Xiaogang Zhang

Switchable hyperbolic metamaterials with magnetic control

A switchable hyperbolic material (SHM) is investigated, with which one can turn on or off the hyperbolic dispersion of the material via magnetic control. The SHM has simple structure, with a one-dimensional periodic stacking of dielectric layer and gyromagnetic layer. The hyperbolic dispersion of SHM is due to the negative effective permeability of gyromagnetic layers, and it can be transformed into a regular circular dispersion when the d.c. magnetic field is switched off. This switchable dispersion transition is reversible, which may have great potential applications in many fields.

Enhanced wavelength sensitivity of the self-collimation superprism effect in photonic crystals via slow light.

We demonstrate that the wavelength sensitivity of a self-collimation superprism in photonic crystals (PhCs) can be greatly improved via slow light. With the help of a saddle point Van Hove singularity, we present an approach to obtain such a wavelength-sensitive self-collimation superprism. Our superprism not only has extremely high wavelength sensitivity, but also can suppress beam divergence, irregular beam generation, and wavelength channel dropout, overcoming the limitations of traditional PhC-based superprisms. Based on our superprism, a high-performance compact demultiplexer is also proposed.

Super-collimation with high frequency sensitivity in 2D photonic crystals induced by saddle-type van Hove singularities

Using detailed numerical simulations, and theoretical modeling, we predict a new super-collimation operation regime which is very sensitive on frequency. This operation regime is predicted to exist in 2D photonic crystals of dielectric rods in low index media. We explain the physical origin of this operation regime, as well as discuss how it could be of interest for implementation of low-power non-linear devices, novel sensors, as well as low-threshold lasers.

Confined one-way mode at magnetic domain wall for broadband high-efficiency one-way waveguide, splitter and bender

We find the one-way mode (OWM) can be well-confined at the magnetic domain wall by the photonic bandgap of gyromagnetic bulk material. Utilizing the well-confined one-way mode at the domain wall, we demonstrate the photonic one-way waveguide, splitter and bender can be realized with simple structures, which are predicted to be high-efficiency, broadband, frequency-independent, reflection-free, crosstalk-proof, and robustness against disorder. Additionally, we find that the splitter and bender in our proposal can be transformed into each other with magnetic control, which may have great potential applications in all photonic integrated circuit.

Disorder dependence of helical edge states in HgTe/CdTe quantum wells

We study the evolutions of the nonmagnetic disorder-induced edge states with the disorder strength in the HgTe/CdTe quantum wells. From the supercell band structures and wave-functions, it is clearly shown that the conducting helical edge states, which are responsible for the reported quantized conductance plateau, appear above a critical disorder strength after a gap-closing phase transition. These edge states are then found to decline with the increase of disorder strength in a stepwise pattern due to the finite-width effect, where the opposite edges couple with each other through the localized states in the bulk. This is in sharp contrast with the localization of the edge states themselves if magnetic disorders are doped which breaks the time-reversal symmetry. The size-independent boundary of the topological phase is obtained by scaling analysis, and an Anderson transition to an Anderson insulator at even stronger disorder is identified, in-between of which, a metallic phase is found to separate the two topologically distinct phases.In recent years, extensive attention has been focused on a new topological phase induced by nonmagnetic disorder, known as the topological Anderson insulator (TAI). In this work, we study the disorder strength dependence of the edge states in TAI phase in disordered HgTe/CdTe quantum wells. It is shown clearly that the disorder-induced edge states appear above a critical disorder strength after a gap-closing phase transition. These edge states are then found to decline with an increase of disorder strength in a stepwise pattern due to the finite-width effect, where the opposite edges couple to each other through the localized bulk states. This is in sharp contrast with the localization of the edge states themselves by time-reversal symmetry breaking. The size-independent phase boundaries are further obtained through scaling analysis, where a metallic phase is found separating two topologically distinct phases, which is due to the Fermi energy and mass renormalization.

Wide-range and tunable diffraction management using 2D rectangular lattice photonic crystals

We propose that 2D rectangular lattice photonic crystals composed of dielectric rods can be utilized for wide-range and tunable diffraction management. The control of diffraction for a normally incident beam is achieved by either properly choosing the operating frequency or changing the refractive index of the dielectric rods. The convergent, collimated, and divergent beam behaviors corresponding to a wide range of diffraction are clearly illustrated using FDTD simulations. The tunability of diffraction around the frequency of super-collimation is also analyzed and demonstrated.