Femtosecond Laser Research Articles

Single-crystal silicon ablation with temporally delayed femtosecond laser double-pulse trains

Abstract A double-pulse femtosecond laser is used to process single-crystal silicon. Modulating the delay time was discovered to increase the ablation depth and improve the morphology of the ablated surface. The hole fabricated by a dual-pulse with a 200 ps interval is 24.4% deeper than that created by a single pulse of the same energy. Moreover, utilizing a dual pulse with an interval ranging from 100 to 1000 ps produces a considerably smoother ablation area as compared to the single pulse. The effect of the sub-pulse energy ratio of the double-pulse femtosecond laser on the size and morphology of the ablated area was also investigated. As the sub-pulse energy ratio decreases from 3:1 to 1:3, the size of the ablation area initially decreases and then increases, while the size of the ablation area is minimized when the sub-pulse ratio is 1:1, enabling precise control over the machining size. As the energy of the second sub-pulse increases, the ablation area becomes smoother due to the plasma heating of the double-pulse femtosecond laser.

Eight-wavelength-switchable single-frequency fiber laser with scattering-enhanced fiber-based wavelength self-adaptive linewidth compression

We demonstrate an eight-wavelength-switchable single-frequency (SF) ultra-narrow linewidth erbium-doped fiber laser (EDFL). A switchable polarization-dependent eight-channel filter (S-PDECF), fabricated using femtosecond laser direct-writing technology, determines the eight wavelengths of the laser output, with four channels concentrated near 1530 nm and the other four near 1550 nm. A dual-coupler ring-based compound-cavity filter is utilized to achieve single-longitudinal-mode selection for the EDFL. The introduction of a femtosecond laser direct-written scattering-enhanced fiber (SEF) effectively narrows the laser linewidth, enabling ultra-narrow linewidth outputs lasing at all eight wavelengths, and the wavelength self-adaptive linewidth compression of the SEF is verified for the first time. Furthermore, the incorporation of SEF significantly enhances the overall performance of the EDFL. The eight switchable single-wavelength laser outputs all exhibit exceptional characteristics, including spectral wavelength fluctuations of ≤0.011 nm, spectral peak power fluctuations of ≤0.54 dB, optical signal-to-noise ratios of ≥71 dB, linewidths of ≤248 Hz, output power fluctuations of ≤0.252 dB over a 10-min measurement period, and relative intensity noise of ≤−150.17 dB/Hz @≥ 3 MHz.

Quantification of locked mode instability triggered by a change in confinement

Abstract This work presents the first analysis of the disruptive locked mode (LM) triggered by the dynamics of a confinement change. It shows that, under certain conditions, the LM threshold during the transient is significantly lower than expected from steady states. We investigate the sensitivity to a controlled n = 1 error field (EF) activated prior to the L-H transition in the COMPASS tokamak, at q 95 ∼ 3, β N ∼ 1, and using EF coils on the high-field side of the vessel. A threshold for EF penetration subsequent to the L-H transition is identified, which shows no significant trend with density or applied torque, and is an apparent consequence of the reduced intrinsic rotation of the 2/1 mode during this transient phase. This finding challenges the assumption made in theoretical and empirical works that natural mode rotation can be predicted by global plasma parameters and urges against using any parametric EF penetration scaling derived from steady-state experiments to define the EF correction strategy in the entire discharge. Furthermore, even at EFs below the identified penetration threshold, disruptive locking of sawtooth-seeded 2/1 tearing modes is observed after about 30% of L-H transitions without external torque.

Generation of Quantum Vortex Electrons with Intense Laser Pulses.

Accelerating a free electron to high-energy forms the basis for studying particle and nuclear physics. Here it is shown that the wave function of such an energetic electron can be further manipulated with the femtosecond intense lasers. During the scattering between a high-energy electron and a circularly polarized laser pulse, a regime is found where the enormous spin angular momenta of laser photons can be efficiently transferred to the electron orbital angular momentum (OAM). The wave function of the scattered electron is twisted from its initial plane-wave state to the quantum vortex state. Nonlinear quantum electrodynamics (QED) theory suggests that the GeV-level electrons acquire average intrinsic OAM beyond at laser intensities of 1020Wcm-2 with linear scaling. These electrons emit γ-photons with two-peak spectrum, which sets them apart from the ordinary electrons. The findings demonstrate a proficient method for generating relativistic leptons with the vortex wave functions based on existing laser technology, thereby fostering a novel source for particle and nuclear physics.

Ultra-smooth processing of lithium niobate for outstanding mid-infrared transmittance.

