Tunable and polarization-sensitive perfect absorber with a phase-gradient heterojunction metasurface in the mid-infrared

Multifunctional Meta-Devices for Full-Polarization Rotation and Focusing in the Near-Infrared

The creation of multi-channel focused beams with arbitrary polarization states and their corresponding optical torques finds effective applications in the field of optical manipulation at the micro-nanoscale. The existing metasurface-based technologies for polarization rotation have made some progress, but they have been limited to single functions and have not yet achieved the generation of full polarization. In this work, we propose a multi-channel and spatial-multiplexing interference strategy for the generation of multi-channel focusing beams with arbitrary polarization rotation based on all-dielectric birefringent metasurfaces via simultaneously regulating the propagation phase and the geometric phase and independently controlling the wavefronts at different circular polarizations. For the proof of concept, we demonstrate highly efficient multi-channel polarization rotation meta-devices. The meta-devices demonstrate ultra-high polarization extinction ratios and high focusing efficiencies at each polarization channel. Our work provides a compact and versatile wavefront-shaping methodology for full-polarization control, paving a new path for planar multifunctional meta-optical devices in optical manipulation at micro-nano dimensions.

A polarization-dependent perfect absorber with high Q-factors enabled by Tamm phonon polaritons in hyperbolic materials

As a natural biaxial hyperbolic material, α-phase molybdenum trioxide (α-MoO3) is highly anisotropic, making it an ideal candidate for polarization-dependent devices. Herein, using a Tamm configuration where one-dimensional photonic crystal is coated on an α-MoO3 substrate separated by a dielectric interlayer, we demonstrate the perfect absorption effect in the mid-infrared band governed by Tamm phonon polaritons. The resultant absorption peak exhibits an ultra-narrow bandwidth due to the polaritonic resonance with a high quality factor of up to 181. By varying the thickness of the interlayer, we demonstrate that near-unity absorption resonances can be tuned to a wider range of wavelengths. In addition, due to the in-plane anisotropy of α-MoO3, the device exhibits an outstanding polarization-dependent absorption performance, rendering it highly useful for various applications. Also, we show that the electronic tunability of the device is through addition of a graphene monolayer. These excellent results suggest that the designed structure could be promising in applications such as infrared absorbers, polarization detectors, sensors and energy harvesting devices.

Tunable metasurface for independent controlling radar stealth properties via geometric and propagation phase modulation

Metasurfaces have been verified as an ideal way to control electromagnetic waves within an optically thin interface. In this paper, a design method of a tunable metasurface integrated with vanadium dioxide (VO₂) is proposed to realize independent control of geometric and propagation phase modulation. The reversible conversion of VO₂ between insulator phase and metal phase can be realized by controlling the ambient temperature, which enables the metasurface to be switched quickly between split-ring and double-ring structures. The phase characteristics of 2-bit coding units and the electromagnetic scattering characteristics of arrays composed of different arrangements are analyzed in detail, which confirms the independence of geometric and propagation phase modulation in the tunable metasurface. The experimental results demonstrate that the fabricated regular array and random array samples have different broadband low reflection frequency bands before and after the phase transition of VO₂, and the 10 dB reflectivity reduction bands can be switched quickly between C/X and Ku bands, which are in good agreement with the numerical simulation. This method realizes the switching function of metasurface modulation mode by controlling the ambient temperature, which provides a flexible and feasible idea for the design and fabrication of stealth metasurfaces.

Polarization-controlled varifocal metalens with a phase change material GSST in mid-infrared

Detection of aldehyde carbonyl radiation plays an essential role in guaranteeing the safety of fried food. However, the radiation of low-content aldehyde carbonyl is always weak and includes polarized light. Focusing the weak radiation with polarization-sensitive configurations provides an efficient way to improve the signal-to-noise ratio of detection. The advent of dynamic metasurfaces based on phase-change materials (PCMs) have demonstrated superiorities over their traditional counterparts in tunability and miniaturization. In this paper, we propose two reflected varifocal metasurfaces, which combine Ge₂Sb₂Se₄Te₁ (GSST) with two materials that have close optical constants with amorphous and crystalline GSST. The first one realizes a four-spot focal system with linearly-polarized incidence based on polarization multiplexing. It adds a new polarization degree of freedom compared with traditional varifocal metasurfaces. Compared with traditional spatial-multiplexing method, our second metasurface enables the independent control of the polarization and phase profiles of circularly-polarized light. Remarkably, it reduces energy loss and crosstalk. We believe the novel scenarios of combing GSST with similar materials provide a new direction for tunable metasurfaces based on PCMs.

Phase-only femtosecond optical pulse shaping based on an all-dielectric polarization-insensitive metasurface

Recently, metasurfaces capable of manipulating the amplitude and the phase of an incident wave in a broad frequency band have been employed for femtosecond optical pulse shaping purposes. In this study, we introduce a phase-only pulse shaper based on an all-dielectric CMOS-compatible polarization-insensitive metasurface, composed of Si nano cylinders sitting on a fused silica substrate. The required phase profile of the metasurface for desired waveforms are calculated using an iterative Fourier transform algorithm, and the performance of the pulse shaper metasurface in implementing the phase masks was assessed using full-wave simulations. Such approach for realizing a polarization-insensitive metasurface-based phase-only pulse shaper has never been investigated to the best of our knowledge. It is demonstrated that the simulated results of the proposed metasurface-based pulse shaper is in great agreement with the results of the algorithm, while exhibiting a very high transmission efficiency. This work indicates yet another exciting but not fully examined application of meta-structures that is the optical pulse shaping.

Chalcogenide-based, all-dielectric, ultrathin metamaterials with perfect, incidence-angle sensitive, mid-infrared absorption: inverse design, analysis, and applications

The demand for miniature, low-cost, utmost efficient optical absorbers triggered ongoing research efforts to minimize the overall design thickness, particularly the photo-active layer, while still maintaining a high optical absorptance. In this study, we present all-dielectric nanophotonic metamaterials of optimized, fabrication compatible and tolerant, architecture for perfect mid-wave infrared absorptance. Overall sub-vacuum-wavelength thick designs are intended to couple and confine light inside an ultrathin 100 nm PbTe photo-absorbing film. Three application-oriented structures, with dimensions inversely designed to provide diverse requirements, are introduced: a two-dimensional metasurface embedded design for unpolarised wide-band absorption and two, one-dimensional metasurface embedded designs for s-polarised wide-band and non-polarised narrow-band absorption. A comprehensive study of the structures' spectral absorptance under normal- and oblique-incidence irradiation is performed. The conical-mounting absorptance analysis elucidates that the high absorption can be continuously spectrally tuned with the azimuthal component of the incidence angle. To the best of our knowledge, this property is discussed for the first time for all-dielectric metamaterials. Also, the ranges of geometrical tuning of the peak absorptance are investigated in detail, and usage of another prospective semiconductor absorber is explored. To unfold the mutual, and essentially different, physical mechanisms that fuel the perfect absorptance, an elaborated analysis is presented. The electromagnetic power transport, portrayed by the Poynting vector, displays three-dimensional singular flows around points, such as vorticity centers, saddles, sinks, and spirals. The potential mid-infrared applications which can benefit from the peculiar properties of the designed structures, such as spectroscopy, sensing, thermal radiation manipulations, and communication, are also discussed.