Short Laser Pulses Research Articles

Theoretical model of passive mode-locking in terahertz quantum cascade lasers with distributed saturable absorbers

Abstract In research and engineering, short laser pulses are fundamental for metrology and communication. The generation of pulses by passive mode-locking is especially desirable due to the compact setup dimensions, without the need for active modulation requiring dedicated external circuitry. However, well-established models do not cover regular self-pulsing in gain media that recover faster than the cavity round trip time. For quantum cascade lasers (QCLs), this marked a significant limitation in their operation, as they exhibit picosecond gain dynamics associated with intersubband transitions. We present a model that gives detailed insights into the pulse dynamics of the first passively mode-locked QCL that was recently demonstrated. The presence of an incoherent saturable absorber, exemplarily realized by multilayer graphene distributed along the cavity, drives the laser into a pulsed state by exhibiting a similarly fast recovery time as the gain medium. This previously unstudied state of laser operation reveals a remarkable response of the gain medium on unevenly distributed intracavity intensity. We show that in presence of strong spatial hole burning in the laser gain medium, the pulse stabilizes itself by suppressing counter-propagating light and getting shortened again at the cavity facets. Finally, we study the robustness of passive mode-locking with respect to the saturable absorber properties and identify strategies for generating even shorter pulses. The obtained results may also have implications for other nanostructured mode-locked laser sources, for example, based on quantum dots.

On the Importance of Low-Frequency Signals in Functional and Molecular Photoacoustic Computed Tomography.

In photoacoustic computed tomography (PACT) with short-pulsed laser excitation, wideband acoustic signals are generated in biological tissues with frequencies related to the effective shapes and sizes of the optically absorbing targets. Low-frequency photoacoustic signal components correspond to slowly varying spatial features and are often omitted during imaging due to the limited detection bandwidth of the ultrasound transducer, or during image reconstruction as undesired background that degrades image contrast. Here we demonstrate that low-frequency photoacoustic signals, in fact, contain functional and molecular information, and can be used to enhance structural visibility, improve quantitative accuracy, and reduce spare-sampling artifacts. We provide an in-depth theoretical analysis of low-frequency signals in PACT, and experimentally evaluate their impact on several representative PACT applications, such as mapping temperature in photothermal treatment, measuring blood oxygenation in a hypoxia challenge, and detecting photoswitchable molecular probes in deep organs. Our results strongly suggest that low-frequency signals are important for functional and molecular PACT.

Diagnosis of indirectly driven double shell targets with point-projection hard x-ray radiography

We present an application of short-pulse laser-generated hard x rays for the diagnosis of indirectly driven double shell targets. Cone-inserted double shell targets were imploded through an indirect drive approach on the upgraded SG-II laser facility. Then, based on the point-projection hard x-ray radiography technique, time-resolved radiography of the double shell targets, including that of their near-peak compression, were obtained. The backlighter source was created by the interactions of a high-intensity short pulsed laser with a metal microwire target. Images of the target near peak compression were obtained with an Au microwire. In addition, radiation hydrodynamic simulations were performed, and the target evolution obtained agrees well with the experimental results. Using the radiographic images, areal densities of the targets were evaluated.

Changing the High Strength Steel Surface Properties with Femtosecond Laser Beam

Today, the automotive industry is undergoing rapid change. While manufacturers are constantly switching to fully or semi-electric hybrid models, the weight of the vehicles also increases significantly due to the extra weight of the batteries. Since the weight of vehicles has the most significant influence on their consumption and, with it, harmful emissions (even indirectly far from the place of use), manufacturers strive to keep weight under control with continuous improvements. One of the main directions of the developments is the use of new light but at the same time heavy-duty materials with a wide variety of material combinations. These new material pairings pose challenges to knitting technology solutions, which need to be developed similarly. In the course of our research, we are investigating how the surface properties can be optimized in the case of steel with increased strength for the automotive industry, without additional added material and changes visible to the naked eye. We subject the examined DP600 material to a short-pulse laser beam treatment, and we manage to change the surface structure so that the interface properties measured by wetting are significantly improved. The results are confirmed by electron microscopic examinations.

