Real-time synchronized monitoring for the overlap accuracy of the combining beam spot in a wavelength beam combination based on confocal planes.

Beam overlap accuracy in a wavelength beam combination system determines the beam quality and efficiency, so systematic monitoring of overlap accuracy is essential. In this work, a method of performing real-time synchronized monitoring and recording overlap accuracy for a combining beam spot is proposed. Firstly, theoretical calculations for monitoring different wavelength sub-beam positions and angular errors are established. Then, an optical design and grayscale centroid algorithm are developed to analyze and simulate the combination spots. A monitoring device was designed and constructed to meet the requirements of combining system applications, which achieved an accuracy of 8.86 µrad. Finally, the method successfully monitored the system spot fluctuation range within ±22 µrad. This study resolves the issue of distinguishing the different wavelength sub-beams and their response delays in traditional combining beams. It offers precise error data for real-time synchronized calibration of the overlap accuracy in laser beam combining technology.

Group delay dispersion monitoring for computational manufacturing of dispersive mirrors.

We present a computational manufacturing program for monitoring group delay dispersion (GDD). Two kinds of dispersive mirrors computational manufactured by GDD, broadband, and time monitoring simulator are compared. The results revealed the particular advantages of GDD monitoring in dispersive mirror deposition simulations. The self-compensation effect of GDD monitoring is discussed. GDD monitoring can improve the precision of layer termination techniques, it may become a possible approach to manufacture other optical coatings.

Production of ultra-steep dichroic filters with broad band optical monitoring.

The feasibility of using direct broad band optical monitoring control in the fabrication of the ultra-steep dichroic filters based on resonant structures is investigated. Using computational manufacturing and deposition experiments, the role of the errors self-compensation effect is clarified by comparing the results of direct broad band optical monitoring and time monitoring. The errors correlation strength of ultra-steep dichroic filter is analyzed and it shows that the correlation calculated by the current model is not strong. The relationship between errors correlation and errors self-compensation effect for the ultra-steep dichroic filter is discussed.

Mechanisms of the Jian Pi Tiao Gan Yin in the treatment of simple obesity revealed by network pharmacology.

Background: This study sought to examine the mechanism of the Jian Pi Tiao Gan Yin in the treatment of obesity by network pharmacology. Methods: The active components and corresponding targets of the Jian Pi Tiao Gan Yin were identified using the traditional Chinese medicine systems pharmacology database and analysis platform, and the obesity-related targets were acquired from the Online Mendelian Inheritance in Man database. The drug and disease targets were also identified. Cytoscape software was used to construct the "active component target" network diagram. The protein-protein interaction network was drawn using the Search Tool for the Retrieval of Interacting Genes/Proteins platform, and the Cytoscape MCODE plugin was used to find clusters for the protein cluster analysis. The gene annotation and analysis were performed with the Metascape database via functional databases, such as the Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG), and Autodock and PyMOL were used for the molecular docking. Results: The GO analysis identified 244 target genes of the Jian Pi Tiao Gan Yin, 1,378 targets of obesity, and 123 targets of drug and disease. Additionally, 208 biological process items, 38 molecular function items, and 33 cell component items were also identified. The KEGG pathway analysis identified the hypoxia-inducible factor, forkhead box O, cyclic adenosine monophosphate, and vascular endothelial growth factor signaling pathways. The results of the molecular docking showed that the main active components of the Jian Pi Tiao Gan Yin in the treatment of obesity were quercetin, kaempferol, stigmasterol, luteolin, isorhamnetin, ß-sitosterol, sapogenin, tanshinone, and formononetin, all of which have been proven to bind to core obesity-related proteins, such as AKT1, interleukin-6 (IL-6), vascular endothelial growth factor A (VEGFA), tumor necrosis factor (TNF), tumor protein 53 (TP53), prostaglandin-endoperoxide synthase 2 (PTGS2), caspase-3 (CASP3), mitogen-activated protein kinase 1 (MAPK1), JUN, and epidermal growth factor (EGF). Thus, our study revealed the potential mechanism of the Jian Pi Tiao Gan Yin as a multi-component, multi-target, and multi-channel treatment for obesity. These findings lay the foundation for further studies on the mechanism of the Jian Pi Tiao Gan Yin in obesity treatment. Conclusions: The Jian Pi Tiao Gan Yin can be used as a multi-component, multi-target, and multi-channel treatment for obesity.

