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  = 0.4 ± 0.1 μJ (peak laser intensity  ≈ 8 TW/cm), visible bright micro-voxels emerged inside PMMA at the 100 ÷ 300-μm 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 Δ = +(4 ÷ 10) × 10, and a new 1640-cm peak of C=C vibrations emerged in the Raman spectra. Pump self-transmittance measurements demonstrated the predominating eight-photon absorption (excited energy level ≈ 9.7 eV, coefficient  ≈ 3 × 10 cm/TW) at the sub-threshold  <  , implying photoionization of the PMMA chains (the ionization potential of MMA molecule ≈ 9.7 eV). At higher peak intensities  >  , inverse brems-strahlung absorption (coefficient ∼10cm) of near-critical micro-plasma (density >5 × 10 cm) predominates over the multi-photon PMMA absorption, providing the bulk energy density >6 × 10 J/cm and the temperature rise Δ > 2.2 × 10 K, which are sufficient for PMMA (de)polymerization near the equilibrium bulk temperature  ≈ 220°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.