Periodic Bell-shaped Upheaval Structure on Surface of Polycarbonate by Irradiation of Femtosecond Laser Pulse

Abstract

A periodic bell-shaped upheaval structure was formed on the surface of polycarbonate (PC) by line irradiation of a near-infrared (NIR) femtosecond laser pulse onto PC bulks. The upheaval structures were controlled by the irradiation depth and irradiation energy in PC. For example, the increase in irradiation energy at a constant target depth (50 µm) induced the discontinuous change in structures: a small periodic bell-shaped upheaval structure, an intermediate continuous band-shaped structure without a clear upheaval structure and a final large periodic bell-shaped upheaval structure. The size of the bell-shaped upheaval structure varied from approximately 15 µm to 45 µm in diameter of a basal plane upon changing the irradiation conditions. Using the cross-sectional images of the irradiated PC bulk samples observed using an optical microscope, a scanning electron microscope (SEM) and a differential interference laser scanning microscope, we confirmed the position of cracks and voids, and the structure with refractive index modulation formed in PC bulk. The mechanism of the formation of the upheaval structures in PC was investigated by changing the irradiation conditions. We proposed a mechanism of the formation of the upheaval structure in PC on the basis of multiphoton absorption, anisotropic thermal gradient, growth of microcracks by microexplosion at the focused point and movement of the grown cracks toward the surface along the anisotropic thermal gradient.