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Localized foaming of polymers is promising to fabricate polymeric composite structures for applications in biomedical, aerospace, automotive, etc. However, the severe pyrolysis and carbonization of polymer chains during localized foaming may degrade the bulk strength. Herein, a mild localized foaming strategy is proposed to fabricate poly(lactic acid) (PLA) micro‐foams within dense PLA matrices using ultrafast lasers. The PLA substrates doped with graphene (Gr) and azodicarbonamide (AC) are developed for mild foaming. A picosecond laser is applied to pattern PLA micro‐foams on the PLA/Gr/AC substrates to fabricate porous/nonporous micro‐composite structures. The thermal behavior of the PLA/Gr/AC substrates is characterized to evaluate their capability in photothermal conversion and mild foaming during ultrafast laser pulses. To optimize the laser foaming process, the microstructure of the laser‐foamed PLA processed with different laser parameters is studied. It demonstrates that the oxidation of the functional groups dominates the side reactions on the PLA chains with slight variation in molecular weight during laser foaming, indicating that the PLA chains keep almost intact and mild localized foaming is realized. Furthermore, the developed technology is demonstrated for micro‐patterning and texturing, material toughness programming, and shape memory programming, showing promising applications for smart materials, metamaterials, micro‐robotics, and biological scaffolds. [ABSTRACT FROM AUTHOR]

Commercialization of photoelectrochemical (PEC) water-splitting technology is hindered by the slower kinetics of oxygen evolution in the currently available photoanodes. Light absorption, charge-separation, and charge-transfer efficiencies determine the kinetics of the oxygen evolution reaction (OER) at semiconductor photoanode surfaces. All these parameters can be maximized by nanotexturing the photoanode surface through morphological (cones, pyramids, grids, etc.) control and crystallographic (facets) orientation. The primary objective of texturing on conducting substrates is to improve the photoconversion efficiency in PEC devices through an increased surface area and light absorption. Laser-assisted ablation and melt-fusion additive techniques allow for rapid iteration of morphological architecture designs for optimized light absorption properties. This work reviews various ultrafast laser techniques employed for fabricating nanotextured surfaces and their impact on PEC performance. Compared with conventional electrode fabrication techniques, laser-based surface nanotexturing techniques provide a cost-effective, rapid design, and validation method for the research and development of photoelectrochemical water splitting technologies. [ABSTRACT FROM AUTHOR]

Optical frequency combs have revolutionized frequency metrology and spectroscopic measurements, enabling the most precise measurements of all physical quantities. However, precision frequency metrology heavily relies on mode‐locked laser combs, which are only directly available for a few selected near‐infrared wavelength ranges. Recently, a strong tendency emerged for combs in the mid‐infrared molecular fingerprint region. To this end, several methods for wavelength conversion have been proposed and demonstrated, which nevertheless cost a degradation of coherence properties. Here a first approach is presented toward measuring resulting phase noise fluctuations, exploiting the temporal resolution of the dispersive temporal interferometry (DTI) technique for experimentally unveiling the carrier‐envelope phase dynamics of a cw‐seeded femtosecond optical parametric amplifier (OPA). Particularly, it is experimentally demonstrated for the first time that unexpectedly low seed power levels suffice to exceed vacuum fluctuations by more than an order of magnitude, resulting in pulse‐to‐pulse phase fluctuations of 82 mrad at rather moderate cw seed levels of 8 mW. Additionally, a formula is provided that allows exact prediction of the experimentally observed noise levels. This study therefore provides new insight into the role of vacuum fluctuations in OPAs, which open up an avenue for coherent dual‐comb systems in the mid‐infrared and beyond. [ABSTRACT FROM AUTHOR]

Based on the ultrafast and extremely strong interaction between laser pulses and materials, ultrafast laser irradiation can break the solid solubility constraints and enable hyperdoping of impurities. This process overcomes the bandgap constraints of crystalline silicon, resulting in heightened absorption across a broad spectral range spanning from ultraviolet to infrared wavelengths, therefore commonly referred to as black silicon (b‐Si). The b‐Si demonstrates significant changes in optoelectronic properties, making it highly promising for applications in silicon photonics. Specifically, b‐Si photodetectors exhibit distinct advantages in terms of high photoelectric gain at low voltage, ultrabroadband spectral responsivity, large dynamic range, and suitability for operation over a wide temperature range. These properties address the limitations of traditional silicon photodetectors, showcasing great potential for applications in optoelectronic integration, artificial intelligence, information technology, energy devices, and beyond. This review focuses on b‐Si achieved through ultrafast laser processing, with a special emphasis on its applications in photodetectors. The mechanism of ultrafast laser irradiation and the properties of hyperdoped silicon are discussed. Then, the research progresses and state‐of‐the‐art b‐Si photodetectors are introduced, as well as working mechanism and potential application expansion. Finally, the development prospects of b‐Si photodetectors based on ultrafast laser hyperdoping are predicted. [ABSTRACT FROM AUTHOR]

This study reports new types of passive mode-locked Er-doped fiber laser (EDFL) based on a segment of doped fiber saturable absorber (DFSA) with Tm/Ho-doped fiber (THDF), Yb-doped fiber (YDF), and Er-doped fiber (EDF). By employing THDF-SA, a bright pulse sequence with a fundamental repetition rate of 17.86 MHz was obtained. In addition, various mode-locked output states, including dark pulses, dark–bright pulse pairs, bright–dark pulse pairs, and second-harmonic pulses, were obtained through polarization modulation and gain modulation, and the orthogonality of dark–bright pulses in both polarization directions was verified. Furthermore, using EDF-SA and YDF-SA, dark pulses and dark–bright pulses were obtained. A comparison of the three experiments revealed that THDF-SA effectively reduces the mode-locked threshold and improves the average output power. Compared with bright pulses, dark pulses offer several advantages such as resisting noise, increasing propagation speed, and suppressing nonlinear scattering (such as pulse-intrinsic Raman scattering); thus, the EDFL can find broad application in long-distance transmission, precision measurement, and other fields. [ABSTRACT FROM AUTHOR]

