Femtosecond laser microfabrication in polymers towards memory devices and microfluidic applications

Abstract

dnrsp@uohyd.ernet.in , Phone: 91 040 23134335 Abstract We have investigated femtosecond laser induced microstructures, gratings, and craters in four different polymers: poly methyl methacrylate (PMMA), poly dimethyl siloxane (PDMS), polystyrene (PS) and poly vinyl alcohol (PVA) using Ti:sapphi re laser delivering 800 nm, 100 femtosecond (fs) pulses at 1 kHz repetition rate with a maximum pulse energy of 1 mJ. Local chemical modifications leading to the formation of optical centers and peroxide radicals which were studied using UV-Visible absorption and emission, confocal micro-Raman and Electron Spin Resonance (ESR) spectroscopic techniques. Keywords Diffraction grating, Electron spin resonance, Emission, Free radicals, Laser direct writing. 1. Introduction Different materials including polymers and semiconductors have been tried for fabricating different micro- and nano-patterns by laser abla tion method [1, 2]. The ablation of polymers after interaction with fs laser pulses is a subject that has been studied in detail [3, 4]. The interaction mechanism is slightly different in case of fabricating structures inside bulk samples. Nonlinear absorption takes place while transferring the energy fr om laser pulse to the material of interest as photon energy is less than the band gap of the material. Each 800 nm photon has 1.55 eV energy while band gap of the most of the transparent pol ymers lie in between 4 to 7 eV. There are three different nonlinear ionizations that take place when femtosecond (fs) pulses interact with materials namely tunneling, intermediate and avalanche ionizations which can be predicted by Keldysh parameter [5]. X. Liu et al. have shown that for 100 fs pulses, the dominant absorption process is avalanche ionization which produces an exponential increase in conduction band electrons. As this requires seed electrons in the conduction band [6 ], thermally excited carriers from defects and/ or traps form the necessary seed electrons. At large intens ities as in the present case, multiphoton absorption leads to ionization to seed the avalanche ionization [5, 7]. The