Laser-Induced Periodic Surface Structures on Layered GaSe Crystals: Structural Coloring and Infrared Antireflection.

We study structural and morphological transformations caused by multipulse femtosecond-laser exposure of Bridgman-grown ϵ-phase GaSe crystals, a van der Waals semiconductor promising for nonlinear optics and optoelectronics. We unveil, for the first time, the laser-driven self-organization regimes in GaSe allowing the formation of regular laser-induced periodic surface structures (LIPSSs) that originate from interference of the incident radiation and interface surface plasmon waves. LIPSSs formation causes transformation of the near-surface layer to amorphous Ga2Se3 at negligible oxidation levels, evidenced from comprehensive structural characterization. LIPSSs imprinted on both output crystal facets provide a 1.2-fold increase of the near-IR transmittance, while the ability to control local periodicity by processing parameters enables multilevel structural color marking of the crystal surface. Our studies highlight direct fs-laser patterning as a multipurpose application-ready technology for precise nanostructuring of promising van der Waals semiconductors, whose layered structure restricts application of common nanofabrication approaches.

Silicon microprotrusions with tailored chirality enabled by direct femtosecond laser ablation.

Here, we report on formation of nanoprotrusions on the surface of a bulk crystalline silicon wafer under femtosecond-laser ablation with a donut-shaped laser beam. By breaking circular symmetry of the irradiating donut-shaped fs-pulse beam, a switch in geometry of the formed surface nanoprotrusions from regular to chiral was demonstrated. The chirality of the obtained Si nanostructures was promoted with an asymmetry degree of the laser beam. An uneven helical flow of laser-melted Si caused by asymmetry of the initial intensity and temperature pattern on the laser-irradiated Si surface explains this phenomenon. Chirality of the formed protrusions was confirmed by visualizing cross-sectional cuts produced by focused ion beam milling as well as Raman activity of these structures probed by circularly polarized light with opposite handedness. Our results open a pathway towards easy-to-implement inexpensive fabrication of chiral all-dielectric nanostructures for advanced nanophotonic applications and sensing of chiral molecules.

Characterization of azimuthal and radial velocity fields induced by rotors in flows with a low Reynolds number.

We theoretically and experimentally investigate the flow field that emerges from a rodlike microrotor rotating about its center in a nonaxisymmetric manner. A simple theoretical model is proposed that uses a superposition of two rotlets as a fundamental solution to the Stokes equation. The predictions of this model are compared to measurements of the azimuthal and radial microfluidic velocity field components that are induced by a rotor composed of fused microscopic spheres. The rotor is driven magnetically and the fluid flow is measured with the help of a probe particle fixed by an optical tweezer. We find considerable deviations of the mere azimuthal flow pattern induced by a single rotating sphere as it has been reported by Di Leonardo et al. [Phys. Rev. Lett. 96, 134502 (2006)]. Notably, the presence of a radial velocity component that manifests itself by an oscillation of the probe particle with twice the rotor frequency is observed. These findings open up a way to discuss possible radial transport in microfluidic devices.

Dielectric barrier discharges in analytical chemistry.

The present review reflects the importance of dielectric barrier discharges in analytical chemistry. Special about this discharge is-and in contrast to usual discharges with direct current-that the plasma is separated from one or two electrodes by a dielectric barrier. This gives rise to two main features of the dielectric barrier discharges; it can serve as dissociation and excitation device and as ionization mechanism, respectively. The article portrays the various application fields for dielectric barrier discharges in analytical chemistry, for example the use for elemental detection with optical spectrometry or as ionization source for mass spectrometry. Besides the introduction of different kinds of dielectric barrier discharges used for analytical chemistry from the literature, a clear and concise classification of dielectric barrier discharges into capacitively coupled discharges is provided followed by an overview about the characteristics of a dielectric barrier discharge concerning discharge properties and the ignition mechanism.

Self-organized nanopatterns in thin layers of superheated liquid metals.

In this paper experimental observations of self-organized patterns in resolidified thin films of liquid superheated metals are reported. The superheated melt layers represent an example of a system driven far from equilibrium, which undergoes explosive boiling and solidifies afterward. The melts appear in the course of single-shot femtosecond laser heating of metal samples. Self-organized cells, solitonlike structures, periodic stripes, and transient patterns are observed. Pattern properties and mechanisms leading to the pattern formation as well as possible applications for nanotechnology are discussed.

Rotating hexagonal pattern in a dielectric barrier discharge system.

Here, we report on the experimental observation of a rotating hexagonal pattern in a continuous dissipative medium. The system under investigation is a planar dielectric barrier gas-discharge cell. The pattern consists of a set of current filaments occupying the whole discharge area and rotating as a rigid body. The symmetry of the rotating hexagons is lower than the symmetry of the stationary hexagonal pattern. We study the dynamics of the pattern, especially peculiarities of its rotational velocity. The temperature of the gas is found to be an important quantity influencing the rotating hexagons.

Role of surface charges in dc gas-discharge systems with high-ohmic electrodes.

Surface charge of the electrodes is investigated for planar dc gas-discharge systems. Both analytical estimates and experimental data show that such a charge plays an important role for the dc systems with a high-ohmic electrode. This is demonstrated by several experiments concerning discharge establishment and pattern formation phenomena. The surface charge has an inhibitory role, as it diminishes the electric field in the gas. Due to the low mobility of the surface charges, their distribution can be nonuniform giving rise to the observable filamentary structure of the discharge. It is also shown that the surface charge effect can be naturally incorporated in existing phenomenological models of the planar discharge. Thereby one can explain several observable phenomena, such as stability, multiplicity, and motion of the localized structures.

Concentric-ring patterns in a dielectric barrier discharge system.

We report on the first experimental observation of a concentric-ring pattern in a short planar dielectric barrier gas-discharge system and study its spatiotemporal behavior. While increasing the gas pressure the destabilization of the rings into a filamentary structure is observed. The charge carriers deposited on the dielectric electrodes determine the spatiotemporal behavior of the pattern.