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AIP conference proceedings

Multiresonant two‐photon‐absorption‐induced four wave mixing in crystalline rare earth insulators

R. L. Cone, David A. Ender, Michael S. Otteson, Paula L. Fisher, Joel M. Friedman, Howard J. Guggenheim

Published Jan 1, 1982• 1 citation
Publisher

Abstract

Coherent nonlinear optical generation of ω4=ω+ω2−ω3 exhibits strong sharp intermediate (ω1) and two‐photon (ω1+ω2) resonances in crystalline Tb(OH)3 and LiTbF4, providing a novel method for high resolution coherent measurements of both excited electronic configurations and intermediate 4 fn states of rare earth ions. New regions of the UV and VUV are thus made accessible to existing tuneable visible and near ultravisible lasers. Selection of sharp features from broad overlapping absorptions, line narrowing due to phase matching selectivity, and coherent transient applications are discussed.

DOI
10.1063/1.33746

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Doubly-resonant two-photon-absorption-induced four-wave mixing in Tb(OH)₃and LiTbF₄

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Journal of the Optical Society of America B•Mar 1, 1984•38 citations
Coherent nonlinear optical generation of ω4=ω+ω2−ω3 exhibits strong sharp intermediate (ω1) and two‐photon (ω1+ω2) resonances in crystalline Tb(OH)3 and LiTbF4, providing a novel method for high resolution coherent measurements of both excited electronic configurations and intermediate 4 fn states of rare earth ions. New regions of the UV and VUV are thus made accessible to existing tuneable visible and near ultravisible lasers. Selection of sharp features from broad overlapping absorptions, line narrowing due to phase matching selectivity, and coherent transient applications are 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 . .
We report the first observation of intermediate state enhancement of four wave mixing induced by two photon absorption in rare earth insulators. Coherent generation of ω4=ω1+ω2−ω3 exhibits strong intermediate state resonances when ω1 sweeps through a single photon transition to the 5D4 states arising from the 4f8 ground configuration of Tb3+ while (ω1+ω2) is simultaneously resonant with an allowed transition to the excited 4f75d configuration. Measurements are presented for Tb(OH)3 and LiTbF4.
Many of the most impressive demonstrations of the efficient generation of vacuum ultraviolet (VUV) light have made use of two‐photon resonantly enhanced four‐wave mixing to generate light at ωVUV =2ωL1±ωL2. The two‐photon resonance state is coupled to the ground state both by two photons from the first laser, or by a photon from the second laser and one from the generated VUV beam. We show here that these two coherent pathways destructively interfere once the second laser is made sufficiently intense, thereby leading to an important limiting effect on the achievable conversion efficiency.
Resonant enhancement by a factor of over 20 of direct two-photon absorption from the ground state 7F6^{7}\mathrm{F}_{6}7F6​ to the excited state 5G6^{5}\mathrm{G}_{6}5G6​ of the 4f8{f}^{8}f8 configuration of the Tb3+{\mathrm{Tb}}^{3+}Tb3+ 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 7F6^{7}\mathrm{F}_{6}7F6​${\mathrm{\ensuremath{-}}}^{5}$D4{\mathrm{D}}_{4}D4​ resonance, a much stronger contribution arose from the 4f8{f}^{8}f8 configuration 5D4^{5}\mathrm{D}_{4}5D4​ intermediate state. Two-photon absorption was clearly isolated from two-step (incoherent) excitation by varying the timing of two separately triggered lasers.
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]).
A pulsed dye-laser beam (ωL) pumped by a frequency-doubled Nd:YAG laser is focused into a heat-pipe oven containing potassium vapor (nk ≈ 1016 cm−3). Two-photon resonant four-wave and six-wave mixing processes using the potassium 8s 2S1/2 state are investigated. Thirty-two coherent UV radiation lines are generated within the range of 300–400 nm through the processes ωUV = 2ωL ± ωIR and ωUV = 2ωL − ω′IR − ω″IR ± ω‴IR, where ωIR are frequencies of laser-induced IR waves generated in the vapor near the transitions 8S–7P, 7P–7S, 7P–5D, 8S–6P, 7S–6P, 5D–6P, etc. The influence of resonant enhancement on the UV line intensities and the phase-matching condition are analyzed.