Dielectric reflectors which can be found in the shells of some gold beetles (e.g. Anopognathus parvulus and Aspidomorpha tecta) consist of a stack of alternating layers of two high- and low-refractive index materials. Inspired by these structures, here we describe irregularly staggered reflectors which can be designed to feature high reflectance in a broad wavelength band that includes entirely the visible (VIS) and partially the nearinfrared (NIR) regions ([400, 1400] nm). Given the materials and the number of layers, we use a dedicated genetic algorithm to find out the reflector configuration (i.e., the sequence of the thicknesses of the layers) maximizing the reflectance in the considered wavelength band. Some results are reported showing the characteristics of such reflectors.
Three numerical studies were undertaken involving the interactions of plane waves with topological insulators. In each study, the topologically insulating surface states of the topological insulator were represented through a surface admittance. Canonical boundary-value problems were solved for the following cases: (i) Dyakonov surface-wave propagation guided by the planar interface of a columnar thin film and an isotropic dielectric topological insulator; (ii) Dyakonov–Tamm surface-wave propagation guided by the planar interface of a structurally chiral material and an isotropic dielectric topological insulator; and (iii) reflection and transmission due to the planar interface of a topologically insulating columnar thin film and vacuum. The nonzero surface admittance resulted in asymmetries in the wave speeds and decay constants of the surface waves in studies (i) and (ii). The nonzero surface admittance resulted in asymmetries in the reflectances and transmittances in study (iii).
Temperature-induced changes in the propagation of electromagnetic surface waves guided by the planar interface of a temperature-sensitive isotropic material (namely, InSb) and a temperature-insensitive anisotropic material were investigated theoretically in the terahertz frequency regime. Two types of anisotropic partnering material were considered: (i) a homogeneous material and (ii) a periodically nonhomogeneous material. As the temperature increases, the isotropic partnering material is transformed from a weakly dissipative dielectric material to a plasmonic material. As a consequence, the surface waves change from Dyakonov surface waves to surface-plasmon-polariton waves for case (i), and change from Dyakonov–Tamm surface waves to surfaceplasmon-polariton waves for case (ii). Numerical investigations demonstrated that dramatic changes in the numbers of propagating Dyakonov or Dyakonov–Tamm surface waves, their angular existence domains, their propagation constants, and their decay constants, could arise from modest changes in temperature.
Dielectric multilayers consisting of alternating layers of two different materials with thicknesses irregularly decreasing with depth in the structure are included in the cuticle of some beetles whose shell exhibits broadband reflection in the optical wavelength range. Emulating these structures, we propose and numerically analyze irregularly chirped dielectric multilayers. Analysis was performed using a dedicated genetic algorithm (GA) that searches for the multilayer configurations maximizing the reflection for normal incidence over a large wavelength range. We found that the GA leads to the irregularly chirped reflectors that significantly outperform the regularly chirped ones proposed and analyzed in the literature.
Surface-plasmon-polariton waves can be compounded when a sufficiently thin metal layer is sandwiched between two half spaces filled with dissimilar periodically nonhomogeneous dielectric materials. We solved the boundary-value problem for compound waves guided by a layer of a homogeneous and isotropic metal sandwiched between a structurally chiral material (SCM) and a periodically multilayered isotropic dielectric (PMLID) material. We found that the periodicities of the PMLID material and the SCM are crucial to excite a multiplicity of compound guided waves arising from strong coupling between the two interfaces.
Inspired by the multilayer structure of the cuticle of some gold beetles whose shell exhibits broadband reflection in the visible wavelength range, we numerically analyzed irregularly chirped dielectric mutilayers. The analysis was performed using a dedicated genetic algorithm which searches for the multilayer configurations that maximizes the reflection bandwidth. We found that the genetic algorithm leads to irregularly chirped structures which significantly outperform the regularly chirped ones proposed and analyzed in literature.
