If a metalens integrates the circular polarization (CP) conversion function, the focusing lens together with circular-polarizing lens (CPL) in traditional cameras may be replaced by a metalens. However, in terahertz (THz) band, the reported metalenses still do not obtain the perfect and strict single-handed CP, because they were constructed via Pancharatnam-Berry phase so that CP conversion contained both left-handed CP (LCP) and right-handed CP (RCP) components. In this paper, a silicon based THz metalens is constructed using dynamic phase to obtain single-handed CP conversion. Also, we can rotate the whole metalens at a certain angle to control the conversion of multi-polarization states, which can simply manipulate the focusing for incident linear polarization (LP) THz wave in three polarization conversion states, including LP without conversion, LCP and RCP. Moreover, the polarization conversion behavior is reversible, that is, the THz metalens can convert not only the LP into arbitrary single-handed CP, but also the LCP and RCP into two perpendicular LP, respectively. The metalens is expected to be used in advanced THz camera, as a great candidate for traditional CPL and focusing lens group, and also shows potential application in polarization imaging with discriminating LCP and RCP.
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.
Vortex beam has attracted much attention for carrying orbital angular momentum (OAM). It has a helical phase structure described by exp (ilθ) and an annular intensity distribution, where l is the topological charge corresponding to the OAM of each photon, and θ is the azimuth angle. [1] Vortex beam is widely used in OAM communications, [2,3] optical tweezers, [4] and quantum information coding. [5,6] In particular, the superposition of OAM states is very significant in metrology, [7,8] quantum science, [6,9] and OAM communications. The superposition of high-order OAM modes can be used for ultra-sensitive angle measurement. [8] In Bose-Einstein condensate, multiple OAM states can also be used to generate arbitrary superpositions of atomic rotation states. [10] Vortex beam carrying OAM has shown promising prospects in increasing the data capacity of communication systems, because multiple orthogonal modes can be transmitted at the same frequency in a single communication channel simultaneously.As we all know, photons carry two different kinds of angular momentums, namely spin angular momentum (SAM) and OAM. [11] The SAM represents the polarization state of light, and the OAM is related to the spatial distribution of light. [1,12] Although mode conversion, [13] holograms, [14] spiral phase plates, [15] and spiral zone plates [16][17][18] can be used to generate vortex beam, they usually do not involve the interaction between SAM and OAM. [19] Different from the above, the geometric phase elements can establish a connection between SAM and OAM, but only two conjugate vortex states can be output. Devlin presented a method realizing arbitrary spin-to-OAM conversion, overcoming this limitation mentioned above. [20] In addition, when the paraxial beam is tightly focused, a strong longitudinal component will be generated in the focal area. [21] It is demonstrated that, in a highly focused system, a circularly polarized beam partly transfers its incident SAM to OAM and a helical phase in the longitudinal component of the electric field can be generated. [22] Due to their potential breakthroughs in optical manipulation, metasurfaces have attracted widespread attention in the scientific community. [23,24] Metasurfaces have been widely used in many applications, such as lenses, [25,26] spin Hall effect, [27] holograms, [28] and vortex generators. [29][30][31][32][33] In order to further Vortex beams, carrying orbital angular momentum (OAM), have plenty of applications ranging from particle manipulation to high-capacity data transmissions. In particular, the superpositions of OAM patterns are significant in classical physics and quantum science. The flexible control of spin angular momentum (SAM) to OAM can provide more freedom for the design of multifunctional devices. Here, a kind of dielectric metasurface is proposed that generates polarization-controllable superpositions of OAM patterns in the terahertz (THz) band, which is achieved by the interleaving of anisotropic and isotropic meta-atoms. The conversions of arbitrary S...
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