Pulsed Regime Research Articles

Parametric study of ultra-intense laser interaction with uniform and nano-porous near-critical plasmas

Responses of the uniform near-critical plasma (UNCP) and nano-porous near-critical plasma (NPNCP) upon interaction with a short-intense laser have been scrutinized using two-dimensional (2D) particle-in-cell simulations. Maximum proton energy variation by the deposition of uniform and nano-porous layers in front of a solid target for a wide range of laser intensities (normalized amplitude a0 = 5–25) and average densities of the front layer ne = 0.3 − 3nc (where nc is the critical density) has been parametrically studied. It is found that the proton maximum energy for the front layers with sub-10 µm thicknesses is independent of the target porosity and density. However, in the relatively thick targets, the nano-porous structure decreases the laser energy absorption and, subsequently, the maximum proton energy compared to the uniform one. The results indicate that by employing UNCPs instead of NPNCPs, at the moderate laser intensity, the maximum proton energy reveals a 23% enhancement. This increment could be explained by rapid self-focusing of the laser pulse and dominant direct laser electron acceleration regime on the well-formed plasma channel in the UNCP layer. However, in the case of NPNCPs, the laser scattering from the plasma structure makes it less intense and more disordered, which influences the efficient laser energy coupling to the electrons.

Corrosion behavior of aluminum oxide coatings created by electrolytic plasma method under different potential regimes

The electrolytic plasma coating is affected by various factors such as electrolyte conductivity, voltage, and current. However, there has not been much attention to the effect of the current regime. The main objective of the present study is to investigate the potential of Al2O3 coatings deposited by the electrolytic plasma method. Aluminum Series 2 was used in this study and the electrolyte was composed of sodium silicate, sodium tetraphosphate, sodium aluminate and potassium hydroxide. The results showed that, in general, according to the impedance diagrams, the corrosion resistance of the coated specimens greatly increases with the immersion time. Therefore, the unit of resistance increased on average to about 10 MHz after 72 hours. In the case of pulsed potential application regime, the corrosion behavior of the samples in the working cycle of 30% was better than that of 70%, which can be related to the thickness of the formed coatings and their porosity. This allows the coating to degrade the coating faster by increasing the thickness and decreasing the porosity of the aggressive chloride ion.

Formation of a transonic region in a variable-section channel for different stagnation temperatures of the flow

Initiation of operation of a ramjet engine with distributed fuel injection along the combustion chamber is studied numerically. A principally important issue is the presence of a compressed air jet producing a throttling effect and preliminary deceleration of the flow to transonic velocities. The Reynolds-averaged Navier-Stokes equations closed by the SST k-ω turbulence model are solved. Hydrogen is used as a gaseous fuel. Transverse injection of the fuel is considered. A pulsed transonic flow regime is obtained. It is demonstrated that the characteristic scale of vortex regions increases with an increase in the stagnation temperature approximately to 1700 K; consequently, the degree of air-hydrogen mixing is significantly enhanced.

Delay-induced instability in phase-locked dual-polarization distributed-feedback fiber lasers

We study experimentally and theoretically a dual-polarization fiber laser submitted to time-delayed frequency-shifted optical feedback. In addition to the usual frequency-locking regime that is expected when the frequency shift is close to the polarization beat frequency, we observe at high pumping rates a dynamical pulsing regime inside the locking range. This regime is experimentally evidenced in a full-fibered experiment based on an erbium-doped distributed-feedback fiber laser in which polarization beat frequency is about 1 GHz. A rate-equation model including the frequency-shifted feedback term reproduces well the experimental bifurcation maps, provided that both the time delay and a phase-amplitude coupling parameter ($\ensuremath{\alpha}$ factor) are taken into account. The impact on microwave-photonics applications is discussed.

Heterogeneous Catalytic Ozonation of Phenol over Iron-based Catalysts in a Trickle Bed Reactor

The use of continuous reactors for heterogeneous catalytic ozonation is yet to be investigated in order to develop a viable technology for industrial applications. This paper presents hydrodynamic and degradation studies on the use of a co-current down flow trickle bed reactor for heterogeneous catalytic ozonation of phenol (as model pollutant) over Fe-Diatomite pellets and Fe-coated glass beads. It was found that the reactor can operate under trickle or pulsing flow regimes, promoting mass transfer augmentation. Residence time distribution data, fitted with n-CSTR and axial dispersion (ADM) models, showed low axial dispersion and high flow distribution. Just the Fe-diatomite pellets showed important phenol adsorption (16 %). Degradation experiments demonstrated that phenol conversion was substantial when using both catalysts, up to 19,7 % pollutant conversion with liquid-phase space times of just 6 s. Compared to direct ozonation, the use of the Fe-diatomite pellets and Fe-coated glass beads enhanced the reactor performance by 48 % and 23 %, respectively. It was confirmed that mass transfer is an important factor that restricts this reaction system performance; consequently, further improvement in mass transport rate is necessary for system optimization.

