Sub-wavelength Laser-induced Periodic Surface Structures Research Articles

Direct Fabrication of Te-Doped Black Si with an Enhanced Photoelectric Performance by Femtosecond Laser Irradiation under Water.

Tellurium (Te)-doped black silicon (Si) with enhanced absorption and photoelectric performance over a broad wavelength range of 0.2-2.5 μm was obtained using femtosecond (fs) laser irradiation in liquid water. Prior to laser irradiation, the Si sample was covered with a Te thin film (thickness 200 nm) over an adhesion layer of Cr (thickness 5 nm). Surface analyses by scanning electron microscopy and three-dimensional confocal microscopy evidence the presence of hierarchical surface structures combining quasi-periodic stripes with a spatial period of about 5 μm and subwavelength laser-induced periodic surface structures directed in directions parallel and perpendicular to the direction of the laser polarization, respectively. Moreover, the incorporation of Te generates intermediate levels within the Si bandgap. The Te-doped black Si shows a significant enhancement of the absorption, which reaches values of about 48% in the UV and visible (0.2-1.1 μm) and 70% in the near-infrared (1.1-2.5 μm) spectral ranges, respectively, due to the synergistic effects of multiscale surface structures and Te incorporation. Moreover, the surface reflectance is reduced to almost zero across the entire spectrum. The Te-doped black Si sample is used to realize a photodetector which displays an impressive photoelectric capability, being characterized by a responsivity of 328 mA/W, and an external quantum efficiency of 49.27% at a voltage bias of -10 V for 1064 nm light illumination, with rising and falling times of 55 and 67 ms, respectively. These figures remarkably outperform the response of unprocessed Si under the same experimental conditions.

Gold nanoparticles coated LIPSS on GaAs for trace detection of RDX and Tetryl

Femtosecond laser-induced periodic surface structures (LIPSS) on GaAs were accomplished using ablation in the air by varying the input pulse energy (5–100 μJ). A single laser scan line was drawn at each energy, and sub-wavelength LIPSS were observed within each line. Their morphology was thoroughly analysed, and their formation mechanisms were discussed with appropriate theoretical support (SIPE-Drude model). These GaAs-LIPSS have good quality over large ablation areas, and the periodicity (663±16 nm) was nearly the same for all energies. Further, a set of GaAs-LIPSS was produced within an area of 2 × 2 mm2 utilizing an optimized energy of 5 μJ. Photoluminescence and Raman studies were performed to understand the laser-induced damage on these structures. Subsequently, these samples were coated with a 25 nm Au layer and annealed at 400 °C in ambience. The resulting plasmonic Au nanoparticles decorated GaAs-LIPSS were used as SERS substrates for detecting dye (MB) and explosive (RDX, Tetryl) molecules. Enhancement factors of ∼105 and ∼104 (lowest detected concentrations of 10−9 M and 10−5 M) were recorded for dye and explosive molecules, respectively. We believe that these results will aid in further developing GaAs-LIPSS with smaller feature sizes for improved sensing and optoelectronic applications.

Polarization conversion using customized subwavelength laser-induced periodic surface structures on stainless steel

Stainless steel is a basic raw material used in many industries. It can be customized by generating laser-induced periodic surface structure (LIPSS) as subwavelength gratings. Here, we present the capabilities of an LIPSS on stainless steel to modify the polarization state of the reflected radiation at the IR band. These structures have been modeled using the finite element method and fabricated by femtosecond laser processing. The Stokes parameters have been obtained experimentally and a model for the shape has been used to fit the simulated Stokes values to the experimental data. The birefringence of the LIPSS is analyzed to explain how they modify the polarization state of the incoming light. We find the geometry of the subwavelength grating that makes it work as an optical retarder that transforms a linearly polarized light into a circularly polarized wave. In addition, the geometrical parameters of the LIPSS are tuned to selectively absorb one of the components of the incoming light, becoming a linear axial polarizer. Appropriately selecting the geometrical parameters and orientation of the fabricated LIPSS makes it possible to obtain an arbitrary pure polarization state when illuminated by a pure linearly polarized state oriented at an azimuth of 45°. The overall reflectance of these transformations reaches values close to 60% with respect to the incident intensity, which is the same reflectivity obtained for non-nanostructured stainless steel flat surfaces.

Deep Subwavelength Laser-Induced Periodic Surface Structures on Silicon as a Novel Multifunctional Biosensing Platform.

Strong light localization inside the nanoscale gaps provides remarkable opportunities for creation of various medical and biosensing platforms stimulating an active search for inexpensive and easily scalable fabrication at a sub-100 nm resolution. In this paper, self-organized laser-induced periodic surface structures (LIPSSs) with the shortest ever reported periodicity of 70 ± 10 nm were directly imprinted on the crystalline Si wafer upon its direct femtosecond-laser ablation in isopropanol. Appearance of such a nanoscale morphology was explained by the formation of a periodic topography on the surface of photoexcited Si driven by interference phenomena as well as subsequent down-scaling of the imprinted grating period via Rayleigh-Taylor hydrodynamic instability. The produced deep subwavelength LIPSSs demonstrate strong anisotropic anti-reflection performance, ensuring efficient delivery of the incident far-field radiation to the electromagnetic "hot spots" localized in the Si nanogaps. This allows realization of various optical biosensing platforms operating via strong interactions of quantum emitters with nanoscale light fields. The demonstrated 80-fold enhancement of spontaneous emission from the attached nanolayer of organic dye molecules and in situ optical tracing of catalytic molecular transformations substantiate bare and metal-capped deep subwavelength Si LIPSSs as a promising inexpensive multifunctional biosensing platform.

Surface Superconductivity Changes of Niobium Sheets by Femtosecond Laser-Induced Periodic Nanostructures

Irradiation with ultra-short (femtosecond) laser beams enables the generation of sub-wavelength laser-induced periodic surface structures (LIPSS) over large areas with controlled spatial periodicity, orientation, and depths affecting only a material layer on the sub-micrometer scale. This study reports on how fs-laser irradiation of commercially available Nb foil samples affects their superconducting behavior. DC magnetization and AC susceptibility measurements at cryogenic temperatures and with magnetic fields of different amplitude and orientation are thus analyzed and reported. This study pays special attention to the surface superconducting layer that persists above the upper critical magnetic field strength Hc2, and disappears at a higher nucleation field strength Hc3. Characteristic changes were distinguished between the surface properties of the laser-irradiated samples, as compared to the corresponding reference samples (non-irradiated). Clear correlations have been observed between the surface nanostructures and the nucleation field Hc3, which depends on the relative orientation of the magnetic field and the surface patterns developed by the laser irradiation.

Superwavelength, wavelength, and subwavelength laser-induced periodic surface structures on zinc and their energy-dispersive x-ray analysis.

In this work, we demonstrated simultaneous generation of three types of laser-induced periodic surface structures (LIPSSs) on a metal surface by processing Zn with a linearly polarized femtosecond laser pulse of 100 fs duration at a wavelength of 800 nm. In the center of the laser-processed region (a line), the period was found to be as high as 1470 nm (superwavelength LIPSS), whereas at the outer region, the LIPSS period was found to be as small as 200 nm (subwavelength LIPSS). In between these two regions, the LIPSSs were found to have period of 800 nm (wavelength LIPSS). In the energy-dispersive x-ray (EDX) study, the Zn:O atomic ratio was found to be 70%∶30% in the LIPSS-containing region. In contrast to this, in the region containing no LIPSS, the Zn:O atomic ratio was found to be 90%:10%. Discussion is given on the significance of the observed LIPSS, in terms of their possible growth mechanism and comparison with previously published results.