Time-modulated near-field radiative heat transfer

Near-field radiative heat transfer has recently attracted increasing interests for its applications in energy technologies, such as thermophotovoltaics. Existing works, however, are restricted to time-independent systems. Here, we explore near-field radiative heat transfer between two bodies under time modulation by developing a rigorous fluctuational electrodynamics formalism. We demonstrate that time modulation can result in the enhancement, suppression, elimination, or reversal of radiative heat flow between the two bodies, and can be used to create a radiative thermal diode with an infinite contrast ratio, as well as a near-field radiative heat engine that pumps heat from the cold to the hot bodies. The formalism reveals a fundamental symmetry relation in the radiative heat transfer coefficients that underlies these effects. Our results indicate the significant capabilities of time modulation for managing nanoscale radiative heat flow.

Metasurfaces: physics and applications in wireless communications

The ever increasing number of wireless devices and systems has led to a crowded spectrum and increased the demand for versatile and multi-functional wireless apparatuses. Recently, metasurfaces have been explored as a prominent technological solution to the current paradigm of spectrum scarcity by opportunistically sharing the spectrum with various users. In general, metasurfaces are passive/dynamic, ultra-compact, multi-functional and programmable structures that are capable of both reciprocal and nonreciprocal signal-wave transmissions. The controllability and programmability of such metasurfaces are governed through DC bias and occasionally a radio-frequency modulation applied to the active components of the unit cells of the metasurface, e.g. diodes and transistors. This article overviews some of the recently proposed passive and dynamic metasurfaces and shows that metasurfaces can enhance the performance of wireless communication systems thanks to their unique physical features such as real-time signal coding, nonreciprocal-beam radiation, nonreciprocal beamsteering amplification and advanced pattern-coding multiple access communication.

Space-Time-Modulated Reconfigurable Metamaterial Based on a Field-Focused Cavity for Nonreciprocal Transmission Control and Frequency Conversion

Lorentz reciprocity is a fundamental physical property limiting advanced wave propagation control. Previously, special materials and magnetic bias were used to break the reciprocity; however, the approaches are limited by the bulky and costly implementation. To achieve nonreciprocity without magnetic bias, space-time-modulated metamaterials have been investigated for far-field wave propagation control. The metamaterial can also support wave propagation based on near-field coupling between the periodically arranged unit cells, i.e., magneto-inductive waves (MIWs). Near-field wave propagation control via the metamaterial has various significant applications; nevertheless, the potential for near-field wave propagation control has not been fully explored. Therefore, it is necessary to investigate the potential of the space-time-modulated near-field metamaterial. This paper demonstrates nonreciprocal MIW propagation control using a space-time-modulated metamaterial. To achieve field manipulation, we propose a tunable unit cell suitable for creating a cavity mode at a deep subwavelength scale (∼λ/103). Spatial field modulation, achieved by breaking the translational symmetry of the unit cells, allows for the creation of reconfigurable waveguides on the metamaterial. Temporal field modulation, achieved by breaking the capacitive symmetry of the varactor, allows for direction-dependent transmission in the waveguide. This spatiotemporal modulation successfully achieves nonreciprocal wave propagation and frequency conversion, investigated under various conditions. The proposed space-time-modulated metamaterial may provide significant advances for a wide range of systems that require dynamic, nonreciprocal, near-field wave propagation control.