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Photo-Induced Self-Growing of Microstructures in Photopolymerizable Resin | |
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Introduction Optical self-action effects occur when a beam induces a refractive-index change in nonlinear materials through which it propagates. Self-trapping or so called "spatial optical soliton" is one of the typical phenomena. We found that similar effects could occur in photopolymerizable resin, in which several fiber structures are self-grown with time. Self-growing of a single fiber structure Fig 1 shows the photograph of a narrow single fiber which was "self-grown" into a photopolymerizable resin(PR) when a UV laser was focused onto this resin. For this experiment (as well as the other experiments here), a He-Cd laser operating at 441.6 nm was focused onto the entrance face of a glass cell filled by a liquid solution of the PR. The refractive index of the PR before polymerization is 1.53, and that after photopolymerization is 1.55. The length of the self-grown fibers increases proportionally to the exposure time, but its diameter keeps a constant value of 3µm. Numerical investigation of self-growing processes The mechanism of self-growing is similar to that of the spatial optical solitons, and it is based on the self-action of light. When a laser beam is launched into the liquid photopolymerizable resin, the photopolymerization reaction occurs according to the light intensity and thereby a solid structure is formed in the material. Since the refractive index increases as the photopolymerization reaction, the solid structure affects the propagation of the light and the incident light begins to be confined to the solid structure which itself has created. The incident light travels inside the structure as a waveguide mode, which giving rise to a single fiber structure. We proposed an analytical model that describes the relation between the refractive index change of the resin and the light intensity, and simulated the self-growing process of the fiber structure. You can see the detail of the numerical models and simulation results in Ref. 2, 3. Collision and merging of two self-growing single fibers Fig 2 shows that two independent single fibers can merge to form a single fiber when they intersect each other after a simultaneous growth induced by two different beam spot at the entrance of the cell. The merging occurs only when the collision angle is smaller than a critical value of ~9°, otherwise the fibers cross each other. We found that the intensity ratio or the time delay of the two input beams influences the growing direction of the fiber after merging. Numerical simulations in Ref. 2, 3 show good agreement with these experimental results. Multiple fibers We experimentally discovered that multiple fiber structures can be self-grown by choosing much higher power of the laser light and higher NA of the focusing objective lens. The fibers are grown radially from the focus point of the incident beam, as shown in Fig. 4. Reference
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 Fig.1 Optically-grown single fiber into the photopolymerizable resin. The diameter and the length of the fiber are 3µm and ~1mm, respectively.  Fig.2 Schematic of the fiber structure formation. The single fiber growth is associated with the formation of spatial optical soliton(or self-trapped optical beam) in the refractive index change of the photopolymerizable resin.  Fig.3 Interaction and merging of two single fibers. The two self-growing fibers merge to form a single fiber under the collision angle lower than a critical value.  Fig.4 Multiple fiber structures induced by a high NA objective lens(NA:1.0). |