Femtosecond Laser Research Articles

Ultrafast thermal modulation dynamics during ripple formation induced by a femtosecond laser multi-pulse vortex beam

Abstract This work theoretically investigated the ultrafast thermal modulation dynamics during early formation of ripples on an Au film induced by femtosecond laser multi-pulse vortex beam irradiation. An extended two-temperature dynamics model that comprehensively considers optical interference modulation for the formation of seed ripples, transient reflectivity and non-equilibrium thermal transfer was self-consistently built to predict high-contrast ripple formation. The two-dimensional evolution of electron and phonon temperature modulations during ripple formation in a high non-equilibrium state of Au film were obtained via femtosecond laser multi-pulse vortex beam irradiation. It was revealed that ripple contrast can be significantly amplified by shortening the laser wavelength, increasing the pulse number, or enlarging the laser fluence of the vortex beam. Moreover, the electron–phonon coupling time during ripple formation is fully explored in detail. This study provides valuable insights into optimizing laser parameters for controlled high-contrast ripple formation on Au films.

Femtosecond Laser Direct-writing Tilted Fiber Bragg Gratings in Multi-core Fiber

Femtosecond Laser Direct-writing Tilted Fiber Bragg Gratings in Multi-core Fiber

Advances in the research of femtosecond laser-assisted lenticule intrastromal keratoplasty for the correction of hyperopia

Lenticule intrastromal keratoplasty (LIKE) was introduced clinically in the 1980s as a refractive surgery. With the advancements of femtosecond laser, LIKE has been significantly revitalized. Currently, femtosecond laser-assisted LIKE has been demonstrated to be a safe and effective surgical procedure to correct hyperopia, but the predictability is still a major limitation. In this article, we review the history, procedures, influencing factors of predictability and challenge of LIKE to provide reference for clinical practice of femtosecond laser-assisted LIKE.

Lead‐Sulfide‐Based Hybrid Inorganic‐Organic Superlattice Particles for Prominent Nonlinear Optical Absorption

AbstractSimultaneously optimizing nonlinear absorption coefficient and modulation depth is a considerable challenge for nonlinear optical materials. Here we present an effective solution through constructing PbS2‐based inorganic‐organic superlattice. PbS2/Cn superlattice particles are synthesized, where n (4, 6, and 8) denotes the number of carbon atoms in the organic component. First‐principles simulations indicate the formation of covalent bonds and van der Waals interaction between PbS2 unit and interlayer organic molecules. All samples exhibit strong nonlinear absorption under femtosecond laser excitation with a wavelength range between 515 nm and 900 nm, and the nonlinear absorption coefficient increases with interlayer distance. The optimized sample, PbS2/C8, demonstrates outstanding nonlinear absorption and substantial modulation depth under 800 nm (the third‐order nonlinear absorption coefficient βeff, 10449 ± 609 cm GW−1; modulation depth, 39.1%) and 900 nm (the fifth‐order nonlinear absorption coefficient γeff, 6465 ± 68 cm3 GW−2; modulation depth, 88.1%), which exhibits saturable absorption under 515 nm (βeff, ‐4932 ± 818 cm GW−1; modulation depth, 48.1%). It exhibits a small optical limiting threshold of 1.23 mJ cm−2. These performances surpass those of typical single‐/few‐layer metal chalcogenides. Structural and spectral analyses elucidate that the remarkable optical nonlinearity can be attributed to quantum confinement in the inorganic layer and the dielectric enhancement of the superlattice.

Sub-picosecond mode-locked laser operation in a femtosecond-laser-inscribed Yb:CaF2 channel waveguide

We present the successful demonstration of both Q-switched mode-locked (QSML) and continuous-wave mode-locked (CWML) operation in a femtosecond-laser-inscribed Yb:CaF2 double-track waveguide (WG) structure. A semiconductor saturable absorber output coupler (SOC) was used as the mode locker with fine tuning of the intracavity group delay dispersion (GDD) achieved through the Gires-Tournois interference (GTI) effect induced by an air gap. To ensure sufficient fluence on the saturable absorber, the cavity was extended to 50 cm by inserting two lenses between the SOC and WG, resulting in a repetition frequency of ∼300 MHz. In the QSML regime, the laser exhibited an amplitude modulation period of 65 kHz within Q-switched pulses of 3-µs duration. Notably, in the purely CWML regime, the laser generated a maximum output power of 51 mW near 1036 nm with a pulse width of 979 fs.

