Future Prospects For Visible And Infrared Color Center Lasers

Abstract

The term "color center" refers to a broad class of point defects in crystalline lattices. The name originated because many of these defects impart characteristic colors to the otherwise colorless pure crystals. Only a small fraction of the many distinct types of color centers exhibits laser action. The systems that do lase are all characterized by homogeneously broadened emission bands with high oscillator strengths and high quantum efficiencies. This leads to laser oscillators with large tuning ranges, high output powers and low threshold pump powers. As a class, color center lasers can tune continuously from 0.36 microns to 4 microns. To span this range requires the use of several different types of color centers as well as several members of each type. A single crystal may have a tuning range of from 500cml to 2000cml. The width of the tuning band and its wavelength peak depend strongly on the color center type and the lattice host, but there is very little variation among various samples of the same system. The shift of the tuning band among the various host lattices can be as small as 10% for the TI°(1) defect and as large as 100% for the F₂+_A defect. Almost all color center systems require cryogenic operation to achieve lasing. However, the best systems only require storage at 0°C to be free from any long term degradation effects, while the least desirable must always be maintained at liquid nitrogen temperatures even when not in the laser. Except for the requirement for cryogenic cooling, most laser cavity designs are quite similar to the techniques used for dye lasers. The maximum laser output power depends most strongly on the efficiency of the color center fluorescence and the Stokes shift between the pump and laser wavelengths. In turn, the fluorescence efficiency is a product of the quantum efficiency and geometrical factors relating to the pump and laser polarizations. The Stokes shift is additionally important because the fluorescent quantum efficiency decreases at elevated temperature. The larger the Stokes shift, the greater the fraction of the pump power that is dissipated as heat in the crystal. For F_A(II) systems with large Stokes shifts and temperature-dependent quantum efficiencies, the maximum obtainable powers are 50 to 200 mw, corresponding to an energy efficiency of 5 to 10%. For the F₂+ systems with fairly small Stokes shifts, output powers of several watts with efficiencies of greater than 60% have been obtained. Some color center systems degrade under the action of the pump laser, much as laser dyes experience photodegradation. Susceptibility to this process varies widely and the best representatives, the F_A(II), exhibit no measurable deterioration over several thousand hours of use. Other systems, such as F₂+, can degrade completely in a few milliseconds. Because color center lasers are solid state systems with no moving parts, they exhibit excellent intrinsic frequency stability. With careful engineering, color center laser systems can achieve narrow linewidths (< 1MHz) and low drive rates (< 1MHz/minute) even in passive, unstabilized operation. With active stabilization schemes, color center lasers have produced linewidths of less than 10 KHz and drift rates of 1 MHz/hour.