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We present a detailed design treatment for a concentrating photovoltaic mini module subsystem with a specific power of up to 4.1 kW/kg for integration into a space solar power system. Concentrating designs are required to achieve specific power over 1 kW/kg with current high-efficiency III-V multijunction solar cells. The 15 sun, linear concentration concept detailed here reduces the system mass by replacing cell and radiation shield area with ultralight carbon fiber reinforced polymer (CFRP) optics. Reducing the cell size to 1 mm width as well as careful optimization of cell architecture and CFRP material and thickness are critical for maintaining cell temperatures under 100 C despite the concentration. We also describe ultralight multilayer optical coatings to increase the thermal emissivity of the concentrator surfaces and enhance radiative transfer for cell cooling, which is a critical technological component of the total system design.

Scattering particles constitute a key light trapping solution for thin film photovoltaics where either the particles are embedded in the light absorbing layer or a thick layer of them is used as a reflector. Here we introduce a monolayer of wavelength-scale core-shell silica@Ag particles as a novel light trapping strategy for thin film photovoltaics. These particles show hybrid photonic-plasmonic resonance modes that scatter light strongly and with small parasitic absorption losses in Ag (1.5%). In addition, their scattering efficiency does not vary significantly with the refractive index of the surrounding medium. A monolayer of these particles is applied as the top-scattering layers in a dye-sensitized solar cells and it improves the short-circuit current density of a cell with 7 μm-thick dye-sensitized layer by 38%. Optical measurements of the scattering properties of these particles confirm that the strong scattering and low-parasitic absorption losses constitute the main reason for this efficient light trapping.

We report the design, fabrication, and characterization of ultralight highly emissive structures with a record-low mass per area that emit thermal radiation efficiently over a broad spectral (2 to 30 microns) and angular (0-60°) range. The structures comprise one to three pairs of alternating metallic and dielectric thin films and have measured effective 300 K hemispherical emissivity of 0.7 to 0.9 (inferred from angular measurements which cover a bandwidth corresponding to 88% of 300K blackbody power). To our knowledge, these micron-scale-thickness structures, are the lightest reported optical coatings with comparable infrared emissivity. The superior optical properties, together with their mechanical flexibility, low outgassing, and low areal mass, suggest that these coatings are candidates for thermal management in applications demanding of ultralight flexible structures, including aerospace applications, ultralight photovoltaics, lightweight flexible electronics, and textiles for thermal insulation.

We have fabricated a functional prototype of an ultralight power converter tile; a modular building block for a space-based solar power system. The tile is $\sim 10\mathrm \times 15$ cm in area, and weighs $\sim 1.5$ kg/$\text{m}^{\mathbf {2}}$. It comprises a photovoltaic (PV) solar energy collector, a radio-frequency (RF) power converter, and an array of transmission antennas. The PV collector subassembly utilizes $\sim 15\mathrm {x}$, 1D parabolic trough reflective concentrators with triple-junction (3J) solar cells. It has areal mass of $\sim 0.8$ kg/$\textbf{m}^{\mathbf {2}}$, 74% optical efficiency, and a peak specific power of $\sim 230\mathrm {W} /$kg. We demonstrated wireless power transmission over a distance of $\sim 50$ cm in our lab. Analysis of the sources of mass and inefficiency suggest a path towards achieving dramatically higher performance with future designs.

Concentrator photovoltaic systems can provide supplementary shielding against high energy particles. In this paper we compare the radiation environment that the same solar cell would experience in a flat-plate module versus in a parabolic mirror concentrator system. We have observed that the shielding provided by the concentrator system is remarkable. In order to obtain an accurate prediction of the overall shield needed in our concentrator system triple-junction space solar cells have been irradiated on the edge with 350-keV protons at a fluence of 1012P + cm−2. A mild degradation of the open circuit voltage was measured (~70 mV).

Lightweight parabolic mirrors for solar concentrators have been fabricated using carbon fiber reinforced polymer (CFRP) and a nanometer scale optical surface smoothing technique. The smoothing technique improved the surface roughness of the CFRP surface from ~3 {\mu}m root mean square (RMS) for as-cast to ~5 nm RMS after smoothing. The surfaces were then coated with metal, which retained the sub-wavelength surface roughness, to produce a high-quality specular reflector. The mirrors were tested in an 11x geometrical concentrator configuration and achieved an optical efficiency of 78% under an AM0 solar simulator. With further development, lightweight CFRP mirrors will enable dramatic improvements in the specific power, power per unit mass, achievable for concentrated photovoltaics in space., Comment: IEEE Photovoltaic Specialist Conference (PVSC), DC, USA, 2017

We report the design, fabrication and characterization of ultrathin metasurfaces that exhibit wideband 300 K thermal emissivity. The emissive behavior of these structures is almost independent of the emission angle. Our ultralight subwavelength-thickness metasurfaces can be fabricated relatively easily and are excellent candidates for radiative cooling in space applications.

