Optical Sensitivity of Waveguides Inscribed in Nanoporous Silicate Framework.

Laser direct writing technique in glass is a powerful tool for various waveguides' fabrication that highly develop the element base for designing photonic devices. We apply this technique to fabricate waveguides in porous glass (PG). Nanoporous optical materials for the inscription can elevate the sensing ability of such waveguides to higher standards. The waveguides were fabricated by a single-scan approach with femtosecond laser pulses in the densification mode, which resulted in the formation of a core and cladding. Experimental studies revealed three types of waveguides and quantified the refractive index contrast (up to Δn = 1.2·10-2) accompanied with ~1.2 dB/cm insertion losses. The waveguides demonstrated the sensitivity to small objects captured by the nanoporous framework. We noticed that the deposited ethanol molecules (3 µL) on the PG surface influence the waveguide optical properties indicating the penetration of the molecule to its cladding. Continuous monitoring of the output near field intensity distribution allowed us to determine the response time (6 s) of the waveguide buried at 400 µm below the glass surface. We found that the minimum distinguishable change of the refractive index contrast is 2 × 10-4. The results obtained pave the way to consider the waveguides inscribed into PG as primary transducers for sensor applications.

Formation of the Submicron Oxidative LIPSS on Thin Titanium Films During Nanosecond Laser Recording.

Laser-induced periodic surface structures (LIPSSs) spontaneously appearing on the laser-treated (melted or evaporated) surfaces of bulk solid materials seem to be a well-studied phenomenon. Peculiarities of oxidative mechanisms of LIPSS formation on thin films though are far less clear. In this work, the appearance of oxidative LIPSSs on thin titanium films was demonstrated under the action of commercially available nanosecond-pulsed Yb-fiber laser. The temperature and energy regimes favoring their formation were revealed, and their geometric characteristics were determined. The period of these LIPSSs was found to be about 0.7 λ, while the modulation depth varied between 70 and 110 nm, with high stability and reproducibility. It was shown that LIPSS orientation is rather easily manageable in the regimes of our interest, which could provide a way of controlling their properties.

Fluorescent Bulk Waveguide Sensor in Porous Glass: Concept, Fabrication, and Testing.

In this work, we suggest the new concept of sensing elements-bulk waveguides (BWGs) fabricated by the laser direct writing technique inside porous glass (PG). BWGs in nanoporous materials are promising to be applied in the photonics and sensors industries. Such light-guiding components interrogate the internal conditions of nanoporous materials and are able to detect chemical or physical reactions occurring inside nanopores especially with small molecules, which represent a separate class for sensing technologies. After the writing step, PG plates are impregnated with the indicator-rhodamine 6G-which penetrates through the nanoporous framework to the BWG cladding. The experimental investigation proved the concept by measuring the spectral characteristics of an output signal. We have demonstrated that the BWG is sensitive to ethanol molecules captured by the nanoporous framework. The sensitivity of the peak shift in the fluorescence spectrum to the refractive index of the solution is quantified as 6250 ± 150 nm/RIU.

Nondestructive Femtosecond Laser Lithography of Ni Nanocavities by Controlled Thermo-Mechanical Spallation at the Nanoscale.

We present a new approach to femtosecond direct laser writing lithography to pattern nanocavities in ferromagnetic thin films. To demonstrate the concept, we irradiated 300 nm thin nickel films by single intense femtosecond laser pulses through glass substrate. Using a fluence above the ablation threshold, the process is destructive, leading to the formation of an ablation crater. By progressively lowering the laser fluence, the formation of closed spallation cavities below the ablation threshold is achieved. Systematic studies by the electron and optical interferometric microscopies, supported by molecular dynamics simulations, enabled us to gain an understanding of the thermo-mechanical spallation mechanism at the solid-molten interface. We achieved the fabrication of periodic arrangements of closed spallation nanocavities. Due to their topology, closed magnetic nanocavities can support unique couplings of multiple excitations (magnetic, optical, acoustic, spintronic). Thereby, they offer a unique physics playground for emerging fields in magnetism, magneto-photonic, and magneto-acoustic applications.

