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.
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.
Intrinsic optical bistability is of great interest and has potential applications in all-optical communication. In the present work, we have extended occurrences of bistable chromatic switching to a new rare-earth-doped system, namely single Tm-doped laser crystal pumped at 648 nm avalanche wavelength. Intrinsic bistability and chromatic switching of fluorescence emission are theoretically predicted, based on the rate equation theory. By using a four-order Runge-Kutta technique, chromatic switching spectra and bistable hysteresis loop are studied numerically in detail. The numerical results show that intrinsic chromatic switching of visible-infrared spectra is achievable experimentally corresponding to the transitions of Tm 1D2→3H6 (357 nm), 1G4→3H6 (469 nm), 3H4→3H6 (787 nm), 3F4→3H6 (1716 nm), as pump power increases far beyond avalanche threshold from low power level. Furthermore, we have found that the chromatic switching appears bistable in the emission spectra for pump intensity in the vicinity of avalanche threshold. The bistable hysteresis loops of luminescence are also demonstrated numerically for the dominant emission band. Intrinsic bistable chromatic switching in Tm-doped crystal is mediated by resonant excited-state absorption up-conversion.
This research is an outgrowth of earlier experiments that demonstrated bistable luminescence in heavy metal halide crystals doped with trivalent ytterbium ions. This type of instability has importance as a fundamentally new physical phenomenon with a potential application for fast all-optical switching as well as a limitation on compact solid state laser performance. In this thesis, the investigation of luminescent instabilities is extended to bistable energy transfer processes in crystals and to the observation of bistable blackbody emission in rare earth nanopowders. High resolution laser spectroscopy was used to study bistable luminescence and energy transfer in Yb,Er:CsCdBr3 crystals at cryogenic temperatures. For the first time, it was found that bistable behavior associated with Yb 3+ ions was transferred to Er 3+ through resonant energy transfer. Bistability of the resulting sensitized luminescence caused sufficiently dramatic changes in the crystal dynamics so as to change the color of emission from yellow to green. This color changing phenomenon is fully explained in the present work and is referred to as chromatic switching. Temperature is a critical variable that is known to govern luminescent instabilities in all current theories. Therefore, in a search for new systems with luminescent instabilities at high temperatures, materials with extreme thermal properties were investigated as part of this research. Yb,Er:Y 2O3 nanopowders were selected for this purpose. Nanopowders exhibit greatly reduced thermal conductivity and were verified during the course of this work to cause enhanced absorption as the result of multiple scattering. Significant spectral differences between Yb,Er:Y2O 3 nanopowders and single crystals also emerged. Measurements of erbium upconversion luminescence versus pump intensity in resonance with the ytterbium absorption transition revealed striking new optical phenomena: strong luminescent quenching, intense bistable blackbody emission, and the formation of single crystal micro-tubes directly from powder samples. A simple theoretical model was successfully developed to explain all these optical effects by a detailed balance approach to thermal transport that explicitly included erbium and ytterbium atomic level occupation probabilities, blackbody radiation and radiation trapping. Calculations of nanopowder sample luminescence accurately reproduced experimentally observed quenching, the onset of blackbody emission, bistability and details of hysteresis loops. The model also predicted that absorbed powers of less than 15 mW could melt yttria nanopowders (melting point = 2410°C), again in agreement with experiments. This work is expected to enable new approaches to laser processing of ceramics and laser machining of reflective metals, notably aluminum, using low power lasers.
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.
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.
We predict and theoretically study intrinsic optical bistability (IOB) mediated by nonlinear energy transfer processes in rare-earth-doped laser-related crystals. In particular, we investigate Tm-Ho and Tm-Yb systems, in which avalanche pumping is overimposed by energy transfer up-conversion. We predict that IOB can be experimentally observed in (Tm,Yb):BaY2F8 crystals in a wide range of experimentally achievable parameters.
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.