Yingjie Chai

Laser-resistance sensitivity to substrate pit size of multilayer coatings

Nanosecond laser-resistance to dielectric multilayer coatings on substrate pits was examined with respect to the electric-field (E-field) enhancement and mechanical properties. The laser-induced damage sensitivity to the shape of the substrate pits has not been directly investigated through experiments, thus preventing clear understanding of the damage mechanism of substrate pits. We performed a systematic and comparative study to reveal the effects of the E-field distributions and localized stress concentration on the damage behaviour of coatings on substrates with pits. To obtain reliable results, substrate pits with different geometries were fabricated using a 520-nm femtosecond laser-processing platform. By using the finite element method, the E-field distribution and localized stress of the pitted region were well simulated. The 1064-nm damage morphologies of the coated pit were directly compared with simulated E-field intensity profiles and stress distributions. To enable further understanding, a simplified geometrical model was established, and the damage mechanism was introduced.

Suppression of nano-absorbing precursors and damage mechanism in optical coatings for 3ω mirrors.

Damage precursors in the 3ω (351 nm) mirror for a high-power laser system are investigated as well as the relevant damage mechanisms. The precursors are classified into two ensembles according to the different laser resistance and damage features. The former is nano-absorbing precursors, which are sensitive to the standing wave electric field and vulnerable to the laser irradiation. The latter is submicrometer nodular defects, which have higher laser resistance and are sensitive to the adhesion strength between the fluoride coatings and oxide coatings. The damage due to nano-absorbing precursors is efficiently suppressed with the double stack design that screens the electric field in the oxides. Currently, the nodular seed is major originating from the Al2O3/SiO2 stack. Even for the same defect type and mirror, the final damage features are dependent on the local mechanical properties at the irradiation location. The investigations of the damage mechanisms provide a direction to further improve the laser-induced damage threshold of the 3ω mirror.

Beam modulation caused by the plasma scalds in the multilayer dielectric films.

The near-field phase modulation (NFPM) caused by the plasma scalds is investigated with a newly proposed mixed overcoat layer model. Based on the NFPM, the far-field intensity modulation (FFIM) is calculated and discussed with the scalar diffraction theory. The results indicate that both the NFPM and FFIM are sensitive to the scalding depth. A feature curve is developed to analyze the NFPM for arbitrary scalding depth. The modulation can be ignored when the scalding depth is less than the first feature point in the feature curve. Even though the diffraction intensity in the Fresnel region can be enhanced dozens of times, the FFIM in the Fraunhofer region can recover gradually if the scalding depth is below a critical value. The preliminary experimental results are consistent with the theoretical prediction.

Multilayer deformation planarization by substrate pit suturing

In the pursuit of 1064 nm high-power laser resistance dielectric coatings in the nanosecond region, a group of HfO2/SiO2 high reflectors with and without suture layers were prepared on prearranged fused silica substrates with femtosecond laser pits. Surface morphology, global coating stress, and high-resolution cross sections were characterized to determine the effects of substrate pit suturing. Laser-induced damage resistance was investigated for samples with and without suture layers. Our results indicate considerable stability in terms of the nanosecond 1064 nm laser-induced damage threshold for samples having a suture layer, due to decreased electronic field (e-field) deformation with simultaneous elimination of internal cracks. In addition, a suture layer formed by plasma ion-assisted deposition could effectively improve global mechanical stress of the coatings. By effectively reducing the multilayer deformation using a suture layer, electron-beam high-reflective coatings, whose laser-induced damage resistance was not influenced by the substrate pit, can be prepared.

Origin of the plasma scalds in dielectric coatings induced by 1ω laser

The plasma scalds initiated by a 1053 nm (1ω) nanosecond laser are separated from the defect-induced damage pits, which is verified as a result of the ionization wavefront with the subnanosecond laser. Considering the beam reflection from solid-state absorption fronts during the damage process, a theoretical scalding threshold about 6.84 J/cm2 (12 ns) based on the energy required to start an air avalanche is evaluated and agrees well with the experimental scalding threshold. The occurrence order of the initial explosion and subsequent ionization wavefront is verified to explain most of the damage morphologies caused by the 1ω laser. In addition to the significance in laser conditioning or cleaning for a high-power laser system, the results also indicate that through the occurrence of plasma scalds it is possible to mark the onset time of air plasma during laser-coating interaction.

Study of hafina-silica mixed coatings with different compositions prepared by E-beam co-evaporation

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.

Coupling effect of subsurface defect and coating layer on the laser induced damage threshold of dielectric 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.

Laser-Induced damage properties of 355nm space coatings (Conference Presentation)

Space laser systems are widely used in communication, altimeter and Doppler radar. UV laser, which possesses high spectral resolution and provides with the detection of the parallel polarized molecular (Rayleigh) and particle (Mie) backscattered signals has promising use in atmosphere detection and Doppler radar. No orbiting satellite carrying with 355nm laser has yet been launched owing to the laser induced damage of coatings. Coatings for spaceborne laser system are widely used in spacecraft with laser system to improve the transmittance of the optical system and to adjust the laser beams. An effective way to improve the lifetime of the coatings and the resistance to the environment is to increase laser induced damage threshold (LIDT). The subsurface damage (SSD) of the substrate is one of the major harmful factors in laser induced damage. In our study, 355nm high-reflection (HR) and anti-reflective (AR) coatings deposited by dual-ion beam sputtering (DIBS) were stable and showed lit

Study on Brewster angle thin film polarizer using hafnia-silica mixture as high refractive index material

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.

Improving the laser-induced damage threshold of 532-nm antireflection coating using plasma ion cleaning

Abstract. 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.