Controlling photonic spin Hall effect via exceptional points

Generally, most of the photonic spin Hall effect (SHE) are investigated based on the Gaussian beam, and the research of the photonic SHE in novel beams with non‐Gaussian distribution is relatively rare. The Airy beam is one of the novel beams whose initial field distribution is asymmetric and tunable. Thus, it is necessary to explore the photonic SHE in the Airy beam. Here analytical solutions for the transverse and in‐plane shifts of the Airy beam are derived, and the peculiar photonic SHE of the Airy beam is systematically researched. The theoretical research results show that the transverse shifts of the left‐ and right‐handed circularly polarized beams are asymmetric, and the in‐plane shifts of the left‐ and right‐handed circularly beams are identical. More importantly, the variation rules of the transverse and in‐plane shifts with the incident centroid position are obtained, and the incident centroid position is regulated by the decay factor of the Airy beam. That is, the spin‐dependent transverse and in‐plane shift can be manipulated flexibly and purposefully by adjusting the beam parameter. It is believed these findings will provide more ideas for exploring the application of the photonic SHE based on novel beams.

The surface plasmon resonance (SPR) with the bimetallic film can achieve minimum reflectivity and higher evanescent field enhancement than the SPR configuration with the single gold film, which makes it to be a much better option for enhancing the photonic Spin Hall Effect (SHE). It is found that the thicknesses of silver and gold films have a major impact on the photonic SHE, and the maximal transverse shift of horizontal polarization state can reach to 5.34 μm near the SPR resonant angle by optimizing the thicknesses of silver and gold films in the bimetallic film. Our results are far greater than the previously reported transverse shifts in the conventional SPR structure. This novel phenomenon is attributed to the higher evanescent field enhancement and the larger refractive index variation gradient of silver/gold bimetallic film. These findings provide us an effective method to manipulate the photonic SHE and therefore establish a strong foundation for developing photonic devices based on photonic SHE.

The photonic spin Hall effect (SHE), featured by a spin-dependent transverse shift of left- and right-handed circularly polarized light, holds great potential for applications in optical sensors, precise metrology and nanophotonic devices. In this paper, we present the significant enhancement of photonic SHE in the terahertz range by considering the InSb-supported long-range surface plasmon resonance (LRSPR) effect. The influences of the InSb/ENZ layer thickness and temperature on the photonic SHE were investigated. With the optimal structural parameters and temperature, the maximal spin shift of the horizontal polarization light can reach up to 2.68 mm. Moreover, the spin shift is very sensitive to the refractive index change of gas, and thus a terahertz gas sensing device with a superior intensity sensitivity of 2.5 × 105 μm/RIU is proposed. These findings provide an effective method to enhance the photonic SHE in the terahertz range and therefore offer the opportunity for developing the terahertz optical sensors based on photonic SHE.

The photonic spin Hall effect [1] or spin Hall effect of light [2] is the photonic analog of the spin Hall effect occurring with charge carriers in solid-state systems. Typically, this phenomenon takes place when a light beam refracts at an air-glass interface, or when it is projected onto an oblique plane, the latter effect being known as the geometric spin Hall effect of light [3]. In general, the photonic spin Hall effect leads to a polarization dependent transverse shift of a light peak intensity [3,4]. An example of the latter effect is the transverse Imbert-Federov beam shift [3], which happens for paraxial beams reflected or refracted at a sharp inhomogeneity of an isotropic optical interface. Potential applications of the photonic spin Hall effect in spin-dependent beam splitters, optical diodes [1], and surface sensors are considered in various fields in photonics, such as nanophotonics, plasmonics, metamaterials, topological optics, and quantum optics [1,2].

Parity-time (PT) symmetric systems experience phase transition between PT exact and broken phases at exceptional point. These PT phase transitions contribute significantly to the design of single mode lasers, coherent perfect absorbers, isolators, and diodes. However, such exceptional points are extremely difficult to access in practice because of the dispersive behaviour of most loss and gain materials required in PT symmetric systems. Here we introduce a method to systematically tame these exceptional points and control PT phases. Our experimental demonstration hinges on an active acoustic element that realizes a complex-valued potential and simultaneously controls the multiple interference in the structure. The manipulation of exceptional points offers new routes to broaden applications for PT symmetric physics in acoustics, optics, microwaves and electronics, which are essential for sensing, communication and imaging.

