Experimental results of the 2D metal disk arrays

To characterize the resonant mode and absorptivity A(λ) = 1 - R(λ) of the structure, the reflectivities R(λ) at normal incidence normalized with respect to a bare Ag/Si substrate were measured by using a FTIR spectrometer with a MIR unpolarized source and a resolution of 2 cm^-1. Figure 4.7-1(a) shows the measured resonant wavelengths of the SiO2 cavities with varying disk size when tSiO2 = 80nm.

The resonant wavelength (black open square) is linearly proportional to the MD size, in accordance with our simulation (blue solid line). The wavelength tuning rate is approximately 2.8 nm per nm in D.

4.7-1(a) Comparison of the calculated and measured wavelengths of resonances as functions

of the disk diameter D.

The absorptivities of the MD arrays varying in density and size were also examined.

Figure 4.7-1(b) displays the absorptivities of the 1μm MD absorbers with Λ = 1.5, 2, 2.5, 3, 4, and 6μm. All the absorption peaks are observed at 3.63μm with approximately the same FWHM 0.28μm. The peak absorptivity is increased from 17% to 94% as we diminish the periodicity of the MD array (ie an increase in the MD density). The figure of merit (FOM) of the absorber defined as FOM = AQ [67], where A and Q = λ/ FWHM are absorptivity and quality factor. The FOM of the metal disk array can be increase from 2.2 to 12 by reducing the periodicity from 6 μm to 1.5 μm. Figure 4.7-1(c) shows the absorptivities of the absorbers with the considered disk sizes D ranging from 846nm to 1.88μm when Λ= 3μm. The absorption peaks for D= 846nm, 1.06μm, 1.57μm, 1.36μm, 1.65μm, and 1.88μm are found at λ= 2.96, 3.67, 4.05, 4.65, 5.36, and 5.91μm with a maximum absorptivity of 47%, 66%, 75%, 85%, 90%, and 95% respectively, while other high-order modes for D=1.65μm and 1.88μm at λ= 3.40μm and 3.81μm, CO2 and water vapor absorption features in the atmosphere are also visible. When we expand the disk size proportional to its absorption cross section, the energy absorbed to form the Fabry–Pérot-like resonance in the MD resonator per unit area will increase, resulting a pronounced increase in the absorptivity.

4.7-1(b) Absorptivities at normal incidence of the 1 μm MD arrays with several different

periodicities Λ.

4.7-1(c) Absorptivities of the MD arrays with different diameters D for Λ fixed at 3 μm.

Since the MD density and size are associated with the area-fill factor (F) defined as the ratio of MD size to the unit cell area, the dependence of the absorptivty on the fill factor is shown in Figure 4.7-1(d). The absorptivity is linearly proportional to the fill

factor as F<0.24 and it becomes saturated over 90% of absorption when F is greater than or equal to 0.24. The result that behaves similarly to the Beer Lambert law when F<0.24 therefore allows us to control the absorption of the MD configuration.

4.7-1(d) Experimental peak absorptivity of the MD configuration as a function of its area-fill

factor F.

With the above-mentioned absorption properties, we now turn toward experimental demonstrations of a broadband emitter and a high-performance dual-band absorber. Figure 4.7-2(a) shows the absorptivity of the broadband thermal emitter composed of the sublattices with six different disks from 800nm to 1.35μm, where the distance between the nearest-neighbor disks is kept 1.5μm. The absorbing spectrum is evidently broad as a result of the merging of six close peaks located closely to the resonant peaks for each of the single-sized MD absorbers. Although the

broadband emitter has a low absorptivity approximately 35% with a FWHM of 2000nm centered at the wavelength 4.32μm, high thermal emittance peaks of the emitter can be achieved by easily increasing heating temperature to modify the thermal dependence of the blackbody emittance curve B(λ,T) as demonstrated in chapter 3 (see Figure 3.2-2(b)) and the literatures [31, 65] without changing emissivity.

4.7-2(a) Experimental absorptivity of the broadband absorber composed of the multi-sized

disks with D=800 nm, 900 nm, 1.03 μm, 1.12 μm, 1.21 μm, and 1.35 μm and a SiO₂ spacer

thickness, t_SiO2=32 nm.

To obtain a broadband absorber with higher absorptivity, multi-sized MD disks within one unit cell and large area-fill factors for each of the disks are essentially required. However, if all the disks are patterned on the same layer, there is a design

trade-off between the multi-sizeddisks and fill factors corresponding to a number of absorption bands and absorption intensities. In spite of the trade-off, a high performance angle and polarization independent dual-band absorber can be realized.

4.7-2(b) Absorptivity of the dual-band absorber consisting of two different disk sizes with

dimensions D=825 nm, and 960 nm per unit cell when t_SiO2=80 nm.

Figure 4.7-2(b) exhibits the absorptivity of the dual-band absorber with a SiO2

spacer of 80nm where the incident angle varies from 0° and 15°. The two molecule-like disks D=825nm, and 960nm are arranged to form a 1.5μm face-centered unit cell with the area-fill factors of the disks F=0.24 and 0.32. As can be seen, the two strong absorption bands found at 2.99μm and 3.40μm have a maximum absorptivity of 92% and 90%, exhibiting consistency with that predicted by their area-fill factors in Figure 4.7-1(d). The absorptivity in both peaks remains at

more than 84% at an oblique incident angle. By effectively modifying the disk sizes and area-fill factors as well as the SiO2 spacer, such dual-band absorber configuration not only can achieves high absorptivity, but also can offer wide-angle polarization independent flat absorption bands over an angle of incidence 50° compared with the H-shaped metamaterial coating [22].

