We exploit photonic heat engines, which involve optical cooling and heating of matter, for the development of a new class of optical nanodevices and nanomachines. Working principles and applications of these devices and machines will be discussed.

Photonic coolers, including both electrically- and laser-driven devices, seek to transport entropy away from a solid body using the photon field. Since more entropy can be carried per unit energy when photon mode occupation is more dilute, cooling is more thermodynamically efficient in this case. We discuss the opportunity for more efficient photonic cooling afforded by the large étendue associated with high index semiconductors and the associated challenge of étendue-matching along the photon extraction optical path. We will discuss both fundamental considerations and constraints imposed by the growth and fabrication of high quantum efficiency light-emitting materials and their substrates.

This conference presentation was prepared for SPIE OPTO, Photonics West, 2023.

The fluctuational electrodynamic investigation of thermal radiation from non-equilibrium or non-isothermal bodies remains largely unexplored because of severe numerical difficulties. Here, we employ fast calculations based on modal expansion to accelerate research at this frontier. We employ our formalism on a long silica wire held under temperature gradient within its cross section. We discover that the far-field thermal emission carries a nonzero spin which is constant in direction and sign, and interestingly, is transverse to the direction of the power flow. We establish the origin of this transverse spin as arising from the nonequilibrium intermixing of the modes of the wire, and not from any previously studied or intuitively expected origins such as chiral or nonisotropic materials and geometries, magnetic materials or fields, and mechanical rotations. This finding emitted heat radiation can prove useful for advancing the noninvasive thermal metrology or infrared imaging technique

The photonic heat engine has been researched as a combined area of quantum mechanics and thermodynamics. The quantum coherence in the atomic heat source (like coal) drives the photonic working fluid (like steam) [1] has been shown to yield the usual Carnot efficiency. However, the Carnot bound can be overcome, and work can be extracted with a single thermal reservoir [1]. With collective matter-light interactions like superradiance, the efficiency can reach almost unity [2, 3]. Further related aspects, such as emergence of collective behavior in open systems far from equilibrium [4], are also considered.

[1] M. Scully, M. Zubairy, G. Agarwal, and H. Walther, Science 299, 862 (2003).
[2] M. Kim, A. Svidzinsky, and M. Scully, Nature Photonics 16, 368 (2022).
[3] J. Kim, S. Oh, D. Yang, J. Kim, M. Lee and K. An, Nature Photonics 16, 707 (2022).
[4] Z. Zhang, G. Agarwal, M. Scully Physical Review Letter 122, 158101 (2019).

A record cooling to <125 K of an all-solid-state optical cryocooler by the anti-Stokes fluorescence cooling of a 10% Yb:YLF crystal. The record cooling achievable by the employment of a novel textured-MgF2 thermal link which improves the thermal transport and fluorescence escape. Spectrally selective coatings on the multi-pass pump circulator mirrors tuned to be highly reflective for the pump wavelengths yet transmit longer wavelengths in the Stokes regime. The loss introduced prevents sufficient gain from building up leading to amplified spontaneous emission and lasing. The roles of other potential nonlinearities, such as pump saturation are investigated.

Laser coolers are an innovative class of coolers capable of reaching cryogenics temperatures in a miniaturized, contact-less and entirely vibration free way. Hence they are considered as a potential breakthrough technology for space cryogenics, especially in the field of earth-observation missions. We have developed and successfully operated a fiber-coupled laser cooling prototype. We will present a possible architecture for the laser source that could meet space requirements and review possible alternative solutions like intra-cavity cooling and using Ho-doped fluoride crystals.

We cooled a 10% Yb:YLF crystal to temperatures of approximately 130 K using a fiber-coupled laser diode module emitting at 1020 nm. Previous approaches to optical refrigeration typically relied on high-power fiber lasers and intricate multi-pass schemes to reach cryogenic temperatures. In the current experiments a combination of dielectric mirrors deposited onto the sample and total internal reflection confined the laser light inside the crystal. The divergent beams of laser diodes produce relatively low average intensity inside the crystal, mitigating nonlinearities like absorption saturation and lasing. We have designed a full diode-pumped cryo-cooler with a coldfinger to cool arbitrary loads.