High performance ZnS antireflection sub-wavelength structures with HfO2 protective film for infrared optical windows.

Antireflection sub-wavelength structures (SWSs) on ZnS were designed and ZnS SWSs with HfO2 protective film were prepared, and their properties in long-wave infrared applications were examined and compared to AR coatings. The SWS has good antireflection performance and exhibits less polarization sensitivity than the AR coating. At temperatures above 500 °C, the SWS with HfO2 protective film has a better thermal endurance property than the multilayer AR coating. Moreover, the HfO2 protective film significantly improved the mechanical properties of the ZnS SWS and was similar to HfO2 covered AR coating when the HfO2 film was not broken. This study shows that the ZnS SWS with HfO2 protective film has promising application prospects in infrared optical windows.

Hf1-xSixO2 Nanocomposite Coatings Prepared by Ion-Assisted Co-Evaporation Process for Low-Loss and High-LIDT Optics.

Hf1-xSixO2 nanocomposites with different SiO2 doping ratios were synthesized using an ion-assisted co-evaporation process to achieve dense amorphous Hf1-xSixO2 coatings with low loss and a high laser-induced damage threshold (LIDT). The results showed that the Hf1-xSixO2 nanocomposites (x ≥ 0.20) exhibited excellent comprehensive performance with a wide band gap and a dense amorphous microstructure. High-temperature annealing was carried out to ensure better stoichiometry and lower absorption. Precipitation and regrowth of HfO2 grains were observed from 400 °C to 600 °C during annealing of the Hf0.80Si0.20O2 nanocomposites, resulting in excessive surface roughness. A phenomenological model was proposed to explain the phenomenon. The Hf1-xSixO2 nanocomposites (x = 0.3 and 0.4) maintained a dense amorphous structure with low absorption after annealing. Finally, a 1064-nm Hf0.70Si0.30O2/SiO2 high-performance reflector was prepared and achieved low optical loss (15.1 ppm) and a high LIDT (67 J/cm2).

Ultrabroadband Absorption Enhancement via Hybridization of Localized and Propagating Surface Plasmons.

Broadband metamaterial absorbers (MAs) are critical for applications of photonic and optoelectronic devices. Despite long-standing efforts on broadband MAs, it has been challenging to achieve ultrabroadband absorption with high absorptivity and omnidirectional characteristics within a comparatively simple and low-cost architecture. Here we design, fabricate, and characterize a novel compact Cr-based MA to achieve ultrabroadband absorption in the visible to near-infrared wavelength region. The Cr-based MA consists of Cr nanorods and Cr substrate sandwiched by three pairs of SiO2/Cr stacks. Both simulated and experimental results show that an average absorption over 93.7% can be achieved in the range of 400-1000 nm. Specifically, the ultrabroadband features result from the co-excitations of localized surface plasmon (LSP) and propagating surface plasmon (PSP) and their synergistic absorption effects, where absorption in the shorter and longer wavelengths are mainly contributed bythe LSP and PSP modes, respectively. The Cr-based MA is very robust to variations of the geometrical parameters, and angle-and polarization-insensitive absorption can be operated well over a large range of anglesunder both transverse magnetic(TM)- and transverse electric (TE)-polarized light illumination.

Influence of dry etching on the properties of SiO2 and HfO2 single layers.

A comparative study was performed to investigate how etching methods and parameters affect the properties of SiO2 and HfO2 coatings. SiO2 and HfO2 single layers were prepared by electron-beam evaporation (EBE), ion-beam assisted deposition (IAD), and ion-beam sputtering (IBS). Then, ion-beam etching (IBE), reactive ion etching (RIE), and inductively coupled plasma etching (ICPE) were used to study the influence of ion bombardment energy and chemical reaction on the etching rates and properties of the prepared SiO2 and HfO2 single layers. For SiO2 coatings, chemical reaction plays a dominant role in determining the etching rates, so ICPE that has the strongest CHF3 plasma shows the highest etching rate. Moreover, all three etching methods have barely any influence on the properties of SiO2 coatings. For HfO2 coatings, the etching rates are more dependent on the ion bombardment energy, although the chemical reaction using CHF3 plasma also helps to increase the etching rates to some extent. To our surprise, the ion bombardment with energy as high as 900 V does not change the amorphous microstructure or crystalline states of prepared HfO2 coatings. However, the high-energy ion bombardment in IBE significantly increases the absorption of the HfO2 coatings prepared by all deposition techniques and decreases their laser damage resistance to different extents.

