Laser Repetition Rate Research Articles

Optical nuclear electric resonance in LiNa: selective addressing of nuclear spins through pulsed lasers

Optical nuclear electric resonance (ONER), a recently proposed protocol for nuclear spin manipulation in atomic systems via short laser pulses with MHz repetition rate, exploits the coupling between the nuclear quadrupole moment of a suitable atom and the periodic modulations of the electric field gradient generated by an optically stimulated electronic excitation. In this theory paper, we extend the scope of ONER from atomic to molecular systems and show that molecular vibrations do not interfere with our protocol. Exploring the diatomic molecule LiNa as a first benchmark system, our investigation showcases the robustness with respect to molecular vibration, and the ability to address and manipulate each of the two nuclear spins independently, simply by adjusting the repetition rate of a pulsed laser. Our findings suggest that it might be possible to shift complicated spin manipulation tasks required for quantum computing into the time domain by pulse-duration encoded laser signals.

Laser shock incremental forming by employing high repetition rate laser pulses with pulse duration in the scale of 100 ns

Laser shock incremental forming by employing high repetition rate laser pulses with pulse duration in the scale of 100 ns

Laser-induced stress by multi-beam femtosecond pulses in fused silica

Ultrafast laser technology presents the unique capacity to process glass materials with an outstanding processing quality; however, combining high quality and high throughput is still a crucial issue because glass is brittle and highly heat sensitive. One strategy to overcome this limitation is to split in space the main laser beam into multiple beams for process parallelization. In the present paper, the simultaneous interaction of several femtosecond laser beams at the surface of fused silica targets is addressed experimentally and theoretically. This work is devoted to highlight the beams cooperation for inducing stress in the material. The experiment consists in irradiating the target with multiple laser pulses with a wavelength of 1030 nm and a duration of 500 fs. The induced stress is observed through post-mortem cross-polarized microscopy. A multiscale and multiphysics model describing laser energy deposition into the material and its mechanical response is developed. The influence of various laser parameters is studied: number and position of laser beams, repetition rate, and fluence. Both experimental and modeling results, which are in a good agreement, show significant cooperative effects for stress formation with large enough laser energy deposition, possibly leading to detrimental cracks.

Product-specific reaction kinetics in continuous uniform supersonic flows probed by chirped-pulse microwave spectroscopy.

Experimental studies of the products of elementary gas-phase chemical reactions occurring at low temperatures (<50K) are very scarce, but of importance for fundamental studies of reaction dynamics, comparisons with high-level quantum dynamical calculations, and, in particular, for providing data for the modeling of cold astrophysical environments, such as dense interstellar clouds, the atmospheres of the outer planets, and cometary comae. This study describes the construction and testing of a new apparatus designed to measure product branching fractions of elementary bimolecular gas-phase reactions at low temperatures. It combines chirped-pulse Fourier transform millimeter wave spectroscopy with continuous uniform supersonic flows and high repetition rate laser photolysis. After a comprehensive description of the apparatus, the experimental procedures and data processing protocols used for signal recovery, the capabilities of the instrument are explored by the study of the photodissociation of acrylonitrile and the detection of two of its photoproducts, HC3N and HCN. A description is then given of a study of the reactions of the CN radical with C2H2 at 30K, detecting the HC3N product, and with C2H6 at 10K, detecting the HCN product. A calibration of these two products is finally attempted using the photodissociation of acrylonitrile as a reference process. The limitations and possible improvements in the instrument are discussed in conclusion.

Laser-sound reproduction by pulse amplitude modulation audio streams

Recently, the possibility to reproduce complex continuous acoustic signals via pulsed laser-plasma sound sources was demonstrated. This was achieved by optoacoustic transduction of dense laser pulse trains, modulated via single- or multi-bit Sigma–Delta, in the air or on solid targets. In this work, we extend the laser-sound concept to amplitude modulation techniques. Particularly, we demonstrate the possibility of transcoding audio streams directly into acoustic pulse streams by analog pulsed amplitude modulation. For this purpose, an electro-optic modulator is used to achieve pulse-to-pulse amplitude modulation of the laser radiation, similarly to the multi-level Sigma–Delta method. The modulator is directly driven by the analog input stream through an audio interface. The performance of the system is evaluated at a proof-of-principle level for the reproduction of test audio signals such as single tones, double tones and sine sweeps, within a limited frequency range of the audible spectrum. The results are supported by computational simulations of the reproduced acoustic signals using a linear convolution model that takes as input the audio signal and the laser-generated acoustic pulse profile. The study shows that amplitude modulation allows for significant relaxation of the laser repetition rate requirements compared to the Sigma–Delta-based implementation, albeit at the potential cost of increased distortion of the reproduced sound signal. The nature of the distortions is analyzed and a preliminary experimental and computational investigation for their suppression is presented.

