Search query: Optical Crystals band crossing

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abstract: Abstract In this paper, one dimensional photonic crystal distributed feedback structures were chosen for simulating the photonic modes. The corresponding photonic bands were calculated by using a numerical method for solving the master equation, while the reflectivity spectra of the structures were simulated by using a rigorous coupled wave analysis method. By observing the variation of the photonic band diagram and the reflectivity spectrum versus different geometrical parameters, the variation of the photonic bands was detailedly studied. We observed two kinds of photonic modes: (i) the one related to the vertical structures, and (ii) the other related to the horizontal periodic structures. The detailed analysis of the optical modes was illustrated by proposing T E n X m B Z for indexing all transverse electric modes. An active layer coated on the distributed feedback structures plays an essential role in having radiative non leaky photonic modes. The coupling between these modes, giving to anti crossing, was also identified both by simulation and by modelling. This study can pave a way for further modelling optical modes in distributed feedback structures, and for selecting a suitable one dimensional photonic crystal for optoelectronic applications with a specific active semiconductor layer.
author_names: Ngoc Duc Le

Thuat Nguyen-Tran
title: Simulation of coupling optical modes in 1D photonic crystals for optoelectronic applications
year: 2020

abstract: We present a detailed temperature and frequency dependence of the optical conductivity measured on clean high quality single crystals of URu2Si2 of ac and ab plane surfaces. Our data demonstrate the itinerant character of the narrow 5f bands, becoming progressively coherent as the temperature is lowered below a crossover temperature T*~75 K. T* is higher than in previous reports as a result of a different sample preparation, which minimizes residual strain. We furthermore present the density response (energy loss) function of this compound, and determine the energies of the heavy fermion plasmons with a and c axis polarization. Our observation of a suppression of optical conductivity below 50 meV along both the a and c axes, along with a heavy fermion plasmon at 18 meV, points toward the emergence of a band of coherent charge carriers crossing the Fermi energy and the emergence of a hybridization gap on part of the Fermi surface. The evolution towards coherent itinerant states is accelerated below the hidden order temperature THO=17.5 K. In the hidden order phase the low frequency optical conductivity shows a single gap at ~6.5 meV, which closes at THO.
author_names: Nimrod Bachar

D A Stricker

S R Muleady

K Wang

John A Mydosh

Yun Kai Huang

Dirk van der Marel
title: Detailed optical spectroscopy of hybridization gap and hidden order transition in high quality URu 2 Si 2 single crystals
year: 2016

abstract: Bulk undoped ZnS materials exhibit relatively bright yet diverse luminescence behavior, which has, in recent years, been attributed to intrinsic defects. However, the luminescence also resembles that of doped materials, implying a role of impurities. Luminescence features have also been attributed to oxygen impurities causing defect clusters or energy band anti crossing. Thus, this study couples optical and electrical techniques, such as band edge transmission, photoluminescence (PL) PL excitation, radioluminescence, thermoluminescence, optical deep level transient spectroscopy, and photoinduced current transient spectroscopy, to explore the identity of defect levels. ZnS materials studied are commercial single crystals made by physical vapor transport, high pressure Bridgman, and powder processing. These undoped bulk ZnS exhibit luminescence behavior similar to that of reported doped ZnS powders (10 4 to 10 2 mol. doping for luminescent materials) Dopants (such as Al, Cl, Cu, and Ag) are also commonly found impurities in ZnS; hence, it is reasonable to believe they have a role in the luminescence of nominally undoped ZnS. By comparing the variation in optical and electrical properties between samples to the processing method and the rich literature on intentionally doped ZnS, this study shows a possible dominant contribution of impurities and impurity containing defects on the luminescence of bulk "undoped" ZnS. Hence, there is no need to resort to complex mechanisms to explain the luminescence, but rather the metal and halide impurities and their defect complexes determine the main characteristics of luminescence in this wide bandgap semiconductor.Bulk undoped ZnS materials exhibit relatively bright yet diverse luminescence behavior, which has, in recent years, been attributed to intrinsic defects. However, the luminescence also resembles that of doped materials, implying a role of impurities. Luminescence features have also been attributed to oxygen impurities causing defect clusters or energy band anti crossing. Thus, this study couples optical and electrical techniques, such as band edge transmission, photoluminescence (PL) PL excitation, radioluminescence, thermoluminescence, optical deep level transient spectroscopy, and photoinduced current transient spectroscopy, to explore the identity of defect levels. ZnS materials studied are commercial single crystals made by physical vapor transport, high pressure Bridgman, and powder processing. These undoped bulk ZnS exhibit luminescence behavior similar to that of reported doped ZnS powders (10 4 to 10 2 mol. doping for luminescent materials) Dopants (such as Al, Cl, Cu, and Ag) are also commonl.
author_names: Muad Saleh

