Enhanced and unidirectional photonic spin Hall effect in a plasmonic metasurface with S4 symmetry

Ultrawide and unidirectional enhancements of a photonic spin Hall effect in a tilted uniaxial crystal

Since the enhancement of the photonic spin Hall effect (PSHE) is limited around the Brewster's angle, the scientific problem of how to extend the range of incident angles and to keep them unidirectional for the enhanced PSHE remains open. Here, we propose an effective method to achieve the ultrawide angle and unidirectional enhancement of PSHE via the omnidirectional Brewster's effect in a tilted uniaxial crystal. By properly setting the permittivity and the optical axial angle of the uniaxial crystal, the omnidirectional Brewster's effect can be obtained to realize an ultrawide angle enhancement of the PSHE. Then, by appropriately deviating the optical axial angle, the ultrawide enhancement of the PSHE can be achieved within the maximum incident angle range of 60° with unchanged direction. These findings inspire an unprecedented route to facilitate the applications in precision measurement and spin-dependent devices.

Generalized Brewster angle-enhanced photonic spin Hall effect in an all-dielectric metasurface

The enhancement of the photonic spin Hall effect (PSHE) is usually limited at horizontally polarized incidence and around the nonadjustable Brewster angle. In this Letter, a flexible method for enhancing the reflective PSHE with tunable incident angle under both vertically (V) and horizontally (H) polarized light has been theoretically explored. By using the multipole decomposition method, the variable generalized Brewster angle (GBA) is proven to be obtained under both V- and H-polarized light at different wavelengths in the all-dielectric metasurface. Then, owing to the large ratio of Fresnel coefficients at the GBA, the enhancement of PSHE in this Letter can not only be available for both V- and H-polarization, but also achieved at widely tunable incident angle and different operating wavelengths in the same metasurface. This work provides a simple method to achieve the flexible enhancement of PSHE and offers a novel way for designing a functional spin-based photonic device.

Dark-field spin Hall effect of light

While an optical system's symmetry ensures that the spin Hall effect of light (SHEL) vanishes at normal incidence, the question of how close to the normal incidence can one reliably measure the SHEL remains open. Here we report simulation and experimental results on the measurement of SHEL at $\sim 0.12^\circ$ away from normal incidence in the Fourier plane of a weakly focused beam of light, reflected at an air-glass interface. Measurement of transverse spin-shift due to $< 0.05^\circ$ polarization variation in the beam cross section along the X- and Y-directions is achieved in the dark-field region of the reflected beam. Our ability to measure the SHEL at near-normal incidence with no moving optomechanical parts and significantly improved sensitivity to phase-polarization variations is expected to enable several applications in the retro-reflection geometry including material characterization with significant advantages.

Directional dependent magnetooptical effect and the photonic spin Hall effect in a magnetic Weyl semimetal-based photonic crystal

The ability to generate and manipulate the directional dependent magnetooptical effect and photonic spin Hall effect is essential toward realistic unidirectional optoelectronic devices, but its exploration remains scarce. Here we theoretically identify that the multilayer structure whose unit cell is composed of a new, to the best of our knowledge, emergent magnetic Weyl semimetal layer and two anisotropic dielectric layers has the capability of creating the propagation direction dependent magnetooptical effect and photonic spin Hall effect simultaneously due to its intrinsic lack of space inversion and time reversal symmetries. Specifically, we also realize the continuous manipulation of the magnetooptical effect and photonic spin Hall effect in this structure under two opposite directions by an electrical means, which is contributed by the control of the optical properties in magnetic Weyl semimetals by Fermi energy. Our work enables an alternative strategy to achieve the directional dependent optical as well as magnetooptical effects simultaneously, which provides new perspectives in the fresh field of unidirectional optoelectronics and spin photonics.

Enhancing Goos-Hänchen shift based on magnetic dipole quasi-bound states in the continuum in all-dielectric metasurfaces

Metasurface-mediated bound states in the continuum (BIC) provides a versatile platform for light manipulation at the subwavelength dimension with diverging radiative quality factor and extreme optical localization. In this work, we theoretically propose the magnetic dipole quasi-BIC resonance in asymmetric silicon nanobar metasurfaces to realize giant Goos-Hänchen (GH) shift enhancement by more than three orders of wavelength. In sharp contrast to GH shift based on the Brewster dip or transmission-type resonance, the maximum GH shift here is located at the reflection peak with unity reflectance, which can be conveniently detected in the experiment. By adjusting the asymmetric parameter of metasurfaces, the Q-factor and GH shift can be modulated accordingly. More interestingly, it is found that GH shift exhibits an inverse quadratic dependence on the asymmetric parameter. Furthermore, we theoretically design an ultrasensitive environmental refractive index sensor based on the quasi-BIC enhanced GH shift, with a maximum sensitivity of 1.5×10⁷ μm/RIU. Our work not only reveals the essential role of BIC in engineering the basic optical phenomena but also suggests the way for pushing the performance limits of optical communication devices, information storage, wavelength division de/multiplexers, and ultrasensitive sensors.