Generation of an Array of Birefringent Nanogratings in the Bulk of Fluorite Irradiated by Ultrashort Laser Pulses with Different Durations

Nanohydrodynamic Local Compaction and Nanoplasmonic Form-Birefringence Inscription by Ultrashort Laser Pulses in Nanoporous Fused Silica

“…The measured microtracks exhibit their lengths L , increasing beyond the corresponding Rayleigh length of the focusing 0.25-NA micro-objective (1030 nm–≈12 μm, 515 nm–≈7 μm) versus the increasing pulse energy in the threshold-like manner ( Figure 3 a,b), in good agreement with the nanoplasmonic mechanism of form-birefringence inscription in the form of bulk nanogratings [ 19 , 22 ]. The corresponding thresholds are pulsewidth- and wavelength-dependent quantities ( Figure 3 a,b), while the observed length magnitudes are quite comparable for both the 1030-nm and 515-nm wavelengths.…”

“…In contrast, if the length L of the birefringent microtracks ( Figure 3 ) is explicitly taken into account, the derivation for the first time average refractive index change, Δn (633 nm) = Ret / L , indicates very different dependence on laser pulsewidth τ ( Figure 4 c,d). Here, the maximal value of Δn max ≈ (4 ± 2) × 10 −3 at 1030 nm ( Figure 4 c) and Δn max ≈ (1.5 ± 0.5) × 10 −3 at 515 nm ( Figure 4 d) appear pulsewidth-independent but strongly wavelength-dependent, as expected according to the nanoplasmonic mechanism of the form-birefringence inscription in the form of bulk nanogratings [ 19 , 22 ]. Moreover, the maximal values are achieved in the threshold-like manner versus E ( Figure 4 c,d), as expected within the latter mechanism; they will be discussed below.…”

“…To date, the main challenge in the modeling of birefringent nanograting inscription in bulk dielectrics is the tailoring planar nanoplasma configurations, which could support excitation by the linearly polarized laser pulses and the following propagation of interfacial plasmons or plasmon-polaritons [ 19 ]. In this study, our experimental findings imply that in the linear focusing regime, bulk nanostructuring in fused silica could proceed via the formation of reflective near-critical focal plasma, its significant reflection/backscattering of the incident ultrashort laser pulses, and the interferential formation of a 1D longitudinal standing electromagnetic wave in the pre-focal region [ 19 , 22 ]. According to our full-wave FDTD analysis (see Materials and Methods section), the low-loss plasma, the spherical-shape inclusion with its dielectric function ε* 515 = −2.25 + i0.3 at the 515-nm wavelength, provides pronounced radially longitudinal standing waves in the pre-focal region with a characteristic period of λ/2 n ( Figure 6 a), where n is the refractive index of the dielectric.…”

“…Our side-view optical microscopy studies of blue recombination plasma emission form the filamentary tracks [ 21 ] in the np-FS ( Figure 1 c, inset) enabled us to identify the critical powers P cr for self-focusing and filamentation in this material, corresponding to the bottom limit pulse energy (P cr τ) for the laser filamentation onset of the 1030-nm and 515-nm pulses at different pulsewidths. The entire pulse energy ranges (0.018–0.045 μJ (1030 nm) and 0.05–0.11 μJ (515 nm)) were below the corresponding critical energy P cr τ for the utilized pulsewidths of 0.3–3.8 ps ( Figure 1 c), thus representing the linear (geometrical) focusing regime with the proposed interferential mechanism of nanograting inscription [ 19 , 22 ]. This mechanism implies it is the pre-focal region of interference between the incident laser pulses and their plasma-reflected replica that dictates the nanograting array in-depth length L , rather than the corresponding geometrical Rayleigh length [ 19 , 22 ].…”

“…The entire pulse energy ranges (0.018–0.045 μJ (1030 nm) and 0.05–0.11 μJ (515 nm)) were below the corresponding critical energy P cr τ for the utilized pulsewidths of 0.3–3.8 ps ( Figure 1 c), thus representing the linear (geometrical) focusing regime with the proposed interferential mechanism of nanograting inscription [ 19 , 22 ]. This mechanism implies it is the pre-focal region of interference between the incident laser pulses and their plasma-reflected replica that dictates the nanograting array in-depth length L , rather than the corresponding geometrical Rayleigh length [ 19 , 22 ].…”