Ultrafast Research Articles

Light-Induced Polaronic Crystals in Single-Layer Transition Metal Dichalcogenides.

Light-induced ordered states can emerge in materials after irradiation with ultrafast laser pulses. However, their prediction is challenging because the inverted band occupation confounds our chemical intuition. Hence, we use a recently developed constrained density functional perturbation theory approach to systematically screen single-layer transition metal dichalcogenides (TMDs) for light-induced ordered states. We demonstrate that all examined single-layer TMDs reveal similar light-induced charge orderings. The corresponding reconstructions are periodic arrangements of polarons (polaronic crystals), characterized by triangular metal clusters and having no equivalent at equilibrium conditions. The polarons are accompanied by localized midgap states in the electronic band structure, detectable by experimental methods. We assess the selenides as the most promising candidates for potential photoexcitation experiments because they transition at low critical fluences, have low transition barriers, and maintain an open band gap under photoexcitation. Our work paves the way for innovative material design approaches targeting light-induced phases.

Multi-channel photonic sampled ADC with hybrid deep-learning for distortion recovery

Analog-to-digital converter (ADC) is indispensable components in modern information systems, and photonic technologies have emerged as promising avenues for overcoming bottlenecks in ADC performance. This paper introduces a novel approach: a photonic sampled ADC integrated with a neural network that combines Convolutional Neural Networks (CNNs) and Transformers for data recovery. The proposed system utilizes a multi-wavelength optical sampling pulse train interleaved in the time domain through fiber dispersion. Sampled signals are quantified in parallel channels by electronic ADC (EADC). Through supervised training, the hybrid deep neural network extracts both local and global features of photonic system defects, enabling the recovery of distorted data. Experimental validation showcases the effectiveness of this architecture, achieving successful reconstruction of radio frequency (RF) signals at a sampling rate of 12 Giga-samples per second (Gs/s) with an effective number of bits (ENOB) of 7.13. The results underscore the potential of the proposed architecture in achieving high conversion accuracy in multi-channel photonic sampled ADCs.

Suppression of Kerr-induced satellites in multi-pulse CPA

Amplification of bursts of ultrashort pulses is very challenging when the intraburst repetition frequency reaches the THz range, corresponding to (sub)-ps intervals between consecutive pulses. Periodic interference significantly modifies conditions for chirped pulse amplification (CPA), leading to temporal and spectral distortions during CPA due to optical Kerr nonlinearity. Multi-pulse chirped amplification to mJ energies may lead to a pronounced degradation of burst fidelity and the appearance of periodic temporal satellites after de-chirping the amplified waveform. We study, experimentally and numerically, the limitations of THz burst-mode CPA caused by self- and cross-phase modulation. A number of practical recipes to suppress nonlinear distortions and improve energy scaling by optimizing burst parameters and applying modulation techniques are presented.

Reinforced Magnetic-Responsive Electro-Ionic Artificial Muscles by 3D Laser-Induced Graphene Nano-Heterostructures.

Efficient ion transport and enriched responsive modals via modulating electrochemical properties of conductivity and capacitance are essential for soft electro-ionic actuators. However, cost-effective and straightforward approaches to achieve expedited fabrication of active electrode materials capable of multimodal-responsiveness remain limited. Herein, this work reports the one-step ultrafast laser direct patterning method, to readily synthesize electro- and magneto-active electrode material, derived from the unique cobalt-phosphorus co-doped core-shell heterostructures within 3D graphene frameworks, for fulfilling the dual-mode responsive electro-ionic actuators. The designed nanofiber-structured heterointerfaces across electrodes and electrolytes further promote highly efficient electron/ion transfer. The developed soft actuator exhibits superior actuation performance of peak-to-peak displacement to 13.08mm under an ultra-low ±0.5V, with doubled direct current deflection under 200 mT at 1V, an ultrafast response of 1.38 s and long-term stability (>90% retention for ≈106000 cycles), even detectable bending to ≈280µm under exceptional ±10mV. The promising demonstration of promoting differentiation and proliferation of stem cells under mechanical strain and electrical stimuli, sheds more light as well on the possibility of facilitating biomedical soft robotics with ultrahigh actuation performance.

