Non-steady-state photo-EMF interferometer for detection of mechanical oscillations in transparent scattering objects.

The operation of an adaptive non-steady-state photo-electromagnetic field (EMF) sensor is studied in an interferometric arrangement including a diffuse scattering object-fiber optic plate. The mechanical oscillations of this plate induce the strains and stresses of the medium, which modulates the phase of the propagating light wave across the plate. The resonant frequencies of the mechanical system and the distribution of the phase modulation amplitude across the plate's surface are measured. The minimal detectable stress amplitude is estimated.

Direct laser printing of chiral plasmonic nanojets by vortex beams.

Donut-shaped laser radiation, carrying orbital angular momentum, namely optical vortex, was recently shown to provide vectorial mass transfer, twisting transiently molten material and producing chiral micro-scale structures on surfaces of different bulk materials upon their resolidification. In this paper, we show that at high-NA focusing nanosecond laser vortices can produce chiral nanoneedles (nanojets) of variable size on thin films of such plasmonic materials, as silver and gold films, covering thermally insulating substrates. Main geometric parameters of the produced chiral nanojets, such as height and aspect ratio, were shown to be tunable in a wide range by varying metal film thickness, supporting substrates, and the optical size of the vortex beam. Donut-shaped vortex nanosecond laser pulses, carrying two vortices with opposite handedness, were demonstrated to produce two chiral nanojets twisted in opposite directions. These results suggest optical interference of the incident and reflected laser beams as a source of complex surface intensity distributions in metal films, possessing spiral components and driving both center-symmetric and spiral thermocapillary melt flows to yield in frozen nanoneedles with their pre-determined spiral nanocarving.

Multi-beam pulsed-laser patterning of plasmonic films using broadband diffractive optical elements.

Multi-sector broadband diffractive optical elements (DOEs) were designed and fabricated from fused silica for high-efficiency multiplexing of femtosecond and nanosecond Gaussian laser beams into multiple (up to one 100) optically tunable microbeams with increased high-numerical aperture (NA) focal depths. Various DOE-related issues, such as high-NA laser focusing, laser pulsewidth, and DOE symmetry-dependent heat conduction effects, as well as the corresponding spatial resolution, were discussed in the context of high-throughput laser patterning. The increased focal depths provided by such DOEs, their high multiplexing efficiency and damage threshold, as well as easy-to-implement optical shaping of output microbeams provide advanced opportunities for direct, mask-free, and vacuum-free high-throughput subtractive (ablative) and displacive pulsed-laser patterning of various nanoplasmonic films for surface-enhanced spectroscopy, sensing, and light control.

Modeling of surface plasmon resonance in metalized optical waveguides with low V number by eigenmode expansion method.

The paper reports on the numerical study of surface plasmon resonance excitation in a bent metal-coated single mode optical fiber with a low V-number. It was shown that by choosing a proper combination of normalized frequency, bend radius, and metal film thickness one can achieve strong coupling between the fundamental mode guided by the fiber core, and symmetric surface plasmon mode supported by the metal layer applied to the fiber cladding. The effect is demonstrated to allow precision refractive index measurement, with spectral sensitivity and resolution estimated at 70 µm/refractive index unit and 3⋅10(-7), respectively.

Dynamic laser-induced effects in nanocomposite systems based on the cadmium sulfide quantum dots in a silicate matrix.

In this paper we study the laser-induced modification of optical properties of nanocomposite based on cadmium sulphide quantum dots encapsulated into thiomalic acid shell which were embedded into a porous silica matrix. It was found that exposure to laser radiation at λ = 405.9 nm leads to modification of optical properties of nanocomposite. For this exposed area there is a significant amount of photodynamic changes under subsequent exposure to laser radiation at λ = 405.9 nm, namely photoabsorption and photorefraction which were studied at λ = 633 nm. The value of these effects dependent on the concentration of quantum dots and modifying radiation parameters. Moreover, it has dependence from polarization of exposure radiation.

Flash-imprinting of intense femtosecond surface plasmons for advanced nanoantenna fabrication.

