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Optics Letters

Narrowed intermediate resonance in multiresonant four-wave mixing in LiTbF_4

David A. Ender, Howard J. Guggenheim, Michael S. Otteson, R. L. Cone, M. B. Ritter

Published Dec 1, 1982• 9 citations
Publisher

Abstract

A new technique for overcoming inhomogeneous broadening in condensed matter is reported. Phase-matchinginduced frequency selectivity associated with multiresonant four-wave mixing is explored in detail for the first time to our knowledge and shown to give laser-limited linewidths that are more than 10 times narrower than the inhomogeneous width of an intermediate resonance in LiTbF (4). The anomalous dispersion associated with the resonance. was also measured, and model calculations suggest that significantly greater narrowing is feasible.

DOI
10.1364/ol.7.000611

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Physical Review A•Sep 1, 1996•12 citations
A new technique for overcoming inhomogeneous broadening in condensed matter is reported. Phase-matchinginduced frequency selectivity associated with multiresonant four-wave mixing is explored in detail for the first time to our knowledge and shown to give laser-limited linewidths that are more than 10 times narrower than the inhomogeneous width of an intermediate resonance in LiTbF (4). The anomalous dispersion associated with the resonance. was also measured, and model calculations suggest that significantly greater narrowing is feasible.
Four-wave mixing processes with double and triple resonances are discussed in detail. Explicit expressions for the resonant nonlinear susceptibilities of various cases are derived. It is shown that in some cases, four-wave mixing can yield a spectrum with reduced inhomogeneous broadening. Possible applications of multiresonant four-wave mixing as a spectroscopic technique are considered. They include high-resolution spectroscopy, deduction of transition matrix elements, study of transitions between excited states, measurement of longitudinal relaxation times, distinction between resonant Raman scattering and resonant fluorescence, etc. Selective polarization arrangement allows us to suppress the nonresonant background, separately measure the real and imaginary parts of the resonant nonlinear susceptibility, minimize the effects of absorption and laser intensity fluctuations, and greatly enhance the signal-to-noise ratio.
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 . .
Recent research has expanded the capabilities of four-wave mixing by providing it with component selectivity, site selectivity, and mode selectivity. The selectivity is achieved by taking advantage of the three resonance enhancements that occur in a four-wave mixing process. New spectral scanning strategies allow one to scan a single resonance while maintaining the other two resonances at constant values. The constant resonances can be used to select a specific component, a specific site within an inhomogeneously broadened envelope of a component, and/or a specific vibrational or vibronic mode of that site. The scanned resonance will then contain enhanced features corresponding to the particular component, site, and/or mode that was chosen by the constant resonances. These component and site selective capabilities of the four-wave mixing complement the single vibronic level fluorescence methods. The relative transition intensities from a specific component or site reflect the mode coupling between the vibrational and vibronic modes. Since the four-wave mixing is fully resonant in these experiments, saturation effects play a dominant role and cannot be ignored. This review presents a complete discussion of the theoretical and experimental factors that control the fully resonant four-wave mixing. It shows examples of the component selectivity and the line narrowing capabilities. It also provides examples of how mode selection can identify mode coupling through the relative intensities of different vibrational features.
We explain the utility of third-order nonlinear susceptibility, χ (3) (ω 2 ; ±ω 1 , ∓ω1{\mp}\omega_{1}∓ω1​, ω 2 ), for the observation of resonances with homogeneous widths from inhomogeneously broadened distributions in three-level systems related to Raman or two-photon allowed electronic transitions. The line narrowing and high resolution capabilities are demonstrated theoretically using a model for inhomogeneous distributions of transitions in which we regard an absorption spectrum as the composition of some inhomogeneously broadened substructures. The experimental methods to separate a signal having frequency ω 2 from ingoing beam at the same frequency are outlined.
We present a theoretical and experimental study of the line shape of resonant four-wave mixing induced by broad-bandwidth laser radiation that revises the theory of Meacher, Smith, Ewart, and Cooper (MSEC) [Phys. Rev. A 46, 2718 (1992)]. We adopt the same method as MSEC but correct for an invalid integral used to average over the distribution of atomic velocities. The revised theory predicts a Voigt line shape composed of a homogeneous, Lorentzian component, defined by the collisional rate \ensuremath{\Gamma}, and an inhomogeneous, Doppler component, which is a squared Gaussian. The width of the inhomogeneous component is reduced by a factor of \ensuremath{\surd}2 compared to the simple Doppler width predicted by MSEC. In the limit of dominant Doppler broadening, the width of the homogeneous component is predicted to be 4\ensuremath{\Gamma}, whereas in the limit of dominant homogeneous broadening, the predicted width is 2\ensuremath{\Gamma}. An experimental measurement is reported of the line shape of the four-wave-mixing signal using a broad-bandwidth, ``modeless,'' laser resonant with the Q1{\mathit{Q}}_{1}Q1​(6) line of the A2{\mathrm{}}^{2}2\ensuremath{\Sigma}-X2{\mathrm{}}^{2}2\ensuremath{\Pi}(0,0) system of the hydroxyl radical. The measured widths of the Voigt components were found to be consistent with the predictions of the revised theory. \textcopyright{} 1996 The American Physical Society.