Dezhuan Han

Plasmonic analog of electromagnetically induced transparency in nanostructure graphene

Graphene has shown intriguing optical properties as a new class of plasmonic material in the terahertz regime. In particular, plasmonic modes in graphene nanostructures can be confined to a spatial size that is hundreds of times smaller than their corresponding wavelengths in vacuum. Here, we show numerically that by designing graphene nanostructures in such deep-subwavelength scales, one can obtain plasmonic modes with the desired radiative properties such as radiative and dark modes. By placing the radiative and dark modes in the vicinity of each other, we further demonstrate electromagnetically induced transparency (EIT), analogous to the atomic EIT. At the transparent window, there exist very large group delays, one order of magnitude larger than those offered by metal structures. The EIT spectrum can be further tuned electrically by applying a gate voltage. Our results suggest that the demonstrated EIT based on graphene plasmonics may offer new possibilities for applications in photonics.

Effective plasma frequency in one-dimensional metallic-dielectric photonic crystals

Photonic band structures of one-dimensional (1D) metallic-dielectric photonic crystals (MDPCs) are studied theoretically. We show that a 1D MDPC can be considered as an effective metallic medium with a well-defined effective plasma frequency. This effective plasma frequency is found to be inversely proportional to the optical thickness of the dielectric layer and is independent of either the constituent metal or the thickness of the metallic layer. By increasing the optical thickness of the dielectric layer, the effective plasma frequency of a 1D MDPC can be depressed into extremely low frequencies such as far infrared or even below.

Dirac Spectra and Edge States in Honeycomb Plasmonic Lattices

We study theoretically the dispersion of plasmonic honeycomb lattices and find Dirac spectra for both dipole and quadrupole modes. Zigzag edge states derived from Dirac points are found in ribbons of these honeycomb plasmonic lattices. The zigzag edge states for out-of-plane dipole modes are closely analogous to the electronic ones in graphene nanoribbons. The edge states for in-plane dipole modes and quadrupole modes, however, have rather unique characters due to the vector nature of the plasmonic excitations. The conditions for the existence of plasmonic edge states are derived analytically.

Direct observation of valley-polarized topological edge states in designer surface plasmon crystals

The extensive research of two-dimensional layered materials has revealed that valleys, as energy extrema in momentum space, could offer a new degree of freedom for carrying information. Based on this concept, researchers have predicted valley-Hall topological insulators that could support valley-polarized edge states at non-trivial domain walls. Recently, several kinds of photonic and sonic crystals have been proposed as classical counterparts of valley-Hall topological insulators. However, direct experimental observation of valley-polarized edge states in photonic crystals has remained difficult until now. Here, we demonstrate a designer surface plasmon crystal comprising metallic patterns deposited on a dielectric substrate, which can become a valley-Hall photonic topological insulator by exploiting the mirror-symmetry-breaking mechanism. Topological edge states with valley-dependent transport are directly visualized in the microwave regime. The observed edge states are confirmed to be fully valley-polarized through spatial Fourier transforms. Topological protection of the edge states at sharp corners is also experimentally demonstrated.The photonic valley-Hall effect can enable the unidirectional propagation of edge states, but often require covers which shield the states from direct measurement. Here, Wu et al. realize photonic valley-Hall effect using designer surface plasmons, enabling the direct observation of topological states.

Tunable terahertz radiation from graphene induced by moving electrons

Based on a structure consisting of a single graphene layer situated on periodic dielectric gratings, we show theoretically that terahertz radiation can be generated by low-energy electron bunches moving atop the graphene layer. The THz emission arises from graphene plasmons excited efficiently by the moving electrons. We find that the radiation intensity can be strongly enhanced due to the local field enhancement of graphene plasmons arising from their low losses and high confinement. Importantly, the radiation frequency can be tuned over a wide spectral range by varying the Fermi level of the graphene layer. Our results could find applications in developing tunable and miniature free-electron terahertz radiation sources.

Core–Shell-Structured Dielectric–Metal Circular Nanodisk Antenna: Gap Plasmon Assisted Magnetic Toroid-like Cavity Modes

Plasmonic nanoantennas, the properties of which are essentially determined by their resonance modes, are of interest both fundamentally and for various applications. Antennas with various shapes, geometries, and compositions have been demonstrated, each possessing unique properties and potential applications. Here, we propose the use of a sidewall coating as an additional degree of freedom to manipulate plasmonic gap cavity modes in strongly coupled metallic nanodisks. It is demonstrated that for a dielectric middle layer with a thickness of a few tens of nanometers and a sidewall plasmonic coating of more than ten nanometers, the usual optical magnetic resonance modes are eliminated, and only magnetic toroid-like modes are sustainable in the infrared and visible regime. All of these deep-subwavelength modes can be interpreted as an interference effect from the gap surface plasmon polaritons. Our results will be useful in nanoantenna design, high-Q cavity sensing, structured light-beam generation, and pho...

Extrinsic photonic crystals: Photonic band structure calculations of a doped semiconductor under a magnetic field

Doped semiconductors are intrinsically homogeneous media. However, by applying an external magnetic field that has a spatially periodic variation, doped semiconductors can behave extrinsically like conventional photonic crystals. The authors show this possibility theoretically by calculating the photonic band structures of a doped semiconductor under an external, spatially periodic magnetic field. Homogeneous media, behaving like conventional photonic crystals under some external, spatially periodic fields, define another kind of photonic crystals: extrinsic photonic crystals.

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.

Nonlocal optical properties in periodic lattice of graphene layers

Based on the effective medium model, nonlocal optical properties in periodic lattice of graphene layers with the period much less than the wavelength are investigated. Strong nonlocal effects are found in a broad frequency range for TM polarization, where the effective permittivity tensor exhibits the Lorentzian resonance. The resonance frequency varies with the wave vector and coincides well with the polaritonic mode. Nonlocal features are manifest on the emergence of additional wave and the occurrence of negative refraction. By examining the characters of the eigenmode, the nonlocal optical properties are attributed to the excitation of plasmons on the graphene surfaces.

Enhanced transmission mediated by guided resonances in metallic gratings coated with dielectric layers

Transmission properties of metallic gratings coated symmetrically with a dielectric layer on both sides are studied theoretically. For subwavelength narrow slits, besides cavity resonances in slits and surface plasmon-polaritons, a new kind of mechanisms for enhanced transmission in coated metallic gratings, namely, guided resonances in the dielectric coating layers, is found. Transmission peaks mediated by guided resonances are found to be much sharper than those mediated by cavity or surface plasmon-polariton resonances.