Aperture-size-controlled optical fiber tips for high-resolution optical microscopy

A method is developed to produce chemically etched optical tips for near-field and confocal microscopies with valuable properties such as large transmission and no lateral light leaks. Prior to metal coating, tips are coated with a dielectric material, MgF2, that is refraction-index-matched to silica. It is shown that adjusting the MgF2 layer thickness allows us to control the tip aperture size in the diameter range from 70to500nm. First, near-field fluorescence imaging of latex nanospheres with the smallest optical aperture tips confirms their ability to reach subwavelength optical resolution. In addition, thanks to their high transmission and collection efficiencies as well as their natural achromatism, it is proposed that the large optical aperture tips be advantageously substituted to high numerical aperture objectives in some confocal microscopes operating in constrained environments such as at low temperature.