Pulsed Regime Research Articles

Femtosecond laser induced damage threshold and incubation in l-threonine aminoacid crystal

l-threonine aminoacid crystals exhibit high second-order optical nonlinearities that can be exploited for second harmonic generation, optical parametric amplification, and optical parametric oscillation. In addition, it possesses a large transparency window and low refractive index, making them an attractive material for photonics devices. Among several processing methods used to develop photonic integrated micro-devices, fs-laser micromachining has stood out for its high resolution, either on the surface or in the bulk, as well as to its flexibility to be used with various types of materials. Although organic crystals present relevant linear and nonlinear optical features, studies on fs-laser processing in such materials are yet scarce. Thus, this work presents a study of the fs-laser incubation and damage threshold fluence determination in l-threonine crystals at 515 nm and 1030 nm. The damage threshold fluence was determined for one up to 105 pulses. For the single pulse regime, we obtained Fth = (0.94 ± 0.04) J/cm2 at 1030 nm and Fth = (0.31 ± 0.01) J/cm2 at 515 nm. Such difference is explained by the number of photons involved in each case; 5-photons at 1030 nm and 3-photons at 515 nm. Also, a slower incubation dynamic was observed at 515 nm. Still, micro-Raman spectroscopy revealed that no structural modifications were induced by the fs-laser pulses on the sample upon micromachining. Such results provide relevant data for the processing of l-threonine crystals via fs-laser micromachining to achieve organic photonic integrated devices.

Time-resolved self-heating temperature measurements of AlInN/GaN HEMTs using CeO2 Raman micro-thermometers

The DC and RF electrical performance and reliability of GaN-based transistors depend on their thermal behavior. Therefore, measuring the self-heating temperature of these devices under real operating conditions with high accuracy is an important and challenging issue. For these reasons, we present a time-resolved thermometry technique to measure transient self-heating temperatures in semiconductor components by combining conventional Raman spectroscopy and CeO2 Raman micro-thermometers. Thus, the experimental GaN volumetric and surface self-heating temperatures measured for biased AlInN/GaN HEMTs in both DC and pulsed regime are reported with a submicrometer spatial resolution and a temperature resolution of about 5 °C. Likewise time-resolved self-heating temperature of drain contact surface has also been studied.

Influence of Heat Loss on the Chaotic Dynamics of Reaction Waves in the Model with Chain-Branching Reaction

In this paper, the complex dynamics of the pulsating regime of combustion waves propagation is numerically investigated within the Zel’dovich–Barenblatt–Dold model with two-step chain-branching reaction mechanism. It is shown that there exists a remarkable similarity in the dynamics of oscillations in this system and one-dimensional discrete-time maps such as the logistic map. In particular, both systems exhibit the period doubling route to chaos and the appearance of windows of stability as the bifurcation parameter is increased. For the first time for a model of combustion wave propagation the sequence of windows of stability is demonstrated and classified. Nevertheless, as the activation energy, which is taken as a bifurcation parameter, reaches a critical value corresponding to the dynamical quenching scenario the dynamics of the flame oscillations becomes multidimensional and therefore it cannot be described by a one-dimensional map anymore. The effect of the heat losses is also studied and it is demonstrated that the number of windows of stability decreases, the order of their appearance changes and the dynamical quenching appears for smaller values of the activation energy as the heat losses are increased.

Laser cutting of basalt fibre reinforced polymers by QCW fiber laser: Interaction mechanisms and effect of laser parameters

In the paper, laser cutting of basalt fibre reinforced polymer laminate using a 450 W QCW fiber laser, working in a pulsed regime is presented. Preliminary tests were performed on a basalt laminate, 1.3 mm in thickness, changing the pulse power (Pp) and pulse duration (D) to find the maximum cutting speed. Then a 33 experimental plan was carried out changing the pulse power, the pulse duration, and the cutting speed. The kerf width and the heat-affected zone were measured in the section and on the external surfaces. ANOVA analysis was adopted to assess the process parameters’ influence. The interaction mechanisms were found and discussed. From the results, the BFRP laminate appears easy to cut at maximum cutting speeds up to 3500 mm/min. However, thermal damages occur. Kerf geometry and HAZ are mainly affected by the cutting speed, while the Pp and D seem to have a minor role.

