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In a conventional flat plate solar cell under direct sunlight, light is received from the solar disk, but is re-emitted isotropically. This isotropic emission corresponds to a significant entropy increase in the solar cell, with a corresponding drop in efficiency. Here, using a detailed balance model, we show that limiting the emission angle of a high-quality GaAs solar cell is a feasible route to achieving power conversion efficiencies above 38% with a single junction. The highest efficiencies are predicted for a thin, light trapping cell with an ideal back reflector, though the scheme is robust to a non-ideal back reflector. Comparison with a conventional planar cell geometry illustrates that limiting emission angle in a light trapping geometry not only allows for much thinner cells, but also for significantly higher overall efficiencies with an excellent rear reflector. Finally, we present ray-tracing and detailed balance analysis of two angular coupler designs, show that significant efficiency improvements are possible with these couplers, and demonstrate initial fabrication of one coupler design.

We compare the optical response of periodic nondiffracting metallic nanoparticle and nanohole arrays. Experimental data from both structures show a pronounced minimum in their wavelength-dependent transmittance that, through numerical modeling, we identify as being due to the excitation of localized surface-plasmon resonances associated with the nanoparticles/nanoholes. Our main finding is that, while the optical response of the nanoparticle arrays is largely independent of interparticle separation, the response from nanohole arrays shows a marked dependence on interhole separation. We attribute this effect to coupling between localized surface-plasmon resonances mediated by the symmetric surface plasmon-polaritons associated with the metal film. Further numerical modeling supports this view. [4][5][6] applications that exploit the strongly localized electromagnetic fields associated with these resonant modes. Arrays of nanoparticles have in particular received considerable attention as advances in fabrication techniques allow finer control over structural dimensions. [7][8][9][10][11] It is also known that when metal nanoparticles are brought into close proximity to each other the modes they support may interact, or couple, so as to modify both the resonance shape and frequency of the LSPRs. 12,13 The properties of LSPRs associated with metallic nanoparticles can also be modified by the presence of a nearby metallic surface. 14,15 The extraordinary optical transmission properties of regular arrays of nanoholes in thin metal films were first reported by Ebbesen and co-workers. 16 Since then it has been demonstrated that under appropriate conditions single nanoholes in metallic films may support LSPRs in a manner analogous to that of nanoparticles. [17][18][19][20][21][22] The similarities between the LSPRs of nanoholes and nanodiscs were discussed by Haynes et al. 10 Indeed, Käll and co-workers 17 recently showed that for irregular arrays of such holes the LSPRs of the nanoholes are blueshifted as the hole density is increased, an effect attributed to coupling between LSPRs of neighboring holes. However, as far as we are aware, there has not yet been a comparison of interparticle/interhole coupling in periodic nondiffracting metallic nanoparticle/nanohole arrays.Here we present such a study and show that despite their similarities, these complementary structures show marked differences in their optical response. For the range of periods considered here, we find that the spectral position of the transmittance minimum associated with the nanohole arrays varies with the array period, an effect we attribute to strong LSPR coupling mediated by surface plasmon-polaritons ͑SPPs͒ supported by the intervening flat metallic film. For the complementary nanoparticle arrays there is little shift since SPPs are not supported by this structure. Results from numerical modeling help us identify the role of the symmetric ͑with respect to surface charge distributions 23 ͒ SPP mode supported by the metal film in causing this diffe...

We have fabricated microphotonic parabolic light directors using two-photon lithography, thin-film processing, and aperture formation by focused ion beam lithography. Optical transmission measurements through upright parabolic directors 22 μm high and 10 μm in diameter exhibit strong beam directivity with a beam divergence of 5.6°, in reasonable agreement with ray-tracing and full-field electromagnetic simulations. The results indicate the suitability of microphotonic parabolic light directors for producing collimated beams for applications in advanced solar cell and light-emitting diode designs.