The fabrication of anti-reflection (AR) subwavelength structures (SWSs) of lithium niobate (LN) is a challenging but rewarding task in mid-infrared LN laser systems. However, there are still some issues with the high-quality processing and fabrication of bifacial AR SWSs. Herein, a novel, to the best of our knowledge, approach to the fabrication of SWSs was proposed, which includes femtosecond laser ablation followed by wet etching and thermal annealing. The fabricated structures exhibit high surface quality (Ra = 0.08 nm) and uniformity. According to the experimental and simulated results, the transmittance of the mid-infrared AR SWSs with a period of 1.8 µm could be improved from 78% to 87% in the 3.6-5 µm band. Furthermore, the double-sided construction enabled a transmittance of up to 90%. The results have great potential in the promotion of the development of mid-infrared laser systems and LN-based photonics.

Optimized terahertz generation in BNA organic crystals with chirped Ti:sapphire laser pulses.

We report on the efficient generation of intense terahertz radiation from the organic crystal N-benzyl-2-methyl-4-nitroaniline pumped by chirped Ti:sapphire femtosecond laser pulses. The THz energy and spectrum as a function of the pump fluence and duration of the chirped laser pulses are studied systematically. For the appropriate positively chirped pump pulses, a significant boost in the THz generation efficiency by a factor of around 2.5 is achieved, and the enhancement of high-frequency components (>1 THz) shortens the THz pulse duration. Via complete characterization of THz properties and transmitted laser spectra, this nonlinear behavior is attributed to the extended effective interaction length for phase matching as a result of the self-phase modulation of the intense pump laser pulses. Numerical calculations well reproduce the experimental observation. Our results demonstrate a robust, efficient, strong-field (up to several MV/cm) THz source using the common sub-10 mJ and sub-100 fs Ti:sapphire laser systems without optical parametric amplifiers.

Amplification characteristics of N<i/>2+ lasing at multiple wavelengths generated by polarization-modulated femtosecond laser fields

Multiple gain routes complicate the amplification behaviors of N2+ lasing. A direct comparison of the amplification processes of various lasing lines of N2+ is still lacking to date, mainly because the efficient generation of different lasing lines requires different experimental conditions. In this work, to overcome the limitation, we use an intense polarization-modulated femtosecond laser pulse to simultaneously produce high-intensity N2+ lasing signals at 391 nm and 428 nm, permitting us readily performing their time-domain characterizations. Our results show that the primary amplification of 428-nm lasing is always retarded by a few picoseconds with respect to the probe pulse while the amplification of 391-nm lasing can be basically completed within the probe pulse duration at a relatively high gas pressure, and besides, the time duration of the former sustains several times longer than that of the latter. These observations indicate the non-negligible contribution of the initial electronic coherence established by the pump laser, apart from the external triggering pulse. For the 391-nm lasing, the seed amplification and superradiance can fiercely interplay within the probe pulse duration. However, for the 428-nm lasing, its amplification is in essence Raman-assisted superfluorescence due to the lacking of the initial electronic coherence and a direct triggering pulse. Our findings not only shed light on the physics of N2+ lasing but also promote the relevant studies on the collective emissions in the cascaded multi-level molecular ion system.

Optical quantification of the weak van der Waals coupling between multilayer antimonene and bilayer MoS2 using ultrafast coherent vibration spectroscopy.

Antimonene, a promising conductor for next-generation 2D-based devices, has its contact resistance significantly influenced by the van der Waals (vdW) interaction within its heterostructure. In this study, we report the quantification of the vdW coupling between multilayer antimonene and bilayer MoS2 by ultrafast coherent vibration spectroscopy. By utilizing a femtosecond laser, we excited coherent acoustic vibrations in the multilayer-antimonene on substrate-supported bilayer MoS2, and the relative displacement at the vdW heterojunction was detected with the aid of bilayer MoS2. The photoexcited strain pulse generated in the multilayer-antimonene was observed as it transported to the bilayer MoS2, explaining the distortion at the beginning of the oscillation. By analyzing the thickness-dependent oscillation frequencies, we determine the effective vdW elastic constant between multilayer-antimonene and MoS2 to be (1.9 ± 0.2) × 1018N/m3. This non-destructive optical technique offers a significant advance in the evaluation of vdW interactions at 2D metal-semiconductor interfaces.