In this work, we demonstrate an all-laser method of fabrication of optical nanoantennas (ONAs) with an additional coupling/focusing diffractive element. This method is based on double-shot femtosecond laser nanoablation of a thin supported metallic film, inducing a sequence of electrodynamic (surface plasmon-polariton [SPP] excitation and interference), thermal (melting, ablation and ultrafast cooling), and hydrodynamic processes. In particular, the thermal and hydrodynamic processes are important for ONA formation after the first laser shot, while second spatially shifted laser shot via an induced SPP wave results in a radial surface grating near the nanoantenna. Such gratings provide efficient coupling between incident laser radiation and SPP waves, thus significantly improving the ONA efficiency.

Analysis of surface plasmon resonance in bent single-mode waveguides with metal-coated cladding by eigenmode expansion method.

A numerical study is presented of surface plasmon waves excitation in a metal film applied to the cladding of a standard bent single-mode optical fiber. It was shown that by adjusting the bend radius and metal film thickness one can achieve effective coupling between the fiber fundamental mode and symmetric surface plasmon mode through the intermediary of whispering gallery modes supported by the cladding of the bent fiber. This effect is demonstrated to allow for refractometric measurement both in the wavelength and intensity-modulated regimes with a resolution of up to 10⁻8 RIU. Usage of standard noise reduction techniques for intensity-modulated optical signals promises further increase in accuracy.

Laser-induced photodynamic effects at silica nanocomposite based on cadmium sulphide quantum dots.

In this paper we study the laser-induced modification of optical properties of nanocomposite based on cadmium sulphide quantum dots encapsulated into thiomalic acid shell which were embedded into a porous silica matrix. We found red shift of luminescence of the nanocomposite when exposed to laser radiation at λ = 405 nm. Using pump-probe method and Small-Angle X-ray Scattering technique it was found that laser radiation at λ = 405 nm also increases the absorption coefficient of the nanocomposite in 15 times due to agglomeration of quantum dots. The modification of absorption properties is fully reversible.

High-quality fiber microaxicons fabricated by a modified chemical etching method for laser focusing and generation of Bessel-like beams.

The fabrication method of the high-quality fiber microaxicons (FMAs) on the endface of the optical fiber was developed. Using several types of the commercially available optical fibers we experimentally demonstrated the fabrication of a high-quality FMA focusing a laser beam into a tiny spot with a FWHM≈0.6λ and Bessel-like field distribution. It was also demonstrated that choosing the appropriate chemical composition of the etching solution makes it possible to change the shape of the FMA tip from conical to hemispherical. This allows one to change the spatial distribution of the output laser beam, which can represent both the Bessel-like beam with a depth of focus of up to 49λ and a very tiny focal spot close to the diffraction limit size. Experimentally measured focusing characteristics of the fabricated FMAs obtained using a homemade collection-mode scanning near-field optical microscope setup demonstrate good agreement with numerical simulations based on the 3D finite-difference time-domain simulations.

In situ-Synthesized cadmium sulfide quantum dots in pore-forming protein and polysaccharide matrices for optical biosensing applications.

The main limitation for practical implementation of quantum dots-based sensors and biosensors is the possible contamination of sensing media with quantum dots (QDs) moved out from the sensor structure, being critical for living systems measurements. Numerous efforts have addressed the challenge of pre-synthesized QDs incorporation into porous matrix provide, on the one hand, proper fixation of quantum dots in its volume and preserving a free analyte transfer from the sensing media to them - on the other hand. Here, we propose an alternative insight into this problem. Instead of using preliminary synthesized particles for doping a matrix, we have in situ synthesized cadmium sulfide QDs in porous biopolymeric matrices, both in an aqueous solution and on a mica substrate. The proposed technique allows obtaining QDs in a matrix acting simultaneously as a ligand passivating surface defects and preventing QDs aggregation. The conjugates were used as a photoluminescence sensor for the metal ions and glutathione detection in an aqueous media. Different kinds of sensor responses have been found depending on the analyte nature. Zinc ions' presence initiates the intraband QDs emission increases due to the reduction of non-radiative processes. The presence of copper ions, in contrast, leads to a gradual photoluminescence decrease due to the formation of the non-luminescent copper-based alloy in the QDs structure. Finally, the presence of glutathione initiates a ligand exchange process followed by some QDs surface treatment enhancing defect-related photoluminescence. As a result, three different kinds of sensor responses for three analytes allow claiming development of a new selective QD-based sensor suitable for biomedical applications.

