Conventional Reflector Research Articles

Deflection of Hazardous Near-Earth Objects by High Concentrated Sunlight and Adequate Design of Optical Collector

Some detailed astronomical and applied aspects deflection of hazardous near-Earth objects (NEO) by direct high concentrated sunlight, causing intensive local ablation of their surfaces, are considered. The major requirements to solar concentrating optics within a single collector (a large mirror) approach, along with the asteroid properties being most substantial in achieving the predetermined effect for the period less than a year (mid-thrust action), are discussed. Such a hastened strategy may become topical in the case of late detection of potential danger, and also, if required, in providing the possibility for some additional action. It is also more acceptable in the public perception and keeping the peace for mankind rather than a long-run expectation of the incorrigible deflection resulting shortly ahead of the predicted hazard. The conventional concave reflectors have been graved to be practically inapplicable within the high concentrating geometry. This is primarily because of the dramatic spread of their focal spots at needful inclinations of optical axis from the direction toward the Sun, as well as of problematic use of the secondary optics. An alternative design of a mirrored ring-array collector is presented (as a tested and approved point-focus version of innovative reflective lenses for sunlight concentration within this approach), and comparative analysis was made. The assessment argues in favor of such a type of high-aperture optics having more capabilities than conventional devices. Mainly, this is because of the underside position (as respects the entrance aperture) of its focal area that allows avoidance of target shadowing the reflecting surfaces and minimizes their coating by the ejected debris. By using the modern asteroids database, some key estimations have been obtained. The surface irradiance around 4–5 MW/m2 (average across the focal spot concentration level ~5 × 103) for the ring-array collector size ~0.5 of asteroid diameter might suffice to deflect a 0.5-km-diameter NEO during several months. For the larger diameter NEOs, to 1.3–2.2 km, the required collector sizes are about the asteroid diameters, and they are even greater for more massive objects.

An optimized surface plasmon photovoltaic structure using energy transfer between discrete nano-particles

Surface plasmon enhancement has been proposed as a way to achieve higher absorption for thin-film photovoltaics, where surface plasmon polariton(SPP) and localized surface plasmon (LSP) are shown to provide dense near field and far field light scattering. Here it is shown that controlled far-field light scattering can be achieved using successive coupling between surface plasmonic (SP) nano-particles. Through genetic algorithm (GA) optimization, energy transfer between discrete nano-particles (ETDNP) is identified, which enhances solar cell efficiency. The optimized energy transfer structure acts like lumped-element transmission line and can properly alter the direction of photon flow. Increased in-plane component of wavevector is thus achieved and photon path length is extended. In addition, Wood-Rayleigh anomaly, at which transmission minimum occurs, is avoided through GA optimization. Optimized energy transfer structure provides 46.95% improvement over baseline planar cell. It achieves larger angular scattering capability compared to conventional surface plasmon polariton back reflector structure and index-guided structure due to SP energy transfer through mode coupling. Via SP mediated energy transfer, an alternative way to control the light flow inside thin-film is proposed, which can be more efficient than conventional index-guided mode using total internal reflection (TIR).

Enhanced light trapping for high efficiency crystalline solar cells by the application of rear surface plasmons

In this article, a novel rear structure using Ag nanoparticles to create surface plasmons to enhance light trapping is applied on the rear of planar high efficiency PERT (Passivated Emitter and Rear Totally Diffused) silicon wafer cells, targeting the spectrum range from 1000nm to 1200nm. Variations of this rear structure that combine Ag nanoparticles, dielectric layers and back metal reflectors were studied and analysed. Thickness of the rear surface passivation SiO2 spacer layer was optimised to achieve maximum optical enhancement using surface plasmons but with minimum electronic losses due to recombination effects. The effect of the precursor evaporated Ag film thickness was also studied as a means to vary the size/shape of the nanoparticles. The measured external quantum efficiency (EQE) of the best performing rear reflector shows a maximum enhancement of more than 4-fold at 1160nm. This corresponds to a 16% photocurrent increase (calculated from 900nm to 1200nm) compared to the cell with conventional Al rear reflector. Moreover, from the measured spectral response and optical absorption data, we successfully separated and analysed the electrical and optical properties of the novel rear light trapping designs. Light trapping features were quantified using optical parameters characterised by an effective optical path length factor Z, while electrical parameters such as surface recombination velocity S (cm/s) and effective minority charge carrier lifetime τbulk (μs) were also extracted. Relative errors for these parameters were also calculated. For the cell with the best performing rear structure, we report a maximum Z factor enhancement of around 6-fold using Ag nanoparticles in conjunction with a detached Ag reflector, in comparison to the reference at 1200nm.

