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Plate laser beam splitter (PLBS) coatings for intensity-splitting in high-power laser applications face increasing requirements, where the challenge is to simultaneously satisfy a given transmittance (T)/reflectivity (R) intensity-splitting ratio with a certain bandwidth, and a high laser damage threshold (LIDT). Traditionally, a given T/R ratio is achieved by using a non-quarter-wave design. We propose a quarter-wave design strategy for arbitrary T/R ratios by using mixture layers with tunable refractive index (n) and optical bandgap as high- n layers. Two quarter-wave PLBS coatings based on HfO 2 –Al 2 O 3 mixture with a designed T/R ratio of 50:50 are experimentally demonstrated and compared with the traditional PLBS coating. The mixture-based PLBS coating shows good spectral performance, low surface roughness, and high LIDT. For an s -polarized 8 ns pulse laser at a wavelength of 1064 nm, a mixture-based PLBS coating design can improve LIDT by a factor of ∼ 2 with a slightly wider bandwidth. This tunable mixture-based PLBS coating design strategy provides support for the increasingly high output power of high-power laser systems. [ABSTRACT FROM AUTHOR]

Abstract: Four mirror coatings were designed for transporting 1064nm light, and prepared under the same process parameters. The laser induced damage threshold (LIDT), electric field distribution, as well as the damage morphology of the prepared transport mirror coating was investigated. The results indicate that the SiO2 overcoat layer and electric field distribution have a great impact on the LIDT and damage morphology of the transport mirror coating. A thicker SiO2 overcoat layer can suppress the delamination damage morphology, and significantly increase the LIDT. For the designs with similar SiO2 overcoat layer thickness, the nano-absorbing defects can survive at higher laser fluence in the design with lower electric field intensity. In addition, the HfO2 over SiO2 interface has a better interfacial strength and less “invisible” nano-absorbing defects than the SiO2 over HfO2 interface, suggesting that improving the SiO2 over HfO2 interfacial quality will be an effective way to enhance the laser damage performance of the transport mirror coating. [Copyright &y& Elsevier]

Low dispersion mirrors are important because of their potential use in petawatt (PW) laser systems. The following two methods are known to increase the laser-induced damage threshold of low dispersion optical components: use of a wide-bandgap-material protective layer and control of electric field distribution. By controlling the electric field distribution of composite low-dispersion mirrors (CLDM), we shift the electric field peaks from the material interface into the wide-bandgap material. However, the damage threshold of modified-electric-field composite low dispersion mirror (E-CLDM) does not increase. Damage morphology shows that the initial damaged layer is Ta2O5. An immediate cause is the enhancement of the electric field in internal layers caused by surface electric field regulation. Theoretical calculations show that the damage threshold of CLDM or E-CLDM is determined by the competition results of bandgap and the electric field of layer materials. The CLDM with different materials or different protective layer periods can be optimally designed according to the electric field competition effect in the future. [ABSTRACT FROM AUTHOR]

The high optical performance and laser-induced damage threshold (LIDT) of broadband hafnium oxide/silica (HfO 2 /SiO 2) dichroic coatings with high transmittance and reflectance around 532 and 1064 nm, respectively, are essential for the efficiency of a harmonic separation system. However, the transmittance of dichroic coatings tends to drop rapidly at 532 nm because of the half-wave hole effect. Moreover, an intense electric field penetrates the coating (~6 μm thick) to the substrate, thereby generating a challenge to improve the LIDT at 532 nm. In this paper, we study the methods to improve the optical performance and LIDT of HfO 2 /SiO 2 dichroic coatings prepared by electron-beam evaporation. The inhomogeneity in the growth characteristics of the HfO 2 layer is studied and considered in the design of dichroic coatings, which can improve spectral performance, especially by broadening the transmittance bandwidth around 532 nm. Additionally, the effects of the different treatment procedures, including ultrasonic cleaning, acid-solution etching, and argon-ion-beam etching, of the fused silica substrate on the LIDT of dichroic coatings are investigated by the raster-scan damage test method. We observe that the LIDT of dichroic coatings at 532 nm is limited by the surface and subsurface defects of the substrate. Etching ~200 nm of the surface of the fused silica substrate can effectively inhibit the occurrence of low-energy catastrophic growth damage. Compared with acid-solution etching, argon-ion-beam etching can yield improved substrate surface quality and increased LIDTs of dichroic coatings at 532 nm. • The refractive index inhomogeneity of the HfO 2 single layer is consistent with that of the HfO 2 layer in dichroic coatings. • HfO 2 /SiO 2 dichroic coatings considering the refractive-index inhomogeneity of HfO 2 layer inhibit the half-wave hole effect. • Laser damage at 532 nm mainly originates from the substrate subsurface defects in the redeposition (Beilby) layer. • Argon-ion-beam etching can bring better substrate surface quality and higher LIDTs of dichroic coatings at 532 nm. [ABSTRACT FROM AUTHOR]