Comparison of AlGaInAs-Based Laser Behavior Grown on Hybrid InP-SiO₂/Si and InP Substrates

In this work, we present the fabrication process of a $3~\mu \text{m}$ -thick AlGaInAs-based vertical p-i-n laser diode structure grown on a directly bonded InP-SiO2/Si (InPoSi) substrate. High-quality epitaxial growth on InPoSi is reported. Based on these structures, the fabrication process of Fabry-Perot broad lasers is presented. Device performance is compared to the ones obtained from the same growth run on an InP substrate as a reference. High electrical properties of the n and p contacts are demonstrated. Lasing operation in the pulse regime is shown up to 50°C. Threshold current densities as low as 0.4 kA/cm 2 at 20°C are obtained for the laser on InPoSi, comparable to the ones obtained for the reference on InP substrate.

Autosetting Mode-Locked Laser Using an Evolutionary Algorithm and Time-Stretch Spectral Characterization

There is an increased research effort on the development of smart ultrafast laser systems with the intended goal of converting the high parameter sensitivity of the complex laser dynamics from a practical challenge to an opportunity. Evolutionary algorithms have the potential to find suitable mode-locking parameters and optimize the laser output according to the user specifications. In this article, we present the auto setting of optimized self-starting ultrafast pulse regimes at the optical telecom wavelength. We implement an evolutionary algorithm on a normal-dispersion fiber laser cavity mode-locked through a 4-parameter nonlinear polarization evolution. By using a consistent scheme of intracavity liquid-crystal retarders for polarization control, and the time-stretch dispersive Fourier-transform technique for real-time spectral output analysis, we show that we can achieve an efficient and fast optimization of the mode-locked state, with the ability to tune its spectral width.

All-fiber polarization-maintaining mode-locked laser operated at 980 nm.

For the first time, to the best of our knowledge, we present an all-fiber polarization-maintaining passively mode-locked picosecond laser operated at 980 nm. The laser cavity had a ring configuration with a semiconductor saturable absorber mirror as a mode-locking element. As an active medium, we used a specially designed cladding-pumped Yb-doped fiber with reduced cladding-to-core diameter ratio. The laser was self-starting and operated in the net cavity normal dispersion regime, where a spectral profile of the gain medium acted as a filter element. By intracavity spectral filtering, we achieved about 40 dB dominance of the signal wavelength at 980 nm over 1 µm emission in a highly stable picosecond pulsed regime. The corresponding simulation was performed to extend the knowledge about laser operation.

Resistive switching in HfO2 based valence change memories, a comprehensive 3D kinetic Monte Carlo approach

A simulation study has been performed to analyze resistive switching (RS) phenomena in valence change memories (VCM) based on a HfO2 dielectric. The kernel of the simulation tool consists of a 3D kinetic Monte Carlo (kMC) algorithm implemented self-consistently with the 3D Poisson and heat equations. These VCM devices show filamentary conduction, their RS operation is based on the destruction and regeneration of an ohmic conductive filament (CF) composed of oxygen vacancies. The physics underlying the RS operation is described by means of processes linked to generation of oxygen vacancies, oxygen ion migration and recombination between vacancies and oxygen ions that can be accurately described by using the electric field and temperature distributions in the dielectric. The studied devices consist of TiN/Ti/HfO2/W stacks where the Ti capping layer plays the role of oxygen ion getter material. The simulation tool is useful for obtaining information of internal physical variables, explaining RS dynamics and the CFs evolution from the microscopic viewpoint in terms of their size and shape under different electrical input signals; particularly, the pulsed operation regime has been studied in depth. Furthermore, interesting phenomena, such as partial SETs within overall RESET processes can be accurately reproduced.

