Engineered photonic spin Hall effect of Gaussian beam in antisymmetric parity-time metamaterials

A model of the photonic spin Hall effect(PSHE)in antisymmetric parity-time(APT)metamaterials with incidence of Gaussian beams is proposed here.We derive the displacement expression of the PSHE in APT metamaterials based on the transport properties of Gaussian beams in positive and negative refractive index materials.Furthermore,detailed discussions are provided on the APT scattering matrix,eigenstate ratio,and response near exceptional points in the case of loss or gain.In contrast to the unidirectional non-reflection in parity-time(PT)symmetric systems,the transverse shift that arises from both sides of the APT structure is consistent.By effectively adjusting the parameters of APT materials,we achieve giant displacements of the transverse shift.Finally,we present a multi-layer APT structure consisting of alternating left-handed and right-handed materials.By increasing the number of layers,Bragg oscillations can be generated,leading to an increase in resonant peaks in transverse shift.This study presents a new approach to achieving giant transverse shifts in the APT structure.This lays a theoretical foundation for the fabrication of related nano-optical devices.

Photonic spin Hall effect:fundamentals and emergent applications

The photonic spin Hall effect(SHE)refers to the transverse spin separation of photons with opposite spin angular momentum,after the beam passes through an optical interface or inhomogeneous medium,manifested as the spin-dependent splitting.It can be considered as an analogue of the SHE in electronic systems:the light’s right-circularly polarized and left-circularly polarized components play the role of the spin-up and spin-down electrons,and the refractive index gradient replaces the electronic potential gradient.Remarkably,the photonic SHE originates from the spin-orbit interaction of the photons and is mainly attributed to two different geometric phases,i.e.,the spin-redirection Rytov-Vlasimirskii-Berry in momentum space and the Pancharatnam-Berry phase in Stokes parameter space.The unique properties of the photonic SHE and its powerful ability to manipulate the photon spin,gradually,make it a useful tool in precision metrology,analog optical computing and quantum imaging,etc.In this review,we provide a brief framework to describe the fundamentals and advances of photonic SHE,and give an overview on the emergent applications of this phenomenon in different scenes.

Enhancing terahertz photonic spin Hall effect via optical Tamm state and the sensing application

The photonic spin Hall effect(PSHE),characterized by two splitting beams with opposite spins,has great potential applications in nano-photonic devices,optical sensing fields,and precision metrology.We present the significant enhancement of terahertz(THz)PSHE by taking advantage of the optical Tamm state(OTS)in In Sb-distributed Bragg reflector(DBR)structure.The spin shift of reflected light can be dynamically tuned by the structural parameters(e.g.the thickness)of the InSb-DBR structure as well as the temperature,and the maximum spin shift for a horizontally polarized incident beam at 1.1 THz can reach up to 11.15 mm.Moreover,we propose a THz gas sensing device based on the enhanced PSHE via the strong excitation of OTS for the InSb-DBR structure with a superior intensity sensitivity of 5.873×10^(4)mm/RIU and good stability.This sensor exhibits two orders of magnitude improvement compared with the similar PSHE sensor based on In Sb-supported THz long-range surface plasmon resonance.These findings may provide an alternative way for the enhanced PSHE and offer the opportunity for developing new optical sensing devices.

Modulation and enhancement of photonic spin Hall effect with graphene in broadband regions

The photonic spin Hall effect(SHE)holds great potential applications in manipulating spin-polarized photons.However,the SHE is generally very weak,and previous studies of amplifying photonic SHE were limited to the incident light in a specific wavelength range.In this paper,we propose a four-layered nanostructure of prism-graphene-air-substrate,and the enhanced photonic SHE of reflected light in broadband range of 0 THz–500 THz is investigated theoretically.The spin shift can be dynamically modulated by adjusting the thickness of air gap,Fermi energy of graphene,and also the incident angle.By optimizing the structural parameter of this structure,the giant spin shift(almost equal to its upper limit,half of the incident beam waist)in broadband range is achieved,covering the terahertz,infrared,and visible range.The difference is that in the terahertz region,the Brewster angle corresponding to the giant spin shift is larger than that of infrared range and visible range.These findings provide us with a convenient and effective way to tune the photonic SHE,and may offer an opportunity for developing new tunable photonic devices in broadband range.

