Journal Articles by Liu feng
Topological photonic crystals are designed based on the concept of Zak's phase rather than the to... more Topological photonic crystals are designed based on the concept of Zak's phase rather than the topological invariants such as the Chern number and spin Chern number, which rely on the existence of a nonvanishing Berry curvature. Our photonic crystals (PCs) are made of pure dielectrics and sit on a square lattice obeying the C 4v point-group symmetry. Two varieties of PCs are considered: one closely resembles the electronic two-dimensional Su-Schrieffer-Heeger model, and the other continues as an extension of this analogy. In both cases, the topological transitions are induced by adjusting the lattice constants. Topological edge modes (TEMs) are shown to exist within the nontrivial photonic band gaps on the termination of those PCs. The high efficiency of these TEMs transferring electromagnetic energy against several types of disorders has been demonstrated using the finite-element method.
Received Day Month Day Revised Day Month Day Coherent terahertz radiations based on intrinsic Jos... more Received Day Month Day Revised Day Month Day Coherent terahertz radiations based on intrinsic Josephson junctions from a Bi2Sr2CaCu2O8+δ mesa under a bias voltage were observed in 2007. Later on a novel π phase kink state was proposed which can explain the key experimental results. However, how to drive the system into the π phase kink state efficiently still remains an open problem. In this work we propose several ways to synchronize the system to the π phase kink state. We start from random and typical regular initial phase configurations. It is found that starting from some initial phase configurations the system can be driven efficiently into the π phase kink state. The time spent before entering the π phase kink states for these initial phase configurations is shorter when the initial phase configuration is further away from the uniform phase configuration.
It is found that coherent electromagnetic (EM) radiation in terahertz (THz) band takes place when... more It is found that coherent electromagnetic (EM) radiation in terahertz (THz) band takes place when a single crystal of cuprate high-T c superconductor Bi 2 Sr 2 CaCu 2 O 8+δ (BSCCO) is biased by a dc voltage. In this work we study the inductively coupled sine-Gordon equations which compose a good model for intrinsic Josephson junctions (IJJs) realized in BSCCO due to the strong layer structure. We derive a general solution for the coupled sine-Gordon equations valid for weak and moderate inductive couplings, which can enhance injection of dc energy into IJJs and convert it to THz EM radiation. This solution evolves into the π phase kink state known before for strong inductive coupling.
In order to enhance the radiation power in the terahertz band based on intrinsic Josephson juncti... more In order to enhance the radiation power in the terahertz band based on intrinsic Josephson junctions of Bi 2 Sr 2 CaCu 2 O 8+δ single crystal, we investigate a long cylindrical sample embedded in a dielectric material. Tuning the dielectric constant, the radiation power has a maximum which is achieved when it equals the dissipation caused by the Josephson plasma. This yields the optimal dielectric constant of the wrapping material in terms of the properties of a BSCCO single crystal. The maximal radiation power is found proportional to the product of the critical superconducting current squared and the typical normal resistance, which offers a guideline for choosing superconductors as a source of strong radiation. (Some figures may appear in colour only in the online journal)
It is found that coherent electromagnetic (EM) radiation in terahertz (THz) band takes place when... more It is found that coherent electromagnetic (EM) radiation in terahertz (THz) band takes place when a single crystal of cuprate high-T c superconductor Bi 2 Sr 2 CaCu 2 O 8+δ (BSCCO) is biased by a dc voltage. In this work we study the inductively coupled sine-Gordon equations which compose a good model for intrinsic Josephson junctions (IJJs) realized in BSCCO due to the strong layer structure. We derive a general solution for the coupled sine-Gordon equations valid for weak and moderate inductive couplings, which can enhance injection of dc energy into IJJs and convert it to THz EM radiation. This solution evolves into the π phase kink state known before for strong inductive coupling.
In order to enhance the radiation power in the terahertz band based on intrinsic Josephson juncti... more In order to enhance the radiation power in the terahertz band based on intrinsic Josephson junctions of Bi 2 Sr 2 CaCu 2 O 8+δ single crystal, we investigate a long cylindrical sample embedded in a dielectric material. Tuning the dielectric constant, the radiation power has a maximum which is achieved when it equals the dissipation caused by the Josephson plasma. This yields the optimal dielectric constant of the wrapping material in terms of the properties of a BSCCO single crystal. The maximal radiation power is found proportional to the product of the critical superconducting current squared and the typical normal resistance, which offers a guideline for choosing superconductors as a source of strong radiation.
