Weak anti-localization and quantum oscillations of surface states in topological insulator Bi2Se2Te.

Topological insulators, a new quantum state of matter, create exciting opportunities for studying topological quantum physics and for exploring spintronic applications due to their gapless helical metallic surface states. Here, we report the observation of weak anti-localization and quantum oscillations originated from surface states in Bi2Se2Te crystals. Angle-resolved photoemission spectroscopy measurements on cleaved Bi2Se2Te crystals show a well-defined linear dispersion without intersection of the conduction band. The measured weak anti-localization effect agrees well with the Hikami-Larkin-Nagaoka model and the extracted phase coherent length shows a power-law dependence with temperature (l(Φ)∼T⁻°·44), indicating the presence of the surface states. More importantly, the analysis of a Landau-level fan diagram of Shubnikov-de Hass oscillations yields a finite Berry phase of ∼0.42π, suggesting the Dirac nature of the surface states. Our results demonstrate that Bi2Se2Te can serve as a suitable topological insulator candidate for achieving intrinsic quantum transport of surface Dirac fermions.

A planar four-port channel drop filter in the three-dimensional woodpile photonic crystal.

A compact planar channel four-port drop filter is developed experimentally and theoretically in the three-dimensional woodpile photonic crystal having a complete band gap. This consists of two waveguides separated by a defect in a single layer of the photonic crystal. Frequencies for channel dropping can be tuned throughout the band gap, by changing the size of the defect. Quality factors of approximately 1000 were measured. Simulations demonstrate directional energy transfer between the input and out put waveguides, through excitation of fields in the defect region. The planar nature of the filter is much more amenable to fabrication at optical length wavelengths.

Fine tuning resonant frequencies for a single cavity defect in three-dimensional layer-by-layer photonic crystal.

The resonant frequencies of a single cavity embedded in the three-dimensional layer-by-layer photonic crystal are studied with microwave experiments and transfer-scattering matrix method simulations. The effects of the number of cladding layers and the size of the embedded cavity on resonant frequencies and Q values are carefully examined. The fine increments of cavity size indicate a new pattern of relation between resonant frequencies and cavity sizes.