We report the effects of high energy electron and proton radiation on perovskite solar cells. For irradiation with 1 MeV electrons, at fluence from 1012 to 1016 cm−2, there are no significant changes in the morphology and the crystal phase in the perovskites, and the perovskite solar cells show only slight degradation in photovoltaic performance and spectral response. The results from Monte Carlo simulations show most of the electrons completely penetrate through all of the layers of solar cells with little scattering. In addition, 50 ke V proton radiation with fluence of 1012−2 has no significant impact on the open-circuit voltage or short-circuit current, and degrades only the fill factor. We have further found that the fill factor can be restored with a vacuum annealing process. The results suggest that perovskites have superior electron and proton radiation tolerance, and thus hold particular promise for space applications.

The Space Solar Power Initiative (SSPI) seeks to enable reliable, cost-effective baseload power generation from large-scale solar power stations in space. We propose an ultralight, modular power station, having specific power in the range of 1–10 kW/kg for the photovoltaic (PV) collection subsystem. The building block of the power station is the ‘tile,’ a self-contained element that performs PV energy collection, conversion to radio frequency (RF), and transmission to earth. To minimize PV mass, we select a 1D, 10-20X parabolic trough concentrator geometry, which provides cooling and radiation shielding for the cells, and which folds flat for deployment. Here, we discuss the design, fabrication, and testing of the initial PV tile prototypes.

We investigate the angular behavior of the upper bound of absorption provided by the guided modes in thin film solar cells. We show that the 4n^2 limit can be potentially exceeded in a wide angular and wavelength range using two-dimensional periodic thin film structures. Two models are used to estimate the absorption enhancement; in the first one, we apply the periodicity condition along the thickness of the thin film structure but in the second one, we consider imperfect confinement of the wave to the device. To extract the guided modes, we use an automatized procedure which is established in this work. Through examples, we show that from the optical point of view, thin film structures have a high potential to be improved by changing their shape. Also, we discuss the nature of different optical resonances which can be potentially used to enhance light trapping in the solar cell. We investigate the two different polarization directions for one-dimensional gratings and we show that the transverse magnetic polarization can provide higher values of absorption enhancement. We also propose a way to reduce the angular dependence of the solar cell efficiency by the appropriate choice of periodic pattern. Finally, to get more practical values for the absorption enhancement, we consider the effect of parasitic loss which can significantly reduce the enhancement factor.

Creating arbitrary light patterns finds applications in various domains including lithography, beam shaping, metrology, sensing and imaging. We study the formation of high-contrast light patterns that are obtained by transmission through an ordered optical element based on self-imaging.By applying the phase-space method, we explain phenomena such as the Talbot and the angular Talbot effects. We show that the image contrast is maximum when the source is either a plane wave or a point source, and it has a minimum for a source with finite spatial extent. We compare these regimes and address some of their fundamental differences. Specifically, we prove that increasing the source divergence reduces the contrast for the plane wave illumination but increases it for the point source. Also, we show that to achieve high contrast with a point source, tuning the source size and its distance to the element is crucial.We furthermore indicate and explore the possibility of realizing highly complex light patterns by using a periodic transmission element. These patterns can have more spots in the far field than the number of diffraction orders of the periodic element. We predict that the ultimate image contrast is smaller for a point source compared to a plane wave. Our simulations confirm that the smallest achievable spot size in the image is imposed by diffraction regardless of the imaging regime. Our research can be applied to similar domains e.g. quantum systems.

A rigorous, fast and efficient method is proposed for analytical extraction of guided defect modes in two-dimensional photonic crystals, where each Bloch spatial harmonic is expanded in terms of Hermite–Gauss functions. This expansion, after being substituted in Maxwell’s equations, is analytically projected in the Hilbert space spanned by the Hermite–Gauss basis functions, and then a new set of first order coupled linear ordinary differential equations with non-constant coefficients is obtained. This set of equations is solved by employing successive differential transfer matrices, whereupon defect modes, i.e. the guided modes propagating in the straight line-defect photonic crystal waveguides and coupled resonator optical waveguides, are analytically derived. In this fashion, the governing differential equations are converted into an algebraic and easy to solve matrix eigenvalue problem. Thanks to the analyticity of the proposed approach, the eigenmodes of these structures can be extracted very quickly. The validity of the obtained results is however justified by comparing them to those derived by using the standard finite-difference time-domain method.

