Abstract
This paper introduces an advanced wave engineering technique based on unidirectional frequency generation and spatial decomposition in general space-time-modulated (STM) slabs. The extraordinary wave engineering provided by such slabs is comprised of unidirectional wavefront shaping, nonreciprocal frequency generation, and spatial decomposition of ST harmonics (STHs). We first derive a rigorous analytical solution for the wave scattering from general ST permittivity- and permeability-modulated (STP&PM) slabs. The analytical solution considers the most general form of spatiotemporal variation and wave incidence, i.e., oblique incidence and scattering from time-periodic spatially aperiodic/periodic STP&PM slabs. The spatial aperiodicity may be utilized for further spectrum engineering and suppression of undesired STHs. Next, it will be shown that an equilibrated ST refractive-index-modulated slab with equal permittivity and permeability modulation strengths exhibits highly enhanced nonreciprocity. The FDTD numerical simulation of general STM slabs is presented, providing extra leverage for the analyses of wave propagation and scattering in such media. We show that oblique incidence to an STM slab yields nonreciprocal spatial decomposition of generated STHs. The explored peculiar properties of STM slabs under oblique incidence is expected to pave the way for the realization of new class of subharmonic generators, radars, illusion cloaks, and spatial mixers and isolators.
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