Surface waves of different types can be compounded when a homogeneous layer is sandwiched between two half spaces filled with dissimilar periodically non-homogeneous dielectric materials and the intermediate layer is sufficiently thin. We solved the boundary-value problem for compound waves guided by a layer of a homogeneous and isotropic (metal or dielectric) material sandwiched between a structurally chiral material (SCM) and a periodically multi-layered isotropic dielectric material. We found that the periodicity of the SCM is crucial to excite a multiplicity of compound guided waves with strong coupling between the two interfaces.
Multiple p- and s-polarized compound surface-plasmon-polariton (SPP) waves at a fixed frequency can be guided by a structure consisting of a metal layer sandwiched between a homogeneous isotropic dielectric (HID) material and a periodic multilayered isotropic dielectric (PMLID) material. For any thickness of the metal layer, at least one compound SPP wave must exist. It possesses the p-polarization state, and is strongly bound to the metal/HID interface when the metal thickness is large but to both metal/dielectric interfaces when the metal thickness is small. When the metal layer vanishes, this compound SPP wave transmutes into a Tamm wave. Additional compound SPP waves exist, depending on the thickness of the metal layer, the relative permittivity of the HID material, and the period and composition of the PMLID material. Some of these are p-polarized, the others are s-polarized. All of them differ in phase speed, attenuation rate, and field profile, even though all are excitable at the same frequency. The multiplicity and dependence of the number of compound SPP waves on the relative permittivity of the HID material when the metal layer is thin could be useful for optical sensing applications and intrachip plasmonic optical communication.
Surface-plasmon-polariton (SPP) wave excitation in a composite structure obtained by depositing a columnar thin film (CTF) on a metal (silver) substrate decorated periodically by an array of rectangular grooves was analyzed using the rigorous coupled-wave approach. The SPP wave is excited as a Floquet harmonic of different orders by light incident from different directions, the incidence plane being the same as the grating plane. But, in some instances, the SPP wave appears to be excited as a doublet, i.e., a Floquet harmonic of the same order at two closely spaced angles of incidence, the excitation being less efficient for one angle of incidence than for the other. The characteristics of the SPP wave are affected by the vapor incidence angle chosen to deposit the CTF.
The dispersion equation for surface plasmon-polariton (SPP) waves excited in a composite structure obtained by depositing a columnar thin film (CTF) on a metal (silver) substrate decorated periodically by an array of rectangular grooves was analyzed using rigorous coupled-wave approach. Only one solution of this dispersion equation is possible. The characteristics of the SPP wave are affected by the vapor incidence angle chosen to deposit the CTF.
We present a general analytical synthesis method to design quarter-wave dielectric multilayers exhibiting reflection properties which approximate a given reflection spectrum. We show that the method can be used as a design procedure for N-band antireflection coatings. Examples confirming the effectiveness of our procedure in designing dual- and three-band antireflection coatings are illustrated.
Inspired by the apposition compound eyes of some dipterans, we analyzed the effect of texturing the exposed face of a silicon solar cell by ray-tracing simulation. Bioinspired textures of two types were considered: hillock textures and pit textures. A multifrequency analysis-that took into account the broadband nature of solar radiation and its amplitude at the sea-level, as well as the wavelength dependence of the complex-valued refractive index of silicon-showed that both types of bioinspired textures are beneficial. We found that the light-coupling efficiency can be enhanced by as much as 20% and 24% with respect to that of a flat surface when the exposed face of a silicon solar cell is decorated with the bioinspired hillock texture and the bioinspired pit texture, respectively.
In order to improve the light-harvesting capability of silicon solar cells, bioinspired texturing of the exposed surface was investigated. The texture has a pit topology, which is the negative of a previously analyzed bioinspired hillock texture. Multifrequency numerical simulations of the light-coupling efficiency were carried out within the framework of the geometrical optics over a large and relevant portion of the solar spectrum. The bioinspired pit texture exhibits an efficiency significantly greater than that of a flat silicon surface, and somewhat outperforms the bioinspired hillock texture.