Cavity-based Fabry-Perot probe with protruding subwavelength aperture.

We investigate numerically and experimentally the possibility of development of a cavity-based probe for near-field optical microscopy systems based on a fiber Fabry-Perot interferometer with a subwavelength protruding aperture. It was shown that the probe provides a spatial resolution of no worse than λ/37 for λ=1550 nm.

Optical and nonlinear optical properties of sea glass sponge spicules.

Originating in nature, the combination of spongin protein with silicon dioxide extracted from seawater by silicatein protein presents a natural nanocomposite material of unique optical and mechanical properties. Mechanically, it combines the elasticity of protein with the flexibility and durability of silica. The light propagation inside spicules of glass sponges is of substantial interest for developing novel elements for photonics applications. The glass sponge spicules have remarkable light guiding properties. Our experimental research on passing laser pulses through spicules of Hyalonema sieboldi and Pheronema sp. reveals a concentration of guided light in the paraxial region. The multi-layer cladding of glass sponge spicules produced by nature has an obvious analogy with some contemporary artificial microstructured optical fibers. Our researches have shown that the core diameter and cladding layers thickness of the spicules of H. sieboldi and Pheronema sp. glass sponges are appropriate for causing photonic bandgaps in the infrared, visible, and ultraviolet wavelength regions. This enables singlemode waveguide and Bragg light propagation regimes in the spicules and provides exciting prospects of using them for the development of fundamentally new integrated optical elements based on peculiar waveguide properties of such structures, e.g., single-way waveguides (optical diodes) with increased mode field diameter and unique frequency and dispersion characteristics. Also, we have investigated the dynamics of propagation of intensive ultra-short pulses with durations T (0) < 40 fs through various patterns of spicules. Comparative analysis of the spectra of the output signals has shown that chromatic dispersion in spicules is considerably reduced, which can be explained by waveguide dispersion prevailing over material dispersion because of the multilayer structure of the cladding.

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.

Laser printing of resonant plasmonic nanovoids.

Hollow reduced-symmetry resonant plasmonic nanostructures possess pronounced tunable optical resonances in the UV-vis-IR range, being a promising platform for advanced nanophotonic devices. However, the present fabrication approaches require several consecutive technological steps to produce such nanostructures, making their large-scale fabrication rather time-consuming and expensive. Here, we report on direct single-step fabrication of large-scale arrays of hollow parabolic- and cone-shaped nanovoids in silver and gold thin films, using single-pulse femtosecond nanoablation at high repetition rates. The lateral and vertical size of such nanovoids was found to be laser energy-tunable. Resonant light scattering from individual nanovoids was observed in the visible spectral range, using dark-field confocal microspectroscopy, with the size-dependent resonant peak positions. These colored geometric resonances in far-field scattering were related to excitation and interference of transverse surface plasmon modes in nanovoid shells. Plasmon-mediated electromagnetic field enhancement near the nanovoids was evaluated via finite-difference time-domain calculations for their model shapes simulated by three-dimensional molecular dynamics, and experimentally verified by means of photoluminescence microscopy and Raman spectroscopy.

Formation of crownlike and related nanostructures on thin supported gold films irradiated by single diffraction-limited nanosecond laser pulses.

A type of laser-induced surface relief nanostructure-the nanocrown-on thin metallic films was studied both experimentally and theoretically. The nanocrowns, representing a thin corrugated rim of resolidified melt and resembling well-known impact-induced water-crown splashes, were produced by single diffraction-limited nanosecond laser pulses on thin gold films of variable thickness on low-melting copper and high-melting tungsten substrates, providing different transient melting and adhesion conditions for these films. The proposed model of the nanocrown formation, based on a hydrodynamical (thermocapillary Marangoni) surface instability and described by a Kuramoto-Sivashinsky equation, envisions key steps of the nanocrown appearance and gives qualitative predictions of the acquired nanocrown parameters.