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Intrinsic bistability in cooperative upconversion

Markus P. Hehlen, H. U. Güdel, Jagdish Rai, Suranjana Rai, Stephen Colby Rand

Published May 8, 1994• 0 citations
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Abstract

Optical bistability is a well-known phenomenon in optical systems of many kinds. However observations of intrinsic bistability are rare and have not been reported previously in rare earth systems for nonlinear dynamics due to inter-atom coupling which arises at high impurity concentrations. Typically, nonlinear response by this mechanism is too weak to give rise to sizeable nonlinearities. Here we present observations of cooperative upconversion emission from crystals of Yb3+ Cs3Y2Br4 at low temperatures however, in which hysteresis without mirrors provides evidence of strong cooperative nonlinear response.

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Physical Review Letters•Apr 12, 1999•81 citations
Optical bistability is a well-known phenomenon in optical systems of many kinds. However observations of intrinsic bistability are rare and have not been reported previously in rare earth systems for nonlinear dynamics due to inter-atom coupling which arises at high impurity concentrations. Typically, nonlinear response by this mechanism is too weak to give rise to sizeable nonlinearities. Here we present observations of cooperative upconversion emission from crystals of Yb3+ Cs3Y2Br4 at low temperatures however, in which hysteresis without mirrors provides evidence of strong cooperative nonlinear response.
In single crystals of the dimer compound Cs3Y2Br9:10% Yb3+ below 31 K, both visible (VIS) and near-infrared (NIR) luminescence intensities were found to exhibit hysteresis as a function of incident NIR intensity and temperature. The optical bistability is intrinsic to Cs3Y2Br9:10% Yb3+ and not a result of an external feedback. Lowering the temperature to 11 K strongly enhances the all-optical switching behavior. The switching on VIS cooperative upconversion and NIR luminescence transitions occurs simultaneously and with opposite polarity reflecting the competition of both emission processes. On/Off switching ratios of up to 4.8 and 1.7 were observed for VIS and NIR luminescence intensities. Using NIR luminescence spectroscopy, differences in the internal sample temperature of up to 7 K were found between the upper and lower branches of the hystereses. A two-level density-matrix model is developed which includes ground- and excited-state interactions and shows that the intrinsic bistability due to a local field different from the external field is strongly amplified by the nonlinear cooperative upconversion process. Alternatively, a rate-equation model is presented which takes the multilevel nature of the ions into account but is more phenomenological in nature. Formally, the two models are shown to be equivalent, and they qualitatively explain all major experimental observations. It is found both theoretically and experimentally that increasing the coupling within Yb3+ dimers and/or decreasing energy migration through the Yb3+ lattice enhances switching and renders it easier to observe intrinsic optical bistability.
This paper presents theoretical investigations on the dynamics of intrinsic optical bistability (IOB) in Tm3+/Yb3+ codoped systems with an upconversion avalanche mechanism. Numerical simulations based on the rate equation model demonstrate distinct IOB hysteresis and critical slowing dynamics around the avalanche thresholds. Moreover, the unusual IOB characteristics are well interpreted by developing a simplified analytic model which gives the formulations of two critical thresholds and the necessary condition for IOB to occur. The simplified model shows that such IOB phenomena may occur only if the total quantum yields of the dipole–dipole energy transfer processes are much higher than that of the inherent decay losses involved in an upconversion avalanche. The analytic model developed in the paper may be useful for the prediction of new rare-earth-doped materials giving rise to IOB.
This thesis reports experiments and theory on intrinsic mirrorless bistability in the luminescence of ytterbium-doped crystals and glasses. The main results are the observation of intrinsically bistable luminescence at room temperature and chromatic switching at low temperature, reported here for the first time. The nonlinear dynamics responsible for these optical phenomena are shown to arise from ion-ion interactions controlled by the application of resonant electromagnetic fields. On the theoretical side, the classical Lorentz local field correction is shown to be the lowest order term of radiation-driven correlation contributions to local polarization which arise from inter-ion coherences and cooperative relaxation and frequency shifts beyond the dynamic shift are predicted. The nonlinear Maxwell-Bloch equations are extended to describe energy transfer between different species of ions predicting bistability in excited state populations which then emit bistable luminescence as a consequence, and an intensity dependent transfer rate is predicted. Diffusive excitation migration in space is also explored as a possible mechanism for generating multiple instabilities, but is not predicted to give rise to hysteresis loops by itself. On the experimental side, original observations of intrinsic optical switching and hysteresis are reported in the luminescence of Yb:CsCdBr 3 and Yb, Tm:glass. Evidence of cross-coupling between different impurities is also presented, together with spectroscopic details of intrinsic chromatic switching between different luminescent transitions of the acceptor species in Yb, Er:CsCdBr3 excited at the infrared Yb resonance wavelength. Conclusive spectroscopic evidence that excludes thermal effects as responsible for low temperature intrinsic switching is also presented. Room temperature intrinsic optical switching and hysteresis are reported for the first time in bulk Yb,Tm:glass samples. Switching, hysteresis, and clamping of laser output from waveguide channels fabricated in the glass material are also reported and indicate fundamental design limitations for compact optical amplifiers and oscillators.
Intrinsic bistability in active optical compounds doped with rare-earth ions has attracted widespread interest because of its relevance for optical signal switching and routing. In this paper, room temperature strong bistability effect of Ybsensitized Tm upconversion emission is reported in zirconium dioxide nanocrystal host matrix under infrared diode laser irradiation. The distinct optical bistability behavior is simultaneously observed in three emission bands of Tm ion around 480, 650/680 and 800 nm which are assigned to the radiative transitions of 1G4→3H6, 1G4→3F4/3F2,3→3H6, and 3H4→3H6, respectively. The bistability loops exhibit uniformly clockwise cycle characterized by a relatively lower trace of emission intensity at decreasing pump power than the trace at increasing pump power. Multiphoton upconversion processes are analyzed to determine the Yb→Tm energy transfer mechanism. Such strong bistability effect is associated with thermally enhanced nonradiative decay and reduced luminescence quantum yield due to the strong pump-induced local thermal loading.
We describe several new aspects of light–matter interactions for solids in which interatomic coupling of impurity atoms plays a dominant role in population dynamics. We explore the implications of spatial coherence in such multiatom interactions by introducing a density-matrix theory of cooperative upconversion, focusing on pair systems for which analytic results can be obtained. We predict population pulsations in coherent cooperative upconversion, enhanced quantum efficiency, enhanced energy transfer, and pair-mediated instabilities, not only in cooperative upconversion media without external cavities but in upconversion lasers and conventional lasers in highly doped solids as well. These predictions are compared with rate equation solutions and observations in lasers with inversions sustained by cooperative processes, particularly the 2.8-μm Er laser. Rate equations fail to predict the observed steady-state instabilities of this laser, which are well reproduced by density-matrix theory, furnishing evidence of weak coherent delocalizations in a rare-earth system.
We report the first observation of intrinsic optical bistability in CsCdBr3:1{\mathrm{CsCdBr}}_{3}:1%CsCdBr3​:1 Yb3+{\mathrm{Yb}}^{3+}Yb3+ and show, using site-selective spectroscopy, that only the asymmetric, strongly coupled Yb3+{\mathrm{Yb}}^{3+}Yb3+ ion-pair minority site gives rise to hysteresis of near-infrared and cooperative up-conversion luminescence as a function of incident laser intensity below 15 K.