Focused Laser Radiation Research Articles

The tight focusing of laser radiation using 4-sector polarization converter

A laser beam with mixed linear-radial polarization formed by reflection of a linearly polarized light from a 4-sector micropolarizer was focused using a binary zone plate in a focal spot with diameters 0.35±0.02 and 0.38±0.02 of a wavelength.

Conjugated Polymer Blend Microspheres for Efficient, Long-Range Light Energy Transfer.

Highly luminescent π-conjugated polymeric microspheres were fabricated through self-assembly of energy-donating and energy-accepting polymers and their blends. To avoid macroscopic phase separation, the nucleation time and growth rate of each polymer in the solution were properly adjusted. Photoluminescence (PL) studies showed that efficient donor-to-acceptor energy transfer takes place inside the microspheres, revealing that two polymers are well-blended in the microspheres. Focused laser irradiation of a single microsphere excites whispering gallery modes (WGMs), where PL generated inside the sphere is confined and resonates. The wavelengths of the PL lines are finely tuned by changing the blending ratio, accompanying the systematic yellow-to-red color change. Furthermore, when several microspheres are coupled linearly, the confined PL propagates the microspheres through the contact point, and a cascade-like process converts the PL color while maintaining the WGM characteristics. The self-assembly strategy for the formation of polymeric nano- to microstructures with highly miscible polymer blends will be advantageous for optoelectronic and photonic device applications.

Influence of asymmetric injection of laser radiation into capillary waveguides on wake acceleration of electrons possessing various injection energies

Laser wakefield acceleration of electron bunches possessing various initial injection energies in capillary waveguides is studied in the conditions of an asymmetric input of laser radiation into a waveguide (the propagation direction of laser radiation deviates from the capillary axis or the laser spot is not symmetric). The factors determining the critical angle of the laser radiation input into the capillary, within which the wake acceleration of electrons is close to optimal, have been found. It is shown that in acceleration stages where electron energies are high, the requirements to angular concentricity of the capillary axis and laser radiation focusing are substantially weaker due to the relativistic ‘weighting’ of the electron mass.

Simulation of the spherical aberration of focused laser radiation in transparent materials: comparison of different simulation approaches

Creating microvoids, cracks or refractive index modifications within dielectrics focusing intense laser radiation into the transparent material offers many promising applications, such as holographic data storage, laser-written waveguides or optical gratings. For further improvements in the quality of the named applications, a deep understanding of the involved processes during interaction of laser radiation with material is necessary. In this work, the change in the laser intensity distribution of focused laser radiation into PMMA by spherical aberration, caused by the transition of the radiation from air to matter, is discussed. Theoretical and numerical investigations including nonlinear effects of laser radiation material interaction as well as multi-physical approaches, such as combined calculation of the beam propagation, the temperature distribution and induced tensions, require a lot of effort and long computation time. Therefore, simple linear simulations are performed including calculation of the beam propagation without nonlinear optical effects, like self-focusing. Comparing the calculated intensity distributions with experimental data, regarding the lateral and axial size as well as the position of the laser-generated voids within PMMA, the applicability of the simulation approaches is demonstrated. The different simulation approaches are characterized in regard to their calculation time and accuracy depending on the simulation task.

Investigation on the Ablation of thin Metal Films with Femtosecond to Picosecond-pulsed Laser Radiation

Ultrafast-pulsed laser radiation enables defined removing of different materials with very high processing accuracy due to the short interaction time of the radiation with the material, and the reduced heat affected zone compared to the removal with nanosecond laser radiation. For further processing improvements, a deep understanding of the involved processes of the laser radiation material interaction, especially the complex ablation phenomena, is necessary. Therefore, the ablation of metal films (d = 0.3–2μm) of gold, platinum, aluminum, nickel and molybdenum was investigated using single-pulsed focused laser radiation (λ = 1030nm) in dependence on the pulse duration (τH = 200 fs–15ps). The metals were chosen due to their different thermophysical properties, especially their electron-phonon-relaxation time. Two different ablation regimes, so called strong- and gentle-ablation, are observed for all investigated materials. To explain the experimental results, theoretical investigations based on the thermophysical properties as well as on the Two-Temperature-Model, including a temperature dependent reflectance, were performed.

Solving the inverse problem of focusing laser radiation in a plane region using geometrical optics

We proposed a method for calculating the eikonal function of a light field from the condition of generating a desired intensity distribution in a predetermined region of the focal plane. To improve the efficiency of solving the problem of focusing we developed a modification of a method of rectangle matching, which enables designing diffractive optical elements (DOE) intended to focus into a plane region. The novelty of our approach consists in a method of reconstructing the eikonal function from the known ray map of points on the DOE onto points in the focal plane. We presented the results of the DOE-aided focusing into a circle, a rectangle, a rhombus, and a superellipse. The simulation results showed the high quality of focusing, thus corroborating the efficiency of the proposed method. We expect to use the diffractive optical elements (DOE) for lighting the area of interest from low-flying drones used for night-time Earth remote sensing.

