Stanislav O. Gurbatov

Ion-beam assisted laser fabrication of sensing plasmonic nanostructures

Simple high-performance, two-stage hybrid technique was developed for fabrication of different plasmonic nanostructures, including nanorods, nanorings, as well as more complex structures on glass substrates. In this technique, a thin noble-metal film on a dielectric substrate is irradiated by a single tightly focused nanosecond laser pulse and then the modified region is slowly polished by an accelerated argon ion (Ar+) beam. As a result, each nanosecond laser pulse locally modifies the initial metal film through initiation of fast melting and subsequent hydrodynamic processes, while the following Ar+-ion polishing removes the rest of the film, revealing the hidden topography features and fabricating separate plasmonic structures on the glass substrate. We demonstrate that the shape and lateral size of the resulting functional plasmonic nanostructures depend on the laser pulse energy and metal film thickness, while subsequent Ar+-ion polishing enables to vary height of the resulting nanostructures. Plasmonic properties of the fabricated nanostructures were characterized by dark-field micro-spectroscopy, Raman and photoluminescence measurements performed on single nanofeatures, as well as by supporting numerical calculations of the related electromagnetic near-fields and Purcell factors. The developed simple two-stage technique represents a new step towards direct large-scale laser-induced fabrication of highly ordered arrays of complex plasmonic nanostructures.

Fabrication of porous metal nanoparticles and microbumps by means of nanosecond laser pulses focused through the fiber microaxicon

We present a novel optical element - fiber microaxicon (FMA) for laser radiation focusing into a diffraction-limited spot with Bessel-like profile as well as for precision laser nanostructuring of metal film surfaces. Using the developed FMA for single-pulse irradiation of Au/Pd metal films on quartz substrate we have demonstrated the formation of submicron hollow microbumps with a small spike atop as well as hollow spherical nanoparticles. Experimental conditions for controllable and reproducible formation of ordered arrays of such microstructures were defined. The internal structure of the fabricated nanoparticles and nanobumps was experimentally studied using both argon ions polishing and scanning electron microscopy. These methods reveal a porous inner structure of laser-induced nanoparticles and nanobumps, which presumably indicates that a subsurface boiling of the molten metal film is a key mechanism determining the formation process of such structures.

Mapping the refractive index with single plasmonic nanoantenna

As the size of the state-of-the-art optical devices shrinks to nanoscale, the need for tools allowing mapping the local optical properties at deep sub-diffraction resolution increases. Here we demonstrate successful mapping the variations of the refractive index of a smooth dielectric surface by detecting spectral response of a single spherical-shape Ag nanoparticle optically aligned with a supporting optical fiber axicon microlens. We propose and examine various excitation schemes of the plasmonic nanoantenna to provide efficient interaction of its dipolar and quadrupolar modes with the underlying sample surface and to optimize the mapping resolution and sensitivity. Moreover, we demonstrate an lithography-free approach for fabrication of the scanning probe combining the high-quality fiber microaxicon with the Ag spherical nanoparticle atop. Supporting finite-difference time-domain calculations are undertaken to tailor the interaction of the plasmonic nanoantenna and the underlying dielectric substrate upon various excitation conditions demonstrating good agreement with our experimental findings and explaining the obtained results.

Surface and subsurface refractive index mapping by spherical optical antenna

Finite-difference time-domain (FDTD) calculations of the scattering spectra of a 100-nm diameter Ag spherical nanoparticle lying on a transparent substrate were performed under various excitation regimes to estimate the spectral shift of the dipole and quadrupole localized surface plasmon resonances (LSPRs) as a function of the substrate refractive index (RI). The LSPR spectral shift was shown to be defined by the characteristic near-field energy distribution assigned by the polarization direction of the radiation exciting nanoantenna. For the optimized excitation conditions, spectral registration of the dipole shift was shown to provide the lateral resolution of 76 nm and threshold sensitivity to variation of the substrate RI of 2.2·10−4 RIU, while similar values for quadrupole LSPR reach 25 nm and 5·10−4 RIU, respectively. The possibility of subsurface RI mapping at a depth up to 50 nm is demonstrated via registration of the dipole spectral shift of the spherical nanoparticle.

Nanostructured Metal Substrates Fabrication for Surface Enhanced Fluorescence Using the Nanosecond Speckle-Modulated Laser Pulses

Nanosecond speckle-modulated laser pulses were applied to fabricate nanotextured metal substrates for surface enhanced fluorescence (SEF) applications. The speckle-modulated patterns obtained by passing the laser nanosecond pulse through the simplest diffusive object were found suitable for fast large-scale texturing of metal films with micron-sized structures - nanojets, nanobumps and submicron through holes, previous reported only for single-shot nanoablation with tightly focused beams. The optimal regime for pattering the mm-scale areas of Au films with nanojets with an averaged recording density of 105 nanojets per square millimeter were determined and realized. The fabricated nanotextured Au substrates demonstrate spatially uniform SEF signal from the Rhodamine 6G organic dye with averaged 5-fold enhancement factor being compared with unmodified Au surface.

The Spectrum of a Bent Fiber Fabry-Perot Interferometer under Small Variations of the Refractive Index of the Environment

The phase of light propagating through a bent optical fibre is shown to depend on the refractive index of the medium surrounding the fibre cladding when there is resonance coupling between the guided core mode and cladding modes. This shifts the spectral maxima in the bent fibre-optic Fabry–Perot interferometer. The highest phase and spectral sensitivities achieved with this interferometer configuration are 0,71 and 0,077, respectively, and enable changes in the refractive index of the ambient medium down to 5∙10–6 to be detected. This makes the proposed approach potentially attractive for producing highly stable, precision refractive index sensors capable of solving a wide range of liquid refractometry problems.

The spectrum of a bent fiber Fabry-Perot interferometer under small variations of the refractive index of the environment

The phase of light propagating through a bent optical fibre is shown to depend on the refractive index of the medium surrounding the fibre cladding when there is resonance coupling between the guided core mode and cladding modes. This shifts the spectral maxima in the bent fibre-optic Fabry-Perot interferometer. The highest phase and spectral sensitivities achieved with this interferometer configuration are 0,71 and 0,077, respectively, and enable changes in the refractive index of the ambient medium down to 5·10-6 to be detected. This makes the proposed approach potentially attractive for producing highly stable, precision refractive index sensors capable of solving a wide range of liquid refractometry problems.