Zero-broadening slow light from photorefractive two-wave mixing.

The ability to delay short light pulses is a promising solution for all-optical telecommunications, but suffers from a large distortion of the delayed pulse as a consequence of the high material dispersion. In this Letter, we demonstrate the possibility to all-optically control the group delay in a photorefractive (PR) crystal by the use of the two-wave mixing (TWM) effect in the pulse regime at room temperature. Most importantly, we show that a proper choice of the pump pulse width in the TWM process enables us to slow down shorter or longer signal pulses without distortion. The technique is demonstrated both at visible (638 nm) and infrared (1064 nm) wavelengths and for slowed-down pulses with durations ranging from 10 ns up to 30 ms, hence confirming its broad applicability.

Comparative Study of Percussion Drilling in Glasses with a Femtosecond Laser in Single Pulse, MHz-Burst, and GHz-Burst Regimes and Optimization of the Hole Aspect Ratio.

In this contribution, we present a comparative study on top-down drilling in sodalime glass, with a femtosecond laser operating in single-pulse, MHz-burst and GHz-burst modes, respectively. We investigate the hole depth, drilling rate, and hole morphology for these three regimes while keeping the same experimental conditions. We demonstrate that, for both burst regimes, the burst length has to be adapted for optimizing the hole depth. In the GHz-burst regime, the lower the ablation rate the longer the holes. The three drilling regimes lead to different hole morphologies, where the GHz-burst mode results in the best hole quality featuring glossy inner walls and an almost cylindrical morphology. Furthermore, we obtain crack-free holes, the deepest measuring 3.7 mm in length and 25 µm in entrance diameter corresponding to an aspect ratio of 150, which is the highest aspect ratio reported thus far with femtosecond GHz-burst drilling to the best of our knowledge.

Enhanced resonant nonlinear optical traits and optical limiting performance of metal nanoparticles containing borate glasses in nanosecond pulse regime

Enhanced resonant nonlinear optical traits and optical limiting performance of metal nanoparticles containing borate glasses in nanosecond pulse regime

REMOTE MEASUREMENTS OF WATER SURFACE POLLUTION BY THE METHOD OF LASER LOCATION

As it was shown in the nature investigations,the most area of water in the Azerbaijan shore of Caspian Sea is covered by oil film, the square of which depends on direction of wind. The research of pollution's of environment by the mean of optical apparatus of remote sounding, mounted on the board of flying apparatus or space satellites is one of intensely developing directions. The theory of measuring of oil film's thickens by the help of laser location in detail described. The principles of straight¬-line spreading of electromagnetic waves, reflecting of electromagnetic energy by objects and constant speed of their propagation are laid in the basic of location. We have developed lidar for controlling of pollutions of water surface, where helium-neon laser “LGI-102” having power 3mW working in the pulse regime is used as beam radiator. During the experiment the laser was mounted on the plain ground on the distance 1-meter from water surface. The prism was placed in the road of propagation of laser beam and used for direction of radiation under the angle 45 on water surface. Integral photo transducer on the base of local mono-and polycrystalline silicon films with linear output was used in the photo detector device. Photo transducer was made in hybrid form on the base of two crystals: photo sensitive poly-silicon film and unpack operational amplifier 740UD4. Keywords: Pollution, water surface, oil film, laser location, lidar, photo detector.