Investigation on laser absorption and x-ray radiation in microstructured titanium targets heated by short-pulse relativistic laser pulses

The enhancement effect of a microstructured surface on laser absorption and characteristic Kα emission has been investigated by measuring K-shell emission from titanium (Ti) targets irradiated with high-intensity (∼1020Wcm−2), subpicosecond (500 fs) laser pulses. The experimental results indicate a modest enhancement (1.6×) of Kα emission from microstructured targets compared to flat foils, but with a similar intensity and profile of Heα and Li-like satellites. Particle-in-cell (PIC) simulations are implemented to further understand the underlying physical processes in the laser interaction with both targets, interpreting the mechanisms responsible for the Kα enhancement. The reasons for the lower-than-expected enhancement of Kα emission are discussed. The rapid heating of the bulk plasma might result in the premature shutdown of Kα emission before the thermalization of hot electrons or even the end of laser pulses, suggesting that the use of Kα emission as a diagnostic of the hot-electron yield or relaxation could lead to a misinterpretation. Published by the American Physical Society 2024

Third order nonlinearity examined by pulsed and CW lasers: an organic urea barbituric acid (UBA) single crystal for optical limiting application with DFT study

Bulk size of urea barbituric acid single crystal which has not been reported earlier was successfully grown by slow evaporation solution growth method. Single-crystal XRD brought out the lattice constant and the crystal system is observed to be monoclinic with space group P. Functional group of UBA were determined by FT-IR. UBA crystal is entirely visible from 270 to 900 nm with linear optical energy gap value to be 4.50 eV. The observed HOMO–LUMO energy gap was 5.21 eV. Thermally, UBA crystal is found to be stable up to 184˚°C. The Cp value of UBA increases from 1.21 to 1.58 J g−1K−1 with the temperature difference from 30 to 100˚°C. UBA shows good photoconductive nature and it found to be positive and Laser Damage Threshold (LDT) value of UBA crystal is 0.97 GW cm−2. Hardness testing confirms that UBA crystal belongs to soft nature category. Dielectric properties of UBA crystal are studied as a function of frequency and temperature. Third order NLO properties of the UBA crystal were studied under CW and pulsed lasers (ns) regimes using Z-scan technique. Good Optical limiting (OL) response (2.2860 х 1012 W/m2) confirms the efficiency of UBA crystal to be used as optical limiters for protection towards short pulse lasers.

Solid substrate assisted enhanced laser induced breakdown spectroscopy for metal element analysis in aqueous solution.

Due to plasma quenching caused by the dense water medium, laser-induced breakdown spectroscopy (LIBS) faces challenges such as strong continuous background radiation and weak and broadened characteristic spectral lines when directly detecting metal elements in liquids. In this work, we introduced a simple approach to improve underwater LIBS signals with a solid substrate-assisted method, which requires no sample pre-treatment and simple operation and thus has potential for in situ marine applications. In this method, four submerged solid substrates (Zn, Cu, Ni, and Si) were employed to investigate the breakdown characteristics of underwater LIBS and the mechanism of spectral enhancement by using a CaCl2 solution. The results demonstrated a significant improvement in the detection sensitivity of Ca with these substrates even at a short laser pulse with a relatively low laser energy (10 mJ). Among them, the semiconductor Si substrate exhibited the best enhancement effect, with an enhancement factor of over 75 for the Ca ionic lines at 393.4 nm and 396.8 nm and an enhancement factor of 29 for the Ca atomic line at 422.7 nm, respectively. This is mainly because the presence of substrate decreases the breakdown threshold of the liquid sample, and a higher plasma excitation temperature and electron density are obtained, which, in turn, leads to higher signal intensity. Furthermore, significant plasma emission enhancements for a wide range of elements are also achieved from seawater. These findings can contribute to the development of compact underwater in situ LIBS sensors with low power consumption, while ensuring high detection sensitivity.

Влияние добавок карбида и борида вольфрама на структуру и микротвердость эквиатомного CrFeNi-покрытия, сформированного короткоимпульсной лазерной наплавкой

