# Photo-Thermoelectric Conversion of Plasmonic Nanohole Array
## 1 Introduction  
Photodetection by semiconductors occurs via a photocurrent flow associated with electron-hole pair generation via light absorption. The absorption band is strictly limited by the value of the semiconductor bandgap, meaning that the response-wavelength range for a semiconductor photodetector is determined by its bandgap. A straightforward method to tune the response-wavelength range of photodetectors is to utilize color filters, and this method has been adopted for conventional image sensors. However, the use of color filters results in limitations for the minimum size of the detection device, and therefore, different techniques are needed for device miniaturization.  
Plasmons are an alternative approach for realizing the tuning of photodetector response wavelengths [1; 2]. In fact, the response wavelength of a plasmonic photodetector driven by hot carriers can be completely controlled via the design of the metallic nanostructures, regardless of the intrinsic bandgap of the semiconductor [3; 4; 5; 6]. Using this approach, [PERSON] et al. demonstrated that the widths of gold nanoslits on a Si substrate determined the photodetection response wavelengths [3].  
Meanwhile, we demonstrated a plasmonic photodetector driven by thermoelectric conversion--an alternative approach for photodetection--triggered by plasmonic local heating [7]. Our device consisted of a thin film of a thermoelectric material embedded with plasmonic atoms that generated plasmonic local heating and created a thermal gradient inside the film as a consequence of light detection by absorption, resulting in Seebeck voltage generation. Similarly, many studies of light detection viathermoelectric conversion, in, for example, carbon nanotubes [8; 9], graphene [10; 11], carbon sprays [12], and commercial thermoelectric devices [13] triggered by metal-nanostructure plasmon resonance, have been reported. These light detection systems were based on thermoelectric materials combined with plasmonic atoms used as light receivers. However, similar light detection via thermoelectric conversion can be realized by using a conductive single nanofilm that includes plasmonic couplers such as prisms or nanohole arrays. [PERSON] et al. reported thermo-electrical detection of surface plasmon propagation on a gold waveguide [14]. [PERSON] et al. designed the resonant thermoelectric nanostructure to support guided-mode resonances with spectrally sharp absorption profiles, realizing tunable photodetection across wavelengths from the visible to the mid-infrared (IR) region [15]. More recently, room-temperature Seebeck IR detection using patterned graphene was demonstrated [16; 17]. The presence of nanohole arrays on the graphene surface resulted in graphene plasmon excitation, allowing dramatic enhancement of the light-matter interaction [18; 19]. These demonstrations illustrate the thermoelectrical photodetection capabilities of a conductive single nanomaterial that supports plasmons and thermoelectric conversion, and they suggest that thermoelectric nanophotonics have great potential for optoelectronic applications. However, the underlying photodetection mechanisms for single and patterned nanomaterials have not been investigated; for example, the amount of local heating contributing to these effects has yet to be determined. In order to discuss the potential of the patterned conductive nanomaterials as practical photodetectors, further studies are required. Specifically, the amount of plasmonic local heating at the nanoscale should be clarified in order to better examine the mechanisms of operation.  
In this study, we fabricated an ultra-thin silver (Ag) film punctuated by a nanohole array and examined its potential for plasmonic photodetection via thermoelectric conversion. We verified the amount of plasmonic local heating contributing to photodetection experimentally and numerically and compared this with the results of thermoelectric simulations to discuss the mechanism of photodetection.