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.

We show that ablation features in poly-methyl methacrylate (PMMA) induced by a single femtosecond laser pulse are imposed by light polarization. The ablation craters are elongated along the major axis of the polarization vector and become increasingly prominent as the pulse energy is increased above the threshold energy. We demonstrate ∼40% elongation for linearly and elliptically polarized light in the fluence range of 4-20 J cm −2 , while circularly polarized light produced near circular ablation craters irrespective of pulse energies. We also show that irradiation with multiple pulses erases the polarization-dependent elongation of the ablation craters. However, for line ablation the orientation of the electric field vector is imprinted in the form of quasi-periodic structures inside the ablated region. Theoretically, we show that the polarization dependence of the ablation features arises from a local field enhancement during light-plasma interaction. Simulations also show that in materials with high nonlinearities such as doped PMMA, in addition to conventional explosive boiling, sub-surface multiple filamentation can also give rise to porosity.

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Abstract:We show that surface swelling is the first step in the interaction of a single femtosecond laser pulse with PMMA. This is followed by perforation of the swollen structure and material ejection. The size of the swelling and the perforated hole increases with pulse energy. After certain energy the swelling disappears and the interaction is dominated by the ablated hole. This behaviour is independent of laser polarization. The threshold energy at which the hole size coincides with size of swelling is 1.5 times that of the threshold for surface swelling. 2D molecular dynamics simulations show surface swelling at low pulse energies along with void formation below the surface within the interaction region. Simulations show that at higher energies, the voids coalesce and grow, and the interaction is dominated by material ejection.