An issue on novel applications of materials doped with rare-earth (RE) ions can scarcely fail to address lasers, but it need not address all RE-based lasers. Some Nd3+ -doped lasers, particularly Nd:YAG (Y3Al5O12, yttrium aluminum garnet), emitting light with a wavelength of 1064 nm, are very well-established commercial products—by no means novelties.1 Some other near-infrared (NIR) lasers, based on Er3+ or Tm3+, are also available commercially. That wavelength region is relatively easy for RE laser ions, involving energy spacings between initial and final energy levels small enough to give large stimulated emission cross sections for useful, long upper-state life-times, yet large enough to minimize thermal deexcitation mechanisms. On the other hand, RE-doped lasers for ultraviolet (UV) and visible wavelengths are quite novel, since efficient laser operation is more difficult to achieve in these spectral ranges. Intriguing progress on such devices has been made in recent years, driven by several important applications.
In this article, we begin by noting some of the alternative ways to obtain laser light at these wavelengths, including their advantages and drawbacks. We then discuss basic properties of RE-doped laser materials and how these can be advantageous. We then review a few of the most important and recent RE-doped laser materials and techniques for obtaining UV and visible output.