We numerically study the energy band structures and the corresponding wavefunctions of carbon nan... more We numerically study the energy band structures and the corresponding wavefunctions of carbon nanotubes under circularly polarized irradiation perpendicular to the tube axis on the basis of the Floquet–Bloch theory. We focus on two typical irradiation frequencies, ħΩ B γ and ħΩ > γ, where γ ? 3 eV is the hopping energy of graphene. Circularly polarized irradiation is found to open gaps for metallic zigzag nanotubes near the Fermi energy and shift the degenerate points of armchair nanotubes in the energy spectra away from the K and K$ points. Furthermore, high-frequency irradiation localizes the wavefunctions on either side of the nanotubes; in particular, the localized wavefunctions have different valley indices on each side of the nanotubes.
In the main text we displayed the ribbon spectra of nontrivial topological phases for w = 1.0 and... more In the main text we displayed the ribbon spectra of nontrivial topological phases for w = 1.0 and v = 2.0, which did not have obvious band gaps. Here we show the ribbon spectra of topological nontrivial phases for other two typical cases, namely ones with large complete and small local band gaps in Fig. S1. It is noticed that even for a very small local band gap, the edge states still exists, which implies the topological nature of the edge states.
We present a two-dimensional (2D) lattice model that exhibits a nontrivial topological phase in t... more We present a two-dimensional (2D) lattice model that exhibits a nontrivial topological phase in the absence of the Berry curvature. Instead, the Berry connection provides the topological nontrivial phase in the model, whose integration over the momentum space, the so-called 2D Zak phase, yields a fractional wave polarization in each direction. These fractional wave polarizations manifest themselves as degenerated edge states with opposite parities in the model.
Surface plasma waves (SPWs) are usually discussed in the context of a metal in contact with a die... more Surface plasma waves (SPWs) are usually discussed in the context of a metal in contact with a dielectric. However, they can also exist between two metals. In this work we elucidate their properties and derive some experimentally important length scales. We find that the SPW dispersion always cuts the light line, implying the possibility of optical coupling without the need of special structural features to match the momentum of light and that of SPWs. We propose practical schemes to excite them and evaluate the excitation efficiency. Topological aspects of these waves are also discussed.
Thesis Chapters by Liu feng
Papers by Liu feng
EPL (Europhysics Letters), 2016
Surface plasma waves (SPWs) are usually discussed in the context of a metal in contact with a die... more Surface plasma waves (SPWs) are usually discussed in the context of a metal in contact with a dielectric. However, they can also exist between two metals. In this work we study these bimetallic waves. We find that their dispersion curve always cuts the light line, which allows direct optical coupling without surface grating structures. We propose practical schemes to excite them and the excitation efficiency is estimated. We also show that these waves can be much less lossy than conventional SPWs and their losses can be systematically controlled, a highly desirable attribute in applications. Conducting metal oxides are apt for experimental studies. Surface plasma waves (SPWs) are widely known as charge density undulations propagating at the interface between a metal and a dielectric (such as vacuum) [1-4]. Due to highly localized electromagnetic (EM) fields associated with them, SPWs have seen applications in many areas such as Raman scattering [5, 6], near-field spectroscopy [7] and bio-sensing [8-10]. In the past decade or so, SPWs have emerged as a pivotal player in sub-wavelength optics [11, 12], where it is necessary to render efficient coupling between light (propagating EM waves) and SPWs. As the wavelength of SPW is much shorter than that of light at the same frequency (momentum mismatch), the desired coupling can be achieved only via contrived surface structures such as grating [3]. In this Letter, we investigate SPWs supported at the interface between two metals and show that such SPWs can couple to light without gratings. Although these waves were noticed in early studies [13-16] and are garnering interest in the field of core-shell nano-particles [17, 18], a comprehensive understanding, especially of their coupling with light, has yet to emerge [19]. We
Physical Review B, 2018
Topological photonic crystals are designed based on the concept of Zak's phase rather than the to... more Topological photonic crystals are designed based on the concept of Zak's phase rather than the topological invariants such as the Chern number and spin Chern number, which rely on the existence of a nonvanishing Berry curvature. Our photonic crystals (PCs) are made of pure dielectrics and sit on a square lattice obeying the C 4v point-group symmetry. Two varieties of PCs are considered: one closely resembles the electronic two-dimensional Su-Schrieffer-Heeger model, and the other continues as an extension of this analogy. In both cases, the topological transitions are induced by adjusting the lattice constants. Topological edge modes (TEMs) are shown to exist within the nontrivial photonic band gaps on the termination of those PCs. The high efficiency of these TEMs transferring electromagnetic energy against several types of disorders has been demonstrated using the finite-element method.
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Journal Articles by Liu feng
Thesis Chapters by Liu feng
Papers by Liu feng