Photoinduced photon avalanche turns white objects into bright blackbodies

The study investigates whether materials that appear white (with finite visible reflectance) can be transformed into strong broadband absorbers akin to blackbodies under intense light fields. Motivated by long-standing observations of broadband upconversion and white-light emission under near-infrared excitation—often with puzzling blackbody-like spectra and high apparent color temperatures even at cryogenic conditions—the authors hypothesize that intense irradiation can photoinduce new energy states that dramatically enhance absorption across a broad spectrum. This would enable a light-driven transition from quasi-whitebody to quasi-blackbody behavior, accompanied by broadband emission and photon-avalanche dynamics. The work aims to clarify the mechanism, disentangle thermal versus photoinduced contributions, and establish the conditions and characteristics (thresholds, bistability, absorption bandwidth) of this phenomenon, which bears significance for nonlinear photophysics, light–matter interaction, and potential photonic applications.

Broadband upconversion luminescence under NIR excitation was first reported by the authors’ group in TiO₂:Mo (2002), exhibiting photon-avalanche characteristics. Since 2010, multiple groups (e.g., Tanner/Peter, Strek, Song, Liu, Yan, and others) have observed wideband upconversion and white-light emissions in diverse materials, spanning NIR-to-visible continua and including both up- and down-conversion components. Despite varied explanations (e.g., ESA, cross-relaxation, defect-mediated pathways), the physical mechanism remained unclear, especially given reports of blackbody-like spectra and high fitted color temperatures exceeding the thermal limits of samples or holders and occurrences at low temperatures (~10 K). Prior observations suggested an emission similar to thermal (blackbody) radiation but driven by optical processes, motivating the present formulation of photoinduced blackbody radiation (PBR)/photon-avalanche blackbody radiation (PABR) and a systematic investigation of absorption changes, thresholds, and bistability under strong irradiation.

• Materials: White powder samples Y₂O₃ and Yb₂O₃ (99.99%) were primary exemplars. Thin sheets (~12 mm diameter, ~2 mm thick) were pressed for transmission tests.
• Excitation and probes: Focused CW lasers at 980 nm (0–30 W) and 808 nm (0–10 W) served as pumps. Additional probe lasers (low power ~100 mW unless specified): 266, 405, 532, 650, 808 nm, and 1560 nm, co-aligned to assess broadband absorption via scattering reduction. Pulsed 980 nm excitation (various pulse widths up to 40 ms; repetition ~8–10 Hz; peak powers up to ~30 W) was used for dynamics.
• Spectroscopy: Steady-state spectra recorded across 350–2400 nm using multiple spectrometers (HR4000CG-UV-NIR; YOKOGAWA AQ6370D; AQ6375B) and stitched after system calibration. Calibration employed two standard halogen lamps (2432 K, 2856 K) with metrology-traceable irradiance to derive wavelength-dependent compensation factors.
• Bistability and threshold measurement: Emission intensity (integrated 400–800 nm) and scattered pump light near 980 nm were recorded while ramping pump power up and down to map hysteresis loops. Effect of pre-absorption was probed via light Yb³⁺ doping (0.7 mol%) in Y₂O₃.
• Heating tests: Y₂O₃ on a Ni–Cr heater was heated up to ~1500 K while monitoring probe-laser scattering (808 nm) to isolate thermal effects on absorption and evaluate threshold reduction for PBR.
• Cross-wavelength seeding: Yb₂O₃ (weak at 808 nm, strong at 980 nm) was irradiated simultaneously with 5 W CW 808 nm and 980 nm pulsed (10 Hz, 40 ms, 10 W peak) beams to test induction of new absorption channels at 808 nm.
• Time-resolved spectroscopy: Using a 1 m monochromator (SPEX 1000 M) with appropriate detectors and oscilloscope readout, time-resolved PBR spectra and kinetics at selected wavelengths (e.g., 620 nm) were recorded during and after laser pulses to compare with Planck fits over time.
• Temperature measurements: Color (spectral) temperatures were obtained by fitting PBR spectra to Planck’s law. Actual temperatures were measured with a flat Pt/Rh thermocouple embedded within the powder stack under 980 nm irradiation. Control measurements determined laser-heated thermocouple spectral vs actual temperature agreement.
• Photoconductivity: A powder-filled 0.5 mm gap between aluminum electrodes on a ceramic base was irradiated at 980 nm (pulsed). Conductivity was measured with a source meter at 70 V while varying pump powers to correlate conductance with PBR onset and intensity.
• Transmission/absorption under PBR: A lock-in amplifier with a chopped halogen source and PMT detection measured broadband transmission of Yb₂O₃ thin sheets during PBR to quantify absorption across 500–1500 nm while minimizing interference from the incandescent emission.

