Photo-physical mechanism of near-IR femtosecond laser-induced refractive-index change in PMMA

Micromodification in bulk undoped polymethylmethacrylate (PMMA) by single focused (numerical aperture (NA) = 0.25), 1030-nm 250-fs laser pump pulses was explored by pump self-transmittance; optical, 3D-scanning confocal photoluminescence (PL); Raman micro-spectroscopy; and optical polarimetric and interferometric microscopy. Starting from the threshold pulse energy E-th = 0.4 +/- 0.1 mu J (peak laser intensity I-th approximate to 8 TW/cm(2)), visible bright micro-voxels emerged inside PMMA at the 100 divided by 300-mu m depth, with their PL-acquired dimensions increasing versus pulse energy. Optical phase change was interferometrically measured in the voxels at the 532-nm wavelength, exhibiting versus the pulse energy the isotropic refractive index increase Delta n = +(4 divided by 10) x 10(-4), and a new 1640-cm(-1) peak of C=C vibrations emerged in the Raman spectra. Pump self-transmittance measurements demonstrated the predominating eight-photon absorption (excited energy level approximate to 9.7 eV, coefficient beta(8) approximate to 3 x 10(-5) cm(13)/TW7) at the sub-threshold I < I-th, implying photoionization of the PMMA chains (the ionization potential of MMA molecule approximate to 9.7 eV). At higher peak intensities I > I-th, inverse brems-strahlung absorption (coefficient similar to 10(3)cm(-1)) of near-critical micro-plasma (density >5 x 10(20) cm(-3)) predominates over the multi-photon PMMA absorption, providing the bulk energy density >6 x 10(2) J/cm(3) and the temperature rise Delta T > 2.2 x 10(2) K, which are sufficient for PMMA (de)polymerization near the equilibrium bulk temperature T-P approximate to 220 degrees C. These results uncover the quantitative mechanism of fs-laser modification of PMMA, justifying the previous qualitative findings and enabling controllable energy deposition during fs-laser PMMA micromachining of diverse functional applications. (c) 2024 Optica Publishing Group. All rights, including for text and data mining (TDM), Artificial Intelligence (AI) training, and similar technologies, are reserved.