Sapphire is one of the hardest substances available, and optical elements made from it have extreme surface hardness, scratch resistance, and a high refractive index of 1.76. Sapphire is used to improve the resolving power of optical microscopes through a technique using a lens known as a numerical aperture increasing lens (NAIL). In this method, a NAIL is placed on the specimen to be imaged before the microscope objective, resulting in an overall high numerical aperture, thereby improving magnification and resolution. The authors of this Applied Optics article propose using a liquid immersion medium with a refractive index that is tunable from 1.7 to 1.8 and with good optical transmission from the visible to the NIR. By dissolving antimony tribromide (SbBr3
) in diiodomethane (CH2
), they obtained a liquid with a very high refractive index matching that of a sapphire NAIL. They showed that the effective numerical aperture of a microscope objective increases from 0.619 to 1.17 with a sapphire aplanatic lens (aNAIL) immersed in this newly developed index matching liquid. This not only improved the numerical aperture but additionally provided a long working distance (in this case 12mm) that can be of great advantage in certain applications. Refractive index tunability based on the concentration of the index matching liquid and temperature, may allow the development of an adaptive liquid lens with variable focus. Spherical aberration associated with conventional subsurface microscopy can also be eliminated by the NAIL optics with the proposed immersion medium, leading to improved high numerical aperture and diffraction-limited spatial resolution beyond the limits of conventional subsurface microscopy.
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