We have developed a cryogenic characterization platform for ultrafast photodiodes, whose time domain responses are extracted by electro-optic sampling using femtosecond laser pulses in a pump-probe configuration. The excitation of the photodiodes with the pump beam and the electro-optic sampling crystals with the probe beam are realized in a fully fiber-coupled manner. This allows us to use the characterization platform at different temperatures, ranging from cryogenic to room temperature. As an application example, we characterize the time-domain response of commercial p-i-n photodiodes with a nominal bandwidth of 20 GHz and 60 GHz at temperatures of 4 K and 300 K and in a large parameter range of photocurrent and reverse bias. For these photodiodes, we detect frequency components up to approximately 250 GHz, while the theoretical bandwidth of our sampling method exceeds 1 THz. Our measurements demonstrate a significant excitation power and temperature dependence of the photodiodes' ultrafast time responses, reflecting, most likely, changes in carrier mobilities and electric field screening. Since our system is an ideal tool to characterize and optimize the response of fast photodiodes at cryogenic temperatures, it has a direct impact on applications in superconducting quantum technology such as the enhancement of optical links to superconducting qubits and quantum-accurate waveform generators. [ABSTRACT FROM AUTHOR]

Er3+-ions doped SiO2-ZnO-Na2O thin films were fabricated using ultrafast laser plasma doping (ULPD) techniques under different ambient atmospheres; vacuum, nitrogen, oxygen and argon gas. The thickness of the layer produced depends on the ambient atmosphere during fabrication. The layer fabricated under a vacuum is the thinnest among all of the samples. In addition, the surface layer for the sample fabricated under a vacuum environment seems to be relatively smoother compared with those of the others. XRD patterns show that all samples are in a mixed amorphous-crystalline phase. All the Raman spectra exhibited a similar pattern, except for the intensity of the Si peak which depended on the thickness of the obtained layer. The PL intensity for each sample corresponds to the amount of Er3+ ions embedded in the doped layer. However, all samples still exhibited silicate-based characteristics, indicating nitrogen in Si3N4 was lost in the form of nitrogen gas during fabrication. [ABSTRACT FROM AUTHOR]

The modification of the surface topography at the micro- and nanoscale is a widely established as one of the best ways to engineering the surface of materials, to improve the tribological performances of materials in terms of load capacity and friction. The present paper reviews the state of the art on laser surface texturing by exploiting the technique of direct laser writing for tailoring the coefficient of friction, highlighting the effect of the textures' arrangement on the lubricated conformal and non-conformal contact behavior. [ABSTRACT FROM AUTHOR]

Femtosecond laser material processing (FLMP) was used to make an X-ray mask in a 500 µm thick tungsten sheet without the use of any chemical etch methods. The laser produced an 800 nm wavelength at a 1 kHz repetition rate and a pulse width of 100 fs. The laser beam arrival at the tungsten sheet was synchronized to a computer numerically controlled (CNC) stage that allowed for motion in the XYZθ directions. The X-ray mask design was made using CAD/CAM software (Alphacam 2019 R1) and it consisted of linear, circular, and 45° angle features that covered an area of 10 mm × 10 mm. A total of 70 laser beam passes at a moderate laser energy of 605.94 J/cm2 were used to make through-cut features into the tungsten sheet. The morphology of the top view (laser incident, LS) images showed cleaner and smoother cut edges relative to the bottom view (laser exit, LE) images. It was found that the size dimensions of the through-cut features on the LE surfaces were better aligned with the CAD dimensions than those of the LS surfaces. The focused laser beam produced inclined cut surfaces as the beam made the through cut from the LS to the LE of the tungsten sheet. The circular features at the LS surface deviated toward being oval-like on the LE surface, which could be compensated for in future CAD designs. The dependence of the CNC processing speed on the thickness of the etch depth was determined to have a third-order exponential decay relationship, thereby producing a theoretical model that will be useful for future investigators to predict the required experimental parameters needed to achieve a known etch depth in tungsten. This is the first study that has demonstrated the capability of using a femtosecond laser to machine through-cut an X-ray mask in a 500 µm thick tungsten sheet with no involvement of a wet etch or any other such supporting process. [ABSTRACT FROM AUTHOR]

In the last decade, there has been a proliferation of laser sources for time- and angle-resolved photoemission spectroscopy (TR-ARPES), building on the proven capability of this technique to tackle important scientific questions. In this review, we aim to identify the key motivations and technologies that spurred the development of various laser sources, from frequency up-conversion in nonlinear crystals to high-harmonic generation in gases. We begin with a historical overview of the field in Section 1, framed by advancements in light source and electron spectrometer technology. An introduction to the fundamental aspects of the photoemission process and the observables that can be studied is given in Section 2, along with its dependencies on the pump and probe pulse parameters. The technical aspects of TR-ARPES are discussed in Section 3. Here, experimental limitations such as space charge and resultant trade-offs in source parameters are discussed. Details of various systems and their approach to these trade-offs are given in Section 4. Within this discussion, we present a survey of TR-ARPES laser sources; a meta-analysis of these source parameters showcases the advancements and trends in modern systems. Lastly, we conclude with a brief discussion of future directions for TR-ARPES and its capabilities in elucidating equilibrium and non-equilibrium observables, as well as its integration with micro-ARPES and spin-resolved ARPES (Section 5). [Display omitted] [ABSTRACT FROM AUTHOR]

