Photo- and exchange-field controlled line-type resonant peaks and enhanced spin and valley polarizations in a magnetic WSe2 junction

Existing resonant tunneling modes in the shape of line-type resonances can improve the transport properties of the junction. Motivated by the unique structural properties of monolayer WSe2 e.g. significant spin-orbit coupling (SOC) and large direct bandgap, the transport properties of a normal/ferromagnetic/normal (NFN) WSe2 junction with large incident angles in the presence of exchange field (h), off-resonance light (∆Ω) and gate voltage (U) is studied. In a certain interval of U, the transmission shows a gap with optically controllable width, while outside it, the spin and valley resolved transmissions have an oscillatory behavior with respect to U. By applying ∆Ω (h), an optically (electrically) switchable perfect spin and valley polarizations at all angles of incidence have been found. For large incident angles, the transmission resonances change to spin-valley-dependent separated ideal line-type resonant peaks with respect to U, resulting in switchable perfect spin and valley polarizations, simultaneously. Furthermore, even in the absence of U, applying h or ∆Ω at large incident angles can give some spin-valley dependent ideal transmission peaks, making h or ∆Ω a transmission valve capable of giving a switchable fully spinvalley filtering effect. These findings suggest some alternate methods for providing high-efficiency spin and valley filtering devices based on WSe2.

Cascaded PT-Symmetric Artificial Sheets: Multimodal Manipulation of Self-Dual Emitter-Absorber Singularities, and Unidirectional and Bidirectional Reflectionless Transparencies

We herein introduce cascaded parity-time (PT)-symmetric artificial sheets (e.g., metasurfaces or frequency selective surfaces) that may exhibit multiple higher-order laser-absorber modes and bidirectional reflectionless transmission resonances within the PT-broken phase, as well as a unidirectional reflectionless transmission resonance associated with the exceptional point (EP). We derive the explicit expressions of the gain-loss parameter required for obtaining these modes and their intriguing physical properties. By exploiting the cascaded PT structures, the gain-loss threshold for the self-dual laser-absorber operation can be remarkably lowered, while the EP remains unaltered. We further study interferometric sensing based on such a multimodal laser-absorber and demonstrate that its sensitivity could be unprecedentedly high and proportional to the number of metasurfaces along the light propagation direction.

Photonic bandgap engineering using second-order supersymmetry

First-order supersymmetry (SUSY) adapted from quantum physics to optics manipulates the transverse refractive index of guided-wave structures using a nodeless ground state to obtain intended modal content. Second-order SUSY can be implemented using excited states as a seed function, even with the presence of nodes. We apply second-order SUSY to the coupled-mode equations by recasting them as the Dirac equation. This enables the engineering of non-uniform surface corrugation of waveguide gratings and coupling potential, which encapsulates the Bragg interaction between counterpropagating modes. We show that the added bound states appear as transmission resonances inside the bandgap of the finite grating. The probability density of each state provides the longitudinal modal energy distribution in the waveguide grating. The smooth modal energy distribution of the states obtained by SUSY can mitigate longitudinal spatial hole burning in high power laser operation. We demonstrate that degenerate second-order SUSY allows the insertion of two states, which can coalesce into Friedrich-Wintgen type bound states in the continuum (BIC) for one-dimensional grating. We show that the eigenfunctions of BIC states are doubly degenerate with opposite parity, and the corresponding transmission resonances have phase changes of 2π across these states. One-dimensional BIC states can find application as robust high-speed all-optical temporal integrators by lifting restrictions on the length of various sections in the phase-shifted grating.

Scattering of a Klein–Gordon particle by a smooth barrier

We present a study of the one-dimensional Klein–Gordon equation by a smooth barrier. The scattering solutions are given in terms of the Whittaker Mκ,μ(x) function. The reflection and transmission coefficients are calculated in terms of the energy, the height, and the smoothness of the potential barrier. For any value of the smoothness parameter we observed transmission resonances.