  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.
