Microscale filamentation of 0.25 NA-focused, linearly and circularly polarized 1030 nm and 515 nm ultrashort laser pulses of variable pulse widths in fused silica, fluorite, and natural and synthetic diamonds demonstrates the Raman–Kerr effect in the form of critical pulse power magnitudes, proportional to squared wavelength and inversely proportional to laser pulse width of 0.3–10 ps. The first trend represents the common spectral relationship between the quantities, while the second indicates its time-integrated inertial contribution of Raman-active lattice polarization, appearing in transmission spectra via ultrafast optical-phonon Raman scattering. The optical-phonon contribution to the nonlinear polarization could come from laser field-induced spontaneous/stimulated Raman scattering and coherent optical phonons generated by electron–hole plasma with its clamped density in the nonlinear focus. Almost constant product value of the (sub)picosecond laser pulse widths and corresponding critical pulse powers for self-focusing and filamentation in the dielectrics (“critical pulse energy”) apparently implies constant magnitude of the nonlinear polarization and other “clamped” filamentation parameters at the given wavelength.

Ultrafast visible-range excitation in natural diamond produces UV-VIS A-band photoluminescence with marginal zero-phonon line and intense regular multi-peak optical-phonon progression. The A-band photoexcitation can occur via band-center, center-center and band-band transitions, being related to two-photon (center-terminated), or interband three-photon and impact-ionization processes. Minor thermal pre-heating of the diamond (< 55 °C) demonstrates strong damping of A-band progression intensities with two different activation energies.

The ultrashort-laser photoexcitation and structural modification of buried atomistic optical impurity centers in crystalline diamonds are the key enabling processes in the fabrication of ultrasensitive robust spectroscopic probes of electrical, magnetic, stress, temperature fields, and single-photon nanophotonic devices, as well as in “stealth” luminescent nano/microscale encoding in natural diamonds for their commercial tracing. Despite recent remarkable advances in ultrashort-laser predetermined generation of primitive optical centers in diamonds even on the single-center level, the underlying multi-scale basic processes, rather similar to other semiconductors and dielectrics, are almost uncovered due to the multitude of the involved multi-scale ultrafast and spatially inhomogeneous optical, electronic, thermal, and structural elementary events. We enlighten non-linear wavelength-, polarization-, intensity-, pulsewidth-, and focusing-dependent photoexcitation and energy deposition mechanisms in diamonds, coupled to the propagation of ultrashort laser pulses and ultrafast off-focus energy transport by electron–hole plasma, transient plasma- and hot-phonon-induced stress generation and the resulting variety of diverse structural atomistic modifications in the diamond lattice. Our findings pave the way for new forthcoming groundbreaking experiments and comprehensive enlightening two-temperature and/or atomistic modeling both in diamonds and other semiconductor/dielectric materials, as well as innovative technological breakthroughs in the field of single-photon source fabrication and “stealth” luminescent nano/microencoding in bulk diamonds for their commercial tracing.

We present the results of an experimental study of multiphoton ionization in fused silica, using a linearly polarized femtosecond Satsuma fiber laser with an active medium based on Yb⁺³ ions, at 515 -nm and 1030 -nm wavelengths. The radiation transmission in the fused silica was measured as a function of the laser intensity and wavelength and the data were analyzed using a theoretical model based on the Keldysh theory. We determined the multiphoton absorption cross-sections in the fused silica in the case of four- and eight-photon ionization and analyzed the contribution of avalanche ionization. The obtained results provide insight into the fundamental processes involved in multiphoton ionization and have implications for its applications, such as laser micromachining and material processing.

The filamentation of focused 300 fs laser pulses with variable polarization azimuth in bulk of synthetic HPHT diamond demonstrates the possibility of polarization-dependent bandgap control for crystal dielectric photoexcitation. This directly affects the value of the filamentation threshold power, which exhibits the distinct dependence on the polarization azimuth angle. The nonlinear photoluminescence yield, when focusing ultrashort laser pulses with variable polarization in bulk of the synthetic diamond, indicates different polarization-dependent regimes in the dynamics of electron-hole plasma formation, arising due to different processes of photoexcitation and recombination of free carriers during the filamentation process. Thus, at the onset of the filamentation process, at relatively low intensities, the photoluminescence yield rate depends on polarization azimuth controlling bandgap, while at high intensities the resulting dense absorbing plasma exhibits isotropy with respect to laser radiation polarization, and photoluminescence yield weakly depends on polarization azimuth.

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 μJ (peak laser intensity I_th ≈ 8 TW/cm²), 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 Δn = +(4 ÷ 10) × 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 ≈ 9.7 eV, coefficient β₈ ≈ 3 × 10⁻⁵ cm¹³/TW⁷) at the sub-threshold I < I_th, implying photoionization of the PMMA chains (the ionization potential of MMA molecule ≈ 9.7 eV). At higher peak intensities I > I_th, inverse brems-strahlung absorption (coefficient ∼10³cm⁻¹) of near-critical micro-plasma (density >5 × 10²⁰ cm⁻³) predominates over the multi-photon PMMA absorption, providing the bulk energy density >6 × 10² J/cm³ and the temperature rise ΔT > 2.2 × 10² K, which are sufficient for PMMA (de)polymerization near the equilibrium bulk temperature T_P ≈ 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.