Holographic recording materials have the task of representing an interference pattern as a physical structure. They start out as more or less homogenous media, applied to a substrate, and use the energy of the light to effect a change in the layer, which is usually amplified by chemical means, so that the bright and dark fringes of the holographic pattern are represented by differences of some sort in the media.

In the most commonly used holographic recording material, usually described as silver halide emulsion, the medium is not exactly homogenous, as silver bromide crystals, smaller than the wavelengths of light, are dispersed thoughout a gelatin layer, sort of like Jello with a lot of chunks of fruit. In the type of processing that most resembles that used in an analog photographic negative, the bright fringes are represented by developed black silver in the gelatin, while the dim fringes are represented by gelatin that has been cleared of the transparent crystals. More typical of current holographic pracitce is to represent one type of fringe with accumulations of silver bromide crystals versus areas without.

In a holographic photo resist scenario, the energy of the bright fringes breaks bonds of the resin which makes it become soluble in an alkali solution; the dim fringes are represented by unchanged resist, the bright fringes by the absence of resist as the developer dissolved it away.

Gelatin, not unlike that used in the silver halide emulsions, is itself made light sensitive by the addition of a dichromate. The energy of the bright fringes powers the hardeniing of the gelatin in those areas by the response of the dichromate to light. Subsequent processing amplifies these hardness differentials by dissolving and/or forming cracks in the dim fringe areas.

Photopolymers start off as a collection of monomers in a structure of ligands; exposure forces the monomers to link up to each other into polymers, plus moving the monomers out of the dim fringes into the bright ones, rearranging its internal structure of polymers in the bright fringes and voids in the dim.

For more details click on the links below. (Silver halide and photopolymers are the only ones active as of October 2021, don't hold your breath too long for the rest!)