Electrophysiological mechanisms of single-neuron stimulation using a focused femtosecond laser

Electrophysiological mechanisms of single-neuron stimulation using a focused femtosecond laser

High-Efficiency Targeted Gene Transfection of Cells Using Temporal and Spatial Shaping Femtosecond Laser Irradiation.

Laser-irradiation-assisted cell gene transfection is sterile and nontoxic, but the low transfection efficiency cannot meet the application requirements. To improve the efficiency, a temporal and spatial shaping method of a femtosecond laser is proposed. Using the time shaping method, we can segment the pulse into subpulses of varying energies and with a defined delay, thereby influencing the interaction between electrons and photons, ultimately enhancing transfection efficiency. The transfection efficiency is further improved by spatially shaping the laser pulse to extend the focusing beam's working distance and reduce the cell's sensitivity to the focal position. Through the characterization of the viability and transfection efficiency of HEK-293T cells, the method achieved efficient and active transfection, with a maximum transfection efficiency of 45.1% and a cell survival rate of 93.6%. This method provides key technical support for femtosecond laser transfection and promotes its further application in clinical practice.

A multi-method study of femtosecond laser modification and ablation of amorphous hydrogenated carbon coatings

We present a study on femtosecond laser treatment of amorphous hydrogen-containing carbon coatings (a-C:H). The coatings were deposited on silicon wafers by a plasma-assisted chemical vapour deposition (PA-CVD), resulting in two different types of material with distinct properties (referred to as “absorbing” and “semi-transparent” coatings in the following). The samples were laser-treated with single fs-laser pulses (800 nm center wavelength, 35 fs pulse duration) in the ablative regime. Through a multi-method approach using topometry, Raman spectroscopy, and spectroscopic imaging ellipsometry, we can identify zones and thresholds of different fluence dependent effects and have access to the local dielectric function. The two coating materials react significantly different upon laser treatment. We determined the (non-ablative) modification threshold fluence for the absorbing coating as 3.6×10-2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$3.6\ imes {10}^{-2}$$\\end{document} Jcm−2 and its ablation threshold as 0.22 Jcm−2. The semi-transparent coating does not show such a low-fluence modification but exhibits a characteristic interference-based intra-film ablation mechanism with two distinguishable ablation thresholds at 0.25 and 0.28 Jcm−2, respectively. The combination of tailored layer materials and correlative imaging spectroscopic methods delivers new insights into the behaviour of materials when treated with ultrashort-pulse laser radiation.Graphical abstract

Effect of surface treatments and bonding type on elemental composition and bond strength of dentin

The objective of this study was to explore the effect of different surface treatments and bonding types on elemental composition and bond strength of dentin. Under water cooling, 1.5 mm of tooth structure containing just dentin was cut from 39 extracted human molars. Dentin surfaces were untreated (control) or treated by erbium: yttrium aluminum garnet or femtosecond laser (n = 13, each). One sample from each group underwent scanning electron microscopy. Then, dentin surfaces were bonded by Clearfil SE Bond or Clearfil SE Protect (n = 6, each). Energy dispersive X-ray spectroscopy was performed both after surface treatment and bonding application. The dual-polymerized resin cement was applied to dentin surfaces with a special teflon mold (diameter:3 mm × height:3 mm). After polymerization of the resin cement, shear force was applied at the resin cement-dentin interface. Elemental composition value (weight%) of dentin after surface treatment was analyzed by one-way analysis of variance (ANOVA), and the difference value in pre and post-bonding elemental composition by two-way ANOVA. Paired t-tests were executed to compare the weight% values of each element before and after each bonding application. Bond strength was analyzed by two-way ANOVA. The post-hoc test was Tukey’s honest significant difference test. Both laser treatments increased the mineral content of dentin, compared to the controls (P<0.05). Application of bonding agents decreased the mineral content of dentin compared to the surface treated dentin. Bond strength was unaffected by either surface treatment or bonding type (P>0.05). For resin cementation, either surface treatment is suitable. After laser treatment, Clearfil SE Bond is recommended over Clearfil SE Protect.

