title: Omnidirectional and broadband absorption enhancement from trapezoidal Mie resonators in semiconductor metasurfaces.
abstract: Light trapping in planar ultrathin-film solar cells is limited due to a small number of optical modes available in the thin-film slab. A nanostructured thin-film design could surpass this limit by providing broadband increase in the local density of states in a subwavelength volume and maintaining efficient coupling of light. Here we report a broadband metasurface design, enabling efficient and broadband absorption enhancement by direct coupling of incoming light to resonant modes of subwavelengthscale Mie nanoresonators defined in the thin-film active layer. Absorption was investigated both theoretically and experimentally in prototypes consisting of lithographically patterned, two-dimensional periodic arrays of silicon nanoresonators on silica substrates. A crossed trapezoid resonator shape of rectangular cross section is used to excite broadband Mie resonances across visible and near-IR spectra. Our numerical simulations, optical absorption measurements and photocurrent spectral response measurements demonstrate that crossed trapezoidal Mie resonant structures enable angle-insensitive, broadband absorption. A short circuit current density of 12.0 mA/cm(2) is achieved in 210 nm thick patterned Si films, yielding a 4-fold increase compared to planar films of the same thickness. It is suggested that silicon metasurfaces with Mie resonator arrays can provide useful insights to guide future ultrathin-film solar cell designs incorporating nanostructured thin active layers.
authors.affiliation: California Institute of Technology, Thomas J. Watson Laboratories of Applied Physics, Pasadena, 91125, United States.|California Institute of Technology, Kavli Nanoscience Institute, Pasadena, 91125, United States.
authors.forename: Ragip A
authors.initials: RA
authors.lastname: Pala
authors.affiliation: Northwestern University, Department of Electrical Engineering and Computer Science, Evanston, 60208, United States.
authors.forename: Serkan
authors.initials: S
authors.lastname: Butun
authors.affiliation: Northwestern University, Department of Electrical Engineering and Computer Science, Evanston, 60208, United States.
authors.forename: Koray
authors.initials: K
authors.lastname: Aydin
authors.affiliation: California Institute of Technology, Thomas J. Watson Laboratories of Applied Physics, Pasadena, 91125, United States.|California Institute of Technology, Kavli Nanoscience Institute, Pasadena, 91125, United States.
authors.forename: Harry A
authors.initials: HA
authors.lastname: Atwater