Clamshell-type bis-phthalocyanines as colour-changing optical limiters: TDDFT modelling of electrically induced absorption for real-time colour indication of optical limiter efficiency in nonlinear laser protection

Abstract

This paper presents an innovative method for assessing the performance of optical limiters, which are devices that protect humans against laser radiation. The essence of the process is that with increasing laser radiation intensity, the colour of nonlinear absorbers, the working components of these devices, can change in contrast to the original one, which was demonstrated by quantum-chemical linear response time-dependent density functional theory (LR-TDDFT) for molecules excited by static electric finite fields (FF) within the general DFT (density functional theory) approach. Modelling was carried out on clamshell-type bis-phthalocyanines, in which the macrocycles are strapped by a cyclotriphosphazene spacer, suitable candidates for creating nonlinear optical (NLO) dyes for laser technology. As calculations have shown, acting as a “litmus test”, these incredibly stable macrocyclic compounds are not only capable of providing laser protection but also give a real-time visual indication of the protection through changes in the colour of the dye, which correlate with different levels of laser power. Such indication can visually show the current state of optical limiters in practice, allowing users to easily determine whether the limiter material provides adequate protection or already requires replacement. A modelling protocol and program code for calculating the colour of a substance based on its absorption spectrum are presented.