Optimization of the Femtosecond Laser Machining Process for Single Crystal Diamond Using Response Surface Methodology

Femtosecond laser machining offers high precision and minimal thermal impact, making it a promising technique for processing hard and brittle materials like single-crystal diamonds (SCDs). In this study, the femtosecond laser machining process for SCD material was systematically optimized to improve both machining efficiency and quality. Initial single-factor experiments were conducted to explore the effects of key process parameters—laser power, scanning speed, and number of scans—on machining performance. Subsequently, response surface methodology (RSM)-based experiments designed using the Box–Behnken method were employed to comprehensively refine the process. A regression model was developed to analyze the data, and the interaction effects of the parameters were thoroughly evaluated. The validated model identified an optimal set of parameters, resulting in a significant improvement in machining performance. This research provides a comprehensive framework for optimizing femtosecond laser machining processes, offering valuable insights critical for the production of advanced lightweight components in industries such as aerospace, optical instruments, and high-performance electronics.

A biomimetic micro-texture based on shark surface for tool wear reduction and wettability change

Aiming at the problems of severe tool wear and difficulty in ensuring machining surface quality during the cutting process of Ni-base superalloy, a micro-texture design method resembling a shark surface is proposed, which simplifies the skin teeth on the shark surface into a diamond shaped structure with ribs. Different micro-textured samples were prepared using femtosecond laser, and surface wettability characterization and friction and wear experiments were conducted to study the wear reduction and anti wear effects of micro-texture. The parameter combinations with the maximum contact angle and the minimum coefficient of friction were selected. The wear mechanism of YG8 carbide alloy and GH4742, as well as the wear reduction mechanism of microstructure were revealed. Biomimetic cutting tools were prepared and their effectiveness in controlling tool wear, reducing cutting forces, and improving machining quality was verified through milling experiments.

Superhydrophilic Surface Creation and Its Temporal Transition to Hydrophobicity on Copper via Femtosecond Laser Texturing

We analyzed a process to fabricate a superhydrophilic surface on copper by forming various laser-induced periodic surface structures (LIPSS) using a Ti/sapphire femtosecond laser. For these structured surfaces, the correlation between the surface structure and the wetting characteristics was analyzed by scanning electron microscopy, atomic force microscopy, and contact angle (CA) measurement. X-ray photoelectron spectroscopy (XPS) was also employed to analyze variation of the elemental composition of the surfaces. The laser treatment produced micro/nanostructures composed of ripples whose length and width are in microscale and nanoscale, respectively. At specific conditions, the CA of a water droplet was reduced to less than 1°. The superhydrophilcity is attributed to the effect of nanoholes and nanoclusters, which consist of copper (II) oxide and copper hydroxide, having a hydrophilic effect on LIPSS. However, the pristine superhydrophilic surface spontaneously became hydrophobic after being exposed to air at room temperature for about 10 days. According to XPS analysis, the surface’s transition to hydrophobic is attributed not only to the decomposition of Cu(OH)2 but also to the adsorption of oxygen molecules and/or airborne organic molecules containing carbon, which further influences the wettability.

Femtosecond laser writing of durable open microfluidic channels via a mode-switchable strategy

Open microfluidic systems offer significant advantages, including the elimination of external pumps and facilitating fluid access at any point along the channel. However, their deployment in harsh environments is commonly compromised due to the delicate nature of hydrophilic chemical coatings and the vulnerability of open microchannels to clogging and contamination. Here, a bioinspired, demand-responsive mode-switchable strategy is proposed to enhance the mechanical durability of open microfluidic systems. Specifically, under harsh conditions or when long-term storage is necessary, this strategy allows the open microfluidic device to transition to a protective mode simply through releasing the strain, thereby preserving the integrity of the structure and hydrophilic coatings. The stretched open microfluidic mode enables spontaneous liquid spreading along a hydrophilic microchannel scribed by femtosecond laser. This mode-switchable strategy provides the open microfluidic device with robustness to maintain spontaneous liquid flow, even under severe testing conditions such as 2000 cycles of cotton swab rubbing, sand impact, sandpaper abrasion, tape peeling, twisting, and finger rubbing. A proof-of-concept application involving blood type analysis on this mode-switchable open microfluidic device showcases its superior mechanical durability under severe environmental conditions. The proposed strategy paves the way for the broader use of open microfluidic devices in various practical applications.

Targeted Microforming of Borosilicate Glass Induced by a Laser‐Ablation‐Free Process Using Femtosecond Pulses

AbstractThe use of ultrashort pulse lasers opens up a wide range of possibilities for processing dielectric materials. The present study focuses on the investigation of the ablation‐free microscopic surface modification of borosilicate glass using femtosecond laser radiation (350 fs at 515 nm wavelength) in a scanning machining process. Furthermore, its aim is to analyze the relationship between the laser process parameters and the microscopic changes in the surface topography observed. The characteristic of the generated surface structure, which takes place below the ablation threshold, shows both elevations and depressions within the irradiated field (2 × 2 mm2). The realized structures reach a profile height Peak‐to‐Valley (PV) of up to 10 µm. The amount of surface deformation depends on the selected parameters such as laser fluence, number of passes, and scanning strategy. The microdeformation is detected on both the top and bottom sides of the processed glass material with a thickness ≤1 mm. The influence of the temporal and spatial energy distribution on the material modification is discussed, demonstrating the possibilities of microforming of silicate glasses using ultrashort pulsed laser radiation.