The enhanced photonic spin Hall effect (SHE) based on the D-shaped fiber with Ag-Ni alloy/silicon layers is proposed and theoretically investigated under excitation of surface plasmon resonance (SPR). In order to achieve the maximum transverse spin-dependent displacements for practical photonic devices, parameters such as the thickness of the Ag-Ni alloy and silicon layers in the D-shaped fiber are optimized. Theoretical modeling and numerical simulation demonstrate that the multilayer structure can effectively enhance the photonic SHE. The maximum transverse shift of 420 μm obtained with optimized parameters is larger than those in the literature. In addition, a maximum angular sensitivity of 114.6°/RIU is achieved by the wavelength interrogation method. Our concept and theoretical assessment suggest a novel and effective means to enhance the photonic SHE, bring us one step closer to the possibility to characterize parameters of dielectric layers by weak measurements, and accelerate the development of optical fibers based on the photonic SHE.

Parity-time (PT) symmetry was first studied in quantum mechanical systems with a non-Hermitian Hamiltonian whose observables are real-valued. Most existing designs of PT symmetric systems in electronics, optics, and acoustics rely on an exact balance of loss and gain in the media to achieve PT symmetry. However, the dispersive behavior of most loss and gain materials restricts the frequency range where the system is PT symmetric. This makes it challenging to access the exceptional points of the system to observe the PT symmetric transition dynamics. Here, we propose a new path to realize PT symmetric systems based on gyroscopic effects instead of using loss and gain units. We demonstrate that PT symmetry and the occurrence of exceptional points are preserved for inversive, counter-rotating gyroscopic systems even with dispersive sub-units. In a gyroscopic system with two circular rings rotating in opposite directions at the same speed, the spontaneous symmetry breaking across the exceptional points results in a phase transition from a moving maximum deformation location to a motionless maximum point. The motionless maximum point occurs despite the externally imposed rotation of the two rings. The results set the foundation to study nonlinear dispersive physics in PT symmetric systems, including solitary waves and inelastic wave scattering.

Classical open systems with balanced gain and loss, i.e. parity-time ($\mathcal{PT}$) symmetric systems, have attracted tremendous attention over the past decade. Their exotic properties arise from exceptional point (EP) degeneracies of non-Hermitian Hamiltonians that govern their dynamics. In recent years, increasingly sophisticated models of $\mathcal{PT}$-symmetric systems with time-periodic (Floquet) driving, time-periodic gain and loss, and time-delayed coupling have been investigated, and such systems have been realized across numerous platforms comprising optics, acoustics, mechanical oscillators, optomechanics, and electrical circuits. Here, we introduce a $\mathcal{PT}$-symmetric (balanced gain and loss) system with memory, and investigate its dynamics analytically and numerically. Our model consists of two coupled $LC$ oscillators with positive and negative resistance, respectively. We introduce memory by replacing either the resistor with a memristor, or the coupling inductor with a meminductor, and investigate the circuit energy dynamics as characterized by $\mathcal{PT}$-symmetric or $\mathcal{PT}$-symmetry broken phases. Due to the resulting nonlinearity, we find that energy dynamics depend on the sign and strength of initial voltages and currents, as well as the distribution of initial circuit energy across its different components. Surprisingly, at strong inputs, the system exhibits self-organized Floquet dynamics, including $\mathcal{PT}$-symmetry broken phase at vanishingly small dissipation strength. Our results indicate that $\mathcal{PT}$-symmetric systems with memory show a rich landscape.

The optical parity-time (PT) symmetry structure can yield unique properties, including periodicity, discreteness, nonlinearity, and so on. However, the component materials in these PT symmetry structures have been primarily restricted by their lack of tunability. Here, by utilizing the external stimulus-dependent optical properties of the Dirac semimetal, we report the theoretical prediction of the large and controllable photonic spin Hall effect in the PT symmetry structure with the slab of the Dirac semimetal. We provide evidence that the PT symmetry structure with the Dirac semimetal exhibits a large spin shift as high as the half of the waist at a certain incident angle, which is the conventionally theoretical upper limit. Due to the exceptional points, the spin shift can be enhanced effectively. Furthermore, we unravel that a small change in the Fermi energy of the Dirac semimetal on the order of 0.01 eV is able to engineer both the magnitude and sign of the spin shift. In particular, there is a transition in the spectrum of the spin shift when we vary the Fermi energy of the Dirac semimetal, where the number of the spin shift peak changes from one to two. Our results reveal the interplay between the light and the PT symmetry structure with the Dirac semimetal, which offers the possibility of developing Dirac semimetal-based spin-dependent photonic devices.

