Here, we applied direct laser-induced periodic surface structuring to drive the phase transition of amorphous silicon (a-Si) into nanocrystalline (nc) Si imprinted as regular arrangement of Si nanopillars passivated with a SiO₂ layer. By varying the laser beam scanning speed at a fixed pulse energy, we successfully tailored the resulting unique surface morphology of the formed LIPSSs that change from ordered arrangement of conical protrusions to highly uniform surface gratings, where sub-wavelength scale ripples decorate the valleys between near-wavelength scale ridges. Along with the surface morphology, the nc-Si/SiO₂ volume ratio can also be controlled via laser processing parameters allowing the tailoring of the optical properties of the produced textured surfaces to achieve anti-reflection performance or partial transmission in the visible spectral range. Diverse hierarchical LIPSSs can be fabricated and replicated over large-scale areas opening a pathway for various applications including optical sensors, nanoscale temperature management, and solar light harvesting. By taking advantage of good wettability, enlarged surface area and remarkable light-trapping characteristics of the produced hierarchical morphologies, we demonstrated the first LIPSS-based surface enhanced fluorescent sensor that allowed the identification of metal cations providing a sub-nM detection limit unachievable by conventional fluorescence measurements in solutions.

The paper presents a novel three-dimensional quasi-continuous shape sensor based on an FBG array inscribed by femtosecond laser pulses into a 7-core optical fiber with a polyimide protective coating. The measured bending sensitivity of individual FBGs ranges from 0.046 nm/m^-1 to 0.049 nm/m^-1. It is shown that the sensor allows for reconstructing 2- and 3-dimensional shapes with high accuracy. Due to the high value of the core aperture and individual calibration of each FBG we were able to measure the smallest reported bending radii down to 2.6 mm with a record accuracy of ~1%. The temperature resistance of both the inscribed FBG structures and of the protective coating, along with the high mechanical strength of the polyimide, makes it possible to use the sensor in harsh environments or in medical and composite material applications.

This paper presents the characterization of an algorithm aimed at performing accurate fiber optic-based shape sensing. The measurement of the shape relies on the evaluation of the strains applied to an optic fiber in order to identify relevant spatial parameters, such as the curvature radii and bending direction, which define its shape. The measurement system is based on a 7-core multicore fiber, containing up to 9 triplets of fiber Bragg grating sensors (FBGs) organized around a central core used as reference. The proposed study aims at comparing the widely used Frenet-Serret equations with an algorithm based on the homogeneous transformation matrices that are normally used in robotics to express the position of a point in different frames, i.e. from local to global coordinates. The numerical results of the performed experiments (with different multicore fibers and setups) extensively prove the superiority of the alternative method over the Frenet-Serret equations in terms of finding a trade-off between accuracy and execution time.

In this paper, we present a new method of point-by-point femtosecond inscription of fiber Bragg gratings (FBG) arrays of different configurations in a 7-core spun optical fiber. The possibility of FBGs inscription with predefined periods in individual fiber cores allowed us to realize: 1) longitudinal FBG arrays with identical or variable resonant wavelengths in all side cores, 2) longitudinal FBG arrays inscribed only in the central or in the selected side core, and 3) an FBG array in a transverse cross section of a fiber consisting of an FBG inscribed in the central and three side cores. Based on the proposed method, by enabling the inscription through the acrylate protective coating of the fiber, a vector bend sensor has been created. Implementation of this sensor has shown that bending radii less than 4 mm can be measured with a high precision using a single-channel interrogation scheme.

In this paper, we present the results of thermochemical LIPSS formation on a chromium film with a thickness in the range of 28-350 nm induced by femtosecond laser radiation (λ = 1026 nm, ν = 200 kHz, τ = 232 fs). The period, height, morphology and chemical composition of TLIPSS as a function of the metal film thickness and focusing configuration are investigated. The growth of TLIPSS period from 678 nm to 950 nm with increasing thickness of the film has been explained by a formation of oxides with different stoichiometry composition. So, the CrO₂ oxide prevails in the composition for the case of TLIPSS formed on thin films which have the minimal period, whereas Cr₂O₃ oxide is dominant in the case of TLIPSS formed on thick chromium films which have the maximal period value. The results obtained are in agreement with numerical modeling of a period defined by the interference between an incident radiation and a scattered one from a single oxide ridge with a different chemical composition.

In this paper, we present the study of the thermochemical laser-induced periodic surface structures (TLIPSS) formation on a thin chromium film under femtosecond laser ( λ = 1026 nm, τ = 232 fs) irradiation focused on the astigmatic spot with an axis ratio of ≈1/10 and a width along the long side of 155 µ m. The effect of the number of laser pulses and the repetition rate (2–200 kHz) on the formation of the structures is investigated. Variation of the number of pulses allowed us to observe the dynamics of the formation of structures. It has been found that the ordering of TLIPSS structures increases with increasing repetition rate of laser pulses, which is explained by the heat accumulation effect on the oxidation process and, consequently, on the formation of TLIPSS structures in the case of a high repetition rate of pulses, which is confirmed by the results of numerical modelling. This effect leads to a different oxidation dynamics and the formation of various types of oxides: predominance of CrO ₂ in the case of a high-rate impact and an increase in the contribution of Cr ₂ O ₃ in the case of a low repetition rate of pulses.