Ultrafast Research Articles

First-principles study of electron dynamics of MoS2 under femtosecond laser irradiation from deep ultraviolet to near-infrared wavelengths.

Time-dependent density functional theory was employed to investigate the electron dynamics of MoS2 following femtosecond pulse irradiation. The study concerned the effects of laser wavelength, intensities, and polarization and elucidated the ionization mechanisms across the intensity range of 1010-1014 W/cm2. As laser intensity increases, MoS2 irradiated with an infrared (IR) laser (800nm) deviates from single-photon absorption at lower intensities compared to that subjected to an ultraviolet (UV) laser (266nm), and nonlinear effects in the current arise at lower intensities for the 800nm laser. At a wavelength of 266nm, MoS2 irradiated with an a-axis polarized laser deposited more energy and generated more electron-hole pairs compared to c-axis polarization. Rate equations were used to estimate the total number of excited electrons in MoS2 and the corresponding plasma frequency. Simulation results indicate that the damage threshold of the UV laser is higher than that of the IR laser, which contradicts the experimental results. This outcome suggests that the mechanism of material damage induced by the UV femtosecond laser near the damage threshold is independent of optical breakdown. The findings of this research are significant for enhancing the performance of MoS2-based photodetectors and optimizing their stability and reliability in high-power, short-wavelength laser applications.

Pulsed laser directed energy deposition of super duplex stainless steel: engineering microstructure and improving mechanical properties

Alloys developed by fusion-based additive manufacturing often suffer from the coarse columnar grain structure and their effect on properties. This work involves the practical application of pulsed laser in laser-based directed energy deposition (DED-LB) of super duplex stainless steel which led to engineering the microstructure, improving the mechanical properties, and changing the dominant texture. Pulsed laser DED-LB (here P-DED) with laser spot sizes of 1 and 2 mm and different frequencies were used. Refine-grained ferritic steels containing porosity were produced when using a small laser spot size. Ferrite-to-austenite (α → γ) transformation was constrained to the grain boundaries under the effect of small excitation overlaps. Using broader laser, higher energy input, and ultra-short pulse intervals encouraged γ nucleation, promoted the density, and decreased the content of undesirable oxides that are typically formed during the conventional DED-LB (here C-DED). The local ferritization under the fusion lines of C-DED was avoided by P-DED. Directionally solidified α, extending into several layers, was inhibited by P-DED with optimum overlap. Enhanced supercooling resulted in an in situ grain refinement and columnar-to-equiaxed morphological transition. Defect-free microstructure and effective distribution of interphase boundary surface by P-DED, with a laser spot size of 2 mm and ⁓99% excitation overlap, largely improved the toughness and elongation (with acceptable strength). Pulse-induced convection and isotropic heat flow during P-DED with smaller laser spot size subsided the trend of preferred orientation. However, an alignment of < 001 > α with deposition direction during P-DED with the broader laser preserved the typical {001} < 100 > solidification texture and, consequently, the transformation texture.

Superfluorescence and nonlinear optical response of CH3NH3PbBr3-based mesostructures: applications to amplified spontaneous emission and ultrafast lasers

Superfluorescence and nonlinear optical response of CH3NH3PbBr3-based mesostructures: applications to amplified spontaneous emission and ultrafast lasers

Laser melting, evaporation, and fragmentation of nanoparticles: Experiments, modeling, and applications