Transient behaviors of pulse breaking and recovering in a dispersion-managed mode-locked Yb fiber laser

Various pulse regimes have been discovered in mode-locked fiber lasers with carefully designed cavity setups. However, the transitions between different types of stable pulse solutions are rarely observed in a laser with the same cavity configuration. Here, we report on the experimental observation of the transformation from stretched pulses (SPs) to dissipative solitons (DSs) in an all-polarization-maintaining dispersion-managed mode-locked Yb fiber laser for the first time. By means of the time-stretch dispersive Fourier transform technique, the temporal and spectral dynamics of transient behaviors from pulse breaking to pulse recovering are identified in real time. It is revealed that the overdriven nonlinear effect induced by pump increase triggers the wave breaking of SPs, and the improved net-cavity dispersion originated from wavelength shift helps the self-recovering of DSs. Numerical simulations validate the experimental observations and bring insights to the understanding of transient nonlinear dynamics in ultrafast fiber lasers.

Isothermal CO2 injection into water-saturated porous media: Lattice-Boltzmann modelling of pulsatile flow with porosity, tortuosity, and optimal frequency characterization

The Lattice Boltzmann Method (LBM) is used to simulate the isothermal injection of CO2 into a water-saturated, homogeneous porous medium. The complex multiphase flow is modeled using the 2D-pseudopotential Shan-Chen multiphase LBM. This work proposes a unique fluid-pulse technology similar to the one applied to enhance oil recovery with CO2 sequestration. This technology is different from classical CO2 sequestration in aquifers and, to our knowledge, has never been used before. The primary purpose is to investigate whether our LBM simulation allows the introduction of fluid-pulse CO2 injection patterns similar to those reported in the literature on hydrocarbon reservoirs and to evaluate the benefit of using such a technique compared to classical uniform injection. First, our simulations consider a transient homogeneous flow with uniform inlet velocity to determine the natural frequency of the instabilities triggered downstream of the interstitial pore array. Then, CO2 is injected at the inlet in a pulsating regime, varying the forcing frequency near and far from the natural frequency to determine its effect on CO2 penetration. Results demonstrate that a pulsatile injection effectively enhances CO2 sequestration in aquifers when forced with a characteristic frequency. However, this frequency resulting after CO2 is injected to displace the water differed from the natural frequency preliminarily obtained from the homogeneous water flow. Injecting at the characteristic frequency has proven an effective technique for boosting CO2 penetration into a water-saturated porous medium, with a significant enhancement estimated between 1.5% and 16% for a range of pulsatile amplitudes between 1% and 10%.

All-Polarization-Maintaining Linear Cavity Fiber Lasers Mode-Locked by Nonlinear Polarization Evolution in Stretched Pulse Regime

Nonlinear polarization evolution (NPE) is among the most advanced techniques for obtaining ultrashort pulses with excellent optical performance. However, it is challenging to design environmentally stable NPE fiber oscillators using only polarization-maintaining (PM) fibers. Here, we use the same PM fiber and non-reciprocal phase shifter to design two different devices, which are capable of acting as effective NPE saturable absorbers (SAs) in two all-PM linear cavity fiber lasers. These two laser setups differ in the position of the non-reciprocal phase shifter, the presence of which is crucial for the proposed fiber lasers to reduce their mode-locking thresholds and achieve high repetition rates above 100 MHz. The mode-locking principle and pulse evolution in the laser cavity are investigated theoretically. The first all-PM fiber oscillator emits sub-200 fs stretched pulses with low peak powers. The second oscillator, with a simpler architecture, directly delivers stretched pulses with high peak powers, the spectral bandwidth greater than 30 nm, and the pulse duration less than 90 fs. To the best of our knowledge, 79 fs achieved in this design is the shortest pulse duration provided by PM fiber lasers using NPE mode-lockers.

