Tunable acoustic graphene plasmon enhanced nano-infrared spectroscopy

Abstract

Nano-infrared spectroscopy (nano-IR) technology can surpass the diffraction limit of light, achieving infrared spectroscopic detection with a spatial resolution of ~ 10 nm, which is an important technical means for studying the chemical composition and structure of molecules at the nanoscale. However, the weak infrared absorption signals of nanoscale materials pose a significant challenge due to the large mismatch between their dimensions and the wavelength of infrared light. The infrared absorption signals of molecular vibrational modes are proportional to the square of the electromagnetic field intensity at their location, meaning that higher electromagnetic field intensity can significantly enhance molecular detection sensitivity. Acoustic graphene plasmons (AGP), excited by the interaction between free charges in graphene and image charges in metals, exhibit strong optical field localization and electromagnetic field enhancement. These properties make AGP an effective platform for enhancing nano-IR detection sensitivity. However, the fabrication of graphene nanostructures often introduces numerous edge defects due to the limitations of nanofabrication techniques, significantly reducing the electromagnetic field enhancement observed in experiments. Here, using finite element simulation, we theoretically propose a tunable enhanced nano-IR detection platform based on nanocavity-acoustic graphene plasmon (n-AGP), utilizing a graphene/air gap/gold nanocavity structure. This platform avoids the need for nanofabrication of graphene, thereby preventing defects and contamination introduced by processes such as electron beam exposure and plasma etching. By plotting the dispersion of n-AGP, we found that n-AGP has a high wavelength compression capability comparable to AGP (<i>λ</i><sub>0</sub>/<i>λ</i><sub>AGP</sub> = 48). Additionally, due to the introduction of the gold nanocavity structure, n-AGP possess an extremely small mode volume (<i>V</i><sub>n-AGP</sub> ≈ 10<sup>-7</sup><i>λ</i><sub>0</sub><sup>3</sup>, <i>λ</i><sub>0</sub> = 6.25 μm). By calculating the electric field intensity distribution (|<i>E</i><sub>norm</sub>|) and the normalized electric field intensity spectrum (i.e., the relationship between frequency and (|<i>E</i><sub>z</sub>|/|<i>E</i><sub>0</sub>|) of the n-AGP structure, it is evident that due to the high electron density on the gold surface, electromagnetic waves can reflect at the boundaries of the gold nanocavity and be resonantly enhanced within the nanocavity. At the resonant frequency of n-AGP (1800 cm<sup>-1</sup>), the electric field enhancement within the cavity is about 50 times. In contrast, at similar resonant frequencies, the electric field enhancement factors of Graphene plasmon (resonant frequency 1770 cm<sup>-1</sup>) and AGP (resonant frequency 1843 cm<sup>-1</sup>) are approximately 3 and 2 times, respectively, significantly lower than that of n-AGP. Furthermore, by placing a protein film (60 nm wide and 10 nm high) under the graphene, we calculated the spectral dip depths caused by Fano resonance between n-AGP and AGP with the vibrational modes of protein molecules, thereby validating the enhancement factors of different modes for protein vibrational mode infrared absorption. For the amide I band of proteins, the detection sensitivity of n-AGP is about 60 times higher than that of AGP. Additionally, we discovered that by adjusting the structural parameters of the gold nanocavity, including cavity depth, width, and surface roughness, the response frequency band of n-AGP can be modulated (from 1290 to 2124 cm<sup>-1</sup>). Specifically, as the cavity depth increases, the electric field enhancement of n-AGP improves, and the wavelength compression capability of n-AGP decreases, causing the resonant frequency to blue-shift (from 1793 cm<sup>-1</sup> to 2124 cm<sup>-1</sup>). As the cavity width increases, the resonant frequency of n-AGP red-shift (from 1793 cm<sup>-1</sup> to 1290 cm<sup>-1</sup>), and the effectiveness of the gold nanocavity boundary in reflecting the resonant electric field within the cavity diminishes, resulting in a decrease in the electric field enhancement factor. With the gradual increase in the roughness of the gold nanocavity bottom, the effective depth of the gold nanocavity increases, causing a blue shift in the n-AGP resonant frequency (from 1793 cm-1 to 1861 cm<sup>-1</sup>) and an increase in the electric field enhancement factor. Moreover, by adjusting the Fermi level of graphene (from 0.3 eV to 0.6 eV), we achieved dynamic tuning of n-AGP (from 1355 to 1973 cm<sup>-1</sup>). As the Fermi level of graphene increases, the wavelength compression capability of n-AGP decreases, resulting in a blue-shift in the resonant frequency. Finally, by optimizing the structural parameters and Fermi level of n-AGP, and placing protein particles of different sizes (20 nm, 15 nm, and 10 nm wide, all 10 nm high) into the graphene/gold nanocavity structure, we verified the protein detection capability of n-AGP-enhanced nano-IR. We found that n-AGP can detect the vibrational fingerprint features of the amide I and amide II bands of a single protein particle (10×10 nm) with a 15-fold increase in sensitivity. This n-AGP-based enhanced structure holds promise for providing an important detection platform for nanoscale material characterization and single-molecule detection, with broad application potential in biomedicine, materials science, and geology.