"In this work, a transverse spin-dependent shift (SDS) of the horizontal photonic spin Hall effect (PSHE) at a fixed frequency (5 THz) is simulated for detection of refractive index, chemical potential, and applied electrical voltage to a plasmonic sensor with four layers (germanium, organic layer, graphene, and gas medium). The variation of the refractive index of graphene with chemical potential and applied voltage is given from the simulated values. A refractive index resolution (1.73 × 10–12 RIU) is obtained when the refractive index n4 of the analyte layer is changed from 1 RIU to 1.1 RIU when the chemical potential μc is 0.003 eV and the amplified angle ∆ is 0.1°. A chemical potential resolution (6.32 × 10–14 eV) and corresponding voltage resolution (6.83 × 10–13 V) are obtained when the chemical potential is changed from 0.003 eV to 0.03 eV, the refractive index of gas medium n4 is 1.1 RIU and the amplified angle is ∆ = 0.1°. These values are very good in comparison with the situation when the spin Hall effect is not applied (7.47 × 10–6 RIU, 0.001166 eV, and 0.01259 V) for the same values of μc, n4, and ∆."

This work proposes a sensitivity-enhanced film bulk acoustic resonator (FBAR) humidity sensor based on a nonlinear parity-time (PT) symmetric system, which comprises three key components: an FBAR sensor, a series LC network, and a negative resistance circuit implemented with an operational amplifier. The PT -symmetric system operates at the exceptional point (EP), characterized by coalesce eigenfrequencies, leading to enhanced responsiveness to asymmetric perturbations. The nonlinear gain of the negative resistance drives the perturbed PT-symmetric system into a stable oscillatory state. Theoretical analysis demonstrates that the EP-operated PT-symmetric configuration significantly amplifies frequency shifts compared with a traditional single FBAR sensor. Experiments using an electrospun cellulose acetate/ graphene oxide (CA/GO) nanofiber-modified FBAR humidity sensor validated the theoretical predictions, with a 4 %RH perturbation inducing frequency deviations 16-fold higher than those observed in conventional configurations.

Here, we report a photonic spin Hall effect (PSHE), i.e., splitting of opposite spin in a transverse direction using a multi-layered metallo-dielectric heterostructure by hybridization of a Tamm plasmon polariton and a surface plasmon polariton. The underlying mechanism of PSHE is also explained using the concept of superposition of normal and abnormal modes in the circular-polarization basis. It is revealed that the spin-dependent transverse shift for the proposed hybrid structure is enormously high owing to the transverse shift of ∼18 μm. The PSHE is found to be strongly dependent on the number of bi-layers of the structure and the analyte refractive index, which provides an additional degree of freedom for tailoring the device performance. Therefore, this study opens a pathway for developing next-generation photonic spin Hall devices in the fields of spin photonics, optical sensing, and optical communications.

Photonic spin Hall effect: Physics, manipulations, and applications

The photonic spin Hall effect (SHE), manifesting itself as a spin-dependent splitting of left- and right-handed circularly polarized light, exhibits unique potential for nanophotonic devices, precise metrology and optical sensors. This work presents an original design of refractive index sensor for the cancer detection based on novel photonic SHE. The device consists of BK7 prism, monolayer graphene, glass substrate, and the analyte layer, in which the cell solution is introduced. Three groups of human normal (gastric, liver, epidermal) cells and corresponding cancer cells were chosen for the discrimination. The external optical pumping is applied on monolayer graphene in order to flexibly modulate the photonic SHE, and then the sensing performance can be accordingly tuned. It is found that the proposed sensor can not only discriminate the normal cells and cancer cells, but also distinguish the cancer cells for different concentrations. The superior intensity sensitivity of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${6.1} \times {10}^{{5}} \,\,\mu \text{m}$ </tex-math></inline-formula> /RIU for discriminating the gastric normal and cancer cells can be generated under the optimal pumping power, which is four orders of magnitude greater than previous reported sensors based on the resonant optical tunneling effect. These findings may provide practical applications in medical diagnose, drug discovery and cellular pathological analyse.

The photonic spin Hall effect (PSHE) originates from the spin–orbit interaction (SOI) of light. The literature indicates that the transverse spin-dependent shift, δH− (SDS), from the PSHE is weak (in the nanometer range) and difficult to measure directly. This study utilizes a plasmonic structure to improve the δH− in the PSHE. The obtained results of this study demonstrate that the inclusion of silicon nitride (Si3N4) significantly enhances the δH− relative to its absence; however, plasmonic material is present in both cases. The enhanced shifts exhibit a significant dependence on the resonance angle (θr) and the thickness of layers of the PSHE structure to attain the maximum increase in δH− of 350.82 µm at the plasmonic resonance condition. A systematic analysis of the centroid positions of the reflected beam indicates a distinct and constant separation of opposing spin components. Further, the improved δH− is utilized in cancer cell detection, as changes in the refractive index (RI) of cells facilitate the identification of cancer cells from healthy to cancerous. All examined cell types demonstrate that cancerous cells had a greater δH− than normal cells, owing to their elevated effective RI. These results illustrate that the proposed plasmonic-assisted PSHE structure offers significant enhancement and a high sensitivity of 439.30 µm/RIU for label-free detection of cancer cells.