Ultrafast Laser Interaction Research Articles

China's Top 10 Optical Breakthroughs: Ultrafast Laser Interactions with Glass——From Phenomena to Regulation (Invited)

China's Top 10 Optical Breakthroughs: Ultrafast Laser Interactions with Glass——From Phenomena to Regulation (Invited)

Modeling of inhomogeneous heating behavior in ultrafast laser interaction with dielectric–dielectric nanocomposite with band gap contrast

Inspired by the optical data storage and photothermal therapy applications of metal–dielectric nanocomposite materials relying on the difference in laser absorption mechanism between metal and dielectric, the ultrashort single pulse laser irradiation of zirconia–alumina nanocomposite, a dielectric–dielectric nanocomposite with band gap contrast, was investigated. A femtosecond (fs) laser of 250 fs pulse duration and 10 W maximum average power was used for the experiment. A seemingly unusual phase melting phenomenon - whereby the higher melting point zirconia phase (2715 °C) was melted, while the lower melting point alumina phase (2072 °C) remained intact - was observed, thereby suggesting a huge temperature heterogeneity in the composite under ultrashort single pulse laser irradiation. Theoretical modeling indicates that the material band gap plays a significant role in laser energy absorption under an intense laser field. A sharp increase in the total absorbed laser energy and concomitant temperature increase around the laser ablation threshold was revealed. Under appropriate laser fluence, the smaller band gap zirconia phase (5.8 eV) can absorb much more laser energy than the alumina phase (8.8 eV), resulting in a remarkable temperature difference between these two phases. This research demonstrates the capability of generating huge phase specific temperature differences in composite nanostructures composed of phases with band gap contrast by using ultrashort pulsed laser irradiation.

Ultrafast pulsed laser stealth dicing of 4H-SiC wafer: Structure evolution and defect generation

Ultrafast pulsed laser for wafer processing has been promising in the semiconductor industry due to its high precision and low thermal effect. The microstructure and defect during the stealth dicing process is critical to controlling the desired quality of the wafer. The structural evolution and defect formation mechanisms during stealth dicing of 4H-SiC with femtosecond and picosecond lasers were investigated. In order to understand the ultrafast pulsed laser interaction with 4H-SiC wafer, we studied the laser intensity dependence of the nonlinear refractive index, the propagation of laser beam focusing in 4H-SiC, the laser absorption, electron concentration evolution and energy deposition. The effect of two significant processing factors was discussed, including laser fluence and laser scribing times. The propagation paths and energy deposition of femtosecond and picosecond lasers in the internal processing of 4H-SiC were calculated by finite element method. The evolution of electrons temperature, lattice temperature and free electron concentration in the material during laser processing was analyzed. Femtosecond laser stealth dicing had higher processing accuracy due to minimal thermal effects. The defect induced by femtosecond laser was dominated by nanovoids and micro-cracks. The defect structure was affected by laser processing parameters such as scanning passes and laser power density. Multi-focusing of ultrafast pulsed lasers in transparent materials was generally considered to be a dynamic balance of nonlinear self-focusing and plasma defocusing. Picosecond laser processing resulted a higher processing efficiency under the same laser energy density comparing with femtosecond laser processing. However, defects during picosecond laser dicing were dominated by thermal ablation, which impaired the processing accuracy.

Ultrafast laser interaction with transparent multi-layer SiO2/Si3N4 films

We investigate the use of ultrafast lasers exposure to induce localized crystallization and elemental redistribution in amorphous dielectric multi-layers, composed of alternating Si3N4 and SiO2 layers of sub-micrometer thickness. Specifically, we report on the occurrence of a laser-induced elemental intermixing process and the presence of silicon nanocrystals clusters localized within the multi-layers structure. The spatial distribution of these clusters goes significantly beyond the zone under direct laser exposure providing evidence of energy being channeled transversely to the laser propagation axis at the interface of the nanoscale layers. Thanks to the extreme conditions reigning during laser exposure, this process transposed to various materials may offer a pathway for local and selective crystallization of a variety of compounds and phases, difficult to obtain otherwise.