Optimization of femtosecond laser processing parameters of SiC using ANN-NSGA-II

In the field of femtosecond laser machining, it is essential to select the appropriate process parameters to obtain near thermal damage-free and high efficient machining of SiC wafer. In this work, a method of process parameter optimization for femtosecond laser machining of 4H–SiC was proposed by using the predictive ability of the Artificial Neural Network (ANN) and the optimization algorithm of the non-dominated sorting genetic algorithm (NSGA-II). Firstly, the femtosecond laser was used to fabricate microgrooves on SiC wafers, and the effects of process parameters (laser average power, scanning speed and repetition rate) on groove depth, width, heat affected zone and material removal rate were investigated. Secondly, the ANN model is established based on experimental data. Other experiments verify the accuracy of the model, and the average error in the model’s predictions is around 5%. Thirdly, Pareto optimal solutions are obtained by global optimization of the ANN model using the NSGA-II. The experimental results show that the Pareto optimal solutions are effective and reliable. This proposed method offers dependable guidance for the selecting and optimizing process parameters of high hardness and brittle materials in the field of femtosecond laser processing, and reduces the cost of selecting the appropriate processing parameters in the production process. The method can also be extended to other machining means, such as turning and milling.

High-pressure gallium seeder for atomic fluorescence measurements

This work presents the design and testing of a seeder based on laser ablation capable of introducing gallium particles into a gaseous flow at elevated pressures typical of that found in practical combustion devices. The ability to seed such a flow with gallium particles is required to apply Ga-TLAF, a spatially and temporally resolved thermometry imaging technique well suited to harsh combustion environments. The design criteria for this gallium particle seeder are first listed and all the necessary details required to understand and replicate it are then provided. Next, the efficiency of gallium ablation is verified as a function of the ablation laser's fluence and repetition rate and of the pressure using gallium laser induced fluorescence and laser scattering measurements at ∼403 nm. For the conventional 355-nm nanosecond, Nd:YAG laser used for ablation in this study, data show that the quantity of gallium seeded into the flow can be conveniently modulated by varying the fluence of the ablation laser and/or its repetition rate. Data also show that the efficiency of ablation is marginally better for solid gallium than for liquid gallium, but that ablation of liquid gallium should be preferred to avoid a loss of ablation efficiency after some time. The temperature of the liquid gallium feedstock is found to be unimportant. SEM, EDX, and SPMS analyses show that laser ablation yields pure gallium particles with a characteristic size ranging at least from 50 nm to 10 μm and that the size distribution is insensitive to the pressure and to the ablation laser's fluence. Also important for future applications of Ga-TLAF in high-pressure flames, data show that the gallium LIF intensity recorded at room temperature or in the hot products of a flame is not significantly affected by pressure.

Enhanced Predictability of Urea Crystallization by an Optimized Laser Repetition Rate.

Laser-induced crystallization is a novel alternative to classical methods for crystallizing organic molecules but requires a judicious choice of experimental parameters for the onset of crystallization to be predictable. This study investigated the impact of the laser repetition rate on the time delay from the start of the pulsed laser illumination to the initiation of crystallization, the so-called induction time. A supersaturated urea solution was irradiated with near-infrared (λ = 1030 nm) laser pulses of pulse duration τ = 5 ps at a pulse energy of approximately E = 340 μJ while varying the repetition rate from 10 to 20,000 Hz. The optimal rate discovered ranged from 500 Hz to 1 kHz, quantified by the measured induction time (median 2-5 s) and the mean probability of inducing a successful crystallization event (5 × 10-2%). For higher repetition rates (5-20 kHz), the mean probability dropped to 3 × 10-3%. The reduced efficiency at high repetition rates is likely due to an interaction between an existing thermocavitation bubble and subsequent pulses. These results suggest that an optimized pulse repetition rate can be a means to gain further control over the laser-induced crystallization process.

