Thermal emission is the process by which all objects at non-zero temperatures emit light, and is welldescribed by the classic Planck, Kirchhoff, and Stefan-Boltzmann laws. For most solids, the thermally emitted power increases monotonically with temperature in a one-to-one relationship that enables applications such as infrared imaging and non-contact thermometry. Here, we demonstrate ultrathin thermal emitters that violate this one-to-one relationship via the use of samarium nickel oxide (SmNiO3), a strongly correlated quantum material that undergoes a fully reversible, temperature-driven solid-state phase transition. The smooth and hysteresis-free nature of this unique insulator-to-metal (IMT) phase transition allows us to engineer the temperature dependence of emissivity to precisely cancel out the intrinsic blackbody profile described by the Stefan-Boltzmann law, for both heating and cooling. Our design results in temperature-independent thermally emitted power within the long-wave atmospheric transparency window (wavelengths of 8 -14 µm), across a broad temperature range of ~30 °C, centered around ~120 °C. The ability to decouple temperature and thermal emission opens a new gateway for controlling the visibility of objects to infrared cameras and, more broadly, new opportunities for quantum materials in controlling heat transfer. Main text:The total amount of power thermally emitted by a surface in free space can be obtained by integrating its spectral radiance-given by Planck's law and an emissivity-over all wavelengths and hemispherical angles 1 2 . Assuming negligible angular dependence of the emissivity and wrapping the angular integral into the blackbody distribution, ( , ), this relationship can be expressed as:where is the surface area, ( , ) is the spectral emissivity, is the free-space wavelength, is the temperature, and is the Stefan-Boltzmann constant. The total emissivity, ( ), can have a gradual temperature dependence even if the spectral emissivity has no such dependence, due to the integration of ( ) ( , ) 3 ; nevertheless, this dependence is usually dwarfed by the 4 term, and so can often be considered to be approximately constant. Thus, the Stefan-Boltzmann law yields a one-to-one mapping
Thermal emission is the radiation of electromagnetic waves from hot objects. The promise of thermal-emission engineering for applications in energy harvesting, radiative cooling, and thermal camouflage has recently led to renewed research interest in this topic. However, accurate and precise measurements of thermal emission in a laboratory setting can be challenging in part due to the presence of background emission from the surrounding environment and the measurement instrument itself. This problem is especially acute for thermal emitters that have unconventional temperature dependence, operate at low temperatures, or are out of equilibrium. In this paper, we describe, recommend, and demonstrate general procedures for thermal-emission measurements that can accommodate such unconventional thermal emitters.
We designed a nanoscale light extractor (NLE) for the efficient outcoupling and beaming of broadband light emitted by shallow, negatively charged nitrogen-vacancy (NV) centers in bulk diamond. The NLE consists of a patterned silicon layer on diamond and requires no etching of the diamond surface. Our design process is based on adjoint optimization using broadband time-domain simulations and yields structures that are inherently robust to positioning and fabrication errors. Our NLE functions like a transmission antenna for the NV center, enhancing the optical power extracted from an NV center positioned 10 nm below the diamond surface by a factor of more than 35, and beaming the light into a ±30° cone in the far field. This approach to light extraction can be readily adapted to other solid-state color centers.
Optical limiters are nonlinear devices that feature decreasing transmittance with increasing incident optical intensity, and thus can protect sensitive components from high‐intensity illumination. The ideal optical limiter reflects rather than absorbs light in its active (“limiting”) state, minimizing risk of damage to the limiter itself. Previous efforts to realize reflective (rather than absorbing) limiters were based on embedding nonlinear layers into relatively thick multilayer photonic structures, resulting in substantial fabrication complexity, reduced speed and, in some instances, limited working bandwidth. In this paper, these tradeoffs are overcome by using the insulator‐to‐metal transition (IMT) in vanadium dioxide (VO2) to achieve intensity‐dependent modulation of resonant transmission through aperture antennas. Due to the large change of optical properties across the IMT, low‐quality‐factor resonators are sufficient to achieve high on–off ratios in the transmittance of the limiter. As a result, our ultrathin reflective limiter (thickness ≈1/100 of the free‐space wavelength) is broadband in terms of operating wavelength (>2 µm at 10 µm) and angle of incidence (up to ≈50° away from the normal). Our analysis of the experimental results via opto‐thermal simulations provides insight into limiter performance and is a useful guidance for further engineering efforts.
Engineered optical absorbers are of substantial interest for applications ranging from stray light reduction to energy conversion. We demonstrate a large-area (centimeter-scale) metamaterial that features near-unity frequency-selective absorption in the mid-infrared wavelength range. The metamaterial comprises a self-assembled porous structure known as an inverse opal, here made of silica. The structure's large volume fraction of voids, together with the vibrational resonances of silica in the mid-infrared spectral range, reduce the metamaterial's refractive index to close to that of air and introduce considerable optical absorption. As a result, the frequency-selective structure efficiently absorbs incident light of both polarizations even at very oblique incidence angles. The absorber remains stable at high temperatures (measured up to ~900 °C), enabling its operation as a frequency-selective thermal emitter. The excellent performance characteristics of this absorber/emitter and ease of fabrication make it a promising surface coating for passive radiative cooling, laser safety, and other large-area applications.
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