Design and fabrication of a superior nonpolarizing long-wavelength pass edge filter applied in laser beam combining technology.

The wavelength combining technique has been widely used to increase the power scaling of lasers. The edge filter, which is nonpolarizing for wavelength, has proven to be a critical component for the combining process, and should have high spectral efficiency, high laser induced damage threshold, and excellent thermal stability. Here, we design a superior nonpolarizing long-wavelength pass edge filter using a novel depolarized initial film structure based on a tuned Fabry-Perot interference filter. The coating fabricated by electron beam evaporation can withstand 130kW/cm2 of continuous laser energy with high spectral efficiency at 45° and a small temperature rise.

Demonstration of a dual-channel two-dimensional reflection grating filter.

A dual-channel two-dimensional (2D) reflection grating filter operating around the 1.55 µm wavelength region is demonstrated, exhibiting dual-channel reflection peaks at 1.492 µm and 1.647 µm. The sidebands intrinsic to this kind of grating are suppressed by appropriately designed antireflective thin films, and this can be proved by equivalent medium theory. Using the modal analysis method, the excitation modes of the dual-channel reflection peaks are determined to be the TM0 (1.490 µm) and TE0 (1.638 µm) modes. The estimated relative errors in the wavelength determination of these modes are less than 1%. This is found to be in accord with analyses of the reflectivity spectra and electromagnetic fields. The dual-channel reflection peaks are sensitive to the background refractive index and may be useful in biosensing applications.

Near-infrared broadband Si:H/SiO2 multilayer gratings with high tolerance to fabrication errors.

Si:H and TiO2 multilayer dielectric gratings (MDGs) were studied comparatively to highlight the influence of refractive indices on fabrication tolerances, including tolerances for grating width errors and cross-sectional shape errors. The fabrication tolerance of Si:H MDGs is two to four times greater than that of TiO2 MDGs, because the higher refractive index of Si:H has a stronger ability to restrain electric fields. It was further revealed in these studies that a Si:H MDG with positive trapezoidal errors has minor influence on high diffraction efficiency bandwidth. Finally, a Si:H MDG was prepared without iterative corrections. Although large fabrication errors of grating width and cross-sectional shape existed, the MDG still had a 146 nm bandwidth with diffraction efficiency over 97%, which verifies the robustness of the proposed Si:H MDG.

Morphological and damage growth characteristics of shell-type damage of fused silica optics induced by ultraviolet laser pulses.

Under the radiation of a nanosecond pulse laser, the types of damage and damage profile of fused silica optics are closely related to the surface and subsurface defects of the component. Using the raster-scanning mode to measure the laser-induced damage threshold of fused silica optics, three different types and sizes of damage patterns are found, of which shell damage is the intermediate state and is a bridge connecting submicrometer-size damage and catastrophic damage. This paper mainly studies the mechanism and damage growth law of shell damage, analyzes the model of laser-induced shell damage, and discusses the probability and cause of shell-damage growth.

Angle-Insensitive Broadband Absorption Enhancement of Graphene Using a Multi-Grooved Metasurface.

An angle-insensitive broadband absorber of graphene covering the whole visible spectrum is numerically demonstrated, which is resulted from multiple couplings of the electric and magnetic dipole resonances in the narrow metallic grooves. This is achieved by integrating the graphene sheet with a multi-grooved metasurface separated by a polymethyl methacrylate (PMMA) spacer, and an average absorption efficiency of 71.1% can be realized in the spectral range from 450 to 800 nm. The location of the absorption peak of graphene can be tuned by the groove depth, and the bandwidth of absorption can be flexibly controlled by tailoring both the number and the depth of the groove. In addition, broadband light absorption enhancement of graphene is robust to the variations of the structure parameters, and good absorption properties can be maintained even the incident angle is increased to 60°.

Morphology and growth properties of nano- and submicrometer-scale initial damage sites under 355 nm wavelength pulsed laser irradiation.

