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Journal of Experimental and Theoretical Physics

Optical echo spectroscopy and phase relaxation of Nd3+ ions in CaF2 crystals

Tasoltan T. Basiev, Alexander Ya. Karasik, Vladimir Fedorov, Keith W. Ver Steeg

Published Jan 1, 1998• 17 citations
Publisher

Abstract

Accumulated photon echoes have been used to investigate the mechanisms of optical dephasing in CaF2 crystals activated by Nd3+ ions. Tunable picosecond laser radiation, which permits the selective excitation of various Nd3+ optical centers in the 4 I 9/2→4 G 5/2, 2 G 7/2 transition, is used. The optical phase relaxation times measured at temperatures from 9 to 50 K permit determination of the homogeneous widths of the transitions between the low-lying 4 I 9/2 Stark level and three excited 4 G 5/2, 2 G 7/2 levels, and calculation of the constants of the inter-Stark relaxation transitions in the ground and excited multiplets for the rhombic N and M Nd3+ centers in CaF2 crystals. An analysis of the temperature dependence of the homogeneous linewidth of the transitions between low-lying Stark levels of the ground and excited states shows that the mechanism of optical dephasing in the crystals investigated is described well by direct relaxation processes with resonant inter-Stark absorption of one phonon in the ground and excited states. At T=9 K, the homogeneous linewidth Γh in CaF2 crystals is almost an order of magnitude smaller than Γh in disordered CaF2-YF3 crystals. This difference can be attributed to the significantly greater spectral phonon density of states in disordered crystals.

DOI
10.1134/1.558480

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Optics Letters•Feb 1, 1983•66 citations
Accumulated photon echoes have been used to investigate the mechanisms of optical dephasing in CaF2 crystals activated by Nd3+ ions. Tunable picosecond laser radiation, which permits the selective excitation of various Nd3+ optical centers in the 4 I 9/2→4 G 5/2, 2 G 7/2 transition, is used. The optical phase relaxation times measured at temperatures from 9 to 50 K permit determination of the homogeneous widths of the transitions between the low-lying 4 I 9/2 Stark level and three excited 4 G 5/2, 2 G 7/2 levels, and calculation of the constants of the inter-Stark relaxation transitions in the ground and excited multiplets for the rhombic N and M Nd3+ centers in CaF2 crystals. An analysis of the temperature dependence of the homogeneous linewidth of the transitions between low-lying Stark levels of the ground and excited states shows that the mechanism of optical dephasing in the crystals investigated is described well by direct relaxation processes with resonant inter-Stark absorption of one phonon in the ground and excited states. At T=9 K, the homogeneous linewidth Γh in CaF2 crystals is almost an order of magnitude smaller than Γh in disordered CaF2-YF3 crystals. This difference can be attributed to the significantly greater spectral phonon density of states in disordered crystals.
Numerous experimental investigations of homogeneous line broadening (Γh) of RE3+ ions in crystals and glasses in frequency and time domain [1] have shown that Γh for glasses exceed those for crystals by more than one order of magnitude at low temperatures (T) for the same ions and transitions. Discrepancies between theory and experimental results make it difficult to determine an influence of host disordering on the Γh in a wide temperature range. In this work we measured and compared Γh, values for ordered CaF2 and disordered CaF2-YF3 crystals doped with Nd3+ ions.
Accumulated photon echoes were used to measure the dephasing time (T2{\mathit{T}}_{2}T2​) of the 4^{4}4I9/2{\mathit{I}}_{9/2}I9/2​${\ensuremath{\rightarrow}}^{4}$G5/2{\mathit{G}}_{5/2}G5/2​,2{,}^{2},2G7/2{\mathit{G}}_{7/2}G7/2​ transition of Nd3+{\mathrm{Nd}}^{3+}Nd3+ in highly disordered CaF2{\mathrm{CaF}}_{2}CaF2​-YF3{\mathrm{YF}}_{3}YF3​ crystals. T2{\mathit{T}}_{2}T2​ decreased from subnanosecond values for excitation of low-lying Stark levels to subpicosecond values for excitation of high-lying levels. From 9--100 K the measured temperature dependence of the homogeneous linewidth is described by a resonant single-phonon absorption process.
We have observed extremely fast nonradiative relaxation (picosecond to femtosecond) between closely spaced crystal field states derived from the 4G5/2,2G7/2 Stark manifold of Nd3+ in glassy media. The relaxation is significantly faster than that predicted by an indiscriminate application of the energy-gap law, and it is highly wavelength-dependent. Femtosecond accumulated-photon-echo techniques were used at 13 K, revealing single and nonexponential echo decays. Relaxation was found to involve single phonon processes. Generally, the dephasing rate increased as higher-lying crystal field states were excited, reflecting the larger number of available terminal levels for the relaxation and the phonon density of states. However, a detailed plot of the dephasing rate versus wavelength revealed considerable unexpected structure. As higher-lying states were excited, the relaxation rate did not increase in the expected monotonic fashion. Surprisingly, the relaxation rate decreased in various spectral regions as the excitation energy was increased. By using T2-selective dynamic-excitation spectroscopy, we assigned these regions to the spectral locations of the inhomogeneously broadened crystal field states in the Stark manifold. Site-dependent electron-phonon interaction and a strong correlation between the inhomogeneous broadening of the crystal field states are deduced from the data.
Measurements of the homogeneous width of the 4^{4}4 G5/2{\mathrm{G}}_{5\mathrm{/}2}G5/2​ , 2^{2}2 G7/2{\mathrm{G}}_{7\mathrm{/}2}G7/2​ ${\mathrm{\ensuremath{-}}}^{4}$ I9/2{\mathrm{I}}_{9\mathrm{/}2}I9/2​ transition of Nd3+{\mathrm{Nd}}^{3+}Nd3+ in a silicate glass fiber at low temperatures by the technique of accumulated photon echoes show that the dephasing processes are one-phonon processes between the Stark levels of Nd3+{\mathrm{Nd}}^{3+}Nd3+ . These dephasing processes differ from the commonly observed processes of rare-earth ion-doped glasses: the local-configurational-change-induced dephasing process and the Raman process with low-frequency modes. The densely distributed Stark levels of the 4^{4}4 G5/2{\mathrm{G}}_{5\mathrm{/}2}G5/2​ and 2^{2}2 G7/2{\mathrm{G}}_{7\mathrm{/}2}G7/2​ states are responsible for one-phonon processes. The transition-wavelength dependences of the homogeneous widths ${\mathrm{\ensuremath{\Gamma}}}_{\mathrm{h}}$ (4.2 K) and ${\mathrm{\ensuremath{\Gamma}}}_{\mathrm{h}}$ (T\ensuremath{\rightarrow}0) are explained by using the one-phonon process model.