This paper presents the unusual bistable thermal radiation in rare earth doped solids under intense laser excitation. A theoretical model involving the thermal radiation balance is established to discuss the intrinsic bistability of laser-induced thermal radiation. The origin of thermal radiation bistability is attributed to the nonlinear response of radiation quantum efficiency to local temperature.
This paper presents the unusual bistable thermal radiation in rare earth doped solids under intense laser excitation. A theoretical model involving the thermal radiation balance is established to discuss the intrinsic bistability of laser-induced thermal radiation. The origin of thermal radiation bistability is attributed to the nonlinear response of radiation quantum efficiency to local temperature.
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 heating is generally regarded as a secondary consequence of optical excitation in rare-earth (RE3+)-doped materials. Here, we demonstrate that intrinsic thermo-optical feedback can drive the transient dynamics of optically excited RE3+ systems. To isolate this mechanism, we develop a minimal coupled population-temperature model that describes the interplay among optical excitation, temperature-dependent non-radiative relaxation, heat generation, and thermal dissipation. The model predicts the emergence of a threshold excitation power associated with intrinsic thermo-optical feedback, giving rise to pronounced slowing down of the transient dynamics, non-monotonic evolution of the characteristic transient time, population overshoot, and accelerated relaxation above threshold. These phenomena originate from the progressive establishment of a positive feedback loop linking temperature, non-radiative relaxation, quantum efficiency, and heat generation. Our results demonstrate that temperature should no longer be regarded merely as a passive external parameter, but rather as an active dynamical variable capable of continuously reshaping the relaxation pathways of the system. More broadly, this work establishes intrinsic thermo-optical feedback as a general framework for understanding nonlinear transient phenomena in optically excited RE3+ materials and provides a complementary perspective for interpreting threshold-like optical responses beyond conventional photon-avalanche mechanisms.
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.
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.
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.
Materials displaying intrinsic optical bistability (IOB), i.e., allowing the coexistence of two stable steady-state excitation rates for a single given excitation power, are of interest for potential technological applications related to optical data switching and manipulation. The properties of the unusual IOB previously observed in Yb 3+ -doped Cs 3 Lu 2 Br 9 and CsCdBr 3 host materials are studied here using absorption and luminescence spectroscopies. The IOB phenomenon is concluded to derive ultimately from laser heating effects. When combined with a strongly increasing and nonlinear dependence of the material's absorbance on internal temperature, laser heating leads to a positive-feedback absorption amplification process showing a hysteresis in both power- and temperature-sweep experiments. A simple model describing this effect in terms of rates of sample heating and cooling in the irradiated volume reproduces the power, temperature, concentration, and excitation-energy dependence of the Yb 3+ IOB using only the experimental absorption data as input. The temperature dependence of the absorption cross section is correlated with thermal changes in the monomeric YbBr 6 3- geometry, which becomes more asymmetric as the temperature is elevated. The IOB observed in Yb 3+ -doped Cs 3 Lu 2 Br 9 and CsCdBr 3 host lattices is therefore a property of the monomeric Yb 3+ ion in these materials, and not a dimer property as was previously believed. These results also emphasize the more general conclusion that laser heating may contribute significantly to the shape or slope of an excitation power dependence curve, and may even be the dominant aspect of that curve when absorption cross sections are strongly dependent on temperature.