4.8 Summary

In summary, we have demonstrated a 1D plasmonic IR filter/absorber with a localized, angle-independent LSPP mode by using the Ag/SiO2/Ag T-shaped array.

The resonance wavelength can be shifted by varying the geometry, and the angle-independence of LSPP mode was also verified. The T-shaped structure with the LSPP resonance has a strong advantage in the application as angle-independent band-stop filters. To further improve the absorption performance of the T-shaped structure, we therefore studied the two-dimensional round-shaped MD array.

In the study, we have presented a method to control the absorptivity of the MD absorber by modifying its area-fill factor. Because of the LSPP resonances for the MD absorbers not affected by their array periodicities, a broadband emitter made of a multi-sized MD array can be realized. By controlling the disk sizes and fill factor, we have shown an angle and polarization independent dual-band absorber with two

maximal absorptivities over 84%. The incidence angle of the absorber can be achieved near 90° in the operating mid-infrared region from 2.85μm to 3.65μm. The approach is applicable for thermal photovoltaic, sensor, and camouflage applications.

5. Omni-directional mid-infrared (MIR) polarizer made of elliptical metal disk arrays

We have shown in chapter 4 that the metallic round-shaped disk array possesses a LSPP mode for both TE and TM polarizations. In the chapter, we found that by tuning the aspect ratio of the disk structure, both angle-independent TE- and TM- polarized resonant peaks can be separated. Because of its low-cost building materials and strong absorptivities occurring at both peaks, a cheap mid-IR polarizer possibly used can be realized. The elliptical disk structure with an aspect ratio of b/a=0.65 was characterized by using a FTIR spectrometer and simulated by the RCWA method. The disk structure we found has a degree of polarization 99% at wavelength 5.25μm for TE polarization and 91% at 4.12μm for TM polarization, showing a good agreement with our simulation. The extinction ratios for both TE and TM resonances are found to be 20dB and 14dB respectively. The ratios can be further improved by tailoring the aspect ratio of the disk structure.

5.1 Design and simulation of the MIR polarizer

Figure 5.1-1 shows the geometry of the elliptical disk array structure. The array structure consists of three layers: Ag disk layer, SiO2 layer, and Ag reflective layer.

The first Ag layer is an array of elliptical metal disks with two axes: major axis (a) along x-direction and minor axis (b) along y-direction. The periodicity of the array is denoted byΛ^g. The thickness of elliptical disk is given by tAg=100nm. The second layer is a uniform layer of SiO2 with thickness tSiO2=50nm. To block the light transmittance, we used Ag thin film (100 nm) in the third layer to act as a reflector layer. Our simulation based on the RCWA algorithm specially is designed for the arrays.

Fig. 5.1-1 Illustration of the metallic disk array structure.

Figure 5.1-2(a) and 5.1-2(b) show the TM-mode and TE-mode reflectance spectra of an elliptical disk array with parameters of Λ^g=1500nm, a=1000nm, and

b=850nm. The resonance energies (wavelengths) occur at 0.337eV (3.68μm) and at 0.378eV (3.28μm) for TM mode and TE mode, respectively. It is clear that both energy bands for the TM and TE modes are angle-independent due to the strong coupling of SPPs from the top and bottom Ag layers. The |Hy|² and |Hx|² field distributions at the SiO2 layer becomes a linear combination of two exponentially decaying SPPs from the two Ag/SiO2 interfaces, as shown in Figure 5.1-3(a) for the TM modes and 5.1-3(b) for the TE mode. The coupling of SPPs will form a Fabry–Pérot-like resonance in the Ag/SiO2/Ag elliptical metal disk cavity. The resonant mode is also called LSPP mode which is independent of incidence angles.

The TM and TE modes are respectively resonances along the major axis x and minor axis y.

Fig. 5.1-2 Dispersion relations of the metal disk structure for (a) TM mode and TE mode.

Fig. 5.1-3(a) |H_y|² field distribution for TM-mode at 0.33eV in X-Z plane. (b) |H_x|² field

distribution for TE-mode at 0.37eV in Y-Z plane.

One of the useful features in the elliptical metallic disk is that the structure can filter out completely one component of the light polarization and reflects perfectly the other component of the light polarization because of the highly absorptions caused by the TE and TM modes. Therefore, the elliptical disk structure is a good candidate as a polarizer at mid-infrared region. We also found that by modifying the b/a ratio, the extinction ration defined as 10log( )

unwanted wanted

I

I can be tuned, where Iwanted and Iunwanted

are the reflection of the wanted and unwanted intensities. For the TM mode, it can filter out TM-polarized input light and reflect TE-polarized out light. Therefore, Iwanted

and Iunwanted for the mode are ITE and ITM.

Fig. 5.1-4 Calculated extinction ratio versus axial ratio

Fig. 5.1-4 shows the extinction ratio of the TM mode versus the b/a ratio where a=1000nm is fixed. As the ratio of axes decrease from b/a=1, the extinction ratio becomes higher than 20dB and the optimum value can be observed when the axes ratio is 0.85 which can lead to extinction ratio higher than 40dB. In addition to the polarization feature, the building material cost of the elliptical disk structure is cheaper compared with the IR-transmitting building materials such as CaF2, BaF2, and ZnS [68] used for the commercial polarizers.