Plasmonic Nanoantennas for Efficient Infrared (IR) Absorbers, Detectors, and Energy Harvesting Devices

Abstract

An infrared (IR) radiation contributes almost half of the total solar radiation, and such a large IR energy density is only suspected as thermal wasted energy. This increases the opportunity to use IR radiation in various fields such as thermography, heating, night vision, and communications. However, the investigation of converting IR radiation into another form of energy, such as electricity, has not been done before. Therefore, the IR detection method is needed to capture the IR radiation and convert it into another form of energy. Previously, thermal and photon were the common method to detect IR radiation, but these methods suffer from several limitations, such as low detectivity and slow response time. Therefore, an alternative approach for detecting IR radiation with high sensitivity is needed to overcome those limitations. Recently, nanoantenna has attracted significant attention due to their distinctive properties, such as high absorption and field confinement at terahertz frequency range. These characteristics enable the antenna to be widely used to improve the performance of optical and IR detectors by increasing the detector’s photoresponse due to electric energy enhancement. Accordingly, this dissertation emphasized designing nanoantenna with high field enhancement and absorption rate. For application as IR harvesting devices, we developed a thermoelectric nanoantenna with high current density at the antenna center and fabricated the antenna using a common nano-fabrication technique. Finally, nanoantenna measurement was done using an IR measurement set up to determine the fabricated thermoelectric nanoantenna's open-circuit voltage (Voc). First, we proposed a bowtie nanoantenna array integrated with an artificial impedance surface to achieve high field enhancement and perfect absorption at the same time. We implement the artificial impedance surface as a metallic patch array on a grounded 50 nm-thick SiO2 substrate with reactive impedance surface (RIS) or high impedance surface (HIS) characteristic. We designed a bowtie nanoantenna array on an optimum RIS patch array through the proposed design method. We achieved a high field enhancement factor (E/E0) of 228 and a nearly perfect absorption rate of 98% at 230 THz. This novel design outperforms the previously reported nanoantenna structures and the same bowtie nanoantenna array designed using a conventional grounded SiO2. We also show that the HIS-integrated bowtie antenna array cannot realize both goals at the same time because the highly reactive HIS cannot guarantee perfect absorption. The proposed RIS-combined nanoantenna array with high field enhancement and near-perfect absorption can be used for efficient infrared (IR) and optical detectors, sensors, and energy harvesting devices. Second, we proposed a new structure of MIM (metal insulator metal) based IR absorber to overcome the low field enhancement from the standard MIM absorber structure. The proposed absorber uses a reactive impedance surface (RIS) to boost field enhancement without an ultra-thin spacer and maintains near-perfect absorption due to impedance-matching with the vacuum. Unlike conventional metallic reflectors, the RIS is a metallic patch array on a grounded dielectric substrate that can change its surface impedance. The final circular nanodisk array mounted on the optimum RIS offers an electric field enhancement factor of 180 with a nearly perfect absorption of 98% at 230 THz. The proposed absorber exhibits robust performances even with a change in polarization of the incident wave. The proposed RIS-integrated MIM absorber with such good performance can enhance the sensitivity of a localized surface plasmon resonance sensor and surface-enhanced infrared spectroscopy. Last, for IR energy harvesting application, we designed a novel thermoelectric bowtie nanoantenna with a high Seebeck effect and utilized the maximum resonances from the vertical and horizontal dimensions of a SiO2 substrate instead of the membrane. The bowtie nanoantenna was made with a single metal, titanium (Ti), without a thermal conductivity discontinuity at the antenna center to lower the heat spread. Then, a nickel (Ni)-Ti bimetal nano-thermocouple with a high Seebeck coefficient gradient was connected at the center. Finally, the nanoantenna utilized a quarter wavelength-thick SiO2 backed by a metal reflector and an optimum lateral size of the open-ended SiO2 to launch the maximum signal peak of standing waves at the antenna center. The simulation results showed the highest Voc of 2.06 µV from a temperature gap (∆T) of 76.33 mK among the state-of-arts of substrate-mounted thermoelectric nanoantenna operating at λ0 = 10.6 µm. For proof of concept, we fabricated the thermoelectric nanoantennas using typical nano-lithography and deposition methods and showed a similar Voc of 2.03 µV using a CO2 laser-based IR measurement system. In conclusion, we expect that the proven thermoelectric nanoantenna design with the highest Voc from the novel antenna topology and the engineered substrate can be used for a high DC output massive nanoantenna array.

Keywords: Nanoantenna, infrared, terahertz, artificial impedance surface, electric field enhancement, infrared (IR) absorber, high impedance surface (HIS), reactive impedance surface (RIS), thermoelectric nanoantenna, Seebeck coefficient, thermal conductivity, standing waves, open-circuit voltage (Voc).