Controlling aspect ratio of focal spots of high numerical aperture objective lens in multi-photon absorption process

Three types of objective lens (OL), i.e., circular, annular and one-dark-ring, are presented to control the ratio, called aspect ratio (AR), of the longitudinal size and the transverse size of focal spots of micro-focus-region of high numerical aperture (NA) OL. The AR can be varied from 7.1 to 2.7 in the case of using a circular OL with NA changes from 0.7 to 1.4. By employing an annular OL, the transverse size of the central lobe of micro-focus-region decreases but its longitudinal size increases, so that the AR increases several times with respect to the case of using a circular OL. However, using a one-dark-ring OL, one can reduce either both transverse and longitudinal sizes or only the longitudinal size of focal spot, so that the AR obtained with a one-dark-ring OL is 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.