Researchers at Yale University (New Haven, CT) and Digital Instruments (Santa Barbara, CA) have built a solid-immersion-lens (SIL) microscope that reaches a numerical aperture (NA) of 2.0 at visible wavelengths—33% higher than that of oil-immersion objectives and previously reported SILs. At the core of the instrument is a hemispherical SIL made of gallium phosphide, which has a refractive index of 3.42 at 560 nm. The lens has a 500-µm radius, is virtually achromatic, and is used in conjunction with a 0.8-NA commercial microscope objective.
When imaging 40-nm fluorescent polystyrene balls at wavelengths of 560, 645, and 720 nm, the device achieves resolutions of 145, 180, and 183 nm, respectively (defined by the full width at half maximum). Deconvolving the object diameter from the measurements results in resolutions of 139, 175, and 178 nm and effective NAs of 2.05, 1.88, and 2.06, respectively. The researchers hypothesize that a 60-nm gap between the balls and the SIL surface prevents the lens from reaching its theoretical NA of 2.50. If made of higher optical quality and used with a 0.9-NA objective, a spatial resolution of 100 nm should be possible with the lens. Contact Qiang Wu at [email protected].