Kelvin G Lynn

Luiz G Jacobsohn

John S McCloy
title: Luminescence of undoped commercial ZnS crystals: A critical review and new evidence on the role of impurities using photoluminescence and electrical transient spectroscopy
venue: Journal of Applied Physics
year: 2019

abstract: We theoretically demonstrate that tunable exceptional points (EPs) can be realized by using graphene embedded one dimensional (1D) photonic crystals with optical pumping in the terahertz (THz) frequency range. By tuning the Fermi level of graphene sheet, the energy band are altered significantly and the EP appears. In particular, multiple EPs at different frequencies can be selectively produced via subtly adjusting the band structure. Furthermore, topological features of these EPs, such as crossing and anti crossing of the real and imaginary parts of the eigenvalues, have been analyzed in detail. We expect that tunable EPs can provide an instructive method to design active optical devices based on photoexcited graphene sheets in the THz frequency range.
author_names: Shanshan Chen

Weixuan Zhang

Bing Yang

Tong Wu

Xiangdong Zhang
title: Tailoring exceptional points with one dimensional graphene embedded photonic crystals
venue: Scientific Reports
year: 2019

abstract: A two dimensional (2D) material, the holey 2D C2N (h2D C2N) crystal, has recently been synthesized. Here, we investigate the strain effects on the properties of this material by first principles calculations. We show that the material is quite soft with a small stiffness constant and can sustain large strains =12% It remains a direct gap semiconductor under strain, and the bandgap size can be tuned in a wide range as large as 1 eV. Interestingly, for biaxial strain, a band crossing effect occurs at the valence band maximum close to a 8% strain, leading to a dramatic increase of the hole effective mass. Strong optical absorption can be achieved by strain tuning with absorption coefficient ~106 cm 1 covering a wide spectrum. Our findings suggest the great potential of strain engineered h2D C2N in electronic and optoelectronic device applications.
author_names: Shan Guan

Yingchun Cheng

Chang Liu

Junfeng Han

Yunhao Lu

Shengyuan A Yang

Yugui Yao
title: Effects of strain on electronic and optic properties of holey two dimensional C2N crystals
year: 2015

abstract: The CPGE spectrum and optical conductivity in RhSi are measured and discussed, comparing to theoretical predictions. Weyl semimetals are crystals in which electron bands cross at isolated points in momentum space. Associated with each crossing point (or Weyl node) is a topological invariant known as the Berry monopole charge. The circular photogalvanic effect (CPGE) whereby circular polarized light generates a helicity dependent photocurrent, is a notable example of a macroscopic property that emerges directly from the topology of the Weyl semimetal band structure. Recently, it was predicted that the amplitude of the CPGE associated with optical transitions near a Weyl node is proportional to its monopole charge. In chiral Weyl systems, nodes of opposite charge are nondegenerate, opening a window of wavelengths where the CPGE resulting from uncompensated Berry charge can emerge. Here, we report measurements of CPGE in the chiral Weyl semimetal RhSi, revealing a CPGE response in an energy window that closes at 0.65 eV, in agreement with the predictions of density functional theory.
author_names: Dylan Rees

Kaustuv Manna

Baozhu Lu

Takahiro Morimoto

Horst Borrmann

Claudia Felser

J E Moore

Darius H Torchinsky

Joseph Orenstein
title: Helicity dependent photocurrents in the chiral Weyl semimetal RhSi
venue: Science Advances
year: 2020

abstract: Topological semimetals feature protected nodal band degeneracies characterized by a topological invariant known as the Chern number. The magnitude and sign of this invariant determine many of the exotic properties of such semimetals, including the number of topologically protected surface Fermi arcs, the number of chiral landau levels in the presence of a magnetic field, or the direction and strength of topological photocurrents in optical experiments. While formally the Chern number C can take any integer value, any nodal crossing with linear dispersion is expected to have at most |C|=4, realized in fourfold and sixfold nodes in spin orbit coupled, non symmorphic chiral crystals. Despite multiple recent photoemission studies of predicted material candidates, this maximal Chern number, measured by counting the number of Fermi arcs, has not yet been observed experimentally. Using angle resolved photoelectron spectroscopy and ab initio calculations, here we show that the chiral topological semimetal candidate PdGa displays the required multifold nodes, which are connected by long surface Fermi arcs. Due to large spin orbit coupling and the high crystallinity of our sample surfaces, we are for the first time able to resolve 4 independent Fermi arcs, proving experimentally that the observed multifold nodes which are a generic feature of many chiral metals display the maximal Chern number magnitude of 4. Furthermore, by comparing two crystals with mirrored crystal structure, we observe a reversal of the Fermi arc velocities, which demonstrates that the sign of the Chern numbers is tied to the handedness of the sample, which establishes an important control parameter for future experiments probing topological responses in chiral topological semimetals.
author_names: Niels B M Schroter