Recent advances in laser frequency stabilization have enabled lasers to serve as precise probes of fundamental physics and optical local oscillators for all-optical timescales. Current state-of-the-art optical oscillators employ Fabry-Perot cavities made from monocrystalline silicon, designed to operate at cryogenic temperatures where the thermal expansion coefficient cross zero. It is an engineering challenge to design cryostats with low vibration noise and temperature fluctuations that do not compromise the realization of thermal-noise limited instability. Cryostat-free operation of cryogenic reference cavities is key to making the world's best narrow linewidth lasers transportable. In this regard, optical refrigeration is an attractive prospect as cryogenic operation is decoupled from the excess environmental noise associated with most cryostats. With payloads now being cooled to the regime of cryogenic silicon cavity operation, this milestone is significantly closer to realization.

Achieving condensed phase optical refrigeration requires near-unity emission quantum yields (QYs). Colloidal CsPbBr3 nanocrystals (NCs) are promising candidates in this respect given near unity QY-values, achieved by post-synthetic surface treatment with quaternary ammonium bromide ligands. The origin of these QY enhancements, however, is not understood. Systematic nuclear magnetic resonance studies of the organic ligand passivation of near unity QY CsPbBr3 NC surfaces are therefore conducted to better reveal their surface-ligand interactions.

CsPbBr3 perovskite nanocrystals have been identified as a potential medium to realize condensed phase optical refrigeration. This is due to its near unity emission quantum yields and efficient anti-Stokes photoluminescence (ASPL). Despite much work on CsPbBr3’s optical response, the origin of its efficient ASPL remains unclear. We conduct detailed optical spectroscopy measurements in conjunction with theory to establish mechanistic insights into CsPbBr3’s up-conversion process. Experimental techniques utilized include: temperature-dependent and detuning energy-dependent ASPL measurements, temperature-tunable photothermal heterodyne absorption spectroscopy, and ultrafast transient differential absorption (TDA) spectroscopy.

We recently demonstrated laser induced cooling in all oxide silica glass [1], a proof of principle of our materials engineering approach. This technique has the potential of significantly impacting silica photonics by not only improving laser cooling with the preferred rare earth ion, Yb3+, but for the first time, also with different rare earths. The higher rare earth concentration possible in silica without affecting its optical properties, indicates that new amplifiers and laser may be possible. This talk will review our engineering perspective to mitigating serious materials shortcoming in silica and elaborate what may be possible for new applications in photonics. 1. J. Thomas, T. Meyneng, N. Gregoroire, F. Monet, A. Tehranchi, D. Seletskiy, Y. Messaddeq, Raman Kashyap, “Laser Cooling of a Novel GAYY Glass at Atmospheric Pressure”, Advanced Photonics Congress, Maastricht, Holland, Post Deadline paper JTH4A.5, Optica (28 July 2022).

We report optical cooling of a novel rare earth doped oxide-only silica glass fabricated using the modified chemical vapour deposition (MCVD) technique. An ytterbium concentration in silica glass of up to 6.55 x 10^26 ions/m^3 was achieved without compromising its optical properties. Samples were cooled to -0.8 K from room temperature (RT) at atmospheric pressure with pump power of 7 W at 1029 nm. We report experimental results on other RE dopants in oxide-only silica glass. Our approach opens possibilities not only for high efficiency laser cooling, but also in radiation balanced lasers, and compact high-power lasers and amplifiers.

Hexagonal microcavities have been proposed for a wide range of applications including microlasers, levitated optomechanics, quantum information science, and biosensors that make use of both Fabry-Perot and whispering gallery mode cavity resonances. Photothermal heating impacts a number of optical and mechanical properties of hexagonal microcavities based on the temperature dependence of quantities such as radiative lifetime, Young’s modulus, optical index of refraction, and the corresponding wavelength of cavity mode resonances. This talk will present recent results in both analytical and numerical modeling of photothermal heating in hexagonal cavities inspired by recent optomechanical levitation experiments in both aqueous and high-vacuum environments.

Recent advances in the synthesis of inorganic materials have enabled a wide range of control of particle size, composition, phase, and morphology that can be tuned in the design of future levitated optomechanical sensors. This talk will describe recent results in the hydrothermal synthesis and characterization of two-dimensional microprisms of hexagonal sodium yttrium fluoride crystals based on novel molecular ligands. Hexagonal cavities with high aspect with high aspect ratios of ~50 are observed with potential applications in the detection of high frequency gravitational waves, radiation balanced microlasers, and the solid-state laser cooling of quantum sensors.