Reversible and irreversible alterations of the optical thickness of PQ/PMMA volume recording media samples. Part 2: mathematical modeling.

A theoretical study of phenanthrenequinone-doped polymethyl methacrylate (PQ/PMMA) samples parameter changes during irradiation is presented. The research has been carried out on the basis of the experimental study (Part 1, [Appl. Opt.56, 7351 (2017)APOPAI0003-693510.1364/AO.56.007351]) and a mathematical model that describes space-time reversible and irreversible light-induced changes of the PQ/PMMA sample parameters during exposition. Based on the numerical modeling results and the use of analytical dependencies, a number of practically important characteristics of the PQ/PMMA sample-exposure process are considered: conversion-induction period of phenanthrenequinone into a photoproduct, Δtind; bleaching time of a sample, tbl; and thermal nonlocality of photoresponse, δlT. It is shown that the values Δtind and tbl are on the order of tens and hundreds of seconds, respectively, and can be comparable to the exposure time. It is established that temperature increase leads to δlT value change by hundreds of nanometers and can cause considerable deformation of the interference structure under recording. The sample heating during exposition is considered in detail, and an algorithm for estimation of the maximum temperature increase to be used in experiments is proposed. Recommendations are made for the reduction of negative influence of sample heating on the parameters of recorded holograms.

Reversible and irreversible alterations of the optical thickness of PQ/PMMA volume recording media samples. Part I: Experiment.

Experimental research into changes of the optical thickness of phenanthrenequinone-doped polymethyl methacrylate samples in the process of irradiation by a 473 nm laser is presented. It was demonstrated that heating-induced reversible changes lead to a decrease in the optical thickness. The temperature coefficient of the relative changes of the optical thickness was measured to be -1.40×10-5 К-1, which matches the data published by other authors. It is also established that the irreversible changes induced by the photochemical transformation lead to an increase in the optical thickness outside of the absorption band of the samples (λ>530 nm), where the relative change of which at λ=532 nm is +3.7×10-5. It was demonstrated that the reversible and irreversible changes do not compensate for each other in the process of sample exposure.