Deciphering carrier dynamics in polycarbonate following excitation with ultrashort laser pulses

Abstract

Polymers exposed to ultrashort pulsed lasers (UPL) experience a range of physical and chemical changes that play a key role in applications ranging from material processing to advanced photonics, and biomedicine. To elucidate the interaction of UPLs with polymeric materials, ultrafast phenomena such as carrier dynamics, recombination, and relaxation are investigated assuming polycarbonate (PC) as a test material exposed to the laser pulses of moderate energies. A theoretical model developed for dielectric materials is extended to describe the previously unexplored excitation and carrier dynamics for PC, while femtosecond transient absorption spectroscopy is used to elucidate the evolution of the material's response and ultrafast dynamics. Interpreting the experimental measurements using the theoretical model suggests the existence of an energy level that facilitates the formation of self-trapped exciton metastates between the conduction and valence bands (approximately 2.4–2.8 eV below the conduction band). It also predicts the electron-plasma lifetime (around 110–150 fs), the recombination time (about 34 ps), and the non-linear part of the refractive index due to the Kerr effect (with n2 values ranging from 1.1 to 1.5 × 10−13 cm2/W). Furthermore, the dominant character of multi-photon assisted ionization is emphasized while the optical breakdown threshold is also calculated and found to be equal to 2.55 × 1012 W/cm2. The results are expected to support future efforts aimed at elucidating how intense ultrashort laser pulses interact with polymeric materials, which is crucial for optimizing the manufacturing processes of these materials for potential applications.