High Resolution Nonlinear Spectroscopy of Rare Earth Ions in Solids

Abstract

Spectroscopy is one of the most useful techniques for obtaining a fundamental understanding of the electronic structure and dynamical behavior of solids. From it, one can obtain a microscopic understanding of phenomena such as energy migration, trapping and upconversion, nonlinear absorption and bleaching, photorefractive effects and numerous other optical, magnetic and transport phenomena. The information available from spectroscopic studies depends to a great extent on the achievable resolution. At low temperatures this resolution is often limited by inhomogeneous strain broadening, which is intrinsically large in glassy systems but always present to some degree in real crystals, however perfect. We discuss a number of nonlinear spectroscopic techniques appropriate to both the frequency domain and the time domain and illustrate how these can overcome the effects of inhomogencous broadening. Examples will be taken from a wide range of rare earth doped crystals and glasses, where it is possible to achieve a spectral resolution of better than 1 kHz corresponding to a resonance Q greater than 1012 for the optical transition. We describe two-step absorption and avalanche absorption leading to frequency upconversion, spectral holeburning, and photon-gated holeburning, as well as photon echoes and nonlinear optical free-induction decays which measure directly the coherent interaction of light with the materials. The effect of external perturbations such as electric and magnetic fields can be measured with great precision using several different nonlinear spectroscopy techniques. Examples of such measurements will be given. Time and frequency domain techniques can provide different windows on relaxation phenomena by probing different parts of the fluctuation spectrum of a relaxation process. This is manifest by time resolved holeburning measurements of spectral diffusion and non-exponential photon echo decays. Some of the limitations and possibilities for application to other materials will be presented.KeywordsNonlinear AbsorptionBloch VectorPhoton EchoSpectral HoleSpectral DiffusionThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.