Ultrabroad-Band Direct Digital Refractive Index Imaging Based on Suspended Graphene Plasmon Nanocavities

Xiaofei Xiao,Vincenzo Giannini,Stefan A Maier

doi:10.1021/acsanm.0c03100

Abstract

Mid-infrared spectroscopy is essential for chemical identification and compositional analysis because of the existence of characteristic molecular absorption fingerprints. However, it is very chall...

Highlights

The mid-infrared (MIR) spectrum is known for the presence of characteristic molecular absorption fingerprints originating from the intrinsic vibrational modes of chemical bonds, and the MIR spectrum is critical for chemical identification and structural characterization.[1,2]
We report on a broadband tunable sensing technique based on suspended graphene plasmon nanocavities (Figures 1b and 2) and demonstrate its capability for enhancing light−matter interactions, measuring the real part of the refractive index of the analyte and characterizing both types and concentrations of the various molecules
The Graphene plasmons16−19 (GPs) are excited efficiently and the extremely high field enhancement and extraordinary compression of GPs occur simultaneously, thanks to the combination of a shallow cavity and a deep cavity in the same configuration: the shallow one is above the ridge with length L2 and height h2, and the deep one is in the trench with length L1 and height h1 (Figure 2d)

Summary

■ INTRODUCTION

The mid-infrared (MIR) spectrum is known for the presence of characteristic molecular absorption fingerprints originating from the intrinsic vibrational modes of chemical bonds, and the MIR spectrum is critical for chemical identification and structural characterization.[1,2] Conventionally, spectral analysis is achieved using macro systems, such as Fouriertransform infrared (FTIR) spectrometers, which measure the transmittance or emission spectrum of the analyte using gratings.[3] The analyte can be characterized based on either the retrieved refractive index (Figure 1a) or the existence of the characteristic molecular absorption fingerprints This bulk approach usually requires complex and expensive equipment, such as FTIR spectrometers, and suffers from low sensitivity when detecting signals from small volumes of samples, because of the mismatch between MIR wavelengths (∼μm) and the dimensions of molecules (∼nm). These include the ability to measure refractive indices, the ultra-broadband measurable spectral range, the tiny volume of the analyte required, and the deep subwavelength dimensions of the elements

■ RESULTS AND DISCUSSION

■ CONCLUSIONS

■ ACKNOWLEDGMENTS

■ REFERENCES