Photonic Control of Incoherent Emission In and Out of Equilibrium

Abstract

Spontaneous emission is the quantum mechanical process by which matter excited to a higher energy level transitions to a lower one by emitting light in the form of a photon. It is partly characterized by its incoherence (i.e., emitted photons have random frequencies and directions) and can occur in systems that are in or out of thermodynamic equilibrium. Thermal emission is the archetypal example of equilibrium spontaneous emission, while an example of nonequilibrium spontaneous emission is scintillation—light emission when an X-ray passes through a material, notable for its wide use in medical imaging and particle physics. In equilibrium systems, the design of incoherent emitters is enabled by Kirchhoff’s law of radiation, traditionally equating the emissive properties of materials to their absorptive properties, which are much easier to compute. Recently, Kirchhoff’s law was generalized to nonequilibrium systems, but additional complications such as broken time reversal symmetry (via magnetic fields and/or time modulation) require further generalizations that may not take the form of an equality between emission and absorption. Using thermal emission and scintillation as canonical and technologically relevant examples of incoherent emission, this thesis explores the control of both equilibrium and nonequilibrium light emission by applying strategies from nanophotonics within the framework of Kirchhoff’s law and its generalized forms. In the first part, I show how magnetic fields and nanophotonic patterning can be used to tailor thermal emission from nano- to macroscale, for applications in energy and medical imaging. First, I use modeling to show that combining magnetocaloric materials (which change temperature in response to magnetic fields) with magneto-optic materials can enable switchable radiative heat transfer. Then, I experimentally demonstrate nonreciprocal (or directionally asymmetric) reflectivity in a magneto-optic material, which is tantamount to the breakdown of the traditional form of Kirchhoff’s law, i.e., emissivity no longer equals absorptivity. Finally, I revisit X-ray tube design through the lens of photonics and show that nanophotonic patterning can enhance radiative cooling in these systems, leading to superior performance and reduced risk of failure in X-ray imaging. In the second part, I show how nanophotonics provides an avenue to control scintillation, emphasizing approaches that can increase the physical size and dimensionality (i.e., from surface to volumetric nanophotonic patterning) of so-called “nanophotonic scintillators.” I demonstrate the imaging of biological samples using scintillators that are nanophotonically patterned over areas as large as 4×4 cm—competitive with commercially available flat-panel detectors—and measure sixfold scintillation emission enhancement over a relatively large area. Then, I experimentally probe volumetric nanophotonic scintillators that are patterned extremely deep into the material, even all the way through (∼0.5 mm, two orders of magnitude greater than the wavelength of emitted light). In particular, I report a new phenomenon in which the patterns can outcouple light emitted farther away in the bulk without being directly excited themselves, which is promising for applications such as remote particle detection. In summary: using nanophotonic strategies grounded in newly derived generalizations of Kirchhoff’s law, my work demonstrates how incoherent emission can be tailored both in and out of equilibrium. This thesis contributes to the development of more efficient, engineered emitters and absorbers for applications including energy conversion, sensing, and medical imaging.