Nonlinear refraction spectroscopy measurements were performed in resonance with absorption lines in Nd3+ and Cr3+ doped crystals. The observed line shapes can be explained by the interference of resonant and nonresonant contributions to the nonlinear refractive index. These effects were fully discriminated using a pump and probe configuration, in which a dispersive line shape was observed on the top of a plateau that was attributed to the polarizability difference between excited and ground states (Deltaalpha). A theoretical model was used to obtain Deltaalpha from the experimental data, and the results are in agreement with several previous experiments. Slow and fast light propagation are also discussed.
Doubly-resonant two-photon-absorption-induced four-wave mixing has been observed for the first time in a rare earth insulator.The generation of the nonlinear signal exhibits strong intermediate and two-photon resonances in crystalline Tb (OH)3 and LiTbF4 This provides a novel method for high resolution measurements of intermediate 4f^n configurations and excited configurations of rare earth ions.The intermediate resonance showed spectral line narrowing up to ten times narrower than the corresponding inhomogeneously broadened absorption.In LiTbF4, laser-limited line widths were obtained and in Tb(OH)3, a homogeneous limit corresponding to T2 = 50 psec may have been reached.The qualitative behavior of the narrowing is explained by the dispersion of the anamalous index of refraction and its effect on phase matching.Direct measurement of the anamalous dispersion was made to test the model.Results indicate nonlinear contributions to the refractive index may have been present.The two-photon resonance in Tb(OH)3 was measured to be a 230 cm^-1 wide crystal field component of the broad 4f^n 5d excited configuration.Thus high resolution UV spectroscopy with tunable visible lasers is demonstrated.Applications are discussed including the extraction of homogeneous line widths from inhomogeneously broadened spectra, coherent transient measurements, and UV and VUV spectrosocpy.The latter uses different selection rules than linear absorption methods and allows study of transitions between excited states.DOUBLY-RESONANT TWO-PHOTON-AB^ORPTION-INDUCED EOUR-WAVB MIXPNQ IN, .T b .(DR) 3 . .