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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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