Non-collinear bipulse reconstruction via dispersion scan

Light pulses in the femtosecond range require sophisticated methods for their precise temporal characterization. Several techniques have been developed over the past decades that deliver the temporal structure of ultrashort light pulses. Still, there are special cases left that cannot be treated directly by established methods. Here we expand the applicability of existing tools to the case of non-collinear propagation of a pair of identical pulses with an unknown, but fixed temporal spacing. By applying the successful dispersion scan (d-scan) technique to a setup known from the established frequency-resolved optical gating (FROG) technique, we record a rather peculiar measurement trace. A nonlinear signal is only generated outside optimal temporal compression, in contrast to previously used techniques. This feature enables an improved dynamic range for the measurement of the temporal wings of a pulse. We expand a well-established retrieval algorithm to reconstruct the pulse structure from the measurement data. Our results are confirmed by comparison to d-scan and FROG measurements.

Nonlinear dispersion of backward-wave four-wave mixing in a photorefractive semiconductor and its effect on group velocity

Photorefractive backward-wave four-wave mixing may be configured for the amplification of the phase conjugate output and the attenuation of the transmitted output. The nondegenerate-in-frequency interaction is studied in such a configuration, realized in a semiconductor CdTe crystal. The spectra measured at different outputs show the gain and attenuation resonances. The preliminary study of optical pulse propagation demonstrates that, due to different nonlinear dispersions at different outputs of the selected wave mixing configuration, a slowing down of light is observed at the phase conjugate output, while an acceleration of light is detected at the transmitted output for the same input pulse at the same interaction.

Parabolic-like motion of an optical pulse modulated by a temporal boundary in highly noninstantaneous Kerr media

Parabolic-like motion of an optical pulse modulated by a temporal boundary in highly noninstantaneous Kerr media

Scaling of ultrashort-pulsed laser structuring processes for electromobility applications using a spatial light modulator

The structuring of lithium-ion battery (LIB) electrodes and the diffusion media (DM) for polymer electrolyte membrane fuel cells (PEMFCs) with ultrashort laser pulses enables improved performance characteristics of both technologies. However, the transfer of the approaches from a laboratory scale to a commercial use has previously been hindered by the low average output power of ultrashort-pulsed (USP) laser beam sources and the limited productivity of single-beam structuring using scanning optics. Recent advancements in the development of USP laser systems have led to a steady increase in the available output power, thereby enabling new fields of applications. This study aims at accelerating the USP laser structuring of LIB electrodes and DM for PEMFCs to industrially relevant processing rates by comparing a single-beam with a multibeam structuring process regarding ablation characteristics and quality. For the multibeam strategy, the shape of the laser beam was modified by a spatial light modulator (SLM). In addition to microholes, the insertion of microchannels was investigated to demonstrate the high flexibility of state-of-the-art SLMs. The geometry of the created structures was measured with a laser scanning microscope, and the different layers were tested for their geometrical and electrochemical properties to compare both technologies. The results confirmed that applying an SLM enables high-quality microstructures with significantly higher structuring rates. Furthermore, this contribution includes a theoretical analysis of the specifications required for a laser setup to reach an industrially relevant productivity of the structuring processes.

All-Optical Control of Bubble and Skyrmion Breathing.

Controlling the dynamics of topologically protected spin objects by all-optical means promises enormous potential for future spintronic applications. Excitation of bubbles and skyrmions in ferrimagnetic [Fe(0.35 nm)/Gd(0.40 nm)]_{160} multilayers by ultrashort laser pulses leads to a periodic modulation of the core diameter of these spin objects, the so-called breathing mode. We demonstrate versatile amplitude and phase control of this breathing using a double excitation scheme, where the observed dynamics is controlled by the excitation delay. We gain insight into both the timescale on which the breathing mode is launched and the role of the spin object size on the dynamics. Our results demonstrate that ultrafast optical excitation allows for precise tuning of the spin dynamics of trivial and nontrivial spin objects, showing a possible control strategy in device applications.

Precise Nanoparticle Manipulation Using Femtosecond Laser Trapping.

Optical tweezers use strongly focused light for trapping, characterizing, and manipulating objects in the microscopic and nanoscopic regimes. However, fully understanding optical trapping at the nanoscale remains a significant challenge. This holds importance because the nanoscale is the frontier for numerous promising advancements, ranging from enhancing single-molecule investigations in biology to developing hybrid devices for nanoelectronics and photonics and exploring fundamental quantum phenomena in opto-mechanics. We report an experimental and theoretical study of nanoparticles of various sizes, showing the advantages of the immense peak power of ultrashort laser pulses over conventional optical tweezers. We also demonstrate highly stable trapping of nanoparticles for extended durations at low average laser power using femtosecond lasers.

