Abstract
Different types of objective lens (OL), i.e. circular, annular and one-dark-ring, are proposed and demonstrated to control the aspect ratio (AR) of micro-focus-region of high numerical aperture (NA) OL. Namely, the AR decreases from about 7.3 to 2.7 in case of using a circular OL with NA changes from 0.7 to 1.4, respectively. By using an annular OL, the transverse size of the focal spot of micro-focus-region decreases but its longitudinal size increases, so that the AR increases several times with respect to the case of circular OL. In particular, when using the one-dark-ring OL, one can decrease both transverse and longitudinal sizes of the focal spot or decrease only the longitudinal size, so that the AR obtained with a one-dark-ring OL is decreased to about 70% of that obtained with a circular OL. Such lenses can be useful for many applications such as sub-microfabrication and three-dimensional data storage using multi-photon absorption process.
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