A coherent beam combination has the potential to revolutionize high-peak-power laser systems. However, achieving a high-average-power ultrashort pulse is difficult due to the accumulation of a nonlinear phase and gain narrowing. In this article, we demonstrate a coherent beam combination system that does not require pulse shaping or a spectral modulator. By optimizing the gain of each amplifier and using highly integrated optical components, we reduce the limitations caused by the accumulation of a nonlinear phase and gain narrowing. In our study, we used a polarization beam splitter to combine the pulses from two rod-type photonic crystal fibers (PCFs) in a Mach–Zehnder-type interferometer. A piezo-mounted mirror controlled with a Hänsch–Couillaud polarization detecting system was used to stabilize active phase locking. The system produces 165 W with a 91.6% combining efficiency compared to 90 W per amplifier. Compressed pulses with an energy of 132 µJ and Gaussian fitting pulse duration of 330 fs were achieved. [ABSTRACT FROM AUTHOR]

The basic mechanisms of laser interaction with synthetic diamond are reviewed. The characteristics of the main regimes of diamond surface etching are considered. In addition to the well-known graphitization and ablation processes, nanoablation and accumulative graphitization, which have attracted relatively recent attention, are described in detail. The focus is on femtosecond (fs) laser exposure, which allows for the formation of a dense cold electron–hole plasma in the focal zone and minimal overheating in the surrounding area. This potentially opens the way to the development of unique laser-based technologies that combine physical and chemical processes for precise surface treatment and functionalization. The physical limitations that determine how precisely the diamond surface can be treated by short-pulsed laser radiation and possible ways to overcome them with the ultimate goal of removing ultrathin layers of the material are discussed. Special attention is paid to the novel possibility of inducing the local formation of point active defects—nitrogen vacancy (NV) complexes in the laser-irradiated zone. Such defects have been at the forefront of solid-state physics for the past thirty years due to continuous attempts to exploit their unique properties in quantum optics, quantum computing, magnetometry, probing, and other fields. Both regimes of NV center formation with and without graphitization of the diamond lattice are considered. Thus, it is shown that intense pulsed laser irradiation is a perfect tool for the processing of synthetic diamonds at the micro-, nano-, and even at the atomic level, which can be well controlled and managed. [ABSTRACT FROM AUTHOR]

Applications of mode-locked fiber lasers benefit from robust and self-starting mode-locking, spectral tuning, high pulse energy and high average power. All-polarization-maintaining (PM) fiber lasers mode-locked with a phase-biased nonlinear amplifying loop mirror (NALM) have been shown to be very robust and reliably self-starting, and provide either spectral tuning or high pulse energy, but not both. We report on a simple method for concurrent spectral tuning and nanojoule-level pulse energy scaling of an all-PM phase-biased NALM mode-locked Yb:fiber laser, which we demonstrate over a 54 nm tuning range, reaching up to 1.67 nJ pulse energy and 126 mW average power. Unlike other laser configurations, our results show that net normal dispersion is not necessary or optimal for scaling the pulse energy of this type of mode-locked fiber laser.

This study reports new types of passive mode-locked Er-doped fiber laser (EDFL) based on a segment of doped fiber saturable absorber (DFSA) with Tm/Ho-doped fiber (THDF), Yb-doped fiber (YDF), and Er-doped fiber (EDF). By employing THDF-SA, a bright pulse sequence with a fundamental repetition rate of 17.86 MHz was obtained. In addition, various mode-locked output states, including dark pulses, dark–bright pulse pairs, bright–dark pulse pairs, and second-harmonic pulses, were obtained through polarization modulation and gain modulation, and the orthogonality of dark–bright pulses in both polarization directions was verified. Furthermore, using EDF-SA and YDF-SA, dark pulses and dark–bright pulses were obtained. A comparison of the three experiments revealed that THDF-SA effectively reduces the mode-locked threshold and improves the average output power. Compared with bright pulses, dark pulses offer several advantages such as resisting noise, increasing propagation speed, and suppressing nonlinear scattering (such as pulse-intrinsic Raman scattering); thus, the EDFL can find broad application in long-distance transmission, precision measurement, and other fields.

When a laser cavity supports the propagation of several ultrashort pulses, these pulses can form compact bound states called soliton molecules. Soliton molecules are fascinating objects of nonlinear science, presenting striking analogies with their matter molecule counterparts. The relative timing and phase between the copropagating pulses are the most salient internal degrees of freedom of the soliton molecule. The soliton pair, composed of two identical pulses, constitutes the chief soliton molecule of fundamental interest. Its two major internal degrees of freedom allow self‐oscillating soliton molecules, which have been recurrently observed. However, despite theoretical predictions, the low‐dimensional chaotic dynamics of a soliton‐pair molecule remain elusive. This article reports the observation of chaotic soliton‐pair molecules within an ultrafast fiber laser by means of a direct measurement of the relative optical pulse separation with sub‐femtosecond precision in real time. Moreover, it demonstrates an all‐optical control of the chaotic dynamics followed by the soliton molecule by injection of a modulated optical signal that resynchronizes the internal periodic vibration of the soliton molecule. The fast error‐free switching between ordered and chaotic soliton molecules enabled by pump current sweeping and external injection highlights the potential prospects of all‐optical logic gates and chaotic communication using soliton molecules. [ABSTRACT FROM AUTHOR]

