Vortical laser tweezers with predetermined intensity structure

The intensity of spiral beams remains unchanged under propagation and focusing neglecting scaling and rotation. The spiral beam with predetermined intensity in the shape of any planar curve can be generated by use of amplitude and phase elements concurrently. We introduce the new method of singular laser fields formation, close to spiral type, by means of pure phase modulation. Our algorithm is based on the well-known Gerchberg-Saxton phase retrieval algorithm and spiral beams optics. It demonstrates fast convergence and some other advantages: phase distributions obtained are stable to spatial resolution changing (it is enough 128 x 128 pixels for some patterns), theoretical energy efficiency is about 85 % with acceptable intensity homogeneity. We demonstrate theoretical results on fields formation in the shape of closed-curves (triangular, square, "snowflake") and open-ended curve (Archimedes spiral) by means of elements on dichromate gelatin. Besides, the example of experiment on micromanipulation with the use of the square-shaped field is presented.