This review examines the processes of laser heating, melting, evaporation, fragmentation, and breakdown of metal nanoparticles, as well as the dependences and values of the threshold laser parameters that initiate these processes. Literature results are analyzed from experimental studies of these processes with gold, silver, and other nanoparticles, including laser surface melting and evaporation of nanoparticles and Coulomb fragmentation of nanoparticles by ultrashort laser pulses. A theoretical model and description of the thermal mechanisms of mentioned processes with metal (solid) nanoparticles in a liquid (solid) medium, initiated by the action of laser pulses with the threshold fluences, are presented. Comparison of the obtained results with experimental data confirms the accuracy of the model and makes it possible to use them to evaluate the parameters of laser thermal processing of nanoparticles. Applications of the processes include the laser melting, reshaping, and fragmentation of nanoparticles, the formation of nanostructures and nanonetworks, the laser processing of nanoparticles located on substrates, and their cladding on surfaces in various laser nanotechnologies. The use of laser ignition, combustion, and incandescence of nanoparticles is discussed, as is the use of nanoparticle-triggered laser breakdown for spectroscopy. These laser processes are used in photothermal nanotechnologies, nanoenergy, laser processing of nanoparticles, nonlinear optical devices, high-temperature material science, etc. In general, this review presents a modern picture of the state of laser technology and high-temperature processes with nanoparticles and their applications, being focused on the latest publications with an emphasis on recent results from 2021–2024.

Localized Vector Optical Nondiffracting Subcycle Pulses

Structured light is essential in various fields such as imaging, communications, computing, laser microprocessing, and ultrafast and nonlinear optics. The structuring of light can occur in terms of space, amplitude, phase, polarization, time, frequency, and duration. One of the intriguing properties that can be obtained is resistance to the diffractive spread and dispersive broadening of the pulsed beams. This happens when temporal properties such as frequency are coupled with spatial properties like angles of propagation of plane-wave components. In this case, pulsed light beams exhibit characteristics similar to optical bullets, resisting both diffraction and material dispersion. This study questions whether free-space optical bullets that possess nondiffracting and nondispersive properties are possible with subcycle durations. We report on the possibility to create nondiffracting and nondispersing localized subcycle pulsed beams and their complex polarization topologies when controlling the group velocity of these light structures.

Coherence memory and amnesia in a mode-locked Mamyshev oscillator

Self-organization of temporal modes in mode-locked lasers usually starts from quantum noise. In this process, incoherent spontaneous emission is steered into coherent ultrashort pulses by dissipation and nonlinearity. In this work, we investigated self-organization dynamics in a mode-locked Mamyshev oscillator starting from coherent pulse seeds as opposed to quantum noise. We observed that the coherence of the seed can be remembered or forgotten during the mode-locking transition, depending on the initial inverse population. The excessive nonlinearity in the coherence amnesia regime can devastate the seed coherence, while this coherence is retained in the transition for the memory regime. A heterodyne technique was developed to record the fast varying optical phase and characterize these two transition pathways. Dissipative soliton molecules were synthesized from external pulse pair seeds via the coherence memory pathway. In this case, a plateau of the generated pulse spacing independent of seed pulse spacing was observed. Moreover, we show that pulse seeds can be used for laser reconfiguration and optical pulse buffering. Our work paves a way to control transient pulse dynamics and relative pulse timing on demand in mode-locked lasers.

41 W, 206 fs regenerative amplifier based on CPA-free amplification of femtosecond pulses

We present a low-cost method to mitigate the gain narrowing effects during regenerative amplification based on a high-repetition-rate, high-power continuously pumped Yb:YAG thin disk regenerative amplifier (RA). By inserting a 1-mm-thick SiO2 plate on a certain position in the RA, the narrowed bandwidth of the amplified spectrum of 3 nm was significantly broadened to a root-mean-square (RMS) bandwidth of 6.9 nm, which reveals the effectiveness of the method. At a repetition rate of 200 kHz, an output power of 41 W was obtained with a compressed pulse duration of 206 fs. The amplifier displayed stability with a root-mean-square power noise as low as 0.46%. The optical resonator remained nearly diffraction-limited beam quality (M2 &lt; 1.35) until the maximum output power.

Converting Gaussian to line beams with high efficiency by flat liquid crystal optical elements

Line-shaped laser beams are desired for applications ranging from ultrafast laser processing to metrology. Here we demonstrate that line-shaped laser beams with diffraction-limited line widths can be generated from Gaussian beams with high efficiency by using flat liquid crystal optical elements. The liquid crystal optical elements are designed based on the Pancharatnam-Berry (PB) geometric phase acquired by light passing through a half waveplate and fabricated by using a high-resolution molecular orientation photopatterning technique. We show that these liquid crystal PB optical elements can achieve a diffraction efficiency over 99%, yielding no zero-order light leakage. We further demonstrate beam shaping devices for converting Gaussian beams into crosshair-shaped beams. The design and fabrication methods presented in this paper are general and can be extended to meet other complex beam shaping needs.