Laser Thermochemical High-Contrast Recording on Thin Metal Films.

Laser-induced thermochemical recording of nano- and microsized structures on thin films has attracted intense interest over the last few decades due to essential applications in the photonics industry. Nevertheless, the relationship between the laser parameters and the properties of the formed oxide structures, both geometrical and optical, is still implicit. In this work, direct laser interference patterning of the titanium (Ti) film in the oxidative regime was applied to form submicron periodical structures. Depending on the number of laser pulses, the regime of high contrast structures recording was observed with the maximum achievable thickness of the oxide layer. The investigation revealed high transmittance of the formed oxide layers, i.e., the contrast of recorded structures reached up to 90% in the visible range. To analyze the experimental results obtained, a theoretical model was developed based on calculations of the oxide formation dynamics. The model operates on Wagner oxidation law and the corresponding optical properties of the oxide-metal-glass substrate system changing nonlinearly after each pulse. A good agreement of the experimental results with the modeling estimations allowed us to extend the model application to other metals, specifically to those with optically transparent oxides, such as zirconium (Zr), hafnium (Hf), vanadium (V), niobium (Nb), and tantalum (Ta). The performed analysis highlighted the importance of choosing the correct laser parameters due to the complexity and nonlinearity of optical, thermal, and chemical processes in the metal film during its laser-induced oxidation in the air. The developed model allowed selecting the suitable temporal-energetic regimes and predicting the optical characteristics of the structures formed with an accuracy of 10%. The results are promising in terms of their implementation in the photonics industry for the production of optical converters.

Numerical Investigation of Ultrashort Laser-Ablative Synthesis of Metal Nanoparticles in Liquids Using the Atomistic-Continuum Model.

We present a framework based on the atomistic continuum model, combining the Molecular Dynamics (MD) and Two Temperature Model (TTM) approaches, to characterize the growth of metal nanoparticles (NPs) under ultrashort laser ablation from a solid target in water ambient. The model is capable of addressing the kinetics of fast non-equilibrium laser-induced phase transition processes at atomic resolution, while in continuum it accounts for the effect of free carriers, playing a determinant role during short laser pulse interaction processes with metals. The results of our simulations clarify possible mechanisms, which can be responsible for the observed experimental data, including the presence of two populations of NPs, having a small (5-15 nm) and larger (tens of nm) mean size. The formed NPs are of importance for a variety of applications in energy, catalysis and healthcare.

Nanosecond-Laser Generation of Nanoparticles in Liquids: From Ablation through Bubble Dynamics to Nanoparticle Yield.

A comprehensive picture of the nanosecond-laser generation of colloidal nanoparticles in liquids is nowadays the demand of their high-throughput industrial fabrication for diverse perspective biomedical, material science, and optoelectronic applications. In this study, using silicon as an example, we present a self-consistent experimental visualization and theoretical description of key transient stages during nanosecond-laser generation of colloidal nanoparticles in liquids: plasma-mediated injection of ablated mass into the liquid and driving the vapor bubble, finalized by the colloid appearance in the liquid. The explored fundamental transient stages envision the basic temporal and spatial scales, as well as laser parameter windows, for the demanded high-throughput nanosecond-laser generation of colloidal nanoparticles in liquids.

Direct laser writing of barriers with controllable permeability in porous glass.

Barriers were produced in porous glass through its local bulk density modification by direct femtosecond writing accompanied by СО2-laser surface thermal densification, to make functional microfluidic elements separated by such physical barriers with different controlled permeability. The separation of multi-component solutions into individual components with different molecule sizes (molecular separation) was performed in this first integrated microfluidic device fabricated in porous glass. Its application in the environmental gas-phase analysis was demonstrated.