This PDF file contains the front matter associated with SPIE Proceedings Volume 12437, including the Title Page, Copyright information, Table of Contents, and Conference Committee information.

A system consisting of two slabs with different temperatures can exhibit a non-equilibrium lateral Casimir force on either one of the slabs, when Lorentz reciprocity is broken in at least one of them. This system constitutes a photonic heat engine that converts radiative heat energy into work done by the non-equilibrium Casimir force. Inversely, by sliding two slabs at a sufficiently high relative velocity, heat is pumped from the slab at a lower temperature to the other one at a higher temperature. Hence the system operates as a photonic heat pump. In this work, we study the thermodynamic performance of such photonic heat engine and pump via the exact fluctuational electrodynamics formalism. The propulsion force due to the non-reciprocity and the drag force due to the Doppler effect were revealed as the physical mechanism behind the heat engine. We also show that the heat pump can be achieved only by the Doppler effect and non-reciprocal materials can help further reduce the required velocity to achieve heat pumping. Furthermore, we derive a relativistic version of the thermodynamic efficiency for our heat engine and show that the Carnot limit is independent of the frame of reference. We explore an ideal material dispersion to reach that efficiency. Our work serves as a conceptual guide for the realization of photonic heat engines based on fluctuating electromagnetic fields and relativistic thermodynamics and shows the important role of electromagnetic non-reciprocity in operating them.

Recent years have seen silica emerge as a viable material for optical refrigeration with potential applications in directed energy, integrated photonics, and precision metrology. Proper characterization of potential composition profiles is vital for optimization. Here, static photoluminescence spectroscopy of a Yb, Al co-doped silica sample over the temperature range 80 K to 300 K reveals the emission lineshape is dependent on the excitation wavelength. The impact this has on extracted laser cooling parameters is discussed.

Semiconductor optical refrigeration utilizes the energy upconversion of light via anti-Stokes (AS) photoluminescence (PL), which occurs through a phonon-assisted optical absorption process. This process is advantageous for semiconductors with strong electron-phonon (e-ph) interactions, such as lead halide perovskites. We discuss the significance of short-range e-ph interactions in the AS-PL process, rather than the long-range Fröhlich interactions. Short-range e-ph interactions induce Urbach tail absorption, which is experimentally evaluated via the steepness parameter. We simulate the impact of the steepness parameter on the photo-cooling gain and efficiency. We also focus on halide perovskite CsPbBr₃ quantum dots embedded in a Cs₄PbBr₆ host crystal, which possess both near-unity PL quantum efficiency and strong e-ph interactions, and quantitatively evaluate their AS-PL properties. Based on the spectroscopic results, we discuss the possibility of semiconductor optical refrigeration using halide perovskite quantum dots.

For as long as light and matter have partnered, impurities have played a role in optical system performance. This remains generally true for photonic heat engines and especially the case for optical refrigeration. Building on the history of light and glass, including the materials development of low loss telecom fibers, this paper briefly discusses the sources of heat generation in materials and all-material means for their reduction. Particularly attention will be paid to active optical fibers and connect thermal management to parasitic optical nonlinearities, both critical to high and low power amplifier and laser systems.

Various rare-earth doped solids can be cooled by anti-Stokes fluorescence, but only a few, particularly ytterbium (Yb)-doped LiYF₄ (YLF), showed the potential to reach the cryogenic temperature regime (below 123 K). We propose to adopt cubic Yb-doped KY₃F₁₀ (Yb:KYF) for reaching sub-100 K cooling temperatures. The temperature-dependent spectroscopy of Yb:KYF and the comparison with Yb:YLF indicate its high potential to achieve lower cooling temperatures. The calculated figure-of-merit of laser cooling of Yb:KYF is higher than that of Yb:YLF by a factor of five at 100 K. This is because Yb:KYF has a significantly shorter mean fluorescence wavelength of 991 nm compared to the value of 1004 nm for Yb:YLF at 100 K. We grew Yb:KYF crystals by the Czochralski method with varied growth parameters, and experimentally compared their laser cooling performance with an Yb:YLF also grown at our institute. We observed efficient laser cooling in the Yb:KYF crystals at room temperature. Laser-induced thermal modulation spectroscopy tests determined their external quantum efficiencies to be higher than 98.5% and background absorption coefficients to be as low as 1.0•10^-4 cm^-1. The minimal achievable temperature (MAT) of our best Yb:KYF sample was calculated to be ≈90 K, attractive to be used in optical cryocoolers.

Optical cooling in Yb-doped silica fibers using anti-Stokes fluorescence has become a subject of great interest in the fiber laser community. This paper provides an update on the development of silica fibers designed specifically to enhance their cooling properties. This growing list includes a new, nearly single-mode fiber with a borophosphosilicate core that produced –65 mK of cooling with only 260 mW of 1040-nm pump power. The silica compositions that have now been successfully cooled at atmospheric pressure by anti-Stokes fluorescence by our team include aluminosilicate, aluminofluorosilicate, borophosphosilicate, and aluminosilicate doped with one of three different alkali-earth nanoparticles (Ba, Sr, and Ca). By fitting the measured temperature dependence of the cooled fiber on pump power, two key parameters that control the degree of cooling are inferred, namely the critical quenching concentration and the absorptive loss due to impurities. The inferred values compiled for the fibers that cooled indicate that the extracted heat is highest when the Yb concentration is 2 wt.% or more (to maximize heat extraction), the Al concentration is ~0.8 wt.% or greater (to reduce quenching), and the absorptive loss is below approximately 15 dB/km, and ideally below 5 dB/km (to minimize heating due to pump absorption). Only two of the reported fibers, an LaF₃-doped and an LuF₃-doped nanoparticle fiber, did not cool, because their Yb and Al concentrations were not sufficiently high. This analysis shows that through careful composition control (especially the Al and Yb concentrations) and minimization of the OH contamination, a new generation of Yb-doped silica fibers is emerging with higher Yb concentrations, greater resistance to quenching, and lower residual loss than commercial Yb-doped fibers. They can be expected to have a significant impact not only on optically cooled devices but also on a much broader range of fiber lasers and amplifiers.