The distribution and characteristics of surface cracking (i.e. sub-surface damage or SSD) formed during standard grinding processes has been investigated on fused silica glass. The SSD distributions of the ground surfaces were determined by: (1) creating a shallow (18-108 {micro}m) wedge/taper on the surface by magneto-rheological finishing; (2) exposing the SSD by HF acid etching; and (3) performing image analysis of the observed cracks from optical micrographs taken along the surface taper. The observed surface cracks are characterized as near-surface lateral and deeper trailing indent type fractures (i.e., chatter marks). The SSD depth distributions are typically described by a single exponential distribution followed by an asymptotic cutoff in depth (c{sub max}). The length of the trailing indent is strongly correlated with a given process. Using established fracture indentation relationships, it is shown that only a small fraction of the abrasive particles are being mechanically loaded and causing fracture, and it is likely the larger particles in the abrasive particle size distribution that bear the higher loads. The SSD depth was observed to increase with load and with a small amount of larger contaminant particles. Using a simple brittle fracture model for grinding, the SSD depth distribution has been related tomore » the SSD length distribution to gain insight into ''effective'' size distribution of particles participating in the fracture. Both the average crack length and the surface roughness were found to scale linearly with the maximum SSD depth (c{sub max}). These relationships can serve as useful rules-of-thumb for nondestructively estimating SSD depth and to identify the process that caused the SSD. In certain applications such as high intensity lasers, SSD on the glass optics can serve as a reservoir for minute amounts of impurities that absorb the high intensity laser light and lead to subsequent laser-induced surface damage. Hence a more scientific understanding of SSD formation can provide a means to establish recipes to fabricate SSD-free, laser damage resistant optical surfaces.« less

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Sub-surface mechanical damage distributions during grinding of fused silica

https://iopscience.iop.org/article/10.1088/1468-6996/13/5/053002/pdf

Photonic metamaterials: a new class of materials for manipulating light waves

There is controversy as to whether a one-dimensional (1D) electron gas can spin polarize in the absence of a magnetic field. Together with a simple model, we present conductance measurements on ultra-low-disorder quantum wires supportive of a spin polarization at B=0. A spin energy gap is indicated by the presence of a feature in the range (0.5-0.7)x2e(2)/h in conductance data. Importantly, it appears that the spin gap is not constant but a function of the electron density. Data obtained using a bias spectroscopy technique are consistent with the spin gap widening further as the Fermi level is increased.

Density dependent spin polarisation in ultra low-disorder quantum wires

We have studied the dispersion relations of multilayers of silver and a dye-doped dielectric using four methods: standard effective-medium theory (EMT), nonlocal-effect-corrected EMT, nonlinear equations based on the eigenmode method, and a spatial harmonic analysis method. We compare the validity of these methods and show that metallic losses can be greatly compensated by saturated gain. Two realizable applications are also proposed. Loss-compensated metal-dielectric multilayers that have hyperbolic dispersion relationships are beneficial for numerous applications such as subwavelength imaging and quantum optics.

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Loss-compensated and active hyperbolic metamaterials

Magnetic imprinted PEDOT/CdS heterojunction photocatalytic nanoreactors are synergistically synthesized by the microwave-assisted surface imprinting technique. The coating of the PEDOT imprinted layer is conducive to the three-dimensional (3D) specific recognition and selective photocatalysis of danofloxacin mesylate due to the formation of imprinted cavities. The representative specific degradation selectivity coefficient of the developed nanoreactor relative to CdS/Fe3O4/HNT and magnetic non-imprinted PEDOT/CdS heterojunction is 2.08 and 2.11, respectively, clearly elaborating the excellent selectivity. The introduction of Fe3O4 into PEDOT/CdS heterojunction enhances the transfer of photoexcited electrons, thereby, the photocatalytic activity has been significantly boosted up to ∼84.84 %. Meanwhile, the existence of PEDOT imprinted layer not only extracts the photoinduced holes produced by CdS, but also encapsulates CdS, which effectively inhibits the CdS photocorrosion and hinders its secondary pollution. With the improved selectivity and photocatalytic activity, our work paves the way to efficiently remove the target pollutant aiming at the practical application requirements.

Development of magnetic imprinted PEDOT/CdS heterojunction photocatalytic nanoreactors: 3-Dimensional specific recognition for selectively photocatalyzing danofloxacin mesylate

Researchers in China have studied how the electric-field distribution of light affects the laser-induced damage of coatings. Xinbin Cheng and co-workers from Tongji University in Shanghai fired 10-ns-duration laser pulses from a 1,064 nm Nd:YAG laser onto a multilayer dielectric coating on BK7 glass substrate. To simulate the role of nodular defects, which act as precursors for damage, the team introduced 0.3–1.9 μm silica microspheres to the glass substrate prior to application of the coating. They then modelled the electric-field distribution at different nodule sites using finite-difference time-domain simulations, and experimentally analysed the laser-induced damage using microscopy and focused ion-beam technology. The results are expected to aid the development of coatings to provide superior resistance to laser damage.

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The effect of an electric field on the thermomechanical damage of nodular defects in dielectric multilayer coatings irradiated by nanosecond laser pulses

A reactive electron beam evaporation process was used to fabricate 1.064 μmHfO2/SiO2 high reflectors. The deposition process was optimized to reduce the nodular density. Cross-sectioning of nodular defects by a focused ion-beam milling instrument showed that the nodule seeds were the residual particles on the substrate and the particulates from the silica source “splitting.” After optimizing the substrate preparation procedure and the evaporation process, a low nodular density of 2.7/mm2 was achieved. The laser damage test revealed that the ejection fluences and damage growth behaviors of nodules created from deep or shallow seeds were totally different. A mechanism based on directional plasma scald was proposed to interpret observed damage growth phenomenon.

Laser damage study of nodules in electron-beam-evaporated HfO 2 /SiO 2 high reflectors

The light scattering of interference coatings is strongly dependent on the wavelength. In addition to the general strong increase of scattering as the wavelengths get shorter, dramatic scatter effects in and around the resonance regions can occur. This is discussed in detail for highly reflective and chirped mirrors. A new instrument is presented which enables spectral angle resolved scatter measurements of high-quality optical components to be performed between 250 and 1500 nm.

Spectral angle resolved scattering of thin film coatings

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.

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

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.

Jinlong Zhang

Papers

The effect of an electric field on the thermomechanical damage of nodular defects in dielectric multilayer coatings irradiated by nanosecond laser pulses

Laser damage study of nodules in electron-beam-evaporated HfO 2 /SiO 2 high reflectors

Spectral angle resolved scattering of thin film coatings

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

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