An innovative approach to improve the performance of photovoltaic solar cells is presented. Until recently, the fabrication of grating layers has been well proven using bulk micromachining techniques, but lately low-cost dip-pen nanolithography (DPN) has been proposed as a method for printing nanostructures on different substrates and has matured to become one of the most versatile patterning techniques available at the nanoscale. However, this technique has scarcely been studied and tested for fabricating grating layers. In this research, submicron grating patterns from high refractive index polymers are fabricated on a few types of solar cells, significantly improving their efficiency. The appropriate geometries and materials for the grating patterns are obtained via numerical optimization using rigorous coupled wave analysis for electromagnetic simulations of the grating multilayer. Possible light-confinement schemes are analyzed, and their figures of merit are assessed. The simulation of the electrical characteristics is integrated with postdesign electromagnetic simulation. The corresponding theoretical and experimental studies shed light on the impact of the merger of the grating structure with the light harvester on the device’s optical and electrical properties. Success in using DPN paves pathways to low-cost fabrication of light harvesting devices with improved performance.
A new dielectric Fabry-Perot cavity is considered for enhanced optical absorption in a thin semiconductor layer embedded within the resonant cavity. In this design, the front (furthest from the illuminated side) mirror is a grating structure with nearly perfect retroreflection. Proof of concept, including semianalytical calculation, and computer-aided design and simulation is performed for application in a midinfrared wavelength band based on a HgCdTe absorbing layer. The results indicate that this new type of cavity meets the combined challenges of significantly increasing the absorption efficiency and reducing the overall complexity and size of the entire device, in comparison to a conventional resonant cavity, in which both mirrors are formed from quarter-wavelength multilayer stacks.
Based on the experimental results and comparison between analytical and rigorous calculations, we have found that dual-surface plasmon (SP) waves are excited at both interfaces of a periodic array of thin metallic nanoslits: one at the grating-substrate interface and one at the grating-superstrate interface. Dual plasmons are excited for each diffraction order at two different wavelengths when the substrate differs from the superstrate. The splitting of the plasmons was investigated as a function of the refractive index difference between the substrate and superstrate. Verification of the extended nature of the double SPs is presented by comparing the rigorous calculation and analytic dispersion relation of extended SPs.
Lead salt materials are of high interest for midinfrared optical emitters and detectors for molecular spectroscopy. The IV-VI narrow gap semiconductors have a multivalley band structure with band extrema at the L point of the Brillioun zone. Due to the favorable mirrorlike band structure, the nonradiative Auger recombination is reduced by one or two orders of magnitude below that of narrow gap III-V and II-VI semiconductor compounds1. The photoluminescence in the midinfrared range for PbSe film structures, excited by a semiconductor laser diode, is investigated.
The PbSe films were prepared by Physical Vapor Deposition (PVD) using an electron gun. A PbSe crystal doped with 0.1 at% Bi was used as a source for the fabrication of thin layers. Starting from the assumption that the rate of nucleation is a predominate factor in determining grain size, thin films were fabricated on substrates that had been maintained at various temperatures of deposition process2. Amorphous glass and Kapton polyimide film was used as substrate. The growth rate was 0.2 nm/s. Films were thermally treated at high oxygen pressure in a heated encapsulated system. Microstructure has been studied using XRD, AFM and HRSEM.
For PbSe structures photoluminescence at temperature as high as 300 K is demonstrated.
Surface plasmon resonance (SPR) and guided wave SPR (GWSPR) are widely used in sensing. Efforts to improve the
sensitivity and stability of these sensors are done by using different multilayer nanostructures such as with the long
range SPR and the use of combinations of materials such as gold and silver. Silver based SPR sensors have high
sensitivity but a poor stability because of the interactions with water and air. We have shown both theoretically and
experimentally that by using the silver based SPR sensor with a 10-15 nm top layer of dielectric film with a high value
of the real part ε' of the dielectric function, it is possible to improve the sensitivity of the sensor by few times. The
imaginary part ε'' of the top nano layer's permittivity needs to be small enough in order to reduce the losses and get
sharper dips. The stability of the sensor is also improved because the nano layer is protecting the silver from
interacting with the environment. The calculated evanescent field is enhanced near the top layer - analyte interface,
thus the enhancement is due to this and due to an increase of the interaction length as a waveguiding effect.
New investigations are carried out on the optical spectral response of grating based nanophotonic structures and their
sensitivity to refractive index variations of a liquid like analyte embedded within and on top of nanometer-sized grating
structure. The phenomena examined are guided wave resonances in dielectric grating/waveguide structures,
respectively, and scatterometric effects in non-resonant structures. Both resonant and non-resonant configurations are
shown to allow refractive index detection limit on the level of 10-6 - 10-5. The spectroscopic scatterometry approach
offers also specificity in particular when the analyzed materials are dispersive and absorptive. The planarity and
operation at normal incidence as well as possibility of fabrication using silicon technologies are advantages for these
structures that permit building arrays of sensors for biochip applications.
Resonant and non-resonant phenomena in grating based photonic structures were investigated as biological and biochemical sensors particularly for water contaminants. Detectivity on the order of 10-6 RIU was found to be possible. Spectroscopic scatterometry in conical mounting at normal incidence was found to offer high sensitivity and it allows specificity in addition to refractive-index-variation measurement.
Periodic structures with minimum feature sizes in the scale of the mean radiation wavelength or less attract considerable interest due to the peculiarity of their electromagnetic (EM) response. When interference and diffraction effects become sufficiently strong, novel and interesting phenomena emerge in reflectivity, transmissivity, absorbance and even infrared thermal emission. The nanotechnology processing enables the production of high-efficiency diffraction gratings with quite small periods, down to the nanometer range, with aspect ratios higher than in spectroscopic gratings. In this paper we present the spectral measurements (transmission and thermal emission) of GaAs and silicon samples with lamella 1D gratings and mesa 2D structures. We also present the theoretical and simulation tools developed for the design and analysis of multilayer lamellar grating structures.
