Influence of laser-induced air breakdown on femtosecond laser ablation of aluminum

Ultrafast observation of the abnormal time delay of femtosecond laser pulses in a quartz crystal

Light with different frequencies has different propagation speeds when propagating in a medium. Nonlinear optics indicates that the refractive index of the medium also varies with the intensity of light. In this article, we have discovered an anomalous slow light phenomenon that is independent of light intensity through multi-frame ultrafast imaging. The experimental results show that when coaxial 800-nm and 400-nm pulses simultaneously enter the quartz crystal, the time delay between the 800-nm and 400-nm laser pulses is up to 5 times the normal value calculated by the dispersion theory. Moreover, this delay is independent of the laser intensity, indicating that the anomalous slow light phenomenon does not originate from refractive index changes in nonlinear optics. This abnormal time delay effect may find more applications in different fields, and its physical mechanism still needs further exploration.

Single-Shot Multi-Frame Imaging of Femtosecond Laser-Induced Plasma Propagation

Single-shot ultrafast multi-frame imaging technology plays a crucial role in the observation of laser-induced plasma. However, there are many challenges in the application of laser processing, such as technology fusion and imaging stability. To provide a stable and reliable observation method, we propose an ultrafast single-shot multi-frame imaging technology based on wavelength polarization multiplexing. Through the frequency doubling and birefringence effects of the BBO and the quartz crystal, the 800 nm femtosecond laser pulse was frequency doubled to 400 nm, and a sequence of probe sub-pulses with dual-wavelength and different polarization was generated. The coaxial propagation and framing imaging of multi-frequency pulses provided stable imaging quality and clarity, as well as high temporal/spatial resolution (200 fs and 228 lp/mm). In the experiments involving femtosecond laser-induced plasma propagation, the probe sub-pulses measured their time intervals by capturing the same results. Specifically, the measured time intervals were 200 fs between the same color pulses and 1 ps between the adjacent different. Finally, based on the obtained system time resolution, we observed and revealed the evolution mechanism of femtosecond laser-induced air plasma filaments, the multifilament propagation of femtosecond laser in fused silica, and the influence mechanism of air ionization on laser-induced shock waves.

Investigation of multi-timescale processing phenomena in femtosecond laser drilling of zirconia ceramics

Femtosecond lasers have been applied to machining of zirconia (ZrO₂) ceramics because of their ultrashort pulse duration and high peak power. However, an unclear understanding of the ultrafast laser-material interaction mechanisms limits the achievement of precision processing. In this study, a pump-probe imaging method comprising a focusing probe beam integrated with a high-speed camera was developed to directly observe and quantitatively evaluate the multi-timescale transient processing phenomena, including electron excitation, shockwave propagation, plasma evolution, and hole formation, occurring on the picosecond to second timescales, inside a ZrO₂ sample. The variation mechanism in the shapes, lifetimes, and dimensions of these phenomena and their impacts on the drilling performance under different laser parameters were explored. The clear imaging and investigation of the above phenomena contribute to revealing the ultrafast laser-material interaction mechanisms and precision processing in the laser-drilling of zirconia ceramics.

Effect of ablation behavior on the matrix effect in nanosecond laser ablation inductively coupled plasma mass spectrometry

To investigate the effect of ablation behavior on the matrix effect, nanosecond laser ablation inductively coupled plasma mass spectrometry is used to analyze variations in element signal intensities of NIST 610 and GSE-1G standard samples with different laser fluence. Scanning electron microscopy and super depth-of-field microscopy are used to capture the morphology of the ablation crater and obtain depth information, respectively. A pump-probe shadowgraph is used to record the dynamic process of plasma plume evolution during sample ablation. Experimental results show that the proportion of refractory elements to volatile elements in the ablation materials with two different matrices increases with an increase in laser fluence. For the GSE-1G matrix, this range of increase is relatively small, and the signal loss of refractory elements occurs at a higher laser fluence. Combined with the morphology of the ablation crater and evolution of the plasma plume, this potential cause is related to the plasma shielding, which is beneficial to form and deposit large particles, resulting in the loss of refractory elements at high energy fluence.

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.

