Hongfei Jiao

Nanosecond laser-induced damage of nodular defects in dielectric multilayer mirrors [Invited]

Our recent studies on nodular damage in dielectric multilayer mirrors were first reviewed, and the main findings are taken as a foundation to further investigate the influence of seed absorptivity and asymmetrical boundary on the laser-induced damage of nodules. Experimental results showed that the seed absorptivity had a big influence on laser-induced damage thresholds (LIDTs) of the prepared nodules. A direct link between the cross-sectional |E|2 distributions and damage morphologies of nodules was found, which can perfectly explain the observed dependence of LIDTs on seed absorptivity. Another series of asymmetrical nodules were also studied in this work. The measured LIDTs of asymmetrical nodules were about 40%-70% lower than the LIDTs of the symmetrical nodules initiating from the same-sized SiO2 seeds. The weaker mechanical stability and the nonuniform |E|2 distributions are two main reasons for the lower laser damage resistance of the asymmetrical nodules.

Physical insight toward electric field enhancement at nodular defects in optical coatings.

Although the finite-difference time-domain (FDTD) technique has been prevailingly used to calculate the electric field intensity (EFI) enhancement at nodular defects in high-reflection (HR) coatings, the physical insight as to how the nodular features contribute to the intensified EFI is not explicitly revealed yet, which in turn limits the solutions that improve the laser-induced damage threshold (LIDT) of nodules by decreasing the EFI enhancement. Here, a simplified model is proposed to describe the intensified EFI in nodules: 1) the nodule works as a microlens and its focal length can be predicted using a simple formula, 2) the portion of incident light that penetrates through the HR coating can be estimated by knowing the angular dependent transmittance (ADT) of the nodule, 3) strong EFI enhancement is created when the focal point is within the nodule and simultaneously a certain portion of light penetrates to the focal position. In the light of the proposed model, a broadband HR coating was used to reduce the EFI enhancement at the seed by a factor about 10, which leads to a 20 times increment of the LIDT. This work therefore not only deepens the physical understanding of EFI enhancement at nodules but also provides a new way to increase the LIDT of multilayer reflective optics.

Nanosecond pulsed laser damage characteristics of HfO 2 /SiO 2 high reflection coatings irradiated from crystal-film interface

The nano-precursors in the subsurface of Nd:YLF crystal were limiting factor that decreased the laser-induced damage threshold (LIDT) of HfO(2)/SiO(2) high reflection (HR) coatings irradiated from crystal-film interface. To investigate the contribution of electric-field (E-field) to laser damage originating from nano-precursors and then to probe the distribution of vulnerable nano-precursors in the direction of subsurface depth, two 1064 nm HfO(2)/SiO(2) HR coatings having different standing-wave (SW) E-field distributions in subsurface of Nd:YLF c5424181043036123rystal were designed and prepared. Artificial gold nano-particles were implanted into the crystal-film interface prior to deposition of HR coatings to study the damage behaviors in a more reliable way. The damage test results revealed that the SW E-field rather than the travelling-wave (TW) E-field contributed to laser damage. By comparing the SW E-field distributions and LIDTs of two HR coating designs, the most vulnerable nano-precursors were determined to be concentrated in a thin redeposition layer that is within 100 nm from the crystal-film interface.

Thin-film thickness-modulated designs for optical minus filter

We proposed an analytical method to design optical minus filters by the thickness modulation of discrete, homogeneous thin-film layers of a two-material multilayer coating. The main stack provides the narrow, second-order rejection band, and the correct thickness-modulation apodization and match layers can effectively suppress the sidelobes of the passband. Using this approach, we can design minus filters with layer thicknesses close to half-wave of the rejection wavelength, making this method well suited for accurate monitoring during the deposition.

Broadband absorption enhancement in elliptical silicon nanowire arrays for photovoltaic applications

Semiconducting nanowire arrays have emerged as a promising route toward achieving high efficiencies in solar cells. Here we propose a perpendicular elliptical silicon nanowire (PEE-SiNW) array for broadband light absorption in thin film silicon solar cells. Simulation results reveal that light absorption enhancement is originated from the split of the principal modes as well as the excitation of high order modes caused by the asymmetry of the elliptical nanowires and the enhanced mode coupling between adjacent elliptical nanowires attained by the appropriate arrangement of nanowires. An ultimate efficiency of 29.1% is achieved for the optimal PEE-SiNW array, which is 16.4% higher than that of the circular SiNW array with the same fill fraction.

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.

Using monodisperse SiO2 microspheres to study laser-induced damage of nodules in HfO2/SiO2 high reflectors

Nodules have been proved to play an important role in the activation of laser damage in 1.053 μm HfO2/SiO2 high reflectors. However, some damage test results revealed that the ejection fluences of some big nodules with height around 1 μm were abnormally high. To find the correlation between the surface dimensions of nodules and their susceptibility to nano-second pulsed laser radiation, monodisperse SiO2 microspheres with five different sizes were used to create engineered nodules in 1.053 μm HfO2/SiO2 high reflectors. The defect density of nodules that were created from SiO2 microspheres was purposely controlled to be around 20-40 mm2 and special care was taken to minimize clusters of SiO2 microspheres as less as possible. This enabled us to take a raster scan test and to get the statistical value of ejection fluences of these engineered nodules. The height and width dimensions of the engineered nodules, especially the discontinuity of nodular boundary, were measured by cross-sectioning of nodular defects using a focused ion-beam milling instrument. Based on the above information, the damage test results were interpreted from the aspects of electric field enhancement model and mechanical stability of nodular structures.

Using engineered defects to study laser-induced damage in optical thin films with nanosecond pulses

By creating nodules from artificial seeds, the damage behaviors of engineered nodules were systematically studied from experimental approaches. The seed diameters, seed absorption and film absorption were varied independently to uncover a single factors influence on the damage behavior of nodules. First, non-absorbing monodisperse SiO2 microspheres with five different sizes were used to create engineered nodules in 1.053 μm HfO2/SiO2 high reflectors that were prepared by EB process. Laser damage test results showed that the ejection fluences of nodules monotonically decreased with the increase of silica microsphere diameters. And to our surprise, nodules initiating from 0.3 and 0.6 μm silica seeds survived the maximum fluence of 170 J/cm2 (10 ns). Film absorption also has big influence on the damage behaviors of nodules. Compared to the nodules in low absorbing reflectors that were prepared by EB process, the nodules in high absorbing reflectors that were prepared by IAD process exhibited a much lower ejection fluences, although the seed diameters for the comprising nodules were same. Moreover, aluminum seeds were also used to create engineered nodules. Laser damage test results showed that the ejection fluences of nodules initiating from aluminum seeds were around 2 J/cm2 (10 ns), which is more than an order of magnitude less than the ejection fluences of nodules created from nonabsorbing silica seeds. This result revealed that the seed absorption played a very important role in the laser damage of nodules.