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Research Papers: Experimental Techniques

Wideband Tunable Omnidirectional Infrared Absorbers Based on Doped-Silicon Nanowire Arrays

[+] Author and Article Information
X. L. Liu

George W. Woodruff School of Mechanical Engineering,
Georgia Institute of Technology,
Atlanta, GA 30332

L. P. Wang

School for Engineering of Matter,
Transport and Energy,
Arizona State University,
Tempe, AZ 85287

Z. M. Zhang

Fellow ASME
George W. Woodruff School of Mechanical Engineering,
Georgia Institute of Technology,
Atlanta, GA 30332
e-mail: zhuomin.zhang@me.gatech.edu

1Corresponding author.

Manuscript received October 15, 2012; final manuscript received January 13, 2013; published online May 16, 2013. Assoc. Editor: Leslie Phinney.

J. Heat Transfer 135(6), 061602 (May 16, 2013) (8 pages) Paper No: HT-12-1564; doi: 10.1115/1.4023578 History: Received October 15, 2012; Revised January 13, 2013

The present study considers the directional and spectral radiative properties of vertically aligned, heavily doped silicon nanowires for applications as broadband infrared diffuse absorbers. The nanowire array is modeled as a uniaxial medium whose anisotropic dielectric function is based on an effective medium theory. The approximation model is verified by the finite-difference time-domain method. It is found that the radiative properties of this type of nanostructured material could be tailored by controlling the doping concentration, volume filling ratio, and length of the nanowires. Increasing the wire length yields a broadening of the absorption plateau, while increasing the doping concentration results in a shift of the plateau to shorter wavelengths. Moreover, two kinds of omnidirectional absorbers/emitters could be realized based on the doped-silicon nanowire arrays. The first one is a wavelength-tunable wideband absorber, which may be important for applications in thermal imaging and thermophotovoltaic devices. The second acts as a quasi-blackbody in the wavelength region from 3 to 17 μm and, therefore, is promising for use as an absorber in bolometers that measure infrared radiation and as an emitter in space cooling devices that dissipate heat into free space via thermal radiation.

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Figures

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Fig. 1

Schematic of a D-SiNWs array (medium 2) on a Si film (medium 3), where the top and bottom media (1 and 4) are assumed to be air: (a) cross-section view; (b) top view

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Fig. 2

Comparison of the spectral absorptance of a freestanding D-SiNWs array, with H2 = 100 μm and N = 1020 cm-3, predicted from the EMT and the FDTD simulation: (a) normal incidence for filling ratios f = 0.15 and 0.30; (b) incidence angle at 30 deg for both TE and TM waves with f = 0.15

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Fig. 3

Optical constants of doped silicon and D-SiNWs for both ordinary and extraordinary waves with a volume filling ratio of 0.15 and a doping level of 1020 cm-3: (a) the refractive index; (b) the extinction coefficient

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Fig. 4

Dependence of the resonance wavelength on the doping concentration for f = 0.15 and 0.30

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Fig. 5

Absorptance at normal incidence for a freestanding D-SiNW array (f = 0.15) compared to a D-Si film with the same thickness (100 μm) and doping concentration of 1020 cm-3

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Fig. 6

Parametric effects on the absorptance of D-SiNW arrays: (a) thickness; (b) volume filling ratio; (c) doping level. The base set of parameters is H2 = 100 μm, f = 0.15, and N = 1020 cm-3.

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Fig. 7

Contour plots of the absorptance as a function of wavelength and incidence angle for (a) TE waves; (b) TM waves. The parameters are the same as those for Fig. 5.

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Fig. 8

Spectral absorptance for D-SiNWs on D-Si substrate at normal incidence: (a) comparison of the EMT and FDTD results (note that the y-scale is from 0.7 to 1.0); (b) effect of substrate thickness

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Fig. 9

Angular dependence of the absorptance for both TE (right side) and TM (left side) waves at representative wavelengths: λ = 6 μm (blue dotted curve), 10 μm (red solid), and 16 μm (black dashed)

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