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The Journal of Chemical Physics

Theory and experiment of coherent wave packet dynamics in rare earth solids: Absorption spectrum vs femtosecond fringe-resolved interferogram

Qi Luo, D. C. Dai, G. Q. Wang, Valerică Ninulescu, Xiangyang Yu, L. Luo, Jianying Zhou, YiJing Yan

Published Jan 22, 2001• 8 citations
Publisher

Abstract

Coherent dynamic property of neodymium yttrium aluminum garnet (Nd:YAG) crystal at 77 K is studied via the conventional absorption, the femtosecond fringe-resolved wave packet interferometry, and the related difference-phase spectrum. The recorded interferogram exhibits beatings in subpicosecond time scale arising from the interferences among various weakly split 4f-electronic states and the coupled vibronic optical phonon sidebands. The electron–phonon coupling in Nd:YAG can be well described by multiple Brownian oscillators model involving in each individual electronic transition. The parameters for characterizing material coherence and relaxation are determined via the theoretical modelings of both the frequency and the time-domain experimental signals.

DOI
10.1063/1.1335659

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Coherent dynamic property of neodymium yttrium aluminum garnet (Nd:YAG) crystal at 77 K is studied via the conventional absorption, the femtosecond fringe-resolved wave packet interferometry, and the related difference-phase spectrum. The recorded interferogram exhibits beatings in subpicosecond time scale arising from the interferences among various weakly split 4f-electronic states and the coupled vibronic optical phonon sidebands. The electron–phonon coupling in Nd:YAG can be well described by multiple Brownian oscillators model involving in each individual electronic transition. The parameters for characterizing material coherence and relaxation are determined via the theoretical modelings of both the frequency and the time-domain experimental signals.
Yttrium Aluminum Garnet (YAG, Y_3 〖Al〗_5 O_12) crystals doped with Erbium have posed an interesting position in the field of rare earth solid state lasers as they possess the property of self-saturation, in which the upper energy level in a laser has a much shorter lifetime than that of the lower level. This is the case of the 2.94μm transition in Er:YAG. That property at first seems to make this material unfit for use as a laser gain medium, however further research into these classes of rare earth materials revealed interesting non-liner energy transfer mechanisms that allow it to be a useful mid wave infra-red (MWIR) coherent source in the continuous wave (CW) and pulsed regimes. This came to fruition due to decades of modeling and spectroscopic research investigating the non-linear energy transfer mechanisms of excited state absorption (ESA), energy transfer up-conversion (ETU), and cross relaxation (XR). These effects show up in the rise and fall times of the levels observed through fluorescence. Resulting in the non-exponential rise and fall characteristics and having a squared or even cubed relation to the population. The population evolution for each of the lasing levels is now affected by ions recycling energy and in turn cause the lifetimes of the levels to “effectively” change to a point where simple linear models do not adequately describe the system and its performance. The non-linear energy transfer dynamics of Er:YAG are modeled under high resonant pump conditions. By pumping with selective resonant pumps, the interaction dynamics of the (_ ^4)I_(11/2) and the (_ ^4)I_(13/2) levels in the Erbium ion reveal the contributions from generally ignored non-linear energy transfer mechanisms. Specifically, the multi-photon effect known as excited state absorption (ESA) is modeled by measuring and characterizing the cross section in single crystal Er:YAG samples utilizing a pump and probe technique coupled with transient fluorescence measurements. The measured ESA cross section is then included in the rate equation modeling.
The 4f electronic transition accompanied by electron-phonon interaction was studied through the Brownian oscillator model in Nd: YAG. The expression of electron-phonon coupling was derived, the excitation spectroscopies of different electron-phonon coupling constants were calculated and the results indicated the absorption peak of phonon in two sides of electronic peak is produced by electron-phonon coupling. The stronger electron-phonon coupling, the more phonon peaks. Through excitation spectroscopy simulation, the electron-phonon coupling constant and phonon frequency were obtained. From these data, the theoretical time resolved coherent spectroscopy was calculated and is good agreement with experiment result. It also indicated that the electron-phonon coupling in rare earth ion's 4f electronic transition is weak because of the smaller constant of electron-phonon coupling compared with that of semiconductor.
It is proved that both amplitude and phase in instantaneous optical processes can be detected from the number density of the excited state populations of coherent carriers when excited with femtosecond laser pulse pairs. Experimental technique of phase spectroscopy is used for investigations of semiconductor GaAs and rare earth solids. Phase spectra of GaAs, crystal Nd:YVO 4 and powdered Nd:YAG are obtained. Dephasing time of GaAs exciton is 190fs at 77K and the Nd:YVO 4 and Nd:YAG are 300fs at room temperature.
Yttrium Aluminum Garnet (YAG) crystals doped with Erbium have posed an interesting position in the field of rare earth solid state lasers as they possess the property of self-saturation, in which the upper energy level in a laser has a much shorter lifetime than that of the lower level.The non-linear energy transfer dynamics of Er:YAG are modeled under high resonant pump conditions. Specifically the multi-photon effect known as excited state absorption (ESA) is modeled by measuring the cross section in a single crystal Er:YAG sample utilizing a pump and probe technique. The measured ESA cross section is then included in the rate equation modeling that has typically been ignored such an effect when modeling the non-linear ion to ion interactions. By pumping with selective resonant pumps, the interaction dynamics of the upper and the lower laser levels in the Erbium ion reveal the contributions of the non-linear energy transfer mechanisms towards the populations of the levels respectively. This thesis work shows the added accuracy of the rate equation modeling by its inclusion of the measured ESA cross-section and its effect on the population evolution in the Er:YAG system. The model will be tested with measurements made with select Er:YAG crystals being pumped by combinations of resonant infra-red lasers with a chopper spinning at a rate on the order of magnitude of the lower lasing level lifetime.
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.