Numerical Study on Supercontinuum Generation in an Active Highly Nonlinear Photonic Crystal Fiber With Anomalous Dispersion

We numerically study supercontinuum (SC) generation (SCG) in a rare-earth-doped highly nonlinear photonic crystal fiber (HNL-PCF) with anomalous dispersion (AD) in the sub-picosecond pulse regime. We develop a semi-classical numerical model based on the generalized Ginzburg-Landau equation in order to take account of ultrafast interactions between gain ions and ultra-broadband SC radiation encompassing sub-100-femtosecond solitons. Based on the numerical model, we analyze SCG characteristics of an active HNL-PCF in comparison with a passive-type counterpart, unveiling novel optical gain effects in a highly nonlinear optical fiber with AD. We rigorously investigate gain-induced soliton dynamics, such as soliton-cascade-like behaviors, soliton-quasi-soliton collisions, and phase-matched dispersive wave generation, which eventually contributes to enhancement of energy scaling of SC radiation without incurring considerable degradation of its spectral flatness. We also verify that such superior performance characteristics of an active HNL-PCF make it suitable for the use as a boost amplifier for SC radiation. We think that the findings from this study will incite other subsequent studies on unveiling detailed nonlinear pulse dynamics in various gain-embedded nonlinear optical media.

Deposition of vertical carbon nanosheets by MPECVD at atmospheric pressure

The deposition process is studied of vertical carbon nanosheets by microwave (MW) plasma-enhanced chemical vapor deposition (PECVD) at atmospheric pressure. A coaxial MW plasma source with a surface-wave discharge at 2.45 GHz produces plasma in a gas mixture (Ar/H2/CH4) in the processing chamber. The emissive spectra of the plasma column in pulsed regime of the source are registered by an iHR550 spectrometer. The light from the plasma column is collected by a lens system connected to the spectrometer by an optical fiber. The dependence of the gas temperature in the plasma column on the absorbed MW power is obtained from the recorded OH-band and CN-band spectra by using the LIFBASE program. The plasma density is estimated from the Stark broadening of the Hβ-line, while the electron temperature is estimated by the line-ratio method using argon lines. The composition of the outlet gases from the chamber is measured and analyzed by an Agilent Micro gas chromatograph. The results obtained demonstrate the high efficiency of the methane decomposition process in the hot region of the plasma column (T g ~ 3000 K). The deposition of vertical carbon nanosheets is carried out at fixed plasma parameters and controlled substrate temperature. Cu plate and Ni-foam are used as substrates. The carbon nanostructures formed on the metallic substrates are studied by SEM; the dependence is thus obtained of their morphology on the plasma parameters, gas temperature and substrate temperature. The structures are confirmed as graphene sheets via Raman spectroscopy. The results demonstrate the viability of our system for deposition of vertical carbon sheets at atmospheric pressure.

Potential of plasma treatment as water reclamation process for irrigation

A plasma-ozonation system was investigated as a possible method for the reclamation of pesticide contaminated water. A multi-wire to plate corona discharge in oxygen was used, operated in pulsed regime (110 ns full width at half maximum). The synthetic contaminated water contained 25 mg l−1 2,4-dichlorophenoxyacetic acid (2,4-D) in tap water. Complete degradation of 2,4-D and over 90% mineralization were achieved during treatment. The toxicity of the reclaimed water was assessed using seeds of Solanum lycopersicum, known to be sensitive to 2,4-D. It was found that plant growth was inhibited by the presence of the pesticide, however, the treatment reduced the toxicity of the contaminated water. The plasma-ozonation method has proved thus far a good candidate for water reclamation for agriculture.

Saturable plasmonic metasurfaces for laser mode locking

Metamaterials are artificial materials made of subwavelength elementary cells that give rise to unexpected wave properties that do not exist naturally. However, these properties are generally achieved due to 3D patterning, which is hardly feasible at short wavelengths in the visible and near-infrared regions targeted by most photonic applications. To overcome this limitation, metasurfaces, which are the 2D counterparts of metamaterials, have emerged as promising platforms that are compatible with planar nanotechnologies and thus mass production, which platforms the properties of a metamaterial into a 2D sheet. In the linear regime, wavefront manipulation for lensing, holography, and polarization control has been achieved recently. Interest in metasurfaces operating in the nonlinear regime has also increased due to the ability of metasurfaces to efficiently convert incident light into harmonic frequencies with unusual polarization properties. However, to date, the nonlinear absorption of metasurfaces has been mostly ignored. Here, we demonstrate that plasmonic metasurfaces behave as saturable absorbers with modulation performances superior to the modulation performance of other 2D materials and exhibit unusual polarimetric nonlinear transfer functions. We quantify the link between saturable absorption, the plasmonic resonances of the unit cell and their distribution in a 2D metasurface, and finally provide a practical implementation by integrating the metasurfaces into a fiber laser cavity operating in pulsed regimes driven by the metasurface properties. As such, this work provides new perspectives on ultrathin nonlinear saturable absorbers for applications where tunable nonlinear transfer functions are needed, such as in ultrafast lasers or neuromorphic circuits.