A coating based on a single-phase medium-entropy CrFeNi alloy with a face centered cubic structure has good ductility, relatively high anti-corrosion properties, low cost, but insufficient strength for its widespread use. It is assumed that adding strengthening particles in the form of tungsten carbides and borides to the CrFeNi equiatomic coating will lead to an increase in its mechanical properties. This work studies the influence of tungsten carbide and boride additives on the structure and microhardness of a CrFeNi equiatomic coating. The coatings were formed by layer-by-layer short-pulse laser cladding with preplaced powder on a multifunctional laser installation equipped with a solid-state laser with a lamp pump based on an Nd:YAG crystal. The change in phase composition when adding strengthening particles was detected using X-ray diffraction analysis and transmission electron microscopy (TEM). Both methods confirmed the precipitation of Cr23C6 chromium carbide in the deposited coatings. TEM photographs indicate that the precipitated phase is distributed along the grain boundaries of the -solid solution. The study found that the addition of 6 wt. % WC and 3 wt. % WB increases the level of microhardness of the CrFeNi coating by 26 % (from 340±6 to 430±12 HV 0.025). This occurs due to the presence of Cr23C6, WC particles in the structure and possible microdistortions of the crystal lattice of the -phase as a result of doping with tungsten atoms released during the dissolution of tungsten borides and carbides in the process of high-temperature short-pulse laser heating.

Role of neodymium laser in surgery: stimulation of postoperative surgical wounds healing. Results of clinical studies

To prove the effectiveness of the low-intensity laser radiation application in the treatment of wounds of different origin. The clinical study involved 110 persons, divided into 55 subjects in both the study and control groups. The patients of the study group were exposed to the long-wave short-pulse neodymium laser immediately and within 35 days after interventions with a skin incision using it, in a way that wound treated with laser received low-level laser therapy. The control group patients' wounds were treated with standard methods by the means of topical drugs corresponding to the clinical manifestations of the wound process in each particular case. The study was carried out in the hospital of the department of maxillofacial and plastic surgery of the dental complex of the «Russian University of Medicine» from 2019 to 2022, and further conservative treatment was conducted in the department of dermatology and cosmetology of the University Hospital of the Medical Graduate School (Institute) of the RSSU. All wounds were classified into three groups for the convenience of systematization and formation of a generalized treatment protocol for postoperative surgical wounds. The objectivity and optimality of the chosen by us actions were confirmed in the conducted work. The formed scars were visually assessed on the POSAS scale at the end of the treatment by patients and 4 independent doctors, as well as each scar was visually assessed by four independent doctors and patients. At the end of the study we formed and proposed an algorithm for the treatment of surgical wounds of various origins. The parameters of the Aerolase Neo Light Pod neodymium laser for the treatment of patients with different types of skin wounds were clinically determined. Experimentally proven properties of the Aerolase Neo Light Pod neodymium laser on accelerating the healing process of surgical wounds through photobiomodulation mechanism support their regeneration with the formation of negligible normotrophic scars, as well as reduce the length of patients' treatment in surgical hospitals, as compared to patients receiving standard external drugs.

Characterization of blast waves induced by femtosecond laser irradiation in solid targets

Abstract Blast waves have been produced in solid target by irradiation with short-pulse high-intensity lasers. The mechanism of production relies on energy deposition from the hot electrons produced by laser–matter interaction, producing a steep temperature gradient inside the target. Hot electrons also produce preheating of the material ahead of the blast wave and expansion of the target rear side, which results in a complex blast wave propagation dynamic. Several diagnostics have been used to characterize the hot electron source, the induced preheating and the velocity of the blast wave. Results are compared to numerical simulations. These show how blast wave pressure is initially very large (more than 100 Mbar), but it decreases very rapidly during propagation.

Extraordinary transmission of intense short laser pulse through thin foil with near- and sub-wavelength slits

To supplement our earlier study [Lu et al., AIP Adv. 10, 015209 (2020)] on the propagation of intense short laser pulse through foils with multiple sub-wavelength sized slits, here, we consider the effects of the interaction parameters on the occurrence of extremums in the light transmission. Our results suggest that for periodic slit distribution, maximum and minimum transmissions occur for L ± d = κλ and L = κλ, respectively, where L and d are the period and width of the slits, respectively, λ is the laser wavelength, and κ is a positive integer or a rational number.

Methods of Radiation Wavelength Tuning in Short-Pulsed Fibre Lasers

Methods of output wavelength tuning in short-pulsed fibre lasers are analysed. Many of them rely on spectral selection principles long used in other types of lasers. For compatibility with the fibre-optical format, the corresponding elements are sealed in compact, airtight volumes with fibre-optical radiation input and output. A conclusion is presented about the relatively small number of inherently “fibre-optical” ways of tuning the wavelength of radiation. It is demonstrated that the range of output wavelength tuning in short-pulsed fibre lasers may span hundreds of nanometres (even without extension beyond the active medium gain contour through nonlinear effects). From the presented review results, it may be concluded that the search for the optimal tuning method complying with the user-preferred all-PM-fibre short-pulsed laser design is not yet complete.

