Toward High-Throughput Nearfield Scanning Optical Microscopy
Stephan Koev, Alec Talin, and Vladimir Aksyuk
Affiliation: Center for Nanoscale Science and Technology, NIST, Gaithersburg, MD 20899
Nanoscale research and manufacturing require improved optical imaging methods with high resolution and high speed. Farfield optical microscopy is fast, but its resolution is limited by diffraction to approximately the wavelength of light. Nearfield scanning optical microscopy (NSOM) significantly improves resolution by using a nanoscale aperture that locally samples the surface. However, it has very low speed due to the limited light transmission efficiency of the aperture.
We are developing a new nearfield imaging method that can significantly enhance the speed of nearfield optical microscopy. Instead of a single nanoscale aperture, we use an array of nanoscale scatterers to illuminate the sample surface. The scatterers are mounted on a glass probe chip, which is scanned over the sample to acquire an image. Since light is collected from multiple points simultaneously, the scanning speed can be increased considerably compared to the single aperture approach. An optical microscope with evanescent laser illumination and darkfield filtering is used to collect light from the scatterers while blocking specular reflections from planar surfaces. The probe chip is positioned a few tens of nanometers from the sample surface by a piezoelectric actuator with feedback control.
Two different types of scatterers were investigated: colloidal gold nanospheres deposited by spin casting and gold nanorods formed by electron beam lithography and electroplating. It was found that the former have superior optical properties, while the latter allow for better control of dimensions and position. Nearfield scanning with resolution better than 200 nm was demonstrated with both types of scatterers. In this presentation, we discuss the fabrication of the probe chips, their optical characterization, and the initial nearfield scanning results.