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.
Multicontrollability is just beginning to emerge as an engineering paradigm. It is necessary for fault-tolerant operation because multiple agents become available to perform a specific function. This built-in redundancy promotes seamless operation in variable conditions. Inspired by biological multicontrollability, multicontrollable metasurfaces have been conceptualized for terahertz applications. Comprising electrically small elements called MetaAtoms made of diverse pixels each of which is variously controlled, a metasurface could be either homogeneous or graded on the wavelength scale. As an example, terahertz transmission of a normally incident plane wave through a metasurface with subwavelength MetaAtoms containing diverse pixels of magnetostatically controllable material (InAs) and thermally controllable material (CdTe) was analyzed. The co-polarized transmission coefficients were found to exhibit stopbands that shift by switching on/off the magnetostatic field and/or increasing/decreasing the temperature.
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.
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).
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.
A new type of tissue, based on the combination of tobacco cells and multi-walled carbon nanotubes exhibited good mechanical stability even after more than 2 years of room temperature storage. The original intermediate value of electrical conductivity in coplanar configuration, decreased strongly after high electric field stress before the storage period. In the visible wavelength range optical transmission values between 0.1% and 1% have been measured for the more than 100 μm thick sample. The bio/nano-composite in the low-conductivity state revealed to be a sensitive photoreceiver in the visible wavelength range, when illuminated with white LED light. A notable decrease of the low-frequency current noise amplitude with LED illumination has been observed.
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.
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.
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.
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