This article [J. Photon. Energy. 3, , 034599 (2013)] was originally published on 23 January 2013 with errors in the divisors of four ratios, appearing in the fifth paragraph of Sec. 4.1. In each instance the divisor and the quotient were correctly stated. In the order of appearance, the four ratios are: 0.42/0.38≈1.10 , 0.50/0.48≈1.04 , 0.42/0.33≈1.27 , and 0.50/0.43≈1.16 .
We devise a method to design quarter-wave dielectric multilayers, which exhibit reflection properties approximating a given reflection spectrum. The method can be used as a design procedure for N-band antireflection coatings. Dual- and three-band antireflection coatings are designed using this method.
Investigating the use of prismatic lenses with cross-sectional shapes inspired by the apposition compound eyes of some dipterans, we found through numerical simulations that the exposed surfaces of silicon solar cells should be textured as arrays of bioinspired compound lenses in order to improve performance. We +used a ray-tracing algorithm to evaluate the light-coupling efficiency over a large and relevant portion of the solar spectrum and determined the array configuration that maximizes the coupling of light. Our simulation results show that the light-coupling efficiency can be significantly enhanced with respect to that of a silicon cell with a flat surface.
KEYWORDS: Silicon, Lenses, Solar cells, Silicon solar cells, Reflection, Solar energy, Numerical simulations, Ray tracing, Fractal analysis, Solar radiation
Investigating the use of prismatic lenses with cross-sectional shapes inspired by the apposition compound eyes
of some dipterans, we found through numerical simulations that the exposed surfaces of silicon solar cells should
be textured as arrays of bioinspired compound lenses (BCLs) in order to improve performance. We used a raytracing
algorithm to evaluate the light-coupling efficiency over a large wavelength range of the solar spectrum.
Thus, the array configuration maximizing the light-coupling was determined. Results show that the light-coupling
efficiency can be enhanced by at least 16% with respect to that of a silicon cell with a flat surface.
KEYWORDS: Silicon, Solar cells, Silicon solar cells, Lenses, Coating, Fractal analysis, Antireflective coatings, Image enhancement, Geometrical optics, Solar energy
A silicon prismatic lens, whose shape is inspired by the apposition compound eyes of some dipterans, was
investigated to improve the light-harvesting capability of silicon solar cells. The bioinspired compound lens
(BCL) has a fractal construction procedure in which the cross-section of the lens is a frustum of a infinitely
long circular cylinder at the zeroth-stage, and is decorated with sections of smaller cylinders at higher stages
of construction. We found that the light-coupling efficiency of the best performing first-stage BCL cannot be
enhanced by higher-order stages and is significantly superior to other kinds of textured surfaces. By coating the
lens with a double layer anti-reflection coating further enhancement of efficiency can be obtained. Our strategy
can also be adopted for solar cells not made of silicon.
A composite structure obtained by depositing a Columnar Thin Film (CTF) on a transparent substrate with a
periodic array of rectangular grooves of infinite length can function as a narrowband, linear-polarization rejection
filter. This device discriminates between the linear-polarization states of an incident plane wave, so that linearpolarization
high-quality filters with extremely narrow bandwidths can be designed. Furthermore, the filter response can be tuned by varying the angle of incidence.
A design paradigm for Bragg multilayers made of columnar thin films
(CTFs) is provided
approximating both the ordinary and extraordinary refractive
indexes of CTFs as Gaussian functions of the
vapor incidence angle. Furthermore, we use a quadratic approximation
of the same two quantities to
design a cascade of chiral sculptured thin films (STFs) with
successive pitches forming a geometric series to function as a wideband
circular polarization filter.
We highlight the self-similar properties of the reflection coefficient of optical-Cantor prefractals multilayers. These properties are first illustrated by means of numerical results obtained using the characteristic matrix method and then are analytically investigated by resorting to the small reflection approximation.
A numerical variational procedure for determining the field propagating in an optical fiber near to the cutoff frequency is presented. The obtained field representation is used to evaluate the cutoff frequency shift of the LP11 mode occurring in optical fibers when cabling effects are taken into acount.
