Session: 17-15-01: Society-Wide Micro/Nano Poster Forum

Paper Number: 99600

99600 - Direct Thermal Emission Calculation and Validation of Kirchhoff’s Law for Nonreciprocal Material Using Fluctuational Electrodynamics 

Nonreciprocal materials have been intensively studied in recent years due to the capabilities of providing new opportunities to improve the efficiency of thermal energy harvesting applications. To determine the emissivity of a thermal emitter, two different approaches, “indirect” and “direct” methods, are commonly used. As an indirect method, spectral angular emissivity is obtained by the spectral angular absorptivity and the transmissivity of the body according to energy conservation provided by Kirchhoff’s law. While the direct method uses a combination of fluctuation-dissipation theorem (FDT) and dyadic Green’s function (DGF) to directly evaluate the thermal emission. The indirect method by using Kirchhoff’s law, however, can be violated by breaking the Lorentz reciprocity with the nonreciprocal materials such as magneto-optical material or Weyl semimetals. The relations between angular emissivity and angular absorptivity of a nonreciprocal material could be nonexistent. Several groups have provided modified Kirchhoff’s law for the emissivity computation that considers the nonreciprocity of the emitter, but the equivalence between the indirect method of using modified Kirchhoff’s law and the direct method has yet to be demonstrated.   
In this work, fluctuational electrodynamics is used as a direct method to calculate thermal emission. Since the direct method demonstrates the statistical nature of thermal emission, it can be applied to both reciprocal and nonreciprocal materials. Correlation functions of the electric field are given by FDT in terms of DGFs. Since DGFs of anisotropic media are extremely mathematically intensive, the reciprocity theorem is used to simplify the expressions. Though the far-field emission behaves globally like an outgoing spherical wave, the electric field at one point behaves locally like a plane wave propagating in one specific direction. Therefore, the correlation matrix of the electric field is described as a function of a single wave vector that corresponds to the direction of emission. The emissivity of a given polarization is determined by comparing the coherent matrix to the optical intensity of an ideal blackbody. A semi-infinite structure is taken as an example to illustrate the emissive and absorptive properties of the nonreciprocal emitter. It shows that there is no relation between spectral angular emissivity and spectral angular absorptivity. However, specific relations can be found between spectral angular absorptivity and spectral angular emissivity of the same structure but with transposed dielectric functions. The results obtained in this work have validated the modified Kirchhoff’s laws provided by several groups during these years, such as adjoint Kirchhoff’s law and spin-resolved Kirchhoff’s law. 

Presenting Author: chiyu yang georgia Institute of Technology

Presenting Author Biography: Chiyu Yang is a third year Ph.D student in the department of Mechanical engineering at Georgia Tech under the supervision of Prof. Zhuomin Zhang. His research interests include nanoscale heat transfer, polarizationdependent thermal emission, etc.

Authors:

chiyu yang georgia Institute of Technology
Zhuomin Zhang Georgia Institute of Technology
Wenshan Cai Georgia Institute of Technology