Creation of a narrow Bessel-like laser beam using a nematic liquid crystal

large number of applications in physics, chemistry, bilogy, and engineering would greatly benefit from a laser eam that had a sharp focus but, simultaneously, a negliible spreading due to diffraction. Examples of such aplications include laser drilling and writing, trapping nd guiding of microparticles or nanoparticles and neural atoms, precise alignment and metrology echniques, and high-resolution three-dimensional maging. The usability of conventional, focused laser eams for these purposes is limited owing to diffraction; n optical field focused to a small spot quickly diverges. he Rayleigh range of, e.g., an ideal Gaussian beam with waist radius of 5 m is only 160 m for the wavelength f 500 nm. The beam divergence can be reduced by converting the aussian laser beam into a Bessel-like beam, which has a early propagation-invariant intensity profile within a ong distance of propagation. Typically, this is realized by sing an axicon-based optical system, by inserting a arrow ring-shaped aperture in front of a positive ens, or by applying a two-element diffractive ystem. Certain approximate Bessel modes can also be enerated directly in a laser resonator, as demonstrated n Refs. 19–23. Axicons are perhaps the most widely used lements in the realization of Bessel-type laser beams, ut their use has some drawbacks. For example, to obtain narrow and long focal line, the axicon has to be comined with an annular aperture or an apodizing transmitance mask that inevitably blocks a significant part of he incident light. Furthermore, the quality of the focal ine is sensitive to alignment and, in the case of diffracive axicons, to fabrication errors. Bessel-like beams reated inside a laser resonator, for their part, are diffiult to make narrow at the laser output, and focusing of uch a beam does not result in a long depth of focus. In this work, we investigate the possibility to create a ong and narrow focal line by using self-phase modulation