Advances in and Future Perspectives on High-Power Ceramic Lasers

Advancements in laser glass compositions and manufacturing techniques has allowed the development of a new category of high-energy and high-power laser systems which are being used in various applications, such as for fundamental research, material processing and inertial confinement fusion (ICF) technologies research. A ceramic laser is a remarkable revolution in solid state lasers. It exhibits crystalline properties, high yields, better thermal conductivity, a uniformly broadened emission cross-section, and a higher mechanical constant. Polycrystalline ceramic lasers combine the properties of glasses and crystals, which offer the unique advantages of high thermal stability, excellent optical transparency, and the ability to incorporate active laser ions homogeneously. They are less expensive and have a similar fabrication process to glass lasers. Recent developments in these classes of lasers have led to improvements in their efficiency, beam quality, and wavelength versatility, making them suitable for a broad range of applications, such as scientific research requiring ultra-fast laser pulses, medical procedures like laser surgery and high-precision cutting and welding in industrial manufacturing. The future of ceramic lasers looks promising, with ongoing research focused on enhancing their performance, developing new doping materials and expanding their functional wavelengths. The ongoing progress in high-power ceramic lasers is continuously expanding the limits of laser technology, therefore allowing the development of more powerful and efficient systems for a wide range of advanced and complex applications. In this paper, we review the advances, limitations and future perspectives of ceramic lasers.

Simultaneous Probing of Electron Density and Temperature Dynamics Inside Air Plasma with Intense Terahertz Pulses

AbstractAir plasma induced by ultrafast laser pulses is an extraordinary source of electromagnetic waves, emitting microwave, terahertz (THz) radiation, and cavityless lasing in the near‐infrared and visible ranges. The temporal dynamics of the electron density have been revealed by optical pump‐probe techniques, while the evolution of the electron temperature remains elusive due to a lack of suitable methods. Here, it is demonstrated that the intense THz‐field‐enhanced fluorescence emission from the excited molecules of nitrogen is a novel tool that allows to explore the complex dynamics of the plasma density and electron temperature simultaneously. Two relaxation times of electrons in air plasma are observed and interpreted as a competition between the excitation of a triplet state by laser or THz‐field‐heated electrons and the dissociative recombination of nitrogen molecular ions. Based on the theoretical simulations, the tens of picoseconds relaxation process is attributed to the ultrafast temperature decrease, while the longer relaxation in the range of hundreds of picoseconds is ascribed to the decay of electron density. The temporal relaxation of both the electron density and temperature revealed by applying an intense THz field provides further insights into the laser‐air plasma interaction and will benefit the engineering of this exceptional source.

Time-resolved cathodoluminescence measurement of the effects of α -particle-related damage on minority hole lifetime in free-standing n-GaN

Time-resolved cathodoluminescence using 30 keV ultrafast electron pulses has been used to perform direct measurements of the minority hole lifetime τh as a function of 3.7 MeV α-particle fluence in high-quality free-standing n-type GaN substrates. The lifetime damage factor K calculated from these measurements was found to monotonically decrease from 6.9 × 10−2 to 6.4 × 10−4 cm2 s−1 ion−1 with increasing α-fluence from 108 to 1012 cm−2, implying a reduction in trap cross section and/or an aggregation of α-induced traps. The small, ∼200–300 nm, hole diffusion length estimated from the minority hole lifetime for the highest α-fluence necessitates the deployment of α-voltaic device strategies and architectures that emphasize depletion and drift over diffusion for effective charge collection and optimal power conversion efficiency.

Octave-wide broadening of ultraviolet dispersive wave driven by soliton-splitting dynamics

Coherent dispersive wave emission, as an important phenomenon of soliton dynamics, manifests itself in multiple platforms of nonlinear optics from fibre waveguides to integrated photonics. Limited by its resonance nature, efficient generation of coherent dispersive wave with ultra-broad bandwidth has, however, proved difficult to realize. Here, we unveil a new regime of soliton dynamics in which the dispersive wave emission process strongly couples with the splitting dynamics of the driving pulse. High-order dispersion and self-steepening effects, accumulated over soliton self-compression, break the system symmetry, giving rise to high-efficiency generation of coherent dispersive wave in the ultraviolet region. Simultaneously, asymmetric soliton splitting results in the appearance of a temporally-delayed ultrashort pulse with high intensity, overlapping and copropagating with the dispersive wave pulse. Intense cross-phase modulations lead to octave-wide broadening of the dispersive wave spectrum, covering 200–400 nm wavelengths. The highly-coherent, octave-wide ultraviolet spectrum, generated from the simple capillary fibre set-up, is in great demand for time-resolved spectroscopy, ultrafast electron microscopy and frequency metrology applications, and the critical role of the secondary pulse in this process reveals some new opportunities for all-optical control of versatile soliton dynamics.