Molecular-dynamics study of multi-pulsed ultrafast laser interaction with copper

Ultrafast laser has an undeniable advantage in laser processing due to its extremely small pulse width and high peak energy. While the interaction of ultrafast laser and solid materials is an extremely non-equilibrium process in which the material undergoes phase transformation and even ablation in an extremely short time range. This is the coupling of the thermos elastic effect caused by the pressure wave and the superheated melting of the material lattice. To further explore the mechanism of the action of ultrafast laser and metal materials, the two-temperature model coupling with molecular dynamics method was used to simulate the interaction of the copper and laser energy. Firstly, the interaction of single-pulsed laser and copper film was reproduced, and the calculated two-temperature curve and the visualized atomic snapshots were used to investigate the influence of laser parameters on the ablation result. Then, by changing the size of the atomic system, the curve of ablation depth as a function of laser fluence was obtained. In this paper, the interaction of multi-pulsed laser and copper was calculated. Two-temperature curve and temperature contour of copper film after the irradiation of double-pulsed and multi-pulsed laser were obtained. And the factors which can make a difference to the incubation effect were analyzed. By calculating the ablation depth under the action of multi-pulsed laser, the influence of the incubation effect on ablation results was further explored. Finally, a more accurate numerical model of laser machining metal is established and verified by an ultra-short laser processing experiment, which provides a new calculation method and theoretical basis for ultra-fast laser machining of air film holes in aviation turbine blades, and has certain practical guiding significance for laser machining.

Experimental observation of the effect of pulse duration on optical properties in ultrafast laser micro-processing of polymers

Polymers are important materials for both industrial and scientific applications. However, it is challenging to efficiently process polymers with an ultrafast laser due to their low melting point, high bandgap, and different absorption mechanics with different laser parameters. It is common practice in industries to use different kinds of lasers and, therefore, different laser parameters, such as pulse duration, wavelength, pulse energy, etc. In particular, the effect of pulse duration during ultrafast laser interaction with polymers is significant as the absorption mechanism can be different with different pulse durations. In this study, the effect of pulse duration is investigated during the ultrafast laser interaction with transparent polypropylene (TPP), which is an important polymeric material widely used in many industrial applications. This study is based on the experimental measurement of the optical properties of TPP during ultrafast interaction, where optical properties, i.e., reflectance, transmittance, and absorptance, are determined by performing time-resolved measurements in single-pulse configurations. This experiment is carried out by collecting in situ data of the reflection and transmission of each laser pulse in an ellipsoidal reflector-based experimental setup, which enables the collection of both specular and diffusive reflection with nearly full coverage, and absorption is calculated from the experimental results. It is found that TPP undergoes a dramatic morphological change with different pulse durations ranging from 167 fs to 1 ps, which is correlated with the change of optical properties during the ultrafast laser interaction with TPP for different pulse durations. This result will be useful for controlling the processing of polymers with ultrafast lasers for industrial applications.

Genesis of Nanogratings in Silica Bulk via Multipulse Interplay of Ultrafast Photo‐Excitation and Hydrodynamics

AbstractStructuring below diffraction limit is key to developing new laser processing technologies as well as to understanding light‐induced processes on mesoscopic scales, notably self‐organization. Here, an advanced numerical perspective on the generation of embedded self‐arranged sub‐wavelength periodic patterns is developed, describing multipulse ultrafast laser interaction with bulk silica glass. Combining light and material dynamics, the approach couples self‐consistently nonlinear propagation, electronic excitation, and fluid dynamics resulting in irreversible phase transitions and localized damage. With increasing the number of applied pulses, the modification changes from localized nanovoids and elongated random nanopatterns toward regular void nanogratings dominantly covering the spot of the focused laser beam. Driven by local and collective scattering events, the order imposed by electric field patterns is then amplified and stabilized by the material response. The model predicts the gradual evolution of the optical properties considering the complex interplay between material arrangement and the electromagnetic field distribution. It allows thus to define light transport optical functions optimizing losses and anisotropic effects.

