My PhD thesis and part of my Postdoc in MPIP were devoted to the establishment of a detailed phenomenology of the unanticipated effect described above. We discovered that depending on the solvent used, polydiene solutions responded to the applied optical field by lowering or increasing the local polymer concentration inside the irradiated volume. The decoupling of the refractive index difference between the macromolecules and the solvent, Dn, and the optical contrast of the formed structures, dn, resulted in a variety of different patterns, such as high-contrast polymer-rich fibrilar structures or solvent-rich stripe-like formations of much lower contrast.

Despite the rich and clear phenomenology, the physical mechanism behind the interaction between the optical field and the polydiene-solvent systems still remains mysterious, as common effects fail to provide a satisfactory explanation. Research on this subject is going on.

In collaboration with G. Fytas and B. Loppinet, Polymer and Colloid Science Lab, IESL-FORTH, Crete, Greece

What happens when you shine light from a laser pointer on a transparent semidilute solution of poly(isoprene)? Normally, apart from weak light scattering, nothing else should occur…however, performing this simple experiment will definitely surprise you!

Light-Polymer Solution Interactions

The initially well-defined Gaussian profile of the transmitted laser beam widens up to a richly structured light pattern. This observation, combined with the fact that the scattered light strongly increases suggests that light induces refractive index changes in the irradiated fluid. Indeed, a simple microscopy observation reveals that a material pattern is ‘written’ along the laser propagation axis. This surprising light-soft matter coupling was discovered approximately 15 years ago by Sigel et al. [1] and was later utilized for more complex patterning using holographic gratings by Loppinet et. al [2].

In order to open possibilities for 3D micropatterning, more complex micellar solutions of diblock copolymers  in selective solvents were further investigated. In particular, low power irradiation of turbid solutions led to the occurence of the astonishing self-induced transparency (figure below, left part), where a rapid enhancement in the light transmission was achieved, through a complex patterning process (figure below, right part).

By means of real-time phase contrast microscopy, we quantified the kinetics of the fibrilar (polymer-rich) pattern formations and its dependence on various material and irradiation parameters. The reversibility of the writing process was found to depend on irradiation time. For short exposures, written structures of higher polymer concentration re-dissolved in the solutions, while prolonged irradiation gave rise to insoluble structures. 

Interestingly, an analogy between light-induced fibrilar pattern formations and Optical Spatial Solitons and Modulation Instabilities (MI) was demonstrated. Both (1+1)D and (2+1)D MI were evidenced as 1D and 2D arrays of filaments formed by using a cylindrical lens or by defocusing the LASER beam. The experiments was clearly showed that polydiene solutions offer an unexpected, simple and versatile system for non-linear optics.