Linear and Nonlinear Excitation of P3HT Induced by Spectral-Shaped Ultrafast Mid-IR Laser Radiation

Abstract

This article discusses the linear vibrational and nonlinear electronic excitation of the organic material P3HT during processing by ultrafast laser radiation in the mid-IR spectral range. Therefore, the center wavelength 3.4 μm of the laser radiation is set equal to the resonance of the C–H stretching, and the ablation threshold fluence is determined depending on the pulse duration τH and bandwidth Δλ of the laser radiation. Both parameters are varied, using a spectral pulse shaper. At τH < 400 fs, the threshold fluence depends on the pulse duration, indicating nonlinear excitation, and remains constant at τH > 400 fs, due to only linear vibrational excitation. A theoretical model based on the Keldysh formalism, considering the nonlinear photoionization, impact ionization, and linear absorption, supports the experimental results.