Violating Kirchhoff's Law of Thermal Radiation in Semitransparent Structures

TL;DR

This paper demonstrates a novel nonreciprocal semitransparent emitter that surpasses Kirchhoff's law constraints by using magneto-optical effects and guided-mode resonance, enhancing energy harvesting efficiency.

Contribution

It introduces a design for a nonreciprocal, nearly ideal emitter based on magnetic Weyl semimetals and photonic crystals, breaking Kirchhoff's law constraints.

Findings

Achieves near-perfect unidirectional absorption and emission.

Utilizes magneto-optical effects and guided-mode resonance.

Provides a practical design with realistic materials.

Abstract

Kirchhoff's law of thermal radiation imposes a constraint on photon-based energy harvesting processes since part of the incident energy flux is inevitably emitted back to the source. By breaking the reciprocity of the system, it is possible to overcome this restriction and improve the efficiency of energy harvesting. Here, we design and analyze a semitransparent emitter that fully absorbs normally incident energy from a given direction with zero backward and unity forward emissivity. The nearly ideal performance with wavelength-scale thickness is achieved due to the magneto-optical effect and the guided-mode resonance engineered in the emitter structure. We derive the general requirements for the nonreciprocal emitter using the temporal coupled mode theory and the symmetry considerations. Finally, we provide a realistic emitter design based on a photonic crystal slab consisting of a magnetic Weyl semimetal and silicon.