Polarization-dependent microstructural evolution induced by a femtosecond laser in an aluminosilicate glass

Manipulation of femtosecond laser induced microstructures in glass by tuning the laser polarization has great potential in optics. Here we report two different polarization-dependent microstructures and their evolution with pulse repetition rate in an aluminosilicate glass induced by femtosecond laser irradiation. A V-shaped crack oriented parallel to the laser polarization plane is induced at the bottom of modified regions by pulses operated at 200 kHz, 1030 nm, and 300 fs. Further increasing the pulse repetition rate to 500 kHz leads to the formation of a dumbbell-shaped structure, which is elongated perpendicularly to the laser polarization, at the top of the modified region. The size of the coloration area and the dumbbell-shaped structure can be controlled by tuning the pulse duration. Further investigation indicates that higher numerical apertures are in favor of the presence of the polarization effects in femtosecond laser irradiation. The possible mechanism responsible for the formation of the two microstructures is discussed. These results could be helpful for understanding of ultrafast laser interaction with glass.

Terahertz generation from surface of the bulk and monolayer tungsten diselenide

The study of ultrafast laser interaction with graphene-like materials based on transition metal dichalcogenides attracts most scientific groups. It is connected with potential use of these materials in flexible optoelectronic devices of visible and THz range. In this paper the parameters of generation of terahertz field from the surface of bulk layered crystal and monolayer film of tungsten diselenide are analyzed. Generation of terahertz radiation from the surface of experimental samples was studied by the terahertz time-domain spectroscopy in reflection geometry. Bulk layered crystals of tungsten diselenide were grown by gas transport reactions. Monolayers of tungsten diselenide crystals were grown by chemical vapor deposition on a silicon substrate. The bandwidth of the generated terahertz radiation from the surface of the bulk layered tungsten diselenide crystal was ~ 3.5 THz. For tungsten diselenide monolayer the spectrum bandwidth of the generated THz radiation was ~ 2.5 THz. The peak amplitude of the generated terahertz field for both samples was at a frequency of ~ 1 THz. Research of the influence of the angle of rotation of a polarization plane of optical femtosecond pump on peak-to-peak amplitude of the generated terahertz field from the surface of investigated samples was carried out. Symmetry analysis of the azimuthal dependence of THz radiation made it possible to separate the mechanisms of THz radiation and evaluate their contribution. The analysis results confirm that the only possible contribution to the generation of terahertz radiation in a tungsten diselenide monolayer crystal is the second order nonlinear optical effect – optical rectification. One of the contributions to the generation of tungsten diselenide is a nonlinear-optical effect of the third order – surface optical rectification.

Absorption and temperature distribution during ultrafast laser microcutting of polymeric materials

Material processing by ultrafast lasers is an attractive technology for high-precision fabrication, such as cutting, drilling, and surface modification, of a wide range of material, including dielectrics, semiconductor, metals, and polymer composites. However, it is still challenging to apply ultrafast laser processing in many applications because some key processes, such as absorption and heat accumulation, are not fully understood, especially for polymeric materials, which have a low melting temperature and, therefore, are more vulnerable to thermal damage. In this study, an analytical solution to a transient, two-dimensional thermal model is derived using Duhamel's theorem and Hankel’s transform method to understand the effect of laser parameters during ultrafast laser interactions with polypropylene (PP), which is a material widely used in many industrial applications. To correlate with theoretical calculation, laser cutting experiments are carried out on PP sheets. This study found that the total energy absorbed in the material and the laser intensity are two important factors to estimate the laser processing performance. In addition, time-resolved measurements are performed by using fast photodiodes and an oscilloscope to understand the dynamics of ultrafast laser interactions during the laser cutting process. Transmitted and reflected signals are monitored and analyzed to extract information on nonlinearity and the absorption coefficient.

Ultrafast Plasma Electron Dynamics: A Route to Terahertz Pulse Shaping

Intense ultrafast laser interaction with solid-density plasma can lead to relativistic transient electron dynamics that are favorable for the generation of high-field terahertz (THz) pulses. We investigate the temporal characteristics of such THz pulses. Single-shot electro-optic measurements enable us to record the complete temporal profiles of the THz pulses emanating from planar $\mathrm{Cu}$ and aligned $\mathrm{Cu}$-nanorod-array plasma. The temporal properties of the THz pulses exhibit several transients. Fully relativistic two-dimensional particle-in-cell simulations corroborate experimental observations and reveal that these features originate from electron microbunch emission from the target. Our results demonstrate that target nanostructuring provides a route to control such ultrafast electron dynamics and hence THz-pulse properties in the time domain.

