Role of defects in laser-induced modifications of silica coatings and fused silica using picosecond pulses at 1053 nm: II. Scaling laws and the density of precursors

Simultaneous label-free autofluorescence multi-harmonic microscopy driven by the supercontinuum generated from a bulk nonlinear crystal

Nonlinear microscopy encompasses several imaging techniques that leverage laser technology to probe intrinsic molecules of biological specimens. These native molecules produce optical fingerprints that allow nonlinear microscopes to reveal the chemical composition and structure of cells and tissues in a label-free and non-destructive fashion, information that enables a plethora of applications, e.g., real-time digital histopathology or image-guided surgery. Because state-of-the-art lasers exhibit either a limited bandwidth or reduced wavelength tunability, nonlinear microscopes lack the spectral support to probe different biomolecules simultaneously, thus losing analytical potential. Therefore, a conventional nonlinear microscope requires multiple or tunable lasers to individually excite endogenous molecules, increasing both the cost and complexity of the system. A solution to this problem is supercontinuum generation, a nonlinear optical phenomenon that supplies broadband femtosecond radiation, granting a wide spectrum for concurrent molecular excitation. This study introduces a source for nonlinear multiphoton microscopy based on the supercontinuum generation from a yttrium aluminum garnet (YAG) crystal, an approach that allows simultaneous label-free autofluorescence multi-harmonic imaging of biological samples and offers a practical and compact alternative for the clinical translation of nonlinear microscopy. While this supercontinuum covered the visible spectrum (550-900 nm) and the near-infrared region (950-1200 nm), the pulses within 1030-1150 nm produced label-free volumetric chemical images of ex vivo chinchilla kidney, thus validating the supercontinuum from bulk crystals as a powerful source for multimodal nonlinear microscopy.

Influence of the multilayer dielectric mirror design on the laser damage growth in the sub-picosecond regime

The peak power of high-power laser facilities is limited by the laser-induced damage to the final optical components. Also, when a damage site is generated, the damage growth phenomenon limits the lifetime of the component. Many studies have been performed to improve the laser-induced damage threshold of these components. The question now arises as to whether improvement of the initiation threshold leads to a reduction of the damage growth phenomenon. To address this question, we performed damage growth experiments on three different multilayer dielectric mirror designs exhibiting different damage thresholds. We used classical quarter-wave designs and optimized designs. The experiments were carried out with a spatial top-hat beam, spectrally centered at 1053 nm with a pulse duration of 0.8 ps in s- and p-polarization. The results showed the impact of design on the improvement of the damage growth thresholds and a reduction of the damage growth rates. A numerical model was used to simulate damage growth sequences. The results reveal similar trends to those observed experimentally. On the basis of these three cases, we have shown that improvement of the initiation threshold through a modification of the mirror design can lead to the reduction of the damage growth phenomenon.

Ablation threshold of GaN films for ultrashort laser pulses and the role of threading dislocations as damage precursors

The laser-induced ablation threshold of c-plane GaN films upon exposure to ultrashort laser pulses was investigated for different wavelengths from the IR to the UV range and pulse widths between 0.34 and 10 ps. The one-pulse ablation threshold ranges between 0.15 and 3 J/cm² and shows an increase with the wavelength and the pulse width, except for deep UV pulses. Based on a rate equation model, we attribute this behavior to the efficiency of seed carrier generation by interband absorption. In addition, the multi-pulse ablation threshold was analyzed. Accumulation effects are more prominent in case of IR than with UV pulses and are closely linked to damage precursors. By a thorough structural investigation, we demonstrate that threading dislocations, especially those with a screw component, significantly contribute to laser damage, since they provide a variety of dispersed states within the band gap.

Modulation of laser damage by temporal shaping of double picosecond pulses

We propose a temporally shaped double-picosecond-pulse train at a sub-nanosecond scale to control the damage dynamics of optical glass. Both damage threshold and morphology are significantly modulated by pulse-train shaping. The ramp-up-shaped train effectively increases its damage threshold and decreases the damage density and size, which clearly shows that a pump pulse with optimized fluence has a strong positive modification of damage precursors. Furthermore, the temporal evolution of damage modulation is experimentally revealed by varying the interval of pump-probe pulses, and after pump exposure with optimized fluence, enhancement of the probe threshold reaches the maximum at a delay of about 260 ps.

The Damage Threshold of Multilayer Film Induced by Femtosecond and Picosecond Laser Pulses

Laser-induced damage threshold (LIDT) is an essential factor in measuring the anti-laser damage of optical films. The damage threshold and morphology of the Ta2O5/SiO2 multilayer film prepared by electron beam evaporation were studied by femtosecond (50 fs) and picosecond (30 ps) laser irradiations. The results showed that the LIDT of the film was 1.7 J·cm−2 under the femtosecond laser. The damage morphology developed from surface damage to a clear layered structure, and the outline has become more transparent and regular with an increase in the laser fluence. Under the picosecond laser irradiation, the LIDT of the film was 2.0 J·cm−2. The damage morphology developed from small range to thin film layer separation, and the outline changed from blurry to clear with an increase in laser fluence. Therefore, the LIDT of the film decreased with a decrease in the laser pulse width.

