Abstract
Abstract We discuss the state-of-the-art and remaining challenges in the fundamental understanding and technology development for controlling light-matter interactions in nanophotonic environments in and away from thermal equilibrium. The topics covered range from the basics of the thermodynamics of light emission and absorption to applications in solar thermal energy generation, thermophotovoltaics, optical refrigeration, personalized cooling technologies, development of coherent incandescent light sources, and spinoptics.
Highlights
Photons are gauge bosons for electromagnetism and are characterized by their energy ω = hc/λ and momentum k, where h = 2π is the Planck’s constant, c is the vacuum speed of light, λ is the photon wavelength, ω is the angular frequency, and k is the photon wavevector [17, 18]
Thermodynamic treatment of electromagnetic radiation began over a century ago with Planck applying the thermodynamic principles established for a gas of material particles to an analogous “photon gas” [1]
He showed that thermodynamic parameters, such as the energy, volume, temperature, and pressure can be applied to electromagnetic radiation, reflecting the dual wave-particle nature of photons
Summary
Control and optimization of energy conversion processes involving photon absorption and radiation require detailed understanding of thermodynamic properties of radiation as well as its interaction with matter [1,2,3,4,5]. Upon substituting the modified photon distribution function into the Planck radiation law, we can observe that photons with a positive chemical potential carry higher energy per photon state than thermally emitted photons at the same emitter temperature. This effect is illustrated, which compares photon spectra from sources at varying temperatures and with varying positive chemical potential. Another important distinction from the blackbody spectrum is that the spectra of photons emitted with a chemical potential have a low-energy cut-off, which is defined by, for example, the electronic bandgap of the emitter material (Figure 1a). This situation is typical for the operation of thermoradiative (TR) [37] cells (see section 1.2), hot-electron energy converters [38] and optical rectennas [39, 40]
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