• Photoinduced blackbody radiation (PBR): White powders (Y₂O₃, Yb₂O₃) transition under intense focused NIR light into bright broadband emitters while simultaneously developing full-band strong absorption, akin to blackbodies.
• Broadband absorption: Upon entering PBR (e.g., Yb₂O₃ under 30 W 980 nm pump), scattering of six probe lasers (266, 405, 532, 650, 808, 1560 nm) dropped sharply; absorption ratios for all probes exceeded 90%, indicating quasi-blackbody absorption across UV–NIR. Transmission measurements on Yb₂O₃ sheets in PBR showed strong absorption from 500–1500 nm that deepened with pump power.
• Avalanche and bistability: Y₂O₃ exhibited a sharp threshold for PBR at ~22 W (980 nm, ~1 mm² spot), with instantaneous slope n = log I/log P reaching ~70.5 at onset; emission intensity then scaled with n ≈ 1.6 from 22–30 W. The PBR state self-sustained down to ~7 W (bistable hysteresis). Light Yb³⁺ doping (0.7 mol%) increased initial 980 nm absorption, lowering the threshold to ~13 W while the sustaining power remained ~7 W.
• Non-thermal origin of absorption switching: During power down, emission faded gradually but scattered pump light remained low until a sudden step increase when exiting PBR, evidencing abrupt disappearance of photoinduced absorption transitions (non-thermal bistability). Heating alone (room T to 1500 K) did not change 808 nm absorbance of Y₂O₃ but significantly reduced the PBR threshold (e.g., enabling PBR at 5 W 808 nm when heated), showing temperature affects threshold but is not the cause of broadband absorption.
• Cross-wavelength seeding of new absorption: In Yb₂O₃, 5 W 808 nm alone could not induce PBR. Adding a 980 nm pulsed beam created additional absorption at 808 nm (scattering decreased), enabling PBR that then persisted with 808 nm alone until briefly interrupted, demonstrating formation and maintenance requirements of PBR-specific excited states (PBR levels).
• Spectral vs thermal temperature: PBR spectra at high pump powers fit Planck’s law well; at lower powers, deviations from Planck increased (particularly 400–600 nm), attributed to a non-Boltzmann ‘lighting population’ of PBR states. Spectral (color) temperatures consistently exceeded actual thermocouple temperatures by >600 K in some cases, while a laser-heated Pt/Rh thermocouple showed spectral and actual temperatures agreeing within <100 K, confirming PBR deviations from purely thermal emission.
• Dynamics: Time-resolved measurements showed microsecond-scale buildup of PBR during pulses; spectra recorded during pulses deviated from Planck, whereas post-pulse spectra rapidly relaxed to Planck-like shapes as populations redistributed thermally.
• Photoconductivity: Prior to PBR, conductivity decreased with pump power due to heating. At PBR onset (~4 W pulsed example), conductivity spiked, then decreased due to photothermal lattice scattering. At higher powers (e.g., 7 W pulsed), photoinduced conductance exceeded thermal resistance effects, yielding a 2.45× conductivity increase that tracked PBR intensity during pulses, implicating generation of free carriers (electrons/ions) closely tied to PBR levels and broadband absorption.

The findings demonstrate that intense optical fields can photoinduce new energy states (PBR levels) that dramatically enhance broadband absorption and enable a white-to-blackbody-like transition accompanied by photon-avalanche emission and optical bistability. Measurements decouple thermal and photoinduced contributions: heating alone neither creates broadband absorption nor PBR states, though it lowers thresholds; in contrast, strong irradiation produces sharp, reversible changes in scattering/absorption and increased photoconductivity, indicating free carrier creation and non-thermal population of states. Spectral analyses show deviations from Planck’s law during illumination (lighting population dominates), converging to thermal behavior post-illumination (Boltzmann redistribution), which explains the consistent overestimation of temperature by spectral fitting. The correlation between initial absorption and reduced thresholds (e.g., Yb-doped Y₂O₃) and cross-wavelength seeding of new absorption channels substantiate a positive feedback avalanche mechanism: weak initial absorption enables excitation into PBR levels, which in turn broaden and strengthen absorption, sustaining the state at lower power. Collectively, these results address the research question by establishing PBR as a light-driven, non-equilibrium blackbody-like state with distinct kinetics and carrier dynamics, relevant to understanding extreme optical nonlinearities and potential photonic conversion technologies.

Focused laser irradiation can transform nominally white materials into photoinduced blackbodies that strongly absorb across a broad spectrum and emit intense broadband radiation. This PBR exhibits photon-avalanche behavior, optical bistability, and full-band absorption, arising from the formation of PBR quantum states under strong illumination that create a positive feedback between absorption and population. Time-resolved spectra and photoconductance confirm that PBR deviates from purely thermal radiation: during illumination, a non-Boltzmann lighting population elevates the color temperature above the actual temperature, while post-illumination spectra revert toward Planckian shapes. Beyond revealing a new light–matter interaction regime, the results suggest caution in interpreting spectral temperatures in systems where non-thermal populations may coexist with thermal emission, potentially including astrophysical objects. Future work should elucidate the microscopic nature of PBR levels, map material dependencies and thresholds across broader classes, and harness PBR for broadband optical conversion, sensing, and light management.

• The microscopic identity and formation pathways of PBR quantum states are inferred from optical and electrical signatures but not directly resolved; the mechanism diagram remains a proposed model.
• Temperature measurements, while carefully controlled with embedded thermocouples, can suffer from gradients and indirect laser heating of the sensor; although the large spectral–actual temperature gaps are unlikely due to measurement error alone, exact local temperatures at the PBR spot may differ.
• Transmission/absorption measurements during PBR required lock-in techniques to mitigate strong self-emission, which may limit absolute accuracy in some spectral regions.
• Demonstrations focus on oxide powders (Y₂O₃, Yb₂O₃) and select conditions; generality across materials, morphologies, and wavelengths, and long-term stability or damage thresholds under continuous operation, require further study.