Red diode-pumped SESAM and Kerr-lens modelocked alexandrite lasers

We report quantum-well SESAM and Kerr-lens modelocked alexandrite lasers pumped by red laser diodes emitting at 638 nm. The SESAM modelocked laser emits 95 fs pulses with an average output power of 315 mW at a pulse repetition rate of 84 MHz with a peak power of 35 kW and a pulse energy of 3.8 nJ. The Kerr-lens modelocked laser emits 44 fs pulses with an average output power of 110 mW at 100 MHz pulse repetition rate, corresponding to 22 kW of peak power and a pulse energy of 1.1 nJ. The combination of direct diode pumping with robust SESAM modelocking or sub-50 fs pulses generated by Kerr-lens modelocking enables various ultrafast laser applications.

In situ characterization of two unknown ultrashort laser pulses using four-wave mixing in gas

Accurate characterization of two ultrashort laser pulses is of great interest in many ultrafast pump-probe experiments. We demonstrate a method based on four-wave mixing (FWM) in a gas which could be easily implemented into many existing pump-probe setups with minimal modifications for accurate, in situ characterization of both unknown pulses. This technique is tested on pairs of unknown pulses at wavelengths of 400/800 nm, and 266/400 nm. We measured the spectrogram of the pulse generated through FWM of the two unknown pulses by scanning the delay between two unknown pulses. The retrieval algorithm converges to accurately predict the intensity and the phase profiles of both unknown pulses with a trace error of &lt; 1% and the accuracy is verified using an independent pulse characterization device.

Phase-cycling and double-quantum two-dimensional electronic spectroscopy using a common-path birefringent interferometer

Ultrafast spectroscopy provides unique access to the coherent dynamics of atomic, molecular, and solid state quantum systems. A most powerful, yet experimentally challenging tool for this is two-dimensional electronic spectroscopy (2DES), allowing to isolate excitation pathways and to selectively probe coherent and incoherent couplings by controlling the phase of the ultrashort pulses that interact with the system. Its experimental implementation can in principle greatly be simplified by employing inherently phase stable birefringent in-line interferometers (TWINS), which, however, are thought to lack sufficient phase control. Here, we demonstrate an adaptation of TWINS providing full phase-cycling capabilities for 2DES. This is demonstrated by recording rephasing, non-rephasing, zero-quantum, and double-quantum 2DES on a molecular J-aggregate. This easy-to-implement extension opens up new experimental possibilities for TWINS-based 2DES in multidimensional all-optical and photoemission spectroscopy and microscopy.

Tailoring the pulse characteristics of LiB3O5-based femtosecond optical parametric oscillators by temperature tuning.

We show that the phase mismatch in LiB3O5 (LBO) crystals is approximately proportional to the temperature deviation from the phase-matching temperature of crystals. Moreover, a fixed temperature deviation would result in approximately the same amount of phase mismatch over a large range of wavelengths. Based on this, we demonstrate that the pulse characteristics of LBO-based femtosecond optical parametric oscillators (OPOs) can be tailored by simply tuning the crystal temperature. Experimentally, at a temperature deviation of 10°C, nearly transform-limited optical pulses were obtained over a tuning range of 780-910 nm, and at a temperature deviation of -9°C, linearly chirped pulses with broadened spectra were obtained, which were de-chirped to be nearly transform-limited ones outside the cavity. The capability of generating femtosecond pulses with different pulse characteristics and at various wavelengths would benefit many applications, including spectroscopy and Raman microscopy.