Ultrafast mid-infrared fiber lasers have been intensely studied in the last decade for the generation of high harmonics, molecular spectroscopy, material processing and remote sensing. Different designs have been investigated but most of them lacked the ease of use and reliability needed for their democratization. In this paper, we demonstrate a self-starting mode-locked mid-IR erbium-doped fiber laser based on a SESAM and a broadband uniform FBG that produces pulses as short as 15 ps. Different laser cavities were tested with varying FBG peak reflectance, spectral bandwidth and active fiber length. In addition, one cavity uses a pump combiner instead of injecting free-space the pump power through the fiber tip. The results of this study confirm that the FBG spectral bandwidth can efficiently control the duration of the almost Fourier-transform-limited pulses up to a limit seemingly dictated by the presence of water vapor in the laser cavity acting as narrow spectral filters. To a lower effect, the active fiber length influences the pulse duration. Finally, the use of an all-fiber pump combiner allows for a more compact and rugged design without altering the laser performances. This study represents a step towards the development of robust mid-infrared ultrafast all-fiber lasers. [ABSTRACT FROM AUTHOR]

In this submission, we report a maximum 638-W fiberized chirped-pulse-amplification (CPA) laser system with a bulk reflection grating compressor. The system is able to emit 338-W compressed pulses of 624 fs centered at 1 µm, corresponding to a pulse energy of 1.3 µJ at a pulse repetition rate of 258 MHz. The output beam quality factor M2 is measured to be

Hybrid supercapacitors use electric double‐layer capacitance and Faradaic pseudocapacitance as energy storage mechanisms. This type of supercapacitor is becoming a prime candidate for next‐generation energy storage devices, with advantages in terms of energy density, specific capacitance, and life cycle. However, reducing the electrode area and increasing the specific capacitance of hybrid supercapacitors remain challenging. In this study, a MoCl5 Precursor‐assisted Ultrafast Laser Carbonization (MPAULC) method to fabricate symmetric hybrid supercapacitors with improved capacitance and reduced size is proposed. The method uses an ultrafast laser to induce the formation of carbon/MoO3 composite with the assistance of the MoCl5 precursor. This ultrafast laser carbonization method exhibited high processing precision. The role of the precursor in laser processing is studied using time‐resolved imaging and temperature calculations. The specific area capacitance of the C/MoO3 hybrid supercapacitor is 11.85 mF cm−2, 9.2 times higher than that of the laser‐induced carbon supercapacitor without precursor. The MPAULC method provides a reliable pathway for fabricating miniaturized hybrid supercapacitors with carbon/metal oxide composite electrodes on polymer substrates. [ABSTRACT FROM AUTHOR]

Abstract Ultrafast Raman fiber laser has been proved to be an effective method to obtain ultrafast optical pulses at special wavelength. Yet, compared with conventional rare-earth doped counterparts, it is challenging for Raman fiber lasers to generate pulses with high pulse energy and short pulse duration. Here, we review three categories of ultrafast Raman fiber laser technologies and give detailed discussions on the advantages and challenges of each. In regards to mode-locking, different saturable-absorbers-based fiber lasers are compared and their common problem resulting from long cavity length are discussed. In terms of synchronously-pumping, several approaches to match the repetition rate of pulsed pump with the length of Raman fiber cavity are discussed, while the technical complexity of each method is analyzed. Moreover, the recently developed technology termed as nonlinear optical gain modulation (NOGM) is introduced, which turns out to be a simple and quality solution to generate high-energy femtosecond pulses with wavelength agility. Compared with the others, NOGM gathers various advantages including simple structure, long-term stability, high pulse energy and short pulse duration, which may effectively promote application expansion of ultrafast Raman fiber laser in the near future.

Superficial modifications on silicon wafers produced by single-shot focused femtosecond laser irradiation having a 1030 nm wavelength and 300 fs pulse duration were experimentally and theoretically analyzed. The laser fluence window when the amorphous silicon phase develops, resulting in a ring-like modification shape, was experimentally estimated to be between 0.26 J/cm 2 and 0.40 J/cm 2 and was independent of the silicon dopant type and laser focusing conditions; however, the window was narrower when compared to results reported for shorter pulse durations. In addition, we present a simplified numerical model that can explain and predict the formation of these patterns based on the caloric coefficients of silicon and the energy distribution of the deposited material. [ABSTRACT FROM AUTHOR]

The electron response of nanoplasmon and nanoplasma in the laser field is greatly important for improving the functionality and efficiency of many potential applications. However, how and under what conditions the nanoplasmon‐nanoplasma transition works remains poorly understood due to radiation damage and charge buildup. Utilizing the combined aerodynamic lens and velocity map imaging spectrometer, this transition mediated by different mechanisms are demonstrated, as verified by the distinct photoelectron momentum distributions from Cu nanoparticles. Initially the polarization‐dependent distributions emphasize the domination of surface emission driven by the plasmonic field. Subsequently the transition starts and the competition of volume‐multiphoton and thermionic emission plays the intermediate state dominating the transition process. Finally, a complete plasma is generated with the leading role of thermionic emission, which is confirmed by first observing the correlated electronic decay in metal nanoparticles. These findings bridge the gap between electron emission in nanoplasmonic and nanoplasma states and identify the mechanism‐specific contributions, which will offer general principles for designing nanostructure and laser fields to control electron excitation and emission in practice. [ABSTRACT FROM AUTHOR]