A set of orthonormal Laguerre-Gauss functions was used to approximate the field of the fundamental mode of a weakly guiding fiber. The parameters of the expansion were evaluated by a variational method which maximizes the mode propagation constant. It was verified that for fiber with both triangular and parabolic refractive index profile, expansion with few terms only allows to construct an accurate field approximation. The proposed approximation does not require the knowledge of the actual field of the fiber and then avoids to look for a numerical solution of the scalar wave equation. In addition, due to the elementary form of the involved functions, the parameters characterizing the fiber performance can be analytically evaluated in a closed form.
Glass integrated optics represents a viable approach to the development of cheap and robust passive components for communication, signal processing and sensing systems. Ion- exchange, in turn, is one of the most widespread technologies for the fabrication of optical waveguides in glass substrates. The choice of the substrate glass may affect critically the performance of waveguides and devices; thus, the results of an extensive characterization of ion- exchanged waveguides fabricated in a number of cheap, common commercial glass slides are reported here, with the aim of assessing the glass quality and therefore providing indications on the most effective substrate for components fabrication.
A monolithic optical component able to focus a medium or high power laser beam in three different points on the same plane has been designed and constructed. The three-spot lens is constituted by an asymmetrical refractive lens. A face has been conventionally rounded in a convex shape, while the other face has a prismatic shape in order to split the laser beam in three parts. The optical characteristics of the three-spot lens (focal length, focal spot sizes, mutual distance between spots an spherical aberration) were evaluated in first approximation by extending the matrix optics method to misaligned systems. An experimental characterization of the lens has substantially confirmed the results of the theoretical analysis and it has shown that the lens can be conveniently used to design a device for simultaneous coupling of a number of large-diameter optical fibers to a medium or high power laser source (laser- multifiber coupler) or to a single launching fiber (fiber- multifiber coupler). The laser power can be uniformly coupled into the receiving fibers; it was verified that an asymmetrical power distribution between the fibers can be obtained as well.
A set of orthonormal functions is used to expand the field of the fundamental mode of a weakly-guiding optical fiber. The parameters of the expansion are evaluated by minimizing the norm of the difference between the actual field within the fiber and the approximating one. The simple elementary form of the functions involved into the expansion allows an easy evaluation of parameters which need to correctly design an optical fiber based transmission system. For fibers with both triangular and parabolic refractive index profile, a satisfactory field approximation is obtained by retaining only a few terms into the expansion. In particular, by using only the first three even order terms, the accuracy is greater than that of the classical gaussian field approximation and is comparable with the accuracy of other approximations based on more complicate functions.
The realization of low-loss optical waveguides by the sol-gel method and the subsequent production of strip waveguides by direct laser densification are described. These waveguides have been characterized by both the optical and structural point of view, and the results are discussed.
The preliminary results of a round-robin test to check the reproducibility of the measurement of the main optical characteristics of gradient-like planar waveguides are reported. The test involves seven research laboratories in Europe and Canada. The effective indexes of a set of planar waveguides produced by K+/Na+ ion-exchange have been measured in each laboratory and the refractive-index profiles have been reconstructed by various numerical methods. The comparison between the reconstructed profiles and the directly measured one is also shown for one of the waveguides under test.
Laser heating can be used to fully densify selected areas of a partially densified sol-gel film deposited onto a glass substrate, thus substituting the conventional heat treatment in an oven. Here the topographic and optical characteristics of strip waveguides produced in a SiO2-TiO2 film by direct writing with a focused CO2 laser are presented, and the critical parameters of the process discussed.
Graded-index waveguides in glass have been produced by different ion-exchange processes, and their optical characteristics measured to check the consistency of two numerical methods for a proper modelling of the refractive index profile: the results obtained by the two methods appear to be in quite good agreement in most of the cases. An external reference was provided by direct RBS measurements of the concentration of exchanged ions.
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