The evolutionary trend of the polarization state can completely reflect the vectorial information of the optical field. [7,8] The optical angular momentum (OAM) includes both spin and orbital components, which are determined by the polarization and spatial degrees of freedom of the light, respectively. [9,10] As an eigenpolarization state of the cylindrical vector optical field, the charming vector properties have led to the increasing interest in vector vortex beams (VVBs) in many fields. [11,12] Similar to scalar vortex beams (SVBs), VVBs also have a helical phase wavefront exp(ilϕ) and a doughnut-shaped intensity profile, and the number of topological charges is determined by the key parameter l. [13,14] The polarization state of a VVB is not uniformly distributed in its transmitted crosssection and exhibits a distinct change with spatial plane, including radially polarized and azimuthally polarized beams with different focusing characteristics. [12] In addition, the polarization state at the center of the cross-section cannot be uniquely determined; and is thus, also known as the polarization singularity. [15,16] VVBs have been widely used for trapping and rotating particles, [17,18] quantum information, [19,20] and superresolution imaging. [21] Metasurfaces, as quasi-periodic arrays in the subwavelength scale range, have attracted extensive research interest in opto-electronics, such as focusing, [22,23] structured light, [7,15] and polarization control. [24][25][26] Metasurfaces mainly exploit the discontinuity of phase abruptness of the structured surface to Polarization plays a key role in fundamental science, and manipulating the evolutionary trends of longitudinal polarization states can provide new implements for light-matter interaction. However, existing 3D optical devices can only manipulate the polarization conversion in a single transverse plane. Recently, methods to control polarization by cascading bulky optical systems are miniaturized using metasurfaces. Nevertheless, it is quite challenging to generate focused beams with vectorial properties for metasurfaces carrying inhomogeneous polarization profiles. Here, a single-layer all-silicon metasurface operating in the terahertz (THz) band is demonstrated to address all those aforementioned challenges in one go, and pencil-like beams with inhomogeneous polarization profiles can be imparted along the propagation direction. The introduction of a latitudinal polarization control factor representing the initial phase difference enables near-even manipulation of the evolutionary trend of the longitudinal polarization state according to the extended focal depth. By manipulating the topological charges within two orthogonal circularly polarized channels to accomplish coherent superposition, a focused vortex beam with vectorial properties can be generated in desired planes. Two functional demonstrations based on this approach are established experimentally, offering promising opportunities for structured light on meta-optics.
The manipulation of polarization states is reflected in the tailoring of light–matter interactions and has great applications in fundamental science. Nevertheless, the conventional polarization‐separated detection behavior in the terahertz (THz) band is very challenging when applied to visualize the incident polarization state since its measurement requires sophisticated instrumentation. Here, the feasibility of its reconstruction of the full‐Stokes parameter matrix in the THz band is explored by establishing an all‐silicon decoupled metasurface based on the polarization multiplexing encoding technique. The pixelated focal spots gathered in the target plane allow us to employ more elaborate methods to extract the characteristic parameters of the incident polarization states. The resolvability of the THz polarization detection behavior with a single focal spot is further optimized benefiting from the longitudinal polarization component (Ez) generated by the tightly focused beam in the propagation direction. The capability of the Ez‐component in determining the key parameters that compose the polarization ellipse is evaluated by predefining the random incident polarization on a standard Poincaré sphere. Thus, the proposed scheme offers significant advantages in future THz communications, providing opportunities for ultra‐compact, high‐resolution full‐Stokes polarization imaging and multidimensional information processing.
Terahertz metamaterial biosensors have gained significant attention in the biological field due to their label-free, real-time and in-situ detection advantages. In this paper, a highly sensitive metamaterial sensor with semi-ring mirror symmetry based on toroidal dipole resonance was designed for a new metamaterial biosensor. It is shown that 337.5 GHz/RIU refractive index sensitivity can be achieved under the analyte of saturated thickness near 1.33 THz transmission dip. For biosensor samples where aflatoxin B1 was spotted on the metamaterial surface in our experiment, the dip amplitudes of transmission varying from 0.1904 to 0.203, 0.2093 were observed as aflatoxin B1 concentrations alter from 0 to 0.001 μg/mL, 0.01 μg/mL respectively. Furthermore, when aflatoxin B1 concentrations are 0.1 μg/mL, 1 μg/mL,10 μg/mL, 100 μg/mL, dip amplitudes of 0.2179, 0.226, 0.2384, 0.2527 and dip redshifts of 10.1 GHz, 20.1 GHz, 27.7 GHz, 37.6 GHz are separately provided. These results illustrate high-sensitivity and label-free detection of aflatoxin B1, enriching the applications of sensors in the terahertz domain.
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