Optical Frequency Conversion Luminescence: Cooperative Transitions Of Yb³⁺ Ion Clusters And Photoinduced Blackbody Radiation

Optical frequency conversion luminescence is a special form of photoluminescence,which can be divided into optical frequency up-conversion luminescence and optical frequency down-conversion luminescence,corresponding to two different luminescent phenomena in which the emitted frequency of the light is greater than and less than the excitation frequency,respectively.In previous studies,rare earth-doped materials were commonly used to achieve optical frequency up-conversion luminescence under near-infrared light excitation and optical frequency down-conversion luminescence under ultraviolet light excitation.In recent years,our research group has used cooperative transitions of Yb³⁺ion cluster materials to achieve both optical frequency up-conversion and optical frequency down-conversion luminescence and realized luminescence through photothermal radiation using various inorganic materials.Photothermal radiation is a special form of optical frequency conversion luminescence,which can simultaneously achieve both optical frequency up-conversion and optical frequency down-conversion luminescence.During the doctoral degree,the author conducted a systematic study on the issues of multi-ion cooperative transitions in optical frequency conversion luminescence and photoinduced blackbody radiation,and achieved innovative results as follows:1.A new type of nano-effect,the nano-shell effect in bulk materials,has been discovered.Zr O₂ shells were coated onto the surface of Ca F₂:Yb³⁺microparticles using a hydrothermal method,with the thickness of the Zr O₂ shell controlled by adjusting the Zr O₂ and Ca F₂:Yb³⁺feed ratio.When the Ca F₂:Yb³⁺and Zr O₂ feed ratio was 1:0.1,the cooperative frequency upconversion luminescence intensity of clusters in the material reached a maximum value,and the cooperative luminescence intensities of Yb³⁺ion pairs and trimers increased by a factor of 1.97 and 2.19 respectively.The significant enhancement of cooperative luminescence comes from the surface modification of the Zr O₂ coating,and this surface modification only leads to notable fluorescence enhancement in nanomaterials.Kinetic analysis shows that this novel nanoshell effect in bulk materials plays an important role in improving cooperative luminescence of multi-ion clusters.2.The Ca F₂:Yb³⁺@Ca F₂:Ce³⁺core-shell ion cluster material was prepared using a two-step method combining high temperature and co-precipitation.By doping Yb³⁺and Ce³⁺separately into the core and shell,the destruction of Yb³⁺cluster structures caused by Ce³⁺ions were effectively avoided,and the effective doping concentration of Ce³⁺ions and the absorption intensity to the excitation light were improved.Spectral analysis showed that the ion cluster material emitted near-infrared light at～1000 nm under excitation by light in the range of 260-400 nm.A testing system based on a silicon solar cell was constructed to study the quantum cutting frequency conversion phenomenon of the material under light of different wavelengths and to measure the enhancement effect of the frequency-down-conversion luminescence on the silicon solar cell.Experimental results showed that when the doping concentration of Ce³⁺ions in the core-shell ion cluster material was 0.1 mol%,the photoelectric conversion efficiency of the silicon solar cell could be effectively improved in the range of 250-350 nm.When the 280 nm light source is used,the measured solar cell photocurrent is increased by 12.1 times.When testing with a 310 nm Led light source,the measured solar cell photocurrent was increased by a factor of 2.2.3.Photoinduced blackbody phenomenon was discovered.In the experiment,the phenomenon of white inorganic material transforming into blackbody under light irradiation was observed for the first time.Under the irradiation of high power laser,the white powder material with weak absorption will enter a special optical frequency conversion luminescence state.In this state,the material emits broad-spectrum luminescence similar to blackbody radiation,which includes both up-conversion and down-conversion luminescence,and exhibits broad-spectrum strong absorption characteristics similar to blackbody.We refer to this blackbody-like state as photoinduced blackbody（PB）,and the luminescence and strong absorption of blackbody-like under this state are respectively called photoinduced blackbody radiation（PBR）and photoinduced blackbody absorption（PBA）.The occurrence of photoinduced blackbody phenomenon has an avalanche effect,with the threshold power density for avalanche initiation decreasing as the initial absorption rate of the material increases.4.Dual intrinsic optical bistability phenomena of PB radiation and PB absorption were observed for the first time and studied in detail.By changing the material’s absorption rate of the pumping light,vacuum degree of the environment,temperature,and using auxiliary pumping light,multiple factors affecting the intrinsic optical bistability of photoinduced blackbody were confirmed.The results showed that increasing the sample’s temperature,increasing the environment’s vacuum degree,and loading auxiliary pumping light can reduce the threshold power density for avalanche luminescence of photoinduced blackbody and change the shape of the dual intrinsic optical bistability hysteresis loop.Further analysis indicates that the characteristics of the avalanche luminescence of photoinduced blackbody and the dual intrinsic optical bistability require the existence of new（photo-induced）quantum transitions and corresponding new states in the PB material.We refer to this new state as photoinduced blackbody quantum state（PB quantum state）.5.PB radiation spectra of Y₂O₃ powder from 350 nm to 4000 nm were measured under 980 nm laser pumping and corrected.By comparing it with the Planck law,it was found that when the pumping power was low,the PB radiation spectrum deviated significantly from the description of the Planck law.As the pumping power gradually increased,the PB radiation spectrum tended to conform to the Planck law.The actual temperature of Yb₂O₃ powder entering the photoinduced blackbody radiation state was measured using a platinum-rhodium thermocouple,and it was found that there was a significant deviation between the temperature measured by the thermocouple and the spectral color temperature fitted by the Planck law,with a temperature deviation of more than 600 K.6.The mechanism of photoinduced blackbody effect was proposed.Under high-power laser action,PB quantum states with broadband absorption were generated in the photoinduced blackbody material,which led to new light absorption transitions,and new light absorption produced more PB quantum states,forming a positive feedback,ultimately leading to avalanche-type photoinduced blackbody radiation and absorption.According to the above mechanism,there are two ways for PB quantum states to be populated in photoinduced blackbody:one is photoinduced population under excitation light pumping,and the other is population generated after light energy is converted into lattice thermal vibration.The thermal population follows the Boltzmann distribution law.When the thermal population cannot completely obscure the photoinduced population,the PB radiation spectrum will deviate from the Planck law;meanwhile,due to the existence of photonic population,the spectral fitting temperature of photoinduced blackbody will be higher than the actual measured temperature of the thermocouple.