Passively Q-switched Nd:GSGG laser at 936.3 nm

An efficient continuous-wave (CW) and pulse Nd:GSGG laser at 936.3 nm is reported with the help of a simple and compact two-mirror cavity. The CW power at 936.3 nm is 2421 mW at a pump power of 10.1 W with a conversion efficiency of 23.7 %. In the pulse regime, a passively Q-switched quasi-three energy Nd:GSGG at 936.3 nm is firstly achieved. The maximum peak power reaches 2.29 kW with an output power of 1089 mW and the narrowest pulse width of 15.3 ns.

Robust simultaneous multiwavelength bulk laser based on Yb:LaMgB5O10 crystal: from continuous wave to ultrashort pulses

We present a robust simultaneous multiwavelength bulk laser based on Yb:LaMgB5O10 (Yb:LMB) crystal, including its continuous-wave (CW) and ultrafast pulsed regimes. CW dual wavelengths at 1077 nm and 1091 nm with an average output power of 2.67 W were achieved with a 1% output coupling (OC). A maximum output power of 4.42 W with triple wavelengths of 1056, 1077, and 1091 nm was generated with 3% OC. In the case of 25% OC, the Yb:LMB laser can operate with four wavelengths (1025.1, 1031.4, 1033.2, and 1053.3 nm), producing an average output power of up to 4.44 W. Furthermore, 568 fs pulses with an average power of 470 mW were obtained at 1057.4, 1079.2, and 1092.4 nm from a synchronous tri-wavelength mode-locked Yb:LMB laser. These pulses are the shortest ever reported from a synchronous tri-wavelength mode-locked bulk laser. The detected frequency beating pulses had a primary interval of 0.11 ps and a full width at half maximum of 77 fs. Results indicated that equal spectral separation between each wavelength is not an essential factor for establishing a synchronous tri-wavelength mode-locking operation.

Morphology study and threshold measurement of laser induced damage of nano-porous antireflective silica thin films in nano- and femtosecond pulse regimes

Laser induced damage threshold (LIDT) of nano-porous antireflective silica thin films is measured using 10 ns laser pulses at λ = 532 nm. The thin films are prepared by dip-coating of BK7 substrates and then drying them by three different heating methods, to see how their LIDT and transmission are affected. Furthermore, using SEM imaging, the morphology of the laser induced damages is inspected and compared to those of similar samples, previously irradiated by femtosecond laser pulses at λ = 800 nm. The images evidently show that in the regions damaged by nanosecond pulses, the silica nanoparticles are melted and fused to each other, while in the samples irradiated by femtosecond pulses, the silica nanoparticles are sputtered and dispersed around, in the damaged area. The SEM images clearly demonstrate the different damage mechanisms involved in the nanosecond and femtosecond regimes of interaction.

Effectiveness of nebulizers in the treatment of respiratory tract infections in children of different ages

