Nonreciprocal electromagnetic scattering from a periodically space-time modulated slab and application to a quasisonic isolator

Abstract

Scattering of obliquely incident electromagnetic waves from periodically space-time modulated slabs is investigated. It is shown that such structures operate as nonreciprocal harmonic generators and spatial-frequency filters. For oblique incidences, low frequency harmonics are filtered out in the form of surface waves, while high-frequency harmonics are transmitted as space waves. In the quasi-sonic regime, where the velocity of the space-time modulation is close to the velocity of the electromagnetic waves in the background medium, the incident wave is strongly coupled to spacetime harmonics in the forward direction, while in the backward direction it exhibits low coupling to other harmonics. This nonreciprocity is leveraged for the realization of an electromagnetic isolator in the quasi-sonic regime and is experimentally demonstrated at microwave frequencies.

Electrically tunable harmonics in time-modulated metasurfaces for wavefront engineering

“…The effect of loss can be compensated by using stronger modulation depth which is equivalent to the increment in the pumping strength of modulation signal. It should be remarked that the timemodulated metasurface is an open system in which the power of transmitted and reflected harmonics is provided in part by the external agent responsible for modulation [32,33,43,44]. Here, we have limited ourselves to the range E f =0-0.6 eV for the modulation of Fermi energy level of graphene which has been shown to be accessible by electrical gating in experiments [19].…”

“…It should be noted that despite the high conversion efficiency and directionality afforded in the Huygens regime, it is strongly dependent on the incident angle and wavelength [73,74]. Higher conversion efficiencies may be maintained over the spectral and angular regime of operation using elements with more exotic geometries exhibiting multimodal directionality [75] or using multigate active elements [25,27,76] with cascaded time-modulated layers which can also potentially serve as isolators by engineering the bandgaps to realize an asymmetric dispersion [43,44].…”

“…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.…”

“…Introducing time-modulation in a structure leads to generation of higher-order frequency harmonics and breaks the time-reversal symmetry [32][33][34][35]. A significant effort has been devoted into developing magnetless nonreciprocal devices by adopting a periodic spatiotemporal modulation along the propagation direction which mimics a traveling wave motion and enables isolation and circulation of light by imparting different momentums to forward and backward propagating waves [36][37][38][39][40][41][42][43][44][45]. Moreover, timemodulated metasurfaces hold a great potential to extend the degree of light manipulation.…”

Theory and Design of Multifunctional Space-Time Metasurfaces

“…Spatiotemporal modulation can impart synthetic linear or angular momentum in the operating systems that emulate high-speed mechanical motion, and therefore produce Doppler-like nonreciprocity [22][23][24]. Based on this principle, many interesting phenomena have been explored [25], such as wave isolation [26][27][28][29][30][31][32][33][34] and circulation [35,36], one-way beam splitting [37], frequency conversion [38][39][40], and nonreciprocal antennas [23,[41][42][43]. However, the nonreciprocal applications are usually fixed and unadaptable to changing environments or application requirements.…”