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

Abstract

The inscription regimes and formation mechanisms of form-birefringent microstructures inside nano-porous fused silica by tightly focused 1030- and 515-nm ultrashort laser pulses of variable energy levels and pulsewidths in the sub-filamentary regime were explored. Energy-dispersion X-ray micro-spectroscopy and 3D scanning confocal Raman micro-spectroscopy revealed the micro-tracks compacted by the multi-shot laser exposure with the nanopores hydrodynamically driven on a microscale to their periphery. Nearly homogeneous polarimetrically acquired subwavelength-scale form-birefringence (refractive index modulation ~10−3) was simultaneously produced as birefringent nanogratings inside the microtracks of wavelength-, energy- and pulsewidth-dependent lengths, enabling the scaling of their total retardance for perspective phase-modulation nanophotonic applications. The observed form-birefringence was related to the hierarchical multi-scale structure of the microtracks, envisioned by cross-sectional atomic-force microscopy and numerical modeling.

Hierarchical Multi-Scale Coupled Periodical Photonic and Plasmonic Nanopatterns Inscribed by Femtosecond Laser Pulses in Lithium Niobate

“…These trends are perfectly consistent with the threshold appearance of the ablative nanopatterns at the fs-laser pulse energies, exceeding the overall ablation threshold pulse energy value ≈50 nJ (fluence—1.2 J/cm 2 ). Furthermore, similar to other sub-filamentary fs-laser-inscribed birefringent nanopatterns in dielectrics [ 14 , 15 , 16 ], the nanopatterned regions in CLN exhibit high pulse-energy tunable retardance magnitudes up to λ/5, measured by a birefringence imaging system Thorlabs LCC7201B (not shown).…”

“…Recently, a new ultrashort-pulse laser inscription modality was proposed for hierarchical nanopatterning of bulk dielectrics [ 14 , 15 ], via self-organization of birefringent nanograting arrays ( Figure 1 ), utilizing the flexible combinations of laser wavelengths λ, pulsewidths τ, pulse energies E, focusing conditions and diverse dielectric materials—fluorite and fused silica [ 15 , 16 ]. Very surprisingly, such birefringent nanopatterns highly extended along the laser beam waist were observed in a linear (sub-filamentary) focusing regime [ 14 , 15 , 16 ], rather than along the extended non-linear (filamentary) focus of ultrashort laser pulses, possessing peak powers well above the critical one for Kerr self-focusing [ 7 , 17 , 18 ].…”

“…Recently, a new ultrashort-pulse laser inscription modality was proposed for hierarchical nanopatterning of bulk dielectrics [ 14 , 15 ], via self-organization of birefringent nanograting arrays ( Figure 1 ), utilizing the flexible combinations of laser wavelengths λ, pulsewidths τ, pulse energies E, focusing conditions and diverse dielectric materials—fluorite and fused silica [ 15 , 16 ]. Very surprisingly, such birefringent nanopatterns highly extended along the laser beam waist were observed in a linear (sub-filamentary) focusing regime [ 14 , 15 , 16 ], rather than along the extended non-linear (filamentary) focus of ultrashort laser pulses, possessing peak powers well above the critical one for Kerr self-focusing [ 7 , 17 , 18 ]. The self-organized hierarchical nanopatterns—periodical sub-wavelength longitudinal stacks of transverse nanogratings—were assumed to proceed via four schematic main steps: (1) formation of reflective electron-hole plasma of near-critical density in the linear focus ( Figure 1 a); (2) longitudinal interference of the reflected and incident linearly polarized pulse parts in the pre-focal region, formation of the near-plane standing electromagnetic wave and the corresponding ionization wave (plasma sheets) with the period Λ L ≈ λ/(2n) ( photonic nanostructure ) [ 14 , 15 ] ( Figure 1 a,b); (3) excitation and interference of interfacial (boundary between weakly/strongly photoexcited dielectric layers) sub-wavelength plasmons (wavelength Λ P ~ λ/n 2 ≪ λ [ 19 ]), counter-propagating along or normal to the laser polarization [ 20 ] in the pre-focal stack of the near-plane plasma sheets separated by the distance Λ L ( Figure 1 c); (4) periodical structural modification of the dielectric material and the corresponding modulation of the refractive index in the standing electromagnetic/ionization wave of the interfering plasmons (period Λ T ≈ Λ P /2, ultrafine plasmonic sub-structure of the photonic one ) [ 19 , 21 ].…”

Multi-Parametric Birefringence Control in Ultrashort-Pulse Laser-Inscribed Nanolattices in Fluorite

“…Such nanolattices are known to be the product of ultrashort-pulse laser–dielectric interactions involving near-field scattering, plasmonics and material modification [ 4 , 5 , 6 ]. The resulting one-dimensional interferential backscattered [ 4 ] or plasmonic [ 5 , 6 ] “hot spots” imprinted in the dielectric materials as nanoscale periodic structural modification stripes [ 7 , 8 , 9 , 10 ], exhibit anisotropic periodical modulation of the refractive index (form birefringence) [ 1 , 3 ]. A buried dynamic near-critical electron-hole plasma, supporting interfacial plasmon-polaritons during the bulk refractive-index difference (RID) laser inscription, could be produced either in pre-filamentation (linear/geometrical focusing) [ 11 ] or in filamentation (non-linear self-focusing) regimes [ 12 ].…”

Ferroelectric Nanodomain Engineering in Bulk Lithium Niobate Crystals in Ultrashort-Pulse Laser Nanopatterning Regime

“…During the last two decades, direct laser writing emerged as versatile tool in nano- and micro-fabrication of integrated photonic circuits, Bragg gratings, optical memory bits, microfluidic and optofluidic channels, phase and polarizing elements and devices in bulk dielectrics [ 1 , 2 , 3 , 4 , 5 ]. Inscription in a homogeneous dielectric medium requires a formation of a new high-contrast refractive-index interface, possible via densification in silica materials [ 6 , 7 ] and rarefaction in other dielectrics [ 8 ], boosting empty nanovoids [ 9 ] or drilling of hollow microchannels [ 10 ]. Meanwhile, rather recently—about one decade ago—a new fundamental high-NA (numerical aperture) laser inscription process (“bulk nanopatterning”, BN), relying on nanoplasmonic self-organization of birefringent grating arrays in bulk dielectrics as functional microbits, was matured for design and fabrication of innovative optical elements and devices.…”