Zhao-xian Chen

Tunable Fano resonance in hybrid graphene-metal gratings

Hybrid graphene-metal gratings with tunable Fano resonance are proposed and theoretically investigated in THz band. The grating contains alternately aligned metal and graphene stripes, which could be viewed as the superposition of two kinds of gratings with the same period. Due to different material properties, the resonance coupling between the metal and graphene parts forms typical Fano-type transmitting spectra. The related physical mechanism is studied by inspecting the induced dipole moment and local surface charge distributions at different wavelengths. Both of the resonance amplitude and frequency of the structure thus are adjustable by tuning graphenes Fermi energy and the gratings geometrical parameters. Furthermore, the Fano-type spectra are also quite sensitive to environmental indices, which supply another kind of tunability. All these features should have promising applications in tunable THz filters, switches, and modulators.

Hybrid plasmonic waveguide in a metal V-groove

We propose and investigate a type of hybrid plasmonic waveguide in a metal V-groove. A high-permittivity nanowire was placed in the metal channel covered with a dielectric film of lower permittivity. Deeper sub-wavelength confinement and much longer propagation distance were achieved in comparison with conventional channel plasmonic waveguides. The overall performance was improved as compared with the conventional hybrid plasmonic structure based on a flat metal surface. Finite element analysis showed that both the mode propagation and field profile can be adjusted by changing the nanowire radius and film thickness. Some benefits, such as a reduced scattering loss caused by the surface roughness, are also expected owing to the unique mode profile. The proposed approach has potential for application in high-level photonic integration.

Bridging the terahertz near-field and far-field observations of liquid crystal based metamaterial absorbers*

Metamaterial-based absorbers play a significant role in applications ranging from energy harvesting and thermal emitters to sensors and imaging devices. The middle dielectric layer of conventional metamaterial absorbers has always been solid. Researchers could not detect the near field distribution in this layer or utilize it effectively. Here, we use anisotropic liquid crystal as the dielectric layer to realize electrically fast tunable terahertz metamaterial absorbers. We demonstrate strong, position-dependent terahertz near-field enhancement with sub-wavelength resolution inside the metamaterial absorber. We measure the terahertz far-field absorption as the driving voltage increases. By combining experimental results with liquid crystal simulations, we verify the near-field distribution in the middle layer indirectly and bridge the near-field and far-field observations. Our work opens new opportunities for creating high-performance, fast, tunable, terahertz metamaterial devices that can be applied in biological imaging and sensing.

Tailoring entanglement through domain engineering in a lithium niobate waveguide

We propose to integrate the electro-optic (EO) tuning function into on-chip domain engineered lithium niobate (LN) waveguide. Due to the versatility of LN, both the spontaneously parametric down conversion (SPDC) and EO interaction could be realized simultaneously. Photon pairs are generated through SPDC, and the formation of entangled state is modulated by EO processes. An EO tunable polarization-entangled photon state is proposed. Orthogonally-polarized and parallel-polarized entanglements of photon pairs are instantly switchable by tuning the applied field. The characteristics of the source are theoretically investigated showing adjustable bandwidths and high entanglement degrees. Moreover, other kinds of reconfigurable entanglement are also achievable based on suitable domain-design. We believe tailoring entanglement based on domain engineering is a very promising solution for next generation function-integrated quantum circuits.

Generation of N00N State With Orbital Angular Momentum in a Twisted Nonlinear Photonic Crystal

We investigate wavefront engineering of photon pairs generated through spontaneous parametric down conversion in lithium niobate-based nonlinear photonic crystals (NPCs). Due to the complexity of domain structures, it is more convenient to describe photon interaction based on the nonlinear Huygens-Fresnel principle than conventional quasiphase matching regime. Analytical expressions are obtained to describe the transverse properties of down-converted photon states. The convenience of domain engineering in LiNbO3 crystals provides a potential platform for flexible wavefront manipulation of multiphoton states. The generation of N00N state with orbital angular momentum in a twisted NPC is studied utilizing this method. The obtained state is of great value in quantum cryptography, metrology, and lithography applications.

Tunable waveguide bends with graphene-based anisotropic metamaterials

We design tunable waveguide bends filled with graphene-based anisotropic metamaterials to achieve a nearly perfect bending effect. The anisotropic properties of the metamaterials can be described by the effective medium theory. The nearly perfect bending effect is demonstrated by finite element simulations of various structures with different bending curvatures and shapes. This effect is attributed to zero effective permittivity along the direction of propagation and matched effective impedance at the interfaces between the bending part and the dielectric waveguides. We envisage that the design will be applicable in the far-infrared and terahertz frequency ranges owing to the tunable dielectric responses of graphene.

On-chip nonmaximally entangled photon source through domain-engineering of nonlinear optical waveguide

We propose to integrate the electro-optic (EO) tuning function into entangled photons generation process in a domain-engineered lithium niobate (LN) waveguide. Through suitable domain designs, we realize the generation of electro-optically tunable nonmaximally mode-entangled photon state.