High photoinduced birefringence in thermally treated layers of the azopolymer PAZO with significantly changed absorbance spectrum

In this experimental work we present the effect of thermal treatment of azopolymer thin films on their absorbance spectra and how that influences the photoinduced birefringence of the azopolymer. The azopolymer we use – PAZO (poly[1-[4-(3-carboxy-4-hydroxyphenylazo)benzenesulfonamido]-1,2ethanediyl, sodium salt]) is commonly investigated material for polarization holography because it is easily accessible and has well pronounced photoanisotropic properties. In a recent study it was discovered that its absorbance spectrum gradually changes upon heating to 250°C and as a result the absorbance in the range 400-600 nm is increased. That raises the question is it possible to increase the photoinduced birefringence for wavelengths that are far away from the peak of absorbance of the azopolymer PAZO, located approximately at 360 nm. For example, there are powerful lasers at 532 nm, which could be used for optical recording instead of traditionally used lasers in the blue and UV range. To answer this question, we investigated thin film samples of PAZO deposited on quartz substrate and measured continually their spectra of absorbance while heating the samples from room temperature to 250°C. We also measured the photoinduced birefringence using pump laser at 532 nm before and after the thermal procedure. Thus, we report significant increase of the birefringence at 532 nm after the thermal treatment and mention some potential applications.