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Planck's law of thermal radiation depends only on the temperature T and emissivity $\varepsilon$. It is one of the most fundamental discoveries about light-matter interaction that led to the development of quantum physics. Another basic property of a body is its ability to absorb incoming light, characterized by absorptivity $\alpha$. Kirchhoff's law of thermal radiation equals these two properties at thermodynamic equilibrium, i.e., $\varepsilon$=$\alpha$. The generalized Planck's equation extends Kirchhof's law out of equilibrium by scaling the absorptivity with the pump-dependent chemical potential $\mu$, obscuring emissivity as a material property. Quantum efficiency (QE) is a material property, defined out of equilibrium, describing the statistics of absorption followed by emission of a photon. Both emissivity and QE depend on the interplay between radiative and non-radiative rates. Here we theoretically and experimentally demonstrate a prime equation for emissivity as a material property in and out of equilibrium in the form of $\varepsilon$=$\alpha$(1-QE), which at equilibrium is reduced to Kirchhoff's law. Our work lays out the fundamental evolution of non-thermal emission with temperature, which is critical for the development of lighting and energy devices., Comment: 10 pages, 5 figures. arXiv admin note: text overlap with arXiv:2101.06435

Photoluminescence (PL) is a light-matter quantum interaction associated with the chemical potential of light formulated by the Generalized Planck's law. Without knowing the inherent temperature dependence of chemical potential, the Generalized Planck's law is insufficient to characterize PL(T). Recent experiments showed that PL at low temperatures conserves the emitted photon rate, accompanied by a blue-shift and transition to thermal emission at a higher temperature. Here, we theoretically study temperature-dependent PL by including phononic interactions in a detailed balance analysis. Our solution validates recent experiments and predicts important relations, including i) An inherent relation between emissivity and the quantum efficiency of a system, ii) A universal point defined by the pump and the temperature where the emission rate is fixed to any material, iii) A new phonon-induced quenching mechanism, and iv) Thermalization of the photon spectrum. These findings are relevant to and important for all photonic fields where the temperature is dominant., Comment: including supplementary material. arXiv admin note: text overlap with arXiv:2007.00310

The challenge in solar energy today is not the cost of photovoltaic (PV) electricity generation, already competing with fossil fuel prices, but rather utility-scale energy storage and flexibility in supply. Low-cost thermal energy storage (TES) exists but relies on expensive heat engines. Here, we introduce the concept of luminescent solar power (LSP), where sunlight is absorbed in a photoluminescent (PL) absorber, followed by red-shifted PL emission matched to an adjacent PV cell's band-edge. This way the PV cell operates nearly as efficiently as under direct illumination, but with minimal excessive heat. The PL-absorber temperature rises due to thermalization, allowing it to store the excessive heat, which can later be converted into electricity. Tailored luminescent materials that support an additional 1.5kWh PV-electricity for every 1 kWh of (virtual) heat engine-electricity, with a dynamic shift between the two sources are experimentally demonstrated. Such an ideal hybrid system may lead to a potential reduction in the cost of electricity for a base-load solution.

Photoluminescence (PL) is a light matter quantum interaction associated with the chemical potential of light formulated by the Generalized Planck's law. Without knowing the inherent temperature dependence of chemical potential, the Generalized Planck's law is insufficient in order to characterize PL vs. T. Recent experiments showed that PL at a critical temperature abruptly shifts from a conserved rate, accompanied by a blue-shift, to thermal emission. Here, we theoretically study temperature dependent PL by including phononic interactions in a detailed balance analysis. We show that in a three-level system, both chemical potential and T are defined in the case of fast thermalization. Our solution validates recent experiments and predicts new features, including an inherent relation between emissivity and external quantum efficiency of a system, a universal point defined by the pump and the temperature where the emission rate is fixed to any material, a new phonon induced quenching mechanism, and thermalization of the photon spectrum. Our high temperature luminescence solution is relevant to and important for all photonic fields where the temperature is dominant

Frequency up-conversion of a few low-energy photons into a single high-energy photon contributes to imaging, light sources, and detection. However, the up-converting of many photons exhibits negligible efficiency. Up-conversion through laser heating is an efficient means to generate energetic photons, yet the spectrally broad thermal-emission and the challenge of operating at high temperatures limit its practicality. Heating specific modes can potentially generate narrow up-converted emission; however, so far such "hot-carriers" have been observed only in down-conversion processes and as having a negligible lifetime, due to fast thermalization. Here we experimentally demonstrate up-conversion by excitation of a steady-state non-thermal-equilibrium population, which induces steady, narrow emission at a practical bulk temperature. Specifically, we used a 10.6um laser to generate 980nm narrow emission with 4% total efficiency and up-converted radiance that far exceeds the device s possible black-body radiation. This opens the way for the development of new light sources with record efficiencies.