A laser ablation carbon cluster ion source for the FRS Ion Catcher

A Laser Ablation Carbon Cluster Ion source (LACCI) has been developed and commissioned at the FRS Ion Catcher at GSI. It is the first laser ablation ion source for long-term measurements with a multiple-reflection time-of-flight mass spectrometer (MR-TOF-MS). This is enabled by two key developments: (i) a two-dimensional movable target platform that ensures the long-term stable production of ions and (ii) a mass filter for the isolation of the ions of interest. LACCI is capable of providing closely-spaced reference ions in the mass range up to ∼300 u for accurate mass measurements of exotic nuclei (relative mass uncertainty ∼10−8) and systematic studies of the mass uncertainties with the multiple-reflection time-of-flight mass spectrometer. Investigations of the laser energy and repetition rate and long-term measurements have been carried out with carbon targets (Sigradur®, Fullerene) and metallic targets (silver, gold, copper, tungsten, and erbium). In a test experiment, LACCI has delivered a stable ion current for more than 20 h at a laser repetition rate of 100 Hz, corresponding to an operation for one week at 10 Hz. It thus fully satisfies the requirements for long-term accurate mass measurements of exotic nuclei with an MR-TOF-MS. This enables the access of exotic ions, detected with rates as low as 0.1 per hour at the FRS Ion Catcher.

Femtosecond laser-induced ultrafast growth of volcanic-shaped graphene micropillars

Surface micropillars are capable of significantly altering material properties, including anti-reflection, structural color, wettability, and adhesion. Conventional methods for fabricating micropillars are both time-intensive and material-consuming. Recently, a nature-inspired light-induced self-growth method has been proposed to avoid the drawbacks of conventional methods. However, this method suffers from slow growth rates (slower than 3.3 × 102 μm/s) and restricted precursor material (limited to heat-shrinkable shape-memory polymer). Herein, we developed the light-induced method based on femtosecond laser thermal accumulation engineering and realized the self-growth of micropillars on thermal-stable polymers. By inducing localized carbonization rather than heat shrinkage with a high repetition rate laser, given the instantaneous release of large amounts of gases and highly localized stresses during laser carbonization, unique graphene micropillars grow rapidly and spontaneously from the polyimide surface with a growth rate of 4.5 × 103 μm/s. The growth rate of micropillars is increased by an order of magnitude compared to previously reported light-induced methods. Furthermore, the dimensions of these micropillars, including height, diameter, and spacing, can be precisely controlled by modulating the heat accumulation. Last but not least, solar absorbers and light-driven actuators are flexibly fabricated using this method, demonstrating its practical applicability.

Acousto-optic holography for pseudo-two-dimensional dynamic light patterning.

Optical systems use acousto-optic deflectors (AODs) mostly for fast angular scanning and spectral filtering of laser beams. However, AODs may transform laser light in much broader ways. When time-locked to the pulsing of low repetition rate laser amplifiers, AODs permit the holographic reconstruction of 1D and pseudo-two-dimensional (ps2D) intensity objects of rectangular shape by controlling the amplitude and phase of the light field at high (20-200kHz) rates for microscopic light patterning. Using iterative Fourier transformations (IFTs), we searched for AOD-compatible holograms to reconstruct the given ps2D target patterns through either phase-only or complex light field modulation. We previously showed that phase-only holograms can adequately render grid-like patterns of diffraction-limited points with non-overlapping diffraction orders, while side lobes to the target pattern can be cured with an apodization mask. Dense target patterns, in contrast, are typically encumbered by apodization-resistant speckle noise. Here, we show the denoised rendering of dense ps2D objects by complex acousto-optic holograms deriving from simultaneous optimization of the amplitude and phase of the light field. Target patterns lacking ps2D symmetry, although not translatable into single holograms, were accessed by serial holography based on a segregation into ps2D-compatible components. The holograms retrieved under different regularizations were experimentally validated in an AOD random-access microscope. IFT regularizations characterized in this work extend the versatility of acousto-optic holography for fast dynamic light patterning.

1.2 kW, 20 kHz Nanosecond Nd:YAG Slab Laser System

In this paper, we develop a kW-level high-repetition-rate nanosecond master oscillator power amplifier (MOPA) laser system, employing a structure of fiber, Nd:YVO4, and Nd:YAG hybrid amplification. A tunable fiber seed source is used for adjustable pulse repetition frequency and pulse width. The Nd:YVO4 pre-amplifier, which is dual-end-pumped, achieves high gain while maintaining good beam quality, and the high-power side-pumped Nd:YAG slab main-amplifier enables efficient power amplification. The repetition rate of the output laser can be adjusted within the range of 1~20 kHz, and the pulse width can be tuned within the range of 10~300 ns. The seed output is 6 mW at a repetition frequency of 20 kHz; we achieve an average output power of 1240 W with a total power extraction efficiency of 39.1% and single-pulse energy of 62 mJ at a pulse width of 301 ns. This parameter-controllable high-power laser holds promise for applications in the laser cleaning of complex surface contaminants.

