Dynamic polarization flip in nanoripples on photoexcited Ti surface near its surface plasmon resonance

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

The ultrafast interaction of tightly focused femtosecond laser pulses with bulk dielectric media in direct laser writing (inscription) regimes is known to proceed via complex multi-scale light, plasma and material modification nanopatterns, which are challenging for exploration owing to their mesoscopic, transient and buried character. In this study, we report on the first experimental demonstration, analysis and modeling of hierarchical multi-period coupled longitudinal and transverse nanogratings in bulk lithium niobate inscribed in the focal region by 1030 nm, 300 fs laser pulses in the recently proposed sub-filamentary laser inscription regime. The longitudinal Bragg-like topography nanogratings, possessing the laser-intensity-dependent periods ≈ 400 nm, consist of transverse birefringent nanogratings, which are perpendicular to the laser polarization and exhibit much smaller periods ≈ 160 nm. Our analysis and modeling support the photonic origin of the longitudinal nanogratings, appearing as prompt electromagnetic and corresponding ionization standing waves in the pre-focal region due to interference of the incident and plasma-reflected laser pulse parts. The transverse nanogratings could be assigned to the nanoscale material modification by interfacial plasmons, excited and interfered in the resulting longitudinal array of the plasma sheets in the bulk dielectric material. Our experimental findings provide strong support for our previously proposed mechanism of such hierarchical laser nanopatterning in bulk dielectrics, giving important insights into its crucial parameters and opening the way for directional harnessing of this technology.

Controllable ablative machining of Al/Ti and Ti/Al nano-layers on a Si substrate by single-pulse femtosecond laser irradiation

Results concerning the controllable ablation of nano-layered thin films (NLTF) by femtosecond laser pulses are presented. Investigated samples were titanium-aluminum bilayers, deposited on a silicon substrate, with the top titanium or aluminum layer of variable thickness on the surface. Irradiation was done in ambient air with single femtosecond laser pulses under standard laboratory conditions. The samples were analyzed by complementary methods of optical and scanning electron microscopy and optical profilometry, exhibiting laser-fluence-dependent ablative removal either of the top layer or the entire bilayer or even partial ablation of the underlying silicon substrate. The removal (spallation) threshold fluences for the topmost layer are scalable versus its thickness almost irrespectively of its material, being rather selective for the Ti-coated samples and much less selective for the Al-coated samples. The removal of the entire bilayers was found to be strongly influenced by electronic properties of the underlying metallic layer, dictating the NLTF-Si adhesion, heat conduction, and capacity in the NLTFs toward the NLTF-Si interface and beyond, as well as by their thermophysical characteristics, e.g., almost twice higher melting temperature and enthalpy for Ti. As a result, precise fs-laser machining of the entire NLTFs is pronounced and selective for the samples with the fusible Al at the low-adhesion Al-Si interfaces, compared with the incomplete NLTF removal from the high-adhesion and refractory Ti-Si interfaces.

Oxidation ruled transition from normal to anomalous periodic structures with femtosecond laser irradiation on Cr/Si films

Elucidation of the underlying physics for laser-induced periodic surface structures (LIPSSs) is of great importance for their controllable fabrication. We here demonstrate a periodic structure transition from normal to anomalous morphology, upon femtosecond laser irradiation on 50-nm thick Cr/Si films in an air pressure-tunable chamber. As the air pressure gradually decreases, the amount of surface oxide induced by preceding laser pulses is found to reduce, and eventually triggering the structure evolution from the anomalously oriented subwavelength to normally oriented deep-subwavelength LIPSSs. The intriguing structure transition is explained in terms of the competitive excitation between the transverse-electric scattered surface wave and transverse-magnetic hybrid plasmon wave, which is ruled by the thickness of the preformed oxide layer indeed.

Multiscale Hierarchical Micro/Nanostructures Created by Femtosecond Laser Ablation in Liquids for Polarization-Dependent Broadband Antireflection

In this work, we present the possibility of producing multiscale hierarchical micro/nanostructures by the femtosecond laser ablation of transition metals (i.e., Ta and W) in water and investigate their polarization-dependent reflectance. The hierarchical micro/nanostructures are composed of microscale-grooved, mountain-like and pit-rich structures decorated with hybrid laser-induced periodic surface structures (LIPSSs). The hybrid LIPSSs consist of low/high and ultrahigh spatial frequency LIPSSs (LSFLs/HSFLs and UHSFLs). LSFLs/HSFLs of 400–600 nm in a period are typically oriented perpendicular to the direction of the laser polarization, while UHSFLs (widths: 10–20 nm and periods: 30–50 nm) are oriented perpendicular to the curvatures of LSFLs/HSFLs. On the microstructures with height gradients, the orientations of LSFLs/HSFLs are misaligned by 18°. On the ablated W metasurface, two kinds of UHSFLs are observed. UHSFLs become parallel nanowires in the deep troughs of LSFLs/HSFLs but result in being very chaotic in shallow LSFLs, turning into polygonal nanonetworks. In contrast, chaotic USFLs are not found on the ablated Ta metasurfaces. With the help of Fourier transform infrared spectroscopy, it is found that microgrooves show an obvious polarization-dependent reflectance at wavelengths of 15 and 17.5 μm associated with the direction of the groove, and the integration of microstructures with LSFs/HSFLs/UHSFLs is thus beneficial for enhancing the light absorbance and light trapping in the near-to-mid-infrared (NIR-MIR) range.

Feedback-Driven Plasmonic-Thermal Route to Femtosecond-Laser-Induced Periodic Surface Structures in Silicon Indicated by Pump-Probe Scattering and Diffraction

The self-organized formation of nanoscale laser-induced periodic surface structures (LIPSS) is still not fully understood with respect to the dynamics and interplay of contributing complex mechanisms. The transition from randomness to order and the specific role of nano-feedback are of fundamental interest because of their general aspects. In our study, the very first steps of the surface reconfiguration are demonstrated by analyzing the topology of evolving nano-crater maps. The evolution of spatial frequencies and directional arrangement indicate a feedback-driven adaptation of k-vectors to the required excitation conditions of elementary dipoles in the linearly polarized laser field. The time-dependent structure formation was studied by pump-probe diffraction and scattering experiments. The ratio of the contributions of characteristic light patterns enables plasmonic and non-plasmonic mechanisms to be distinguished, which subsequently act at distinctly different time scales. Recently developed multistage models for the dynamics of material modification are confirmed. The influence of accumulation effects is clearly demonstrated by characteristic changes in scattering and diffraction with an increasing number of preceding pulses. It is assumed that the thermal and plasmonic contributions to accumulation are coupled and thus generate spatially variable modifications.