The Investigation Of Upconversion Processes And Electromagnetically Induced Transparency Phenomena In Rare-earth-ion-doped Crystals

In this thesis, the studies have been focused on several different research aspects of rare-earth ions doped in solid including fluorescence decay properties, upconversion mechanisms and electromagnetically induced transparency phenomena. A brief summary will be described here.1. Decay properties of rare earth ions doped in solidThe fluorescence decays of Er³⁺ ions from ⁴I_9/2, ⁴F_9/2, ⁴S_3/2, ²G_9/2 and ²P_3/2 in 8.4 and 10.5 at. % Er³⁺:YAG crystals as well as in 1 at. % Er³⁺:YAlO₃ crystal, and of Tm³⁺ ions from ³H₄, ¹G₄, ¹D₂ and ¹I₆ in Tm:ZBLAN have been studied using the time-resolved laser-induced fluorescence method (see, e.g., table 1).In this experiment, a tunable OPO laser with 6ns pulse duration wasused to excite selectively the Er3+ and Tm3+ ions and sequent fluorescence was monitored. The radiative lifetimes for corresponding levels have also been calculated using Judd-Ofelt theory. The nonradiative decay rate was estimated from the experimental and theoretical data. The fluorescence decay rates for the measured excited states were not only dramatically affected by the host Table 1. The states studied and the lifetime measured.ConcentrationLifetimes ( /j. s)of ions4, '9/2%aS3/2G9/22P3/2ErYAG8.40.064t 1.215.90.1412.510.50.0531.211.590.121.22ErYA10310.41510.1113.60.5648.7Tm:ZBLAN1JH4'D2'I67231754564material, but by the concentration of doped ions. The measured fluorescence decays are much faster than the theoretical calculations, which demonstrate that the nonradiative decay including multi-phonon and cross relaxation are dominant for these states. The results will be useful for investigatingupconversion and lasing process.2. Upconversion pheomonena of rare earth ions doped in solid(1). Ultraviolet and violet upconversion signals at 271nm, 317nm, 381nm and 407nm were observed when an erbium-doped YAG crystal (30 at.%) was pumped by an Ar+ laser (488nm). In order to understand the mechanism of the upconversion luminescence, the dependence of intensity of luminescence emitting from the 4S3/2 state and the 2P3/2 state on pump power (/) was experimentally investigated. Changes from /' down to /'/2for the 4S3/2 state and from I2 down to /' for the 2P3/2 state were observed. Two possible upconversion processes were discussed by means of the rate equations. Based on theoretical and experimental results, it appears that energy-transfer upconversion (ETU) is a dominant process for the Er3+:YAG crystal used in our experiment.(2). The ultraviolet and violet upconverted luminescence around 320nm, 410nm and 470nm in Er3+-YAG (8.4 at. %) has also been observed upon 45"3/2 excitation (?545nm) at room temperature with a tunable OPO pulsed laser excitation (Fig.l). The upconversion mechanisms are investigated bymeans of the decay profiles of upconvered luminescence, and excitation_<sub><sub><sub>AS.o_" 543.5nm excitationS545.4nm excitation300350 400Wavelength (nm]450500spectra. The relative intensities of upconverted luminescence are discussed using Judd-Ofelt theory. It appears that, depending on the excited wavelength, the upconverted luminescence is dominantly contributed either by energy-transfer upconversion or by excited-state absorption, as shown in Fig.2.(a) under 543.5nm excitation, Er3+ ions in the ground state were pumped to the 4 S3I2 state by GSA process, then through ETU process, two ions in the A S3/2 state to produce an ion in the 2 H9/2 state and the other into the ground state. Subsequent nonradiative relaxation from the 2 Hgn statepopulates the 4-P3/2 state. An additional upconversion processalso occurs at this excitationwavelength, (b). under 545.4nm ! excitation, Er3+ ions can also be -excited from the ground state to3+the 53/2 state subsequently ErJion in the 4Sl3/2 state absorbs ^another photon to 2H9/2 , $1 resulting in the ESA upconversionluminescence..475.3nmWe also observed the Fig.2. Decay profiles of upconverted, . , luminescence under 543.5 nm (top) andupconversion luminescence around r/545.4 nm (bottom) excitations. 560 nm, 475 nm, 410 nm and320nm in a YAG: 10.5%Er3+ crystal when pumped by a 647 nm laser excitation, which is resonant with the excited state 4F9/2. The upconversion mechanism was discussed. It should be pointed out that the upconvertedsignal at 320 nm is due to a three-photon process.(3). Using the same technique mentioned above, the dynamics ofvisible-to-ultraviolet upconversion in YAIO3: 1% Er^+ crystal has also been investigated. The results are similar to those obtained in Er3+:YAG.The YAG and YAIO3 crystals are excellent host materials for upconversion luminescence compared with other compounds, such as RbGd2X7 (X=C1, Br), K2LaX5 (X=C1, Br), and Cs3Lu2X9 (X=C1, Br, I), which are all very sensitive to moisture. The observation of ultraviolet and violet luminescence in YAG:Er3+ and YA103:Er3+ is benefit for the investigation of upconversion lasing in short wavelength.(4). The upconversion signal around 800nm was observed when a Tm3+:ZBLAN glass sample was irradiated by 1203nm laser light, which is resonant with the 3Hs state. The dependence of luminescence intensities versus pump power and its decay profile indicates that ESA is dominant for the upconversion process. The results could be practically applied to many fields, such as, fiber upconversion laser and optical communication.3. Electromagnetically induced transparency (EIT) pheomonena in the rare-earth-ion-doped crystalsPrevious studies on quantum interference were mainly focused on gases, such as, alkali metal vapor. We studied the effect of Er3+ concentration on electromagnetically induced transparency in Er3+:YAG. using density matrix equations of interaction between the ions and the field in two types of three-level schemes - the ladder and the A models. The numerical calculations show that, under the same intensities of the probe and the coupling fields, the absorption and the dispersion were dramatically changed with Er3+ concentration. There exists an optimal concentration to realize the EIT in Er3+:YAG. It was also found that there exists a concentration region with large dispersion and small absorption when the coherent field was applied. Compared these two models, we found that it is easier to obtain EIT in A model than in the ladder model. These results would be helpful to perform EIT for practical application in solid.