Blackbody radiation is an important concept in physics and has many applications, including in astronomy, where it is used to determine the temperature and composition of stars and other celestial objects. In fact, an absolute blackbody is only a physical abstraction that does not exist in nature. Few objects have the absorptive properties of the ideal blackbody - strong absorption at all wavelengths. The optical absorption properties of most materials are intermediate between those of whitebody and blackbody, and the possibility of interconversion between whitebody and blackbody has never been studied.
In 2002, our research group first reported the wideband upconversion luminescence of TiO2:Mo excited by near-infrared light (NIR), and confirmed that the upconversion luminescence had the characteristics of photon avalanche.1 Since 2010, many research groups have reported this white light emission phenomenon,2,3 but no one has noticed the photon avalanche feature. For the past 20 years, I have been thinking that this wide-band white light emission looks a lot like blackbody radiation,4 but the spectrum deviates from Planck's law, and the physics of how it happens has been a mystery. Until we shone another weak laser on the sample, and the fog cleared!
Figure 1 Photoinduced blackbody radiation (PBR) and generated broadband optical absorption. a, Variations of the scattering intensities of 5 probe lasers (266 nm, 405 nm, 532 nm, 650 nm, 808 nm and 1560 nm) before (red) and after (blue) the occurrence of PBR phenomenon. b, In the PBR state, the sample’s relative absorption (absorption ratios) to the six probe lasers are above 90%.
When the sample suddenly emitted intense white light under 980 nm intense laser, the scattering probe lasers from the Y2O3 sample suddenly faded greatly, as shown in Fig. 1a, indicating that the broad-spectrum emission was accompanied by a sudden sharp increase of optical absorption in broad spectral range. The newly generated absorption transitions cause the switch of the sample from a quasi-whitebody into a quasi-blackbody in an avalanche fashion, as shown in Fig. 2. At the same time, the sample emits a broadband electromagnetic radiation, becoming a bright blackbody.
Figure 2 Photon avalanche and optical bistable luminescence of PBR. a, The PBR of Y2O3 powder has characteristics of photon avalanche and optical bistable luminescence. The occurrence of PBR coincides with a sudden drop in the intensity of the irradiation laser scattered by the sample. The red and black lines show the process of increasing and decreasing pump power, respectively. b, Doping 0.7 mol% Yb3+ increased the initial absorption of Y2O3 powder to 980 nm laser and reduced the threshold power of PBR occurrence, resulting in a shrunk bistable ring.
We have accurately recorded the PBR spectrum using a calibrated measurement system, and the results indicate that the PBR spectrum does not always conform to the curve described by Planck's law, as shown in Fig. 3a. When the pump power was low, the PBR spectrum deviated from the Planck's law curve, and this deviation became more apparent as the power decreased. Additionally, the temperature measurements provided unexpected results. The actual temperature measured by a Pt/Rh thermocouple was lower than the corresponding color temperature obtained by fitting the spectrum, as illustrated in Fig. 3b. The difference is more than 600 degrees Kelvin. However, the spectrum temperature of the Pt/Rh thermocouple, when heated by lasers, demonstrated a good match to the actual temperature.
Figure 3 Spectral deviation, temperature deviation, and the formation mechanism of PBR. a, PBR spectra of Y2O3 pumped by 980 nm lasers with high power densities are compared with the fitted curves of Planck’s law. b, When the PBR occurred under the irradiation of 980 nm laser with low power densities, the spectral fitting temperature of Yb2O3 powder was very different from the actual temperature measured by using a thermocouple. The red and blue spheres represent spectral temperature and actual temperature, respectively.
To account for the deviation, we propose a possible mechanism for the PBR generation and the temperature deviation: strong light irradiation creates new energy states and associated absorption transitions in the sample. Therefore, in the PBR phenomenon, there are two population mechanisms simultaneously: (1) the lighting population caused by the optical absorption transitions under intense light irradiation and (2) the thermal population which tends to satisfy the Boltzmann distribution. When the pump power is low, the thermal vibration between cations and anions is at a relative low level, the photoinduced absorption transitions produce higher population proportions in the higher energy states, the thermal equilibrium between energy states is destroyed, and the population proportion of each energy state deviates from the Boltzmann distribution, resulting to the PBR spectrum deviates from the description of Planck’s law. Photon avalanche luminescence is also a result of photoinduced PBR quantum states, which further increases the optical absorption in a wider spectral range and generates more PBR states, resulting in a positive feedback effect. The experimental results of time-resolved spectra and photoconductance supported the above judgments. Due to the existence of lighting population, the color temperature of a photoinduced blackbody is higher than the temperature measured by a thermocouple. This discrepancy in temperature measurements also makes us speculate if the temperatures of stars (such as the Sun) obtained by spectral fitting need to be corrected.
Reference
1. Wu, C., Qin, W., et al. Near-infrared-to-visible photon upconversion in Mo-doped rutile titania. Chem. Phys. Lett. 366, 205-210 (2002).
2. Wang, J. & Tanner, P.A. Upconversion for White Light Generation by a Single Compound. J. Am. Chem. Soc. 32 (2010).
3. Wu, J., Zheng, G., Liu, X. & Qiu, J. Near-infrared laser driven white light continuum generation: materials, photophysical behaviours and applications. Chem. Soc. Rev. 49, 3461-3483 (2020).
4. Qin, W. & Wang, H. The Scientific Theory of Trungscin and Its Applications, 1 edn, p. 152 ~ 154. The Science Press: Beijing, China, 2019.
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