Integration of Resonant Plasmonic Nanostructures with 2D Materials: Strategies and Applications beyond the Visible regime

Abstract

Plasmonics deals with the interaction of incident electromagnetic waves with the free electron gas within metal nanostructures, enabling rapid signal processing and the confinement of light into subwavelength scales, leading to significant field enhancements. Noble metals like gold, silver, and copper are commonly used for their ability to support Localized Surface Plasmon Resonance (LSPR), effectively manipulating UV and visible light at the nanoscale. However, these metals suffer from high optical losses in the infrared (IR) region, primarily due to interband transitions, limiting their utility to the UV-Vis spectrum. This limitation underscores the importance of investigating alternative plasmonic materials that exhibit large negative permittivity and reduced optical losses in the IR regime. In our research, we identified transparent conducting oxides (TCOs), particularly aluminum-doped zinc oxide (AZO) and gallium-doped zinc oxide (GZO), as promising materials with minimal optical losses. The LSPR frequency in such materials is determined by the free charge carrier density, effective mass, and geometric factors. Unlike traditional metals that rely solely on electrons for charge oscillation, these materials can utilize both electrons and holes, offering greater versatility. This adaptability makes them promising candidates for IR photodetectors, particularly as an alternative to conventional mercury cadmium telluride (MCT) detectors, which are constrained by toxicity, cryogenic requirements, high cost, and complex fabrication. Using pulsed DC sputtering and forming gas annealing, we successfully fabricated highly conductive thin films with elevated carrier densities and low sheet resistance. These TCO-based films were comprehensively analyzed through Hall measurements, ellipsometry, and X-ray diffraction to assess their carrier concentration, stoichiometry, electronic state, and crystalline phase. A custom ellipsometry model based on the Drude-Lorentz oscillator was developed to characterize their optical properties, revealing that these low-loss TCO films possess a significantly negative real permittivity, positioning them as suitable candidates for mid-infrared (mid-IR) plasmonic applications. By leveraging such alternative plasmonic materials to sensitize graphene in the IR range, we aim to enhance photocarrier generation efficiency across the graphene layer through the creation of hotspots within the device channel. This approach significantly boosts the performance of graphene-based photodetectors. Using a combination of analytical models and COMSOL Multiphysics simulations, we systematically explored the relationship between geometric parameters and mid-IR plasmonic responses, characterizing resonance peaks and FWHM from AZO permittivity values obtained via ellipsometry. Our analysis revealed the onset of Fano resonance in a specific oligomer structure, the nonamer, which showed significant mid-IR enhancement and a distinctive Fano lineshape. These findings highlight the potential of alternative plasmonic materials in developing efficient and versatile mid-IR photodetectors for advanced sensing applications. Further to amplify light-matter interactions, we employed various advanced patterning techniques, including focused ion beam (FIB) milling, enabling the creation of submicron gaps (50-100 nm) in complex nanostructures such as bowties, nonamers, and dipole antennas. Optical transmittance measurements using Fourier transform infrared (FTIR) spectroscopy and a custom-built IR spectrometer demonstrated that nonamer geometries achieved the highest absorption efficiency, with the emergence of the first reported Fano resonance in the mid-IR region. Additionally, nanosphere lithography (NSL) was utilized to fabricate plasmonic arrays of hexagonal nanoholes, resulting in a 23% reduction in transmission, further exhibiting the highest resonance levels in the mid-IR range. Subsequently, we integrated these patterned TCO materials with single-layer graphene to develop a mid-IR photodetector. Under mid-IR illumination at a frequency of 0.05 Hz, the detector exhibited a significantly enhanced photoresponse, achieving an effective responsivity of 49.6 mA/W at a bias voltage of 900 mV. This enhancement underscores the robust electromagnetic near-field supported by the alternative plasmonic material. Our work marks the first demonstration of a localized surface plasmon resonance (LSPR)-assisted mid-IR photodetector, highlighting AZO’s vast potential for critical applications in photodetection, sensing, telecommunications, and security within the mid-IR spectrum. Moreover, we have investigated the optical and electronic responses of different hybrid 2D material- Ag-based UV photodetectors depending on the position of Van Der Waal’s heterostructures. Raman Spectroscopy is utilized to analyze the effects of inherent strain and defect concentration on the graphene monolayer in these hybrid devices. Insights into the intermediate trapping and de-trapping states can be obtained from the photoresponse studies indicating that the positioning of the 2D materials significantly influences the anomalous response of these detectors. In conclusion, our research focuses on the optimal sensitization of resonant plasmonic nanostructures and their integration with 2D materials for enhanced light-matter interaction, enabling sensing applications beyond the conventional visible spectrum.