Optical spin-symmetry breaking for high-efficiency directional helicity-multiplexed metaholograms.

Helicity-multiplexed metasurfaces based on symmetric spin-orbit interactions (SOIs) have practical limits because they cannot provide central-symmetric holographic imaging. Asymmetric SOIs can effectively address such limitations, with several exciting applications in various fields ranging from asymmetric data inscription in communications to dual side displays in smart mobile devices. Low-loss dielectric materials provide an excellent platform for realizing such exotic phenomena efficiently. In this paper, we demonstrate an asymmetric SOI-dependent transmission-type metasurface in the visible domain using hydrogenated amorphous silicon (a-Si:H) nanoresonators. The proposed design approach is equipped with an additional degree of freedom in designing bi-directional helicity-multiplexed metasurfaces by breaking the conventional limit imposed by the symmetric SOI in half employment of metasurfaces for one circular handedness. Two on-axis, distinct wavefronts are produced with high transmission efficiencies, demonstrating the concept of asymmetric wavefront generation in two antiparallel directions. Additionally, the CMOS compatibility of a-Si:H makes it a cost-effective alternative to gallium nitride (GaN) and titanium dioxide (TiO2) for visible light. The cost-effective fabrication and simplicity of the proposed design technique provide an excellent candidate for high-efficiency, multifunctional, and chip-integrated demonstration of various phenomena.

Engineering tunability through electro-optic effects to manifest a multifunctional metadevice.

Ultrafast modulation of the refractive index exhibits either linear or nonlinear electro-optic (EO) effect, which is extensively utilized in tunable photonic circuits. Silicon, a mature material for on-chip devices, lacks a strong electro-optic (EO) Pockels effect. Utilization of the Pockels effect alters the intrinsic property (refractive index) of the material that manifests tunability and offers expanded functionalities. Driven by the limited space constraints in data storage, sensing, and imaging applications, we propose an electrically tunable meta-device whose resonance wavelength and focal length can be tuned by varying applied electric fields in the visible range. The fundamental unit of a metalens is the barium titanate (BTO) diffractive optical element placed on indium titanium oxide (which serves as an electrode) with SiO2 as the substrate. The metalens' tunability is characterized by point spread function (PSF), full-width half-maximum (FWHM), and imaging bandwidth that demonstrates the tuning of resonance wavelength and focal length. Moreover, polarization-insensitive meta-holograms are realized at a wavelength of 633 nm without utilizing propagation and Pancharatnam-Berry (PB) phase. The proposed study can find exciting applications in machine vision, broadband microscopy, and spectroscopy.

Planar Achiral Metasurfaces-Induced Anomalous Chiroptical Effect of Optical Spin Isolation.

Planar chiral structures respond differently for oppositely handed incident light, and thus can produce extraordinary chiroptical effects such as circular conversion dichroism (CCD) and asymmetric transmission (AT). Such chiroptical effects are powerful tools to realize the fundamental principle of optical spin isolation, which leads to a plethora of applications such as optical conversion diodes, chiral imaging, and sensing. Here, we demonstrate the chiroptical effects of simultaneous CCD and AT through meticulously designed single-layered achiral nanofins. Our metamolecule consists of four achiral hydrogenated amorphous silicon (a-Si:H) nanofins that are carefully oriented and optimized to exhibit considerable CCD and AT. The device demonstrates a circular conversion dichroism of 55% and an asymmetric transmission of 58% at a wavelength of 633 nm. Right-hand circularly polarized light (RHCP) is completely absorbed, while left-hand circularly polarized light (LHCP) is transmitted with a polarization conversion, making it a perfect circular polarization wave isolator with negligible backscattering (due to low reflectance). This unique design and its underlying working mechanism are described comprehensively with three different techniques. These methods validate the proposed design and its methodology. For practical applications such as imaging, the proposed design realizes the Pancharatnam-Berry (PB) phase, achieving a 0-2π phase coverage for transmitted circular polarization. For the proof of concept, a metahologram is designed and demonstrated by employing the achieved full-phase control. The measured response of the fabricated metadevice not only validates the CCD and AT but also exhibits a simulated polarization conversion efficiency of up to 71% and measured efficiency up to 52%, comparable to state-of-the-art metahologram demonstrations.

Highly efficient generation of Bessel beams with polarization insensitive metasurfaces.

We present a generic approach for the generation of pseudo non-diffracting Bessel beams using polarization insensitive metasurfaces with high efficiency. Cascaded unit cells, which are fully symmetric, are designed for the complete 2π phase control in the transmission mode. Based on the topological arrangements of such unit cells, two metasurfaces for the generation of zero-order (i.e., single phase profile) and first-order (i.e., merger of two distinct phase profiles) Bessel beams are designed and characterized. Both numerical simulations and experimental measurements are in agreement with each other, confirming the electromagnetic characteristics of the reported Bessel beams. Owing to the isotropy of the unit cells and the rotational symmetry of the arrangements, the proposed metasurfaces are polarization insensitive, providing a promising avenue for achieving such wave manipulations with any linear or circular polarization.