Nonlinear optical studies of Bismuth-doped Titanium di-oxide colloids achieved by femtosecond Z-Scan technique

In this paper, we explored the nonlinear optical (NLO) properties of Bismuth (Bi) doped titanium dioxide (TiO2) nanoparticle (NP) colloids in ethanol, with laser pulses of duration ∼ 150 femtoseconds (fs). The Bi-doped TiO2 NPs were synthesized via the chemical route, sol-gel method. The increased photo response of TiO2 upon doping it with metals has become a burning issue to extend the applications of metal-doped TiO2 NPs as integral parts in solar cells, catalysts, phototherapy, etc. The bandgap of anatase TiO2 NPs was observed to be reduced to 2.89 eV upon doping Bi. The modified band structure of the Bi-doped TiO2 NPs demonstrates novel chemical, mechanical, and optical properties. NLO studies were conducted on Bismuth-doped TiO2 NPs submerged in ethanol employing femtosecond laser input pulses with incoming wavelengths 700, 750, 800, 850, 900, and 950 nm. It has been detected that open aperture (OA) studies performed at an input peak intensity of 100 MW/cm2 on Bi-doped TiO2 NPs in ethanol were exhibiting complex NLO behavior, i.e., reverse saturable absorption (RSA) in saturable absorption (SA) and SA in RSA with mostly effective two-photon absorption (1 + 1) coefficients. Particularly, at ∼800 nm, the behavior observed was so complex, i.e., RSA in SA in RSA in SA, with an effective 3 PA (2 + 1) co-efficient (γ) ∼6.5 × 10−24 m3/W2. The closed aperture (CA) studies at ∼37 MW/cm2 exhibited self-defocusing effects, i.e., an intensity-dependent refractive index (n2) with a negative signature.

Time-domain velocimetry via femtosecond laser electronic excitation tagging

Abstract The application is explored of a linear photodiode array with femtosecond laser electronic excitation tagging (FLEET) to determine the streamwise velocity of flow at Mach 8 in the Sandia hypersonic wind tunnel. Incident light emitted by nitrogen gas upon FLEET excitation is collected by intensifying optics that amplify and transmit the light to the photodiode elements as the gas advects downstream. In contrast to previous studies that track FLEET lines utilizing image sensors to measure the line displacement in space, instead for the first time the time-delay between the initial excitation and convection to the measurement location is used to determine the velocity temporally.

Surface preparation by femtosecond laser - An adhesion strength perspective for suspension plasma sprayed ceramic coatings

Surface preparation is very important for the adhesion of thermal sprayed coatings. In this work, the effect of surface preparation on the adhesion strength of suspension plasma sprayed (SPS) Al2O3 coatings is investigated. Aluminum (Al-2024) substrates were micromachined by femtosecond laser (FSL) and also mechanically blasted by conventional grit blasting for surface preparation prior to SPS coating. Surface morphology results showed laser-induced periodic surface structures (LIPSS) produced by FSL, whereas grit blasting produced random irregularities with peaks and undercuts. Surface chemistry analysis showed that the FSL-prepared substrates were functionalized by -OH and O, while the grit-blasted substrates were not functionalized. The prepared substrates were coated by the SPS process under two spray conditions, where one condition was selected to produce a dense coating and the other condition was selected to produce a porous coating. The dense coating showed poor adhesion regardless of surface preparation, while the porous coating showed good adhesion for both grit-blasted and FSL-treated substrates. The adhesion strength of the porous coating was evaluated using the ASTM C-633 pull test protocol. A significant improvement (4.5 factors higher) in adhesion strength was achieved for the FSL-prepared substrates compared to the grit-blasted substrates. The improved adhesion of the FSL-prepared substrates was assessed by surface chemistry and surface morphology analyses of the prepared substrates, as well as by further analysis of the failed samples after the adhesion test and splat analysis. Results from this research showed that FSL has potential as an effective surface preparation technique for SPS.

