# Photo-Thermoelectric Conversion of Plasmonic Nanohole Array
## 3 Results and Discussion  
### Optical and Electrical Properties of Ag Thin Film with Nanotubes  
Figure 2a shows an extinction spectrum of the Ag nanohole array with a periodicity of 300 nm and a hole diameter of (192 \(\pm\) 3.22) nm (sample number: 12). The extinction peak was observed at 687 nm, indicating excitation of surface plasmon polaritons on the Ag nanohole array. Finite-element method simulations (COMSOL Multiphysics) produced an extinction spectrum (Supplementary Materials Figure S2) similar to the measured spectrum. The photocurrent flowing through the Ag thin film under illumination was examined. A wavelength of 690 nm, which was close to the plasmon resonance wavelength for the nanoholes, was used to induce an electric current of 173 nA by illuminating of the nanohole array. By contrast, illumination of the Ag film in the vicinity of the nanoholes generated an electric current of 11 nA, which was comparable to the level of the multimeter noise, approximately 8 nA (Supplementary Materials Figure S3). Furthermore, the amount of the electric current was proportional to the optical power, which is consistent with the results of previous investigations [7]. Therefore, we concluded that light illumination of the nanohole array is indispensable for the electric current generation. External quantum efficiency (EQE) was calculated by dividing the number of detected electrons by the number of incident photons. The EQE at the wavelength of 690 nm was \(3.1\times 10^{-3}\%\). The wavelength dependences of the EQEs of the Ag nanoholes were examined, as shown in Figure 2a (red circles). The maximum EQE was observed at a wavelength of 690 nm, and the EQEs at other wavelengths were in the range of 1.8 to \(2.2\times 10^{-3}\%\). The wavelength dependence of the EQEs were similar to the extinction spectra of the Ag nanoholes. In contrast, illumination of the Ag film in an area away from the nanoholes resulted in a maximum EQE of \(6.1\times 10^{-4}\%\), and no significant wavelength dependence was observed in this case (Figure 2, blue triangles). These results suggest the following mechanism for the light detection in these systems: Light illumination on the nanoholes  
Figure 1: Ag thin-film detector concept and nanostructure. (**a**) Schematic of the Ag thin-film photodetector, and (**b**) top-view scanning electron microscopy (SEM) images of the periodic plasmonic nanohole arrays. The inset in (**b**) is a magnified image of a section of the image in the main panel. Figure 2: Plasmonic nanohole array extinction spectra and quantum efficiencies. Measured spectra: (**a**) 300, (**b**) 250, and (**c**) 400 nm. Measured external quantum efficiencies of plasmonic photo-thermoelectric conversion, for illuminations of the nanohole array (red circles) and for illumination of a reference position (blue triangles), are also plotted for each array on the respective panels. excites surface plasmon polaritons, leading to plasmonic local heating as a consequence of plasmon loss. This creates a temperature gradient across the Ag film, resulting in a potential difference between its edges. Consequently, electric currents flow through the Ag thin film, owing to the Seebeck effect. In order to verify this hypothesis, the wavelength dependencies of the EQEs of Ag nanoholes with different periodicities were investigated. The Ag nanohole arrays with periodicities of 250 nm and 400 nm were fabricated and evaluated for comparison with the 300 nm-spaced nanohole array (Figure 2b,c). The peak wavelengths of the extinction spectra of the nanoholes with periodicities of 250 and 400 nm were 608 nm and 801 nm, respectively. These results suggest that the peak wavelength of the Ag nanoholes is determined by periodicity of the nanoholes. In each case, the wavelength dependence of the EQEs for illumination of the nanoholes followed a similar trend to the wavelength dependence of the nanohole extinction spectra, whereas, for illumination of an area of the Ag film unpunctuated by nanoholes, no significant wavelength dependence was seen.