Research Papers

Measurement of Coherent Thermal Emission Due to Magnetic Polaritons in Subwavelength Microstructures

[+] Author and Article Information
Z. M. Zhang

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

1Corresponding author.

Contributed by the Heat Transfer Division of ASME for publication in the Journal of Heat Transfer. Manuscript received July 9, 2012; final manuscript received January 16, 2013; published online July 26, 2013. Assoc. Editor: Pamela M. Norris.

J. Heat Transfer 135(9), 091505 (Jul 26, 2013) (9 pages) Paper No: HT-12-1362; doi: 10.1115/1.4024469 History: Received July 09, 2012; Revised January 16, 2013

Spectral and directional control of thermal emission is critically important for applications such as space cooling and energy harvesting. The effect of magnetic polaritons (MPs) on spectral modulation has been analyzed in metallic grating structures with a dielectric spacer on a metallic film. It has been predicted that the spectral emission peaks exhibit omnidirectional characteristics when MPs are excited. The present work provides an experimental demonstration of coherent thermal emission from several microfabricated grating structures in the infrared region from room temperature to elevated temperatures. The emittance at elevated temperatures is directly measured using an emissometer, while the room-temperature emittance is indirectly obtained from the reflectance measurement. The rigorous coupled-wave analysis and an LC-circuit model are employed to elucidate the mechanisms of various resonant modes and their coupling effect, taking into consideration the temperature-dependent electron scattering rate of the metals.

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Fig. 1

Schematic of the fabrication processes for the subwavelength microstructures as coherent thermal emitters: (a) thin film deposition; (b) exposure with photomask; (c) resist development; (d) metal evaporation; and (e) lift-off (resist stripping)

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Fig. 2

SEM images of Pattern 1 in (a) top view and (b) cross-sectional view before being coated with the protective SiO2 layer. (c) Three-dimensional schematic of the coherent emitter made of a gold grating and SiO2 spacer on an optically opaque Au film with geometric parameters: grating period Λ, grating height h, spacer thickness d, and strip width w. (d) Optics layout of the emissometer setup that is used to measure the emittance at elevated temperatures.

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Fig. 3

Spectral reflectance measured at room temperature of all three samples for TM waves incident at angles of (a) 10 deg and (b) 30 deg

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Fig. 4

Comparison between the measurements (solid curve with markers) and the RCWA calculation (dashed curve) on the reflectance for (a and b) Pattern 1, (c and d) Pattern 2; and (e and f) Pattern 3. The RCWA calculation is based on the geometric parameters listed in Table 1, and specular reflectance (0th order) is only compared.

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Fig. 5

(a) Contour plots of emittance calculated from the RCWA for TM waves as a function of frequency and the parallel component of the wavevector. (b) The equivalent LC circuit model for predicting the magnetic resonant conditions inside the structure. The triangles indicate the resonant frequencies predicted by the LC model for the MP1 mode. The geometry of Sample 1 is used in the calculations.

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Fig. 6

Contour plots of the normal emittance (a) as a function of wavenumber and strip width, and (b) as a function of wavenumber and grating period. The calculation is from RCWA with parameters of Pattern 1. The triangles indicate the LC model prediction on the MP1 resonance conditions.

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Fig. 7

The emittance at 700 K for different directions measured with the high-temperature emissometer using (a) a DTGS detector and (b) an InSb detector. An IR polarizer is used and the results are for TM waves only.

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Fig. 8

(a) The normal emittance measured at temperatures of 700 and 750 K using the high-temperature emissometer. The results at ν < 2000 cm−1 were taken with a DTGS detector, while those at ν > 2000 cm−1 were collected with an InSb detector. (b) Theoretical calculation of the normal emittance at 700 K with the parameters of Pattern 1. The average over the emission angle from −3 deg to 3 deg is also shown for better comparison with experiments.




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