Browsing by Author "Gu, BY"

Now showing 1 - 10 of 10

Analysis of the focal characteristics of cylindrical lenses made of anisotropically dielectric material based on rigorous electromagnetic theory
(2003-05-01)
Dong, BZ
;
Ye, JS
;
Liu, J
;
Gu, BY
;
Yang, GZ
The focal characteristics of refractive cylindrical lenses made of anisotropically dielectric material (uniaxial crystal) are analysed based on rigorous electromagnetic theory and the boundary element method. The performances of the lenses with different f numbers are appraised for both incident waves of the TE (transverse electric) and TM (transverse magnetic) polarizations. Numerical results show that the focal performance of this kind of lens for the TE polarization and the TM polarization of incident light wave is a difference, in particular, different focal lengths, owing to the anisotropy of the material. However, for the conventional isotropic lens, the focal features for both the TE and TM polarizations are the same. It is anticipated that this new kind of lens proposed for the first time may serve as a light switching device with high speed used in the micro-optical communication.
Andreev scattering in semiconductor-superconductor junctions containing a finite width semiconductor region applied by magnetic fields
(2004-05-01)
Hsue, YC
;
Yang, TJ
;
Gu, BY
;
Wang, J
;
電子物理學系
We present numerical studies of conductance of quasi-particles (QPs) in junctions consisting of semiconductor (Sm) two-dimensional electronic gas (2DEG) and superconductor (S) containing a finite width 2DEG region (referred to magnetic barrier) applied by a magnetic field perpendicular to the 2DEG plane. Total conductance as a function of beta = mu(N)/homega(c), (where mu(N) is the chemical potential of semiconductor and omega(c) = eB/cm(N)(*) is the cyclotron frequency) strongly depends on the normal or superconducting state of the superconductor. It demonstrates that the Andreev reflection (AR) plays a dominant role. The total conductance of G((NBN)) or G((NBS)) as a function of beta for the NBN or NBS junction displays monotonically increasing behavior with different rising slopes. We observe the existence of a critical magnetic field for switching off the conductance. This critical field of G((NBS)) is a half of that of G((NBN)) in the matching case of the Fermi level and effective mass of QPs in materials forming the junctions. G((NBS)) is usually larger than G((NBN)) for a given magnetic field. The 3D plot of the conductance G((NBN)) and G((NBS)) as functions of beta and the incident angle theta of the QPs displays a flat plateau over a finite domain region of [beta,theta]. However, in the mismatching case of the Fermi energy and effective mass of QPs, G((NBS)) is lower than G((NBN)) because the AR of the holelike QPs now becomes incomplete and normal reflection (NR) of the electronlike QPs is considerably enhanced. The corresponding 3D plot of conductance against beta and theta exhibits heavy spike oscillations around its occupied boundary. We present explicit analytic expressions of the conductance of QPs for this model junction on the basis of classical cyclotron orbit of the QPs in magnetic fields and the quantum mechanical calculation. The characteristics of conductance can be well interpreted by this phenomenological physical picture.
Characteristics of transmission of electrons in a bent quantum waveguide with inhomogeneous magnetic fields
(1999-07-15)
Gu, BY
;
Lin, YK
;
Chuu, DS
;
電子物理學系
;
Department of Electrophysics
We present numerical studies of the transmission properties in an L-shaped quantum waveguide (LQW) subject to an inhomogeneous magnetic field perpendicular to the LQW plane. The magnetic field remains zero at the corner region, thus, a magnetically defined cavity is formed in this LQW. We find that transmission characteristics of electrons in the LQW depend strongly on geometric parameters and magnetic configurations. Sharp peaks with unity amplitude and deep dips are observed in transmission as a function of the Fermi energy of the incident electron at some high fields. The mode-mode coupling between the wires and the cavity and multiple reflection of electrons in the cavity lead to the appearance of these structures in transmission spectrum. We also study the variation of transmission with magnetic field for different magnetic confinement configurations when fixing incident electron energy. In the magnetic depletion process of the propagating modes in wires, the transmission exhibits various patterns, such as stepped drop, wide valley, deep dips, large oscillations, or without any structure, sensitively dependent on incident electron energy and magnetic confinement configurations. It is expected that one can flexibly modify transmission spectrum of the LQW by applying an inhomogeneous magnetic field to match practical requirements. (C) 1999 American Institute of Physics. [S0021-8979(99)05714-X].
Characteristics of transmission of electrons in a quantum wire tangentially attached to a superconductor ring threaded by magnetic flux
(2002-04-01)
Gu, BY
;
Lu, Y
;
Yang, TJ
;
電子物理學系
;
Department of Electrophysics
We present numerical investigations of the transmission properties of electrons in a normal quantum wire tangentially attached to a superconductor ring threaded by magnetic flux. A point scatterer with a delta-function potential is placed at node to model scattering effect. We find that the transmission characteristics of electrons in this structure strongly depend on the normal or superconducting state of the ring. The transmission probability as a function of the energy of incident electrons, in the case of a superconductor ring threaded by one quantum magnetic flux, emerges one deep dip, imposed upon the first broad bump in spectrum. This intrinsic conductance dip originates from the superconductor state of the ring. When increasing the magnetic flux from one quantum magnetic flux to two, the spectrum shifts toward higher energy region in the whole. This conductance dip accordingly shifts and appears in the second bump. In the presence of a point-scatterer at the node, the spectrum is substantially modified. Based on the condition of I he formation of the standing wave functions in the ring and the broken of the time-reserve symmetry of Schrodinger equation after switching magnetic flux, the characteristics of transmission of electrons in this structure can be well understood.