Period doubling is a universal bifurcation of central importance in all disciplines of nonlinear science, which generally signals the existence of chaotic dynamics in the vicinity of the system parameters. Although observed in diverse ultrafast laser configurations, there is still no consensus on its physical origin. Real‐time spectral characterization techniques have recently allowed revisiting pulsating dynamics, from period‐2 to long‐period pulsations. Following a contextual review, this article presents a variety of bifurcation sequences entailing the spectral pulsations of dissipative solitons. These experiments, using ultrafast fiber lasers operated in both chromatic dispersion regimes, are confronted with numerical simulations to demonstrate that self‐phase modulation represents a general mechanism triggering period‐2 bifurcations. In addition, by ramping up the pump power, original sequences of period‐doubling bifurcations intertwined with more complex bifurcations are presented, where entrainment phenomena are unveiled. These findings provide a more general understanding of the period‐doubling bifurcation in ultrafast laser systems while highlighting their potentially intricate combinations with complex bifurcations, which may be exclusively observable from the spectral domain. [ABSTRACT FROM AUTHOR]

The laser‐irradiated aerosolized nanostructures with complex geometries offer unique opportunities to steer orientation‐dependent photoion emission for multiple applications of directional ion sources and photocatalysis. The orientation‐dependent photoion emission is characterized by measuring the distribution and probability on differently oriented nanostructures. This unique capability is achieved by introducing momentum‐to‐space orientation discrimination in the single‐shot velocity map imaging technique with the aerodynamic lens. The effect of geometry, polarization, and concentration to control the photoion emission from differently oriented nanostructures is introduced for optimizing the potential applications of photoion emission including directional tunable nanoanode and optical sensors. The mechanism behind controlling the photoion emission is visualized by the variations in the regions of high laser intensity inside the nanostructures responding to the orientation, geometry, and polarization. This work represents an advance in characterizing the photoion emission from the randomly oriented aerosolized nanostructures and facilitates the potential for combining with the pump–probe methods to provide an intuitive understanding of their real‐time ionization dynamics. [ABSTRACT FROM AUTHOR]

Ultrafast laser ablation is widely used as a versatile method for accurate micro-machining of polymers, glasses and metals for a variety of industrial and biomedical applications. We report on the use of a novel process parameter, the modulation of the laser pulse energy during the multi-scan texturing of surfaces. We show that this new and straightforward control method allows us to attain higher and lower roughness ( R a ) values than the conventional constant pulse energy irradiation sequence. This new multi-scanning laser ablation strategy was conducted on metals that are commonly used in the biomedical industry, such as stainless steel, titanium, brass and silver samples, using a linear (increasing or decreasing) gradient of pulse energy, i.e., varying the pulse energy across successive laser scans. The effects of ablation were studied in terms of roughness, developed interfacial area ratio, skewness and ablation efficiency of the processed surfaces. Significantly, the investigation has shown a global trend for all samples that the roughness is minimum when a decreasing energy pulse sequence is employed, i.e., the irradiation sequence ends up with the applied laser fluences close to threshold laser fluences and is maximum with increasing energy distribution. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) analysis on single craters with the three different energy deposition conditions revealed a chaotic and random material redistribution in the cases of uniform and increasing energy distributions and the presence of regular laser-induced periodic surface structures (LIPSS) at the bottom of the ablation region in the case of decreasing energy distribution. It is also shown that the ablation efficiency of the ablated surfaces does not significantly change between the three cases. Therefore, this novel energy control strategy permits the control of the roughness of the processed surfaces without losing the ablation efficiency. [ABSTRACT FROM AUTHOR]

Recent advances in additive manufacturing technologies have already led to the wide-scale adoption of 3D-printed parts in the aerospace, medical, automotive, tooling, and electronics industries. The expansion in choice of materials that can be processed, in particular, using fused deposition modeling (FDM), selective laser sintering/melting, and stereolithography, and the steady advancements in dimensional accuracy control, have extended the range of applications beyond rapid prototyping. However, additive manufacturing still has considerable limitations compared to traditional and subtractive manufacturing processes. This work addresses limitations associated with the as-deposited surface roughness of 3D-printed parts. The effects of roughness-induced stress concentrations on the mechanical strength were studied, and ultrafast laser postprocessing was utilized to reduce the surface roughness of 3D-printed parts. The samples were manufactured using a commercial desktop FDM system and standard ASTM flat dogbone geometries. The samples were then postprocessed with a high-repetition-rate ultrafast Yb-fiber laser using a multi-layer scan approach. This novel postprocessing method enables high-efficiency material removal without inducing excessive thermal residual stresses into the material and, therefore, is suitable for postprocessing thermally sensitive materials, such as PLA and other polymers as well as parts with engineered porosity. In this work, we vary laser process parameters, such as average power and number of laser-processed layers, to achieve various levels of surface roughness. Values of tensile strength of the specimens were compared between 3D-printed samples featuring initial roughness and laser postprocessed samples with different values of surface roughness. The results indicate that the laser-processed samples exhibit an almost 10% increase in tensile strength depending on specific laser processing parameters. [ABSTRACT FROM AUTHOR]

Two-dimensional (2D) carbides of transition metals, so called "MXenes", maintain excellent optical properties and unique layer stacking form via hydrogen bonds in contrast to other low-dimensional materials. These promising features have attracted increasingly research interest in the field of ultrafast photonics. In this work, titanium carbide (Ti3C2Tx) and tantalum carbide (Ta4C3Tx), as two promising MXene materials for photonic applications, are experimentally synthesized by the ultrasound-assisted liquid phase exfoliation technique. The morphology of prepared few-layer Ti3C2Tx and Ta4C3Tx has been systematically characterized by transmission electron microscopy and X-ray diffraction analysis. Density functional theory is performed to obtain their electronic band structures, demonstrating metallic properties and verifying their optical absorption at 1 μm. Their tempting optical modulation properties for multi-gigahertz pulsed laser emission are demonstrated by using them as saturable absorbers in a waveguide laser cavity. Particularly, high-performance Q-switched mode-locked lasers with/without laser mirrors are realized based on the hybrid waveguide laser configuration, delivering 1 μm laser pulses with durations as short as 30 ps. The results presented in this work show the great potential of metallic MXenes and waveguide structures for applications in functional photonic devices. [ABSTRACT FROM AUTHOR]

