NASA University Research Centers Technical Advances in Aeronautics, Space Sciences and Technology, Earth Systems Sciences, Global Hydrology, and Education
Anisotropic Optical-Response of Eu-doped Yttrium Orthosilicate
Huimin Liu, M. L. Santiago, Weiyi Jia, Shoudu Zhang
Eu-doped yttrium orthosilicate (Eu(3+) : Y2SiO5) had been a subject being investigated for coherent time-domain optical memory and information processing applications since its ultraslow optical dephasing was discovered several years ago. In this crystal the weakly allowed (7)F0 - (5)D0 transition of europium ions exhibits a sufficient long dephasing time and no spectral difli.usion on a time scale of several hours at low temperature, thus an information pattern or data can be stored as a population grating in the ground state hyperfine levels. On the other hand, the study on photon-echo relaxation shows that the dephasing time T2 of Eu (3+) and other rare-earth ions doped YAG, YAlO3 strongly depends on the intensity of the excitation pulses. In Eu (3+) :YAlO3, an exponential decay of photon-echo with T2 = 53 microseconds if the excitation pulses are weak (5 vJ/pulse) was observed. However, when the excitation pulses are strong (80 pJ/pulse) they observed a much shortened T2 with a highly nonexponential decay pattern. The conclusion they derived is that the intensity-dependent dephasing rate effects are quite general, and it depends on how much the excitation intensity varies. In this paper we use transient grating formation technique showing that a temporal lattice distortion may only occur along crystal c axis, caused by EU (3+) excitation. At high excitation level the produced exciton in conduction band may also couple to the dynamical lattice relaxation process, giving rise to an apparently much shortened dephasing time.
Eu-doped yttrium orthosilicate (Eu(3+) : Y2SiO5) had been a subject being investigated for coherent time-domain optical memory and information processing applications since its ultraslow optical dephasing was discovered several years ago. In this crystal the weakly allowed (7)F0 - (5)D0 transition of europium ions exhibits a sufficient long dephasing time and no spectral difli.usion on a time scale of several hours at low temperature, thus an information pattern or data can be stored as a population grating in the ground state hyperfine levels. On the other hand, the study on photon-echo relaxation shows that the dephasing time T2 of Eu (3+) and other rare-earth ions doped YAG, YAlO3 strongly depends on the intensity of the excitation pulses. In Eu (3+) :YAlO3, an exponential decay of photon-echo with T2 = 53 microseconds if the excitation pulses are weak (5 vJ/pulse) was observed. However, when the excitation pulses are strong (80 pJ/pulse) they observed a much shortened T2 with a highly nonexponential decay pattern. The conclusion they derived is that the intensity-dependent dephasing rate effects are quite general, and it depends on how much the excitation intensity varies. In this paper we use transient grating formation technique showing that a temporal lattice distortion may only occur along crystal c axis, caused by EU (3+) excitation. At high excitation level the produced exciton in conduction band may also couple to the dynamical lattice relaxation process, giving rise to an apparently much shortened dephasing time.
Linear- and nonlinear-optical properties of Eu3+:Y2SiO5 have been investigated. All the energy levels of the7F0,1,2,… ground-state manifold and the 5D0,1,2 excited-state manifold were identified, and they were found to be doubly structured because of the two inequivalent optical sites. Two absorption lines of the 7F0–5D0 transition were studied in detail by using nonlinear spectroscopic techniques, photon-echo and stimulated-photon-echo methods. It was found by the photon-echo measurement that the optical dephasing time for one site is extremely long and is excitation-intensity dependent. With the smallest possible intensity we observed the longest ever dephasing time in a solid, 822 μs, or a 387-Hz homogeneous linewidth. From the stimulated-photon-echo measurement, the linewidth was found to be constant over a wide range of time scales, implying the absence of spectral diffusion in this crystal. These quite advantageous properties of this crystal were used for the demonstration of two novel types of time-domain optical data readout: time- and frequency-domain hybrid optical memory and spectrally programmed coherent transients.
Photon-echo-relaxation measurements made on the 3H4${\mathrm{\ensuremath{-}}}^{3}$P0 transition of 0.01 at. % Pr3+:YAG (where YAG represents yttrium aluminum garnet), 3H4${\mathrm{\ensuremath{-}}}^{1}$D2 transition in 0.1 at. % Pr3+:YAlO3, and 7F0${\mathrm{\ensuremath{-}}}^{5}$D0 transition in 0.25 at. % Eu3+:YAlO3 show that the photon-echo relaxation rate increases when the intensities of the excitation pulses are increased. Although a part of the relaxation-rate increase in Pr3+:YAG may be attributed to an instantaneous spectral diffusion (ISD) in which the presence of excited neighboring Pr3+ ions change the local field and the absorption frequency of the rare-earth ions, our data deviate significantly from the ISD-model predictions. An additional intensity-dependent relaxation mechanism is required to explain the results.
