Femtosecond self-reconfiguration of laser-induced plasma patterns in dielectrics

Abstract

Laser-induced modification of transparent solids by intense femtosecond laser pulses allows fast integration of nanophotonic and nanofluidic devices with controlled optical properties. Experimental observations suggest that the local and dynamic nature of the interactions between light and the transient plasma plays an important role during fabrication. Current analytical models neglect these aspects and offer limited coverage of nanograting formation on dielectric surfaces. In this paper, we present a self-consistent dynamic treatment of the plasma build-up and its interaction with light within a three-dimensional electromagnetic framework. The main finding of this work is that local light-plasma interactions are responsible for the reorientation of laser-induced periodic plasma patterns with respect to the incident light polarization, when a certain energy density threshold is reached. Plasma reconfiguration occurs within a single laser pulse, on a femtosecond time scale. Moreover, we show that the reconfigured sub-wavelength plasma structures actually grow into the bulk of the sample, which agrees with the experimental observations of self-organized volume nanogratings. We find that mode coupling of the incident and transversely scattered light with the periodic plasma structures is sufficient to initiate the growth and self-organization of the pattern inside the medium with a characteristic half-wavelength periodicity.

The Role of the Laser-Induced Oxide Layer in the Formation of Laser-Induced Periodic Surface Structures

“…Note that the type-m ("mixed") features observed in FDTD (associated with LSFL parallel to the polarization and periods close to the wavelength λ) can be ruled out here as origin of the LSFL since the necessary condition of n = Re( n) ≈ Im( n) = k [20] on the optical properties of the oxide material is not fulfilled in our case here (n = 2, k = 0 [37]). Interestingly, the spatial periods of the sub-surface LSFL patterns simulated by FDTD rather lie between λ and λ/n as it was previously reported in [21]. The deviation between the experimentally observed and numerically simulated periods Λ LSFL may arise from the fact that in the experiments the laser-generated oxide layer may not a have a sharp interface to the bulk but may exhibit a graded composition [26] between the sample surface and the CrN bulk material.…”

“…In the latter work, the formation mechanisms of different types of LIPSS including LSFL parallel and HSFL perpendicular to the polarization were presented consistently with experimental demonstrations and for the LSFL in line with an analytical theory [19]. Based on the intrinsic optical properties of the irradiated materials, FDTD simulations allow to classify the periodic structures into specific categories [16,20,21], including type-d ("dissident") structures corresponding to LSFL being parallel to the polarization with periodicities Λ LSFL~λ /n (with n being the refractive index of material), type-s ("scattering") for LSFL perpendicular to the polarization with Λ LSFL~λ , type-m ("mixed") for a special kind of LSFL parallel to the polarization with periodicities Λ LSFL~λ , and type-r ("roughness") for HSFL structures that typically are perpendicular to the laser polarization and Λ HSFL << λ. Fuentes-Edfuf et al [8] recently published a study on the formation of LIPSS in metals where the key parameters analyzed were the beam incident angle and the role of surface roughness for the generation of surface plasmons that are ultimately responsible for the formation of LIPSS. The experiments were done in air atmosphere at room temperature using fluences above the ablation threshold.…”

Laser-induced periodic surface structures: Arbitrary angles of incidence and polarization states

“…Despite these successful applications, a complete understanding of their physical origin under various material properties and irradiation conditions is still unestablished and remains to be a subject of intense investigations and debates. Theoretical framework in the electromagnetic theory [7][8][9][10][11][12][13] and mechanisms based on self-organization and surface instability [14][15][16][17] were equally developed to explain the origin of LIPSSs. There is plenty of experimental evidence that the period and orientation of LIPSSs show a clear correlation to laser irradiation parameters such as polarization, angle of incidence, and fluence [8,[18][19][20][21][22], favoring the electromagnetic nature of LIPSSs, at least in their earliest formation stage.…”

“…To overcome theses limitations, a numerical approach [10][11][12][13][33][34][35][36][37][38] adopting the finite-difference time-domain (FDTD) method was developed within the last decade to simulate the formation of LIPSSs. The main advantages of the FDTD approach over the pioneering efficacy theory are the flexibility to model light-matter interactions for any geometries directly from Maxwell's equations, a natural and straightforward incorporation of both topography-driven interpulse feedback effect [11,12,35], and intra-pulse feedback of permittivity dynamics [13,33,34]. The FDTD approach has shown great potential and success in unravelling the origin of many types of LIPSSs including HSFLs.…”

“…Rudenko et al elucidated the respective role of quasicylindrical and surface plasmon waves in the formation of LSFLs by electromagnetic calculations adopting the FDTD method [35]. FDTD simulations with the incorporation of permittivity dynamics [13,33] were also used to understand the formation of volumetric nanograting in glass. It has been shown that the complexity of LIPSSs is intrinsically initiated by the coherence of the laser field via superposition between topography-driven scattered field and the incident field [12].…”