Breaking temporal symmetries for emission and absorption

Abstract

Time-reversal symmetries impose stringent constraints on emission and absorption. Antennas, from radiofrequencies to optics, are bound to transmit and receive signals equally well from the same direction, making a directive antenna prone to receive echoes and reflections. Similarly, in thermodynamics Kirchhoff's law dictates that the absorptivity and emissivity are bound to be equal in reciprocal systems at equilibrium, e(ω, θ) = a(ω, θ), with important consequences for thermal management and energy applications. This bound requires that a good absorber emits a portion of the absorbed energy back to the source, limiting its overall efficiency. Recent works have shown that weak time modulation or mechanical motion in suitably designed structures may largely break reciprocity and time-reversal symmetry. Here we show theoretically and experimentally that a spatiotemporally modulated device can be designed to have drastically different emission and absorption properties. The proposed concept may provide significant advances for compact and efficient radiofrequency communication systems, as well as for energy harvesting and thermal management when translated to infrared frequencies.

Electrically tunable harmonics in time-modulated metasurfaces for wavefront engineering

“…In this section, we look at one of the highly applauded features of space-time gradient metasurfaces which is the magnetless nonreciprocal response [34,[36][37][38][39][40][41][42][43][44][45] and explore the utility of modulation-induced phase shift in obtaining a strong nonreciprocal response and isolation via spatial decomposition of frequency harmonics. The origin of nonreciprocity in space-time gradient metasurfaces is the difference in the imparted momentum by the metasurface to the forward and backward propagating waves.…”

Surface-wave-assisted nonreciprocity in spatio-temporally modulated metasurfaces

“…Breakdown of Lorentz reciprocity has also been demonstrated using spatio-temporally modulated waveguides [10][11][12] and leaky-wave antennas [13][14] . The space-time modulation approach can be particularly attractive when applied in metasurface platforms due to its advantages in reduced size, improved integrability, and for attaining extreme nonreciprocal behavior as we demonstrate in this work.…”

Broadband Nonreciprocal Amplification in Luminal Metamaterials

“…We show how the concept of "luminal grating" can yield giant nonreciprocity, achieve efficient one-way amplification, pulse compression and frequency up-conversion, proposing a realistic implementation in double-layer graphene.Temporal control of light is a long-standing dream, which has recently demonstrated its potential to revolutionize optical and microwave technology, as well as our understanding of electromagnetic theory, overcoming the stringent constraint of energy conservation [1]. Along with the ability of time-dependent systems to violate electromagnetic reciprocity [2][3][4], realising photonic isolators and circulators [5][6][7][8], amplify signals [9], perform harmonic generation [10,11] and phase modulation [12], new concepts from topological [13][14][15] and non-Hermitian physics [16,17] are steadily permeating this field.However, current limitations to the possibility of significantly fast modulation in optics has constrained the concept of time-dependent electromagnetics to the radio frequency domain, where varactors can be used to modulate capacitance [18], and traveling-wave tubes are commonly used as (bulky) microwave amplifiers [19]. In the visible and near IR, optical nonlinearities have often been exploited to generate harmonics, and realize certain nonreciprocal effects [20].…”