Experimental investigation of long time irradiation in polydiene solutions: reversibility and instabilities

Polydiene solutions have recently been shown to possess a refractive index/concentration optical nonlinearity upon irradiation by CW-laser of a power as low as a few mW with typical timescales for solitonic formation in the order of seconds. Here we report on the effects of longer time laser irradiations which unveiled a variety of light-induced patterns. At extended irradiation times, rather than saturation or steady state of the single-filament pattern initially formed, the formation of more complex structures was systematically observed. The reversibility of the laser-induced concentration increase was shown to depend on irradiation time. Whereas exposures of the order of the inverse of the rate of formation resulted in reversible patterns, prolonged irradiation led to surprisingly long lasting irreversible structures that even resisted dissolution in a pure solvent. An intermediate case of semi-reversible patterns was observed giving rise to a pearl-necklace formation. Despite the as yet unidentified coupling, polydiene solutions offer a rich playground for soft matter nonlinear optics and potentially provide a readily available system for micro-patterning in a lithographic-like manner