Phonon-induced coherence loss (PICL) is produced by a monoenergetic nonequilibrium distribution of phonons in
LaF3:
Pr3+. The phonons are generated by relaxation between excited states of the
Pr3+ ion after selective excitation with a pulsed, tunable dye laser. The optical dephasing is observed from the effect on the free-induction decay of
Pr3+ ions whose excited-state level separation is resonant with the phonons. It is shown that PICL is a sensitive detector of monoenergetic phonons, and can be used to study phonon dynamics. The observed rate of optical dephasing is reduced relative to an equivalent occupation of 23-${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ phonons produced thermally.
We demonstrate that the excitation-induced excess dephasing of photon echoes in
Pr3+:
YA1O3(3H4${\mathrm{\ensuremath{-}}}^{1}$
D2) is predominantly induced by the 51-${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ resonant phonons associated with nonradiative relaxations of the excited ions. Instantaneous diffusion via direct ion-ion coupling is found to be much less important. The dephasing mechanism discussed should be applicable to all other systems where the relaxation of excited states involves nonradiative processes.
We introduce a technique utilizing the evanescent field of an optical waveguide to examine the dynamics of nonequilibrium high-frequency phonons resonant with optically excited centers in zones of submicrometer dimension. The technique is used to create a nearly saturated zone of excited
Cr3+ ions at the surface of a 700-at. ppm ruby crystal at 1.5 K by optical pumping through the broad bands. At the same time, the optical pumping serves to generate resonant 29-${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ phonons. The decay of the combined spin-phonon system, measured via the
R2 luminescence following interruption of the optical pumping, is explained by elastic spatial diffusion in parallel with spectral wipeout. The time constant for loss by spatial diffusion is derived rigorously from rate equations. Spectral wipeout is treated according to the model developed by Goossens et al. based on one-site inelastic scattering by weakly exchange-coupled
Cr3+ pairs, but modified to include the appreciable elastic diffusion remaining after wipeout under the present extreme conditions. Spectral wipeout turns out to predominate at high excited-state concentration. The optical feeding into 2A\ifmmode\bar\else\textasciimacron\fi{}
(2E) is found to result mainly from excited-state absorption starting from E\ifmmode\bar\else\textasciimacron\fi{}
(2E), despite the low probability of the latter at the light wavelength used (514.5 nm).
A number of optical techniques have been developed in recent years which have been applied to study the dynamics of high frequency phonons. Monoenergetic phonons were generated by spin-lattice relaxation between two electronically excited states of an impurity ion. Resonant trapping provided the means to detect these phonons with good temporal resolution. VSPS was used to detect high frequency phonons over a wide frequency range with excellent temporal, spectral and spatial resolution. The life-time of a monoenergetic non-equilibrium distribution of phonons was determined by the observation of optical dephasing, using free induction decay(FID), which results from phonon induced coherence loss(PICOLO). Non-resonant fluorescence line narrowing(FLN) techniques provided a means to determine the spectral distribution of non-equilibrium phonons generated by spin lattice relaxation. Keywords: Phonons; Phonon decay; Coherent phonon generation; Phonon spectroscopy; High frequency phonons and Monoenergetic phonons.