Laser-induced damage of optical thin films is one of the main obstacles, which prevents laser technology from being developed toward high power. Many experimental results indicated that microdefect and absorption of films are the two major factors that influence laser induced damage threshold (LIDT). To reduce microdefect density and absorption, and improve LIDT of thin films, researchers have developed not only novel film deposition techniques, but also novel film post-treatment techniques. Though film deposition techniques have been highly developed, microdefect still remains to be the main limited factor of LIDT. Because of this, posttreatment techniques as a novel way to reduce defect density and improve LIDT has (been) attracted much attentions. One of the most frequently used posttreatment methods is laser conditioning and another is ion posttreatment. By comparing the treatment mechanism of two posttreatment techniques, it is easy to find their similarities and differences. Though laser conditioning is a classical posttreatment technique, its shortages such as low efficiency, rigorous requirement of equipment stability, and uncertain treatment results are inevitable. As a novel technique, ion posttreatment has great potential to improve LIDT of thin films. This technique not only has high treatment efficiency, but also has convenience and easily adjusted parameters. So it should be a promising posttreatment technique in improving LIDT of optical thin films.
Investigation of laser-induced damage (LID) of dielectric optical coatings was reviewed in this paper. Several methods for evaluating characters of LID were developed, especially for the determinations of laser-induced damage threshold (LIDT) and the detections of absorption based on surface thermal lensing (STL) technology of optical coatings. Defect was deemed to be the initial source of several previous damage mechanisms, and was the main factor restricting the laser damage resistance of optical coatings. A pulsed laser induced damage model with a spherical absorptive inclusion was proposed in order to obtain the nature, size and distribution of defects. Attentions were paid to find out the origins of damage mechanism transformation from one laser mode to another. Moreover, interests were focused on distinct damage behaviors of ultraviolet (UV) lasers. Deposition temperature and annealing process in vacuum chamber had obvious influences on LIDT of the third harmonic Nd:YAG laser coatings. At the end of this paper, several effective methods for improving LIDT were put forward, such as cleaning substrate, improving deposition process, adding protective layers, optimizing coating stacks based on temperature field theory, as well as laser conditioning.
Laser conditioning effects of the dielectric oxide mirror coatings with different designs were investigated. Simple quarter-wave ZrO2:Y2O3/SiO2 mirrors and half-wave SiO2 over-coated ZrO2:Y2O3/SiO2 mirror coatings at 1064nm were fabricated by E-beam evaporation (EBE). The absorbance of the samples before and after laser conditioning was measured by surface thermal lensing (STL) technology and the defects density was detected under Nomarski microscope. The enhancement of the laser damage resistance was found after laser conditioning. The dependence of the laser conditioning on the coating design was also observed and the over-coated sample obtained greatest enhancement, whereas the absorbance of the samples did not change obviously.
In (2+1) dimension, growth process of thin film on non-planar substrate in Kuramoto-Sivashinsky (K-S) model is studied with the numerical simulation approach. 15×15 semi-ellipsoids arranged orderly on the surface of substrate are used to represent initial rough surface of substrate. The results show that at the initial stage of growth process, the interface width constantly decreases with the growth time, then it reaches minimum. However, with the increase of growth time, the granules of different sizes distributes evenly on the surface of thin film. Whereafter the size of granules and the interface width gradually increase with the growth time, and the surface of thin film presents fractal properties.
Zirconium films were deposited by ion beam sputtering method. A novel substrate holder was designed to realize the approximately in-situ observation the process of the nodule growth. The method of pre-setting particle seeds on the substrates was used in the film deposition. Optical microscope and SEM were presented in observation the specimen before and after annealing. An interesting growth mode of nodules which different from the reported in science literature was found in our experiment. The molecule dynamics theory and diffusion limited aggregation (DLA) model was presented to analysis this phenomenon.
The ZrO2 thin films were deposited on BK7 glass substrates by electron beam evaporation method at room temperature. The influences of post-deposition annealing between 200°C and 400°C on the structural and mechanical properties of the films have been investigated by X-ray diffraction and atomic force microscopy. It was found that a monoclinic phase formed at lower temperature of 200°C. With the increase of the annealing temperature, the tetragonal phase appeared. The stress in ZrO2 films showed a transition from tensile to compressive which could be explained as the evolution of the microstructure as function of annealing temperatures. At the same time, the refractive index of the ZrO2 films increased with the increasing of annealing temperatures, which may be ascribed to the microstructure densification of the ZrO2 films.