Passively harmonic mode-locked erbium-doped fiber laser with a gold nanofilm saturable absorber

We demonstrate a 1.5 GHz harmonic mode-locked erbium-doped fiber laser by incorporating gold nanofilm as a saturable absorber (SA). The high-quality gold nanofilm SA fabricated by the physical vapor deposition method possesses a high modulation depth of 12.9% and a low saturation intensity of 1.69 MW/cm2 at 1.56 µm, facilitating the generation of harmonic mode-locking. The fundamental mode-locked operation was obtained at 1564.7 nm, with a pulse duration of 586 fs and a repetition rate of 34.235 MHz. At the pump power of 610 mW, 44th-order harmonic mode-locking with a repetition rate of 1.506 GHz was achieved, which is the highest yet reported in mode-locked fiber lasers using gold nanomaterials as SAs. Moreover, the gold nanofilm-based harmonic mode-locked fiber laser shows relatively high signal-to-noise ratios, high output power, and good stability. These results highlight the advantage of the gold nanofilm-based SA in realizing high repetition rate laser sources.

Thermal Birefringence-Based beam shaping in high energy lasers

Repetition-rate high-energy laser systems with high beam quality have been attracting growing attentions in various applications. However, the degradation of beam quality caused by thermal effects is nearly unavoidable in high-energy laser amplifiers operating at high repetition rates. One primary thermal effect is thermal-induced birefringence, which typically induces polarization changes at various spatial positions within the beam. This study explores the potential of leveraging the thermal-induced birefringence effect to manipulate the polarization distribution of the beam and, consequently, control the spatial intensity distribution of the beam by using an additional polarizer. This method demonstrates promise for beam pre-compensation, mitigating thermal-induced depolarization, and other laser applications in high-energy lasers with high repetition rates.

Investigating the third-order nonlinear optical properties of a Schiff base (DBAP) and its Co/Cu metal complex using Z-scan and DFT methodology

Utilizing UV-visible, Fluorescence spectroscopic, and Z-scan methods, the optical and structural properties of 2-(4-(dimethyl amino)benzylidene amino) phenol (DBAP) and their Co/Cu metal complexes were studied. The Z-scan method has been used to investigate the nonlinear absorption coefficient (β), nonlinear refractive index (n2), third-order susceptibility (χ(3)), and optical limiting (OL). In dimethyl sulfoxide (DMSO) and methanol (MeOH) solvents, the DBAP and their Co/Cu metal complexes display positive nonlinear absorption and a positive nonlinear refractive index, according to the Z-scan data. Using the Z-scan approach and an 800 nm central wavelength, femtosecond amplified 48 fs pulse, 1kHz repetition rate laser, the compound's third-order nonlinear optical (NLO) properties are measured. According to the Z-scan data, we may modify the structure and bonding of legend with metals to improve the NLO capabilities of metal complexes, making this material a promising choice for upcoming optoelectronic and photonic devices. The geometrical structure of the compound is optimized using theoretical computations at the B3LYP that are functional with 6-31G and the LanL2DZ on a mixed basis, as determined by the density functional theory (DFT) framework. They provide insight into the charge transfer interactions taking place in the molecule through their HOMO-LUMO analysis.

Factors affecting the quality and reproducibility of MALDI-TOF MS identification for human Capnocytophaga species

Reproducibility and quality of MALDI-TOF MS spectra are critical in the identification process, however, information on the factors affecting the identification scores are scarce. Here, we studied the influence of various factors during the identification process of human oral Capnocytophaga species. The influence of two incubation times, plate-spotting reproducibility of two examiners, extraction technique, storage period of plates, and different laser repetition rates on the quality of MALDI-TOF MS identification of 34 human Capnocytophaga strains (including C. gingivalis, C. granulosa, C. haemolytica, C. leadbetteri, C. ochracea, C. sputigena, and Capnocytophaga genospecies AHN8471) was examined. The identification rate did not show a significant difference (P = 0.05) between the two incubation times, except that C. haemolytica needed a longer incubation time to be recognized at the genus level. The reproducibility of spotting between two examiners was ensured by following the manufacturer's instructions. At the species level, formic acid extraction improved the identification of species with limited representation in the database, such as C. haemolytica and C. granulosa. The storage of plates for one week decreased the identification scores. No significant difference (P = 0.39) was observed between the 60 Hz and 120 Hz laser repetition rates for identifying Capnocytophaga species to the genus or species level. In conclusion, the MALDI TOF MS offers a reliable Capnocytophaga identification after following the universal protocol, while the formic acid extraction is restricted to species with a limited number of strains in the database.

