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Annual Meeting Optical Society of America

Ultrafast nonradiative relaxation of Nd3+

Howard W. H. Lee, R. S. Hughes

Published Jan 1, 1990• 0 citations
Publisher

Abstract

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.

DOI
10.1364/oam.1990.mbb5

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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.
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.
Accumulated photon echoes (APE) were used to measure the dephasing rates of Nd3+ in various inorganic glasses as a function of excitation wavelength, sample temperature, and laser power. Although dephasing processes of optical transitions in solids are of considerable interest, they are poorly understood, largely due to the lack of experimental data on femtosecond and picosecond time scales. The APE technique using sub-picosecond laser sources has made it possible to measure in the time domain dephasing rates of inhomogeneously broadened transitions in amorphous systems.1,2
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.
This work presents new results of experimental studies of exciton relaxation in GaAs/AlGaAs multiple quantum well materials, using transient nonlinear laser spectroscopy techniques. In the low excitation regime, effective optical Bloch equations similar to the density matrix formalism of a two-level system describe the response. Solutions of these equations appropriate for three-pulse scattering studies are presented, along with a discussion of new experimental techniques used in this work. Non-ideal conditions present in the molecular beam epitaxy growth process introduce monolayer fluctuations in the quantum well thickness, leading to inhomogeneous broadening and exciton localization at sites of minimum energy. Localized excitons are separated in energy from extended, quasi-delocalized states by the mobility edge. Studies of localized exciton relaxation indicate that these excitons relax on a much slower timescale than delocalized excitons. The temperature dependence of the localized exciton relaxation rate shows that above 15K, the excitons are thermally activated to extended states above the mobility edge. Below 15K the excitons relax by phonon assisted migration between localization sites. For temperatures above 5K, and for exciton energies greater than 1 meV below line center, an extra dephasing mechanism characterizes the exciton relaxation. The inhomogeneous broadening of the exciton leads to the emission of stimulated photon echoes in the experimental geometry used in this work. The time-resolved emission demonstrates the stimulated photon echo nature at low excitation densities. At higher excitation densities, the decay dynamics is more complex. The time-resolved emission shows the presence of a stimulated photon echo and a free polarization decay. This unusual result suggests that both a homogeneous and an inhomogeneous exciton distribution comprise the exciton resonance. Exciton relaxation at low excitation densities represents the interaction of the exciton with the lattice. At higher excitation densities, exciton-exciton and exciton-free carrier interactions modify the relaxation rates. The dependence of the exciton relaxation rate on excitation density shows that many-body effects play an important role in the exciton physics. Free carrier interactions are more efficient at increasing the exciton relaxation rate; both types of interactions are more important in quantum well systems than in bulk materials.
In the present work, ultrafast photon echo spectroscopy has been applied to investigate dephasing dynamics and excited state electronic structure of impurity N-V centers in diamond and Tm$\sp{3+}$-Tm$\sp{3+}$ centers. In the N-V center, quantum beat measurements revealed modulation frequencies of 0.63 ±\pm± 0.02 GHz, 1.650 ±\pm± 0.003 GHz, and 2.298 ±\pm± 0.006 GHz which furnished excited state spin-spin splittings directly for the first time. Wavelength dependent studies of dephasing times showed conclusively the existence of a larger splitting of 46 ±\pm± 2 cm$\sp{-1}$ in the excited E state. These results were found to be consistent with temperature dependent studies which reveal dephasing, near liquid-He temperatures, is dominated by a one-phonon direct dephasing process in the upper state with an activation energy of 68 ±\pm± 16 cm$\sp{-1}$ has a two-phonon Orbach contribution to dephasing in the lower state with an activation energy of 39 ±\pm± 5 cm$\sp{-1}.$ Experiments with circularly polarized light also revealed an interesting polarization selection rule originating from Zeeman coherence in the doublet E spin state of the $\sp3A$ ground state. A complete multi-electron model is presented which makes use of a previously undetected Jahn-Teller effect to explain all the observed excited state structure. An extension of these ultrafast techniques were applied to Tm$\sp{3+}$-Tm$\sp{3+}$ centers in YAlO$\sb3,$ permitting the first preliminary investigations of coherent dynamics on highly cooperative transitions. Dephasing times on the $\sp3F\sb4(\Gamma\sp2)$ →\to→ $\sp1G\sb4(\Gamma\sp1)$ excited state transitions were found to increase from 35 ±\pm± 2 ps to 61 ±\pm± 6 ps with decreasing intensity of a resonant auxiliary cw laser which populates this transition in a controllable fashion. Temperature dependence studies revealed dephasing on this cooperative transition was dominated by a direct process with an activation energy of 157 ±\pm± 16 cm$\sp{-1}$ at liquid-He temperatures. These results were consistent with expected dephasing rate dependence for interacting Tm$\sp{3+}$-Tm$\sp{3+}$ centers.