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Physical review. B, Condensed matter

Resonant enhancement of direct two-photon absorption in Tb3+ : LiYF4

Huang Jin, G. K. Liu, R. L. Cone

Published Apr 1, 1989• 20 citations
Publisher

Abstract

Resonant enhancement by a factor of over 20 of direct two-photon absorption from the ground state $^{7}\mathrm{F}_{6}$ to the excited state $^{5}\mathrm{G}_{6}$ of the 4${f}^{8}$ configuration of the ${\mathrm{Tb}}^{3+}$ ion at 40 200 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ has been observed in time-resolved experiments with two separate pulsed lasers. The results provide clear evidence for resonant enhancement of two-photon absorption in rare-earth compounds and imply a similar enhancement for Raman scattering and nonlinear-optical mixing. Two separate contributions to the direct transition moment were observed. When a single laser frequency was used, the intermediate states which made the largest contribution were from excited configurations of opposite parity; those intermediate states were far from resonance. Detailed two-frequency experiments showed, however, that near a single photon $^{7}\mathrm{F}_{6}$${\mathrm{\ensuremath{-}}}^{5}$${\mathrm{D}}_{4}$ resonance, a much stronger contribution arose from the 4${f}^{8}$ configuration $^{5}\mathrm{D}_{4}$ intermediate state. Two-photon absorption was clearly isolated from two-step (incoherent) excitation by varying the timing of two separately triggered lasers.

DOI
10.1103/physrevb.39.6348

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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 . .
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Resonant and near-resonant two-photon excitations with ultrashort laser pulses of forbidden atomic transitions give access to a wide class of nonlinear-optical processes such as two-photon absorption, stimulated Raman scattering, four-wave sum- and difference-frequency mixing. The latter has proofed to be a powerful tool for efficient femtosecond pulse generation in the VUV-spectral range [1]. Nevertheless, up to now only few experimental results have been obtained which allow the direct observation of the coherent dynamics in two-photon induced radiation-matter interactions (see experimental papers [2-5] and monograph [6]).
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