We report a spectroscopic study of the 7F0–5D1 transition of europium ions in crystal yttrium orthosilicate (Eu3+:Y2SiO5) with a focus on low-temperature properties relevant to its use in time-domain optical data storage. The optical dephasing time T2 for this green transition was found to be approximately 50 μs for Eu3+ in one optical site (site 1) and 56 μs for Eu3+ in the other site (site 2), both of which exceeded their respective excited-state lifetimes (32 μs for site 1 and 42 μs for site 2). Of the two optical sites, the transition on site 2 has a slightly larger inhomogeneous linewidth (~2.7 GHz) but a significantly larger oscillator strength (~6 × 10−8), making it a good candidate for time-domain optical storage. The hyperfine structures of the transition for both sites were also measured by spectral hole burning.
Most of the experiments related to quantum information applications, involving rare-earth doped inorganic crystals, are performed on yttrium orthosilicate single crystals. The work presented here is motivated by the search of new compounds which can be used in the field of quantum computing and/or quantum storage. Relaxation times and hyperfine structure of the $^{3}H_{4}(0)\ensuremath{\rightarrow}^{1}D_{2}(0)$ transition in 1.4% Pr3+:La2(WO4)3 at 4K have been measured by photon-echo and spectral-hole-burning techniques. The hyperfine splittings of the ground 3H4(0) and the excited 1D2(0) states are $14.9\ifmmode\pm\else\textpm\fi{}0.1\phantom{\rule{0.3em}{0ex}}\mathrm{MHz}$, $24.6\ifmmode\pm\else\textpm\fi{}0.1\phantom{\rule{0.3em}{0ex}}\mathrm{MHz}$ and $5.0\ifmmode\pm\else\textpm\fi{}0.1\phantom{\rule{0.3em}{0ex}}\mathrm{MHz}$, $7.3\ifmmode\pm\else\textpm\fi{}0.1\phantom{\rule{0.3em}{0ex}}\mathrm{MHz}$, respectively. An inhomogeneous linewidth of $18.8\ifmmode\pm\else\textpm\fi{}0.1\phantom{\rule{0.3em}{0ex}}\mathrm{GHz}$ was measured. A homogeneous linewidth of $25.3\ifmmode\pm\else\textpm\fi{}2.0\phantom{\rule{0.3em}{0ex}}\mathrm{kHz}$ was obtained with or without an external magnetic field of about 14mT. The fluorescence dynamics of the 1D2 level obtained by a direct excitation in the $^{3}H_{4}\ensuremath{\rightarrow}^{1}D_{2}$ transition gives a nonexponential decay which indicates energy-transfer processes. This decay can be accurately fitted by the Inokuti-Hirayama model [J. Chem. Phys. 43, 1978 (1965)] with a radiative lifetime of $61\ifmmode\pm\else\textpm\fi{}1\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{s}$ giving a minimal homogeneous linewidth of 2.6kHz. The spectral-hole lifetime due to population redistribution within the ground hyperfine levels is $16\ifmmode\pm\else\textpm\fi{}2\phantom{\rule{0.3em}{0ex}}\mathrm{s}$. The results obtained for the La2(WO4)3 compound make this crystal an interesting host for quantum applications.
Detailed site-selective spectroscopy has been performed as a function of temperature on the ${}^{7}{F}_{0}{\ensuremath{\leftrightarrow}}^{5}{D}_{0}$ transition of Eu3+:Y2SiO5 for Eu3+ concentrations of 0.02%, 0.1%, 0.5%, and 1%. Time-domain optical dephasing, spectral hole lifetimes, anisotropic absorption coefficients, inhomogeneous linewidths, and fluorescence lifetimes for Eu3+ ions at both crystallographic sites were measured. The temperature dependence of the optical dephasing, transition energy, and linewidth of the ${}^{7}{\stackrel{\ensuremath{\rightarrow}}{{F}_{0}}}^{5}{D}_{0}$ absorption was measured and interpreted in terms of Raman scattering of phonons. Photon echo measurements of optical dephasing gave T2 values as long as 2.6 ms, approaching the limit set by the fluorescence decay time. Spectral hole lifetimes were measured for temperatures from 2 K to 18 K, with observed lifetimes varying from 1 s at 18 K to an estimated value of greater than 20 days at 2 K. Anisotropic absorption coefficients were measured, and an increase in Eu3+ concentration from 0.02% to 7% produced an increase in the inhomogeneous linewidth ${\ensuremath{\Gamma}}_{\mathrm{inh}}$ from 0.5 GHz to \ensuremath{\sim}150 GHz, indicating that Eu3+ doping induces significant strain in the crystal. New determinations of many energy levels of 7FJ multiplets have been made for J=0 to 6.
The backward stimulated echo has been considered as a novel high-speed high-density optical data storage method. But its potential application has been confined to transient dynamic memory because of relatively short lifetime of optical coherence inside low-temperature solids. Here we report the longest observed storage time of at least six hours in the 581.8 nm7F0-5D0 transition of eu 3+ -doped YAlO3 crystal at 2°K. The experiments are performed using YAG-pumped pulsed dye lasers, beamsplitters and optical delay lines to produce five DATA pulses, at 20 nsec intervals, and WRITE and READ pulses. The measured echo intensity as a function of storage time has exponential decay time of 3.8 hours. This very long storage time is due to the very small effective nuclear magnetic moments of Eu3+ ions, and is consistent with the long lifetime of spectral hole in this system. We also observed that the signal fidelity is not noticeably affected by the number of DATA pulses and the long storage time.