Red-sensitive imaging systems for holography

Abstract

The primary objective of the work described in this thesis was to devise a red-sensitive photoresist imaging process for use in the replication of diffraction optics.

In the introduction the chemistry of conventional photopolymer systems and photoresists used for holographic recording and the fabrication of diffraction gratings and diffracting optical elements is reviewed. The limitations of commercially available photoresist systems, particularly for applications requiring the use of red light are discussed.

A polymer system has been investigated which could be imaged by photochemically generated free radicals, followed by a simple aqueous development procedure as required by the original specification. The polymer chosen for study was a copolymer of methyl methacrylate, methacrylic acid and 2-hydroxyethyl methacrylate. This was derivatized using methacryloyl chloride or methacrylic anhydride in order to introduce cross-linkable units to the polymer backbone.

Polymers have been characterized by a number of techniques and the effect of varying composition on aqueous base solubility has been thoroughly studied. Various methods of derivatization have been employed. The ease of imaging has been found to be very sensitive to both the composition of the polymer and the extent of functionalization. High quality images have been obtained from this polymer using an organic solvent developer.

Imaging experiments have been carried out on thin films of the photopolymer coated on glass using phenylazotriphenylmethane (PATM) as photoinitiator. Good images of 100 lines permillimetre (1 mm-1) have been recorded by contact printing. Interferometry has been used to demonstrate that interference patterns having 600 and 1200 1 mm-1 can be recorded using this polymer with PATM as initiator, exposed to an argon ion laser operating at 458nm.

A number of two component photoredox initiator systems have been investigated, the light absorbing species of such systems being a dye such as methylene blue or certain cyanine dyes. The second component of these initiators may be an aryl sulphinate salt, a 1,3-diketone or some alkyl sulphides. The red light-initiated phatopolymerization of acrylamide has been demonstrated using some of these initiators and a low resolution photopolymer image has been recorded using Azure A and perinaphth-1,3-indandione as the photoinitiator system.

This polymer can, in principle, produce images over a wide range of wavelengths depending on the nature of the initiator used.