Abstract As one of the most fascinating phenomena in dissipative systems, soliton explosions have been intensively investigated in the field of nonlinear optics owing to their intriguing dynamics. However, few efforts have been dedicated to precisely manipulating the dynamics of soliton explosion in fiber lasers. Herein, the exploding dynamics of dissipative soliton in a passively mode‐locked fiber laser by engineering the intracavity dispersion with a spectral pulse shaper is investigated. It is found that the exploding interval of the dissipative soliton can be well manipulated by dynamically engineering the cavity dispersion. The real‐time spectral dynamics of the soliton explosions are investigated by virtue of the dispersion Fourier transform (DFT) technique. The experimental results are further reproduced by the numerical simulations. These findings demonstrated that the dispersion engineering approach is a promising way to manipulate the soliton exploding behavior, and will also shed new light into the dynamics of soliton explosions in dissipative optical systems.

We report on the generation of two-octave mid-infrared supercontinuum in stock, unprocessed, large core chalcogenide fiber pumped by a fiber-based laser delivering 35 kW, 180 fs pulses at 4.53 μm.

The modification of the surface topography at the micro- and nanoscale is a widely established as one of the best ways to engineering the surface of materials, to improve the tribological performances of materials in terms of load capacity and friction. The present paper reviews the state of the art on laser surface texturing by exploiting the technique of direct laser writing for tailoring the coefficient of friction, highlighting the effect of the textures’ arrangement on the lubricated conformal and non-conformal contact behavior.

Femtosecond laser material processing (FLMP) was used to make an X-ray mask in a 500 µm thick tungsten sheet without the use of any chemical etch methods. The laser produced an 800 nm wavelength at a 1 kHz repetition rate and a pulse width of 100 fs. The laser beam arrival at the tungsten sheet was synchronized to a computer numerically controlled (CNC) stage that allowed for motion in the XYZθ directions. The X-ray mask design was made using CAD/CAM software (Alphacam 2019 R1) and it consisted of linear, circular, and 45° angle features that covered an area of 10 mm × 10 mm. A total of 70 laser beam passes at a moderate laser energy of 605.94 J/cm2 were used to make through-cut features into the tungsten sheet. The morphology of the top view (laser incident, LS) images showed cleaner and smoother cut edges relative to the bottom view (laser exit, LE) images. It was found that the size dimensions of the through-cut features on the LE surfaces were better aligned with the CAD dimensions than those of the LS surfaces. The focused laser beam produced inclined cut surfaces as the beam made the through cut from the LS to the LE of the tungsten sheet. The circular features at the LS surface deviated toward being oval-like on the LE surface, which could be compensated for in future CAD designs. The dependence of the CNC processing speed on the thickness of the etch depth was determined to have a third-order exponential decay relationship, thereby producing a theoretical model that will be useful for future investigators to predict the required experimental parameters needed to achieve a known etch depth in tungsten. This is the first study that has demonstrated the capability of using a femtosecond laser to machine through-cut an X-ray mask in a 500 µm thick tungsten sheet with no involvement of a wet etch or any other such supporting process.

A coherent beam combination has the potential to revolutionize high-peak-power laser systems. However, achieving a high-average-power ultrashort pulse is difficult due to the accumulation of a nonlinear phase and gain narrowing. In this article, we demonstrate a coherent beam combination system that does not require pulse shaping or a spectral modulator. By optimizing the gain of each amplifier and using highly integrated optical components, we reduce the limitations caused by the accumulation of a nonlinear phase and gain narrowing. In our study, we used a polarization beam splitter to combine the pulses from two rod-type photonic crystal fibers (PCFs) in a Mach–Zehnder-type interferometer. A piezo-mounted mirror controlled with a Hänsch–Couillaud polarization detecting system was used to stabilize active phase locking. The system produces 165 W with a 91.6% combining efficiency compared to 90 W per amplifier. Compressed pulses with an energy of 132 µJ and Gaussian fitting pulse duration of 330 fs were achieved.

The development of optical metrology suited to ultrafast lasers has played a key role in the progress of these light sources in the last few decades. Measurement techniques providing the complete E-field of ultrashort laser beams in both time and space are now being developed. Yet, they had so far not been applied to the most powerful ultrashort lasers, which reach the PetaWatt range by pushing the chirped pulse amplification (CPA) scheme to its present technical limits. This situation left doubts on their actual performance, and in particular on the peak intensity they can reach at focus. In this article we present the first complete spatio-temporal characterization of a PetaWatt femtosecond laser operating at full intensity, the BELLA laser, using two recently-developed independent measurement techniques. Our results demonstrate that, with adequate optimization, the CPA technique is still suitable at these extreme scales, i.e. it is not inherently limited by spatio-temporal couplings. We also show how these measurements provide unprecedented insight into the physics and operation regime of such laser systems.