The radiance of thermal emission, as described by Plancks law, depends only on the emissivity and temperature of a body, and increases monotonically with temperature rise at any emitted wavelength. Nonthermal radiation, such as photoluminescence (PL), is a fundamental light matter interaction that conventionally involves the absorption of an energetic photon, thermalization, and the emission of a redshifted photon. Until recently, the role of rate conservation when thermal excitation is significant, has not been studied in any nonthermal radiation. A question: What is the overall emission rate if a high quantum efficiency (QE), PL material, is heated to a temperature where it thermally emits a rate of 50 photons/sec at its bend edge, while in parallel is PL excited at a rate of 100 photons/sec. Recently, we discovered that the answer is an overall rate of 100 blueshifted photons/sec. In contrast to thermal emission, the PL rate is conserved with temperature increase, while each photon is blueshifted. Further rise in temperature leads to an abrupt transition to thermal emission where the photon rate increases sharply. These findings show that PL is an ideal optical heat pump. Here we study, for the first time, the emission rate of PL radiation at moderate QE where nonradiative processes dominate the dynamics. Though conservation of photon rate does not apply, we predict that the emission rate at 100% QE is an upper limit for the overall emission rate regardless of the QE. Also, the transition temperature to thermal emission is QE independent., Comment: 8 pages, 4 figures. arXiv admin note: text overlap with arXiv:1412.6896

Single-junction photovoltaic cells are considered to be efficient solar energy converters, but even ideal cells cannot exceed the their fundamental thermodynamic efficiency limit, first analysed by Shockley and Queisser (SQ). For moderated irradiation levels, the efficiency limit ranges between 30%-40%. The efficiency loss is, to a great extent, due to the inherent heat-dissipation accompanying the process of electro-chemical potential generation. Concepts such as solar thermo-photovoltaics (STPV) and thermo-photonics4 aim to harness this dissipated heat, yet exceeding the SQ limit has not been achieved, mainly due to the very high operating temperatures needed. Recently, we demonstrated that in high-temperature endothermic-photoluminescence (PL), the photon rate is conserved with temperature increase, while each photon is blue shifted. We also demonstrated how endothermic-PL generates orders of magnitude more energetic-photons than thermal emission at similar temperatures. These new findings show that endothermic-PL is an ideal optical heat-pump. Here, we propose and thermodynamically analyse a novel device based on Thermally Enhanced Photo Luminescence (TEPL). In such a device, solar radiation is harvested by a low-bandgap PL material. In addition to the PL excitation, the otherwise lost heat raises the temperature and allows the TEPL emission to be coupled to a higher bandgap solar cell. The excessive thermal energy is then converted to electrical work at high voltage and enhanced efficiency. Our results show that such a TEPL based device can reach theoretical maximal efficiencies of 70%, as high as in STPV, while the significantly lowered operating temperatures are below 1000C. In addition to the theoretical analysis, we experimentally demonstrated enhanced photo-current in TEPL device pumped by sub-bandgap radiation. This opens the way for a new direction in photovoltaics., Comment: arXiv admin note: text overlap with arXiv:1412.6896

Photoluminescence (PL) is a fundamental light-matter interaction, which conventionally involves the absorption of energetic photon, thermalization and the emission of a red-shifted photon. Conversely, in optical-refrigeration the absorption of low energy photon is followed by endothermic-PL of energetic photon. Both aspects were mainly studied where thermal population is far weaker than photonic excitation, obscuring the generalization of PL and thermal emissions. Here we experimentally study endothermic-PL at high temperatures. In accordance with theory, we show how PL photon rate is conserved with temperature increase, while each photon is blue shifted. Further rise in temperature leads to an abrupt transition to thermal emission where the photon rate increases sharply. We also show how endothermic-PL generates orders of magnitude more energetic photons than thermal emission at similar temperatures. Relying on these observations, we propose and theoretically study thermally enhanced PL (TEPL) for highly efficient solar-energy conversion, with thermodynamic efficiency limit of 70%.

Improving the conversion efficiency of solar energy to electricity is most important to mankind. For single-junction photovoltaic solar-cells, the Shockley-Queisser thermodynamic efficiency limit is extensively due to the heat dissipation, inherently accompanying the quantum process of electro-chemical potential generation. Concepts such as solar thermo-photovoltaics and thermo-photonics, have been suggested to harness this wasted heat, yet efficiencies exceeding the Shockley-Queisser limit have not been demonstrated due to the challenge of operating at high temperatures. Here, we present a highly efficient converter based on endothermic photoluminescence, which operates at relative low temperatures. The thermally induced blue-shifted photoluminescence of a low-bandgap absorber is coupled to a high-bandgap photovoltaic cell. The high absorber's photo-current and the high cell's voltage results in 69% maximal theoretical conversion efficiencies. We experimentally demonstrate tenfold thermal-enhancement of useful radiation for the high-bandgap cell and 107% enhancement in average photon energy. This paves the way for introducing disruptive-innovation in photovoltaics., Comment: 12 pages

Frequency up-conversion of few low-energy photons into a single high-energy photon, greatly contributes to imaging, light sources, detection and other fields of research. However, it offers negligible efficiency when up-converting many photons. This is because coherent process are fundamentally limited due to momentum conservation requirements, while in incoherent up-conversion the finite intermediate states lifetime requires huge intensities. Thermodynamically, conventional incoherent up-conversion is driven by the internal energy of the incoming photons. However, a system can also drive work through change in its collective properties such as entropy. Here we experimentally demonstrate entropy driven ten-fold up-conversion from 10.6{\mu} to 1{\mu}m at internal efficiency above 27% and total efficiency above 10%. In addition, the emitted radiance at 1{\mu}m exceeds the maximal possible Black-Body radiance of our device, indicating emitter's effective-temperature that is considerably above the bulk-temperature. This work opens the way for up-conversion of thermal-radiation, and high-temperature chemistry done at room-temperature., Comment: 14 pages