Tunable optical spatial differential operation via photonic spin Hall effect in a Weyl semimetal.

Optical differential operation is the basic principle of optical image edge detection, which has the advantages of high efficiency, simple structure and markerless compared with the traditional digital image processing methods. In this paper, we propose an optical differential operation with high contrast based on the photonic spin Hall effect in a Weyl semimetal, which enables to switch between one- and two-dimensional edge detection. Due to the unique optical and electrical properties of the Weyl semimetal, a transport model for the differential operation is established, which is closely related to the beam shifts. By tuning the incidence conditions, we effectively manipulate the in-plane and transverse shifts to switch differential operations between one and two dimensions. The contrast of the differential operation is further regulated by changing the physical parameters of the Weyl semimetal, and can be improved by two orders of magnitude compared to the conventional differentiator. This study provides new possibilities in edge detection and image processing owing to the advantages of switchable dimension and high contrast.

Theory of quantized photonic spin Hall effect in strained graphene under a sub-Tesla external magnetic field.

The quantized photonic spin Hall effect (PSHE) in the strained graphene-substrate system is predicted under a sub-Tesla external magnetic field, which is two orders of magnitude smaller than required to produce the quantized effect in the conventional graphene-substrate system. It is found that in-plane and transverse spin-dependent splittings in the PSHE, exhibit different quantized behaviors and are closely related to the reflection coefficients. Unlike the quantized PSHE in the conventional graphene-substrate system formed by the splitting of real Landau levels, the quantized PSHE in the strained graphene-substrate system is attributed to the splitting of pseudo-Landau levels caused by the pseudo-magnetic field and the lifting of valley degeneracy of the n ≠ 0 pseudo-Landau levels induced by the sub-Tesla external magnetic field. At the same time, the pseudo-Brewster angles of the system are also quantized with the change of Fermi energy. The sub-Tesla external magnetic field and the PSHE appear as quantized peak values near these angles. The giant quantized PSHE is expected to be used for direct optical measurements of the quantized conductivities and pseudo-Landau levels in the monolayer strained graphene.

Visualization of transparent particles based on optical spatial differentiation.

Optical analog computing operates on the amplitude, phase, polarization, and frequency distributions of the electromagnetic field through the interaction of light and matter. The differentiation operation is widely used in all-optical image processing technology, such as edge detection. Here, we propose a concise way to observe transparent particles, incorporating the optical differential operation that occurs on a single particle. The particle's scattering and cross-polarization components combine into our differentiator. We achieve high-contrast optical images of transparent liquid crystal molecules. The visualization of aleurone grains (the structures that store protein particles in plant cells) in maize seed was experimentally demonstrated with a broadband incoherent light source. Avoiding the interference of stains, our designed method provides the possibility to observe protein particles directly in complex biological tissues.

Lattice-dependent spin Hall effect of light in a Weyl semimetal.

We systematically study the lattice-dependent spin Hall effect of light (SHEL) in a Weyl semimetal (WSM) by considering left-handed polarization of the incident beam, and propose a new simple method to sense the lattice spacing precisely. It is revealed that the lattice spacing plays as essential a role as the Weyl points separation in the influences on the SHEL, and the variations of SHEL shifts are closely related to the real part of Hall conductivity. Specifically, the SHEL shifts increase to the peak values first and then decrease gradually with the increase of lattice spacing, and a quantitative relationship between the SHEL and the lattice spacing is established. By simulating weak measurement experiments, the lattice-dependent SHEL shifts are amplified and measured in desirable accuracies. Subsequently, we propose a method of precisely sensing the lattice spacing based on the amplified SHEL shifts. These researches provide theoretical basis for manipulating the SHEL in WSMs, and may open the possibility of fabricating the WSM parameter sensors.