Design of thermal metamaterials beyond the effective medium theory: Direct numerical simulation via the Thermal Discrete Dipole Approximation (T-DDA)

Design of thermal metamaterials beyond the effective medium theory: Direct numerical simulation via the Thermal Discrete Dipole Approximation (T-DDA) 1. INTRODUCTION 1.1. Objective, broader impacts and intellectual merit The objective of this research is to establish a computational toolbox for designing metamaterials, with user-defined thermal radiative properties, beyond the effective medium theory. This will be accomplished by direct calculation of near-field thermal emission via a novel approach called the Thermal Discrete Dipole Approximation. The establishment of thermal metamaterials with unique designer properties will expedite the development of technologies such as infrared cloaking and nanoscale-gap thermophotovoltaic power generation. Metamaterials are composite materials that display exotic electric and magnetic properties resulting from sub-wavelength functional inclusions (" meta-atoms ") [1]. The ability to engineer metamaterials with specific electric permittivity and magnetic permeability enables tailoring media with unique thermal radiative properties [2]. PI's group demonstrated via the effective medium theory (EMT) that quasi-monochromatic near-field thermal emission in the near infrared is achievable at a temperature as low as 400 K via Mie resonance-based metamaterials made of dielectric inclusions, while, according to Wien's law, a temperature greater than 1000 K is required for dominant emission in the near infrared via naturally occurring materials [3,4]. The application of such metamaterials to nanoscale-gap thermophotovoltaic (nano-TPV) power generators will allow low temperature waste heat recovery in a variety of electronic devices, such as cell phones and photovoltaic cells [5-11]. Among the devices that can be powered by nano-TPV systems or whose functionality can be improved are several used by the Army in its mission to provide prompt, sustained land dominance in military operations: computers, communication devices (e.g., radios) and night vision cameras. Worth noting is that the design of thermal metamaterials with user-defined properties will have other applications, such as optical cloaking. In particular, the ability to control the thermal spectrum will pave the way for developing materials invisible to infrared cameras. Such passive infrared camouflage is also relevant to military operations. Metamaterials' electromagnetic properties are usually predicted via the EMT, where a heterogeneous medium is conceptualized as homogeneous with effective electric permittivity and magnetic permeability [1]. However, when considering the near-field electromagnetic spectrum emitted at a distance smaller than the size of the meta-atoms or their separation distance, approximating a heterogeneous layer as homogeneous may lead to significant errors. The validity of the EMT was verified recently in the near field of hyperbolic metamaterials made of thin …