Femtosecond Laser-induced Damage Research Articles

Modeling of the femtosecond pulsed laser-induced damage of multi-layer dielectric gratings

The laser damage resistance of multi-layer dielectric gratings (MDGs) and metal multi-layer dielectric gratings (MMDGs) for femtosecond lasers directly determines the performance and life span of the high-energy laser system. It remains a significant challenge to predict the laser-induced damage threshold (LIDT) of MDGs and MMDGs for femtosecond lasers. Based on the nonlinear ionization and electric field intensity model, an effective method is proposed to calculate the LIDT of MDGs and MMDGs by introducing the dual temperature model. We take into account the evolutions of thermophysical parameters including the absorption coefficient, heat capacity, and thermal conductivity of the dielectric material as a function of the free-electron density and temperature. The free electron density of the conduction band combined with the electric field strength inside the grating can be calculated by the laser-induced ionization theory, more importantly, the transient distribution of the lattice temperature field generated by femtosecond laser irradiated grating excitation is obtained by the two-temperature model (TTM). This work is beneficial for guiding the design and fabrication of MDGs and MMDGs with high-LIDTs for ultra-short ultra-intense laser systems.

Dual-wavelength femtosecond laser-induced single-shot damage and ablation of silicon

An experimental and theoretical study of laser-induced damage and ablation of silicon by two individual femtosecond pulses of different wavelengths, 1030 and 515 nm, is performed to address the physical mechanisms of dual-wavelength ablation and reveal possibilities for increasing the ablation efficiency. The produced craters and damaged areas are analyzed as a function of laser fluence and time separation between the pulses and are compared with monochromatic irradiation. The order of pulses is demonstrated to be essential in bi-color ablation with higher material removal rates when a shorter-wavelength pulse arrives first at the surface. Simulations based on the two-temperature model show that the visible pulse is profitable for the generation of the electron-hole plasma while the delayed IR pulse is efficiently absorbed in the plasma enhancing energy coupling to the target. At long delays of 30–100 ps, the dual-wavelength ablation is found to be particularly strong with formation of deep smooth craters. This is explained by the expansion of a hot liquid layer produced by the first pulse with a drastic decrease in the surface reflectivity at this timescale. The results provide insight into the processes of dual-wavelength laser ablation offering a better control of the energy deposition into material.

Influence of erbium doping on the femtosecond laser damage characteristics of fluorozirconate glasses

In this study, we systematically investigated femtosecond laser-induced damage characteristics of ZHBLAN (27ZrF4–25HfF4–10BaF2–5SrF2–5CaF2–4LaF3–14AlF3–10NaF), and the impact of Er3+ doping on the optical characteristics and damage characteristics of the matrix material. The glasses were subjected to a comparative analysis with respect to typical ZBLAN glasses. The sizes and morphology of damaged craters changed with the power of the 800 nm femtosecond laser. The samples were analyzed using digital and scanning electron microscopes, and their femtosecond laser-induced damage thresholds (LIDTs) were calculated by linear regression. The LIDTs decreased obviously from 612 mJ/cm2 to 532 mJ/cm2 after Er3+ was doped into the ZHBLAN glass. This is related to the absorption of Er3+ near the wavelength of the femtosecond laser and the change in the bandgap structure of the glass due to Er3+ doping. In addition, we have analyzed the electron convergence rate within the material during laser irradiation. The combination of physical models and simulation results reveals the association between the ionization rate of optical materials and the laser damage threshold.

Optical and femtosecond laser-induced damage-related properties of Ta2O5-based oxide mixtures

In this study, four Ta2O5-based mixture coatings, including Ta2O5-TiO2, Ta2O5-HfO2, Ta2O5-Al2O3, and Ta2O5-SiO2, are deposited via plasma-ion-assisted electron-beam co-evaporation and compared with a Ta2O5 monolayer coating. The chemical compositions, microstructures, and optical and femtosecond laser-induced damage threshold (LIDT) properties of the coatings are investigated. For different doping materials, when the refractive indices of the mixture coatings are close, the femtosecond LIDTs exhibit a considerable difference. The experimental results indicate that the Ta2O5-HfO2 mixture coating exhibits a larger optical bandgap and higher femtosecond LIDT than other coatings. The 1-on-1 LIDT of the Ta2O5-HfO2 mixture coating is approximately 1032.3 mJ/cm2 (170 fs), which is 25% higher than that of the Ta2O5 monolayer coating. We believe the Ta2O5-HfO2 mixture will be useful for various multilayer coatings such as chirped mirrors and other mirrors in femtosecond laser applications.

