Ganghua Bao

Study of HfO2/SiO2 dichroic laser mirrors with refractive index inhomogeneity

HfO2/SiO2 dichroic mirrors, having high reflectance at 1064 nm and high transmittance at 532 nm, play an important role in high-power laser systems. However, the half-wave hole effect, caused mainly by the refractive index inhomogeneity of hafnia, affects the spectra and application of these mirrors. Two approaches to eliminate the half-wave hole effect have been proposed. Both approaches attempt to shift the location of the half-wave hole in comparison with the original wavelength. One approach broadens the reflectance band of the first harmonic wavelength and simultaneously adjusts the central reflectance band to a longer wavelength, whereas the other approach combines the two stacks to adjust the location of the half-wave hole far away from the wavelength of interest. Two kinds of dichroic mirrors have been successfully fabricated; moreover, it was found that the method of a two-stack combination, 0.9(HL)8 and 1.1(HL)8, provides designs that can be fabricated more easily and with better quality spectral characteristics.

Interfacial damage in a Ta 2 O 5 /SiO 2 double cavity filter irradiated by 1064 nm nanosecond laser pulses

The Laser-Induced Damage Threshold (LIDT) and damage morphologies of a Ta(2)O(5)/SiO(2) double cavity filter irradiated by 1064-nm, 10-ns pulses were investigated. The depths of flat bottom pits were examined by an optical profiler and then calibrated according to the Electric-Field Intensity (EFI) distributions and the cross-sectional micrographs obtained using the Focus Ion Beam (FIB) technology. The statistics for depths of 60 damage sites suggested that the Ta(2)O(5) over SiO(2) interface was more vulnerable to Laser-Induced Damage (LID) than the SiO(2) over Ta(2)O(5) interface. After examining the morphologies of interfacial delaminations carefully, we found that the Ta(2)O(5) over SiO(2) interface instead had stronger mechanical strength. So, the higher density of susceptible defects at the Ta(2)O(5) over SiO(2) interface was considered to be the reason that LID was preferentially initiated at this type of interface. Based on the above findings, a phenomenological model was proposed to describe the formation of flat bottom pits.

Effects of substrate temperatures on the structure and properties of hafnium dioxide films

Different HfO2 monolayers under different deposition conditions, such as substrate temperature and oxygen partial pressure, were prepared from metal hafnium using the reactive electron beam evaporation method. X-ray diffraction was applied to determine the crystalline phase of these films, the surface morphology of the samples was examined by atomic force microscopy, and the optical properties were analyzed using a spectrophotometer and the surface thermal lens technique. The relationship between substrate temperature and film characteristic was investigated, and the correlation between the observed film properties and the laser damage threshold was also discussed.

Study for Improvement of Laser Induced Damage of 1064 nm AR Coatings in Nanosecond Pulse

For the conventionally polished fused silica substrate, an around 100 nm depth redeposition polishing layer was formed on the top of surface. Polishing compounds, densely embedded in the redeposition polishing layer were the dominant factor that limited the laser induced damage threshold (LIDT) of transmission elements in nanosecond laser systems. Chemical etching, super-precise polishing and ion beam etching were employed in different ways to eliminate these absorbers from the substrate. After that, Antireflection (AR) coatings were deposited on these substrates in the same batch and then tested by 1064 nm nano-pulse laser. It was found that among these techniques only the ion beam etching method, which can effectively remove the polishing compound and did not induce extra absorbers during the disposal process, can successfully improve the LIDT of AR coatings.

Characterization of grain sizes and roughness of HfO_2 single layers

The grain sizes and their influence on the roughness of an HfO2 single layer prepared with ion-assisted deposition were investigated. Three methods, x ray diffractometry, atomic force microscopy, and the k-correlated power spectral density function model, were used to obtain the grain sizes in two HfO2 single layers with 16 and 20 nm thicknesses. X ray diffractometry showed that the grain sizes were about 7 and 9 nm, respectively, whereas the other two methods demonstrated that the grain sizes were about 14 and 16 nm. It was thought that x ray diffractometry underestimated the grain size due to micro strain or a shallow penetration depth. The grains in an HfO2 single layer lead to a rough surface, which had a significant bulge at the middle-high frequency range in a power spectral density function curve. The coating intrinsic roughness of the HfO2 single layer was separated from the substrate roughness.

Study of the HfO2/SiO2 dichroic laser mirrors having refractive index inhomogeneity

Half-wave-hole problem, mainly caused by refractive index inhomogeneity of hafnia, had greatly influenced the spectra and application of HfO2/SiO2 dichroic laser mirrors. Two approaches to eliminate the half-wave-hole effect had been proposed.

The thickness correction of sol-gel coating using ion-beam etching in the preparation of antireflection coating

For the sol-gel method, it is still challenging to achieve excellent spectral performance when preparing antireflection (AR) coating by this way. The difficulty lies in controlling the film thickness accurately. To correct the thickness error of sol-gel coating, a hybrid approach that combined conventional sol-gel process with ion-beam etching technology was proposed in this work. The etching rate was carefully adjusted and calibrated to a relatively low value for removing the redundant material. Using atomic force microscope (AFM), it has been demonstrated that film surface morphology will not be changed in this process. After correcting the thickness error, an AR coating working at 1064 nm was prepared with transmittance higher than 99.5%.

HfO2-SiO2 mixed film deposited by Ion Assisted Deposition Coevaporation

Low absorption, low scattering and high-density HfO2 coatings lead to an important improvement of high power laser systems. In order to suppress the crystallization of HfO2 coatings fabricated with ion assisted deposition (IAD), we used double electron-beam (EB) coevaporation with ion beam assisting method to fabricate HfO2-SiO2 mixtures from two independent material sources. Crystallization following the different mixture ratios was investigated. Several prototypes were designed, featuring different HfO2-SiO2 ratios with similar physical thickness (500nm). The samples were deposited on fused silica and silicon substrates. X-ray diffraction patterns show that the degree of crystallization gradually fades away with increasing SiO2 contents and when SiO2 component reach 18%, the mixture film becomes almost amorphous. To decrease the high absorption originating from IAD method, thermal annealing in air at progressive temperatures was subsequently performed. It was found that post-annealing treatment at 600°C could eliminate the absorption at 1064nm. However, high temperature annealing would induce the crystallization of these initial amorphous coatings. In order to suppress the crystallization further to obtain amorphous structure even after 600°C annealing, the 25% SiO2 sample was fabricated and it successfully obtain low roughness and low absorption equivalent to the bare substrate. Finally, a 1064 nm HR coating using SiO2 and mixed film of 25% SiO2 concentration was prepared and annealed to prove its practical application in low loss and high LIDT optical elements.

Fabrication of broadband antireflection coating using quartz monitor

Through reverse engineering of the fabricating the broadband antireflection coating, the error compensation strategy in quartz crystal monitoring was proposed. And its accuracy and importance throughout the fabrication process is also demonstrated.

Fabrication of the broadband absorbing coatings for ASE suppression

Based on accurate characterization of Cr metal thin films，a metal/dielectric broadband absorbing coating for 1064nm YAG slab lasers to suppress ASE from 0 degree to 53 degree was designed and fabricated successfully.