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TECHNICAL PAPERS

# Coherent Thermal Emission From Modified Periodic Multilayer Structures

[+] Author and Article Information
B. J. Lee

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

Z. M. Zhang1

George W. Woodruff School of Mechanical Engineering, Georgia Institute of Technology, Atlanta, GA 30332zzhang@me.gatech.edu

1

Corresponding author.

J. Heat Transfer 129(1), 17-26 (Apr 05, 2006) (10 pages) doi:10.1115/1.2401194 History: Received November 06, 2005; Revised April 05, 2006

## Abstract

Enhancement of thermal emission and control of its direction are important for applications in optoelectronics and energy conversion. A number of structures have been proposed as coherent emission sources, which exhibit a large emissivity peak within a narrow wavelength band and at a well-defined direction. A commonly used structure is the grating, in which the excited surface polaritons or surface waves are coupled with propagating waves in air, resulting in coherent emission for $p$ polarization only. One-dimensional photonic crystals can also support surface waves and may be modified to construct coherent emission sources. The present study investigates coherent emission from a multilayer structure consisting of a $SiC$ film coated atop a dielectric photonic crystal (PC). By exciting surface waves at the interface between $SiC$ and the PC, coherent emission is predicted for both $p$ and $s$ polarizations. In addition to the excitation of surface waves, the emission from the proposed multilayer structure can be greatly enhanced by the cavity resonance mode and the Brewster mode.

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## Figures

Figure 1

Schematic of the multilayer structure made of a SiC layer coated on a semi-infinite 1D PC for an s-polarized wave incident from air. The unit cell of the PC consists of a dielectric (type a) on both sides of a dielectric (type b) with a total thickness (lattice constant) Λ=da+db, where da=a1+a2. The surface termination is determined by the thickness of the dielectric (a1) next to the SiC film with a dielectric function of εs.

Figure 2

Schematic of the amplitudes of forward and backward waves in a semi-infinite 1D PC, in the right half space. The unit cell of the 1D PC is made of two dielectric layers, type a and type b, and has a period Λ=da+db.

Figure 3

Band structure of the 1D PC shown in Fig. 2, with da=db and na=2.4 and nb=1.5 in the reduced ω−kx domain: (a)p polarization; and (b)s polarization. The dash-dot line denotes the light line in air. The crossovers for p polarization are due to the Brewster angle between media a and b.

Figure 4

The optical constants of SiC predicted by the phonon oscillator model given in Eq. 10 at wavelengths from 9 to 14μm. The inset is for wavelength from 10 to 10.5μm.

Figure 5

Identification of the important regimes where the radiative properties are dominated by different mechanisms: (a) the reflectance of the 1D PC with a 1.45-μm-thick SiC layer, where the dotted line is for the 1D PC without SiC; and (b) the regime map in λ−θ space. CR stands for the cavity resonance mode, SW for surface wave, and BR for the Brewster mode. The shaded areas are corresponding to the first band gap of the 1D PC for s polarization (lighter) and p polarization (darker). The phonon absorption band of SiC is between the two dashed horizontal lines.

Figure 6

Contour plot of the spectral–directional emissivity of the SiC–PC structure: (a)s polarization; and (b)p polarization. The thickness of SiC is set to be 1.45μm.

Figure 7

The spectral–directional emissivity of the SiC–PC structure when surface wave is excited. (a) Spectral dependence at θ=0deg, 30deg, and 60deg for s polarization with different locations of the surface termination. The upper panel illustrates the emissivity when surface termination occurs at a1=0.6da. The middle and lower panels show the cases when a1=0.5da and a1=0.4da, respectively. (b) Angular distributions at λc=11.479μm, 11.293μm, and 10.929μm for s polarization when a1=0.5da.

Figure 8

The spectral–directional emissivity of the SiC–PC structure for the cavity resonance mode: (a) spectral dependence at θ=0deg (solid line) and at θ=30deg for s polarization (dashed line) and p polarization (dash-dot line); and (b) angular distributions at λc=12.921μm for s polarization

Figure 9

Effects of the dielectric located next to the SiC film: (a) spectral–directional emissivity of the SiC–PC structure for two cases when (i) dielectric type a (solid line) or (ii) type b (dashed line) is next to the SiC layer; and (b) the square of the electric field, normalized by the incident, inside the SiC–PC structure for wavelengths corresponding to two cavity resonance modes. The upper panel of (b) is enlarged by 25 times.

Figure 10

The spectral–directional emissivity of the SiC–PC structure for the Brewster mode: (a) Spectral dependence when ds is 0.5μm, 1.45μm, and 2.5μm at θ=30deg for p polarization. The solid line with circular marks is for the case when ds=1.45μm for s polarization. (b) Magnitude of the complex reflection coefficient at the SiC–PC interface for p polarization with (dashed line) or without (solid line) damping. (c) Angular distributions at λc=10.29μm when ds=1.45μm for p polarization.

Figure 11

The spectral–hemispherical emissivity of the SiC–PC structure with the same parameters as in Fig. 6

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