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Annual Meeting Optical Society of America

Laser-induced grating spectroscopy of Cr3+-doρed garnets

Richard C. Powell, Faqir M. Hashmi, A. Suchocki, Frederic M. Durville, G. Boulon

Published Jan 1, 1988• 0 citations
Publisher

Abstract

Laser-induced grating measurements were made as a function of laser power and write beam crossing angle for various temperatures between 10 and 300 K on Cr3+ ions in Gd3Ga5O12 (GGG), Gd3Sc2Ga3O12 (GSGG), and GGG mixed with Ca, Mg, and Zr ions. Population gratings were produced in the Cr3+ ions, and the diffusion coefficient describing energy migration among the Cr3+ ions was determined from the grating decay rate. The energy migration was found to increase as the temperature was raised for GGG and to decrease with decreasing temperature for GSGG. This difference is attributed to the differences in the crystal field splittings of the excited states in the two hosts. Enhanced energy diffusion was observed in the multiply doped GGG sample. Contributions to the laser-induced gratings from absorption and dispersion changes were determined, and this was used to provide information on radiationless relaxation and excited state absorption processes. For long writing times, long-lived changes in the refractive index were observed and attributed to optically induced color centers.

DOI
10.1364/oam.1988.wk3

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Physical review. B, Condensed matter•Dec 1, 1989•29 citations
Laser-induced grating measurements were made as a function of laser power and write beam crossing angle for various temperatures between 10 and 300 K on Cr3+ ions in Gd3Ga5O12 (GGG), Gd3Sc2Ga3O12 (GSGG), and GGG mixed with Ca, Mg, and Zr ions. Population gratings were produced in the Cr3+ ions, and the diffusion coefficient describing energy migration among the Cr3+ ions was determined from the grating decay rate. The energy migration was found to increase as the temperature was raised for GGG and to decrease with decreasing temperature for GSGG. This difference is attributed to the differences in the crystal field splittings of the excited states in the two hosts. Enhanced energy diffusion was observed in the multiply doped GGG sample. Contributions to the laser-induced gratings from absorption and dispersion changes were determined, and this was used to provide information on radiationless relaxation and excited state absorption processes. For long writing times, long-lived changes in the refractive index were observed and attributed to optically induced color centers.
Four-wave-mixing techniques were used to establish and probe population gratings of Cr3+{\mathrm{Cr}}^{3+}Cr3+ ions in alexandrite crystals at temperatures between 10 and 300 K. The results were interpreted in terms of the interaction of the laser radiation with a two-level atomic system. They provide information about the characteristics of four-wave-mixing signals for this physical situation as well as being useful in characterizing the properties of energy transfer and dephasing within the ensemble of Cr3+{\mathrm{Cr}}^{3+}Cr3+ ions. The patterns of the transient four-wave-mixing signals are consistent with a model based on the pumping dynamics of ions in the mirror and inversion crystal-field sites. The variation of the signal intensity with laser power is strongly affected by beam depletion. The characteristics of exciton migration among Cr3+{\mathrm{Cr}}^{3+}Cr3+ ions in mirror sites were determined from the results of measuring the variation of the signal decay rate with grating spacing. The temperature dependences of the ion-ion interaction rate, the exciton-phonon scattering rate, and the diffusion coefficient were determined. These are found to be essentially the same for pumping into the 4T2^{4}\mathrm{T}_{2}4T2​ and 2E^{2}\mathrm{E}2E levels, but the effects of scattering from a grating of ions in inversion sites is much stronger for 4T2^{4}\mathrm{T}_{2}4T2​ pumping. The dephasing times for the atomic system were found from analyzing the variation of the signal intensity with grating spacing. For pumping into the 4T2^{4}\mathrm{T}_{2}4T2​ level the dephasing is dominated by radiationless decay processes. A model is presented for the decay channel that provides a theoretical explanation for the decay process which is consistent with the measured temperature and frequency dependences of the results as well as their variation with crystal-field strength. For pumping into the 2E^{2}\mathrm{E}2E level the dephasing is dominated by dephasing processes associated with the inhomogeneous linewidth of the transition.
We report what is to our knowledge the first direct measurement of spatial energy migration among the Tm(3+) ions in a Tm,Ho:YAG crystal at room temperature. Four-wave mixing techniques were used to establish and probe transient laser-induced population gratings in the Tm(3+) ions. The grating decay rate was measured as a function of the grating spacing. The results show the presence of spatial energy migration among the Tm(3+) ions, and the diffusion coefficient of this process was measured to be 4 x 10(-7) cm(2) s(-1) for a sample containing 8.3 x 10(20) cm(-3) Tm(3+) ions.