• The PI alignment layer is pinned on the ITO film to imitate the structure of an LCD. • The damage of the samples are attributed to the high absorption of the ITO film. • The mechanical properties of the PI are primarily responsible for the LIDT result. This report focuses on the damage characteristics of the indium-tin-oxide (ITO) layer and the polyimide (PI) layer, which are two constituent components of a LCD. This investigation is different from the previous study, in which the alignment layer was deposited directly on a glass substrate. The PI alignment layer is pinned on the ITO film to imitate the structure of the LCD as much as possible in our current study. The damage process of the ITO/Glass sample involves melting, vaporization near the laser-induced damage threshold (LIDT), and removal at a higher fluence. However, the damage process of the PI/ITO/Glass sample involves thermally induced plastic deformation, followed by cooling when the irradiation fluence is near the LIDT, and rupture when the irradiation fluence is higher. The LIDTs of the PI/ITO/Glass samples, as determined by the on-line CCD detection technique, are higher than those of the ITO/Glass samples. The favorable mechanical properties of the PI are primarily responsible for this result. [ABSTRACT FROM AUTHOR]

Antireflection (AR) coatings are deposited on UV grade fused silica substrates, which are cleaned in the dual ion beam sputtering device. Compared to ultrasonic and acid etching cleaning progress, ion beam etching improves the laser-induced damage threshold (LIDT) of substrates and AR coatings significantly at 355 nm. Ion beam etching declines the low LIDT defects drastically and removes lots of the impurity elements (Ce, Fe, K, and Na). Roughness test shows that the AR coatings and substrates with ion beam etching are very flat and of low roughness. Ion beam etching reduces the density of deep defect from substrates greatly. Damage morphologies show double layers delamination, which is explained via calculation of layer stress. This study will be helpful for preparation of high LIDT optical coatings. • The LIDT of the 355 nm AR coatings and substrates are improved greatly with ion beam etching of substrates. • The damage curves show that ion beam etching declined the low LIDT defects drastically. • The impurity elements (Ce, Fe, K, and Na) are removed efficiently, and the surfaces are quite smooth. • Damage morphologies show two different defects in AR coatings and correspond to the laser damage curves. • Laser damage mechanism of AR coatings is explained. [ABSTRACT FROM AUTHOR]

Abstract Laser-induced damage of ultraviolet (UV) coatings poses a great challenge and restricts its application in space. Coatings prepared by ion beam sputtering are dense and possess high laser-induced damage threshold (LIDT), which are absence of structural nodule defects. Subsurface damage defects easily exposed by etching are inevitable under substrate grinding and polishing processes, which are usually ignored in electron-beam evaporation coatings. In our experiments, we find that LIDT of 355-nm high-reflection (HR) coatings sputtered on etched substrates declines dramatically, meanwhile defect density increases. Pits are found on the surface of the substrates and the coatings, and the pits that induce laser damage are very small (submicron-scale). Laser damage morphologies reveal that damage sites are prone to occur around the pits caused by acid etching. Finite element analysis shows consistent results, enhancement of electric field and rise of temperature are obvious, and the most significant part is around the pits, which is prone to induce damage. To conclude, small structural defects are sensitive to laser-induced damage of HR coatings prepared by dual ion beam sputtering (DIBS). This work contributes to finding new ways of improving the LIDT of sputtering coatings. Highlights • The structural defects on substrates are relatively small and shallow, the depth and the diameter of the pits are in the range of ∼50–120 nm and ∼400–600 nm, respectively. • The diameters of the pits on the surface of the coatings (DIBS) are larger than the substrate, which is different from the EBE coatings and indicates different growth process. • The LIDTs of our samples are sharply declined by the presence of the pits, the damage morphologies show thermal ablation. • High electric field and temperature by a FEA method are found at the edge of the pits on the surface of the coatings, which agrees with the experimental result. • The pits strongly enhance the thermal effect, which increases the possibility of damage of 355 nm IBS coatings. [ABSTRACT FROM AUTHOR]

The plasma scald damage is the most typical laser damage morphology presented on the laser conditioned HfO 2 /SiO 2 mirrors. In order to provide guidance for defining the acceptance quantitative specification for the plasma scald on these laser conditioned mirrors, the simulations about the downstream beam modulations of the plasma scald are investigated based on its damage characteristics and the diffraction theory. The single plasma scald with the typical size, induced by the laser conditioning, can hardly cause damage to the downstream optics and its influence on the beam contrast can be reduced easily with the increment of the propagation distance. Those with small size and large depth, or those with big size and shallow depth can also hardly induce downstream damage, but the beam contrasts become worse with the increment of the size of the concave part as well as its depth. For a maximum plasma scald observed on the laser conditioned mirror, the intensity modulation peak is close to 1.3 and the beam contrast reaches to 25% even at 2 m downstream from the mirror. For multi plasma scalds, more diffraction rings show up in the downstream beam and further decrease the overall uniformity of the beam. The uniformity of the beam gets worse as the number of the plasma scald is increased. When the density of the plasma scald with the typical size in these 1ωHfO 2 /SiO 2 mirrors reaches 20 per 1 cm 2 , the beam contrast is less than 6%. [ABSTRACT FROM AUTHOR]