The generation and manipulation of vector light fields are of great significance for both fundamental research and industrial applications of polarized optics. In recent years, the spatial domain control of structured vector fields has gradually expanded from two-to three-dimensional, including traditional optics and meta-optics. Here, a new method to generate and manipulate structured vector light fields along the propagation direction is proposed, and the functionality in terahertz band using all-silicon metasurfaces is demonstrated. The coherent superposition of orthogonal circularly polarized terahertz waves through long focal depth and multifocal metalens is completed, and varying phase differences between them in the propagation direction via path accumulation or initial phase design are introduced, thereby continuous variation or independently designed vector polarization distributions in multiple planes are obtained. It is worth mentioning that the proposed scheme is not only for the design of transverse electric field components, but also shows a strong ability for manipulation of the longitudinal component. This scheme realizes the polarization distribution designs of three-dimensional vector fields in three-dimensional space, and provides a new inspiration for the generation and manipulation of vector beams based on meta-optics.

Polarization control is crucial for tailoring light-matter interactions. Direct manipulation of arbitrarily incident polarized waves could provide more degrees of freedom in the design of integrated and miniaturized terahertz (THz)...

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.

exploring novel polarization generation methods has always been a key strategy to improve the efficiency of optoelectronic devices. [1,2] A linear polarizer is a specific optical device that can separate a desired linear polarization from unpolarized light, widely used for measurement, [3] detection, [4] and imaging, [5] and provides great convenience for establishing polarization optics systems. [6] Natural light contains a variety of polarization modes, and traditional cascaded optics can separate the light of the desired polarization state. Once the output polarized light has a certain polarization state defined on the surface of the Poincaré sphere, the channel can be independently modulated by introducing a phase modulation profile. [7,8] Conventional phase modulation principles include the propagation phase and geometric phase, the former can act on the circularly polarized and linearly polarized channel, respectively, while the latter is merely for circularly polarized light with conjugate locking distribution. [9] However, traditional optical polarizers based on natural materials, such as half-wave plates (HWPs), quarter-wave plates (QWPs), and beam splitters, are generally bulky and far from the goal of advanced integration and Polarization plays a key role in fundamental science, and the improvement in miniaturization and practicability of polarization conversion devices could provide more degrees of freedom for light-matter interactions. Metasurfaces that can manipulate arbitrary polarization states at subwavelength scales can significantly reduce the complexity of meta-optical systems. Here, a general design of an all-silicon diatomic metasurface operating in the terahertz band that can generate a tailorable linear polarization state by the superposition of two metaatoms with individual geometric parameters is experimentally demonstrated. By periodically arranging polarization-converting and polarization-maintaining meta-atoms, the existence of interference effects enables the proposed diatomic meta-platform to act as an optimal linear polarization operator. The gradient arrangement of the meta-molecules under the profile of the propagation phase is deduced by using the advanced Jones matrix, so that the polarization filtering and wavefront manipulation can be realized simultaneously, including the generation of tightly converged vortex and bifocal focusing beams. This demonstration of generating tailorable linear polarization states located on the Poincaré sphere directly from arbitrarily polarized waves can significantly facilitate the development of functional polarization meta-devices.