Optical Absorption Caused by Periodic Antenna Array-induced Surface Plasmons in the Mid-infrared Range

It is known that resonant excitation of surface plasmon polaritons (SPPs) manifests itself in the specular reflection on the surface of an antenna array. Ohmic loss of SPPs on the surface of a metallic film becomes small in... [ view full abstract ]

It is known that resonant excitation of surface plasmon polaritons (SPPs) manifests itself in the specular reflection on the surface of an antenna array. Ohmic loss of SPPs on the surface of a metallic film becomes small in the mid-infrared range compared to the visible region. In addition, attenuation depth of the SPP, at which the electric field of SPP decreases to 1/e of the intensity at a metal-material interface, also increases as wavelength, for example, being as deep as ~0.5 μm at 10 μm in the case of Au/n-GaAs. Thus, SPPs can be utilized as a low-loss optical spectral filter detecting at different mid-infrared wavelengths. SPP characteristics appearing in an antenna array in the mid-infrared have been reported primarily for semiconductor materials. However, very little experimental work is currently available based on this viewpoint. Here, we prepared two kinds of devices with 10 x 10 circular slot antennas formed on the top of SiC and n-GaAs (doping concentration: 2 x 10¹⁸ cm^-3) substrates with different absorption coefficients. Figure 1 shows the measured reflectivity of SiC having openings with a dimeter of 6 μm and three different spacings. The results considering different filling factors are shown in Fig. 2. Reflectivity decreases at 940 and 980 cm^-1 corresponded to signals arising from surface-phonon polariton and the resonance of one circular slot antenna, following a Fabry-Perot condition. It should be noted that the reflectivity for the sample with a separation of 9 μm decreased slightly at 1020 cm^-1. This was ascribed to SPP generation at the interface of metal/SiC, whose wavelengths were in agreement with the calculated results. Figure 3 shows both reflectivities for n-GaAs (diameter: 1.4 μm; spacing: 2.5, 2.9, and 3.9 μm). Antenna resonance for 1.4 μm-diameter was calculated to be ~1640 cm^-1, and much different from the measured SPP wavenumbers when compared to SiC samples. However, reflection dips, caused by SPPs, were clearly observed because of free carrier absorption in this case. Experimental results confirmed that SPPs excited resonantly from the periodic antenna array act as a tailored optical filter rather than loss generation in the mid-infrared region.