Objective: to study the effectiveness of compressor nebulizers in the treatment of respiratory tract respiratory infections in children of different age groups. Material and methods : the main group consisted of 57 children - the average age was 9,7 ± 4,3 years, who were treated with nebulizer therapy using the compressor nebulizer during treatment, 32 children were included in the control group - the average age was 10,8 ± 4,7 years who have not been administered inhalation therapy. Along with conventional therapy, children of the main group underwent inhalation therapy using the compressor nebulizer in two modes, depending on the clinical symptoms of the patient: for bronchitis, the course was 5-6 days, for pneumonia - up to 8-9 days. Using the compressor nebulizer, physiological saline, ipratropium bromide, berodual, pulmicort, lasolvan were injected in a constant flow mode, and in cases of diagnosed rhinosinusitis, a pulsed aerosol delivery regime was used. Results: when analyzing the clinical picture, it was revealed that in all patients the process took a torpid course with prolonged unproductive cough, viscous sputum and persistently retained auscultatory data with insignificant changes in the general blood test. Analysis of the obtained data on the use of various treatment regimens for nosological forms of diseases in the examined children showed that in the patients of the main group the clinical symptoms of a combination of pulmonary diseases and rhino-sinusitis were stopped when applying the pulsating delivery regimen in terms of 6,1 ± 0,7 days (p<0.05), in cases of isolated variants of bronchitis and pneumonia, the use of a constant flow regime reduced the hospitalization of children to 82 ± 7,4% of the standard indicators of bed days. Conclusion: our own studies on the effectiveness of inhalation therapy in children revealed a wide range of clinical effects using the compressor nebulizer, regardless of the age of the patient: the clinical symptoms of a combination of pulmonary diseases and rhinosinusitis were stopped by using a pulsating delivery regimen in a shorter time compared to the control group without inhalation therapy, which reduced the hospitalization of children in the hospital.

APPLICATION OF LASER SURFACE POLISHING ON ADDITIVE MANUFACTURED PARTS OF INCONEL 718 NICKEL-BASED SUPERALLOY

This article deals with the influence of laser polishing parameters on the final surface quality of nickel-based superalloy Inconel 718 produced by metal additive manufacturing. The aim is to optimize the laser polishing process parameters to reduce the surface roughness of the sample. The investigated parameters of the laser are scanning speed, frequency, focal length, and pulsed and continuous wave regimes are considered. The samples of Inconel 718 are produced by direct metal laser sintering technology. Depending on the printing orientation, the roughness after AM is Ra 5 – 12 μm. The samples were machined to ensure uniformity of the monitored and tested surface. An IFM – G4 3D optical microscope is used for the surface evaluation. The initial roughness of the samples before applying laser polishing was Ra 1 - 2 μm. The roughness after laser polishing is significantly reduced to Ra 0.369 μm.

Potential and morphological transitions during bipolar plasma electrolytic oxidation of tantalum in silicate electrolyte

Surges in the cell potential, due to an increased overpotential for hydrogen evolution, and transitions in ceramic oxide coating morphology during plasma electrolytic oxidation (PEO) of tantalum under a pulsed bipolar current regime at 1000 Hz in a silicate electrolyte are investigated using real-time imaging of gas evolution, analytical scanning electron microscopy, X-ray photoelectron spectroscopy and supplementary potential-controlled electrochemical measurements. The coatings, which contained Ta2O5, TaO and incorporated silicon species, revealed a nodular morphology that transformed with treatment time to a “pancake” type and then a “coral reef” type. The first potential surge occurred only in the cathodic potential, coinciding with an increased spark intensity, more vigorous gas evolution, emergence of “pancake” structures and a reduction in the coating porosity. The later increases in both the anodic and cathodic potential, coincided with intensification of the sparking, the establishment of silicon-rich “coral reef” structures, and formation of a comparatively thick coating. The kinetics of coating growth differed significantly between the three morphological stages. Electrochemical measurements showed that anodic discharges increased the overpotential for hydrogen evolution in the subsequent cathodic pulse, which is proposed to be due to gas impeding the coating and at and near the coating surface increasing the resistance to ionic transport.

Wave plate-dependent lasing regimes transitions in an all-normal-dispersion fiber laser mode-locked by nonlinear polarization rotation

We address the transitions between three different working regimes of an all-normal-dispersion pulsed fiber laser, mode-locked by nonlinear polarization rotation (NPR). In particular, we investigate numerically the dynamics and study in details the transitions induced by solely rotating the cavity wave plates. We analyze the phemomenon by putting forward a model for all-normal-dispersion NPR-moded-locked fiber laser, based on the coupled Ginzburg-Landau equations and Jones matrix. Numerical results show that depending on the values of the cavity parameters, the transitions between the dissipative soliton regime and amplifier similariton one, or the transition between the dissipative soliton regime and the splitting pulse one may be realized by simply rotating the cavity wave plates. We found that the transitions are intrinsically related to the NPR mode-locking mechanism by explicitly evaluating the polarization states across the pulse at different locations of the cavity. Our numerical results are in very good agreement with experimental results reported in the relevant literature, and provide evidence that besides gain saturation and spectral filtering, the orientations of wave plates may be exploited as a suitable degree of freedom to obtain the desired pulse regimes in all-normal-dispersion fiber lasers.