We try to find out the details of how light fields behind the structures of photomasks develop in order to determine the best conditions and designs for proximity printing. The parameters that we use approach real situations like structure printing at proximity gaps of 20 to 50 μm and structure sizes down to 2 μm. This is the first time that an experimental analysis of light propagation through a mask is presented in detail, which includes information on intensity and phase. We use high-resolution interference microscopy (HRIM) for the measurement. HRIM is a Mach–Zehnder interferometer, which is capable of recording three-dimensional distributions of intensity and phase with diffraction-limited resolution. Our characterization technique allows plotting the evolution of the desired light field, usually called the aerial image, and therefore gives access to the printable structure until the desired proximity gap. Here, we discuss in detail the evolution of intensity and phase fields of elbow or corner structures at different positions behind a phase mask and interpret the main parameters. Of particular interest are tolerances against proximity gap variation and the theoretical explanation of the resolution in printed structures.

We calculate the maximal absorption enhancement obtainable by guided mode excitation in a weakly absorbing dielectric slab over wide wavelength ranges. The slab mimics thin film silicon solar cells in the low absorption regime. We consider simultaneously wavelength-scale periodicity of the texture, small thickness of the film, modal properties of the guided waves and their confinement to the film. Also we investigate the effect of the incident angle on the absorption enhancement. Our calculations provide tighter bounds for the absorption enhancement but still significant improvement is possible. Our explanation of the absorption enhancement can help better exploitation of the guided modes in thin film devices., accepted for publication in JOSA B

We explore the limit of absorption enhancement in thin film solar cells obtained by guided mode excitation and we discuss the geometrical and angular dependence of absorption. We consider the modal properties of the cell.

Finding the optimal structure to enhance light trapping in thin film silicon solar cells has attracted much attention in the previous decades. However, because of problems in integrating theory and experiment, there are only few comprehensive contributions that provide guidelines for the optimal design of such structures. In this work, a realistic thin film solar cell with almost conformal layers based on a one-dimensional metallic grating back-reflector is investigated through experiment and theory. The external quantum efficiency of the cell is obtained with the aid of both theory and experiment for different angles of incidence and in both polarizations to validate the computational method and to show the impact of guided mode excitation. Different substrate shapes that are compatible with solar cell fabrication are then considered and the effect of geometrical parameters on the short circuit current density of the device is investigated. Calculations show that among the investigated shapes, trinagular gratings with a very sharp slope in one side, so called sawtooth gratings, are the most promising one-dimensional grating for light trapping. Furthermore, the role of material property is discussed specifically in the back-reflector by simulating aluminum and silver back-reflectors. It is shown that the blue response of the solar cells is similar almost regardless of the back-reflector material but their red response is viable to change due to variation in resonant properties of the structure.

Effect of relaxation time on the performance of photonic crystal optical bistable switches based on Kerr nolinearity is discussed. This paper deals with optical pulses with the duration of about 50 ps. In such cases the steady state response of the optical device can be used to approximate the pulse evolution if the nonlinearity is assumed instantaneous, hence analytical solutions such as the coupled mode theory can be used to obtain the time evolution of the electromagnetic fields. However if the relaxation time of the material nonlinear response is also considered, changes in the power levels and in the shape of the hystersis loop is observed. In this case, we use the nonlinear finite difference time domain method (NL-FDTD) to follow the system dynamics and get the bistability hystersis loop. Codes are developed to analyze the instantaneous Kerr materials and the Kerr materials with finite response times. Depending on the material, the relaxation times of the order of 1-10fs should be considered in studying bistability to obtain the right shape of the output pulses. It is observed that the relaxation leads to larger input power and threshold and hence degrades the performance of the switch in pulse shaping.

An optical amplifier followed by a bistable element is statistically analyzed. To this end, the birth-death-immigration model and the coupled mode theory are used to characterize the amplifier and the bistable element, respectively.

In this work, an optimum frequency is found for the operation of single cavity photonic switches. At this optimum point, the transmission contrast of ON and OFF states takes its highest value, while keeping the device power threshold relatively low and the device speed acceptably high. Then, the dynamic behavior of a typical single cavity all optical switch is investigated in the optimum operation point through temporal Coupled Mode Theory. Switching speed and power are discussed, and the device is shown to be applicable for telecommunication and data processing applications. The analysis is quite general, and can be used for resonant structures, such as photonic crystals and microring resonators, in both side coupled and direct coupled configurations.

