Fabrication of optical fiber probes for scanning near-field optical microscopy

Abstract

Many areas of cell biology have remained unexplored due to the limitations of conventional optical microscopy for image structures smaller than the diffraction limit of light. Scanning near-field optical microscopy (SNOM) is an emerging technique which allows sub-diffraction limit optical resolution and hence access to nanoscale structures such as those in biological cells. Since the success of a scanning probe microscopy techniques depend on the tip, we present a method for the fabrication of SNOM aperture probes and antenna probes capable of high resolution imaging of delicate liquid based samples such as neurons. The procedure includes thinning and tapering optical fibers by chemical etching, followed by the deposition of a thin aluminium film, and micromaching using focused ion beam (FIB). Tip geometry, antenna resonances, excitation conditions, and field localizations have been examined. The probes have a resonant frequency of 18-28 kHz, a Q of 200-400 in air and up to 100 in water, and a spring constant of on the orders of 140 N/m. Successful optical and topographical imaging up to a resolution of 250 nm was achieved with the probes. Together with adjustments in equipment and instrumentation, the probe’s optical and mechanical properties allow for a low imaging force and high tip sensitivity which are necessary conditions for the imaging of biological cells.