Femtosecond laser–induced damage characteristics of the novel fluorozirconate glasses

In this study, we systematically investigated the femtosecond laser–induced damage and the physical characteristics of a series of multicomponent fluorozirconate (MFZ) glasses: xZrF4–(52–x) HfF4–10BaF2–5SrF2–5CaF2–4LaF3–14AlF3–10NaF (x = 27, 29, 31, 33 mol%, named MFZ27, MFZ29, MFZ31, and MFZ33), and these MFZ glasses were compared with typical ZBLAN glasses. The sizes of damaged craters and the morphology changed under different powers of 800 nm fs laser. They were measured and analyzed with a digital microscope and scanning electron microscopes. The femtosecond laser-induced damage thresholds (LIDTs) of all the samples were calculated using the linear regression method. The highest LIDT of 616.9 mJ/cm2 was obtained in MFZ27. We compared the hydrolytic resistance of ZBLAN glass with that of MFZ27 glass. The laser-induced damage characteristics and the LIDT of the samples after hydrolysis were also studied. To the best of our knowledge, this study is the first to investigate in detail the femtosecond laser–induced damage characteristics using the fluorozirconate glass.

Mid-infrared femtosecond laser-induced damage in TeO2-BaF2-Y2O3 fluorotellurite glass

Fluorotellurite glasses based on TeO2-BaF2-Y2O3(TBY) with a broadband transmission window of 350–6000 nm, relatively low phonon energy, stable chemical and physical characteristics compared to fluoride glasses have been investigated to develop high-powered mid-infrared (MIR) fiber lasers. In this work, a series of xTeO2-(90-x)BaF2−10Y2O3 (TBYx) (x = 60,65,70,75 mol%) fluorotellurite glasses were prepared with the conventional melting-quenching method in an argon glove box. Then, laser damage characteristics of glass samples under 3000 and 4000 nm MIR femtosecond laser with different pulses was compared and studied. The TBY60 glass has the highest damage threshold, which reaches 1.08 J/cm2 and 0.852 J/cm2 at 4000 nm and 3000 nm, respectively. The femtosecond laser-induced damage threshold (LIDT) of TBY glasses decreased from 1.08 to 0.782 J/cm2 as TeO2 content increased from 60 to 75 mol% at the wavelength of 4000 nm. In addition, the effect of pulse numbers is consistent with exponential defect accumulation model.

Laser damage threshold of Ge8As23S69 films irradiated under single- and multiple-pulse femtosecond laser

In this study, the single- and multiple-pulse laser-induced surface damage characteristics of the Ge8As23S69 chalcogenide film under the femtosecond laser irradiation were experimentally investigated. The 1-on-1 and S-on-1 methods were utilized to measure the femtosecond laser damage by using single and multiple pulses, respectively. The femtosecond laser-induced damage threshold (LIDT) was obtained using linear regression method by measuring the damage morphology under different pulse energies and pulse numbers of the femtosecond laser. Results show that the LIDT of the film under single-pulse radiation is higher than that under multiple-pulse radiation because of accumulation and defect effects. For the single-pulse radiation, LIDT is attributed to the field enhancement effect, multiphoton ionization, and avalanche ionization. The single-pulse damage threshold of Ge8As23S69 films is 232.548 mJ/cm2. For the multiple-pulse radiation, the LIDT of the film decreases with increasing pulse number due to accumulation and defect effects.

