Waterproof coatings for high-power laser cavities

With the ever-increasing laser power and repetition rate, thermal control of laser media is becoming increasingly important. Except for widely used air cooling or a bonded heat sink, water cooling of a laser medium is more effective in removing waste heat. However, how to protect deliquescent laser media from water erosion is a challenging issue. Here, novel waterproof coatings were proposed to shield Nd:Glass from water erosion. After clarifying the dependence of the waterproof property of single layers on their microstructures and pore characteristics, nanocomposites that dope SiO2 in HfO2 were synthesized using an ion-assisted co-evaporation process to solve the issue of a lack of a high-index material that simultaneously has a dense amorphous microstructure and wide bandgap. Hf0.7Si0.3O2/SiO2 multifunctional coatings were finally shown to possess an excellent waterproof property, high laser-induced damage threshold (LIDT) and good spectral performance, which can be used as the enabling components for thermal control in high-power laser cavities.

Although hydrophobic coatings have been well studied regarding their water repellency or moisture proof property 1-3 , they are not applicable under cooling water conditions [4][5][6][7][8][9] , where coatings are immersed in water over long periods of time. For waterproof coatings, Murahara has proposed a hard-water-resistant coating that can protect KDP from dissolving 10 . However, his approach was limited to the use of SiO 2 single layers and failed to meet the requirement of multilayers in which both highand low-index materials are needed 11,12 . Although the microstructures of widely used optical coatings prepared using physical vapor deposition techniques have been extensively studied regarding stress, the refractive index, humidity-induced spectral shift and so on [13][14][15] , the correlation between the coating microstructure and waterproof property has not been specifically addressed. To meet the challenging requirement of waterproof cavity mirrors, the dependence of water resistance on the microstructure of single-and multilayers was studied first. On the basis of these results, microstructure and bandgap engineering were performed to synthesize nanocomposite-based cavity mirrors with exceptional multifunctionality, which has been previously unattainable. The proposed multifunctional coating can be used as an enabling technology to realize high-repetition-rate laser inertial confinement fusion, which can then be used as a fusion power plant.
The schematic of a water-cooled Nd:Glass laser cavity is shown in Fig. 1. SiO 2 and HfO 2 are the dominant low and high index materials for the near-infrared region 16,17 . The water resistance of single layers was investigated first. SiO 2 and HfO 2 single layers were prepared using electron beam evaporation (EBE) and ion assisted deposition (IAD) processes, respectively. The coatings prepared by the EBE process had a porous microstructure and failed to protect Nd:Glass from water erosion. The SiO 2 layers peeled off of Nd:Glass substrates with heating in a water bath over a period of several hours. The eroded morphologies of the SiO 2 layer were recorded using a camera and an optical microscope, as shown in Fig. 2a1, a2. A cross-sectional transmission electron microscopy (TEM) image of a Ta 2 O 5 coating was used instead to reveal the microstructure of the SiO 2 coating, where the brighter areas represent pores. It can be seen that an abundance of pores exist between the columns, which are open, elongated, and oriented perpendicular to the coating surface, as shown in Fig. 2a3. Water can quickly penetrate into the Nd:Glass surface through these open microscopic channels by capillarity, which is schematically illustrated in Fig. 2a4. Erosion of the Nd:Glass surface led to the delamination of SiO 2 coatings. For porous HfO 2 layers on Nd: Glass substrates, visible damage was observed after testing the samples in a hot water bath for several days, as shown in Fig. 2b1, b2. The relatively longer survival time of these layers is attributed to their different microstructure and pore characteristics. Figure 2b3 shows that there is a transition from an amorphous microstructure to a polycrystalline microstructure as the coating grows thicker. Compared to the elongated pores in the amorphous coatings, the polycrystalline microstructure in the upper part of the HfO 2 coating results in a substantially lower number of long and open pores. Water diffusion through the HfO 2 layer is delayed due to these crisscrossing channels, which are illustrated in Fig. 2b4. This comparison shows that for porous coatings, the polycrystalline  Figure S2 and S3 in the Supplementary Information microstructure offers