Dielectric chiral metasurface is a new type of planar and efficient chiral optical device that shows strong circular dichroism or optical activity, which has important application potential in optical sensing and display. However, the two types of chiral optical responses in conventional chiral metasurfaces are often interdependent, as their modulation of the amplitudes and phases of orthogonal circularly polarized components is correlated, which limits the further progress of chiral meta-devices. Here we propose a new scheme for independently designing the circular dichroism and optical activity of chiral metasurfaces to further control the polarization and wavefront of transmitted waves. Inspired by mixtures of chiral molecular isomers, we use the dielectric isomer resonators to form “super-units” instead of single meta-atoms for chiral responses in terahertz band, which is called racemic metasurface. By introducing two levels of Pancharatnam-Berry phases between meta-atoms and “super-units”, the polarization rotation angle and wavefront of the beam can be designed without the far-field circular dichroism. We demonstrate the strong control ability on terahertz waves of this scheme through simulation and experiments. In addition, this new type of device with near-field chirality but no far-field circular dichroism may also have important value in optical sensing and other technologies.

Moiré meta-devices facilitate continuous and precise modulation of optical properties through the alteration of the relative alignment, such as twisting, sliding, or rotating of the metasurfaces. This capability renders them particularly suitable for dynamic applications, including zoom optics and adaptive imaging systems. Nevertheless, such designs often sacrifice more complex functionalities, such as polarization manipulation, in favor of simplicity and tunability. Here, we propose and experimentally validate a design strategy for a twisted bilayer metasurface that exhibits both varifocal capabilities and polarization filtering properties. By selecting silicon pillars with polarization-maintaining properties for Layer I and polarization-converting properties for Layer II, the designed Moiré metasurface can become sensitive to specific polarization states. Experimental results demonstrate that the proposed design can generate on-demand terahertz (THz) focused beams, achieving an average focusing efficiency exceeding 35% under x-linearly polarized (x-LP) illumination. This is accomplished by systematically varying the twisting angles p and q of Layer I in relation to Layer II in increments of 30°. Additionally, we provide numerical evidence that the focal length of the transmitted vortex beam can be adjusted using the same approach. The Moiré meta-device platform, which is engineered to modulate optical properties via mechanical twisting, obviates the necessity for external power sources or active materials. This generalized design strategy has the potential to significantly expedite the commercialization of multifunctional metasurfaces, which can produce high-precision optics across various practical applications.

Diatomic metasurfaces designed for interferometric mechanisms possess significant potential for the multidimensional manipulation of electromagnetic waves, including control over amplitude, phase, frequency, and polarization. Geometric phase profiles with spin-selective properties are commonly associated with wavefront modulation, allowing the implementation of conjugate strategies within orthogonal circularly polarized channels. Simultaneous control of these characteristics in a single-layered diatomic metasurface will be an apparent technological extension. Here, spin-selective modulation of terahertz (THz) beams is realized by assembling a pair of meta-atoms with birefringent effects. The distinct modulation functions arise from geometric phase profiles characterized by multiple rotational properties, which introduce independent parametric factors that elucidate their physical significance. By arranging the key parameters, the proposed design strategy can be employed to realize independent amplitude and phase manipulation. A series of THz metasurface samples with specific modulation functions are characterized, experimentally demonstrating the accuracy of on-demand manipulation. This research paves the way for all-silicon meta-optics that may have great potential in imaging, sensing and detection.