We present a comparison between spectrally-resolved reflection measurements of gratings and theoretical predictions from RCWA approach. Diffraction intensities vary significantly with polarization, and surface plasmon absorption is observed around the onset of the diffraction orders.

We use the 3D vectorial Gaussian beam tracing method to model millimeter wave propagation. We simulate the propagation of Gaussian beams in a large scale environment at 62.4 GHz. First, a known far field pattern of the transmitter antenna is expanded into a set of Gaussian beams. We chose Gabor expansion based method to expanding radiating field into Gaussian beams. Then, a Gaussian beam tracing model considering reflections and transmission is used to represent urban microcell propagation. We use a phase matching method to find reflected Gaussian beam The results of the Gaussian beam tracing simulations including line-of-sight (LOS) and non-line-of-sight (NLOS) propagation are compared with the results of the ray tracing method. The results are in good agreement with results of ray tracing technique on the same geometry. Overall coverage map shows the dominant effect of shadowing in the millimeter wave band. The tracing technique is faster and more promising compared to exhausted ray tracing methods.

Low power operation and high speed have always been desirable in applications such as data processing and telecommunications. While achieving these two goals simultaneously, however, one encounters the well-known powerbandwidth trade-off. This is here discussed in a typical bistable switch based on a two-dimensional photonic crystal with Kerr type nonlinearity. The discussion is supported by the nonlinear finite difference time domain (FDTD) simulation of a direct coupled structure with a home-developed code. Two cases of working near resonant and offresonant are simulated to compare the power and the speed of the device in the two cases. It is shown that working nearresonance reduces the power levels at the expense of reducing the settling time, i.e. the bandwidth limitation. The hystersis loops for the device are also obtained with both coupled-mode theory and quasi-steady state FDTD simulation. The impact of operating near/off resonance on the shape of the hystersis loop is discussed as a confirmation of the previous results. Alternative ways of reducing the power while saving the bandwidth are also examined. The discussion is general and one may investigate other optical switches to obtain similar results.

The Goos-Haenchen shift of a totally reflected beam at the planar interface of two dielectric media, as if the incident beam is reflected from beneath the interface between the incident and transmitted media, has been geometrically associated with the penetration of the incident photons in the less-dense forbidden transmission region. This geometrical approach is here generalized to analytically calculate the Goos-Haenchen shift in one- and two-dimensional periodic structures. Several numerical examples are presented, and the obtained results are successfully tested against the well-known Artman's formula. The proposed approach is shown to be a fast, simple, and efficient method that can provide good physical insight to the nature of the phenomenon.

We propose and study an integrated refractive index sensor which is based on a plasmonic slot cavity integrated on silicon on insulator wafers. In this device the guided mode is vertically coupled to the cavity which is a metal dielectricmetal waveguide and is separated from a photonic wire waveguide by a silicon dioxide spacer. We perform an in depth study that links the geometrical parameters to the coupling and sensitivity. The strong coupling from the dielectric waveguide to the plasmonic slot waveguide cavity allows a local change in refractive index to be detectable with a high sensitivity of around 600 nm/RIU in a femto liter volume. These results are obtained with three dimensional time domain simulations made with the CST Microwave Studio. The sensing performance of the devices are presented and compared to the practical needs to achieve localized biomolecular sensing. © 2014 SPIE.

We report on the Gouy phase anomaly of light in the focus of cylindrical and spherical microlenses. The prime subject of our study concerns a discussion of how the very small size of microlenses affects the phase properties of light in their foci. We put emphasis on determining the amount of the Gouy phase shift for line and point foci within the limited axial space. Contrary to macroscopic lenses the optical properties of microlenses are strongly governed by the effect of diffraction when their size tends to be comparable to the operation wavelength. In our study we clearly show how such diffraction features affect the axial phase shift. For instance phase singularities which occur at discrete points on the optical axis where the total intensity vanishes for spherical microlenses cause an additional axial phase shift when compared to the cylindrical microlens where those axial phase singularities are absent. The rotational symmetry of the Fresnel zones is the origin of such a difference between point and line foci. © 2013 IOP Publishing Ltd.

