers the basic spectroscopic concepts and particularly the spectroscopic properties, structural characteristics, and physical parameters of Cr 4+ doped various solids. The effects on the laser performance and on the quantum efficiency of non-radiative relaxation from the first excited state level will be critically reviewed, and possible proposed models will be introduced and analyzed. Knowing the spectroscopic properties, the physical parameters, and the crystal field effects on the position of the energy levels, we shall discuss the use of various Cr 4+-doped solids such as forsterite and garnets as room temperature, tunable mid-IR lasers in the spectral range 1.3–1.6 μm, with emphasis on the crystal field and coordination effects on the spectroscopy and laser performance of Cr 4+-doped solids. The last section deals with the role of several Cr 4+-doped garnets as saturable absorbers in various Nd- and Yb-doped crystals, both in lamp- and diode-pumped systems. We shall also focus on understanding the physics of the saturation process in saturable absorbers for improving the utilization of these materials. Several systems of passively Q-switched systems and their current applications will be discussed.