| For biosensing applications a useful device needs at least two properties: high sensitivity and high selectivity. Optical spectroscopy offers unique advantages over other sensing techniques however one big challenge to overcome is the mismatch between wavelength and the size of biologically relevant molecules. In order to have high enough sensitivity to approach the single-molecule limit, the interaction between the light and the molecule should be strong. However, the diffraction limit of light is approximately half the incidence wavelength, on the order of 100 nm for the smallest nondestructive wavelengths. This presents a significant mismatch between the size of the molecule and the smallest focus spot of the light. The photo-excitation should be compressed more than 100 fold to interact strongly. We must use metallic antennas that convert the incidence radiation into plasmonic modes which can then be compressed well below the wavelength diffraction limit. Studying the near field characteristics of these metallic nanostructures will help us gain insight into this emerging field and allow us to better use them in developing next generation devices.;We have developed different geometries of these antennas and simulated their performance using Finite Difference Time Domain software. We have concentrated our efforts in the mid-infrared because that is the natural molecular vibration frequency region and also the near infrared because at these frequencies there exists a mature industry for compact sources, detectors, and fiber optic components.;Our simulations show a 6,000 fold mode compression for a bowtie antenna and a million fold compression for a plasmonic photonic crystal (ppc) antenna. The bull's-eye antenna does not have as a high a mode compression but it has a natural geometry for molecular sensing due to the central metallic disc. Experimentally, we have measured the near field of these antennas with a custom back reflection apertureless NSOM setup in both non-contact and constant current AFM mode and found excellent qualitative agreement with our simulations for the bowtie and bull's-eye. Further development on these experimental methods will help bring about advances in medical devices, MEMS, NEMS, and revolutionize the way we engineer nanoscale optical components. |