Investigation of High‐Power Spatiotemporal Mode‐Locking with High Beam Quality

AbstractThe developed spatiotemporal mode‐locked (STML) laser has emerged as an effective platform for investigating high‐dimensional nonlinear spatiotemporal dynamics. Additionally, it offers a novel avenue for the design of fiber oscillators capable of operating at high power levels. At present, the focus of STML laser research primarily revolves around this aspect. However, considering practical applications, there is a strong desire to conceive a simple all‐fiber STML oscillator with both high beam quality and high power. In this study, an all‐fiber, high‐power STML oscillator based on multimode fibers is constructed and investigated. By manipulating the pump power and the polarization state, the laser could operate in a dissipative soliton or noise‐like pulse regime, both with average output powers of two operation states reaching the Watt level. Simultaneously, high‐quality output beam profiles are observed in both operation states. To the best of knowledge, this is the first demonstration of high‐power spatiotemporal mode‐locking with high beam quality. The study holds great benefits for advancing the investigations of compact all‐fiber STML lasers which deliver high‐power outputs with superior beam quality and ultimately propels the application of STML lasers. Furthermore, this study contributes to the understanding of the behavior of STML lasers with high power.

Wide dynamic range signal detection for underwater optical wireless communication using a PMT detector.

In the underwater optical wireless communication (UOWC) scenario, a photomultiplier tube (PMT) with higher sensitivity, lower noise, and a larger receiver area is employed as the photon detector to further extend the transmission distance. Due to the complex underwater environment, the high directionality of the light beam, and the vibration of a transceiver, the incident optical power usually spans a very wide dynamic range, and the PMT may operate in any one of the three regimes: pulse, transition, and waveform. While it is difficult to obtain the analytical characterization of the output electric signals across these regimes, this paper resorts to experimental measurements of the upsampled discrete samples within a training symbol duration. Among different statistical distribution fitting options, generalized extreme value (GEV) distribution is found to show excellent performance in fitting the probability density function (PDF) of either multiple samples or the superimposition of all samples within a symbol duration. Then joint sample distribution (JSD) based and superimposed sample distribution (SSD) based symbol detection methods are proposed by adopting the GEV distribution and log-likelihood ratio (LLR) testing criterion. The proposed methods are experimentally evaluated under different received signal optical powers, data rates, and sampling rates. They are shown to outperform the Poisson and Gaussian based maximum likelihood detection methods which are employed for the pulse regime and waveform regime respectively. Furthermore, the effectiveness of the proposed methods in alleviating strong ambient radiation is experimentally verified.

Field Emission in Emerging Two-Dimensional and Topological Materials: A Perspective

Since the first field emission model or the well-known Fowler-Nordheim (FN) law was formulated about a century ago (in 1928), it remains an active topic to discover different aspects of field emission due to new materials and structures, geometrical effects, high current space charge effects, short pulse regime, and analytical models. Despite its simplicity, the original FN law, or its improvement, the Murphy-Good (MG) model remains as the important equations to characterize different field emitters. With the emergence of two-dimensional (2-D) materials such as graphene, transition-metal dichalcogenide (TMD) materials, and topological materials (such as topological insulators and semimetals) in the past two decades, new experiments and theoretical models are produced to study the validity of FN law and MG model on field emission from these quantum materials and to explore their applications as compact field emitters operating at low turn-on field. In this short perspective, after a very brief introduction of FN law and some recent (selected) improvements, we proceed to review the experimental works measuring field emission from the abovementioned quantum materials. The key performances, such as experimental growth methods, and emission characteristics, such as turn-on field, field enhancement factor, and field emission current density, are summarized and compared. During the discussion, we also highlight some recent models proposed to account for the effects of nonparabolic dispersion and topologically nontrivial bandstructures within these quantum materials. We suggest the importance of having consistent physics-based models to understand the field emission from these quantum materials and reinforce the presence of a non-FN scaling law due to their unique properties absent from the traditional bulk materials. Finally, we conclude by highlighting some possible new directions that may be further explored in future.

