Influence of different substrates on laser induced damage thresholds at 1064 nm of Ta2O5 films

Ta2O5 films are prepared on Si, BK7, fused silica, antireflection (AR) and high reflector (HR) substrates by electron beam evaporation method, respectively. Both the optical property and laser induced damage thresholds (LIDTs) at 1064 nm of Ta2O5 films on different substrates are investigated before and after annealing at 673 K for 12h. It is shown that annealing increases the refractive index and decreases the extinction index, and improves the O/Ta ratio of the Ta2O5 films from 2.42 to 2.50. Moreover, the results show that the LIDTs of the Ta2O5 films are mainly correlated with three parameters: substrate property, substoichiometry defect in the films and impurity defect at the interface between the substrate and the films. Details of the laser induced damage models in different cases are discussed.

Investigation of laser-induced damage on multi-layer dielectric gratings

We investigate mechanisms of laser induced damage thresholds (LIDTs) of multi-layer dielectric gratings (AIDG,). It is found that the laser damage thresholds of MDGs and unstructured dielectric multi-layer coatings (the substrate of MDG) are 3.15J/cm(2) and 9.32 J/cm(2), respectively, at 1064nm (12ns) with the Littrow angle 51.2 degrees and the TEM00 mode. The laser-induced damage mechanism of multi-layer dielectric is presented with the analysis of the following factors: The dominant factor is the pollution on the corrugated surface, which is induced by the complex manufacture process of multi-layer dielectric gratings; another is the electric field distribution along the corrugated surface. The third reason is due to the reduction in stoichiometry of oxide films, resulting from the manufacture process of etching.

High laser-induced damage threshold HfO2 films prepared by ion-assisted electron beam evaporation

HfO2 films were deposited by electron beam evaporation with different deposition parameters. The properties such as refractive index, weak absorption, and laser induced damage thresholds (LIDTs) of these films have been investigated. It was found that when pulsed Nd:YAG 1064 nm laser is used to investigate LIDT of films: Metallic character is the main factor that influences LIDTs of films obtained from Hf starting material by ion-assisted reaction, and films prepared with higher momentum transfer parameter P have fewer metallic character; The ion-assisted reaction parameters are key points for preparing high LIDT films and if the parameters are chose properly, high LIDT films can be obtained. (c) 2004 Elsevier B.V. All rights reserved.

Comparison of femtosecond and nanosecond laser-induced damage in HfO2 single-layer film and HfO2-SiO2 high reflector

HfO2 single layers, 800 run high-reflective (HR) coating, and 1064 ran HR coating were prepared by electron-beam evaporation. The laser-induced damage thresholds (LIDTs) and damage morphologies of these samples were investigated with single-pulse femtosecond and nanosecond lasers. It is found that the LIDT of the HfO2 single layer is higher than the HfO2-SiO2 HR coating in the femtosecond regime, while the situation is opposite in the nanosecond regime. Different damage mechanisms are applied to study this phenomenon. Damage morphologies of all samples due to different laser irradiations are displayed. (c) 2007 Optical Society of America.

Effect of SiO2 protective layer on the femtosecond laser-induced damage of HfO2/SiO2 multilayer high-reflective coatings

Two kinds of HfO2/SiO2 800 nm high-reflective (HR) coatings, with and without SiO2 protective layer were deposited by electron beam evaporation. Laser-induced damage thresholds (LIDT) were measured for all samples with femtosecond laser pulses. The surface morphologies and the depth information of all samples were observed by Leica optical microscopy and WYKO surface profiler, respectively. It is found that SiO2 protective layer had no positive effect on improving the LIDT of HR coating. A simple model including the conduction band electron production via multiphoton ionization and impact ionization is used to explain this phenomenon. Theoretical calculations show that the damage occurs first in the SiO2 protective layer for HfO2/SiO2 HR coating with SiO2 protective layer. The relation of LIDT for two kinds of HfO2/SiO2 HR coatings in calculation agrees with the experiment result. (c) 2006 Elsevier B.V. All rights reserved.

Influence of substoichiometer on the laser-induced damage characters of HfO2 thin films

HfO2 is one of the most important high refractive index materials for depositing high power optical mirrors. In this research, HfO2 thin films were prepared by dual-ion beam reactive sputtering method, and the laser-induced damage thresholds (LIDT) of the sample were measured in 1-on-1 mode for laser with 1064 nm wavelength. The results indicate that the LIDT of the as-grown sample is only 3.96 J/cm(2), but it is increased to 8.98 J/cm(2) after annealing under temperature of 200 degrees C in atmosphere. By measuring the laser weak absorption and SIMS of the samples, we deduced that substoichiometer is the main reason for the low LIDT of the as-grown sample, and the experiment results were well explained with the theory of electronic-avalanche ionization. (C) 2008 Elsevier B.V. All rights reserved.