Asymmetrical photonic spin Hall effect based on dielectric metasurfaces

The photonic spin Hall effect has attracted considerable research interest due to its potential applications in spincontrolled nanophotonic devices.However,realization of the asymmetrical photonic spin Hall effect with a single optical element is still a challenge due to the conjugation of the Pancharatnam-Berry phase,which reduces the flexibility in various applications.Here,we demonstrate an asymmetrical spin-dependent beam splitter based on a single-layer dielectric metasurface exhibiting strong and controllable optical response.The metasurface consists of an array of dielectric nanofins,where both varying rotation angles and feature sizes of the unit cells are utilized to create high-efficiency dielectric metasurfaces,which enables to break the conjugated characteristic of phase gradient.Thanks to the superiority of the phase modulation ability,when the fabricated metasurface is under normal incidence with a wavelength of 1550 nm,the lefthanded circular polarization(LCP)light exhibits an anomalous refraction angle of 28.9°,while the right-handed circular polarization(RCP)light transmits directly.The method we proposed can be used for the flexible manipulation of spin photons and has potentials in high efficiency metasurfaces with versatile functionalities,especially with metasurfaces in a compact space.

Breakdown of effective-medium theory by a photonic spin Hall effect

Effective-medium theory pertains to the theoretical modelling of homogenization,which aims to replace an inhomogeneous structure of subwavelength-scale constituents with a homogeneous effective medium.The effective-medium theory is fundamental to various realms,including electromagnetics and material science,since it can largely decrease the complexity in the exploration of light-matter interactions by providing simple acceptable approximation.Generally,the effective-medium theory is thought to be applicable to any all-dielectric system with deep-subwavelength constituents,under the condition that the effective medium does not have a critical angle,at which the total internal reflection occurs.Here we reveal a fundamental breakdown of the effective-medium theory that can be applied in very general conditions:showing it for deep-subwavelength all-dielectric multilayers even without a critical angle.Our finding relies on an exotic photonic spin Hall effect,which is shown to be ultrasensitive to the stacking order of deep-subwavelength dielectric layers,since the spin-orbit interaction of light is dependent on slight phase accumulations during the wave propagation.Our results indicate that the photonic spin Hall effect could provide a promising and powerful tool for measuring structural defects for all-dielectric systems even in the extreme nanometer scale.

Giant photonic spin Hall effect in momentum space in a structured metamaterial with spatially varying birefringence

The photonic spin Hall effect(SHE)in the reflection and refraction at an interface is very weak because of the weak spin-orbit interaction.Here,we report the observation of a giant photonic SHE in a dielectric-based metamaterial.The metamaterial is structured to create a coordinate-dependent,geometric Pancharatnam–Berry phase that results in an SHE with a spin-dependent splitting in momentum space.It is unlike the SHE that occurs in real space in the reflection and refraction at an interface,which results from the momentum-dependent gradient of the geometric Rytov–Vladimirskii–Berry phase.We theorize a unified description of the photonic SHE based on the two types of geometric phase gradient,and we experimentally measure the giant spin-dependent shift of the beam centroid produced by the metamaterial at a visible wavelength.Our results suggest that the structured metamaterial offers a potential method of manipulating spin-polarized photons and the orbital angular momentum of light and thus enables applications in spin-controlled nanophotonics.

Photonic spin Hall effect on the surface of anisotropic two-dimensional atomic crystals

We examine the spin-orbit interaction of light and photonic spin Hall effect on the surface of anisotropic two-dimensional atomic crystals. As an example, the photonic spin Hall effect on the surface of black phosphorus is investigated. The photonic spin Hall effect manifests itself as the spin-dependent beam shifts in both transverse and in-plane directions. We demonstrate that the spin-dependent shifts are sensitive to the orientation of the optical axis, doping concentration, and interband transitions. These results can be extensively extended to other anisotropic two-dimensional atomic crystals. By incorporating the quantum weak measurement techniques, the photonic spin Hall effect holds great promise for detecting the parameters of anisotropic two-dimensional atomic crystals.

Ultrasensitive Protein Concentration Characterization Based on Weak Measurements

An optical rotation bio-sensor based on the photonic spin Hall effect was established and applied to detecting the concentration varieties of chiral molecules.The optical rotation,introduced by sample solutions,was exploited to modulate the postselected polarization of a weak measurement system.Much work has been done in the case of glucose and fructose.However,little attention has been paid for biomolecules,such as proteins and amino acids.With this modulation,the optical rotation can be determined through the direction and spin accumulation of light spots,thus mirroring the concentration of solutions.A resolution of 2×10^(-4) degree was achieved.