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

TL;DR

This paper introduces a nanophotonic imaging technique using suspended graphene plasmon cavities to measure the refractive index of analytes across a broad mid-infrared spectrum with high sensitivity and minimal sample volume.

Contribution

It presents a novel two-dimensional array of graphene plasmon cavities enabling broadband, high-resolution refractive index imaging in the mid-infrared range, surpassing existing nanophotonic methods.

Findings

Achieved ultra-broadband spectral measurement of refractive index.

Demonstrated high field enhancement and spatial resolution.

Reduced analyte volume needed for accurate measurement.

Abstract

Mid-infrared spectroscopy is essential for chemical identification and compositional analysis, due to the existence of characteristic molecular absorption fingerprints. However, it is very challenging to determine the refractive index of an analyte at low concentrations using current photonic systems in a broad mid-infrared spectral range. We propose an imaging-based nanophotonic technique for refractive index determination. The technique is based on deeply subwavelength graphene plasmon cavities and allows for the retrieval of molecular concentration. This method features a two-dimensional array of suspended graphene plasmon cavities, in which the extremely high field enhancement and extraordinary compression of graphene plasmons can be realized simultaneously by combining shallow and deep cavities. This enables resonant unit cells to be read out in the spatial absorption pattern of the array at multiple spectral points, and the resulting information is then translated into the refractive index of the analytes. The proposed technique gives complementary information compared with the current nanophotonic techniques based on molecular absorption, including the ability of the refractive index measurement, the ultra-broadband measurable spectral range, and the small volume of the analyte required, thereby pushing the potential for refractometric sensing technologies into the infrared frequencies.