Study on superhydrophobicity and corrosion resistance of the micro-nano structure prepared by femtosecond laser

The wettability as well as the corrosion resistance of metal is closely related to the surface microstructure. A surface structure with micro protrusion and nano ripple was designed and constructed on 316L stainless steel by femtosecond pulse laser. Superhydrophobic surfaces with a great contact angle over 150° were obtained by surface modifications of heat treatment or fluorination treatment. The corrosion resistance of the superhydrophobic micro-nano structured surfaces was studied through electrochemical testing. The result of polarization curve revealed that the superhydrophobic micro-nano structured surfaces possessed a higher corrosion potential, signifying the susceptibility to corrosion was reduced. Besides, the results of impedance indicated that the superhydrophobic micro-nano structured surfaces exhibited greater total resistance when compared to the original surface, demonstrating the corrosion resistance was enhanced. According to the results, the superhydrophobic surface, characterized by its micro-nanostructure, facilitates the formation of a protective gas film. Additionally, the intricate micro-nano rough structure, in conjunction with the gas film, effectively shields the underlying surface from infiltrative processes, thereby mitigating the potential exacerbation of corrosion phenomena.

Widely wavelength-tunable high-repetition-rate femtosecond pulse source with highest average power up to 28 W

We have proposed and demonstrated a high-repetition-rate ultrashort pulse fiber amplification system based on a wavelength-tunable oscillator. This fiber amplification system produces an average power exceeding 20 W in bursts of 200 pulses with a 578 MHz intra-burst pulse repetition rate and a 1 MHz burst repetition rate. The center wavelength of the amplified pulses can be tuned from 1030 to 1080 nm. By utilizing pre-chirp management nonlinear amplification technique, the achieved shortest pulse duration is 27 fs with an average power of 25 W at 1032 nm. For all the amplified pulses with different wavelengths, the pulse duration after optimal compression is below 60 fs. To the best of our knowledge, this is the first time that a widely wavelength-tunable high-power laser with a repetition rate exceeding 100 MHz and a pulse duration of several tens of femtoseconds has been realized. Additionally, using only common double-cladding Yb-doped fiber as the gain fiber, without any large-mode-area Yb-doped photonic crystal fiber or rod-type Yb-doped fiber, makes the system compact and reliable due to the simple fusion operation.

Intelligent controllable ultrafast fiber laser via deep learning and adaptive optimization algorithm

Ultrafast fiber lasers based on nonlinear polarization rotation can generate femtosecond pulses with different pulse durations and high peak powers, which are powerful tools for engineering applications and scientific research. However, achieving a precise and repeatable polarization state for generating the ultrashort pulses with the shortest pulse duration remains a significant challenge. In this paper, we extend the use of recurrent neural networks and adaptive optimization algorithms, specifically designed to optimize repetitive processes in optical systems, to facilitate intelligent search and control aimed at achieving the minimum pulse duration within a mode-locked fiber laser cavity. Our multi-algorithm-based intelligent system can fully simulate and optimize the processes involved in hands-on experiments. Our intelligent system identified a mode-locked fiber laser with the shortest pulse duration of 465 fs, which was experimentally verified. The proposed intelligent algorithm not only identifies the shortest pulse but also holds significant potential for selecting related laser characteristic parameters. We believe this work opens up a novel avenue for exploration and optimization in mode-locked lasers and the intelligent laser can find practical applications in engineering and scientific research.

Ultrafast laser triggering nanocrystallization inside Nd-doped photo-thermo-refractive glass and its application in Q-switched laser

Ultrafast laser triggering nanocrystallization inside Nd-doped photo-thermo-refractive glass and its application in Q-switched laser

Dissipative soliton resonance in a normal dispersion all-polarization-maintaining thulium-doped fiber laser

We report a first demonstration of dissipative soliton resonance (DSR) in a normal dispersion thulium-doped fiber laser (TDFL) constructed entirely of polarization-maintaining fibers. A nonlinear fiber loop mirror with a combination of normal and anomalous dispersion fibers inside the loop ensures a self-starting mode-locking operation in the DSR regime. The oscillator generated linearly polarized optical pulses with a fundamental repetition frequency of 5.735 MHz and a nearly constant peak power clamped at ∼ 4.35 W. The pulse duration extends from 217 ps to 6 ns, when the pump power increases from 417 mW to 1.86 W. The maximum average output power of the DSR pulses was 151 mW, corresponding to the single pulse energy of 26 nJ.

Impact of higher order dispersion in the propagation of optical pulse in the photonic crystal fibers - A Projection Operator approach

Impact of higher order dispersion in the propagation of optical pulse in the photonic crystal fibers - A Projection Operator approach