Third order nonlinear optical spiropiran containing polymer solutions and layers

Transparent in visible region spyropirans (SP) undergo photochromic effect due to ultraviolet radiation changing to open form - merocyanine (MC). This may be the base to use them as molecular switches. As both SP and MC molecules generate strong NLO this switching may be seen also for second and third order processes. Nonlinear properties of both SP and MC molecules in polymer matrix were simulated within PCM model at DFT level using different functionals - namely B3LYP, Cam-B3LYP, LC-BLYP - and different basis sets. It occurred that polarizability and hyperpolarizabilites tensors values are several times greater than those calculated for urea which is often used as an reference. Experimental evaluation is based on z-scan measurements. Data are collected for SP / MC solutions in mixture of polyvinyl alcohol (PVA) or polystyrene (PS) in tetrahydrofuran (THF). The same was done for layers of SP / PVA or SP / PS composites on quartz plates. Z-scan spectra were recorded both for open and for closed aperture leading to the possibility to calculate nonlinear absorption coefficient and nonlinear refraction index. Values of these parameters confirm pretty well that investigated materials have very good NLO properties. Samples with specific SP concentration reveal untypically strong effects both NLO and in absorption spectra. This was explained as a result of SP clustering within matrix, leading to occurrence of nanograins of this active species. (author)