advantages with respect to water resistant compared to an amorphous microstructure. IAD can significantly reduce the number of pores in coatings via energetic ion bombardment during film growth. The densified SiO 2 layers did not peel off of Nd: Glass substrates with water bath heating for several months, as shown in Fig. 2c1, c2. Although no visible pores can be identified in the cross-sectional TEM image in Fig. 2c3, some nanometer-sized closed pores are assumed to be embedded in the SiO 2 coatings 18 . These closed nanopores are isolated from each other, as schematically presented in Fig. 2c4. There are no channels for water diffusion in dense amorphous SiO 2 coatings, so Nd: Glass substrates can be protected from corrosion. HfO 2 coatings prepared by the IAD process exhibited a polycrystalline microstructure. Although these coatings were also very dense, they failed to protect Nd:Glass from water erosion. Figure 2d1, d2 shows that HfO 2 coatings were severely damaged after immersion in a hot water bath for several weeks. The cross-sectional TEM image shown in Fig. 2d3 shows an abundance of complex grain boundaries with nanopores. These nanopores are connected with each other, resulting in a network of zigzag channels from which water can find ways to reach and erode the Nd: Glass, as shown in Fig. 2d4. This comparison reveals that for dense single layers, an amorphous microstructure rather than a crystallized microstructure exhibits a good waterproof property.
The water resistance of multilayers was further studied. HfO 2 /SiO 2 multilayers were prepared using the IAD process. After immersing samples in a hot water bath for several months, severe delamination was observed. The erosion morphologies of HfO 2/ SiO 2 multilayers are shown in Figure S1 in the Supplementary Information. This result means that a multilayer does not possess a good waterproof property when one coating material is water resistant while another is not. Although water penetration perpendicular to the surface might be effectively blocked by dense amorphous SiO 2 layers, water can diffuse along zigzag channels in HfO 2 layers that are parallel to the surface, as illustrated in Fig. 3a1. Since there are always extrinsic defects in real multilayer coatings 19 , water molecules in HfO 2 layers could reach and corrode the Nd:Glass substrate along defectinduced channels, which could be linked to the Nd:Glass surface. To verify the influence of defects, artificial nodules created from 2μm silica microspheres on Nd:Glass were tested in a hot water bath 20 . Figure 3a2 shows that blisters were observed at artificial nodules after serval hours of water bath heating. The cross-sectional scanning electron microscopy (SEM) image shown in Fig. 3a3 shows that large pores at nodular boundaries are open to the Nd: Glass surface, through which water can reach and erode the Nd:Glass. For comparison, artificial nodules in dense amorphous Ta 2 O 5 /SiO 2 multilayers were also prepared using the IAD process. Because both Ta 2 O 5 and SiO 2 have a dense amorphous microstructure with closed nanopores, there are no channels for water diffusion along defect-induced channels, as shown in Fig. 3b. The Ta 2 O 5 / SiO 2 multilayers did not peel off from the Nd:Glass substrate for the same water bath time. However, the LIDT for the Ta 2 O 5 /SiO 2 multilayers is much lower than that for the HfO 2 /SiO 2 multilayers because the bandgap of Ta 2 O 5 is~30% less than that of HfO 2 21 . It is desirable to find novel coating materials to achieve an excellent waterproof property without sacrificing the LIDT.
Nanocomposites that dope SiO 2 into HfO 2 were synthesized using an ion-assisted co-evaporation process to obtain a dense amorphous microstructure, wider bandgap and higher LIDT. The higher the SiO 2 concentration in Hf x Si 1−x O 2 nanocomposites, the better the resistance to crystallization. SiO 2 (20%) in Hf x Si 1−x O 2 nanocomposites was not enough to suppress crystallization. Figure 4a shows that severe crystallization occurred after an initial amorphous growth phase. Hf x Si 1−x O 2 nanocomposites with a 30% SiO 2 concentration or higher maintained a dense amorphous microstructure, as shown in Fig. 4b. From the aspects of coating design and electric-field control, it is optimal to use an amorphous Hf x Si 1−x O 2 nanocomposite, which has the highest refractive index. Therefore, an Hf 0.7 Si 0.3 O 2 nanocomposite layer was used. The ultraviolet transmission spectra of the Hf 0.7 Si 0.3 O 2 , SiO 2 , HfO 2 and Ta 2 O 5 layers are compared in Fig. 4c. Together with the reflection spectrum, the bandgap of the Hf 0.7 Si 0.3 O 2 nanocomposite layer was derived to be 6.4 eV using the Tauc algorithm 22 . The bandgap of this is much wider than that Ta 2 O 5 and its LIDT should also be much higher.
