Higher-order topological states in T-graphene and their realization in photonic crystals.

Higher-order topological states extend the power of nontrivial topological states beyond the bulk-edge correspondence. Here we study the higher-order topological states (corner states) in an open-boundary two-dimensional T-graphene lattice. Unlike the common zero-energy corner states, our findings reveal non-zero energy corner states in such lattice systems, and the energy could be controlled by modifying the hopping parameters. Moreover, the corner states could be transferred away from the lattice corners by designing the position-specific vacancy defects. The strong robustness of the corner states is also demonstrated against the uniaxial strain and vacancy defects, respectively. A plasmonic crystal is constructed to testify to the theory, in which the corner states are realized in optical modes and their higher-order topological properties are verified. Our results open the avenue of corner-states engineering, which holds significant physical implications of higher-order topological states for the design of photonic and electronic devices with specialized functionalities.

Excitonic terahertz photoconductivity in intrinsic semiconductor nanowires.

Excitonic terahertz photoconductivity in intrinsic semiconductor nanowires is studied. Based on the excitonic theory, the numerical method to calculate the photoconductivity spectrum in the nanowires is developed, which can simulate optical pump terahertz-probe spectroscopy measurements on real nanowires and thereby calculate the typical photoconductivity spectrum. With the help of the energetic structure deduced from the calculated linear absorption spectrum, the numerically observed shift of the resonant peak in the photoconductivity spectrum is found to result from the dominant exciton transition between excited or continuum states to the ground state, and the quantitative analysis is in good agreement with the quantum plasmon model. Besides, the dependence of the photoconductivity on the polarization of the terahertz field is also discussed. The numerical method and supporting theoretical analysis provide a new tool for experimentalists to understand the terahertz photoconductivity in intrinsic semiconductor nanowires at low temperatures or for nanowires subjected to below bandgap photoexcitation, where excitonic effects dominate.

The Berry phase in the nanocrystal complex composed of metal nanoparticle and semiconductor quantum dot.

We investigate the Berry phase in the nanocrystal complex made of a metal nanoparticle and a slowly rotating semiconductor quantum dot under the radiation of a circularly polarized light. The Berry phase in the dynamic system is found to be more effective to manifest the interaction between the plasmon in the metal nanoparticle and the exciton in the quantum dot. The dependences of the Berry phase on the interparticle distance and the relative position are studied in the weak field condition. The methods to observe the Berry phase are also given.

Quantum plasmon model for the terahertz photoconductivity in intrinsic semiconductor nanowires.

A quantum plasmon model for the terahertz photoconductivity in intrinsic semiconductor nanowires is developed. The classical plasmon model assumes the excited electron in semiconductors feels a restoring force generated by a harmonic-oscillator potential. Although it is successfully applied to explain the terahertz photoconductivity in semiconductor nanowires, the classical treatment of the potential weakens accurate theoretical analysis. Here I treat the potential in a full quantum way and present an exact analytical formula for photoconductivity. The formula not only gives more reasonable photoconductivity, but also has the same conciseness when compared with that of the classical plasmon model. The validity of the quantum plasmon model is proved independently by numerical calculations in real space.

Optical properties of excitons in metal-insulator-semiconductor nanowires.

The theoretical model for the metal-insulator-semiconductor nanowires is established and the optical properties are investigated. The linear absorption of the hybrid excitons, formed due to the exciton-plasmon interaction, shows obvious red shift on the magnitude of several meVs. The mechanism of the red shift is found to be the joint action of the increased excitonic binding energy attributed to the indirect Coulomb interaction and the decreased effective bandgap caused by the additional self-energy potential. The conclusion is also supported by the evolution of the absorption spectra with the adjustable structural parameters.

Ring-like solitons in plasmonic fiber waveguides composed of metal-dielectric multilayers.

We design a plasmonic fiber waveguide (PFW) composed of coaxial cylindrical metal-dielectric multilayers in nanoscale, and constitute the corresponding dynamical equations describing the propagation modes in the PFW with the Kerr nonlinearity in the dielectric layers. The physics is connected to the discrete matrix nonlinear Schrödinger equations, from which the highly confined ring-like solitons in scale of subwavelength are found both for the visible lights and the near-infrared lights in the self-defocusing condition. Moreover, when increasing the intensity of the input light the confinement can be further improved due to the cylindrical symmetry of the PFW, which means both the width and the radius of the ring are reduced.