Damage characteristics of dual-band high reflectors affected by nodule defects in the femtosecond regime

The influence of nodule defects on the characteristics of femtosecond laser-induced damage has not been fully investigated. In this study, two types of 800 nm/1064 nm dual-band HfO2/SiO2 high-reflection films with different configurations were analyzed. Combined with finite-difference time-domain electric field simulation and focused ion beam analysis, the initial state and growth process of femtosecond laser damage of nodules were explored. In particular, the sequence of blister damage determined by the film design and the inner damage caused by nodules were clarified. The rule of the laser-induced damage threshold of different size nodules was obtained. The difference in the damage behavior of nodules in the two types of films was elucidated.

The mutual influence between rare earth element doping and femtosecond laser-induced effects in Ga-As-Sb-S chalcogenide glass

Femtosecond laser-induced damage thresholds (LIDTs) of Ga0.8As29.2Sb10S60 glasses doped with gradient Tm3+ concentrations and the effects of laser-induced damage on the glass' luminescence properties were studied in this work. Tm3+ doping in the glass considerably decreased the LIDT, from 3394.8 to 1881.8 mJ/cm2, when the Tm3+ concentration increased from 0 to 5000 ppmw. This was related to the absorption of Tm3+ around the femtosecond laser's wavelength and microstructural changes caused by the Tm3+ doping. On the other hand, the femtosecond laser changed the glass matrix's elemental distribution and microstructure. Although the laser damaged the glass, the luminescence properties were barely affected. Based on the changes, femtosecond laser damage mechanism of chalcogenide glass doped with rare earth element was firstly proposed.

Femtosecond Laser-Induced Damage Characterization of Multilayer Dielectric Coatings

The laser-induced damage threshold (LIDT) of optical components is one of the major constraints in developing high-power ultrafast laser systems. Multi-layer dielectric (MLD) coatings-based optical components are key parts of high-power laser systems because of their high damage resistance. Therefore, understanding and characterizing the laser-induced damage of MLD coatings are of paramount importance for developing ultrahigh-intensity laser systems. In this article, we overview the possible femtosecond laser damage mechanisms through damage morphologies in various MLD optical coatings tested in our facility. To evaluate the major contributions to the coating failure, different LIDT test methods (R-on-1, ISO S-on-1 and Raster Scan) were carried out for a high reflective hybrid Ta2O5/HfO2/SiO2 MLD mirror coating at a pulse duration of 37 fs. Different LIDT test methods were compared due to the fact that each test method exposes the different underlying damage mechanisms. For instance, the ISO S-on-1 test at a higher number of laser pulses can bring out the fatigue effects, whereas the Raster Scan method can reveal the non-uniform defect clusters in the optical coating. The measured LIDT values on the sample surface for the tested coating in three test methods are 1.1 J/cm2 (R-on-1), 0.9 J/cm2 (100k-on-1) and 0.6 J/cm2 (Raster Scan) at an angle of incidence of 45 deg. The presented results reveal that the performance of the tested sample is limited by coating defects rather than fatigue effects. Hence, the Raster Scan method is found to be most accurate for the tested coating in evaluating the damage threshold for practical applications. Importantly, this study demonstrates that the testing of different LIDT test protocols is necessary in femtosecond regime to assess the key mechanisms to the coating failure.

Femtosecond laser-induced damage on the end face of an As2S3 chalcogenide glass fiber

In this study, we investigated femtosecond laser-induced damage on the end face of an As2S3 chalcogenide (ChG) glass fiber by repeatedly inducing 1 kHz ultrashort pulses with 150 fs duration at 800 nm wavelength in comparison with the bulk As2S3 glass. The damage size, geometries, and morphology changes under different laser powers were measured and analyzed with optical, and scanning electron microscopes (SEM). The laser-induced damage threshold on the end face of three As2S3 fibers calculated using the linear regression method was 32.15 mJ/cm2, whereas that of As2S3 bulk glass was approximately 24.32 mJ/cm2. The femtosecond laser-induced damage on the As2S3 bulk glass at 800 nm wavelength was also compared with that in the mid-infrared (MIR) region, which indicates a decrease in the damage threshold and an aggravation in the morphologies of damage craters under an irradiation of 800 nm femtosecond pulsed laser. To the best of our knowledge, this study is the first to investigate the interaction of femtosecond laser with the end face of ChG glass fiber. This work provides scientific guidance for the fabrication of novel photonic devices based on ChG glass fibers.