Raman spectroscopy and electrical properties of InAs nanowires with local oxidation enabled by substrate micro-trenches and laser irradiation

The thermal gradients along indium arsenide nanowires were engineered by a combination of fabricated micro-trenches in the supporting substrate and focused laser irradiation. This allowed local spatial control of thermally activated oxidation reactions of the nanowire on the scale of the diffraction limit. The locality of the oxidation was detected by micro-Raman mapping, and the results were found to be consistent with numerical simulations of the temperature profile. Applying the technique to nanowires in electrical devices the locally oxidized nanowires remained conducting with a lower conductance as expected for an effectively thinner conducting core.

Photo–induced processes in Ag and Eu β-diketonates incorporated into aerogel matrix of silicon dioxide by supercritical fluid impregnation

Samples of silicon dioxide aerogel with embedded Ag and Eu β-diketonate molecules are obtained by impregnation in supercritical carbon dioxide (SC-CO2). The sample impregnated by Eu(tta)3 molecules possesses photoluminescence properties. Moreover, adsorption of Eu(tta)3 on the walls of the pores results in a strong broadening of the Stark components of its photoluminescence spectra. It is found that aerogel impregnation by AgFOD molecules followed by laser irradiation causes the formation of Ag nanoparticles in the sample volume as a result of AgFOD photolysis and subsequent diffusion self-assembly. The Ag nanoparticles assemble into filament structures due to self-organization as they focus laser radiation.

Water at the graphene – substrate interface: interaction with short laser pulses

We have investigated the role of adsorption water in the local transformation of multilayer graphene deposited on an oxidised silicon substrate, which was exposed to nanosecond low-intensity focused laser radiation with a wavelength of in the air. Experimental data obtained for a laser energy density suggest that the formation of micropits (craters) is a consequence of the multipulse removal of the layer of a water adsorbate, which is intercalated between graphene and the substrate, from the zone of laser irradiation of the graphene sheet. The energy threshold of graphene damage in the regions devoid of water was found to be higher in comparison with the initial one (0.058 against ). According to computer simulations of the heating dynamics of the sample and the heat distribution in the substrate – adsorbate – graphene multilayer system, at energy densities corresponding to the experimental ones the water adsorbate layer heats to a temperature sufficiently high to form an increased-pressure vapour cavity under the graphene film.

Multifrequency Stimulated Raman Scattering in Light and Heavy Water

We establish the regularities of changes in the spectra of stimulated Raman scattering in light water (H2O) and heavy water (D2O) under changing laser excitation regimes. We find several satellites occurring in Stokes and anti-Stokes regions at high pumping intensities. We study the spectra of stimulated Raman scattering in H2O and D2O under excitation by the second optical harmonic (532 nm) of a YAG: Nd3+ laser generating ultrashort (60 ps) pulses with a recurrence frequency of 20 Hz and a peak power of 10-100 MW. We show that intense spectral bands determined by both intramolecular modes of light/heavy water and intermolecular modes - librations or translations of the molecular characteristic of ice - occur in stimulated Raman scattering spectra upon an increase in exciting radiation intensity. Our experiments demonstrate the prospects of using the phenomenon of stimulated Raman scattering for developing coherent waves corresponding to intramolecular and lattice modes of light and heavy water. The phenomenon can also be used for developing a deuterium plasma with high effective temperature in the region of laser radiation focusing in heavy water and low-temperature water in the boundary region.

Highly sensitive nonlinear luminescent ceramics for volumetric and multilayer data carriers

The interaction of optical ceramics based on wide-band-gap crystals with near-IR femtosecond laser radiation is studied experimentally. The formation of luminescent centres in LiF and ceramics under the action of single laser pulses is considered. Two interaction regimes are used. In the regime of low-aperture focusing of laser radiation , multiple self-focusing and filamentation in the samples are observed. The luminescent centres are formed in thin channels induced by light filaments. The average effective self-focusing length is ; the formation of luminescent centres begins at this length and ceases at a wavelength of about . The luminescent trace (spur) induced by a single laser filament was long and in diameter. The second regime of light interaction with the sample was based on high-aperture focusing with a simultaneous decrease in the laser pulse energy. This led to the formation of single pits with a diameter smaller than the optical diffraction limit. The luminescent centres induced by the laser radiation were aggregated colour centres. The mechanism of their creation included the highly-nonlinear generation of electron – hole pairs in the filamentation region, their recombination with the formation of anion excitons and the decay of excitons into Fresnel defects by the Lushchik – Vitol – Hersh – Pooley mechanism, as well as their recharging, migration and aggregation.

Productivity and quality of cladding with a coaxial supply of laser radiation and a gas—powder mixture (Jet)

A production process of laser cladding using a coaxial supply layout of powder in the processing zone is investigated. Main attention is paid to investigating the joint influence of parameters of the focused gas—powder jet and focused laser radiation on the properties of the facing layer (sizes, porosity, etc.). The investigation was performed in two directions, notably: 1) numerical modeling (with experimental verification of the results) of focusing the gas—powder mixture by jets of various configurations and 2) modeling by the methods of experimental design. The results of numerical modeling were used when describing the cladding method by methods of experimental design. The data found from the active performance strategy of the investigations showed the statistically valuable influence of these parameters (mutual arrangement of the caustic constriction, shape of the gas—powder jet, treated surface, etc.) on the productivity (sizes) and quality (number of pores) in the facing track.