Optimizing sub-nanosecond laser conditioning of DKDP crystals by varying the temporal shape of the pulse

We propose a strategy to optimize the laser conditioning of DKDP crystals by varying the temporal shape of sub-nanosecond pulses. Four sub-ns temporally shaped pulses with nearly the same full width at half maxima of ∼600 ps but different rising-falling statuses were designed to conduct laser-induced damage (LID) and laser conditioning experiments on DKDP crystals. The shape of the pulse substantially influences the damage pinpoints size and LID threshold (LIDT) of the crystals in the sub-nanosecond range. After sub-nanosecond laser conditioning, the ns R-on-1 LIDT showed that slow-rising fast-falling pulse (R400-F200 and High-foot pulses) conditioning achieved a 14%-20% LIDT enhancement than the traditional Gaussian pulse (R300-F300 pulse). The 8-ns laser damage morphologies after slow-rising fast-falling pulse conditioning showed cracks, whereas those after fast-rising slow-falling pulse (R200-F400 pulse) conditioning were pinpoint core, as usual. These results suggest that the rising front plays an important role in the LID and laser conditioning of the DKDP crystals. A pulse with a slower rising front is beneficial for thermal modification, thereby leading to better LID properties. This strategy greatly expands and enriches the manipulation methods to improve the LIDT of DKDP crystals, and sheds light on understanding the laser damage mechanisms.

Beam-size effects on the measurement of sub-picosecond intrinsic laser induced damage threshold of dielectric oxide coatings

Laser-induced damage experiments on HfO₂ and Nb₂O₅ thin films were performed with 500 fs pulse duration at 1030 nm wavelength. Threshold fluences as a function of beam size have been determined for effective beam diameters ranging from 40 to 220 µm, in a single shot regime. The results suggest no beam-size effect related to material properties in the investigated range, but size effects related to the metrology. The results indicate the importance of appropriate focusing conditions and beam measurement to qualify the optics for use in lasers with large beam sizes.

Implications of laser beam metrology on laser damage temporal scaling law for dielectric materials in the picosecond regime

We report on the implications that the temporal and spatial beam metrologies have on the accuracy of temporal scaling laws of Laser Induced Damage Threshold (LIDT) for dielectric materials in the picosecond regime. Thanks to a specific diagnostic able to measure the temporal pulse shape of subpicosecond and picosecond pulses, we highlight through simulations and experiments how the temporal shape has to be taken into account first in order to correctly understand the temporal dependency of dielectrics LIDT. This directly eases the interpretation of experimental temporal scaling laws of LIDT and improves their accuracy as a prediction means. We also give numerically determined benchmark temporal scaling laws of intrinsic LIDT for SiO₂ (thin film) based on the model developed for this work. Finally, we show as well what kind of spatial metrology is needed during any temporal scaling law determination to take into account potential variations of the spatial profile.

Mechanisms of picosecond laser-induced damage in common multilayer dielectric coatings

The physical mechanisms and ensuing material modification associated with laser-induced damage in multilayer dielectric high reflectors is investigated for pulses between 0.6 and 100 ps. We explore low-loss multilayer dielectric SiO₂/HfO₂ mirrors which are commonly employed in petawatt-class laser systems. The spatial features of damage sites are precisely characterized, enabling the direct correlation of the observed damage morphology to the location of energy deposition and the corresponding standing-wave electric-field intensities within the layer structure. The results suggest that there are three discrete damage-initiation morphologies arising from distinctly different mechanisms: the first prevailing at laser pulse lengths shorter than about 2.3 ps, while the other two are observed for longer pulses. Modeling of the thermomechanical response of the material to localized laser-energy deposition was performed for each type of damage morphology to better understand the underlying mechanisms of energy deposition and subsequent material response.

Polarization dependent laser damage growth of optical coatings at sub-picosecond regime

We report the influence of polarization on the damage mechanism of oxide thin films submitted to multiple pulses in the sub-picosecond regime. We have exposed single layer coatings of oxide materials and multilayer stacks (mirrors) to multiple laser pulses at 1030nm, 500fs, and the events on the tested sample sites were recorded in situ with high resolution microscopy. For multiple shots while keeping the fluence below the single shot threshold, damage on the film begins to form and for some of the samples the damage growth follows polarization dependent patterns. This damage growth was investigated and our results match with the assumption that the existence of nano-defects contributes to the early stage of the formation of damage, in which the energy absorption in a defect site causes local nanoablation at a laser fluence under the intrinsic ablation threshold and nanovoid formation. Based on the simulation of the interference of the scattered wave by the nanovoid with the incident wave, we obtain good correlation between simulated and observed damage growth behavior. This process leads to the formation of specific damage morphology that is strongly dependent on the polarization of the incident wave.

Thermochromic and Femtosecond-Laser-Induced Damage Performance of Tungsten-Doped Vanadium Dioxide Films Prepared Using an Alloy Target

Thermochromic tungsten-doped VO₂ thin films were successfully fabricated using a W-V alloy target. X-ray diffraction analyses showed that the W-doped VO₂ film had a preferred orientation of (011), and that the doping did not degrade the film crystallinity compared with that of the pure film. X-ray photoelectron spectroscopy and energy-dispersive spectroscopy showed that the doped 0.81 atom% tungsten replaced vanadium in the lattice of the film. The metal⁻insulator transition temperature of the W-doped VO₂ film was reduced to 35.5 °C, which is close to room temperature. Additionally, the infrared transmittance modulation of the W-doped film at λ = 2500 nm reached 56%, indicating an excellent switching efficiency. The damage behavior of the W-doped VO₂ film under a femtosecond-laser irradiation was experimentally investigated. Our results revealed that defect-related damages induced by the femtosecond laser are relevant for W-doped VO₂ films. This study provides valuable insights into VO₂ films for potential applications in laser protection.