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Optics and Spectroscopy

Cascade-avalanche up-conversion in Tm3+:YLF crystals

E. Yu. Perlin, Alexandra M. Tkachuk, M.‐F. Joubert, R. Moncorgé

Published May 1, 2001• 31 citations
Publisher

Abstract

New efficient mechanisms of long-wavelength excitation of short-wavelength luminescence in a system of rare-earth impurity ions are proposed. The model studied involves a multilevel system of electronic states of a rare-earth ion interacting with the long-wavelength emission that is in resonance with one or several transitions between the excited levels. The concentration of impurity ions is assumed to be sufficiently high, so that the interionic cross-relaxation and the up-conversion play a significant role. Moreover, the model includes processes similar to photon avalanche, in which the photoinduced excitation of a single ion is converted into lower-level excitations of several ions, each of them reentering the process after photon absorption. Numerical solutions of the system of balance equations for a multilevel system are obtained with calculated and semiempirical parameters of self-quenching and up-conversion. It is shown that, in a system of Tm 3+ impurity ions, short-wavelength luminescence at λ≥0.29 μm can be efficiently excited by radiation at λ≅1.11 or 0.649 μm with a moderate intensity.

DOI
10.1134/1.1374657

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New efficient mechanisms of long-wavelength excitation of short-wavelength luminescence in a system of rare-earth impurity ions are proposed. The model studied involves a multilevel system of electronic states of a rare-earth ion interacting with the long-wavelength emission that is in resonance with one or several transitions between the excited levels. The concentration of impurity ions is assumed to be sufficiently high, so that the interionic cross-relaxation and the up-conversion play a significant role. Moreover, the model includes processes similar to photon avalanche, in which the photoinduced excitation of a single ion is converted into lower-level excitations of several ions, each of them reentering the process after photon absorption. Numerical solutions of the system of balance equations for a multilevel system are obtained with calculated and semiempirical parameters of self-quenching and up-conversion. It is shown that, in a system of Tm 3+ impurity ions, short-wavelength luminescence at λ≥0.29 μm can be efficiently excited by radiation at λ≅1.11 or 0.649 μm with a moderate intensity.
Avalanche upconversion dynamics and associated nonlinear optical response was also reported for the first time in Tm:YALO crystals at room temperature. The avalanche mechanism involves a cooperative down conversion (cross relaxation) step and is a promising method of pumping rare earth lasers at entirely new (diode compatible) wavelengths, corresponding to excited state absorptions rather than ground state absorptions. Quantum theory describing the threshold for photo-darkening and the role of energy migration in avalanches was developed, and semi quantitative agreement obtained with preliminary measurements of nonlinear susceptibilities. Cooperative dynamics in general, and avalanche dynamics in particular, were shown to furnish a promising new mechanism not only for solid state lasers but for nonlinear optical interactions as well, opening the door to practical signal processing and switching applications exploiting the low intensity thresholds of avalanche processes.
This thesis reports original observations and theory of strong, induced excited state absorption and emission, known as avalanche upconversion, in thulium doped crystals. The nonlinear dynamics responsible for this phenomenon and its characteristic threshold are studied in great detail, and shown to consist of two distinct cooperative processes, namely near-neighbor cross relaxation(s) and excitation migration among more distant ion pairs. The temperature dependence of avalanche upconversion and cross relaxation in these materials is explained for the first time. Also, phase conjugation with threshold behavior and potential motion sensitivity for novelty filtering is demonstrated utilizing an avalanche nonlinearity. Complex, transverse ring structures observed in Gaussian beam propagation through avalanche media are explained on the basis of numerical simulations of avalanche-induced absorption and dispersion. Evidence is presented of coherent delocalization in Tm-doped solids, in the form of unexpected oscillations in the transient response to incident light pulses. Two theoretical approaches for describing avalanche behavior were compared. A rate equation analysis suitable for weak-coupled impurity ions was used to predict basic avalanche features, including increasing absorption and upconversion emission above a threshold, temporal delay of absorption, and temperature dependence of avalanche threshold. A density matrix approach suitable for arbitrary