Analysis of laser induced thermal mechanical relationship of HfO_2/SiO_2 high reflective optical thin film at 1064 nm

A numerical model is developed for the calculation of transient temperature field of thin film coating induced by a long-pulsed high power laser beam. The electric field intensity distribution of HfO2/Si02 high reflective （HR） film is investigated to calculate the thermal field of the film. The thermal-mechanical relationships are discussed to predict the laser damage area of optical thin film under long pulse high energy laser irradiation.

Enhanced laser induced damage threshold of dielectric antireflection coatings by the introduction of one interfacial layer

A new method for increasing laser induced damage threshold （LIDT） of dielectric antireflection （AR） coating is proposed. Compared with AR film stack of H2.5L （H：HfO2, L：SiO2） on BK7 substrate, SiO2 interracial layer with four quarter wavelength optical thickness （QWOT） is deposited on the substrate before the preparation of H2.5L film. It is found that the introduction of SiO2 interfacial layer with a certain thickness is effective and flexible to increase the LIDT of dielectric AR coatings. The measured LIDT is enhanced by about 50%, while remaining the low reflectivity with less than 0.09% at the center wavelength of 1064 nm. Detailed mechanisms of the LIDT enhancement are discussed.

Contamination process and laser-induced damage of HfO_2/SiO_2 coatings in vacuum

The performances of HfO2/SiO2 single- and multi-layer coatings in vacuum influenced by contamination are studied. The surface morphology, the transmittance spectrum, and the laser-induced damage threshold are investigated. The results show that the contamination in vacuum mainly comes from the vacuum system and the contamination process is different for the HfO2 and SiO2 films. The laser-induced damage experiments at 1064 nm in vacuum show that the damage resistance of the coatings will decrease largely due to the organic contamination.

Annealing effects on residual stress of HfO_2/SiO_2 multilayers

HfO2/SiO2 multilayer films were deposited on BK7 glass substrates by electron beam evaporation method. The effects of annealing at the temperature between 200 and 400 ℃ on residual stresses have been studied. It is found that the residual stress of as-deposited HfO2/SiO2 multilayers is compressive. It becomes tensile after annealing at 200 ℃, and then the value of tensile stress increases as annealing temperature increases. And cracks appear in the film because tensile stress is too large when the sample is annealed at 400 ℃. At the same time, the crystallite size increases and interplanar distance decreases with the increase of annealing temperature. The variation of residual stresses is corresponding with the evolution of structures.

Influence of APS bias voltage on properties of HfO_2 and SiO_2 single layer deposited by plasma ion-assisted deposition

HfO2 and SiO2 single layer is deposited on glass substrate with plasma ion assistance provided by Leybold advanced plasma source （APS）. The deposition is performed with a bias voltage in the range of 70-130 V for HfO2, and 70-170 V for SiO2. Optical, structural, mechanical properties, as well as absorption and laser induced damage threshold at 1064 nm of HfO2 and SiO2 single layer deposited with the plasma ion assistance are systematically investigated. With the increase of APS bias voltage, coatings with higher refractive index, reduced surface roughness, and higher laser-induced damage threshold （LIDT） are obtained, and no significant change of the absorption at 1064 nm is observed. For HfO2, a bias voltage can be identified to achieve coatings without any stress. However, too-high bias voltage can cause the increase of surface roughness and stress, and decrease the LIDT. The bias voltage can be properly identified to achieve coatings with desired properties.

Influence of purity of HfO_2 on reflectance of ultraviolet multilayer

The impurities in two kinds of HfO2 materials and in their corresponding single layer thin films were determined through glow discharge mass spectrum technology and secondary ion mass spectrometry （SIMS） equipment respectively. It was found that ZrO2 was the main impurity in the two kinds of HfO2 either in the original HfO2 materials or in the electron beam deposited films. In addition, the difference of Zr content in the two kinds of HfO2 single layer films was much larger than that of the other impurities such as Ti and Fe, which showed that it was just ZrO2 that made the difference between the optical performance of the film products including the two kinds of HfO2. With these two kinds of HfO2 and the same kind of SiO2, we deposited HfO2/SiO2 multilayer reflective coatings at the wavelength of 266 nm. Experimental results showed that the reflectances of these two mirrors were about 99.85% and 99.15% respectively, which agreed well with the designed results what were based on the optical constants obtained from the corresponding single layer thin films.

Experimental study of sweep control in e-beam evaporated optical coatings

High performance optical coating requires excellent uniformity of thin-film.Keeping the surface of evaporation material flat is propitious for the stability of vapor plume,and can improve the uniformity of thin-film.Based on the principle of electron beam spot sweep,a pattern controller in domestic coater is designed.For the purpose of even evaporation during auto-sweep,the influence of the depth of material surface in the crucible on the evaporation characteristic is considered.Pre-melting and evaporation experiments are performed on melting material（Ti3O5）,subliming material（SiO2）,and semi-melting, semi-subliming material（HfO2）.The sweeping experimental results show that using the designed sweep controller can make good performance on evaporation and pre-melting for the above materials.

Influence of laser conditioning on defects of HfO_2 monolayer films

The influence of laser conditioning on defects of HfO2 monolayer films prepared by electron beam evaporation （EBE） is investigated utilizing the spot-size effect of the laser-induced damage.It is found that the laser-induced damage threshold of HfO2 monolayer films can be increased by a factor of 1.3-1.6.It is also found that the defects with low threshold can be removed by laser conditioning and defects with higher threshold may be removed partially.