Efficient mode coupling technique between photonic crystal heterostructure waveguide and silica waveguides
(2005-09-15)
Chau, YF
;
Yang, TJ
;
Gu, BY
;
Lee, WD
;
電子物理學系
We report an efficient mode coupling technique between silica waveguides (SWGs) and a planar photonic crystal heterostructure waveguide (PPCHWG) composed of two semi-infinite two-dimensional photonic crystals (PCs) with different filling fractions. By calculating the transmittance at output end of SWG, we find that the designed structures possess the merits of high coupling efficiency and relaxing fabricating tolerance. The maximal transmittance exceeds 90% at a wavelength near 1.65 mu m. The high transmission efficiency of PPCHWGs can be achieved without introducing any defect structures for optimal mode matching. Besides, the width of PPCHWGs can be flexibly adjusted via changing the separation of two component PCs in PC heterostructures. (c) 2005 Elsevier B.V. All rights reserved.
Magnetoconductance oscillations in semiconductor-superconductor junctions with a laterally isolating barrier layer inside semiconductor region
(2003-07-01)
Hsue, YC
;
Yang, TJ
;
Gu, BY
;
Wang, J
;
電子物理學系
The oscillatory characteristics of magnetoconductance for a junction composed of a superconductor and a semiconductor, in which two parallel quantum wave guides are coupled with each other through a potential barrier layer, are studied systematically. To model the imperfectness of the interface, we introduce a delta-function scattering potential barrier lying close to the interface of the junction. The magnetoconductance oscillations (MCO) in this system stem from two sources: one is the interference of wave functions of quasi-particles due to multiple Andreev reflections at the interface; the other is attributed to the variation of the number of the propagation modes when introducing the isolating barrier layer. The introduction of the isolating layer in the quantum wave-guides strongly modifies MCO. We also present a physical picture for the MCO based on a phenomenological argument. The theoretically fitted results are in good agreement with numerical ones.
Oscillations of magnetoconductance in semiconductor-superconductor junctions with a laterally potential barrier isolator inside semiconductor region
(2003-05-01)
Yang, TJ
;
Hsue, YC
;
Gu, BY
;
電子物理學系
;
Department of Electrophysics
Magnetoconductance (MC) injunctions of a superconductor and a semiconductor (Sm) consisting of two parallel quantum waveguides (QW) coupled through a potential barrier under a perpendicular applied magnetic field in the semiconductor region is studied theoretically. The results of MC showing a series of oscillations with stepwise platform and spikes may be understood well within the phenomenological argument similar to Y. Asano (Phys. Rev. B 61 (2000) 1732) proposed. The oscillatory peaks of the MC in the dual QW are found essentially due to the variation of the number of the propagation modes. (C) 2003 Elsevier Science B.V. All rights reserved.
Sensitivity of complete bandgaps to the shift of movable dielectric rod in two-dimensional photonic crystals with complex lattices
(2004-06-30)
Liu, WL
;
Yang, TJ
;
Gu, BY
;
電子物理學系
;
Department of Electrophysics
Two-dimensional tunable photonic crystals (TPCs) with movable components are proposed, which consist of a circular dielectric rod inserted into a square lattice of square dielectric cylinders in air. The band gap structures of TPCs can be tuned by adjusting the shift of the position of the circular rod in each unit cell. Band gap structures are calculated with the use of the plane-wave expansion method for various structural parameters, such as different filling fractions, ratio of the radius of the circular rod to the side-length of the square cylinder, different shifts or shifting orientations of the circular rod, etc. We find that there is a region of parameters in which the ratio of the gap width to the midgap is insensitive to the shift of the position of the circular dielectric rod. It is anticipated that the proposed TPC may become a favourable candidate of PCs owing to the benefit of facilitating fabrication, allowing a large tolerance.
Significantly enhanced coupling efficiency in 2D photonic crystal waveguides by using cabin-side-like tapered structures at two terminals
(2004-08-15)
Chau, YF
;
Yang, TJ
;
Gu, BY
;
電子物理學系
;
Department of Electrophysics
We report that using a cabin-side-like tapered structure, containing only one pair of defects, can markedly enhance the coupling efficiency at the entrance and exit terminals of planar photonic crystal waveguides (PPCWGs). The PPCWGs are composed of circular dielectric rods set in 2D triangular lattices. By means of simulations, we find that the transmission efficiency reaches up to 90%. Moreover, the suggested structures possess other advantages, such as the shortness of the taper, the ease of fabrication, and its low cost. It is anticipated that the proposed structures might be feasibly be applicable to the integrated optical circuits compatibility.
Transmission properties of electrons in a T-type opened quantum waveguide with inhomogeneous magnetic fields
(1999-03-30)
Chen, K
;
Gu, BY
;
Lin, YK
;
Chuu, DS
;
電子物理學系
We calculate the quantum-mechanical transmission of electrons in a T-shaped opened quantum waveguide (TOQW) subjected to an inhomogeneous magnetic field perpendicular to the TOQW plane with the use of the mode-matching technique. An electron incident through one branch of the waveguide can travel into the other two channels in the quantum ballistic regime. We find that transmission coefficients in two channels exhibit quite difference from each other and depend sensitively on geometric parameters and magnetic configurations. Sharp peaks with unity amplitude and deep dips are observed in transmission spectrum as a function of the Fermi energy of the incident electrons at some high fields. We also study the variation of transmission with magnetic field and find that the transmission exhibits various patterns such as step drop, deep dips, wide valley, large oscillations, or without any structure, sensitively depending on the incident electron energy and magnetic confinement configurations. It is expected that by applying inhomogeneous magnetic fields to the TOQW with appropriate magnetic configuration one may provide an available way to control transmission property of the TOQW to match practical requirements in devices.