Arvind Sundaramurthy

Optical antennas: Resonators for local field enhancement

Electromagnetic field enhancement in optical antenna arrays is studied by simulation and experiment at midinfrared wavelengths. The optical antennas are designed to produce intense optical fields confined to subwavelength spatial dimensions when illuminated at the resonant wavelength. Finite difference time domain (FDTD) method simulations are made of the current, charge, and field distributions in the antennas. The influence of antenna shape, length, and sharpness upon the intensity of the optical fields produced is found. Optical antennas arrays are fabricated on transparent substrates by electron beam lithography. Far-field extinction spectroscopy carried out on the antenna arrays shows the dependence of the resonant wavelength on the antenna length and material. The FDTD calculated and experimentally measured extinction efficiencies of the optical antennas are found to be in good agreement.

Exploring the chemical enhancement for surface-enhanced Raman scattering with Au bowtie nanoantennas

Single metallic bowtie nanoantennas provide a controllable environment for surface-enhanced Raman scattering (SERS) of adsorbed molecules. Bowties have experimentally measured electromagnetic enhancements, enabling estimation of chemical enhancement for both the bulk and the few-molecule regime. Strong fluctuations of selected Raman lines imply that a small number of p-mercaptoaniline molecules on a single bowtie show chemical enhancement >10(7), much larger than previously believed, likely due to charge transfer between the Au surface and the molecule. This chemical sensitivity of SERS has significant implications for ultra-sensitive detection of single molecules.

Field enhancement and resonance in optical antennas

Optical antennas are studied as probes for near-field optical microscopy. Finite difference time domain calculations indicate an intensity enhancement of more than 3 orders of magnitude. Optical antennas are fabricated and tested with good agreement between experiment and theory.

Nearfield optics with solid immersion lenses and sharp metal probes

We discuss two methods for achieving optical resolution beyond the diffraction limit in air. The first method, the solid immersion lens (SIL), improves optical resolution by increasing the numerical aperture (NA) beyond 1.0, the usual limit in air, to a maximum of n, the refractive index of the SIL. We present experimental results with a scanning optical microscope (/spl lambda/=400 nm) based on a micromachined silicon nitride SIL that demonstrates optical resolution /spl sim/1.9 times better with the SIL than without. Specifically, the microfabricated silicon nitride SIL improves the optical edge response from /spl sim/190 nm to /spl sim/100 nm. The second method for improving resolution is based on the strongly enhanced electric field at the surface of a nanoparticle illuminated with light whose wavelength corresponds to the particles plasmon resonance. We present finite difference time domain (FDTD) calculations showing that the electric field intensity at the sharp apex of a triangular nanoparticle is enhanced by more than two orders of magnitude over the incident intensity in a very small region (< 10 nm).