Laser irradiation of various materials including metals, polymers, and semiconductors with vortex beams was previously shown to “twist” transiently molten matter providing the direct easy-to-implement way to obtain chiral surface relief. Specifically for metals, this effect was attributed to transfer of an orbital angular momentum (OAM) carried by a vortex beam. In this Letter, we report the formation of twisted metallic nanoneedles on surfaces of silver and gold films under their irradiation by a zero-OAM laser beam with a spiral-shaped intensity distribution. Our comparative experiments clearly demonstrate, for the first time to the best of our knowledge, that the formation of the chiral nanoneedles on the noble-metal films is mainly governed by the temperature-gradient-induced chiral thermocapillary mass transfer, rather than by OAM-driven rotation of the molten matter.

Ultrashort laser pulses deliver electromagnetic energy to matter causing its localized heating that can be used for both material removal via ablation/evaporation and driving interface chemical reactions. Here, it is shown that both mentioned processes can be simultaneously combined within straightforward laser nanotexturing of Si wafer in a functionalizing solution to produce a practically relevant metal–semiconductor surface nano‐morphology. Such unique hybrid morphology represents deep‐subwavelength Si laser‐induced periodic surface structures (LIPSSs) with an extremely short period down to 70 nm and high‐aspect‐ratio nano‐trenches loaded with controllable amount of plasmonic nanoparticles formed via laser‐induced decomposition of the precursor noble‐metal salts. Moreover, heat localization driving reduction process is utilized to produce surface morphology locally decorated with dissimilar plasmon‐active nanoparticles. Light‐absorbing deep‐subwavelength Si LIPSSs loaded with controllable amount of noble‐metal nanoparticles represent an attractive architecture for plasmon‐related applications such as optical nanosensing where efficient coupling of the propagating optical waves to highly localized electromagnetic “hot spots” is a mandatory requirement. To support this statement, applicability of such hybrid morphology for fluorescence‐based detection of nanomolar concentrations of mercury cations in solution is demonstrated.

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.

Nonlinear infrared (IR) crystals for radiation conversion are of paramount importance for realization of advanced laser spectrometers for medical diagnostics, environmental monitoring, and advanced sensing. However, performance of such crystals suffers from substantial surface reflectivity coming from rather high (over 2.5) refractive index of the key nonlinear materials used. Here, based on the example of promising BaGa4Se7 nonlinear crystal, we attested direct surface patterning with ultraviolet (257 nm) femtosecond laser pulses used to engrave anti-reflective microstructures (ARMs) directly on both output sides of the crystal. Imprinted surface nanotrenches arranged into a fish-net morphology with a periodicity down to 500 nm was found to increase transmittance of the crystals from 65% to 84% within a practically relevant shortwave IR spectral range. Formation of the ARMs with an optimized geometry is expected to weakly reduce the laser damage threshold of a pristine crystal material as it was also evidenced from supporting full-wave simulations and tests.