Shaped liquid drops generate MeV temperature electron beams with millijoule class laser

MeV temperature electrons are typically generated at laser intensities of 1018 W cm−2. Their generation at non-relativistic intensities (~1016 W cm−2) with high repetition rate lasers is cardinal for the realization of compact, ultra-fast electron sources. Here we report a technique of dynamic target structuring of micro-droplets using a 1 kHz, 25 fs, millijoule class laser, that uses two collinear laser pulses; the first to create a concave surface in the liquid drop and the second, to dynamically-drive electrostatic plasma waves that accelerate electrons to MeV energies. The acceleration mechanism, identified as two plasmon decay instability, is shown to generate two beams of electrons with hot electron temperature components of 200 keV and 1 MeV, respectively, at an intensity of 4 × 1016 Wcm−2, only. The electron beams are demonstrated to be ideal for single shot high resolution (tens of μm) electron radiography.

Analog multiplexing of a laser clock and computational photon counting for fast fluorescence lifetime imaging microscopy.

The dynamic range and fluctuations of fluorescence intensities and lifetimes in biological samples are large, demanding fast, precise, and versatile techniques. Among the high-speed fluorescence lifetime imaging microscopy (FLIM) techniques, directly sampling the output of analog single-photon detectors at GHz rates combined with computational photon counting can handle a larger range of photon rates. Traditionally, the laser clock is not sampled explicitly in fast FLIM; rather the detection is synchronized to the laser clock so that the excitation pulse train can be inferred from the cumulative photon statistics of several pixels. This has two disadvantages for sparse or weakly fluorescent samples: inconsistencies in inferring the laser clock within a frame and inaccuracies in aligning the decay curves from different frames for averaging. The data throughput is also very inefficient in systems with repetition rates much larger than the fluorescence lifetime due to significant silent regions where no photons are expected. We present a method for registering the photon arrival times to the excitation using time-domain multiplexing for fast FLIM. The laser clock is multiplexed with photocurrents into the silent region. Our technique does not add to the existing data bottleneck, has the sub-nanosecond dead time of computational photon counting based fast FLIM, works with various detectors, lasers, and electronics, and eliminates the errors in lifetime estimation in photon-starved conditions. We demonstrate this concept on two multiphoton setups of different laser repetition rates for single and multichannel FLIM multiplexed into a single digitizer channel for real-time imaging of biological samples.

Stable, intense supercontinuum light generation at 1 kHz by electric field assisted femtosecond laser filamentation in air

Supercontinuum (SC) light source has advanced ultrafast laser spectroscopy in condensed matter science, biology, physics, and chemistry. Compared to the frequently used photonic crystal fibers and bulk materials, femtosecond laser filamentation in gases is damage-immune for supercontinuum generation. A bottleneck problem is the strong jitters from filament induced self-heating at kHz repetition rate level. We demonstrated stable kHz supercontinuum generation directly in air with multiple mJ level pulse energy. This was achieved by applying an external DC electric field to the air plasma filament. Beam pointing jitters of the 1 kHz air filament induced SC light were reduced by more than 2 fold. The stabilized high repetition rate laser filament offers the opportunity for stable intense SC generation and its applications in air.

Parallel Diffractive Multi‐Beam Pulsed‐Laser Ablation in Liquids Toward Cost‐Effective Gram Per Hour Nanoparticle Productivity

Nanoparticles (NPs) generated by pulsed‐laser ablation in liquids (PLAL) have benefited many key applications due to their versatility, enlarged surface area, and high purity. However, scaling up NPs production represents one of the main requisites to commercialize this technology. The established upscaling strategy demands high power and repetition rate laser source with fast scanning systems, which are not widely available and costly. Herein, a cost‐effective alternative is proposed, the addition of static diffractive optical elements to achieve parallel processing through the multi‐beam PLAL (MB‐PLAL). In MB‐PLAL, the optimum repetition rate is reduced to compensate laser energy splitting, hence achieving a higher interpulse distance, reducing pulse shielding, and increasing NPs productivity. MB‐PLAL with 11 beams reached a factor 4 productivity increase for iron–nickel alloy (Fe50Ni50) NPs compared to the single‐beam setup (0.4–1.6 g h−1), and a factor 3 increase for gold (Au) NPs (0.32–0.94 g h−1). The scalability of the proposed MB‐PLAL technique setup is confirmed by Au and Fe50Ni50 NPs productivity experiments using 1, 6, and 11 beams, showing a linear increase in productivity.