Electrostatic engineering of charge clouds around DNA inhibits strand breakages

Femtosecond laser-induced damage to plasmid DNA in aqueous medium manifests itself as strand breaks arising from interactions with slow OH-radicals (and electrons) in the laser-induced plasma generated in water. Such strand breakages are found to be significantly inhibited upon addition of salts (in physiologically significant concentrations) to the aqueous medium. A simple model rationalizes our observations and suggests the feasibility of electrostatically “engineering” DNA's ion atmosphere so as to prevent damage-inducing free radicals accessing DNA strands. These findings may aid development of new strategies for cancer treatment.

Femtosecond laser damage of germanium from near- to mid-infrared wavelengths.

Femtosecond laser-induced damage and ablation (fs-LIDA) is a rich field in extreme non-perturbative nonlinear optics with wide ranging applications, including laser micro- and nano-machining, waveguide writing, and eye surgery. Our understanding of fs-LIDA, however, is limited mostly to visible and near-infrared wavelengths. In this work, we systematically study single-shot, fs-laser ablation (fs-LIA) of single-crystal germanium from near- to mid-infrared wavelengths, and compare the fs-LIA wavelength scaling with two widely used models. We show that these models are inadequate, particularly at mid-infrared wavelengths. Instead, a hybrid model is proposed involving Keldysh ionization rates, a constant free-carrier density threshold, and multi-band effects, which yields good agreement with experimental observations. Aspects of this model may be applied to understanding other strong-field non-perturbative phenomena in solids.

Mid-infrared femtosecond laser-induced damages in As2S3 and As2Se3 chalcogenide glasses

In this paper, we report the first measurements of mid-infrared (MIR) femtosecond laser-induced damage in two typical chalcogenide glasses, As2S3 and As2Se3. Damage mechanism is studied via optical microscopy, scanning electron microscopy and elemental analysis. By irradiating at 3, 4 and 5 μm with 150 fs ultrashort pulses, the evolution of crater features is presented with increasing laser fluence. The dependence of laser damage on the bandgap and wavelength is investigated and finally the laser-induced damage thresholds (LIDTs) of As2S3 and As2Se3 at 3 and 4 μm are calculated from the experimental data. The results may be a useful for chalcogenide glasses (ChGs) applied in large laser instruments to prevent optical damage.

Wavelength dependence of femtosecond laser-induced damage threshold of optical materials

An experimental and numerical study of the laser-induced damage of the surface of optical material in the femtosecond regime is presented. The objective of this work is to investigate the different processes involved as a function of the ratio of photon to bandgap energies and compare the results to models based on nonlinear ionization processes. Experimentally, the laser-induced damage threshold of optical materials has been studied in a range of wavelengths from 1030 nm (1.2 eV) to 310 nm (4 eV) with pulse durations of 100 fs with the use of an optical parametric amplifier system. Semi-conductors and dielectrics materials, in bulk or thin film forms, in a range of bandgap from 1 to 10 eV have been tested in order to investigate the scaling of the femtosecond laser damage threshold with the bandgap and photon energy. A model based on the Keldysh photo-ionization theory and the description of impact ionization by a multiple-rate-equation system is used to explain the dependence of laser-breakdown with the photon energy. The calculated damage fluence threshold is found to be consistent with experimental results. From these results, the relative importance of the ionization processes can be derived depending on material properties and irradiation conditions. Moreover, the observed damage morphologies can be described within the framework of the model by taking into account the dynamics of energy deposition with one dimensional propagation simulations in the excited material and thermodynamical considerations.

Direct holographic imaging of ultrafast laser damage process in thin films

Dynamic process of femtosecond laser-induced damage formation in dielectric thin films is reconstructed from a series of time-resolved images. Ta2O5 single-layer coatings of four different thicknesses have been investigated in transmission mode by means of time-resolved off-axis digital holography. Different processes overlapped in time were found to occur; namely, the Kerr effect, free-electron generation, ultrafast lattice heating, and shockwave generation. The trends in contribution of these effects are qualitatively reproduced by numerical models based on electron-rate equations and Drude theory, which take into account transient changes in the films and interference effects of the pump and probe pulses.