Effects of various laser applications on surface roughness and bond strength to veneering composites of polyether ether ketone (PEEK) and polyether ketone ketone (PEKK) materials.

The aim of this study was to investigate the bond strength between PEEK/PEKK and composite resins after various laser treatments and to compare the effectiveness of lasers on these polymers. 130 disc-shaped PEEK and PEKK blocks were obtained (10mm diameter-4mm height). One sample from each group (10 in total) was selected for scanning electron microscopy (SEM) analysis. The samples were randomly divided into 5 different surface treatment groups for each material (PEEK and PEKK): Er: YAG laser, Nd: YAG laser, diode laser, femtosecond laser and control (no surface treatment) (n = 12). Baseline and post-treatment surface roughness measurements were performed using a profilometer. The composite resin was bonded and SBS was measured. Comparisons among the groups were conducted via Kruskal-Wallis, one-way ANOVA; Tukey and Dunn tests were used as a post hoc test (p ≤ 0.05). All lasers significantly increased the roughness values of the PEEK and PEKK samples. In terms of shear bond strength; the Er: YAG and femtosecond laser groups had the highest values and the Nd: YAG, diode and control groups had the lowest values of the PEEK samples (p ≤ 0.05). The control group had the highest bond strength values and the femtosecond group had the lowest values for PEKK samples (p ≤ 0.05). All laser treatments increased the surface roughness of the PEEK and PEKK. Lasers increased the bond strength of PEEK to the veneering composite resin and decreased the bond strength values of PEKK. This shows that lasers behave differently in PEEK and PEKK materials.

Evaluation of a Novel Femtosecond Laser Ablation System for In Situ Analysis Based on Two-Dimensional Galvanometer Scanners.

Uneven energy distribution of femtosecond lasers presents a significant challenge for single-spot analysis, which often leads to concave ablation craters. This study assesses the performance of a femtosecond laser ablation system for in situ analysis using novel galvanometer scanners. A galvanometer rapidly moved the laser beam focus to create craters with a small beam spot. We first examined the inductively coupled plasma mass spectrometry (ICP-MS) signal sensitivity to laser parameters, establishing a strong linear correlation with the laser energy, repetition rate, scanning ablation area, and galvanometer scanning frequency. Elemental fractionation analysis of NIST SRM 610 suggests minimal bias, with fractionation indices of different elements approaching unity. Subsequently, the elemental concentration of six reference material glasses was measured by fsLA-ICP-MS to evaluate the elemental quantification capabilities of the Galvo-femtosecond laser (Galvo-fsLA). The laser's capability for in situ U-Pb dating was demonstrated by concordant U-Pb ages of five zircon reference materials that are highly consistent with the ID-TIMS ages reported previously. Finally, the reliability of the new Galvo-fsLA for isotope analysis was verified by the accurate determination of radiogenic Hf, Pb isotopes, and stable Cu isotopes, all agreeing well with their reference values within uncertainties. These assessments underscore the significant potential of Galvo-fsLA for enhanced accuracy and precision of single-spot in situ analysis.