Ultrafast Laser Applications in Manufacturing Processes: A State-of-the-Art Review

Abstract With the invention of chirped pulse amplification for lasers in the mid-1980s, high power ultrafast lasers entered into the world as a disruptive tool, with potential impact on a broad range of application areas. Since then, ultrafast lasers have revolutionized laser–matter interaction and unleashed their potential applications in manufacturing processes. With unprecedented short pulse duration and high laser intensity, focused optical energy can be delivered to precisely define material locations on a time scale much faster than thermal diffusion to the surrounding area. This unique characteristic has fundamentally changed the way laser interacts with matter and enabled numerous manufacturing innovations over the past few decades. In this paper, an overview of ultrafast laser technology with an emphasis on femtosecond laser is provided first, including its development, type, working principle, and characteristics. Then, ultrafast laser applications in manufacturing processes are reviewed, with a focus on micro/nanomachining, surface structuring, thin film scribing, machining in bulk of materials, additive manufacturing, bio manufacturing, super high resolution machining, and numerical simulation. Both fundamental studies and process development are covered in this review. Insights gained on ultrafast laser interaction with matter through both theoretical and numerical researches are summarized. Manufacturing process innovations targeting various application areas are described. Industrial applications of ultrafast laser-based manufacturing processes are illustrated. Finally, future research directions in ultrafast laser-based manufacturing processes are discussed.

Time-resolved measurements of optical properties in ultrafast laser interaction with polypropylene.

Time-resolved, single-shot measurements are performed to determine the reflectance, transmittance, and absorptance in ultrafast laser interaction with polypropylene for a wide range of laser pulse energies. An ellipsoidal mirror is used to collect the majority of the reflected light, enabling the detection of plasma emission starting at about 40 ns after the incident pulse. The measured transmittance is explained by a model that takes into account different effective absorption channels, and the non-linear absorption coefficient is estimated, which suggests that the non-linear absorption originates from the two-step or two-photon absorption through overtone. The results are useful for selecting laser parameters in the processing of polymeric materials.

Coupling of material point method and molecular dynamics for modeling ultrafast laser interaction with metals

A hybrid numerical method that couples molecular dynamics (MD) with material point method (MPM) for atomistic simulations of thermomechanical responses of metals irradiated by ultrafast lasers is developed. In this coupled MD-MPM method, the MPM is used for the non-Fourier heat conduction in the electron subsystem of the heated metallic targets, and the lattice responses of the metals are described by the MD method with the inclusion of additional damping forces accounting for the energy transfer between the electrons and the lattice. The MD atoms are partitioned based on their topological relation with the MPM background grid and the MPM model for the electron subsystem are built with respect to the grouped atoms. The validity and robustness of the proposed MD-MPM scheme have been demonstrated through the representative examples for femto-/picosecond laser heating of metal thin films in one and two dimensions. Due to the inherent nature of the MPM, the presented MD-MPM algorithm could better handle the large deformations involved in the heated metals and be used for modeling the heating of metals by a wider range of ultrafast laser pulses as compared to the procedure combining the MD with the mesh-based methods.

Ultrafast laser burst-train filamentation for non-contact scribing of optical glasses.

A systematic study of glass scribing is presented on the benefits of ultrafast laser burst trains in generating filamentation tracks to guide cleaving of glass substrates. The interplay of Kerr self-focusing, plasma defocusing, and burst-train accumulation effects in filament formation was characterized by time-resolved in-situ microscopic imaging. Various filament-track scribing geometries were compared with and without assistance from burst-train pulse delivery or surface V-groove ablation. The cleaving guidance and reproducibility were examined together with the breaking force, facet morphology and flexural strength of cleaved substrates to assess the overall scribing and cleaving quality. The reported results attest to the benefits and flexibility of burst-mode ultrafast laser interactions to assist cleaving of optically transparent materials along well formed filament arrays.

Thermal effects of ultrafast laser interaction with polypropylene.