Femtosecond laser-induced damage in reflector

We have investigated the damage for ZrO2/SiO2 800 nm 45 degrees high-reflection mirror with femtosecond pulses. The damage morphologies and the evolution of ablation crater depths with laser fluences are dramatically different from that with pulse longer than a few tens of picoseconds. The ablation in multilayers occurs layer by layer, and not continuously as in the case of bulk single crystalline or amorphous materials. The weak point in damage is the interface between two layers. We also report its single-short damage thresholds for pulse durations ranging from 50 to 900 fs, which departs from the diffusion-dominated tau(1/2)(p) scaling. A developed avalanche model, including the production of conduction band electrons (CBE) and laser energy deposition, is applied to study the damage mechanisms. The theoretical results agree well with our measurements. (c) 2005 Elsevier B.V. All rights reserved.

Mechanisms of femtosecond laser-induced damage in magnesium fluoride

We studied the single-shot damage in magnesium fluoride irradiated by 800 nm femtosecond (fs) laser. The dependence of damage thresholds on the laser pulse durations from 60 to 750 fs was measured. The pump-probe measurements were carried out to investigate the time-resolved electronic excitation processes. A coupled dynamic model was applied to study the microprocesses in the interaction between fs laser and magnesium fluoride. The results indicate that both multiphoton ionization and avalanche ionization play important roles in the femtosecond laser-induced damage in MgF2. (C) 2006 Elsevier Ltd. All rights reserved.

Influence of oxygen post-treatment on laser-induced damage of antireflection coatings prepared by electron-beam evaporation and ion beam assisted deposition

Antireflection coatings at the center wavelength of 1053 nm were prepared on BK7 glasses by electron-beam evaporation deposition (EBD) and ion beam assisted deposition (IBAD). Parts of the two kinds of samples were post-treated with oxygen plasma at the environment temperature after deposition. Absorption at 1064 nm was characterized based on surface thermal lensing (STL) technique. The laser-induced damage threshold (LIDT) was measured by a 1064-nm Nd:YAG laser with a pulse width of 38 ps. Leica-DMRXE Microscope was applied to gain damage morphologies of samples. The results revealed that oxygen post-treatment could lower the absorption and increase the damage thresholds for both kinds of as-grown samples. However, the improving effects are not the same. (c) 2008 Elsevier B.V. All rights reserved.

Quantifying pulsed laser induced damage to grapheme

As an emerging optical material, graphene’s ultrafast dynamics are often probed using pulsed lasers yet the region in which optical damage takes place is largely uncharted. Here, femtosecond laser pulses induced localized damage in single-layer graphene on sapphire. Raman spatial mapping, SEM, and AFM microscopy quantified the damage. The resulting size of the damaged area has a linear correlation with the optical fluence. These results demonstrate local modification of sp2-carbon bonding structures with optical pulse fluences as low as 14 mJ/cm2, an order-of-magnitude lower than measured and theoretical ablation thresholds.

Theoretical and experimental study on femtosecond laser induced damage in CaF2 crystals

We report on the damage threshold in CaF2 crystals induced by femtosecond laser at wavelengths of 800 nm and 400 nm, respectively. The dependences of ablation depths and ablation volumes on laser fluences are also presented. We investigate theoretically the coupling constants between phonon and conduction band electrons (CBE), and calculate the rates of CBE absorbing laser energy. A theoretical model including CBE production, laser energy deposition, and CBE diffusion is applied to study the damage mechanisms. Our results indicate that energy diffusion greatly influences damage threshold and ablation depth.

The ultrafast excitation processes in femtosecond laser-induced damage in dielectric omnidirectional reflectors

A pump and probe system is developed, where the probe pulse duration tau is less than 60 fs while the pump pulse is stretched up to 150-670 fs. The time-resolved excitation processes and damage mechanisms in the omnidirectional reflectors SiO2/TiO2 and ZnS/MgF2 are studied. It is found that as the pump pulse energy is higher than the threshold value, the reflectivity of the probe pulse decreases rapidly during the former half, rather than around the peak of the pump pulse. A coupled dynamic model based on the avalanche ionization (AI) theory is used to study the excitation processes in the sample and its inverse influences on the pump pulse. The results indicate that as pulse duration is longer than 150 fs, photoionization (PI) and AI both play important roles in the generation of conduction band electrons (CBEs); the CBE density generated via AI is higher than that via PI by a factor of 10(2)-10(4). The theory explains well the experimental results about the ultrafast excitation processes and the threshold fluences. (c) 2006 American Institute of Physics.

Ultraviolet-infrared femtosecond laser-induced damage in fused silica and CaF2 crystals

The damage in fused silica and CaF2 crystals induced by wavelength tunable femtosecond lasers is studied. The threshold fluence is observed to increase rapidly with laser wavelength lambda in the region of 250-800 nm, while it is nearly a constant for 800 damage in dielectrics irradiated by the visible and near ultraviolet femtosecond lasers. Our theory explains well the experiments.