This article proposed a metastructure device that can realize polarization conversion (PC) and absorption function switching in the terahertz (THz) range based on the photoconductivity effect. The photoconductance is formed by exposing silicon to different intensities of light, then the PC and absorption function can be switched. At the same time, the absorption bandwidth is expanded by inserting air resonant cavities into the dielectric substrate, changing the thickness of the dielectric locally, and cutting rectangular slots at the metal bottom plate. When the device works as a polarization converter, linear-to-linear PC with a polarization conversion rate of over 90% at 0.96-1.47 THz can be achieved, and its relative bandwidth is 42%. And when the silicon conductivity is fixed at 3500 S/m through illuminating, the device switches to an ultra-broadband absorber with over 90% absorption at 0.75-1.73 THz and a relative bandwidth of 79%. The designed device can be applied efficiently in many fields, such as electromagnetic cloaking and communication.

Switchable ultra-broadband absorption and polarization conversion metastructure controlled by light.

It is essential to independently control the amplitude, phase, and polarization of electromagnetic waves, and there is an urgent need to integrate multiple functions independently on a single device. Here, an all‐silicon multifunctional metasurface platform that can arbitrarily and independently encode three pairs of amplitude and phase profiles into a single‐layer device is proposed. Three complex amplitude functions can be presented independently by switching the polarization states of the incident and transmitted waves. As a proof of the feasibility and flexibility of the platform, three examples are designed here: the first can generate multipair foci of equal intensity, the second can generate multipair axial bifocals of equal intensity, and the third can generate multipair foci of arbitrary intensity ratios. The experimental results are in good agreement with the simulations. The proposed multifunctional metasurface platform is powerful in manipulating the amplitude, phase, and polarization of electromagnetic waves, which can find applications in particle manipulation, biology, imaging, and other fields.

All‐Dielectric Trifunctional Metasurface Capable of Independent Amplitude and Phase Modulation

Metasurface-mediated bound states in the continuum (BIC) provides a versatile platform for light manipulation at subwavelength dimension with diverging radiative quality factor and extreme optical localization. In this work, we employ magnetic dipole quasi-BIC resonance in asymmetric silicon nanobar metasurfaces to realize giant Goos-Hanchen (GH) shift enhancement by more than three orders of wavelength. In sharp contrast to GH shift based on the Brewster dip or transmission-type resonance, the maximum GH shift here is located at the reflection peak with unity reflectance, which can be conveniently detected in the experiment. By adjusting the asymmetric parameter of metasurfaces, the $Q$-factor and GH shift can be modulated accordingly. More interestingly, it is found that GH shift exhibits an inverse quadratic dependence on the asymmetric parameter. Furthermore, we design an ultrasensitive environmental refractive index sensor based on the quasi-BIC enhanced GH shift, with a maximum sensitivity of 1.5$\times$10$^{7}$ $\mu$m/RIU. Our work not only reveals the essential role of BIC in engineering the basic optical phenomena, but also suggests the way for pushing the performance limits of optical communication devices, information storage, wavelength division de/multiplexers, and ultrasensitive sensors.

Enhancing Goos-H\"anchen shift based on magnetic dipole quasi-bound states in the continuum in all-dielectric metasurfaces

Multifunctional terahertz (THz) devices in transmission mode are highly desired in integration‐optics applications, but conventional devices are bulky in size and inefficient. While ultra‐thin multifunctional THz devices are recently demonstrated based on reflective metasurfaces, their transmissive counterparts suffer from severe limitations in efficiency and functionality. Here, based on high aspect‐ratio silicon micropillars exhibiting wide transmission‐phase tuning ranges with high transmission‐amplitudes, a set of dielectric metasurfaces is designed and fabricated to achieve efficient spin‐multiplexed wavefront controls on THz waves. As a benchmark test, the photonic‐spin‐Hall‐effect is experimentally demonstrated with a record high absolute efficiency of 92% using a dielectric metasurface encoded with geometric phases only. Next, spin‐multiplexed controls on circularly polarized THz beams (e.g., anomalous refraction and focusing) are experimentally demonstrated with experimental efficiency reaching 88%, based on a dielectric meta‐device encoded with both spin‐independent resonant phases and spin‐dependent geometric phases. Finally, high‐efficiency spin‐multiplexed dual holographic images are experimentally realized with the third meta‐device encoded with both resonant and geometric phases. Both near‐field and far‐field measurements are performed to characterize these devices, yielding results in agreement with full‐wave simulations. The study paves the way to realize multifunctional, high‐performance, and ultra‐compact THz devices for applications in biology sensing, communications, and so on.

