Nonlinear lithographic properties by femtosecond laser pulses using a low-NA lens

Refractive indices of a chemically amplified photoresist were changed by femtosecond (fs) laser irradiation without post exposure bake (PEB) treatment. We have proposed a combined process of fs nonlinear lithography and plasma etching for the fabrication of functional photonic devices with 3-D surfaces of inorganic materials. In this study, we report the nonlinear lithographic properties using fs laser pulses focused by a low-NA objective lens. Diffraction gratings were written directly inside the pre-baked resists by fs laser nonlinear absorption. From diffraction efficiencies using He-Ne laser light, refractive indices were changed by 8 x 10(-3) without PEB treatment which was required for cross-linking reaction. In contrast, no changes of refractive index were observed in the case of ultraviolet light exposure (i-line). Considering this large refractive index change and the threshold intensity of nonlinear absorption of the resist, self-guiding of fs laser pulses can occur due to the optical confinement in the radial direction. In fact, filamentary patterns which were optical-axially asymmetric and longer than the focal depth could be obtained without translating the focal spot using this lithographic property.