Two-dimensional (2D) ternary transition-metal dichalcogenides (TMDCs) are of great research interest because their superior layer-dependent optical modulation properties. In this work, three different kinds of TMDC nanosheets, including hafnium diselenide (HfSe2), titanium diselenide (TiSe2) and zirconium diselenide (ZrSe2), are prepared by liquid phase exfoliation (LPE) technique. The high-quality material properties of these TMDC nanosheets are confirmed by Raman spectroscopy and X-ray diffraction analysis. Furthermore, the bandgap information of five-layer MSe2 has been investigated via utilizing density functional theory. The calculation results exhibit ultra-narrow bandgap structure (lower than 1.1 eV) for all these three materials, indicating that MSe2 is suitable for broadband photonic applications. By applying the fabricated MSe2 as saturable absorbers, high-performance Q-switched mode-locked laser operation has been realized. The laser gain media are Nd:GdVO4 cladding waveguides fabricated by femtosecond laser direct writing. As a result, the pulsed waveguide lasers are able to deliver approximately 6-GHz laser pulses with a signal-to-noise ratio of over 45 dB. The minimum pulse width is determined to be as short as 26 ps. The results demonstrated in this work exhibit the great potential of TMDCs and waveguide structures in applications of pulsed lasers with compact footprints.

In this paper, we present a polarization-multiplexed system capable of generating two stable optical frequency combs with tunable frequency differences in the range from 100 to 250 Hz and an extinction ratio of 16.5 dBm. Also, the polarization dynamics of a dual-frequency comb generated from a single mode-locked Er-doped fiber laser are experimentally studied. The obtained results will extend the application to areas such as polarization spectroscopy and dual-comb-based polarimetry.

We present a new scheme of picosecond optical parametric chirped pulse amplification (OPCPA) in which a Fourier-transform-limit 5.0 ps pulse is optically sheared from a single-longitudinal-mode 1064 nm CW laser. The pulse is amplified and frequency-doubled as the pump in order to maintain the pump narrow bandwidth and picosecond duration simultaneously, which is very important to ensure the high temporal contrast for an OPCPA amplifier. Combined with the cross-polarized wave generation (XPW), a compound frontend for the high-power femtosecond laser system that delivers a 1 Hz chirped pulse train is established. The experiments provide an output pulse energy of 17.1 mJ, a spectrum bandwidth 71 nm (FWHM), and a pulse duration 16.4 fs. The pulse contrast reaches 1:10−12 several picoseconds before the peak of the main pulse, which is the best value of the available measuring instruments. [ABSTRACT FROM AUTHOR]

High-power mid-infrared 2.8- $\mu \text{m}$ ultrafast laser has potential applications in the fields of laser surgery due to its wavelength overlap with the absorption peak of water. In this work, we demonstrate a 2.8- $\mu \text{m}$ picosecond Raman laser in a methane-filled hollow-core anti-resonant fiber (HC-ARF) with average power of 4 W, pulse energy of 40 $\mu \text{J}$ , and beam quality of 1.47. The laser beam quality improvement at mid-infrared Raman conversion in methane-filled HC-ARF is firstly observed. This work heralds a promising approach to generate mid-infrared ultrafast Raman laser with multi-Watt level average power and high beam quality. [ABSTRACT FROM AUTHOR]

We report a compact ultrafast solid-state laser source with a pulse repetition rate continuously tunable in the range of 0.56–1.23 GHz. The optical cavity design allows a user to vary the repetition rate only by moving positions of two optical components inside the resonator, without a need to exchange components or realignment. The Yb:KYW crystal based cavity emits 250 femtosecond pulses at a central wavelength around 1040 nm, with a self-starting SESAM modelocking. In a robust modelocked operation mode, an average power up to 130 mW is achieved using a stabilized single mode pump diode at a wavelength of 981 nm, while the laser in continuous wave (CW) mode delivers up to 270 mW. Atmospheric air cooling was sufficient for both CW and modelocked laser operation. Quite low noise performance was observed, with carrier frequency SNR higher than 80 dB, although the measurements were taken with the open breadboard setup. [ABSTRACT FROM AUTHOR]

We report a compact ultrafast solid-state laser source with a pulse repetition rate continuously tunable in the range of 0.56–1.23 GHz. The optical cavity design allows a user to vary the repetition rate only by moving positions of two optical components inside the resonator, without a need to exchange components or realignment. The Yb:KYW crystal based cavity emits 250 femtosecond pulses at a central wavelength around 1040 nm, with a self-starting SESAM modelocking. In a robust modelocked operation mode, an average power up to 130 mW is achieved using a stabilized single mode pump diode at a wavelength of 981 nm, while the laser in continuous wave (CW) mode delivers up to 270 mW. Atmospheric air cooling was sufficient for both CW and modelocked laser operation. Quite low noise performance was observed, with carrier frequency SNR higher than 80 dB, although the measurements were taken with the open breadboard setup.

The former reports of coherent beam combining (CBC) in the ultra-intense ultra-short laser field focus on the phase control technology and two-beam CBC. To study the four-beam tiled-aperture CBC in the ultra-intense and ultra-short laser field, we demonstrate four-beam CBC of femtosecond laser pulses based on chirped pulse amplification (CPA) scheme. In this experiment, the four beams were compressed using two grating compressors to simulate the situation in ultra-intense and ultra-short laser CBC systems. To achieve real-time measurement of the phase error between the four beams, we introduced a continuous reference laser and the phases of beams 2, 3, and 4 were locked to beam 1.A combined efficiency of 57% was achieved. Although the coherent combining efficiency is not very high, the possibility of a four-beam CBC based on the CPA scheme was confirmed, especially with different grating compressors which can reduce the limitation of gratings in single beam petawatt lasers. Finally, the remaining difficulties in the implementation of CBC of four beams and the methods to improve its efficiency were analyzed. The use of CBC in ultra-intense and ultrashort laser systems is of considerable significance.

