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

Mid-infrared spectroscopy
is essential for chemical identification
and compositional analysis because of 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 deep subwavelength
graphene plasmon nanocavities and allows for the retrieval of molecular
concentration. This method features a two-dimensional array of suspended
graphene plasmon nanocavities, 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.