The effect of oxygen vacancy on the laser-induced damage behaviors of
HfO2/SiO2 high reflective coating was investigated by single 355nm-8ns laser.
The oxygen vacancy was tuned by controlling the oxygenation of the outmost
HfO2 layer during the deposition procedure. The reflectivity of the coating with
higher oxygen vacancy drops by 0.2% and the damage threshold drops by 60%,
compared with the normal coating. Damage morphologies of samples were
obtained by optical microscope, AFM, SEM and FIB technology. Typical
morphologies of these coatings show little difference. Average oxygen vacancy
of single HfO2 layer prepared on the BK7 substrate measured by XPS is about
43%. Theoretical analysis with a nonlinear thermodynamics model shows that
the damage can be attributed to nonlinear thermal process. Moreover, the size of
damage crater can be interpreted by a mechanics model.
The growth of laser induced damage on the surface of fused silica plays a major role in determining the operation fluence
and optics lifetime in high power laser system. In this paper, the damage growth characteristic of fused silica and possible
growth mechanisms were investigated. The morphology of damage site was measured by scanning electron microscopy
(SEM) and optical microscopy (OM). The finite difference time domain (FDTD) method was used to calculate the electric
field distribution around the damage site. Furthermore, energy dispersive spectrometers (EDS) micro-analysis technique,
x-ray photoelectron spectrometer (XPS) and Raman spectroscopy were applied to detect the chemical composition, point
defect and microstructure of damage site in order to explore the growth mechanism. It’s found that the growth threshold is greatly affected by the size of damage site, and the growth threshold of damage site is much lower than that of undamaged area. Theoretical calculation demonstrated that the rough damage site can strong modulate the distribution of electric field and result in the enhancement of local light field around the damage site. Results also showed that the oxygen defect was generated and the structure was changed after initial laser damage. Based on the above analysis, a mechanism of laser-induced damage growth on fused silica surface was proposed.
The multilayer dielectrics (MLDs) for broad bandwidth 800nm pulse compression gratings were
fabricated with optimized design by electron beam evaporation using three different kinds of materials
(Ta2O5/SiO2/HfO2), which had more than 99% reflectance with bandwidth larger than 160nm around
the center wavelength of 800 nm and high transmission at the exposure wavelength of 413nm.
Laser-induced damage behaviors of the mirrors were investigated. It was found that the laser-induced
damage threshold (LIDT) of the samples could reach 1.0J/cm2 and 2.0J/cm2 in the normal beam (57
degrees, TE mode) at pulse duration of 50fs and 120fs, respectively. The depth information of the
damage sites at these two cases was explored by atomic force microscope (AFM). The reason of the
sample having so high LIDT was also discussed in this paper. The MLDs provide a solid base for the
high laser threshold 800nm pulse compression gratings and may open a new way for broad bandwidth
800nm reflectance coatings used in the ultrashort pulse laser system.
This paper is devoted to a long-term investigation into the nature of incubation effect of multilayer dielectric HR mirror
coatings. Accumulated damage behaviors of HfO2/SiO2 mirrors for 800nm, 1053nm, and 1064nm, both fabricated by
conventional electron beam evaporation (EBE), were investigated by ultra-short pulse (800nm/~100fs), short pulse
(1053nm/~1ps), and long pulse (1064nm/~10ns) lasers, respectively. Incubation effect was found to be a universal
phenomenon for HfO2/SiO2 mirrors irradiating by the femto-nanosecond lasers. And when the shot number was about
100, the multi-pulse damage threshold of samples decreased to the level of 60~70% of the single-pulse threshold. Typical
damage morphologies and depths information of HR samples were characterized by optical microscope and surface
profiler. The results revealed that the electric field distribution within the mirrors had significant influence on the initial
damage onset of the mirrors. In addition, theoretical simulation was carried out to describe the incubation behaviors of
HfO2/SiO2 mirrors in the femto- and nano-second regions. It seemed reasonable that incubation effect was attributed to the accumulation of native or laser-induced electronic trapping states.
As large amounts of heat need to be dissipated during laser operation, some diode pumped solid state
lasers (DPSSL), especially Yb:YAG laser, operate at cryogenic condition. This work investigated the
laser induced damage of coatings (high-reflective and
anti-reflective coatings) on Yb:YAG crystals at
cryogenic temperature and room temperature. The results show that the damage threshold of coatings at
cryogenic temperature is lower than the one at room temperature. Field-emission scanning electron
microscopy (FESEM), optical profiler, step profiler and Atomic force microscope (AFM) were used to
obtain the damage morphology, size and depth. Taking alteration of physical parameters,
microstructure of coatings and the environmental pollution into consideration, we analyzed the key
factor of lowering the coating damage threshold at cryogenic conditions. The results are important to
understand the mechanisms leading to damage at cryogenic condition.
Two kinds of multilayer HfO2/SiO2 high reflectors, the "standard" and "modified" designs with different electric field
distribution, were investigated in this paper. A femtosecond laser system was applied to test the laser-induced damage
thresholds (LIDTs) of these two kinds of coatings. Optical microscopy and scanning electron microscopy (SEM) were
employed to study the morphology and structural information of the damage site. Compared the LIDTs of "modified"
design with that of "standard" design, it was found that reducing electric field at layer interface could improve the LIDTs
of multilayer coatings. Furthermore, the improvement efficiency showed certain dependency on the pulse width. Diverse
roles of several conceivable ionization mechanisms played in the damage process were discussed to explain the relation
between the LIDTs and electric field distribution.
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