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lateral resolution [1][2][3] and in presbyopia treatment. [4,5] The reported approaches to realizing lenses with an extended focal depth, i.e., forward logarithmic axicons (FLAs), axilenses (AXLs), and light sword optical elements (LSOEs), are mainly based on radial modulation (RM) and angular modulation (AM). [6] However, the phase profiles for both RM-and AM-based lenses should continuously vary from 0 to 2π, indicating that the curvature surface of the designed element must smoothly vary to yield the desired phase, resulting in extreme difficulty in fabrication. Optical metasurfaces, the 2D counterparts of metamaterials, have opened up new avenues in manipulating the phase, amplitude, and polarization of light at subwavelength resolution. Benefiting from the unprecedented ability to manipulate the electromagnetic wavefront and ease of fabrication, a plethora of metalenses, such as dual-polarity plasmonic metalenses, [36] multifoci lenses, [37] multifunctional metalenses, [38] broadband achromatic metalenses, [39][40][41] and metalens arrays, [42] with novel functions that are challenging to achieve by using traditional lenses have been proposed and realized. In addition to the traditional metalens with a limited focal depth, the light sword metasurface lens [43] with an extended focal depth has been demonstrated. Although this approach tackles the technical challenge of fabrication of LSOEs, two intractable issues have to be urgently settled: (i) Metalenses have shown simultaneous extended focal depth and polarization-insensitive functionality.(ii) Imaging with high tolerance in the longitudinal direction has not yet been demonstrated. Although polarization-independent metalenses (with a limited focal range) have been demonstrated, [44][45][46] they are limited to high structural complexity or lose a degree of freedom in the design space. Here, we propose an approach to realize a polarization-insensitive metalens with an extended focal depth using anisotropic dielectric micropillars (geometric metasurfaces). Unlike polarization-dependent LSOEs with AM, [43] a polarization-insensitive terahertz (THz) AXL with RM is demonstrated in this paper. Under the illumination of arbitrarily polarized THz waves, this metalens shows a focal depth of ≈23λ along the propagation direction. Longitudinal high-tolerance imaging is experimentally demonstrated based on such a THz AXL. The polarization-insensitive metalens with a long focal depth may be of interest for a variety of practical applications, such as imaging, lithography, and detection.Lenses with an extended focal depth have crucial applications in highprecision optical alignment systems and optical disk readout systems. However, further development of lenses with an extended focal depth under radial and angular modulation is limited because of fabrication difficulties. Metasurfaces, 2D metamaterials, have shown unprecedented capabilities in the manipulation of the intensity, phase, and polarization of electromagnetic waves. Here, based on geometric metasurfaces, an a...

The control of spin electromagnetic (EM) waves is of great significance in optical communications. Although geometric metasurfaces have shown unprecedented capability to manipulate the wavefronts of spin EM waves, it is still challenging to independently manipulate each spin state and intensity distribution, which inevitably degrades metasurface-based devices for further applications. Here we propose and experimentally demonstrate an approach to designing spin-decoupled metalenses based on pure geometric phase, i.e., geometric metasurfaces with predesigned phase modulation possessing functionalities of both convex lenses and concave lenses. Under the illumination of left-/right-handed circularly polarized (LCP or RCP) terahertz (THz) waves, these metalenses can generate transversely/longitudinally distributed RCP/LCP multiple focal points. Since the helicity-dependent multiple focal points are locked to the polarization state of incident THz waves, the relative intensity between two orthogonal components can be controlled with different weights of LCP and RCP THz waves, leading to the intensity-tunable functionality. This robust approach for simultaneously manipulating orthogonal spin states and energy distributions of spin EM waves will open a new avenue for designing multifunctional devices and integrated communication systems.

Asymmetric transmission, defined as the difference between the forward and backward transmission, enables a plethora of applications for on-chip integration and telecommunications. However, the traditional method for asymmetric transmission is to control the propagation direction of the waves, hindering further applications. Metasurfaces, a kind of two-dimensional metamaterials, have shown an unprecedented ability to manipulate the propagation direction, phase, and polarization of electromagnetic waves. Here we propose and experimentally demonstrate a metasurface-based directional device consisting of a geometric metasurface with spatially rotated microrods and metallic gratings, which can simultaneously control the phase, polarization, and propagation direction of waves, resulting in asymmetric focusing in the terahertz region. These dual-layered metasurfaces for asymmetric focusing can work in a wide bandwidth ranging from 0.6 to 1.1 THz. The flexible and robust approach for designing broadband asymmetric focusing may open a new avenue for compact devices with potential applications in encryption, information processing, and communication.

AbstractThe spin Hall effect of light, associated with spin-orbit interactions, describes a transport phenomenon with optical spin-dependent splitting, leading to a plethora of applications such as sensing, imaging, and spin-controlled nanophotonics. Although geometric meatsurfaces can mimic photonic spin Hall effect by spatially splitting left-hand circularly polarized and right-hand circularly polarized states of electromagnetic waves with anomalous refraction or reflection angles, the geometric phase generated by metasurfaces hinders metalenses to realize simultaneous focusing of different spin states, limiting further applications. Here, we propose and experimentally demonstrate an approach to realizing a spin Hall metalens that can focus terahertz waves with different spin states and flexibly manipulate spin-dependent focal points in multiple spatial dimensions based on a pure geometric phase. A dielectric metasurface consisting of micropillars with identical shape and different in-plane orientations is designed to realize the multidimensional manipulation of photonics spin Hall effect in terahertz region. Furthermore, helicity-dependent imaging is demonstrated by the terahertz spin Hall metalens. The uniqueness and robust approach for manipulating spin photons may have a significant impact on designing ultra-compact and multifunctional devices and spin photonics devices.