Effect of Surface Plasmon Resonance on the Photoluminescence from Si Quantum Dot Structures for Third Generation Solar Cell Applications

ABSTRACTLow dimensional structures like quantum dots (QDs) offers the the ability to tune the absorption properties of standard semiconductor materials. However, QDs are relatively weak light absorbers and hence may benefit significantly from coupling with plasmonic modes in nearby metal structures. In the case of a Si QD absorber layer for photovoltaic applications, enhanced absorption would lead to improved power conversion efficiency. Silver metal nanoparticles (MNPs) were deposited on Si QD structures using the self-assembly method of evaporation and annealing. Room temperature photoluminescence (PL) measurements were used to study the surface plasmon (SP) enhanced emission from the samples. The results were compared to conventional metal back reflectors. Enhanced surface plasmon coupled emission (SPCE) from Si QDs in the vicinity of silver metal nanoparticles (MNPs) is observed with a good correlation between the enhancement and the resonance excitation. Quenching was observed from the same emitter layers placed in close proximity to thin flat silver reflector layers, indicating the importance of the spacer layer between a metal layer and the quantum dots in optimising enhancement. The results have implications for the design of SP-enhanced QD solar cells.

The Motional Design and Analysis for Linear Fresnel Reflector System Combined Three-Movement

This paper designed a new movement for the linear Fresnel reflector system. Conventional linear Fresnel reflector system only contains one-dimension movement that is flat mirror rotation, and the addition movement in new design includes translating mirror field to decrease cosine effect and rotating the secondary reflector to coordinate the translation of mirror field. Based on the theoretical design, the optical simulation has been done, and some motile laws and geometric relationship have been included. The laws would be helpful for advance research and to develop the tracking device fit for the system combined three-movement.

Better than Bragg: Optimizing the quality factor of resonators with aperiodic dielectric reflectors

Periodic dielectric structures, such as Bragg reflectors and photonic band-gap crystals, are used to localize electromagnetic modes. A key parameter for electromagnetic resonators is the quality factor, Q. For conventional Bragg reflectors, losses occur predominantly in the first few reflectors and one of the materials usually has a low dielectric loss tangent. By varying the individual dimensions of the reflecting elements, a significant improvement in the Q-factor can be obtained. We verify this by constructing and measuring proof-of-principle microwave resonators.

Wideband Beam-Steerable Flat Reflectors via Transformation Optics

A conventional parabolic reflector is converted into a flat one based on the discrete coordinate transformation. Instead of general beam-steering techniques, such as off-axis feeding, tilting the feed/reflector, or utilizing phase shift, we show an alternative way to manipulate the reflected emission through tuning transformed dielectrics. The proposed design, only including the conventional dielectric components, has a merit of keeping the flat profile of a compact reflector system while possessing the ability to steer the radiation beams in a wide frequency band.

Raman fiber lasers with a random distributed feedback based on Rayleigh scattering

We demonstrate lasing based on a random distributed feedback due to the Raman amplified Rayleigh backscattering in different types of cavities with and without conventional point-action reflectors. Quasistationary generation of a narrowband spectrum is achieved despite the random nature of the feedback. The generated spectrum is localized at the reflection or gain spectral maxima in schemes with and without point reflectors, respectively. The length limit for a conventional cavity and the minimal pump power required for the lasing based purely on a random distributed feedback are determined.

Field penetrations in photonic crystal Fano reflectors

We report here the field and modal characteristics in photonic crystal (PC) Fano reflectors. Due to the tight field confinement and the compact reflector size, the cavity modes are highly localized and confined inside the single layer Fano reflectors, with the energy penetration depth of only 100nm for a 340 nm thick Fano reflector with a design wavelength of 1550 nm. On the other hand, the phase penetration depths, associated with the phase discontinuity and dispersion properties of the reflectors, vary from 2000 nm to 4000 nm, over the spectral range of 1500 nm to 1580 nm. This unique feature offers us another design freedom of the dispersion engineering for the cavity resonant mode tuning. Additionally, the field distributions are also investigated and compared for the Fabry-Perot cavities formed with PC Fano reflectors, as well conventional DBR reflectors and 1D sub-wavelength grating reflectors. All these characteristics associated with the PC Fano reflectors enable a new type of resonant cavity design for a large range of photonic applications.

