Metasurfaces have been extensively studied in the field of THz wavefront manipulation as an effective means for light field manipulation. Among them, the metasurface with geometric phase shows its natural and robust phase control ability for spin wave beams. However, the phase responses of opposite spins are locked to be opposite to each other, resulting in symmetric transmission of light. The symmetric transmission is not conducive to the multifunctional integration of optical systems, resulting in decreased efficiency of the metasurface. As a phase component of the metasurface, the transmission phase has spin-independent properties. Moreover, the research discovered that combining transmission phase gradients with geometric phase can achieve spin-decoupled manipulation of light. Based on this, we designed three spin-decoupled bifunctional THz metasurface, and spin-selective focusing, asymmetric hologram and vortex beam generation are realized. The proposed metasurface consists of an arrangement of 16 all-dielectric elliptical silicon pillar meta-atoms with different electromagnetic responses. The co-simulation method of MATLAB and CST realizes the efficient design of these meta-atoms. The average cross-polarization transmissivity of these meta-atoms exceeds 87%, and the transmission phase covers 0~2π. Utilizing these meta-atoms allows free design of spin-decoupled bifunctional THz metadevice to independently manipulate opposite spins. This work is expected to promote the development of THz metasurface devices in practical applications.
Terahertz (THz) absorbers have drawn great attention due to their potential applications in high-resolution imaging systems, sensing, and imaging. In particular, metasurface-based THz absorbers have exhibited the exotic advantage in high efficiency and broad bandwidth benefitted from the excellent abilities of metasurface in flexible modulating electromagnetic (EM) waves. However, the interactions between metasurface and EM waves are complex, and the metasurface-based absorbers have many structural parameters to optimize for high performance. Therefore, the absorbers are constrained by the manual design process with limited geometry complexity and tedious parameters sweeping. In this paper, the genetic algorithm (GA) is employed to the design of THz metasurface absorber. The EM responses of metasurface device is calculated by a simple yet powerful analytic method derived from catenary field. The employment of GA can achieve the automatic design process and demand-oriented reverse design for high performance and decreasing time consumption. As a proof-of-concept, the broadband and monolayer metasurface terahertz absorber with absorbance exceeding 80% in the frequency range from 1 to 4 THz is designed by the proposed strategy based on five typical types of metasurface. The investigations of this article present important guidance and a promising approach to design and optimize metasurface-based devices for their practical applications.
The non-ionizing radiation and the high sensitivity to weak interactions of terahertz (THz) waves make THz technology a great applicability in the field of biosensing and medical detection. Besides, benefitted from the flexible capability of metasurface in manipulating the electromagnetic (EM) waves, the high-sensitivity THz sensors can be achieved to promote the development of THz sensing. However, the polarization dependence of hybrid-resonances-based metasurface and the single resonance cause the reduction in sensitivity, which is urgent to be settled. In this paper, we proposed a centrosymmetric metasurface to produce high-quality and polarization-insensitive resonance for improving the sensitivity of THz sensing. The designed metasurface is locally asymmetric, which can induce the high-quality Fano resonance. However, the entire structure is centrosymmetric and thus exhibits polarization-independent characterization. The designed metasurface possesses a polarization-independent resonance peak of transmittance spectrum in 0.1-2THz, which also shows high sensitivity related to ambient refractive index. The advantage of transmitted structure and polarizationindependent resonance can relief the difficulty of measurement. We believe these studies will promote the development of high-sensitivity THz biosensing.
In the past decades, metasurfaces have shown their extraordinary ability to manipulate the wavefront of electromagnetic wave. However, the most of previously proposed designs based on metasurfaces are fixed once design, which are unsuitable for applications where light manipulation needs to be dynamically. In this paper, we proposed a design for dynamic wavefront manipulation achieved by the combination of metasurfaces and phase change materials (PCM) in the near infrared spectral range. Here, we present a metal-insulator-metal (MIM) configuration with the polarization conversion exceeding 80% for circular polarized (CP) light converted to its opposite handedness when PCM is in the amorphous state, but the efficiency turns to 0 when PCM switch into its crystalline state. By utilizing the Pancharatnam- Berry (PB) phase, we can achieve the dynamic wavefront manipulation between the amorphous and crystalline states. As a proof-of-concept, a deflector and focus lens are designed and characterized, and the results further verify the ability for dynamic wavefront manipulation. It is believed that the design in our work may pave the way towards the dynamic manipulation of light.
Chirality is a key molecular structural concept and a ubiquitous phenomenon in nature that has become an increasingly significant research avenue. Here, we show that an all-dielectric metasurface, an array of spatially varying anisotropic nanofins, exhibits asymmetric wavefronts for forwardly and backwardly propagating circularly polarized lights. Similar to the asymmetric transmission phenomena, two wavefronts generated by one circular polarization from both sides of the proposed metasurface are not limited to the same or mirror symmetric, but also can be arbitrarily and independently manipulated. The observation of this novel effect originates from asymmetric photonic spin-orbit interactions. As an example, a metasurface is designed to produce an optical vortex and holographic image, respectively, when a circular polarization propagates through it in opposite directions. Due to its high efficiency and multifunctionality of the proposed metasurface, this work may have potential applications in many fields, such as optical communications, and provide new ideas for studying chiral and functional materials.
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