Optically Modulated Nanofluidic Ionic Transistor for Neuromorphic Functions

AbstractNeuromorphic systems that can emulate the behavior of neurons have garnered increasing interest across interdisciplinary fields due to their potential applications in neuromorphic computing, artificial intelligence and brain‐machine interfaces. However, the optical modulation of nanofluidic ion transport for neuromorphic functions has been scarcely reported. Herein, inspired by biological systems that rely on ions as signal carriers for information perception and processing, we present a nanofluidic transistor based on a metal–organic framework membrane (MOFM) with optically modulated ion transport properties, which can mimic the functions of biological synapses. Through the dynamic modulation of synaptic weight, we successfully replicate intricate learning‐experience behaviors and Pavlovian associate learning processes by employing sequential optical stimuli. Additionally, we demonstrate the application of the International Morse Code with the nanofluidic device using patterned optical pulse signals, showing its encoding and decoding capabilities in information processing process. This study would largely advance the development of nanofluidic neuromorphic devices for biomimetic iontronics integrated with sensing, memory and computing functions.

Multidisciplinary Laser Facility Driven by New-Generation High-Repetition Laser

For many years, high-power laser technology has been divided between industrial applications, which prioritize higher average powers and repetition rates, and academic research, which focuses on achieving higher peak powers and ultrashort pulse durations. The introduction of Yb-doped crystals in laser technology has paved the way for a new generation of laser sources that bridge the gap between industrial and academic requirements, combining high average power with ultrashort pulse capabilities. These advancements enable the integration of compact, adaptable front-end stages, making such lasers versatile for scientific applications. In Lisbon, the Laboratory of Intense Lasers leverages this technology with a system that combines commercial and custom-built front-end stages to enhance operational flexibility. In this paper, we present the current status of this facility and outline upcoming upgrades. We also showcase applications enabled by these high-power laser sources, including semiconductor studies, bi-photonics, and time-resolved spectroscopy.

Ultrafast laser excitation as a pathway to liquid-like transport rates in the solid state

This work presents an ultrafast laser-based mechanism for achieving liquid-like mass transport rates in solids. Transport at these rates was achieved by irradiating GaAs with a femtosecond laser (repetition rate = 1 kHz, τ = 150 fs, and λ = 780 nm). Following irradiation, the material exists in a highly excited state in which the atomic bonds are softened. The probability of hopping between lattice/interstitial sites for atoms within the softened interatomic potentials is greatly increased. Evidence of the enhanced diffusion is observable in the rate at which islands and corrugated structures form on the GaAs surface. Quantification of the mass transport rate necessary to form the structures via Fick's law produces a diffusion coefficient of 1.18×10−6 cm2/s, which is over 10 orders of magnitude higher than that for near-melt vacancy self-diffusion in crystalline GaAs. In fact, the calculated diffusion coefficient is on the order of magnitude of liquid GaAs, while estimates of temperature during mass transport place the crystal well below its melting point. These results suggest that ultrafast laser irradiation can be used to enhance the diffusion rate in materials without inducing a phase transition.

Ultrafast mid-infrared interferometric photocurrents in graphene-based two-terminal devices

We demonstrate that graphene-based two-terminal devices allow autocorrelating femtosecond mid-infrared pulses with a pulse duration of about 100 fs in the wavelength regime of 5.5–14 μm. The results suggest that the underlying ultrafast detection principle relies on an electric field dominated autocorrelation in combination with the optoelectronic dynamics at the metal–graphene interfaces. The demonstrated scheme excels because of the ease in nanofabrication of two-terminal graphene-based optoelectronic devices and their robustness.

Tunable FSRS measurements with reduced background signals: Using an etalon filter to generate picosecond pump pulses in the 460-650nm range.