Four-wave-mixing techniques were used to establish and probe population gratings of Cr3+{\mathrm{Cr}}^{3+}Cr3+ ions in both mirror and inversion sites in BeAl2{\mathrm{BeAl}}_{2}BeAl2​O4{\mathrm{O}}_{4}O4​:Cr3+{\mathrm{Cr}}^{3+}Cr3+ crystals as a function of temperature between about 6 and 300 K. The four-wave-mixing signal intensity and decay rate were monitored as a function of the crossing angle of the laser ``write'' beams. The variation of the signal intensity with crossing angle is explained theoretically with use of a model based on the interaction between the laser and a two-level atomic system. Theoretical fits to the results provide information concerning the relative importance of the absorption and dispersion contributions to the signal, and the dephasing time of the atomic system. The latter is found to be 2.2 psec for the mirror site ions and 80 psec for the inversion site ions. The signal decay rate of the inversion site ions was found to be independent of crossing angle at all temperatures, while the signal decay rate for ions in the mirror site increased with increasing crossing angle at temperatures below about 150 K. This indicates the presence of long-range energy migration, and the diffusion coefficient describing this process was found to increase to about 4.2\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}7}$ cm2{\mathrm{cm}}^{2}cm2?${\mathrm{c}}^{\mathrm{\ensuremath{-}}1}$ at 6 K.
We have performed nondegenerate four-wave mixing measurements on Cr(3+) ions in three crystals: BeAl(2)O(4), Gd(3)Sc(2)Ga(3)O(12), and LaMgAl(11)O(19). Two laser beams of the same frequency were used to resonantly pump one of the absorption bands of Cr(3+), and probe beams with three different wavelengths ranging from 442 to 1064 nm were used to generate a signal from the laser-induced population grating. The observed four-wave mixing signal strength was found to increase as the wavelength of the probe beam was decreased. A model was utilized to explain these results based on the change in polarizability of the Cr(3+) ions when they are in the metastable state versus the ground state, and it was found that the dispersion effects can be described by a single-oscillator model with the lowest-energy charge-transfer transition of Cr(3+) making the dominant contribution to the signal.
The refractive index of most ion-doped materials increases with the excited state population. This effect was studied in many laser materials, particularly those doped with Cr3+ and rare earth ions, using several techniques, such as interferometry, wave mixing, and Z-scans. This refractive index variation is athermal (has an electronic origin) and is associated with the difference in the polarizabilites of the Cr3+ ion in its excited and ground states, Δαp. The Cr3+ optical transitions in the visible domain are electric-dipole forbidden, and they have low oscillator strengths. Therefore, the major contribution to Δαp has been assigned to allowed transitions to charge transfer bands (CTBs) in the UV with strengths ∼3 orders of magnitude higher. Although this CTB model qualitatively explains the main observations, it was never quantitatively tested. In order to further investigate the physical origin of Δαp in Cr3+-doped crystals, excited state absorption (ESA) and Z-scan measurements were thus performed in Cr:Al2O3 (ruby) and Cr:GSGG. Cr:GSGG was selected because of the proximity of its E2 and T24 emitting levels, and thus the possibility to explore the role of the spin selection rule in the ESA spectra and the resulting variations in polarizability by comparing low and room temperature data, which were never reported before. On the other hand, Cr:Al2O3 (ruby) was selected because it is the only crystal for which it is possible to obtain CTB absorption data from both ground and excited states, and thus for which it is possible to check the CTB model more accurately. Thanks to these more accurate and more complete data, we came to the first conclusion that the spin selection rule does not play any significant role in the variation of the polarizability with the E2–T24 energy mismatch. We also discovered that using the CTB model in the case of ruby would lead to a negative Δαp value, which is contrary to all refractive index variation (including Z-scan) measurements.
We have measured the degenerate four-wave-mixing signals from several Cr3+{\mathrm{Cr}}^{3+}Cr3+-doped oxide and fluoride laser crystals. The signals are predominantly generated by scattering of the input beams from a refractive index grating that is due to a spatially modulated excited-state population. From the measured reflectivity, the change in polarizability between the ground and excited states of Cr3+{\mathrm{Cr}}^{3+}Cr3+ is calculated. The polarizability change in the oxide hosts is accounted for by large-oscillator-strength charge-transfer transitions in the energy region of 50 000 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$. The fluoride hosts, however, did not give measurable signal levels, and this is attributed to the lack of high-oscillator-strength transitions in this energy range. The polarizability changes in the oxide hosts are observed to depend on the relative thermal population of the 4T2^{4}\mathrm{T}_{2}4T2​ and the 2E^{2}\mathrm{E}2E excited states. The polarizability change between the 4T2^{4}\mathrm{T}_{2}4T2​ excited state and the 4A2^{4}\mathrm{A}_{2}4A2​ ground state is found to be greater than that between the 2E^{2}\mathrm{E}2E and the 4A2^{4}\mathrm{A}_{2}4A2​.