Reflectors and reflector light and sound source systems

A reflector employs materials and design features that can transfer both light and sound emission simultaneously, from sources to planes or volumes, in an efficient and controlled manner. Compound orthogonal parabolic reflectors employ an extension onto conventional orthogonal parabolic reflectors to efficiently deliver light and/or sound to a focal volume or surface. The extension shapes the output, and can provide inflow and outflow to the focal region, along with a brush. Pulsed sources may be employed, which may emit light, sound or both light and sound, may erode and may be wire initiated with the wire replaced after each pulse by a wire feed.

Surface plasmon reflector based on serial stub structure

Plasmonic reflectors based on serial stub structure are studied in this paper. A general theory of periodic stub structure using transmission line model is developed. The transmission characteristics, e.g., periodicity and symmetry of the spectra, are closely related to the ratio of structure period to stub length. Investigation reveals that the transmission valleys of the spectra could be divided into two categories, which is quite different from conventional Bragg reflectors. Finite-Difference Time-Domain (FDTD) method is used in numerical analysis in this paper.

Optical design of retro-reflectors by coordinate transformation

Transformation optics based on the form-invariant coordinate transformation of Maxwell’s equations offers an unconventional approach for designing devices with unprecedented electromagnetic (EM) behaviors. In this paper, we expand the coordinate transformation method to design omni-directional (OD) retro-reflectors by extending the conventional Luneberg lens reflectors to work in a much wider angular width. The constitutive medium tensors of the transformation devices are derived. Based on full wave simulations combined with Huygen’s principle, the flat and efficient backscattering responses of the resultant devices have been evaluated quantitatively. We confirm that the proposed structures are highly visible to incident EM waves from all directions, and they can be used as good OD radar cross section (RCS) enhancers in radar applications, such as the reference target for RCS measurements, range finding and position identification.

Short-Circuit Current Densities Exceeding 30 $ \hbox{mA/cm}^{2}$ by Use of Chirped Porous-Silicon Reflectors and Shallow Emitters in Thin-Film (20- $\mu\hbox{m}$) Epitaxial Silicon Solar Cells

We demonstrate the use of chirped porous-silicon broadband optical reflectors for thin-film epitaxial silicon solar cells. The benefits of chirped multilayer structures over conventional Bragg reflectors on the cell level are presented. By combining these chirped reflectors with shallow emitters, we show that both low- and high-energy photons are more effectively absorbed in the thin (20-mum ) epitaxial active layer of the cell. This resulted in a high conversion efficiency of 14.2% on screen-printed epitaxial solar cells made on highly doped multicrystalline silicon substrate. The corresponding photogenerated current densities are well above 30 mA/cm2 .

Shaped Circularly Symmetric Dual Reflector Antennas by Combining Local Conventional Dual Reflector Systems

This paper presents a geometrical optics (GO) shaping method for circularly symmetric dual reflector antennas. The method is a one step procedure obtaining a local dual reflector system including both reflector surfaces and caustic simultaneously, such that a given feed power is transformed through a corresponding local reflector system into a desired aperture distribution. A shaped reflector system consists of an electrically small section of distinctive local reflector surfaces and its own caustic. Each caustic location varies and explains how the power is redistributed through the shaped reflector system. Each local reflector system is found iteratively with a previously known local dual reflector system. Several nonlinear algebraic equations are formulated based on GO principals and geometrical properties of a dual reflector system. The method reduces the solution to one simple non-differential equation with one unknown in order to find a local dual reflector system. Physical Optics (PO) analysis is used to verify the solution.

InGaN/AlGaN Ultraviolet Light-Emitting Diode with a Ti3O5/Al2O3 Distributed Bragg Reflector

Here, we report InGaN/AlGaN ultraviolet (UV) light-emitting diodes (LEDs) using distributed Bragg reflectors (DBRs). The DBRs consist of 11 layers of alternating quarter-wave thickness Ti3O5 and Al2O3 deposited onto the indium–tin-oxide ohmic contact layer of a 385 nm UV LED by ion-assisted electron beam evaporation. Numerical calculations showed that the angle averaged reflectivity of the GaN/DBR structure at 385 nm was 12% higher than that of a conventional GaN/Ag structure. The measured light output–current–voltage of the DBR UV LEDs at 20 mA current injection showed an increase of 15% in output power in comparison to a conventional Ag reflector with no degradation in electrical properties. We attributed this light output enhancement to the increased light extraction from the higher DBR reflectivity.