Synergic Effects of Nanosecond Laser Ablation and PVD-Coating on Cemented Carbides: Assessment on Surface and Mechanical Integrity

Emerging laser precision machining, particularly using pulsed lasers, enlightens the innovation and functionalization of cemented carbides. These backbone materials of the tooling industry are usually considered difficult to machine or shape using conventional mechanical approaches. The coating of cemented carbide tools, deemed to improve their mechanical and thermal properties, is a common supplementary surface treatment prior to their application. This work aims to study the synergic effects of nanosecond laser ablation and coating deposition on the surface, as well as the mechanical integrity of cemented carbides. In this regard, two plain WC–Co grades with different metallic binder content (10%wtCo and 15%wtCo) were first processed by a short-pulsed nanosecond laser. Subsequently, an AlTiN film was physically vapor-deposited on the laser-processed surfaces. The resulting surface integrity was assessed in terms of topographical, morphological, and microstructural changes. Mechanical integrity was evaluated in terms of indentation and sliding contact responses using Vickers hardness and scratch tests, respectively, the latter including frictional, penetrating, and sliding performances under selected surface processing conditions. In general, the nanosecond laser ablation proved to be beneficial for the mechanical integrity of coated cemented carbides in most studied cases, as it increased surface hardness, reduced penetration depth, and hindered damage during sliding. This was the case despite a slight increase in surface roughness, as well as minor morphological and microstructural changes at the coating–substrate interface, discerned.

Highly Collimated Monochromatic X-rays Generated by Collision of High-Energy Electrons with Tightly Focused Linearly Polarized Laser Pulse

This article delves into the generation and modulation process of X-rays as high-energy photon sources. Using the principles of classical electrodynamics, this study enables nonrelativistic short pulse lasers to collide with high-energy electrons while the collision center is away from the focal point. This scattering method may produce X-rays with good collimation and monochromaticity, and it progressively approaches inverse Thomson scattering. We studied and analyzed the effects of different electron characteristics and laser parameter settings on the high-energy angular distribution and spectrum of X-rays, especially the setting of the collision center and initial electron velocity, as well as the setting of laser intensity and pulse width. Linear polarized laser pulses with relativistic intensity can generate discrete supercontinuum X-rays with spectral distortion. In addition, the relationships between electronic and laser properties and radiation energy were also studied. Our research can provide valuable insights for manipulating collimated or distorted, monochromatic, or tunable X-rays, as well as understanding their properties.

Influence of material thickness and hatching strategies on laser cutting of epoxy mold composites

Glass-filled composites are used for overmolding of electrical components due to their good electrical isolation properties. Laser cutting is a preferred technology to remove excess mold material to achieve a low surface roughness and reduce tool wear. Hatching strategies improve the laser-cutting process of carbon fiber-reinforced polymers toward lower cutting times and a more homogeneous cut surface. The impact of hatching strategies on epoxy mold compounds has been so far unknown as the laser-cutting strategy was based on multiple single passes in previous studies. This work investigates the effects of hatching strategies such as perpendicular hatching, parallel hatching, and a single line, including the influence of material thickness and filler content regarding the cutting time, kerf taper angle, and heat-affected zone, using a 50 W short-pulsed fiber laser for different highly filled epoxy mold compounds. Results show that the use of a hatching strategy is required to cut workpieces at thicknesses of 4 mm or higher due to the sieving size of the filler. Perpendicular hatching needs to be chosen when the aim is a minimal cutting time. The kerf taper angle at the top of the cut is below 4° while hatching leads to a more pronounced kink of up to 25° occurring toward the bottom of the cut. Meanwhile, an increase in filler concentration leads to an increase in cutting time, because of higher thermal conduction, while no effect on the kerf taper angle or the heat-affected zone can be identified.

