An Experimental Study of the Dynamics and Temporal Evolution of Self-Trapped Laser Beams in a Photopolymerizable Organosiloxane

Self-trapping of a visible, continuous wave laser beam in a photopolymerizable organosiloxane was studied at intensities ranging across 10 orders of magnitude (3.2 × 10−5 to 12 732 W·cm−2). The process was characterized in detail through spatial intensity profiles of the beam, temporal monitoring of its width and peak intensity combined with optical microscopy of the resulting self-written waveguides. These observations revealed a rich diversity of dynamic phenomena during the self-trapping process in different intensity regimes, including (i) complementary oscillations in width and peak intensity of self-trapped beams, (ii) in situ sequential excitation of high-order modes (corresponding to optical fiber modes) in self-written cylindrical waveguides, (iii) variations in modal composition during the transition of self-written waveguides from single to multimode guidance, (iv) generation of spatial diffraction rings, and (v) beam filamentation. Quantitative analyses of parameters such as self-focusing time, self-trapped beam width and light transmittance gave new insight into details of the self-trapping mechanism, particularly the significance of the spatial profile (gradient) of refractive index changes induced in the medium. The results of this comprehensive experimental study provide a deep understanding of the dynamics of self-trapping in a photopolymerizable medium, including the general process of waveguide self-writing, and are consistent with some predictions of earlier theoretical models, the most significant being the rare opportunity to individually observe high-order optical modes during the evolution of the waveguide. In addition, entirely new features such as the emergence of spatial diffraction rings require further theoretical and experimental investigations.