Nonlinear waves in a rivulet falling down a vertical plate

Nonlinear waves in a rectilinear rivulet flowing down a vertical plate are studied based on the developed theoretical model. The model equations are derived by the weighted residual method through projecting the Navier–Stokes equations onto the constructed system of basic orthogonal polynomials. Stability of the rivulet flow is analyzed and dispersion dependences for linear waves are obtained. Nonlinear wave regimes of a rivulet flow are numerically studied within the framework of two different problems, namely, the problem of stationary traveling waves with a given wavelength and the problem of spatial development of forced waves with a given frequency. Characteristics of nonlinear quasi-two-dimensional stationary traveling waves are obtained, and spatial development of forced waves is studied. Waves of various families are identified. It is shown that in a certain narrow range of excitation frequency, there are no stationary traveling waves, but a pulsating regime of flow occurs.

Extreme nonlinear optics in a long pulse regime: High harmonic generation of picosecond mid-IR pulses in polycrystalline zinc selenide.

High harmonic generation (HHG) in semiconductors has been extensively studied recently in the high-intensity limit using middle infrared (mid-IR) femtosecond laser pulses resulting in emission spectra of self-phase modulated harmonics resting on top of a broadband continuum. In this report, a different approach to HHG in polycrystalline zinc selenide (poly-ZnSe) was explored utilizing a relatively low power regime (1-40 GW/cm2) and much longer (30 ps) mid-IR laser pulses. Through a combination of low power, picosecond excitation, and narrowband (<10 nm full width at half maximum) mid-IR excitation, the nonlinear optical effects in poly-ZnSe could be isolated and studied independently. From the clearly distinguishable HHG peaks, harmonic conversion efficiencies of 10-4-10-12 for second to ninth harmonic in poly-ZnSe were measured, and the relationship between the Nth harmonic intensity and excitation intensity (I0) was found to follow a power law, I0x with x ≤ N/2, as a result of the random quasi-phase matching process.

Generation of super high frequencies by graphene on piezoelectric buffer layer upon applied nanosecond electrical pulse

An unforeseen phenomenon occurred when measuring the current–voltage (I–U) characteristics of graphene samples in the pulsed regime. The graphene monolayer of the samples was grown by chemical vapor deposition and transferred onto 50 nm buffer HfO2 and AlO3 layers. Bilayer graphene was obtained from epitaxial graphene grown on SiC and was transferred onto 100 nm SiO2. The distance between contacts varied from 5 to 25 µm. The measurements were performed employing cables of 50 Ω impedance to transmit the pulses and a 0–12 GHz bandwidth sampling scope was used to register them. The transmitted registered pulses through graphene/HfO2 and graphene/SiO2 samples were superposed with super high frequency (SHF) oscillations. The wave packet oscillations of about 2 ns in duration appeared on the pulses passed through graphene/HfO2. On the passed-through graphene/SiO2 pulses, the appearing wave packet turns into continuous generation along the pulse length. Spectral decomposition reveals that the base frequency of the oscillations is about 10 GHz; however, bisected frequencies are also present in the spectra. Preliminary analysis of the SHF oscillations occurrence shows that the wave packets appear as an inverse piezoelectric response to a rapidly rising voltage of the pulses. It is proposed that due to acoustic–electronic interaction, the oscillating wave packet in graphene/SiO2 turns into the continued generation, which consumes three-quarters of the pulse energy. These results show that the coupling between graphene and the piezoelectric buffer layer combined with acoustic–electronic interaction brings forth novel effects that are in demand for practical applications.

Intermediate state between a solitary singlet and a molecule in lasers

As an important tool in science and industry, the ultrafast fiber laser has two paradigmatic pulse regimes of the single-soliton (SS) and the soliton molecule (SM), which have been individually researched thoroughly. However, the mutual transformation between two states is a fuzzy problem with significance in understanding the soliton dynamics and actively manipulating lasing states. Here, we reveal an intermediate state that spontaneously switches between SS and SM without external triggering in experiments and simulations. Two critical processes dominating the switching are the soliton splitting and SM partial annihilation, which are found to be induced by the high-order soliton effect and spontaneous symmetry breaking, respectively. Moreover, statistics of this intermediate state reveal the spectral rogue waves induced by the SM with high spectral brightness. This work sheds light on the switching dynamics between nonlinear states in the out-of-equilibrium systems, and has potential for intelligent manipulation of lasing states.

