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Cheng X.,MOE Key Laboratory of Advanced Micro Structured Materials | Cheng X.,Tongji University | Wang Z.,MOE Key Laboratory of Advanced Micro Structured Materials | Wang Z.,Tongji University
Advanced Optical Technologies | Year: 2014

Defects in optical coatings are a major factor degrading their performance. Based on the nature of defects, we classified them into two categories: visible defects and non-visible defects. Visible defects result from the replication of substrate imperfections or particulates within the coatings by subsequent layers and can increase scattering loss, produce critical errors in extreme ultraviolet lithography, weaken mechanical and environmental stability, and reduce laser damage resistance. Non-visible defects mainly involve a decrease in laser damage resistance but typically have no influence on other properties of optical coatings. In the case of widely used HfO2/SiO2 dielectric coatings, metallic Hf nano-clusters, off-stoichiometric HfO2 nano-clusters, or areas of high-density electronic defects have been postulated as possible sources for non-visible defects. The emphasis of this review is devoted to discussing localized defect-driven laser-induced damage (LID) in optical coatings used for nanosecond-scale pulsed laser applications, but consideration is also given to other properties of optical coatings such as scattering, environmental stability, etc. The low densities and diverse properties of defects make the systematic study of LID initiating from localized defects time-consuming and very challenging. Experimental and theoretical studies of localized defect-driven LID using artificial defects whose properties can be well controlled are highlighted. © 2014 THOSS Media and De Gruyter. Source


Gallais L.,Aix - Marseille University | Cheng X.,MOE Key Laboratory of Advanced Micro Structured Materials | Cheng X.,Tongji University | Wang Z.,MOE Key Laboratory of Advanced Micro Structured Materials | Wang Z.,Tongji University
Optics Letters | Year: 2014

The influence of nodular coating defects on the sub-picosecond laser damage resistance of multilayer coatings is investigated. The study is conducted on engineered nodules from monodisperse silica microspheres in HfO 2/SiO2 high-reflective mirrors, at 400 fs/1030 nm. We demonstrate through an experimental study coupled with 3D finitedifference time-domain simulations that nodules in dielectric multilayer coatings are a main concern for the damage resistance of femtosecond optics. The nodules, and hence possibly other defects that produce E-field enhancement in coating materials, induce damage initiation at very low fluences (0.1 J/cm2 in the case under study) compared to the intrinsic damage threshold of the component (1.4 J/cm2 for the present mirror). After initiation, the damage sites grow catastrophically at a determined threshold, reducing significantly the damage resistance (0.6 J/cm2) but allowing a "safe" operating fluence to be defined. © 2014 Optical Society of America. Source


Zhang L.,MOE Key Laboratory of Advanced Micro Structured Materials | Zhang L.,Tongji University | Cheng X.,MOE Key Laboratory of Advanced Micro Structured Materials | Cheng X.,Tongji University | And 4 more authors.
Hongwai yu Jiguang Gongcheng/Infrared and Laser Engineering | Year: 2015

Using the Si substrate and the HfO2 single layer as the examples, the power spectral density (PSD) of the optical surface was calculated and characterized. First, the method that calculates the 1D power spectral density (PSD1D), 2D power spectral density (PSD2D) and isotropic power spectral density (PSDISO) was introduced. Then, the surface morphologies of the Si substrate and the HfO2 single layer were measured using atomic force microscopy with four scan areas, for example, 1 μm×1 μm, 5 μm×5 μm, 10 μm×10 μm, 20 μm×20 μm. Using a MATLAB program, the PSDISOs of the different scan area were calculated. And the PSDISO-Combined over a larger spatial frequency range was given using the geometric mean of these PSDISOs. The result shows that the PSDISO-Combined of Si substrate before and after HfO2 coating were similar for the low spatial frequency region but quite different for the middle and high frequency region. The poly-crystallized microstructure of the HfO2 coating is the main reason for the observed PSD difference between the Si substrate and the HfO2 coating. The σISO and σSTD were calculated and compared. The results are quite similar, which proves the correctness of the proposed method for the PSD calculation. © 2015, Editorial Board of Journal of Infrared and Laser Engineering. All right reserved. Source


Wei Z.,MOE Key Laboratory of Advanced Micro Structured Materials | Wei Z.,Tongji University | Cao Y.,Shanghai Radio Equipment Institute | Gong Z.,MOE Key Laboratory of Advanced Micro Structured Materials | And 10 more authors.
Physical Review B - Condensed Matter and Materials Physics | Year: 2013

We theoretically and experimentally investigate subwavelength imaging with a flat lens constructed with fishnet metamaterial. By adopting a ring antenna with a diameter of 5 mm, a high-quality image, with the full width at half maximum no more than half of the operational wavelength at 10 GHz, is measured in free space at a distance of 19 mm away from the flat lens. The spatial resolution is about one third of the operational wavelength considering the electrical size of the ring antenna, which is also verified by finite-difference time-domain simulations. The subwavelength imaging arises from the negative coupling between the adjacent spoof plasmonic waveguides of fishnet metamaterial and is proved valid in a wide frequency range. © 2013 American Physical Society. Source


Xu J.,MOE Key Laboratory of Advanced Micro Structured Materials | Xu J.,Tongji University | Wang X.,MOE Key Laboratory of Advanced Micro Structured Materials | Wang X.,Tongji University | And 8 more authors.
Review of Scientific Instruments | Year: 2016

This paper presents a novel lobster-eye imaging system for X-ray-backscattering inspection. The system was designed by modifying the Schmidt geometry into a treble-lens structure in order to reduce the resolution difference between the vertical and horizontal directions, as indicated by ray-tracing simulations. The lobster-eye X-ray imaging system is capable of operating over a wide range of photon energies up to 100 keV. In addition, the optics of the lobster-eye X-ray imaging system was tested to verify that they meet the requirements. X-ray-backscattering imaging experiments were performed in which T-shaped polymethyl-methacrylate objects were imaged by the lobster-eye X-ray imaging system based on both the double-lens and treble-lens Schmidt objectives. The results show similar resolution of the treble-lens Schmidt objective in both the vertical and horizontal directions. Moreover, imaging experiments were performed using a second treble-lens Schmidt objective with higher resolution. The results show that for a field of view of over 200 mm and with a 500 mm object distance, this lobster-eye X-ray imaging system based on a treble-lens Schmidt objective offers a spatial resolution of approximately 3 mm. © 2016 Author(s). Source

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