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Physical review. B, Condensed matter

Persistent UV-induced optical dephasing in Pr-doped yttrium aluminum garnet

Kiwan Jang, Richard S. Meltzer, Joseph Ganem

Published Feb 1, 1994• 0 citations
Publisher

Abstract

Intensity-dependent optical dephasing is shown to arise from nonresonant excitation of 1% ${\mathrm{Pr}}^{3+}$-doped yttrium aluminum garnet (YAG) with UV radiation between 325--355 nm. UV irradiation of the sample with intensities greater than 30 MW/${\mathrm{cm}}^{2}$ produces persistent changes in the optical dephasing. Two-pulse photon echo decays on the $^{3}$${\mathit{H}}_{4}$${\ensuremath{\rightarrow}}^{1}$${\mathit{D}}_{2}$ transition of ${\mathrm{Pr}}^{3+}$ were measured at 1.5 K before and after exposure to UV radiation. Changes persisting for at least several hours are observed in the echo intensity, echo decay rate, and temporal form of the echo decay. Changes persist if the sample is kept below 10 K, but warming to 77 K results in the echo decays resuming their initial intensity and duration. No change is detected in the optical absorption spectrum of the ${\mathrm{Pr}}^{3+}$ ions after UV exposure, indicating that most of the ions remain in that charge state. Although the mechanism giving rise to the increased optical dephasing is not completely understood, the additional dynamics may arise from the UV radiation freeing electrons that become trapped in lattice defects. The presence of the dephasing at low temperatures indicates the availability of low-energy excitations of the electronic system.

DOI
10.1103/physrevb.49.3009

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Physical review. B, Condensed matter•Oct 1, 1996•2 citations
Intensity-dependent optical dephasing is shown to arise from nonresonant excitation of 1% Pr3+{\mathrm{Pr}}^{3+}Pr3+-doped yttrium aluminum garnet (YAG) with UV radiation between 325--355 nm. UV irradiation of the sample with intensities greater than 30 MW/cm2{\mathrm{cm}}^{2}cm2 produces persistent changes in the optical dephasing. Two-pulse photon echo decays on the ^{3}$${\mathit{H}}_{4}$${\ensuremath{\rightarrow}}^{1}$${\mathit{D}}_{2} transition of Pr3+{\mathrm{Pr}}^{3+}Pr3+ were measured at 1.5 K before and after exposure to UV radiation. Changes persisting for at least several hours are observed in the echo intensity, echo decay rate, and temporal form of the echo decay. Changes persist if the sample is kept below 10 K, but warming to 77 K results in the echo decays resuming their initial intensity and duration. No change is detected in the optical absorption spectrum of the Pr3+{\mathrm{Pr}}^{3+}Pr3+ ions after UV exposure, indicating that most of the ions remain in that charge state. Although the mechanism giving rise to the increased optical dephasing is not completely understood, the additional dynamics may arise from the UV radiation freeing electrons that become trapped in lattice defects. The presence of the dephasing at low temperatures indicates the availability of low-energy excitations of the electronic system.
Photon-echo-relaxation measurements made on the 3H4^{3}\mathrm{H}_{4}3H4​${\mathrm{\ensuremath{-}}}^{3}$P0{\mathrm{P}}_{0}P0​ transition of 0.01 at. % Pr3+{\mathrm{Pr}}^{3+}Pr3+:YAG (where YAG represents yttrium aluminum garnet), 3H4^{3}\mathrm{H}_{4}3H4​${\mathrm{\ensuremath{-}}}^{1}$D2{\mathrm{D}}_{2}D2​ transition in 0.1 at. % Pr3+{\mathrm{Pr}}^{3+}Pr3+:YAlO3{\mathrm{YAlO}}_{3}YAlO3​, and 7F0^{7}\mathrm{F}_{0}7F0​${\mathrm{\ensuremath{-}}}^{5}$D0{\mathrm{D}}_{0}D0​ transition in 0.25 at. % Eu3+{\mathrm{Eu}}^{3+}Eu3+:YAlO3{\mathrm{YAlO}}_{3}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+{\mathrm{Pr}}^{3+}Pr3+:YAG may be attributed to an instantaneous spectral diffusion (ISD) in which the presence of excited neighboring Pr3+{\mathrm{Pr}}^{3+}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.
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.
Photon-echo techniques were employed to measure the homogeneous dephasing rate of the 592.5-nm, 3^{3}3${\mathit{H}}_{4\mathrm{\ensuremath{-}}}^{1}$D2{\mathit{D}}_{2}D2​, transition of Pr3+{\mathrm{Pr}}^{3+}Pr3+ as experimental parameters affecting the number of Pr3+{\mathrm{Pr}}^{3+}Pr3+ ions optically excited were varied. These parameters include the dopant concentration and the intensity, carrier frequency, and bandwidth of the excitation pulses. Dephasing rates are found to contain a background contribution whose magnitude is dependent upon the specific crystal sample studied and a second contribution whose magnitude increases with the number of Pr3+{\mathrm{Pr}}^{3+}Pr3+ ions changing state in the course of the echo excitation sequence. The second contribution to the dephasing rate is attributed to the presence of optical instantaneous diffusion, i.e., the redistribution of ionic transition frequencies consequent to optical excitation, in the samples.
Using a laser diode modulated at 250 and 500 MHz, dependence of enhanced absorption on both sample length and temperature was investigated in an erbium doped yttrium aluminum garnet crystal. The transmitted laser intensity decreased with a decay time of 300 ps for sample lengths greater than 3.0 mm and temperatures higher than 50 K when the incident laser intensity increased. In addition, an emission peak was observed near 835 nm corresponding to the transition from the S3/24 level to the I13/24 in Er3+. It was suggested that the enhanced absorption was caused by stimulated emission in three excited states.
Yttrium aluminum garnet (YAG): Pr3+ is synthesized by codeposition me thod. Vacuum ultraviolet (VUV) excitation and emission spectra of YAG: Pr3+ at different temperatures are measured using synchrotron radiation as an excitation source. We analyze the transition process after excitation to 4f5d en ergy band and estimate the Huang-Rhys factor S and phonon energy ηω of YAG: Pr 3+ by multi-phonon relaxation. Emission of self-trapped exciton (STE) and 3P0→3H4,5,6 emission transferr ed from the STE energy appear when YAG: Pr3+is excited by 170nm, whic h is a different transition process from those excited by 240nm and 289nm.
Using two-pulse photon echo excitation a strong resonance in the measured homogeneous dephasing time as a function of magnetic field is observed in the 3^{3}3H4{\mathit{H}}_{4}H4​${\mathrm{\ensuremath{-}}}^{1}$D2{\mathit{D}}_{2}D2​ transition of Pr3+{\mathrm{Pr}}^{3+}Pr3+:YAlO3{\mathrm{YAlO}}_{3}YAlO3​. The resonance could only be observed at low excitation energies since it otherwise was masked by excitation intensity dependent extra dephasing processes in the crystal. Thus intensity dependent dephasing processes may not only affect the apparent dephasing time of a system but may also mask other more subtle interactions in the materials studied. It is reasonable to assume that the type of resonance observed could be caused by Zeeman induced level crossings in the active ion or in the host crystal ions or, possibly, that it could be due to coincidences between Zeeman level transition frequencies. However, based on current data of the Pr:YAlO3{\mathrm{YAlO}}_{3}YAlO3​ system the observed resonance could not be unambiguously tied to such effects. \textcopyright{} 1996 The American Physical Society.