Enhanced Light Trapping in Periodic Aluminum Nanorod Arrays as Cavity Resonator
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Enhanced Light Trapping in Periodic Aluminum Nanorod Arrays as Cavity Resonator Rosure B. Abdulrahman, Arif S. Alagoz, Tansel Karabacak Department of Applied Science, University of Arkansas at Little Rock, Little Rock, AR, U.S.A.
ABSTRACT Metallic nanostructures can exhibit different optical properties compared to bulk materials mainly depending on their shape, size, and separation. We present the results of an optical modeling study on ordered arrays of aluminum (Al) nanorods with a hexagonal periodic geometry placed on an Al thin film. We used a finite-difference time-domain (FDTD) method to solve the Maxwell's equations and predict the reflectance of the nanorod arrays. The thickness of the base Al film was set to 100 nm, and diameter, height and nanorod center-to-center periodicity were varied. Incident light in the FDTD simulations was an EM-circular polarized plane wave and reflectance profiles were calculated in the wavelength range 200-1800 nm. In addition, we calculated spatial electric field intensity distributions around the nanorods for wavelengths 300, 500, and 700 nm. Our results show that average reflectance of Al nanorods can drop down to as low as ~50%, which is significantly lower than the ~90% reflectance of conventional flat Al film at similar wavelengths. In addition to the overall decrease in reflectance, Al nanorod arrays manifest multiple resonant modes (higher-order modes) indicated by several dips in their reflectance spectrums (i.e. multiple attenuation peaks in their absorption profiles). Positions of these dips in the reflectance spectrum and spatial EM field distribution vary with nanorod height and diameter. Multiple reflectance peaks are explained by cavity resonator effects. Spatial EM field distribution profiles indicate enhanced light trapping among the nanorods, which can be useful especially in optoelectronic and solar cell applications. INTRODUCTION Engineering optical properties of nanomaterials can provide lead to high performance optoelectronic devices. In the specific case of solar cells, nanostructured metals can be used to enhance light trapping in solar cell absorbent layer by using surface plasmon effects [1]. Surface plasmons (SP) are collective excitation of free conduction electrons at metal dielectric interface. When metal is illuminated by light, electromagnetic field couples with surface plasmons and propagate along the surface, which is called surface plasmon polariton (SPP). At nanoscale, these waves are confined on the nanostructure surface and called localized surface plasmon resonances (LSPR). LSPR is very sensitive to nanostructure shape and size [2]. It has been shown that, geometries such as nanoparticles, nanoshell [3] and coaxial holes [4] can utilize plasmonic behavior of metals by introducing field enhancement. Crystalline Al exhibits interband absorption at far infrared wavelengths of 826 nm and 2479 nm [5] . Al nanoparticles show two plasmon resonances at 250 nm and 190 nm due to dipolar field and quadrupolar field, respectively, and exhibit red shif
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