Plasmonic metasurfaces act as appealing platforms for flexible and accurate wave regulation, but their functionalities in wavefront or spectral tuning are settled upon manufactured. So the ability to dynamically tailor the amplitude, phase or polarization is highly desirable for many scenarios of micro & nano photonics and the integrated optoelectronics. Here, one type of plasmonic metasurface devices for active wavefront control is demonstrated in the mid-infrared regime. Fundamentally, the switchable and tunable lensing is exemplified by phase change of refractive index and the generalized Snell's law as well as the Pancharatnam-Berry phase. As a proof of concept, the metal-insulator (MI) and metal-insulator-metal (MIM) architectures are employed with the phase change medium and an optimized energy conversion efficiency of cross polarization. In a wide spectral range between 2.8 µm and 3.8 µm, the reflective beam focuses with a cross-polarization efficiency of 80% in the amorphous state, but defocuses in the crystalline state with zero cross-polarization, i.e. the 'on' and 'off' states of the metalens. Further, both device architectures show tunable focal length from 4.4 µm to 4.8 µm (MI) and 3.4 µm and 3.6 µm (MIM) respectively upon switching between the two states. As a result, our scheme potentially paves a way to flexibly construct the active photonic devices for the reconfigurable wavefront engineering in integrated optoelectronics, i.e. beam switch, tunable steering and lensing, etc.

Active spectral tuning of nanophotonic devices offers many fascinating prospects for the realization of novel optical function. Here, switchable spectral response is enabled by the architecture of one-dimensional (1D) photonic crystal (PC) integrated with phase change material of the germanium antimony telluride (GST). Active and precise tuning of the bistable passband and central resonant frequency is demonstrated in the 1D PC composed of alternate SiN and GST nanofilms. An analytical model is derived to specify the tunable spectral features, including the band gap and resonant frequencies. Both the measured and calculated results show distinct red shifts of passband and the resonant minima (or maxima), well confirming theoretical predictions. This work demonstrates a route to construct active photonic devices with the electrically or thermally tunable spectra via 1D PC and potentially extends diverse applications based on the PC platform.

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Pyroelectric photodetector (PPD) has continuously been an attractive candidate due to its noncryogenic and easy-to-fabricate specific. For the detectivity or resolution of pyroelectric detector is closely related with the absorptivity to incident radiation, we propose a scheme of metasurface-based pyroelectric detector (MPPD) by constructing its top electrode with an absorptive metasurface composed of nanogrid-patch units. The resonance modes of Fabry-Perot cavity, magnetic dipole and electric dipole are collectively leveraged for the mid-infrared MPPD with optimal broadband absorption plus three absorptive peaks at 3.13 μm , 3.94 μm and 4.8 μm . The mechanisms and field distributions of those resonance peaks are intensively explored and verified for configuration of the metasurface top electrode with optimized geometries. Finally, our scheme achieves an average absorptivity of 94.2% in the mid-infrared range of 3-5 μm , with its thermal response enhanced by 2.6 times that results in the significantly improved pyroelectric current for the detector.