Magnetic functionalization of ZnO nanoparticles surfaces via optically generated methyl radicals.

The combination of nuclear and electron magnetic resonance techniques, in pulse and continuous wave regimes, is used to unravel the nature and features of the light-induced magnetic state arising at the surface of chemically prepared zinc oxide nanoparticles (NPs) occurring under 120K when subjected to a sub-bandgap (405nm) laser excitation. It is shown that the four-line structure observed around g ∼ 2.00 in the as-grown samples (beside the usual core-defect signal at g ∼ 1.96) arises from surface-located methyl radicals (•CH3), originating from the acetate capped ZnO molecules. By functionalizing the as-grown zinc oxide NPs with deuterated sodium acetate, the •CH3 electron paramagnetic resonance (EPR) signal is replaced by trideuteromethyl (•CD3). For •CH3, •CD3, and core-defect signals, an electron spin echo is detected below ∼100K, allowing for the spin-lattice and spin-spin relaxation-time measurements for each of them. Advanced pulse-EPR techniques reveal the proton or deuteron spin-echo modulation for both radicals and give access to small unresolved superhyperfine couplings between adjacent •CH3. In addition, electron double resonance techniques show that some correlations exist between the different EPR transitions of •CH3. These correlations are discussed as possibly arising from cross-relaxation phenomena between different rotational states of radicals.

Fundamental and applied investigations of the liquid-metal cooled fast reactor thermal hydraulics (achieved results and further investigation issues)

The results of experimental and computational-theoretical investigations in the area of hydrodynamics and heat transfer of fast reactors and accelerator-driven system with liquid metal coolants are presented, and the problems and tasks for further investigations are formulated. Physical phenomena, effects, laws and process characteristics occurring in reactor power plants, including a flow path and a fast reactor core: the velocity and temperature fields, the structure and characteristics of turbulent transfer of momentum and energy, the hydraulic resistance of channels and fuel rod assemblies, the collector system hydrodynamics, the vibroacoustics, the heat transfer in channels and fuel rod assemblies with liquid metal cooling and inter-channel exchange are considered and analyzed. The data of experimental investigations are presented on a single-tube model of a large-module steam generator and on a fragmentary thermohydraulic model of a steam generator of a reactor power plant with twisted steam-generating tubes operating at subcritical and supercritical water pressure. The results of investigations of temperature and velocity fields on a small-scale water model of a fast reactor vessel with an integral layout in nominal, transient and emergency operating regimes are demonstrated. The investigation results on model of a fast reactor vessel with an integral layout are demonstrated that the effect of thermogravitational forces leads to temperature stratification with stagnant and recirculating formations, internal waves appear at the stratified boundaries, temperature pulsations, thermal fatigue of structural materials and a decrease in the service life of the equipment. It is shown that the liquid metal boiling process in fuel rod assemblies is formed under the influence of various factors, has a complex structure, is characterized by both stable (nucleate, annular-dispersed) and pulsation (slug) regimes with significant fluctuations of technological parameters, which can cause a crisis of heat transfer. Heat transfer was studied, a cartogram two-phase flow was obtained for liquid metal boiling in fuel rod assemblies, the effect of the surface roughness of fuel rods on liquid metal heat transfer and boiling regimes in fuel rod assemblies was found. The possibility of long-term stable cooling of the core during sodium boiling was shown by using “sodium cavity” over the reactor core. The kinetic and mechanical characteristics of the degradation process of the simulated fuel assembly in fast reactor core during the thermal interaction of the uranium-containing simulators of fuel with static sodium and their dependence on the parameters and design system were determined.Information about the key problems of thermophysical investigations of a high-temperature fast reactor with a sodium coolant for the production of hydrogen and reactors cooled by water at supercritical pressure is given.