coupling reproduced these features but also predicted oscillations in excited state populations in the presence of significant interion coherence. This theory incorporated both near-neighbor cross relaxation which gives rise to induced absorption, and excitation migration among distant pair ions necessary for immensely magnifying the effect, in a single formalism. Studies of the concentration and temperature dependence of fluorescence decays of Tm ions from $\sp1$G$\sb4$ and $\sp3$H$\sb4$ states provided firm evidence of pair-wise cross relaxations of ions in both these states. Transient four-wave mixing experiments directly revealed energy migration among $\sp3$F$\sb4$ state Tm ions at a rate rapid enough to support the content ion that migration plays an important role in avalanche dynamics. Additional experiments yielded a temperature dependence of avalanche threshold in good agreement with theory, as well as avalanche phase conjugation and transverse propagation effects due to avalanche dynamics in accord with predictions.
Here we report and discuss the avalanche-like up-conversion behavior in absence of the avalanche. We experimentally observed significant changes in the slope of the curve for the intensity dependence of up-conversion luminescence of erbium ions in the green band (520-560 nm) on the pump intensity of the diode laser. Such changes are typical for the photon avalanche. However, the concentration of erbium ions is insufficient for an efficient exchange of energy between them, and the excitation of a photon avalanche is not possible. Using a simple three-level approximation of the up-conversion process model, we have shown that the observed avalanche-like luminescence process can also occur in the absence of a photon avalanche due to the nonlinear relation between the efficiency of two pumping channels of erbium ions caused by the intensity dependence of the pump spectrum.
Intrinsic optical bistability and dynamic hysteresis phenomenon, induced by nonlinear excitation and energy level coupling, are predicted theoretically in single Tm-doped laser crystal pumped at 648nm avalanche wavelength. Taking into account the dominant energy transfer processes including excited state absorption, cross relaxation, energy transfer up-conversion and decay of the metastable level Tm3+ 3F4, the analytical formula of avalanche threshold condition are deduced in the steady-state approximation. Based on the theory of microscopic coupled rate equations, intrinsic optical bistability and influence of system parameters on hysteresis loop are studied numerically in detail by using a four-order Runge-Kutta technique. The numerical results predict that intrinsic optical bistability of near 2 micrometer fluorescence emission relevant to Tm 3F4→3H6 transition is achievable experimentally in single Tm-doped laser crystal. For realization of low threshold optical bistability and notable bistable hysteresis loop, it is beneficial to properly increase Tm3+-doped concentration and suppress energy transfer up-conversion. Furthermore, the interesting results obtained numerically show that laser-induced thermal effect is not the indispensable factor for occurrences of intrinsic optical bistability in rear-earth-doped crystal. Cooperation of nonliear excitation and energy level coupling can essentially lead to intrinsic optical bistability in rear-earth-doped system.
Abstract Controlling the up-conversion luminescence of rare-earth ions in real-time, in a dynamical and reversible manner, is very important for their application in laser sources, fiber-optic communications, light-emitting diodes, color displays and biological systems. In previous studies, the up-conversion luminescence control mainly focused on the weak femtosecond laser field. Here, we further extend this control behavior from weak to intermediate femtosecond laser fields. In this work, we experimentally and theoretically demonstrate that the up-conversion luminescence in Dy 3+ ion doped glass can be artificially controlled by a π phase step modulation, but the up-conversion luminescence control behavior will be affected by the femtosecond laser intensity, and the up-conversion luminescence is suppressed by lower laser intensity while enhanced by higher laser intensity. We establish a new theoretical model (i.e. the fourth-order perturbation theory) to explain the physical control mechanism by considering the two- and four-photon absorption processes, and the theoretical results show that the relative weight of four-photon absorption in the whole excitation process will increase with the increase in laser intensity, and the interference between two- and four-photon absorptions results in up-conversion luminescence control modulation under different laser intensities. These theoretical and experimental works can provide a new method to control and understand up-conversion luminescence in rare-earth ions, and also may open a new opportunity to the related application areas of rare-earth ions.