The initial damage of optical elements under irradiation of a 355 nm wavelength nano-second pulsed laser below the laser-induced damage threshold mainly includes nano- and submicrometer-scale craters. The morphology and growth characteristics of such small-scale damage, which are different from those of larger-scale damage, have not been fully clarified. In this study, laser-induced nano- and submicrometer-scale initial damage of fused silica was investigated by atomic force microscope. The morphology of craters and ejecta was analyzed, and the relationship between the structure characteristics and the irradiation parameters was obtained. In addition, the growth and evolution features under multipulsed laser irradiation were classified.

Interface and material engineering for zigzag slab lasers.

Laser damage of zigzag slab lasers occurs at interface between laser crystal and SiO2 film. Although an additional HfO2 layer could be used to manipulate electric-field on the crystal-film interface, their high absorption and polycrystalline structure were unacceptable. SiO2 was then doped in HfO2 to suppress its crystallization and to achieve low absorption by annealing. HfxSi1-xO2 nanocomposite layers were then inserted between laser crystal and SiO2 film to minimize electric-field at crystal-film interface. Laser damage resistance of this new architecture is two times higher than that of traditional zigzag slab lasers.

Reducing light scattering in high-reflection coatings through destructive interference at fully correlated interfaces.

Light scattering in quarter-wave high-reflection (QWHR) coatings with fully correlated interfaces was reduced by adding Fabry-Perot (FP) cavity structures on top of the multilayer. The properly designed FP cavity can induce destructive interference for fully correlated interfaces and reduce the scattering loss. Compared to QWHR coatings, adding one FP cavity could decrease the scattering at the near specular angles, and two FP cavities have the potential to reduce light scattering in a broad angular range. A low-scattering HR (LSHR) coating using two FP cavities has been realized to suppress light scattering. The numerical scattering calculation illustrated that the total scattering loss of the LSHR was about 30% less than that of the QWHR coatings. The measured angle-resolved scattering of the LSHR coating showed a good correspondence to the numerical calculation, although a small deviation exists in a limited angular range.

Contribution of angle-dependent light penetration to electric-field enhancement at nodules in optical coatings.

The influence of angle-dependent light penetration on electric-field intensity (EFI) enhancement at nodules was investigated in this Letter. An experiment consisting of 3D finite-difference time-domain simulations was conducted on two types of polarizers that prevent light penetration at a low and a high incident angular range (IAR). The EFI at the focal point region is six times lower, and the laser damage resistance is three times higher in the polarizer blocking light penetration in a high IAR. These results reveal for the first time, to the best of our knowledge, that light penetration at a high IAR, rather than at a low IAR, contributes to EFI enhancement at the focal region of the nodules. Our findings may provide useful guidance in selecting optimal designs to suppress EFI enhancement at nodules in multilayer coatings.

Effect of boundary continuity on nanosecond laser damage of nodular defects in high-reflection coatings.

Two types of engineered nodules in Ta2O5/SiO2 high-reflection coatings were prepared using electron beam evaporation and an ion-assisted deposition processes to facilitate poor and good boundary continuity, respectively. The influence of nodular boundary continuity on their nanosecond laser damage characteristics was investigated through experimental studies, combined with 3D finite-difference time domain simulations and photo-thermal micro-characterizations. Better boundary continuity led to improved mechanical stability and higher ejection fluence of nodules, in accordance with the thermomechanical damage model. In contrast, the ejected nodules that initially had better boundary continuity exhibited higher localized absorption and lower damage growth fluence, which is attributed to the creation of mechanically induced electronic defects or stronger electric field intensity enhancement at the ejected nodules.

Characterization of grain sizes and roughness of HfO2 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.

Fabrication of broadband absorbing coatings for amplified spontaneous emission suppression.

Amplified spontaneous emission (ASE) is a critical factor that limits the output power of slab lasers. To suppress the ASE effect in slab lasers, this paper proposes a metal/dielectric broadband absorbing coating between the laser gain medium and the metal heat sink to suppress ASE from zero to total internal reflection angle on the facet from inside the laser medium. Based on accurate characterization of Cr metal thin films, Cr/SiO2 broadband absorbing films for 1064 nm YAG slab lasers are designed and fabricated to suppress ASE. The good agreement between experiment and design shows the reliability to suppress ASE in slab lasers.