Bistable emission of a black-body radiator

Bistability is common in nonlinear dynamical systems, but there have been no reports of intrinsic instabilities in black-body emission of optically excited media. Intrinsically bistable photo-luminescence has been reported however in Yb-doped heavy metal halide crystals at cryogenic temperatures and also in room temperature experiments in rare-earth-doped laser glass and transition metal laser hosts. Since thermal conduction and reflectivity play key roles in the internal heat balance thought to influence luminescent instabilities, we investigated laser crystals prepared as nanopowders to see if their altered transport properties might lead to more pronounced instabilities at high temperatures. Yb3+ :Y2O3 is an example of a laser material currently under development for compact, high-power ceramic lasers, where multiple scattering and attendant luminescent instabilities could restrict designs or impair performance. The rapid spectral quenching of visible luminescence and the abrupt appearance of high-temperature black-body radiation in Er3+ ,Yb3+ :Y2O3 nanopowders reported in this letter indicate that fundamental scaling limits may indeed exist in ceramic and random lasers. These phenomena also enhance laser processing of ceramics and reflective metals. Rare-earth-doped powders were prepared for these experiments by flame spray pyrolysis, and consisted of unaggregated single-crystal particles with average diameters in the range of 10–200 nm. Transmission experiments with free-standing samples pressed lightly into wedges of 1 mm maximum thickness were used to determine the Yb3+ absorption coefficient in the presence of multiple scattering. Disks with measured filling fractions of 15–20% were mounted at the entrance of an integrating sphere and transmission of a laser beam was measured versus sample thickness as shown in Fig. 1, where the F7/2s0d– F5/2s28d infrared absorption resonance of Yb is evident. A direct comparison of attenuation on and off resonance, together with the data in the inset of Fig. 1, showed that the absorption length of the sample at l=905.6 nm was ,a=0.505 mm. (This value was more than an order of magnitude less than the absorption length of an equal concentration of Yb ions in crystalline Y2O3. For an impurity density of NYb=4.131020 cm−3 and a cross section of s=0.35310−20 cm2, the calculated absorption length is ,a8 =7.0 mm at zero porosity.) With low absorption and high reflectivity sR.96% d an incident power of 400 mW tuned to the Yb resonance leads to a total absorbed power of at most 14 mW in a 1 mm thick sample, even when multiple scattering is taken into account. Hence, from the outset, strong thermal interactions are not anticipated at the surface of such weakly absorbing samples with high albedo. In view of this, the difference between the emission spectra of an Er3+ ,Yb3+ :Y2O3 powder sample irradiated at two different laser powers (Fig. 2) is quite remarkable. At an input power of 250 mW, visible emission originates principally from the H11/2 multiplet, whereas at 50 mW it originates almost entirely from S3/2. Yet both emission spectra result from the same basic process of upconversion, involving infrared absorption by two Yb3+ ions followed by energy transfer to the F7/2 level of the secondary dopant ion Er 3+. In Er3+ ,Yb3+ :CsCdBr3, thermally activated upconversion can result in abrupt switching of luminescent spectra. Here, in Er3+ ,Yb3+ :Y2O3 nanopowder, we find that a remarkably sudden, thermally induced spectral change of different origin is observed. First, the F7/2 population is rapidly transferred to the S3/2 state in the Y2O3 host through resonant cross relaxation. Hence, emission normally originating from the S3/2 state at room temperature produces thermally activated H11/2 emission at higher temperatures. This occurs above 1162 K, when thermal energies exceed the S3/2– H11/2 intermanifold energy splitting of 807 cm−1 (see Fig. 3 inset), and the H11/2 state becomes significantly populated. Sample emission changes color. Second, with further increases of input power (additional laser heating), visible luminescence from the sample virtually disappears—it is strongly quenched over a wide interval of pump power. Finally, at the