Developing a near-field scanning optical microscope for the characterization of aluminum nanoantennas

Abstract

Optical antennas localize energy from incoming electromagnetic waves that oscillate at visible frequencies. Because optical wavelengths are smaller than radio wavelengths, optical antennas are substantially smaller than radio antennas - they are on the order of tens or hundreds of nanometers, where light-matter interactions are described by charge density oscillations. The electromagnetic field response of aluminum optical antennas can be studied using near-field scanning optical microscopy. The spatial and intensity characteristics of the electromagnetic field response from an optical antenna are a function of the antenna geometry and the light-matter interaction. This response will have propagating and nonpropagating field components. Far-field microscopes can measure the propagating components of the field, but these components provide little information about features that are smaller than the diffraction limit, or about half the wavelength of the illuminating light. The nonpropagating components of the field can provide more detailed and localized spatial information, but these components are difficult to measure because their amplitude decays exponentially as they move away from the sample. By introducing a sharp probe into the focus, near-field microscopes can scatter nonpropagating field components into the far-field. Our system illuminates the nanoantenna with laser light, controls a sharp probe in the focus, and collects the scattered light. We have implemented a feedback system to maintain the sub-50 nm probe-sample separation as the probe is scanned with respect to the sample. This feedback system allows us to acquire a high-resolution surface profile image together with an optical image. The spatial resolution of a near-field microscope is limited by the sharpness of the probe and the ability to distinguish photons scattered by the presence of the probe. The optical antenna system has been modeled by approximating the nanoscale aluminum antennas as a dipole orthogonal to the optical axis and the probe as a dipole along the optical axis.