Conical Fresnel zone lens for optical trapping

Abstract

The phase of a negative axicon is combined with that of a Fresnel zone lens (FZL) to obtain an element labelled as conical FZL, which can generate a focused ring pattern at the focal plane of the FZL. The phase integration is achieved by modifying the location and width of zones of FZL in accordance with the phase variation of the negative axicon. The element was designed for a high power laser with a wavelength of 1064 nm, focal length and diameter of conical FZL of 30 mm and 8 mm respectively and for a ring diameter of 50 μm. The element was fabricated using photolithography. The pattern was transferred from the resist layer to the borosilicate glass plates by dry etching to achieve an etch depth of 1064 nm. The etch depth measured using confocal microscope was 1034 nm at the central part and 930 nm for the outermost part of the device with a maximum error of 12.5% at the outermost part and 3% at the central part. The element was used in an optical trapping experiment. The ring pattern generated by the conical FZL was reimaged into the trapping plane using a tightly focusing microscopic objective. Polystyrene beads with diameters of 3 μm were suspended in deionized distilled water at the trapping plane. The element was found to trap multiple particles in to the same trap. INRODUCTIONFocused ring patterns are used for many applications like optical trapping 1 , corneal surgery 2 , micro drilling 3 , etc. The scheme employed for the generation of ring pattern varies with the dynamics of the ring pattern required at the application plane. Ref [1] and Ref [3] deals with the generation of a ring pattern with a fixed ring radius while Ref [2] uses a scheme for generation of a ring pattern with variable radius at the application plane. In all the above cases, the optical configuration is bulkier and involves many optical components.In diffractive optics, it is possible to design multifunctional single elements 4,5 . The generation of a ring pattern by combining a binary axicon with a binary Fresnel zone lens (FZL) has been demonstrated earlier 6 . However, due to the method used to combine the independent elements, the generated element had very fine feature sizes 7 making fabrication difficult. Furthermore the maximum efficiency of this element is only 32%, which is 8% smaller than that of a standard binary diffractive element. In order to overcome the above difficulties, a modified scheme to combine an axicon with a FZL to obtain a conical FZL for the generation of focused ring patterns was proposed 7 .The proof of concept was done by fabrication of the device with a device diameter of 2 mm on an electron beam resist (PMMA) as the optical power of the source is only 2 mW. However for optical trapping applications, the optical power is higher with a magnitude of hundreds of mW. Hence, it is necessary to transfer the pattern from the resist layer to the glass substrate through etching. In most of the reported 8-10 etching of borosilicate glass, the etching was carried out with constant period (constant aspect ratio) ...

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