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Near-unity broadband absorption designs for semiconducting nanowire arrays via localized radial mode excitation

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Abstract

We report design methods for achieving near-unity broadband light absorption in sparse nanowire arrays, illustrated by results for visible absorption in GaAs nanowires on Si substrates. Sparse (<5% fill fraction) nanowire arrays achieve near unity absorption at wire resonant wavelengths due to coupling into ‘leaky’ radial waveguide modes of individual wires and wire-wire scattering processes. From a detailed conceptual development of radial mode resonant absorption, we demonstrate two specific geometric design approaches to achieve near unity broadband light absorption in sparse nanowire arrays: (i) introducing multiple wire radii within a small unit cell array to increase the number of resonant wavelengths, yielding a 15% absorption enhancement relative to a uniform nanowire array and (ii) tapering of nanowires to introduce a continuum of diameters and thus resonant wavelengths excited within a single wire, yielding an 18% absorption enhancement over a uniform nanowire array.

© 2014 Optical Society of America

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Figures (4)

Fig. 1
Fig. 1 Absorption vs. wavelength for a 150 nm planar layer of GaAs (black line) compared to a 5% fill fraction uniform array of GaAs nanowires with radii of 65 nm and heights of 3 µm (red line) with an inset demonstrating a radial cross section of power absorption for the TM11 mode at its resonant wavelength of 675 nm
Fig. 2
Fig. 2 (a) Schematic of the mechanism of scattering and coupling into resonant leaky radial optical waveguide modes in the nanowire array with multiple radii; (b) Aerial view of one unit cell of the array with multiple nanowire radii and schematic of radial modes in nanowires of various radii, labeled with their TM11 resonant wavelengths; (c) Absorption vs. wavelength for each individual wire in the optimized multi-radii wire array depicted in (a) with arrows indicating corresponding curve/peak and wire radius
Fig. 3
Fig. 3 (a) Array of optimized GaAs truncated nanocones with tip radii of 40 nm, base radii of 100 nm and heights of 3 µm, labeling x, y, and z dimensions and indicating the vertical cross section shown in (c); (b) Absorption in a single truncated nanocone integrated over x and y, its radial cross section, (red indicating strong absorption and blue indicating little to no absorption) as a function of both wavelength and position along the z axis (labeled in a); (c) xz (vertical) cross sections of absorption for a single nanocone illuminated at wavelengths of 400, 500, 600, 700 and 800 nm
Fig. 4
Fig. 4 (a) Diagrams of sparse arrays of (i) uniform nanowires with radii of 65 nm, (ii) nanowires with varying radii (45, 55, 65, 75 nm) with inset of aerial layout, and (iii) truncated nanocones with tip radii of 40 nm and base radii of 100 nm; (b) Cross sections of normalized power absorbed at the TM11 resonance at 675nm for (i) a 65nm radius nanowire in a uniform array, (ii) a truncated nanocone at r = 65 nm and (iii) a 65 nm radius nanowire in the multi-radii nanowire array (black circles outline the edges of the wire); (c) Simulated absorption vs. wavelength for the geometrically-optimized GaAs arrays of truncated nanocones shown in (a) and the planar equivalent thickness (t = 150 nm). All nanostructured arrays are 3 µm in height, have a 5% fill fraction, sit on top of an infinite silicon substrate and are embedded in 30 nm of silica (not shown).

Equations (1)

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( 1 k 1 2 k 2 2 ) 2 ( β m k 0 a ) 2 = ( n 1 2 k 1 J m ' ( k 1 a ) J m ( k 1 a ) n 2 2 k 2 H m ' ( k 2 a ) H m ( k 2 a ) ) ( 1 k 1 J m ' ( k 1 a ) J m ( k 1 a ) 1 k 2 H m ' ( k 2 a ) H m ( k 2 a ) )
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