Shuhai Fan

Employing oxygen-plasma posttreatment to improve the laser-induced damage threshold of ZrO 2 films prepared by the electron-beam evaporation method

ZrO2 films are deposited by the electron-beam evaporation method. Parts of the prepared samples are post-treated with oxygen plasma at the environment temperature. The laser-induced damage threshold (LIDT) of the films increases from 15.9 to 23.1 J/cm2 after treatment with oxygen plasma. Compared with that of the as-grown samples, significant reduction of the average microdefect density and absorption are found after oxygen-plasma posttreatment. These results indicate that the oxygen-plasma posttreatment technique is an effective and simple method for reducing the microdefect density and absorption to improve the LIDT.

Laser-induced damage threshold of ZrO2 thin films prepared at different oxygen partial pressures by electron-beam evaporation

ZrO2, films were deposited by electron-beam evaporation with the oxygen partial pressure varying from 3 X 10(-3) Pa to I I X 10(-3) Pa. The phase structure of the samples was characterized by x-ray diffraction (XRD). The thermal absorption of the films was measured by the surface thermal lensing technique. A spectrophotometer was employed to measure the refractive indices of the samples. The laser-induced damage threshold (LIDT) was assessed using a 1064, nm Nd: yttritium-aluminium-garnet pulsed laser at pulse width of 12 ns. The influence of oxygen partial pressure on the microstructure and LIDT of ZrO2 films was investigated. XRD data revealed that the films changed from polycrystalline to amorphous as the oxygen partial pressure increased. The variation of refractive index at 550 nm wavelength indicated that the packing density of the films decreased gradually with increasing oxygen partial pressure. The absorptance of the samples decreased monotonically from 125.2 to 84.5 ppm with increasing oxygen partial pressure. The damage threshold, values increased from 18.5 to 26.7 J/cm(2) for oxygen partial pressures varying from 3 X 10(-3) Pa to 9 X 10(-3) Pa, but decreased to 17.3 J/cm(2) in the case of I I X 10(-3) Pa

Development of surface thermal lensing technique in absorption and defect analyses of optical coatings

Absorption is one of the main factors which cause damage to optical coatings, under the radiation of high power lasers. Surface thermal lensing (STL) technique was developed into a practical high-sensitivity apparatus for the weak absorption analysis of optical coatings. A 20 W continuous-wave 1064 nm Nd:YAG laser and a 30 mW He-Ne laser were employed as pump source and probe source, respectively. Low noise photoelectrical components and an SR830 DSP lock-in amplifier were used for photo-thermal deformation signal detection. In order to improve sensitivity, the configuration of the apparatus was optimized through choosing appropriate parameters, that including pump beam spot size, chopper frequency, detection distance, waist radius and position of probe beam. Coating samples were mounted on an x-y stage which was driven by high precision stepper motors. Different processes of absorption measurements, including single spot, linear scan and 2-dimension area scan, could be performed manually or automatically under the control of PC program. Various optical coatings were prepared by both electron beam evaporation and ion beam sputtering deposition. High sensitivity was obtained and low to 10 ppb absorption could be measured by surface thermal lensing technique. And a spatial resolution of 25 micron was proved according to the area scanning which traced out the profile of photo-thermal defects inside optical coatings. The system was employed in the analyses of optical absorption, absorption uniformity and defect distribution, and revealed the relationship between laser-induced damage and absorption of optical coatings.

Absorption measurement for coatings using surface thermal lensing technique

Two absorption measuring methods by a measurement system based on Surface Thermal Lensing theory (STL) are given. The measuring results of coatings indicate a deviation of 5ppm and the sensitivity is better than 10ppm.

The effects of ion cleaning on the roughness of substrates and laser-induced damage thresholds of films

The ion cleaning effect on the roughness of substrates and laser induced damage thresholds (LIDT) of films were investigated. It is found that ion cleaning has different effects on the roughness of substrates with the different ion cleaning time, and it improves the LIDT of single layer films greatly.

Laser conditioning of dielectric oxide mirror coatings at 1064 nm

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.

Absorptivity measurement for thin films based on surface thermal lensing

Two absorptivity measuring methods by a measurement system based on Surface Thermal Lensing theory are given. The measuring results indicate a deviation of 5ppm and the sensitivity is better than 10ppm.

Anisotropic optical properties of ZrO2 films by oblique deposition

The ZrO2 thin films are prepared by oblique deposition. Spectrophotometry yields an obvious anisotropic p-polarized transmittance. On the computation of refractive indices of different incidence angles and polarization states, the anisotropic optical properties are obtained. The refractive indices derived from s-polarized light are around 1.725, but the ones for p-polarized light are from 1.82 to 1.94. The distribution of refractive indices is consistent with the transmittance of s- and p-polarized light. The transmittance of ZrO2 thin films by oblique and normal deposition are compared each other. The results could be qualitative understood from the presence of an inclined column with shape birefringence.