Samuel Stolz

Kaustuv Manna

Fernando de Juan

Maia G Vergniory

Jonas A Krieger1

Ding Pei

Pavel Dudin

Timur K Kim

Cephise Cacho

Barry Bradlyn

Horst Borrmann

Marcus Peter Schmidt

Roland Widmer

Vladimir N Strokov

Claudia Felser
title: Observation and manipulation of maximal Chern numbers in the chiral topological semimetal PdGa
year: 2019

abstract: Optical phonon effects on the magnon resonance (FMR and AFMR) are shown to be negligible in ferromagnetic insulators and most antiferromagnets in present day laboratory fields (H 2 x 105 G) In these fields the magnon frequency lies lower than the optical phonon frequency so that the coupling is to a polarization mode which is mainly photon like. However, at the band edge, the wavelength dependence of the magnon and optical phonon dispersion may allow crossing in low fields. Also, in metamagnetic systems where the anisotropy field is much larger than the exchange field, the resonance in the paramagnetic phase may occur in a frequency range such that the polarization mode will contain a relative large fraction of optical phonons. The theory developed in this paper can also be applied to optical phonon spin resonance in paramagnetic salts. This type of resonance should occur in attainable magnetic fields for materials with large g factors. Die Wirkung optischer Phononen auf die Magnonresonanz (FMR und AFMR) in ferromagnetischen Isolatoren und in den meisten antiferromagnetischen Stoffen ist bei Feldern von H 2 x 105 G zu vernachlassigen. In diesen Feldern ist die Magnonfrequenz niedriger als die optische Phononenfrequenz, so das ihre Kopplung eine Polarisation ergibt, die hauptsachlich der durch Photonen verursachten ahnlich ist. An der Bandkante konnen sich jedoch in niedrigen Feldern die Wellenlangenabhangigkeit der Magnonen und optischen Phononendispersion kreuzen. Auch in metamagnetischen Systemen, bei denen das Anisotropiefeld sehr viel groser ist als das Austauschfeld, kann die Resonanz in der paramagnetischen Phase in einem solchen Frequenzbereich auftreten, das die Polarisation einen relativ grosen Anteil optischer Phononen enthalt. Die in dieser Arbeit entwickelte Theorie kann ebenfalls auf die optische Phonon Spin Resonanz in paramagnetischen Salzen angewandt werden. Dieser Resonanztyp sollte bei erreichbaren Feldern for Stoffe mit grosen g Faktoren auftreten.
author_names: T N Casselmann

Harold N Spector
title: Interaction of Optical Phonons with Spin Waves in Ionic Crystals
year: 1967

abstract: Photonic crystals or photonic band gap materials are materials with modulated refraction index in the wavelength range. In the optical domain, the index modulation must be in the micrometric or sub micrometric range. The structures that we had fabricated are constituted by nanometric cylindrical air holes in thin films of arsenic sulphide chalcogenide (As2S3) and silicon. We obtained these two dimensional structures by a holographic technique that we developed in two steps using an argon ion laser. We also fabricated waveguides, couplers, or Y junctions incorporated into a two dimensional photonic crystal by combining the techniques of photolithography and holography. The functionality of these optical devices was also demonstrated at the wavelength of 1.55 mum. We also simulated the optical properties of these materials in order to understand how a homogenate medium can be profoundly modified when we structure periodically refraction index in the range of the wavelengths crossing it. Band diagrams were calculated for different structures, materials, refractive indices, dimensions, and polarizations. The method of Finite Difference Time Domain (FDTD) was also used to simulate the behavior of the 90deg waveguides, couplers, Y junctions, and a Mach Zehnder interferometer incorporated into a two dimensional structure of cylindrical air holes of AS2S3 or silicon. We particularly demonstrated the functionality of these optical components when operating at telecommunication wavelength at 1.55 mum.
author_names: Ismail Aboudihab

Elmaati Ech-chamikh

Abdelkader Outzourhit
title: Photonic crystal nanomaterials with arsenic sulphide chalcogenide and silicon for optical telecommunications
venue: 2008 2nd ICTON Mediterranean Winter
year: 2008

abstract: Two dimensional photonic crystals have been fabricated by x ray lithography and reactive ion etching on an air/GaAs/AlGaAs asymmetric waveguide. The shape of the lattice unit cell has been varied by exploiting x ray diffraction effects and nonlinear response of resists during the development process. Rings with or without a central pillar have been fabricated with a resolution down to 50 nm. Lithographic details are described to show the accuracy of this fabrication technique. Optical characterization has been performed showing the presence of a well defined photonic band structure as well as band anti crossing. The results are discussed and compared with theoretical calculations of the photonic band dispersion. Structures with high dielectric fraction are shown to yield photonic modes with small line widths and low propagation losses. The reliability and the precise control of the fabricated sample structures make this lithographic method useful for a full investigation of optical properties on the dependence of the lattice unit cell shape.
author_names: Filippo Romanato

Luca Businaro

Enzo Di Fabrizio

Adriana Passaseo

Massimo de Vittorio

Roberto Cingolani

Maddalena Patrini

Matteo Galli

Daniele Bajoni

Lucio Claudio Andreani

Fabrizio Giacometti

Massimo Gentili

D Peyrade

Yong Zhi Chen
title: Fabrication by means of x ray lithography of two dimensional GaAs/AlGaAs photonic crystals with an unconventional unit cell
year: 2002