Time-resolved visualization of high-repetition ultrashort pulse laser ablation

A pump-probe setup including a Robert-cell-type delay stage is calculated and built in the presented study. The goal is to visualize laser beam material interactions upon highly repetitive ultrashort pulse irradiations by shadowgraph imaging, which makes a valuable contribution to clarify the occurring interaction phenomena in this field. Ultrashort laser pulses (λ=1030nm; τ _H =400fs) are irradiated onto a bright-rolled stainless steel metal plate (AISI 316). The high-speed shadowgraph sequences are captured for the time-resolved imaging of plasma and shockwave evolution during material ablation. The captured time frame ranges from the time just before the next pulse irradiates the interaction zone until 2 µs after pulse irradiation. The first part of the experimental study features the shockwave dynamics and evolution of the laser plasma/ablation plume as induced upon single-pulse irradiations. It is shown that the expansion velocity of the shockwave decreases from 10 km/s shortly after pulse irradiation to 6.1 km/s at 41 ns after pulse irradiation. The second part deals with laser pulse trains by irradiating up to 10 pulses at 500 kHz pulse repetition frequency to the substrate. For increasing pulse numbers, the shadowgraphs show a steady increase in height and width of the laser plasma/ablation plume that were measured at 2.4 mm in height and 1.2 mm in width after the 10th pulse.

Dynamics of pressure waves during femtosecond laser processing of glass

Although femtosecond lasers enable microfabrication of transparent materials, precise processing is difficult owing to the inevitable damage caused to the surroundings of the processed region. In the present work, we combine pump-probe imaging with a high-speed camera to capture the dynamics of pressure waves varying from pulse to pulse, before a desired shape is created by hundreds of pulses. The results demonstrate that the pressure waves change their forms and locations as the number of pulses increases. The numerical analyses explain that a tensile stress, much greater than the tensile strength, is distributed around the processed region during the travel of the pressure wave, and that repetitive pressure waves result in cracks. The identified mechanism of the damage generation will contribute to developing strategies to prevent damage and expand the range of applications of femtosecond laser processing.

Structure-Mediated Excitation of Air Plasma and Silicon Plasma Expansion in Femtosecond Laser Pulses Ablation

Femtosecond laser-induced surface structures upon multiple pulses irradiation are strongly correlated with the pulse number, which in turn significantly affects successive laser-material interactions. By recording the dynamics of femtosecond laser ablation of silicon using time-resolved shadowgraphy, here we present direct visualization of the excitation of air plasma induced by the reflected laser during the second pulse irradiation. The interaction of the air plasma and silicon plasma is found to enhance the shockwave expansion induced by silicon ablation in the longitudinal direction, showing anisotropic expansion dynamics in different directions. We further demonstrate the vanishing of air plasma as the pulse number increases because of the generation of a rough surface without light focusing ability. In the scenario, the interaction of air plasma and silicon plasma disappears; the expansion of the silicon plasma and shockwave restores its original characteristic that is dominated by the laser-material coupling. The results show that the excitation of air plasma and the laser-material coupling involved in laser-induced plasma and shockwave expansion are structure mediated and dependent on the pulse number, which is of fundamental importance for deep insight into the nature of laser-material interactions during multiple pulses ablation.

Characteristics of plasma plume in ultrafast laser ablation with a weakly ionized air channel

We report the influence of femtosecond (fs) laser weakly ionized air channel on characteristics of plasma induced from fs-laser ablation of solid Zr metal target. A novel method to create high temperature, low electron density plasma with intense elemental emission and weak bremsstrahlung emission was demonstrated. Weakly ionized air channel was generated as a result of a non-linear phenomenon. Two-dimensional time-resolved optical-emission images of plasma plumes were taken for plume dynamics analysis. Dynamic physical properties of filament channels were simulated. In particular, we investigated the influence of weakly ionized air channel on the evolution of solid plasma plume. Plasma plume splitting was observed whilst longer weakly ionized air channel formed above the ablation spot. The domination mechanism for splitting is attributed to the long-lived underdense channel created by fs-laser induced weakly ionization of air. The evolutions of atomic/molecular emission intensity, peak broadening, and plasma temperature were analyzed, and the results show that the part of plasma entering weakly ionized air channel features higher initial temperature, lower electron density and faster decay.

Sequentially timed all-optical mapping photography (STAMP) utilizing spectral filtering

We propose and experimentally demonstrate a new method called SF-STAMP for sequentially timed all-optical mapping photography (STAMP) that utilizes spectral filtering. SF-STAMP is composed of a diffractive optical element (DOE), a band-pass filter, and two Fourier transform lenses. Using a linearly frequency-chirped pulse and converting the wavelength to the time axis, we realize single-shot ultrafast burst imaging. As an experimental demonstration of SF-STAMP, we monitor the dynamics of a laser ablation using a linearly frequency-chirped broadband pulse (>100 nm) that is temporally stretched up to ~40 ps. This imaging method is expected to be effective for investigating ultrafast dynamics in a diverse range of fields, such as photochemistry, plasma physics, and fluidics.