Chemical vapor deposited (CVD) Zinc Selenide (ZnSe) window are often used in the high power laser system because of their extremely low (near-/far-)infrared absorption, and large transmission range (0.5um-22um). A good CVD process enables us to achieve the extreme purity needed for high-powered optics: 99.999% purity. Zinc vapor and H2Se gas react to form sheets of ZnSe on graphite susceptors. Different grain-sized polycrystalline ZnSe plates are precisely prepared and tooled to form optical windows or optical gain media. In recent years, in the area of few-cycle laser interaction with optic components, many new phenomena were found and investigated. Electron dynamic calculations were clearer and richer in this time domain[1-2]. Due to the potential applications in NIR few-cycle laser or attosecond laser generation, the single shot and multi-shot laser-induced damage threshold are tested in a multiplate supercontinuum-based few-cycle laser platform (12fs 1030nm centered) in CREOL according to ISO 21254. The damage threshold of different pulse numbers was carefully extracted. Laser-induced damage evolution along with the pulse number and grain boundary density are carefully characterized by SEM, AFM, and confocal Raman spectroscopy. Our experimental results will show the impact of grain boundary density on the laser-induced damage threshold of polycrystalline materials, which is of great value for future ultrashort laser pulse applications.
Ultrashort laser pulse interaction with solid contains an abundant electronic dynamic process. When the laser pulse width reached few-cycle area, surface damage behaviors can reflect the initial light-electron interaction. In this work, we employed an ultrashort laser source to investigate damage behavior on solid. We investigate the potential of the multi-plate medium to generate broad supercontinuum and few-cycle pulses from a relatively high peak and average power Yb solid-state laser source in a single-pass double-stage multi-plate supercontinuum setup. The experiments were performed by focusing the output from a Yb3+: KGW regenerative amplifier, producing pulses from energy of 600mJ(1kHz) and a duration of 170 fs(~5ps tunable), through a set of thin fused silica plates with individual thicknesses of 1mm to 2mm. This resulted in pulse width of 15fs-25fs (4 cycles-7 cycles) continuous tunable laser source to investigate the laser-induced damage behavior of ZnSe and Fused silica. The damage mechanism and damage threshold are explained in the context of the Keldysh theory and critical electron density.
Our work simulated the electron dynamic process based on different models, field-cycle-resolved photoionization theory and Keldysh theory. The central idea for predicting laser-induced damage threshold of few-cycle laser pulse based on the total laser energy coupled with the electron energy transfer in the crystal lattice. With this approach, predictions of the physical model start to converge to the available experimental data of 1-on-1 few-cycle laser damage experiments on the semiconductor (e.g. ZnSe) surface.
We report on the experimental and theoretical studies of ultrafast laser-induced optical breakdown on the surface of fused silica to elucidate the mechanism of damage formation and sub-optical-cycle dynamics in material processing using single and a burst of two femtosecond laser pulses. Ionization pathways, including photo-ionization (PI) and avalanche ionization (AI), are investigated by using single-beam and double-beam laser damage threshold measurements, which are used to analyze electron dynamics and extract the avalanche coefficient. The relationship between damage size and laser fluence is interpreted as a result of a combination of PI and AI. Electrical field rather than laser intensity is the fundamental influential factor in PI, and AI is found to play a significant role in creating the free electron density needed for optical breakdown. These findings are verified by a double-pulse delay-scan experiment where two cross-polarized pulses are used to induce damage with delay within a few optical cycles. Variation of the damage diameter is observed within one optical cycle, which is explained by the periodic change of polarization in the combined electric field. This finding shows the potential of controlling laser induced damage by tuning the temporal overlap of a burst of ultrashort laser pulses.
High-power, ultrashort laser-induced periodic surface structures (also referred as ripples), which has been observed on metals, dielectrics and semiconductors surface, could be generated and deliberately modulated by controlling the incident laser pulse. The periodicity, orientation and structure are the typical parameters in the study this near/subwavelength structures. The formation mechanism of LIPSS is still under investigation, and the current formation mechanisms on LIPSS include classical surface scattering model, self-organization, second/third harmonic generation, excitation of surface plasma polaritons, coulomb explosion, and cavitation instability and so on. In our work, 1-on-1 and N-on-1 laser-induced damage experiments were conducted on ZnSe substrate by using 170 fs laser and few-cycle laser to verifying the structure dependence with polarization, periodicity, and laser-induced damage threshold. Damage mechanism based on phenomenon was proposed.
The properties of coatings deposited by electron-beam (e-beam) technique can be easily influenced by environmental humidity, causing spectrum shift, residual stress evolution, and wave front errors. HfO2 / SiO2 multilayer coatings with different overcoat layer deposition parameters have been prepared. The optical spectrum shifts induced by atmosphere-vacuum effect are investigated by a spectrometer. The laser resistance is studied and their damage morphologies are characterized by a scanning electron microscope. The surface morphologies and the global mechanical stresses of the films are analyzed by an atomic force microscope and zygo interferometer, respectively. The experimental results demonstrate that by introducing a dense capping SiO2 layer employed with plasma ion assisted deposition, considerable environmental stability of e-beam coatings can be improved due to retarded water vapor transport. A relatively smaller grain size can be obtained as well. Moreover, the laser-induced damage threshold shows no significant difference.