The basic mechanisms of laser interaction with synthetic diamond are reviewed. The characteristics of the main regimes of diamond surface etching are considered. In addition to the well-known graphitization and ablation processes, nanoablation and accumulative graphitization, which have attracted relatively recent attention, are described in detail. The focus is on femtosecond (fs) laser exposure, which allows for the formation of a dense cold electron–hole plasma in the focal zone and minimal overheating in the surrounding area. This potentially opens the way to the development of unique laser-based technologies that combine physical and chemical processes for precise surface treatment and functionalization. The physical limitations that determine how precisely the diamond surface can be treated by short-pulsed laser radiation and possible ways to overcome them with the ultimate goal of removing ultrathin layers of the material are discussed. Special attention is paid to the novel possibility of inducing the local formation of point active defects—nitrogen vacancy (NV) complexes in the laser-irradiated zone. Such defects have been at the forefront of solid-state physics for the past thirty years due to continuous attempts to exploit their unique properties in quantum optics, quantum computing, magnetometry, probing, and other fields. Both regimes of NV center formation with and without graphitization of the diamond lattice are considered. Thus, it is shown that intense pulsed laser irradiation is a perfect tool for the processing of synthetic diamonds at the micro-, nano-, and even at the atomic level, which can be well controlled and managed.

Two-dimensional (2D) ternary transition-metal dichalcogenides (TMDCs) are of great research interest because their superior layer-dependent optical modulation properties. In this work, three different kinds of TMDC nanosheets, including hafnium diselenide (HfSe2), titanium diselenide (TiSe2) and zirconium diselenide (ZrSe2), are prepared by liquid phase exfoliation (LPE) technique. The high-quality material properties of these TMDC nanosheets are confirmed by Raman spectroscopy and X-ray diffraction analysis. Furthermore, the bandgap information of five-layer MSe2 has been investigated via utilizing density functional theory. The calculation results exhibit ultra-narrow bandgap structure (lower than 1.1 eV) for all these three materials, indicating that MSe2 is suitable for broadband photonic applications. By applying the fabricated MSe2 as saturable absorbers, high-performance Q-switched mode-locked laser operation has been realized. The laser gain media are Nd:GdVO4 cladding waveguides fabricated by femtosecond laser direct writing. As a result, the pulsed waveguide lasers are able to deliver approximately 6-GHz laser pulses with a signal-to-noise ratio of over 45 dB. The minimum pulse width is determined to be as short as 26 ps. The results demonstrated in this work exhibit the great potential of TMDCs and waveguide structures in applications of pulsed lasers with compact footprints. [ABSTRACT FROM AUTHOR]

Ultrafast Raman fiber laser has been proved to be an effective method to obtain ultrafast optical pulses at special wavelength. Yet, compared with conventional rare-earth doped counterparts, it is challenging for Raman fiber lasers to generate pulses with high pulse energy and short pulse duration. Here, we review three categories of ultrafast Raman fiber laser technologies and give detailed discussions on the advantages and challenges of each. In regards to mode-locking, different saturable-absorbers-based fiber lasers are compared and their common problem resulting from long cavity length are discussed. In terms of synchronously-pumping, several approaches to match the repetition rate of pulsed pump with the length of Raman fiber cavity are discussed, while the technical complexity of each method is analyzed. Moreover, the recently developed technology termed as nonlinear optical gain modulation (NOGM) is introduced, which turns out to be a simple and quality solution to generate high-energy femtosecond pulses with wavelength agility. Compared with the others, NOGM gathers various advantages including simple structure, long-term stability, high pulse energy and short pulse duration, which may effectively promote application expansion of ultrafast Raman fiber laser in the near future. [ABSTRACT FROM AUTHOR]

The synthesis of adjacent overlapped Stokes spectra in two stages of a transient stimulated Raman chirped-pulse amplifier, tuned respectively to the vibrational modes at 901 and 767 cm−1 of a potassium gadolinium tungstate [KGd(WO4)2] crystal, is demonstrated. The contribution of the spatio-temporal overlap of seed and pump pulses, as well as the self-phase modulation, was investigated. The noncollinear configuration allows the composite bandwidth at the central wavelength of 1120 nm to be increased by a factor of 23 compared to the pump pulse bandwidth of 1.6 nm. After reaching a conversion efficiency of 35% in the second stage, the compressibility of a chirped Stokes pulse with a tailored spectrum was also investigated. [ABSTRACT FROM AUTHOR]

Ultrafast mid-infrared fiber lasers have been intensely studied in the last decade for the generation of high harmonics, molecular spectroscopy, material processing and remote sensing. Different designs have been investigated but most of them lacked the ease of use and reliability needed for their democratization. In this paper, we demonstrate a self-starting mode-locked mid-IR erbium-doped fiber laser based on a SESAM and a broadband uniform FBG that produces pulses as short as 15 ps. Different laser cavities were tested with varying FBG peak reflectance, spectral bandwidth and active fiber length. In addition, one cavity uses a pump combiner instead of injecting free-space the pump power through the fiber tip. The results of this study confirm that the FBG spectral bandwidth can efficiently control the duration of the almost Fourier-transform-limited pulses up to a limit seemingly dictated by the presence of water vapor in the laser cavity acting as narrow spectral filters. To a lower effect, the active fiber length influences the pulse duration. Finally, the use of an all-fiber pump combiner allows for a more compact and rugged design without altering the laser performances. This study represents a step towards the development of robust mid-infrared ultrafast all-fiber lasers.