Ultrafast lasers have been used for high-precision processing of a wide range of materials, including dielectrics, semiconductors, metals and polymer composites, enabling numerous applications ranging from micromachining to photonics and life sciences. To make ultrafast laser materials processing compatible with the scale and throughput needed for industrial use, it is a common practice to run the laser at a high repetition rate and hence high average power. However, heat accumulation under such processing conditions will deteriorate the processing quality, especially for polymers, which typically have a low melting temperature. In this paper, an analytical solution to a transient, two-dimensional thermal model is developed using Duhamel's theorem and the Hankel transform. This solution is used to understand the effect of laser parameters on ultrafast laser processing of polypropylene (PP). Laser cutting experiments are carried out on PP sheets to correlate with the theoretical calculation. This study shows that, in laser cutting, the total energy absorbed in the material and the intensity are two important figures of merit to predict the cutting performance. Heat accumulation is observed at low scanning speeds and high repetition rates, leading to significant heat-affected zone and even burning of the material, which is supported by experimental data and modelling results. It is found that heat accumulation can be avoided by a proper choice of the processing condition.

Light absorption by surface nanoholes and nanobumps

This paper deals with a numerical investigation of the energy deposition induced by ultrafast laser interaction with nanostructures. We calculate and analyze the intensity near-field reactive and radiative patterns resulted from the interference of the incident light with light scattered by individual subwavelength holes and bumps on the surface of metallic and dielectric materials. The role of light polarization, optical material properties, collective effects and nature of the imperfections in localized energy absorption is elucidated. The results open new perspectives in precise light manipulation by surface inhomogeneities and well-controlled surface nanostructuring by ultrashort laser.

Numerical study of material decomposition in ultrafast laser interaction with metals

A study of ultrafast laser-induced ablation of metals is presented based on an improved two-temperature model. Material decomposition and the resultant energy loss from the sample are considered through the dynamic description of the ablation process. The boiling temperature is adopted as the ablation initiation temperature to capture the contributions from multiple ablation mechanisms. Ablation occurrence period, ablation depth, threshold fluence, residual thermal energy and melting layer thickness have been studied for aluminum, copper, and gold. The simulation results agree well with the experimental measurements for different materials and laser parameters. The electron-phonon coupling strength and thermal conductivity are found to be critical factors determining ablation behaviors. With strong electron-phonon coupling and low thermal conductivity (like aluminum), ultrafast ablation can be triggered within hundreds of femtoseconds, leading to a more efficient laser energy deposition, high ablation depth, and thin melting layer.

Terahertz surface emission from layered semiconductor WSe2

Ultrafast laser interaction with the layered semiconductors has attracted wide interest due to not only the fundamental physical understanding of the light-matter interaction in these advanced materials, but also the potential optoelectronic devices from visible region to THz region based on these emergent semiconductors. Herein, we investigated the THz radiation property from the layered WSe2 due to the d-d photo-transition by an ultrafast laser excitation. We observed strong broadband p-polarized THz radiation under different pump polarization and an evident THz radiation saturation with the pump fluence. The THz radiation demonstrated a cosine function with the polarization angle of the pump beam. Angular dependent THz radiation had a polarity reverse with the opposite incident angle and could be fitted well with a dipole approximation model. These results reveal that the dominant mechanism of THz emission is the photocarrier surging under the surface field. The azimuthal angle dependence of THz radiation suggests that the dominant contribution is due to the surface depletion field rather than surface field induced optical rectification. In addition, we inferred that the laser damage threshold for the WSe2 crystal is 3.11 mJ/cm2 confirmed by both THz emission spectroscopy and Raman Spectroscopy. Our results could provide the fundamental light-matter interaction data for the layered WSe2 and promise the potential applications of this semiconductor for THz devices.

Efficient monoenergetic proton beam from ultra-fast laser interaction with nanostructured targets

The broad energy spectrum of laser-accelerated proton beams is the most important difficulty associated with such particle sources on the way to future applications such as medical therapy, proton imaging, inertial fusion, and high-energy physics. The generation of proton beams with enhanced monoenergetic features through an ultra-intense laser interaction with optimized nanostructured targets is reported. Targets were irradiated by 40 fs laser pulses of intensity 5.5×1020 W cm−2 and wavelength 1 μm. The results of multi-parametric Particle-in-Cell calculations showed that proton beams with considerably reduced energy spread can be obtained by using the proposed nanostructured target. At optimized target dimensions, the proton spectrum was found to exhibit a narrow peak at about 63 MeV with a relative energy spread of ΔE/Epeak∼ 5% which is efficiently lower than what is expected for unstructured double layer targets (∼70%).