Bifunctional Manipulation of Terahertz Waves with High‐Efficiency Transmissive Dielectric Metasurfaces

The ability to simultaneous achieve circular dichroism (CD) and wavefront manipulation is extremely important for many practical applications, especially for detecting and imaging. However, many of the previously observed weakness chiral features are limited to nanostructures with complex three-dimensional building configurations, single narrow-band response, and no active tunability, which are getting farther and away from the goal of integration and miniaturization. Here, a platform of bi-layer all-graphene meta-mirrors with spin-selective full-dimensional manipulation is proposed to simultaneously achieve giant dual-band CD response and wavefront shaping, based on the principle of the hybridization coupling. By simply controlling the structural variables of the meta-mirror and the characteristic parameters of graphene, that is, the combination of passive and active regulation, the proposed design can selectively manipulate the polarization, amplitude, phase, and working frequency of the incident circularly polarized wave near-independently. As a proof of concept, we used the meta-mirror to design two metasurface arrays with spin-selective properties for dynamic terahertz (THz) wavefront shaping and near-field digital imaging, both of which show a high-performance dynamic tunability. This method could provide additional options for the next-generation intelligent THz communication systems.

Dual-band giant spin-selective full-dimensional manipulation of graphene-based chiral meta-mirrors for terahertz waves.

Polarization manipulation of electromagnetic wave or polarization multiplexed beam shaping based on metasurfaces has been reported in various frequency bands. However, it is difficult to shape the beam with multi-channel polarization conversion in a single metasurface. Here, we propose a new method for terahertz wavefront shaping with multi-channel polarization conversion via all-silicon metasurface, which is based on the linear shape birefringence effect in spatially interleaved anisotropic meta-atoms. By superimposing the eigen- and non-eigen-polarization responses of the two kinds of meta-atoms, we demonstrate the possibility for high-efficiency evolution of several typical polarization states with two independent channels for linearly polarized waves. The measured polarization conversion efficiency is higher than 70% in the range of 0.9–1.3 THz, with a peak value of 89.2% at 1.1 THz. In addition, when more other polarization states are incident, combined with the integration of sub-arrays, we can get more channels for both polarization conversion and beam shaping. Simulated and experimental results verify the feasibility of this method. The proposed method provides a new idea for the design of terahertz multi-functional metadevices.

/pdf/terahertz-wavefront-shaping-with-multi-channel-polarization-20hmfxpsvz.pdf

Terahertz wavefront shaping with multi-channel polarization conversion based on all-dielectric metasurface

Chiral metasurfaces based on asymmetric meta-atoms have achieved artificial circular dichroism (CD), spin-dependent wavefront control, near-field imaging, and other spin-related electromagnetic control. In this paper, we propose and experimentally verify a scheme for achieving high-efficiency chiral response similar to CD of terahertz (THz) wave via phase manipulation. By introducing the geometric phase and dynamic phase in an all-silicon metasurface, the spin-decoupled terahertz transmission is obtained. The giant circular dichroism-like effect in the transmission spectrum is observed by using a random phase distribution for one of the circular polarization components. More importantly, the effect can be adjusted when we change the area of the metasurface illuminated by an incident terahertz beam. In addition, we also demonstrate the spin-dependent arbitrary wavefront control of the transmitted terahertz wave, in which one of the circularly polarized components is scattered, while the other forms a focused vortex beam. Simulated and experimental results show that this method provides a new idea for spin selective control of THz waves.

Circular dichroism-like response of terahertz wave caused by phase manipulation via all-silicon metasurface

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Terahertz wavefront shaping with multi-channel polarization conversion based on all-dielectric metasurface

Circular dichroism-like response of terahertz wave caused by phase manipulation via all-silicon metasurface