Interaction of CO2-laser beam with argon plasma of gas tungsten arc

A self-consisted mathematical model was proposed for energy, mass, and charge transfer processes in the arc column plasma and anode region of gas tungsten arc (GTA) with water-cooled anode under the influence of a focused laser beam, propagating along the arc column. This model was the basis for performing detailed numerical analysis of the interaction processes between Gaussian beam radiation emitted by a continuous-wave CO2-laser and the argon plasma of a stationary GTA. It was determined that additional local heating by focused laser radiation changes the thermal and electromagnetic characteristics of the arc column plasma. The influence of laser radiation absorption and refraction in the arc plasma on the characteristics of the laser beam and its thermal effect on the anode metal surface was studied. Laser heating of the arc plasma also leads to a rearrangement of spatial distributions of temperature, electric potential and current density in the near-anode arc plasma, thus changing the distributed and integral characteristics of its thermal impact on the anode surface.

Application of nanostructured silver-palladium resistive films for accurate positioning of focused laser radiation

It was experimentally determined that the electric signal read out of needle-shaped electrodes placed on the surface of the silver-palladium resistive film, when exposed to laser radiation, depends essentially on the position of the irradiated spot relative to the electrodes. When the film surface is scanned by the focused laser beam in the area equidistant from both electrodes, the electric signal changes its sign and vanishes in the position of the spot between the electrodes. In the experiments, the accuracy in positioning the CO2-laser beam with a wavelength of 10.6 μm on the resistive film surface with dimensions of 6.00 × 6.00 × 0.02 mm was 100 μm, when the sensitivity to the intensity was 1 pV m2/W.

Investigation of Laser Induced Structure formation and resultant fluorescence

The formation mechanism of 3D micron-sized fluorescent structures generated in silver nanoparticle containing sodium citrate dihydrate films, during exposure to focused laser radiation, was investigated. Microscopic and thermochemical data indicate that heat accumulates at the nanoparticle surface. The heat causes local melting and an increase in temperature beyond the decomposition point of the immediate surrounding layer. In turn this leads to the rapid release of volatile gases (H2O and CO2). These expanding gases push the melted liquid pool outward, from the center of the focal volume, leaving behind a trough-like structure with elevated edges. It was observed that the edges of the structures were fluorescent. The fluorescence mechanism was investigated using atomic force, scanning electron and Confocal Fluorescence Microscopy. The observed fluorescence was attributed to the decomposition of sodium citrate dihydrate to sodium citrate. The presence of water acts to quench fluorescence in the bulk film, but near regions that experienced heat, the water is driven off.

Binary doe with elongated focal depth to focus terahertz free electron laser radiation (novofel)

A binary silicon diffractive optical element (DOE) focusing laser radiation onto an axial segment (or a DOE with elongated focal depth) for the terahertz spectral range has been designed and characterized using terahertz radiation of the Novosibirsk Free Electron Laser (NovoFEL).

NIR Microscopy Possibilities for the Visualization of Silicon Microelectronic Structure Topology through the Substrate

Experimental setup based on visible and NIR spectral range microscope with laser port and picosecond laser is developed for silicon integrated circuit (IC) failure analysis. The possibility of visualizing the topology of the submicron technology silicon structures from the back side of the crystal through the substrate is shown. Main features of new setup are demonstrated by some results of backside focused pulsed laser beam initiated latchup effect study. The possibility of the localization of the latchup sensitive areas under focused laser irradiation is shown. NIR light emission accompanying the latchup effect is observed and analyzed. The practical aspects of NIR microscopy for failure analysis under backside laser irradiation are discussed.

Fast microstructuring of silica glasses surface by NIR laser radiation

The glass surface microstructuring technology using laser radiation with NIR wavelength (λ=1.064μm) was revealed in this work. Glass plates were placed on the cellular graphite surface. Focused laser radiation passed through the glass plate and interacted with cellular graphite. The radiation heated the graphite surface and thus the high temperature influenced the back side of the glass plate. After consecutive laser scans, having certain periods and interruptions of laser radiation, the microstructures with depth ~0.5μm were formed. Besides, in this work we suggested the method to calculate optical characteristics of formed elements. It was experimentally shown that these microstructures could be used to form phase diffraction gratings (PDGs) and random phase plates (RPPs). We experimentally demonstrated the possibility of these elements being used as RPPs which are suitable for multimode laser radiation homogenization and as PDGs which are suitable for laser simultaneous processing of metal films.

Generation of low-frequency radiation under focused laser irradiation of a semiconductor

The excitation of the nonlinear medium based on the zigzag nanotubes is analyzed. The terahertz emission is demonstrated under excitation in spite of an equidistant spectrum of a single nanotube. The system exhibits relatively high absorbance in the absence of pumping due to a relatively large oscillator strength.

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.