The properties of coatings deposited by electronic beam (e-beam) technique can be easily influenced by environmental humidity, causing spectrum shift, residual stress evolution, and wave front errors. In this work, HfO2/SiO2 multilayer coatings with different overcoat layer deposition process were prepared. The optical spectrum shift caused by atmosphere-vacuum effect of the prepared samples was investigated by spectrometer. The laser-induced damage resistance was studied and the damage morphologies were characterized by Scanning Electron Microscope (SEM). The surface morphology and global coating stress of the films were analyzed by Atomic Force Microscope (AFM) and Zygo interferometer, respectively. The experimental results demonstrate that by a capping SiO2 layer employed by plasma ion assisted deposition (PIAD), considerable stability concerning the environmental stability of e-beam coatings can be improved due to delayed water vapor transport rate. A relatively smaller grain size can be obtained as well. Moreover, the laser- induced damage threshold (LIDT) shows no significant differences.
Two kinds of polarizer coatings were prepared by electron beam evaporation, using HfO2–SiO2 mixture and HfO2 as the high-refractive-index materials, respectively. The HfO2–SiO2 mixture layer was implemented by coevaporating SiO2 and metal Hf, the materials were deposited at an oxygen atmosphere to achieve stoichiometric coatings. The certain HfO2 and SiO2 content ratio is controlled by adjusting the deposition rate of HfO2 and SiO2 using individual quartz crystal monitor. The spectral performance, surface and interfacial properties, as well as the laser-induced damage performance were studied and compared. Comparing with polarizer coating using HfO2 as high-refractive-index material, the polarizer coating using HfO2–SiO2 mixture as high-refractive-index material shows better performance with broader polarizing bandwidth, lower surface roughness, better interfacial property while maintaining high laser-induced damage threshold.
Two kinds of hafnia-silica polarizer coatings were prepared by electron beam evaporation, using hafnia-silica mixture (MPOL) and hafnia (POL) as the high refractive index materials, respectively. The spectral performance, surface and interfacial properties, as well as the laser induced damage performance were studied and compared. The M-POL coating shows better performance with broader polarizing bandwidth, lower surface roughness, better interfacial property, while maintaining high laser induced damage threshold.
BK7 glass substrates were precleaned by different cleaning procedures before being loaded into a vacuum chamber, and then a series of plasma ion cleaning procedures were conducted at different bias voltages in the vacuum chamber, prior to the deposition of 532-nm antireflection (AR) coatings. The plasma ion cleaning process was implemented by the plasma ion bombardment from an advanced plasma source. The surface morphology of the plasma ion-cleaned substrate, as well as the laser-induced damage threshold (LIDT) of the 532-nm AR coating was investigated. The results indicated that the LIDT of 532-nm AR coating can be greatly influenced by the plasma ion cleaning energy. The plasma ion cleaning with lower energy is an attractive method to improve the LIDT of the 532-nm AR coating, due to the removal of the adsorbed contaminations on the substrate surface, as well as the removal of part of the chemical impurities hidden in the surface layer.
KEYWORDS: Annealing, Particles, Dewetting, Silver, Reflectivity, Sputter deposition, Scanning electron microscopy, Antireflective coatings, High power lasers, Solar cells
The structure properties of random mask of antireflective structure prepared by the thermal dewetting process are investigated. As a low-cost and large-scale technique, the mask obtained in our work has a great prospect in the field of solar cell and high power laser system. Ultrathin films of amorphous Ag are deposited on the fused silica by magnetron sputtering. By fast thermal annealing the structures in Ag film are agglomerated on the substrate and form mask. The influence of different thickness and annealing temperature on the structure properties of random mask are studied. The surface morphologies are characterized by scanning electronic microscopy. The suitable conditions to obtain excellent quality Ag nanomasks with the pebble particles are achieved.
Hafnia-silica (HfO2/SiO2) mixed thin films with a wide range of different compositions have been deposited on fused silica substrate by E-beam co-evaporation. The change in composition is achieved by changing the deposition rates of individual materials. The transmittance spectra are measured by the spectrometer, and then the refractive indexes are calculated with Essential Macleod. The vertical uniformity and grain structure of the films are analyzed by X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD), respectively. The experimental results demonstrate that the coatings have good uniformity, and all the films are amorphous except the pure HfO2 film. In addition, the mixed coating with about 16.85% SiO2 content has a higher refractive index than the pure HfO2 coating.
The laser damage resistance of coatings for high power laser systems depends greatly on the surface quality of substrate.
In this work, experimental approaches with theoretical simulation were employed to understand the coupling effect of
subsurface defect and coating on the laser resistance of coating. 1064 nm anti-reflection coating was deposited by
E-beam deposition on fused silica. Substrate with and without micro-scale pits were fabricated precisely by femtosecond
laser processing. Experimental results indicate that impurities induced in the finishing process shifted to the substrate
surface and aggregated during the heating process. Theoretical simulation result shows that the coupling effect of the
aggregated impurities and coating are mainly responsible for the low LIDT of E-beam deposition coating.
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