We studied solar cells with periodic interface texture. For periods of 550 and 833 nm we found that the measured photocurrent increases under oblique incidence and assumes a maximum between 20°and 30°. For the geometries used in our experiments the initial increase occurred for conditions close to those of photonic band gaps. In addition to a forbidden band of energies where waveguide modes cannot propagate we found that the coupling to the high energy branches was suppressed under conditions of high symmetry such as sinusoidal interface geometry and perpendicular incidence. © 2013 AIP Publishing LLC.

We study light scattering and absorption in thin film solar cells using a model system of a sinusoidally textured silver reflector and dielectric layers of ZnO and amorphous silicon. Experimental results are compared to a theoretical model based on a Rayleigh expansion. Taking into account the explicit interface profile the expansion converges fast and can be truncated typically after three or four orders. At the same time the use of realistic permittivity data correctly reproduces the intensity of diffracted orders as well as the coupling to guided modes and surface plasmon polariton resonances at the silver surface. The coupling phenomena behind the light trapping process can therefore be assessed in a simple yet accurate manner. © 2012 American Institute of Physics.

We study absorption enhancement by light scattering at periodically textured interfaces in thin film silicon solar cells. We show that the periodicity establishes resonant coupling to propagating waveguide modes. Ideally, such modes propagate in the high index silicon film where they are eventually absorbed, but waveguide modes exist also in the transparent front contact layer if the product of its refractive index and thickness exceeds half the wavelength. Taking into account that the absorption coefficient of realistic transparent conducing films exceeds the one of silicon close to its band gap, certain waveguide modes will enhance parasitic absorption in the transparent front contact. From an analysis based on the statistic distribution of energy among the available waveguide and radiation modes, we conclude that conventional thin film silicon solar cells with thick and nonideal contacts may fail to reach the previously noted bulk limit of 4n(Si)(2); instead, a more conservative limit of 4(n(Si)(2) - n(TCO)(2)) applies. (C) 2011 American Institute of Physics. [doi: 10.1063/1.3569689]

Despite the progress in the engineering of structures to enhance photocurrent in thin film solar cells, there are few comprehensive studies which provide general and intuitive insight into the problem of light trapping. Also, lack of theoretical propositions which are consistent with fabrication is an issue to be improved. We investigate a real thin film solar cell with almost conformal layers grown on a 1D grating metallic back-reflector both experimentally and theoretically. Photocurrent increase is observed as an outcome of guided mode excitation in both theory and experiment by obtaining the external quantum efficiency of the cell for different angles of incidence and in both polarization directions. Finally, the effect of geometrical parameters on the short circuit current density of the device is investigated by considering different substrate shapes that are compatible with solar cell fabrication. Based on our simulations, among the investigated shapes, triangular gratings with a very sharp slope in one side, so called sawtooth gratings, are the most promising 1D gratings for optimal light trapping.

We study the plasmonic and the non-plasmonic regimes of operation of a thin-film amorphous-silicon solar cell. By rigorous calculation, we discuss the impact of the cell geometry on the nature of its optical resonances.

Thin film silicon solar cells are attractive for photovoltaics; however, the poor charge transport in this material requires that the devices are thinner than the absorption length. Adequate absorption can nevertheless be achieved by light scattering at textured interfaces because light can get trapped inside the absorber layer if it is scattered into angles above the critical angle of total internal reflection. This situation can be identified with the propagation of a guided mode in a waveguide where silicon plays the role of the high index guiding medium and the interface texture serves to couple the incident light to modes via grating coupling. We present an experimental realization of a solar cell structure on a line grating where the enhanced photocurrent can be clearly related to resonant excitation of waveguide modes.

We investigate the influence of patterned metallic back reflector on absorption in thin-film silicon photovoltaics. The impact of symmetry, periodicity, and angle of incidence of light is discussed by simulation and experiment.

We study n i p amorphous silicon solar cells with light scattering nanoparticles in the back reflector. In one configuration the particles are fully embedded in the zinc oxide buffer layer; In a second configuration the particles are placed between the buffer layer and the flat back electrode. We use stencil lithography to produce the same periodic arrangement of the particles and we use the same solar cell structure on top thus establishing a fair comparison between a novel plasmonic concept and its more traditional counterpart. Both approaches show strong resonances around 700 nm in the external quantum efficiency the position and intensity of which vary strongly with the nanoparticle shape. Moreover disagreement between simulations and our experimental results suggests that the dielectric data of bulk silver do not correctly represent the reality. A better fit is obtained by introducing a porous interfacial layer between the silver and zinc oxide. Without the interfacial layer e.g. by improved processing of the nanoparticles our simulations show that the nanoparticles concept could outperform traditional back reflectors. © 2013 Optical Society of America.