Chirped porous silicon reflectors for thin-film epitaxial silicon solar cells

The studies of porous silicon as a one-dimensional photonic crystal have led to solutions allowing the fabrication of broad photonic band gaps as large as several hundreds nanometers for various types of applications. In this work we demonstrate the use of the chirping process, i.e., the gradual increase in the spatial period of the structure, as it is used in image processing, to design porous silicon broad-band reflectors for thin-film silicon solar cells. Modeling of those layers is done using linear design method and a simulation software. Such chirped structures are fabricated by an anodization process. Samples are prepared on mono and multicrystalline silicon substrates with 15, 40, 60, and 80 layers. The reflectance spectra of such prepared porous reflectors are evaluated and the results show an increase in bandwidth of over 50% of the total width in comparison with the conventional, unchirped reflectors. We demonstrate clear advantages of introducing chirped multilayer structures over conventional ones. Although some understanding of chirping exists mainly in the field of image processing and photonics, detailed study of chirped porous structures as broad-band reflectors is needed for their implementation in thin-film epitaxial silicon solar cells. A record efficiency of almost 14% was reached for those types of reflectors on a large area 71 cm2 epitaxial Si solar cell, giving a boost in efficiency of 0.6% absolute in comparison with the solar cells containing conventional reflectors.

Highly efficient diffractive reflector using microgratings for reflective display

A highly efficient diffractive reflector with a rhombus shape angular distribution of reflected light for mobile liquid crystal display is developed. The fabricated diffractive reflector shows enhanced reflectance two times higher than the conventional bumpy reflector. The diffractive reflector composed of 25 surface relief microgratings is fabricated by photoresist patterning and metal deposition. The angular distribution of light that is reflected off the diffractive reflector is measured under specula illumination and diffusive illumination. The sufficient controllability of the angular distribution of the reflected light by the specific design of the period, orientation, and depth of each micrograting is shown.

Focus on lasers and optics

Focus on lasers and optics

Mesoporous Bragg Stack Color Tunable Sensors

Herein we report a novel self-assembly synthesis, structural and optical characterization of mesoporous Bragg stacks (MBS) composed of spin-coated multilayer stacks of mesoporous TiO(2) and mesoporous SiO(2). Investigation of the optical response of MBS to the infiltration of alcohols and alkanes into its pores reveals better sensitivity and selectivity than conventional Bragg reflectors. Furthermore, we demonstrate that the chemical sensing ability can be tuned via layer thickness, composition and surface properties.

Design optimization studies for large-scale contoured beam deployable satellite antennas

Satellite communications systems over the past two decades have become more sophisticated and evolved new applications that require much higher flux densities. These new requirements to provide high data rate services to very small user terminals have in turn led to the need for large aperture space antenna systems with higher gain. Conventional parabolic reflectors constructed of metal have become, over time, too massive to support these new missions in a cost effective manner and also have posed problems of fitting within the constrained volume of launch vehicles. Designers of new space antenna systems have thus begun to explore new design options. These design options for advanced space communications networks include such alternatives as inflatable antennas using polyimide materials, antennas constructed of piezo-electric materials, phased array antenna systems (especially in the EHF bands) and deployable antenna systems constructed of wire mesh or cabling systems. This article updates studies being conducted in Japan of such deployable space antenna systems [H. Tanaka, M.C. Natori, Shape control of space antennas consisting of cable networks, Acta Astronautica 55 (2004) 519–527]. In particular, this study shows how the design of such large-scale deployable antenna systems can be optimized based on various factors including the frequency bands to be employed with such innovative reflector design. In particular, this study investigates how contoured beam space antennas can be effective by constructed out of so-called cable networks or mesh-like reflectors. This design can be accomplished via “plane wave synthesis” and by the “force density method” and then to iterate the design to achieve the optimum solution. We have concluded that the best design is achieved by plane wave synthesis. Further, we demonstrate that the nodes on the reflector are best determined by a pseudo-inverse calculation of the matrix that can be interpolated so as to achieve the minimum deviation from the reflectors’ ideal shape. The RMS characteristics of the “cable network” in any deployable space antenna are constrained from exactly achieving the ideal contour because they ultimately contain very small plane facets. The effect of these plane facets on the characteristics of the contoured beams can be identified, however, by numerical simulations. Then these characteristics can be improved with the node locations being optimized as these simulations are undertaken. This process, however, must take into account a non-dimensional parameter. This parameter is based on the frequencies being employed, the plane facet size, and the angle of plane wave from the z-axis. When this optimization process is undertaken, the optimum location of the nodes in the “cable network” that compose the space antenna reflector can be precisely identified.