Thermal emission is a ubiquitous and fundamental process by which all objects at non-zero temperatures radiate electromagnetic energy. This process is often presented to be incoherent in both space and time, resulting in broadband, omnidirectional light emission toward the far field, with a spectral density related to the emitter temperature by Planck's law. Over the past two decades, there has been considerable progress in engineering the spectrum, directionality, polarization, and temporal response of thermally emitted light using nanostructured materials. This review summarizes the basic physics of thermal emission, lays out various nanophotonic approaches to engineer thermal-emission in the far field, and highlights several relevant applications, including energy harvesting, lighting, and radiative cooling. 2 I: IntroductionEvery hot object emits electromagnetic radiation according to the fundamental principles of statistical mechanics. Examples of this phenomenon-dubbed thermal emission (TE)-include sunlight and the glow of an electric stovetop or embers in a fire. The basic physics behind TE from hot objects has been well understood for over a century, as described by Planck's law 1 , which states that an ideal black body (a fictitious object that perfectly absorbs over the entire electromagnetic spectrum and for all incident angles) emits a broad spectrum of electromagnetic radiation determined by its temperature.Increasing the temperature of a black body results in an increase in emitted intensity, and a skew of the spectral distribution toward shorter wavelengths. A fundamental property of this process is that the thermal emissivity of any object-which quantifies the propensity of that object to generate TE compared to a black body-is determined by its optical absorptivity 1 . The connection between absorption and thermal emission, linked to the time reversibility of microscopic processes, suggests that the temporal and spatial coherence of the TE can be engineered via judicious material selection and patterning.Engineering of TE is of great interest for applications in lighting, thermoregulation, energy harvesting, tagging, and imaging. This review summarizes the basic physics of TE and surveys recent advances in the far-field control of TE via nanophotonic engineering. First, we present the basic formalism that describes TE, and review realizations of narrowband, directional, and dynamically reconfigurable far-field thermal emitters based on nanophotonic structures. Then we review applications of engineered far-field TE, with emphasis on energy and sustainability. II: Theoretical backgroundAt elevated temperatures, the constituents of matter, including electrons and atomic nuclei, possess

The insulator-to-metal transition (IMT) in vanadium dioxide (VO 2 ) can enable a variety of optics applications, including switching and modulation, optical limiting, and tuning of optical resonators. Despite the widespread interest in VO 2 for optics, the wavelength-dependent optical properties across its IMT are scattered throughout the literature, are sometimes contradictory, and are not available at all in some wavelength regions. Here, the complex refractive index of VO 2 thin films across the IMT is characterized for free-space wavelengths from 300 nm to 30 µm, using broadband spectroscopic ellipsometry, reflection spectroscopy, and the application of effective-medium theory. VO 2 films of different thicknesses are studied, on two different substrates (silicon and sapphire), and grown using different synthesis methods (sputtering and sol-gel). While there are differences in the optical properties of VO 2 synthesized under different conditions, these differences are surprisingly small in the 2-11 µm range where the insulating phase of VO 2 also has relatively low optical loss. It is anticipated that the refractive-index datasets from this article will be broadly useful for modeling and design of VO 2 -based optical and optoelectronic components, especially in the mid-wave and long-wave infrared.

Perovskite solar cells (PSCs) have gained tremendous research interest because of their tolerance of defects, low cost, and facile processing. In PSC devices, PbI 2 has been utilized to passivate defects at perovskite film surfaces and GBs; however, a systematic mechanism of PbI 2 in situ passivation for enhancing the solar cells efficiency has not been fully explored. Here, this work, we systematically studies the effect of the precise PbI 2 ratio and the PbI 2 in situ passivation mechanism based on trap density, carrier lifetime, Fermi level, and so forth. This study finds the appropriate ratio of I/Pb to be around 2.57:1 using energy-dispersive spectroscopy. After the moderate excess PbI 2 in situ passivation, the trap density is reduced from 6.12 × 10 16 to 3.38 × 10 16 cm −3 , and the carrier lifetime is extended from 168.35 to 368.77 ps by using fs-TA spectroscopy. This result indicates that the moderate excess PbI 2 in situ passivation can reduce the trap density and suppress the nonradiative recombination. The efficiency of solar cell has shown a nearly 11.3% improvement of 19.55% for an I/Pb ratio of 2.57:1 compared with 2.69:1. It also demonstrates that the efficiency of PSCs can be enhanced effectively by PbI 2 in situ passivation.

With the size of plasmonic devices entering into the nanoscale region, the impact of quantum physics needs to be considered. In the past, the quantum size effect on linear material properties has been studied extensively. However, the nonlinear aspects have not been explored much so far. On the other hand, much effort has been put into the field of integrated nonlinear optics and a medium with large nonlinearity is desirable. Here we study the optical nonlinear properties of a nanometre scale gold quantum well by using the z-scan method and nonlinear spectrum broadening technique. The quantum size effect results in a giant optical Kerr susceptibility, which is four orders of magnitude higher than the intrinsic value of bulk gold and several orders larger than traditional nonlinear media. Such high nonlinearity enables efficient nonlinear interaction within a microscopic footprint, making quantum metallic films a promising candidate for integrated nonlinear optical applications.

We consider propagation of an electromagnetic (EM) wave through a dynamic optical medium whose refractive index varies with time. Specifically, we focus on the reflection and transmission of EM waves from a temporal boundary and clarify the two different physical processes that contribute to them. One process is related to impedance mismatch, while the other results from temporal scaling related to a sudden change in the speed of light at the temporal boundary. Our results show that temporal scaling of the electric field must be considered for light propagation in dynamic media. Numerical solutions of Maxwell's equations are in full agreement with our theory.