Hf 0.7 Si 0.3 O 2 /SiO 2 cavity mirrors were then prepared with a high reflectance at 1053 nm for s-polarization at the Brewster angle and a high transmittance at 802 nm for normal incidence 23 . Their LIDT was 34 ± 4 J/cm 2 according to the method described by Borden et al. 24 , which is almost three times higher than the LIDT of Ta 2 O 5 /SiO 2 cavity mirrors. This value is close to the LIDT value of~42 ± 4 J/cm 2 for a bare Nd:Glass substrate. By carefully controlling defects during the preparation of Nd: Glass substrates and coatings, Hf 0.7 Si 0.3 O 2 /SiO 2 cavity mirrors can protect Nd:Glass from corrosion for over 1 year, even in a hot water bath.
Here, the influence of the pore characteristics on the waterproof property of single layers was determined. Dense amorphous coatings are water resistant because no channels exist for water diffusion. If one material is water resistant while another is not, the multilayer does not possess a waterproof property when extrinsic defects are present. Hf x Si 1−x O 2 nanocomposites were then synthesized to generate Hf 0.3 Si 0.7 O 2 /SiO 2 cavity mirrors with the multifunctionality of an excellent waterproof property, high LIDT and good spectral performance.

Preparation of laser coatings
SiO 2 , HfO 2 and Ta 2 O 5 single layers with a thickness of 500 nm were deposited on Nd:Glass substrates using EBE and IAD processes. The deposition temperature was approximately 393 K, and the chamber was pumped down to a base pressure of 2.3 × 10 −4 Pa. The IAD process employed was based on an RF-type ion source with a working condition of 600 V and 600 mA. Further details of the deposition process can be found in our previous paper 25 . Hf x Si 1−x O 2 nanocomposites were prepared using an ion-assisted co-evaporation process including annealing at 600°C to reduce their absorption. A schematic of the ion-assisted co-evaporation process is shown in Figure S4 in the Supplementary Information. Further details of the process can be found in a previous paper 26 .

Characterization of laser coatings
The water resistance of the prepared coatings was evaluated by immersing samples in a temperaturecontrolled water bath. Compared to a practical watercooling situation, the temperature of the water bath was set to 90°C to accelerate the failure process.
The microstructure of SiO 2 , HfO 2 and Ta 2 O 5 single layers was characterized using TEM. However, due to the low contrast of electron scattering between pores and the SiO 2 matrix, it is difficult to clearly observe its microstructure. Based on the fact that the SiO 2 coating has the same microstructure as a Ta 2 O 5 coating 27 , a crosssectional TEM image of a Ta 2 O 5 coating was used instead to determine the microstructure of the SiO 2 coating. For comparison, TEM images and electron diffraction patterns of SiO 2 coatings are shown in Figures S2  and S3 in the Supplementary Information. The electron diffraction pattern of the crystallized part of the HfO 2 coating is also shown in Figure S2 in the Supplementary Information. The microstructure of the artificial nodules in the HfO 2 /SiO 2 and Ta 2 O 5 /SiO 2 multilayers was determined by cutting them through the middle using focused ion beam technology. Then, the cross-sectional images shown in Fig. 3 were taken using SEM.
The concentration of SiO 2 in Hf x Si 1−x O 2 nanocomposites was determined by fitting the refractive indices of the Hf x Si 1−x O 2 single layers using a Lorentz-Lorentz model 28 .