Spontaneous nano pattern formation under pulsed exposure: relaxation kinetics study

Formation of spontaneous surface relief grating under pulsed exposure on the surface of an azo polymer film has been investigated. We did experiments in different repetition rates and pulse widths by use an electro mechanical chopper and a diode-pumped solid-state laser with wavelength of 532?nm as pump beam. Incidence of pump beam, has been set normal to the surface of azo-polymer film. Spontaneous surface relief grating with grating pitch of ~1,250?nm was formed. For low repetition rate, relaxation due to cis-trans isomerization becomes dominant and growth rate becomes slower. A threshold for spontaneous pattern formation was found. In addition to an increase in the diffraction intensity during illumination, anomalous variation of diffraction intensity by switching on and off the laser was observed. By switching off the recording beam sharp enhancement occurred. By switching on the recording beam, after a sharp decrease in the intensity of diffracted beam, it increased further under illumination. Anomalous variations, strongly depends on polarization of the probe beam and the irradiation time for the formation of spontaneous SRG. Sharp enhancement has been attributed to cis-trans thermal isomerization and sharp decrease has been attributed to isomeration in whole surface under illumination.