Generating wavelength-tunable picosecond laser pulses from an ultrafast laser source is essential for femtosecond stimulated Raman scattering (FSRS) measurements. Etalon filters produce narrowband (picosecond) pulses with an asymmetric temporal profile that is ideal for stimulated resonance Raman excitation. However, direct spectral filtering of femtosecond laser pulses is typically limited to the laser's fundamental and harmonic frequencies due to very low transmission of broad bandwidth pulses through an etalon. Here, we show that a single etalon filter (15cm-1 bandwidth; 172cm-1 free spectral range) provides an efficient and tunable option for generating Raman pump pulses over a wide range of wavelengths when used in combination with an optical parametric amplifier and a second harmonic generation (SHG) crystal that has an appropriate phase-matching bandwidth for partial spectral compression before the etalon. Tuning the SHG wavelength to match individual transmission lines of the etalon filter gives asymmetric picosecond pump pulses over a range of 460-650nm. Importantly, the SHG crystal length determines the temporal rise time of the filtered pulse, which is an important property for reducing background and increasing Raman signals compared with symmetric pulses having the same total energy. We examine the wavelength-dependent trade-off between spectral narrowing via SHG and the asymmetric pulse shape after transmission through the etalon. This approach provides a relatively simple and efficient method to generate tunable pump pulses with the optimum temporal profile for resonance-enhanced FSRS measurements across the visible region of the spectrum.

Palladium Nanocubes as Saturable Absorbers for Mode-Locked Laser Generation at 1.56 μm.

Palladium (Pd) nanocubes, a type of metallic nanostructure, have demonstrated remarkable optoelectronic properties, garnering significant attention. However, their nonlinear optical characteristics and related device applications remain underexplored. In this study, we report the fabrication of a novel saturable absorber (SA) by depositing Pd nanocubes onto a D-shaped fiber (DF). The Pd nanocubes, with an average size of 12 nm, were synthesized and integrated with a DF, resulting in a highly robust SA with broadband saturable absorption characteristics. When incorporated into Er3+-doped laser cavities, the Pd-DF SA enabled the generation of ultrafast pulses with a central wavelength of 1560 nm, a corresponding repetition rate of 26.7 MHz, and a temporal width of 1.85 ps. Our findings highlight the strong potential of the Pd-DF device as a versatile SA for constructing high-energy ultrafast fiber lasers.

Determination of the XUV Frequency Chirp at the Free-Electron Laser FLASH via THz Streaking and Electron Beam Diagnostics

Free-electron lasers (FELs) operating in the extreme ultraviolet (XUV) and X-ray regions deliver ultrashort pulses with unprecedented intensity, enabling groundbreaking research across various scientific disciplines. A potential chirp (frequency change within the pulse) of these pulses influences their spectral properties, directly impacting the experimental outcomes and FEL performance. The accurate characterization of the chirp is, therefore, important for optimizing FEL operation and interpreting experimental results. This study presents a comprehensive comparison of two techniques determining the chirp of the XUV pulses at FLASH by directly measuring the XUV pulses with THz streaking and by detecting the chirp of the electron bunches by a Transverse Deflection Structure (PolariX TDS) to infer the XUV chirp. We conducted simultaneous measurements using both techniques at FLASH2 while tuning the FEL to produce various energy chirps on the electron bunch.

Review on Principal and Applications of Temporal and Spatial Beam Shaping for Ultrafast Pulsed Laser

Ultrafast or ultrashort pulsed lasers have become integral in numerous industrial applications due to their high precision, non-thermal interaction with materials, and ability to induce nonlinear absorption. These characteristics have expanded their use in microfabrication, semiconductor processing, automotive engineering, and biomedical fields. Temporal pulse shaping reduces laser pulse durations, often to shorter timescales than many physical and chemical processes, enabling greater control. Meanwhile, spatial shaping improves efficiency and precision in micro- and nanofabrication and biomedical applications. Advances in optical parametric amplifiers (OPAs) and chirped-pulse amplifiers (CPAs) have allowed for more refined temporal and spatial shaping, ensuring the preservation of high peak power while achieving ultrashort pulse durations. Additionally, spatial light modulators (SLMs) have facilitated sophisticated beam shaping, which, when combined with ultrafast lasers, supports applications like computer-generated holography and nanoscale fabrication. These developments underscore the growing utility and versatility of ultrafast lasers in both research and industrial contexts.