Enhancement of mid-infrared saturable absorption of graphene by compositing with PbSe for ultra-short pulse lasers in the ∼ 2.8 to ∼ 4 μm band

The suitably large modulation depth of the saturable absorber (SA) is crucial to obtaining short pulse lasers via passive Q-switching or mode-locking techniques. Graphene could theoretically be used as a mid-infrared (mid-IR) SA due to its zero band-gap. However, the low modulation depth of graphene in the mid-IR region limits its applications. In this work, to improve the mid-IR modulation depth of graphene, a strategy of functionalizing graphene with narrow-band gap semiconductors was adopted. Graphene/PbSe (G/PbSe) composite was synthesized by a facile hydrothermal process and the reflective G/PbSe SA was prepared by spin-coating method. The saturation absorption properties were investigated by the open aperture Z-scan technique. The results indicated that the saturation absorption properties of G/PbSe SA were largely enhanced, and the modulation depth was increased from 3.02 % to 5.4 % at ∼ 2.8 μm and from 2.5 % to 6.4 % at ∼ 4 μm compared with pure graphene. Moreover, with the as-prepared reflective G/PbSe SA, passively Q-switched Er:YAP laser operation at ∼ 2.8 µm was realized. At the pump power of 6.78 W, the pulse train with pulse width of 326 ns and repetition rate of 107 kHz was obtained, and the average output power was 498 mW. This study presents a way for preparing graphene-based mid-IR SAs for short pulse mid-IR lasers.

Optothermal Needle-Free Injection of Vaterite Nanocapsules.

The propulsion and acceleration of nanoparticles with light have both fundamental and applied significance across many disciplines. Needle-free injection of biomedical nano cargoes into living tissues is among the examples. Here a new physical mechanism of laser-induced particle acceleration is explored, based on abnormal optothermal expansion of mesoporous vaterite cargoes. Vaterite nanoparticles, a metastable form of calcium carbonate, are placed on a substrate, underneath a target phantom, and accelerated toward it with the aid of a short femtosecond laser pulse. Light absorption followed by picosecond-scale thermal expansion is shown to elevate the particle's center of mass thus causing acceleration. It is shown that a 2µm size vaterite particle, being illuminated with 0.5W average power 100fsec IR laser, is capable to overcome van der Waals attraction and acquire 15msec-1 velocity. The demonstrated optothermal laser-driven needle-free injection into a phantom layer and Xenopus oocyte in vitro promotes the further development of light-responsive nanocapsules, which can be equipped with additional optical and biomedical functions for delivery, monitoring, and controllable biomedical dosage to name a few.

First indirect drive inertial confinement fusion campaign at Laser Megajoule

The first indirect drive Inertial Confinement Fusion (ICF) experiments on the Laser Megajoule facility were carried out with approximately 150 kJ of laser energy distributed on 48 beams (12 quads) arranged in two cones. The target consisted of a gold vacuum rugby-shaped hohlraum and a plastic capsule located at its center, filled with deuterium gas fuel. The arrangement of the 12 quads is such that the laser irradiation on the wall generated a three-dimensional (3D) x-ray flux around the capsule creating 3D deformations on the imploding plastic shell. This constraint forced the design of a robust target (relatively thin ablator, around 40 μm) driven by a short laser pulse (3 ns) that delivered about 1011 neutrons. Full-integrated 3D radiation hydrodynamics simulations allowed both the target definition and the data interpretation (mainly radiation temperature, x-ray images, and neutron yield). 3D calculations and experiments compare well.

Generation and Filtering of Ultrashort Pulses When a Short Intense Laser Pulse is Reflected from Solid-State Targets

Generation and Filtering of Ultrashort Pulses When a Short Intense Laser Pulse is Reflected from Solid-State Targets

Experimental study on the implementation method of short pulse laser in distance-selective imaging system

Conventional distance-selective imaging systems use lasers that are large in size, high in power consumption, and high in cost. In order to reduce the system size and reduce the system power consumption and cost, The principles and design methods of two drive circuits for generating narrow pulse lasers based on step recovery diodes SRD (combined with shorted transmission lines) and RF bipolar transistors are discussed, physically fabricated and tested, and the characteristics of the two pulse generators and the factors affecting the pulse width amplitude are analyzed. The experimental results show that the SRD-based method can generate a narrow pulse with a rise time of 456.8 ps, a fall time of 458.3 ps, a pulse width of 1.5 ns, and an amplitude of 2.38 V; the transistor-based method can generate a narrow pulse with a rise time of 903.5 ps, a fall time of 946.1 ps, a pulse width of 824 ps, and an amplitude of 2.46 V, both of which can reach a repetition frequency of 50 MHz. Both design methods can be combined with an external laser diode to achieve excellent short pulse laser output.