Yaping Yang

Heterostructure-based optical absorbers

We have fabricated optical absorbers based on heterostructures composed of thick metallic films and truncated all-dielectric photonic crystals. Under the tunneling mechanism, the light can enter the heterostructure without reflection and is greatly absorbed due to the strong local-field enhancement in the metallic film. With the increase in the thickness of the metal, the absorbance will tend to unity. Experiments, in good agreement with the simulations, demonstrate a maximum of absorbance close to 98%. Possible methods to realize a wide-angle or/and wideband absorption are also given.

Polarization conversion of reflected electromagnetic wave from topological insulator

Based on the special electromagnetic properties of a 3D strong topological insulator (TI), we discuss, theoretically, the reflection of electromagnetic wave at the interface between a dielectric and a TI, and focus on the polarization conversion between the incident field and reflected field. Two cases, linear polarization and elliptical polarization at oblique incidence are considered. We derive the conditions required for the complete polarization conversion from incident s polarization into reflected p polarization, and vice versa. Furthermore, elliptical polarization incidence also can be modulated to linear or circular polarization after reflection, under special conditions, and the corresponding reflectivity can approach 1. All these special polarization behaviors originate from the intrinsic topological magnetoelectric coupling response in TI. This work provides promising applications of TIs on polarized devices and the polarization splitters.

Broadband light absorption in graphene ribbons by canceling strong coupling at subwavelength scale

We theoretically investigate the broadband light absorption in the THz range by canceling the strong coupling in an array of graphene ribbons at subwavelength scale. A series of resonators with different absorption frequencies can achieve a broadband absorber, however, the suppression of absorption always accompanies since the mutual coupling between resonators cause the mode splitting. By adjusting the near- and far-field coupling between the plasmon resonances of the graphene ribbon array to the critical point, the absorption linewidth is broadened for almost one magnitude larger than that of individual graphene ribbon, to be ~1 THz. Our study provides not only insight understanding but also new approaches towards the broadband graphene absorber.

Goos–Hänchen and Imbert–Fedorov shifts at the interface of ordinary dielectric and topological insulator

Using Yasumoto and Oishi’s energy flux method, we evaluated the Goos–Hanchen (GH) and Imbert–Fedorov (IF) shifts of beam incident from the ordinary dielectric upon the topological insulator (TI) with totally internal reflection. Comparing with the case of two ordinary isotropic dielectrics, it is found that the topological parameter Θ of TI can affect two shifts. More important, IF shift appears even for a linear polarized TE or TM beam and achieves the maximum with elliptical polarization, which completely originates from the TI’s intrinsic magnetoelectric coupling effect. This observation provides an optical experimental approach to determine the topological parameters Θ and provide a new way to control the GH shift and IF shift.

Collective multiphoton blockade in cavity quantum electrodynamics

We present a study of collective multi-photon blockade in coherently driven atoms in a single mode cavity. Considering two atoms strongly coupled to an optical cavity, we show that the two-photon blockade with two-photon anti-bunching, and the three-photon blockade with three-photon anti-bunching can be observed simultaneously. The three-photon blockade probes both dressed states in the two photon and three photon spaces. The two photon and three photon blockades strongly depend on the location of two atoms in the strong coupling regime. The asymmetry in the atom-cavity coupling constants opens new pathways for multiphoton blockade which is also shown to be sensitive to the atomic decay and pumping strengths. The work presented here predicts many new quantum statistical features due to the collective behavior of atoms and can be useful to generate non-classical photon pairs.

Optical nonreciprocity of a single two-level atom in a cavity

We analyze the transmission character of a single-mode cavity containing a resonant two-level atom. Such a simple structure exhibits optical nonlinearity and spatial symmetry breaking, as well as time-reversal symmetry breaking. We perform a detailed analysis in the Purcell regime and find out that there is giant optical nonreciprocity in proper parameter space. Therefore, it is feasible to assemble such a simple atom–cavity structure to realize an acceptable optical nonreciprocal device after carefully designing.

Enhanced displacements in reflected beams at hyperbolic metamaterials.

We examine the Goos-Hänchen (G-H) shift of a Gaussian beam reflected on a thin slab of Ag/TiO2 hyperbolic multilayer metamaterial (HMM). The HMM is modeled using the effective medium theory which yields the anisotropic dielectric functions of the HMM. The G-H shifts can be very large on the surface of the HMM. It can be about 40 µm which are far bigger than the G-H shifts on the usual materials like metals and dielectrics. The enhancement is due to the excitation of the Brewster modes in HMM. Such Brewster modes in HMM have a well-defined frequency-dependent line shape. We relate the the half width at half maximum of the G-H shift to the imaginary part of the complex frequency of the Brewster mode. Moreover, we also present results for the Imbert-Fedorov shifts as well as the spin Hall effect of light on the surface of a thin HMM slab. We show that the spin Hall effect on the HMM slab is much more pronounced than that on the surface of metal. Thus a thin HMM slab can be used to enhance the lateral displacements, which can have many interesting applications for optical devices.

High contrast all-optical diode based on direction-dependent optical bistability within asymmetric ring cavity

We propose a simple all-optical diode which is comprised of an asymmetric ring cavity containing a two-level atomic ensemble. Attributed to spatial symmetry breaking of the ring cavity, direction-dependent optical bistability is obtained in a classical bistable system. Therefore, a giant optical non-reciprocity is generated, which guarantees an all-optical diode with a high contrast up to 22 dB. Furthermore, its application as an all-optical logic AND gate is also discussed.

Collective dynamics and entanglement of two distant atoms embedded into single-negative index material

We study the dynamics of two two-level atoms embedded near to the interface of paired meta-material slabs, one of negative permeability and the other of negative permittivity. This combination generates a strong surface plasmon field at the interface between the meta-materials. It is found that the symmetric and antisymmetric modes of the two-atom system couple to the plasmonic field with different Rabi frequencies. Including the Ohmic losses of the materials we find that the Rabi frequencies exhibit threshold behaviour which distinguish between the non-Markovian (memory preserving) and Markovian (memoryless) regimes of the evolution. Moreover, it is found that significantly different dynamics occur for the resonant and an off-resonant couplings of the plasmon field to the atoms. In the case of the resonant coupling, the field does not appear as a dissipative reservoir to the atoms. We adopt the image method and show that the dynamics of the two atoms coupled to the plasmon field are analogous to the dynamics of a four-atom system in a rectangular configuration. A large and long living entanglement mediated by the plasmonic field in both Markovian and non-Markovian regimes of the evolution is predicted. We also show that a simultaneous Markovian and non-Markovian regime of the evolution may occur in which the memory effects exist over a finite evolution time. In the case of an off-resonant coupling of the atoms to the plasmon field, the atoms interact with each other by exchanging virtual photons which results in the dynamics corresponding to those of two atoms coupled to a common reservoir. In addition, the entanglement is significantly enhanced.

Lossless Kerr-phase gate in a quantum-well system via tunneling interference effect for weak fields

We examine a Kerr phase gate in a semiconductor quantum well structure based on the tunnelling interference effect. We show that there exist a specific signal field detuning, at which the absorption/amplification of the probe field will be eliminated with the increase of the tunnelling interference. Simultaneously, the probe field will acquire a −π phase shift at the exit of the medium. We demonstrate with numerical simulations that a complete 180◦ phase rotation for the probe field at the exit of the medium is achieved, which may result in many applications in information science and telecommunication.