Axisymmetric photonic structures with PT-symmetry

Copyright 2016 Society of Photo-Optical Instrumentation Engineers. One print or electronic copy may be made for personal use only. Systematic reproduction and distribution, duplication of any material in this paper for a fee or for commercial purposes, or modification of the content of the paper are prohibited.PT-symmetric structures in photonic crystals, combining refractive index and gain-loss modulations is becoming a research field with increasing interest due to the light directionality induced by these particular potentials. Here, we consider PT-symmetric potentials with axial symmetry to direct light to the crystal central point obtaining a localization effect. The axial and PT-symmetric potential intrinsically generates an exceptional central point in the photonic crystal by the merge of both symmetries. This particular point in the crystal lattice causes field amplitude gradients with exponential slopes around the crystal center. The field localization strongly depends on the phase of the central point and on the complex amplitude of the PT-potential. The presented work analyzes in a first stage 1D linear PT-axisymmetric crystals and the role of the central point phase that determines the defect character, i.e. refractive index defect, gain-loss defect or a combination of both. The interplay of the directional light effect induced by the PT-symmetry and the light localization around the central point through the axial symmetry enhances localization and allows higher field concentration for certain phases. The linearity of the studied crystals introduces an exponential growth of the field that mainly depends on the complex amplitude of the potential. The work is completed by the analysis of 2D PT-axisymmetric potentials showing different spatial slopes and growth rates caused by symmetry reasons.Peer ReviewedPostprint (published version

Fabrication of ZnO based type-II core/shell nanowires for photovoltaic applications

近来，由于持续增长的能源危机和环境保护意识，光伏器件已成为一个热门的课题。许多研究都集中在如何提升这类使用氧化物半导体，如ZnO，TiO2，的太阳能电池和光电化学电池（PEC）的效率上。这些氧化物半导体有这大且固定的带隙，使得它们都不能有效地吸收太阳光的可见光区域，从而导致电池效率低下。一个可选的策略是在纳米线外包裹一层窄带隙半导体，例如ZnSe，CdSe，CdS，PbSe和ZnCdSe，应用这个方法已经把材料的吸收拓展至可见光区域。由于光俘获范围有限以及光激发电子注入效率低等原因，大部分的这种核壳结构的光电转化效率（PCE）都较低。相比起二元材料，通过控制组分，三元合金ZnCdSe具有带隙可...Recently, photovoltaic devices have been a subject of great interest due to the growing awareness of energy crisis and environmental protection. Many studies have been focused on increasing the efficiency of solar cells or photo-electrochemical (PEC) cells using oxide semiconductors, such as ZnO, TiO2. These oxide semiconductors have large and fixed bandgaps, which individually can’t efficiently a...学位：理学博士院系专业：物理与机电工程学院_凝聚态物理学号：1982011015419

Self-collimation in PT -symmetric crystals

We predict the self-collimation phenomena (or equivalently, dynamical localization) in two-dimensional PT-symmetric complex potentials, where the complex modulation is considered in the transverse, longitudinal, or simultaneously in both directions. Nondiffractive propagation is analytically predicted and further confirmed by numerical integration of a paraxial model. The parameter space is explored to identify the self-collimation regime in crystals with different PT symmetries. In addition, we also analyze how the PT-symmetric potentials determine the energy distribution between spatial modes of the self-collimated beams.Peer ReviewedPostprint (published version

Self-collimation in 2D Complex P- and PT-symmetric systems

We predict the self-collimation phenomena (or equivalently, dynamical localization) in 2-dimensional P-symmetric and PT-symmetric complex potentials, with periodic modulations of both gain/loss and refractive index. Non diffractive propagation is analytically predicted and further confirmed by numerical integration of a paraxial model. The parameter space is explored to identify the self-collimation regime in crystals with different complex symmetries.Postprint (published version

PT-axisymmetric VCSELs with linear central defect

Semiconductor Lasers and particularly Vertical-Cavity Surface-Emitting Lasers (VCSELs) are important laser sources used for many purposes. However, the applications of these lasers are mainly restricted by their strongly multimode operation given by the lack of an intrinsic transverse mode selection mechanism [1]. The introduction of an axial PT-symmetric potential within this kind of lasers is expected to induce a field enhancement and localization at the symmetry axis, central part of the laser. The required complex potential, combining a modulated refractive index and gain-loss distributions, may be achieved by different configurations with actual fabrication techniques. The Complex Ginzburg-Landau equation is used as a simple VCSELs model, and the numerical results show important localization effects; due to the asymmetric mode coupling energy converges to the center leading to a strong light confinement. The main consequence is a narrow and bright laser emission from the central part of the device. As the system nonlinearities introduce saturation limiting the maximum intensity of the output beam, the inclusion of a central linear defect in the structure allows a larger field concentration.Postprint (published version

Self-collimated beams in 2D complex periodic lattices from P- to PT-symmetry

We analyze self-collimation in two-dimensional periodic complex lattices. We consider P-symmetric and PT-symmetric complex lattices with different geometries, where the periodic modulations of both refractive index and gain-loss are either in-phase, or dephased a quarter of wavelength of the modulation. The non-diffractive propagation of light beams is analytically predicted using coupled mode approach and further confirmed by numerical integration of a paraxial model.Postprint (published version