Evolution of femtosecond laser-induced damage in doped GaN thin films

Mg- and Si-doped GaN layers deposited by metalorganic chemical vapor deposition method were irradiated with femtosecond pulse duration laser of three different wavelengths 1,030, 515 and 343 nm. Both single and multiple shot laser induced damage thresholds of doped GaN layers were evaluated and discussed. The scanning electron microscopy employed with electron beam induced current and energy dispersive X-ray techniques were used to study laser damage morphology. It was observed that ablated area and laser-induced damage increased with irradiation fluence. The mechanism of damage generation by Gaussian beam profile laser was considered.

Effect of nanosecond laser pre-irradiation on the femtosecond laser-induced damage of Ta2O5/SiO2high reflector

The effect of nanosecond laser pre-irradiation on the femtosecond laser-induced damage behaviors of 800 nm 0° AOI Ta(2)O(5)/SiO(2) high reflectors fabricated by e-beam evaporation was explored. Laser pre-irradiation was carried out by Raster-scanning with scanning mode of 1-on-1 and scanning velocities timed such that there was a beam overlap at 70% of the peak fluence, utilizing 5 Hz 1064 nm 12 ns Nd:YAG fundamental lasers. Femtosecond laser damage was investigated by 1 kHz 800 nm 135 fs Ti: sapphire laser system with 1-on-1 mode test. The results indicated that nanosecond laser pre-irradiation did not promote the femtosecond laser-induced damage threshold of reflectors. Instead, the thresholds of all the samples with various fluence steps for pre-irradiation were reduced by about 20%. Furthermore, the damage morphologies were analyzed by optical microscope, SEM and AFM, which displayed deterministic field induced breakdown characteristics. To explain these phenomena, a theoretical model including photoionization, avalanche ionization, and decays of electrons was built to simulate the evolution of electron density in the conduction band. Field ionization mechanism was considered to dominate the femtosecond laser damage process, while the electronic defects induced by nanosecond laser pre-irradiation accelerated the femtosecond laser damage evolution.

Mapping of femtosecond laser-induced collateral damage by electron backscatter diffraction

The distribution of the collateral damage beneath craters produced by ultrafast (femtosecond) laser single pulses in a nickel base superalloy single crystal has been analyzed using electron backscatter diffraction. The procedure is based on measurements of the localized crystal rotations caused by the dislocations generated by the shock wave that is produced as an effect of laser pulse interaction with a material. Damage in the form of lattice misorientation created by excess dislocations was negligible at laser fluences up to 2 J/cm2. At higher fluences (5.1-41 J/cm2), the depth of the damage zone beneath the crater increased linearly with laser fluence. Based on lattice misorientation, excess dislocation densities as high as 3 × 1010/cm2 are calculated to be present below the ablated surface in the high fluence regime.

Comparison of femtosecond laser-induced damage on unstructured vs. nano-structured Au-targets

The combination of high-field physics with nano-plasmonics has proven to be feasible in producing high harmonics of intense laser radiation from noble gases, assisted by the field-enhancement effect in the proximity of metallic nano-antennas. However, the intensity region where harmonics can be generated without irreversible damage to these delicate structures is rather narrow. We explore the damage threshold of gold targets that exhibit regular structures on a nanoscopic scale, either explicitly resonant to the used laser frequency, or off-resonance. These are compared to values for bulk material in order to gain insight into the role of plasmonic resonances in the response of solid targets on intense laser radiation. We find that the presence of such a resonance lowers the threshold fluence (J/cm2) where global structural damage sets in by about an order of magnitude. Statistical deviations either in local pulse energy of the damage inducing laser radiation or in the exact resonance behaviour of singular structures prove to be limited. These results should serve as a guideline for future experiments working near the damage threshold of more sophisticated antenna designs.