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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.

Upon systematic studies of femtosecond‐laser processing of monocrystalline Si in oxidation‐preventing methanol, it is shown that the electromagnetic processes dominating at initial steps of the progressive morphology evolution define the onset of the hydrodynamic processes and morphology upon subsequent multi‐pulse exposure. Under promoted exposure quasi‐regular subwavelength laser‐induced periodic surface structures (LIPSSs) are justified to evolve through the template‐assisted development of the Rayleigh‐Plateau hydrodynamic instability in the molten ridges forming quasi‐regular patterns with a supra‐wavelength periodicity and preferential alignment along polarization direction of the incident light. Subsequent exposure promotes fusion‐assisted morphology rearrangement resulting in a spiky surface with random orientation, yet constant inter‐structure distance correlated with initial LIPSS periodicity. Along with the insight onto the physical picture driving the morphology evolution and supra‐wavelength nanostructure formation, this experiments also demonstrated that resulting quasi‐regular and random spiky morphology can be tailored by the intensity/polarization distribution of incident laser beam allowing on‐demand surface nanotexturing with diverse hierarchical surface morphologies exhibiting reduced reflectivity at visible and shortwave‐IR wavelengths. Finally, the practical attractiveness of the suggested approach for improving near‐IR photoresponse and expanding operation spectral range of vertical p‐n junction Si photodetector operating under room temperature and zero‐bias conditions via single‐step annealing‐free nanopatterning is highlighted. [ABSTRACT FROM AUTHOR]

We report an easy-to-implement device for surface-enhanced Raman scattering (SERS)-based detection of various analytes dissolved in water droplets at trace concentrations. The device combines an analyte-enrichment system and SERS-active sensor site, both produced via inexpensive and high-performance direct femtosecond (fs)-laser printing. Fabricated on a surface of water-repellent polytetrafluoroethylene substrate as an arrangement of micropillars, the analyte-enrichment system supports evaporating water droplet in the Cassie–Baxter superhydrophobic state, thus ensuring delivery of the dissolved analyte molecules towards the hydrophilic SERS-active site. The efficient pre-concentration of the analyte onto the sensor site based on densely arranged spiky plasmonic nanotextures results in its subsequent label-free identification by means of SERS spectroscopy. Using the proposed device, we demonstrate reliable SERS-based fingerprinting of various analytes, including common organic dyes and medical drugs at ppb concentrations. The proposed device is believed to find applications in various areas, including label-free environmental monitoring, medical diagnostics, and forensics. [ABSTRACT FROM AUTHOR]

• Laser irradiation in liquid allows fabrication of hybrid Au@Si Microspheres (MS). • Au@Si MS properties revealed by SEM, XRD, optical/Raman spectroscopy and full-wave simulations. • Nonlinear hot-carrier-induced photoluminescence of Au@Si MS upon fs-laser sub-nJ pump. Hybrid nanomaterials with chemical composition integrating light-emitting low-loss semiconductors with plasmon-active metals are highly demanded for optoelectronics, nanophotonics and sensors. However, there is a still lack for high-performing and inexpensive methods allowing facile integration of plasmonic and all-dielectric concepts within unified practically relevant nanostructures and ensuring their production at gram-per-hour yield. Here, we report one-step synthesis of hybrid Au@Si nanomaterial with unique composition and morphology via scalable and high-performing pulsed laser irradiation of isopropanol solution containing commercial Si micro-powder and AuCl 4- ions. The resulting hybrid nanomaterial represents sub-micron nanocrystalline Si grains embedded into Au surrounding forming micro-spheres (MSs) additionally decorated with Au nanoparticles. Such unique structure and chemical composition of the Au@Si MSs permits to efficiently absorb and enhance incident radiation within rather broad spectral range spanning from visible to near-IR making the nanomaterial promising for plasmon-mediated amplification of linear and nonlinear optical effects. Efficient generation of broadband hot-carrier-induced photoluminescence of nanocrystalline Si grains upon IR femtosecond-laser sub-nJ pump proves the produced nanomaterial as advanced nanophotonic platform for nano-spectroscopy and sensing. [ABSTRACT FROM AUTHOR]