The metasurface is widely used in wave manipulation because it has an excellent capability of phase modulation in a simple way. In this work, we propose an elastic metasurface that can achieve retroreflection of a flexural wave on a thin metal plate. The elastic metasurface is composed of only two subunits with a simplified design. By carefully selecting the thickness and material of the subunits, the phase of flexural wave is re-modulated, and retroreflection is achieved at different incident angles. Furthermore, a single-phase metasurface and a two-component metasurface with fixed constraints and free boundaries, respectively, are studied. The results show that this type of elastic metasurface conveniently steers reflection of the flexural wave, and it may have potential applications in vibration control, energy harvesting, and piezoelectric sensors.

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It is well known that cats have fascinating eyes with various colors, such as green, blue, and brown. In addition, they possess strong night vision ability, which can distinguish things clearly even in a poor light environment. These drive us to reveal the secrets behind them. In fact, cats' eyes can be considered as special lenses (which we would like to mimic by using a Luneburg lens). We make an analysis of the role of photonic crystals behind the lens and demonstrate that the integration of photonic crystals into Luneburg lens can be regarded as a retroreflector and greatly improve the light focusing intensity of the lens in a broad band of frequencies. This wonderful bioinspired phenomenon is expected to design more interesting and serviceable devices by combining photonic crystals with transformation optics.

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Hyperbolic shear polaritons (HShPs) emerge with widespread attention as a class of polariton modes with broken symmetry due to shear lattices. We find a mechanism of generating quasi-HShPs(q-HShPs). When utilizing vortex waves as excitation sources of hyperbolic materials without off-diagonal elements, q-HShPs will appear. In addition, these asymmetric q-HShPs can be recovered as symmetric modes away from the source, with a critical transition mode between the left-skewed and right-skewed q-HShPs, via tuning the magnitude of the off-diagonal imaginary component and controlling the topological charge of the vortex source. It is worth mentioning that we explore the influence of parity of topological charges on the field distribution and demonstrate these exotic phenomena from numerical and analytical perspectives. Our results will promote opportunities for both q-HShPs and vortex waves, widening the horizon for various hyperbolic materials based on vortex sources and offering a degree of freedom to control various kinds of polaritons.