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A metamaterial layer comprising of a conducting square mesh surrounding subwavelength holes has a largely pure imaginary effective refractive index. We explore the microwave transmissivity of a stack of such metamaterial layers separated by dielectric spacers. As expected, a family of high transmissivity bands is experimentally observed. It is found that the lowest frequency edge is independent of the number of unit cells making up the structure and is highly tunable by appropriate geometrical design of the metamaterial layers. © 2009 American Institute of Physics. ͓doi:10.1063/1.3253703͔Multilayer metal-dielectric structures have been extensively studied at visible frequencies, and their response utilized in areas such as electromagnetic shielding, nonlinear photonics and perfect lensing.1-3 Geffcken 4 in 1939 fabricated metal-dielectric thin film stacks that exhibited transmission features that were significantly narrower than those previously observed in conventional dielectric-dielectric multilayer arrangements. 5The spectral response of metal-dielectric stacks in the visible regime comprises of a series of photonic band gaps where the reflectivity is high ͑and the transmissivity is low͒, separated by a series of peaks of high transmissivity. These transmission peaks correspond to near-standing-wave resonances within each dielectric cavity, coupled together via exponential fields within the metal film. Near the high frequency band edge of the first transmission band, the electric fields are predominantly confined to the dielectric and pass through zero in the metal. In contrast, at the low frequency band edge a significant proportion of the field enhancement occurs inside the metal regions. [6][7][8] An equivalent study of a metal-dielectric layer stack in the microwave domain ͑10 9 -10 10 Hz͒ is at first sight impractical since the real and imaginary parts of the refractive index of metals are both large ͑Ͼ10 3 ͒ and almost equal ͑i.e., the metal is near-perfectly conducting͒. Even a metal film of thickness 20 nm will almost completely screen the incident field 9 because of the large impedance mismatch. Instead, a metal is structured on the subwavelength scale to create a metamaterial with effective electromagnetic properties which replicates the behavior of Drude-like ͑plasmonic͒ metals in the visible regime ͑Ag, Au, etc.͒. 10 The metamaterial layer consists of a non diffracting square metal mesh surrounding an array of identical square holes. At wavelengths greater than the size of the holes, the electromagnetic fields are exponential within the holes with a decay length that is primarily dictated by the metamaterial geometry. Consequently the metamaterial is equivalent to a thin layer with a pure imaginary refractive index. 12,13The sample shown in Fig. 1͑a͒ comprises of eight printed circuit board ͑PCB͒ layers that are originally clad with 18 m of copper on one face. The copper is removed from three of these substrates the remaining five being etched to leave a copper square mesh ͑Fig. 1͑b͒͒ with period...

The electromagnetic resonances of multilayer metal-dielectric stacks are investigated. These structures support periodic bandpass regions, whose band edges may be predicted by considering the character of the fields inside the different layers. It is shown that the response of the structure is largely independent of its overall length, and that only the geometry of the unit cell is important. In the metal layers, the fields may have either a cosh or a sinh distribution function and match to standing waves inside the adjacent dielectric cavities at the metal-dielectric interface. It is shown that the different boundary conditions, imposed by the evanescent fields, result in the dielectric layers having a different effective length for the two modes. The sinh fields result in an effective length being very close to that of the physical length, and adjacent cavities oscillating out of phase, while the cosh fields may result in a significantly larger effective dielectric length and adjacent cavities oscillating in phase. A bandpass region is opened, with its high frequency edge always being near the dielectric Fabry-Perot limit, while the low frequency band edge is significantly redshifted.

We study the propagation of light in a three-dimensional double-periodic Ag/TiO 2 multilayer metamaterial composed of coupled plasmonic waveguides operating in the visible and UV spectral range. For these frequencies, light propagation in the plane of the waveguides is described by a negative phase velocity, while for the orthogonal direction light propagation is described by a Bloch wave composed of a large number of harmonics. As a result, the material cannot generally be described by a single phase index: decomposing the Bloch wave into different harmonics we show that for the wavelength range of interest the positive index m=1 harmonic dominates the propagation of light in the orthogonal direction. These results are corroborated by numerical simulations and optical refraction experiments on a double-periodic Ag/TiO 2 multilayer metamaterial prism in the 380-600 nm spectral range, which show that positive refraction associated with right-handed harmonics dominates. Studying the isofrequency contours we find that despite the occurrence of multiple harmonics the double-periodic structure can act as a flat lens: for a slab consisting of an integer number of unit cells all harmonics are degenerate and constructively interfere at the image plane. This work identifies important considerations relevant to the design of many three dimensional periodic metamaterials.