Infrared spectral transmission, reflection and thermal emission from diffraction gratings with differing periods, groove widths and groove depths were experimentally and theoretically studied. The structural dimensions are comparable to the measured spectral wavelengths in the range 2.5 to 25 microns. For calculating the optical properties (transmission and reflection spectra), we have used an in-house S-Matrix Propagation Algorithm (SMPA) technique which is unconditionally stable versus changes in structural dimensions, optical constants and truncation order. We have experimentally studied the planar angular transmission and reflection spectrum of Si and GaAs grating samples, using FTIR spectrometry over the spectral range from 2.5 μm to 25 μm. At λ < Λ, the transmitted intensity is quasi-periodic with respect to wave number. A similar property also appears in the reflection spectra. The theoretical results for spectral transmission are in good agreement with the experimental results for the wavelength range 2.5 to 25 μm.
In this paper we present a study of infrared spectral thermal emission from varius grating structures. The structures include various lamellar grating layers of metals, silicon or GaAs on the same semiconductor substrate. The gratings have different periods, groove widths and groove depths, with feature sizes comparable to the radiated measurement wavelengths (2.5 - 25 μm). The measurement temperatures for all samples were in the range 27 to 740°K. Lateral and vertical optical confinement in the grating layers can occur. In the semiconductor grating layer in the case where the material is partially transparent lateral optical coupling exist which affect the spectral emission. In addition vertical confinement of the electromagnetic field exists which corresponds to "organ-pipe" like modes. The vertical confinement is enhanced in the case where the grating scructure is coated with metal or degenerate semiconductor. These phenomena resulted in thermal emission spectral oscillation for the wavelength range larger than the grating period.
Theoretical work of our group is placed in the general frame of efforts to improve numerical performance and efficiency of rigorous coupled-wave analysis of grating diffraction. Mathematical transformation of Maxwell equations for a multi-layered structure to evolution equations in functional space is presented. By-construction numerically stable symbolic algorithm to solve these equations using the notion of in-layer scattering operator is proposed. On the base of this algorithm a toolbox for simulation of diffraction from multi-layered grating structures, implemented by a graphical user interface is developed. An example of simulation using this in-house software is exposed.
This paper is devoted to the analysis of the optical performance of movable-beam grating structures with both lateral and vertical displacement of the beams. We show that in the vector-diffraction domain one can design high- efficiency switches (e.g. between beam transmitter and beamsplitter mode) and modulators for polarized radiation. The devices exhibit a good tolerance to fabrication and mounting errors and a functional flexibility. In addition, by simulating the optical performance of polysilicon micro- electro-mechanical grating structures at a visible wavelength, reported in the literature, we demonstrate that our tools for rigorous electromagnetic simulation of grating structures work well in the scalar-diffraction domain.
Quantum efficiency of long-base n+p junction silicon solar cells with SiO2 layer deposited on the top cell, and with one-dimensional gratings etched in the cell's top is considered. It is assumed that the region where the useful absorption occurs is outside the grating region. The efficiency is independently optimized by adjusting the layer depth, and the grating dimensions, respectively. It is shown that optimum efficiency of the cells with grating is higher than that of the cell with SiO2 layer.
A visual tool for optical properties (reflection and transmission) simulation and design of multilayer lamillar grating structures is developed, and reported in this work. The simulations are based on rigorous electromagnetic analyses using S-matrix propagation algorithms which are implemented with Fourier-transform discretization, and are coded in Matlab. Visual C++ software tools are used to feed in the input parameters and feed out the results. The input parameters include: input structure, optical and materials parameters. The tools run on a pc. Examples of some of the screen frames obtained by the tools are presented.
S-matrix propagation algorithm for electromagnetic analysis of multilayer gratings with Rayleigh-Fourier implementation and TM-polarization issue renewal is overviewed. Examples of polarized-antireflection grating design are considered.
The scattering of plane wave incident on multilayer structure, refractive index in each layer being a function of one lateral dimension in layers plane, is considered in symbolic vector- operator form. In this framework a S-matrix propagation algorithm is developed, which rigorously eliminates backscattering from solution procedure. In the case of grating layers with unique-period refractive indices the S-matrix propagation algorithm is implemented with Fourier-transform technique of numerical solution. Stability upon increasing truncation order, layers depths and number, and a high-precision holding of power conservation test within the S-matrix propagation are found. Convergence issue for Fourier-transform implementation in TM polarization is recapitulated and a new recipe of its using is suggested. The examples of optimal designs are considered.
A S-matrix propagation algorithm for multilayer planar structure, refractive index in each layer being a function of one lateral dimension in layer's plane is developed. The algorithm is described in symbolic operator form not tied to specific representation in which boundary value problem for Maxwell equations might be solved in the layers. In the case of grating layers where refractive indices are periodic with the unique period two Fourier transform based techniques of numerical solution are implemented with S-matrix propagation algorithm. The examples of the optimal design simulations: Si grating-based zero reflection surfaces and perfect reflection layer/substrate structures are considered.
The infrared optical constants (index of refraction and extinction coefficient) and reflectance of bulk-doped n-silicon are calculated for electron concentrations up to 1021 cm-3. These calculations are based on generalized Drude-Lorentz form of dynamic dielectric function and current relaxation approach. A nonmonotonic behavior of IR absorption versus electron concentration is found. A connection between the theoretical results and available experimental data is discussed.
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