Liu Liu

The excitation of multiple surface-plasmon-polariton (SPP) waves and waveguide modes was experimentally demonstrated in a structure fabricated as a one-dimensional photonic crystal (PC) on top of a two-dimensional metal grating. The PC had two periods, each period comprising nine layers of silicon oxynitrides of different compositions. The individual excitations of the SPP waves and waveguide modes were also theoretically predicted using the Floquet theory, surface-multiplasmonics theory, and the transfer–matrix approach for multilayered waveguides. Both the theoretical and experimental results indicate broadband coupling of incident light of either linear polarization state to the guided waves of both types over a broad range of the angle of incidence.

The Floquet theory and the transfer-matrix approach were used to investigate the excitation of surfaceplasmon-polariton (SPP) waves and waveguide modes in a structure comprising a one-dimensional photonic crystal (1D PC) of finite thickness on top of a planar thick metallic layer. The solutions of the relevant dispersion equations were used to predict the excitation of multiple SPP waves and waveguide modes when the metallic layer is patterned as a two-dimensional (2D) surface-relief grating. The same structure was experimentally fabricated and optically characterized to validate the theoretical approach. Both the theoretical and experimental results show broadband coupling of incident light of either linear polarization state over a broad range of the angle of incidence. This structure has potential applications in planar optical concentrators.

Nanostructured optical coatings with tailored spectral absorption properties are of interest for a wide range of applications such as spectroscopy, emissivity control, and solar energy harvesting. Optical metamaterial absorbers have been demonstrated with a variety of customized single band, multiple band, polarization, and angular configurations. However, metamaterials that provide near unity absorptivity with super-octave bandwidth over a specified optical wavelength range have not yet been demonstrated experimentally. Here, we show a broadband, polarization-insensitive metamaterial with greater than 98% measured average absorptivity that is maintained over a wide ±45° field-of-view for mid-infrared wavelengths between 1.77 and 4.81 μm. The nearly ideal absorption is realized by using a genetic algorithm to identify the geometry of a single-layer metal nanostructure array that excites multiple overlapping electric resonances with high optical loss across greater than an octave bandwidth. The response is optimized by substituting palladium for gold to increase the infrared metallic loss and by introducing a dielectric superstrate to suppress reflection over the entire band. This demonstration advances the state-of-the-art in high-performance broadband metamaterial absorbers that can be reliably fabricated using a single patterned layer of metal nanostructures.

A tandem solar cell comprising two p-i-n solar cells made of amorphous-silicon alloys and backed by a periodically corrugated metallic back-re ector was theoretically investigated. The intrinsic semiconductor layers in both constituent solar cells were taken to be nonhomogeneous with linearly varying bandgap. An AM1.5 solar irradiance spectrum was incorporated in the nite-di erence-time-domain calculations (Lumerical^TM) to obtain the generation rate of electron-hole pairs, which was then used in Synopsys Sentaurus^TM to compute the electrical properties of the solar cell. The short-circuit current increases when the intrinsic layers are nonhomogeneous as compared to homogeneous intrinsic layers. However, electrical simulations showed that a new approach to modeling is needed that can take into account the continuously varying bandgap instead of considering it as piecewise uniform.

Light incident upon a periodically corrugated metal/dielectric interface can generate surface plasmon polariton (SPP) waves. This effect is used in many sensing applications. Similar metallodielectric nanostructures are used for light trapping in solar cells, but the gains are modest because SPP waves can be excited only at specific angles and with one linear polarization state of incident light. Here we report the optical absorptance of a metallic grating coupled to silicon oxide/oxynitride layers with a periodically varying refractive index, i.e., a 1D photonic crystal. These structures show a dramatic enhancement relative to those employing a homogeneous dielectric material. Multiple SPP waves can be activated, and both s- and p-polarized incident light can be efficiently trapped. Many SPP modes are weakly bound and display field enhancements that extend throughout the dielectric layers. These modes have significantly longer propagation lengths than the single SPP modes excited at the interface of a metallic grating and a uniform dielectric. These results suggest that metallic gratings coupled to photonic crystals could have utility for light trapping in photovoltaics, sensing, and other applications.