Femtosecond laser controllable annealing for color centers based on ion-implanted silicon carbide substrate

As a representative of third-generation semiconductors, silicon carbide serves as a new platform for quantum nano-optics with readable paramagnetic color centers such as silicon vacancies. The potential applications include biomedical imaging, quantum sensing and micro/nano environment detection. Color centers are usually prepared based on ion-irradiated substrates, then annealed at high temperature to increase the yield. Currently, annealing furnaces have been the main method to heat the substrate and achieve annealing of the whole sample. However, further exploration is still necessary for fine processing requirements such as specialized annealing regions with certain shapes. In this work, we applied femtosecond laser to achieve controllable annealing at micron scale, based on silicon carbide substrate after ion injection. The whole process of point, line and pattern annealing was realized. The annealing area demonstrates enhanced photoluminescence, attributed to silicon vacancy color centers. Raman and photoluminescence spectra reveal that the annealing effect is primarily influenced by the number of pulses per unit area. The mechanism of femtosecond laser annealing was introduced. Finally, the controllable annealing areas with various shapes were prepared based on the comprehensive optimal parameters (100 kHz pulse repetition frequency, 105 point annealing pulses, 10 μm/s line scanning speed). This annealing strategy demonstrates excellent stability and uniformity, as well as micron scale accuracy. Additionally, the excited state lifetime of silicon vacancy was extracted and quantified. The result means that femtosecond laser annealing enhances the optical properties and quantum application potential of color centers. This work can be further extended to other semiconductors as well as patterned modification of local surface/planes at some depth, and helpful to explore the mechanism and applications of femtosecond laser annealing.

Dynamics of femtosecond laser-driven liquid jets

The complex dynamics of flowing liquids under external stimuli can lead to significant changes in the direction of liquid jets. In this study, we utilized a femtosecond laser to drive a flowing liquid column and analyzed the effect of varying laser energies on the direction of the liquid flow. Additionally, we examined how the diameter and volume of the liquid column influenced its directional behavior when driven by the laser. By comparing the driving angles under different conditions, we explored the principles governing the femtosecond laser-driven liquid columns. These findings offer valuable insights into the field of liquid dynamics, showcasing femtosecond lasers as a good method for controlling liquid flows.

Laser-zoned treatment of magnesium surfaces with predictable degradation applications

Magnesium metal is a promising material for medical applications due to its biocompatibility and similar modulus of elasticity to human bone. However, its complex corrosion process must be addressed before it can be used clinically to match post-implantation tissue repair. This study aims to regulate material degradation by utilizing laser surface treatment. The surface of pure magnesium was modified using nanosecond and femtosecond laser methods to create various micro-nanostructures, such as chain, streak, column, and groove structures. Surface roughness and wettability tests revealed that the groove structures had higher roughness values. All structures exhibited hydrophilicity, but the femtosecond laser-generated structures were more hydrophilic. Electrochemical tests and immersion experiments demonstrated that femtosecond laser modification significantly improved the corrosion resistance of magnesium metal compared to polished samples. Cytotoxicity experiments showed that the laser-treated magnesium was not cytotoxic. Based on the results, we constructed various structures on the magnesium rods in different regions. As a result, the rods exhibited multi-stage biodegradation behavior in simulated body fluids (SBF). This study presents a novel approach to controlling the degradation sequence of medical metals.

A comparative study for demonstrating the effect of the assist gas and E-field on hole generation characteristics induced by femtosecond laser layered ring hole-cutting of magnesium alloy AZ91D sheets

The widely-used magnesium alloy AZ91D is light-weight and energy-saving with good castability and mechanical performance. The traditional machining way for magnesium and its alloys is the high-speed mechanical contact cutting, which usually leads to uncontrollable intense ignition with some notable heat-induced machining defects. However, it has been rarely reported that the non-contact precision laser machining is used for cutting the magnesium and its alloys. The problems such as the local heat effect, the heat-induced defects and the local ignition as well as the vapor-plasma plume usually encountered during laser machining of magnesium and its alloys. Hence, in this study, a hybrid laser cutting way of femtosecond laser layered ring hole-cutting using the assist gas and E-field is reported to machine micro holes in the AZ91D material. The effect of the lateral E-field and assist gas on the hole formation characteristics was demonstrated for femtosecond laser cutting of the AZ91D sheets. It was indicated that the use of the lateral E-field and assist gas suppressed the ignition as well as the vapor-plasma plume. The hole depth was increased by the external assistance used especially the lateral nitrogen blowing. The hole wall formation quality was improved by using the lateral E-field and compressed air blowing, while it got poorer if using the lateral nitrogen blowing. The ignition-induced carbonization was effectively suppressed by the assist gas used. The hole depth, microstructure and micro hardness were improved by using the lateral E-field and assist gas.