Near-complete extraction of maximum stored energy from large-core fibers using coherent pulse stacking amplification of femtosecond pulses

High field science relies on ultrashort pulse lasers with multi-joule pulse energies for studying light–matter interactions under extreme conditions and for driving particle accelerators and secondary radiation sources of x rays, gamma rays, neutrons, positrons, muons, and protons. Next-generation laser drivers will require a 103−104 times increase in pulse repetition rates, producing multi-joule energies at multi-kilowatt average powers to enable practical applications in nuclear engineering, advanced materials, medicine, biology, homeland security, and high-energy physics. Spatially coherently combined femtosecond fiber lasers are recognized as a pathway to these next-generation drivers, with significant practical advantages including high efficiency and the possibility of compact integration. However, chirped pulse amplification in fibers is capable of extracting only a small fraction (usually ∼1%) of the maximum stored energy. Here we demonstrate near-complete maximum stored energy extraction with low accumulated nonlinearity from a large-core fiber amplifier using coherent pulse stacking amplification. We have amplified a 81-pulse stacking burst in a 85 µm core chirally coupled core Yb-doped fiber, extracting up to 9.5 mJ (∼90% of stored energy) with &lt;4.5 radians of accumulated nonlinear phase, temporally combined this burst into a single pulse, and achieved 4.2 mJ pulses of 313 fs bandwidth-limited duration after compression. This represents, to our knowledge, the highest energy extracted and compressed into a femtosecond pulse from a single fiber amplifier, enabling approximately two orders of magnitude size reduction of future high-energy coherently spatially combined fiber laser arrays.

Ultra-compact on-chip camera based on optoelectronic compound eyes with nonuniform ommatidia

Abstract Compound eyes (CEs) that feature ultra-compact structures and extraordinary versatility have revealed great potential for cutting-edge applications. However, the optoelectronic integration of CEs with available photodetectors is still challenging because the planar charge-coupled device (CCD)/complementary metal oxide semiconductor (CMOS) detector cannot match the spatially distributed images formed by CE ommatidia. To reach this end, we report here the optoelectronic integration of CEs by manufacturing 3D nonuniform ommatidia for developing an ultra-compact on-chip camera. As a proof-of-concept, we fabricated microscale CEs with uniform and nonuniform ommatidia through femtosecond laser two-photon photopolymerization, and compared their focusing/imaging performance both theoretically and experimentally. By engineering the surface profiles of the ommatidia at different positions of the CE, the images formed by all the ommatidia can be tuned on a plane. In this way, the nonuniform CE can be directly integrated with a commercial CMOS photodetector, forming an ultra-compact CE camera. Additionally, we further combine the CE camera with a microfluidic chip, which can further serve as an on-chip microscopic monitoring system. We anticipate that such an ultra-compact CE camera may find broad applications in microfluidics, robotics, and micro-optics.

Buried depressed cladding waveguides in neodymium-doped yttrium aluminate crystals fabricated by femtosecond laser writing: Raman properties and laser performance

Buried depressed cladding waveguides in neodymium-doped yttrium aluminate crystals fabricated by femtosecond laser writing: Raman properties and laser performance

Linear and nonlinear optical properties of femtosecond laser inscribed waveguides into GLS glass

Gallium lanthanum sulfide (GLS) glass is a promising material for mid-infrared photonics due to its wide transmission window and its high nonlinear refractive index that is almost three orders of magnitude higher than that of fused silica. In this paper, we present the results of a detailed study into the linear and nonlinear optical properties of waveguides fabricated in GLS glass via ultrafast laser direct-inscription using three different techniques: cumulative heating in the thermal regime as well as multi-scan and half-scan in the athermal regime. Using quadriwave lateral shearing interferometry, we fully characterized the refractive index profiles of such inscribed waveguides and found no difference between half-scan and multi-scan writing which indicates the absence of laser-induced stress in this soft glass in stark contrast to fused silica. In terms of nonlinearity, we utilized self-phase modulation (SPM)-induced spectral broadening experiments at mid-IR wavelengths to demonstrate that waveguides fabricated in the athermal regime preserve the high intrinsic nonlinearity of the GLS bulk material, outperforming those written in the thermal regime based. These findings pave the way for the fabrication of fiber-coupled optical waveguide chips for nonlinear mid-infrared photonics.

