Design of a plasmonic photonic crystal for single bio-molecule spectroscopy

We present the design and initial characterization of a device geometry that is suitable for measuring the direct mid-infrared absorption of single molecules. The devices are based on a metal-dielectric-metal sandwich with optically thick gold cladding layers which squeeze the gap mode, the optical mode in the dielectric layer, in the electric field direction; with a photonic crystal cavity defect which can squeeze the gap mode in the planar direction. Simulations show a field enhancement of 10000 times the incident field in the cavity defect at an excitation frequency of 87 THz. Experimentally, devices with varying periods were made using a free standing Si₃N₄ film of 15 nm or 50 nm as the inner dielectric. These devices show a red shift as the period is increased and more interesting there is also a red shift for the thinner dielectric devices showing that the field is further squeezed in this layer. By placing a molecule into the cavity, there should be a strong enough interaction between the light and the analyte so that its absorption spectra can be resolved.