Four-level diffractive photon sieves by deep-UV femtosecond laser ablation

Four-level diffractive photon sieves by deep-UV femtosecond laser ablation

Delay drift compensation of an optoelectronic oscillator over a large temperature range through continuous tuning

Phase noise reduces target sensitivity in radar and increases bit error rate in telecommunications systems. Optoelectronic oscillators are known for using optical fibre technology to realise the large delay required to attain superior phase noise performance compared to conventional microwave source technology. However, the long fibre is vulnerable to environmentally induced phase perturbations, while conventional phase shifters have insufficient range to compensate for the phase drift over the operational temperature range without the use of a temperature-controlled enclosure. Here we introduce a vector modulator controlled by a Stuart-Landau integrator, as a solution to non-efficient tuning for the phase shift. The concept is verified by simulation and experimentally demonstrated using an optoelectronic oscillator phase-locked to a system reference. Phase lock is maintained over four free spectral ranges equivalent to a tuning phase range of 1440° when the optoelectronic oscillator is cycled over a 10 °C to 85 °C temperature range. These demonstrations highlight the practical potential of our continuous tuning method.

Femtosecond laser-inscribed in-fiber Mach-Zehnder interferometer for ultra-sensitive small-angle torsion measurement

We propose and demonstrate a novel in-fiber Mach-Zehnder interferometer (MZI), using a femtosecond (FS) laser to inscribe two parallel straight waveguides along both sides of the core-cladding boundary of an optical fiber. Both simulations and experiments demonstrated that constructing a second straight waveguide (WG2) on the opposite side of the fiber core significantly enhances the mode coupling efficiency in fiber, resulting in ultra-sensitive small twist-angle torsion measurement by intensity interrogation. By finely adjusting the waveguide spacing between the WG2 and the fiber core during the FS fabrication process, a large tuning range of ∼ 200 nm for the designable interference wavelength with maximal fringe contrast has been achieved. Through comparative experiments, the proposed in-fiber MZI performs ultra-sensitivity with 17417.92 dB/(rad/mm) over small twist angles from −10° to 0° for the sample “WG1-4/WG2-4” and 19480.57 dB/(rad/mm) over small twist angles from 0° to 10° for the sample “WG1-4/WG2-6”. Besides, the device is insensitive to temperature and strain. The exhibited advantages of high sensitivity, high reproducibility, and low crosstalks promote the proposed in-fiber MZI to be applied in future industrial engineering monitoring and intelligent health monitoring.

All-optical switching in nonlinear topological waveguide arrays

Photonic topological states are prospective in integrated optical devices due to their robustness to perturbations and defects. When taking into account the nonlinear effects of the system, the functionality of topological photonics can be further enhanced. Here, we investigated the interplay between topological edge states and nonlinear effects based on the Su–Schrieffer–Heeger (SSH) model. Relying on the theory prediction that topological edge states would shift upward under the action of nonlinearity, two types of optical switching are designed and experimentally realized in femtosecond laser direct-write waveguide arrays. This work provides a new, to the best of our knowledge, approach to preparing all-optical switches and offers a new perspective on the application of nonlinearity in topological optical devices.

InAs/GaSb superlattice SESAM mode-locked fluoride fiber laser at 2.8 µm

We demonstrate a mode-locked fluoride fiber laser operating at 2.8 µm using an InAs/GaSb superlattice (SL) semiconductor saturable absorber mirror (SESAM). Z-scan measurements show that the SL saturable absorber layer possesses off-bandgap nonlinearity near 2.8 µm, with a saturation fluence of 15.9 µJ/cm2, a single-pass modulation depth of 2.4%, and a damage threshold of 10.4 mJ/cm2. Using this high-damage-threshold SESAM, we achieved a maximum average output power of 1.16 W from the 2.8 µm mode-locked Er:ZBLAN fiber laser, with a pulse duration of 28.1 ps. Continuous mode-locking operation at an average power of 224 mW was sustained for 36 h without interruption, demonstrating the reliability of the InAs/GaSb SL SESAM for the development of mid-infrared mode-locked fluoride fiber lasers.