Dichroic laser mirrors are usually used as harmonic separators [1,2], beam combiners [3], or beam splitters [4] and play an important role in many laser applications, including inertial confinement fusion (ICF) lasers [5], petawatt femtosecond lasers [6], high power fiber lasers [3,7], compact Q-switched or mode-locked lasers [8–10], and other emerging lasers [11]. The ideal dichroic laser mirror for high-power lasers requires a significantly different reflection or transmission property and a high laser-induced damage threshold (LIDT) at two different wavelengths of interest. Unfortunately, a traditional dichroic laser mirror (TDLM) composed of alternating high- and low-refractive-index (n) pure materials often has difficulty achieving excellent spectral performance and high LIDTs at two wavelengths simultaneously [4]. Generally, TDLM for UV-NIR laser applications is achieved by alternately e-beam deposited HfO2 layers and SiO2 layers [12]. Sometimes, Al2O3 is chosen instead of HfO2 as the high-n material, which shows improved LIDT but requires a relatively large total number of coating layers [13,14]. There is a trade-off between the required optical performance and LIDT because suitable candidate coating materials are limited. In recent years, the library of available coating materials is expanded by co-evaporated or co-sputtered oxide mixtures [15–17]. The mixture materials provide us with adjustable optical gap values and optical constants, show superior properties over pure materials [16,18], and are attractive for many applications [19–21]. In addition to the coating material itself, it is also necessary to consider the interface-related issues of the traditional discrete interface, which is one of the key factors affecting LIDT. The co-evaporated interface with a graded-refractive index shows a significant increase in LIDT compared with the traditional discrete interface [22,23]. Therefore, by appropriately designing mixture materials and optimizing the interface, it is expected to realize an ideal dichroic laser mirror suitable for high-power lasers.

All coatings are deposited on fused silica and BK7 substrates using e-beam evaporation, in which the HfO2−Al2O3 mixture layer and the sandwich-like-structure interface are obtained by dual e-beam co-evaporation [18]. BK7 and fused silica substrates are used for stress measurement and other measurements, respectively. Before deposition, the coating chamber is heated to 473 K and evacuated to a base pressure of 9×10−4Pa; then, the substrate is cleaned with plasma ions at a bias voltage of 80 V for 120 s. The deposition rates for HfO2 and Al2O3 in monolayer and nanolaminate coatings and the deposition rates for SiO2 in multilayer coatings are 0.1, 0.1, and 0.2 nm/s, respectively. The deposition rates for HfO2 and Al2O3 in the HfO2−Al2O3 mixture coating are 0.028 and 0.072 nm/s, respectively. The deposition rates for HfO2 and Al2O3 in the mixture layer of the MDLM coating are 0.05 and 0.05 nm/s, respectively. The oxygen pressures of the SiO2 layer and other layers are 5.0×10−3 and 1.3×10−2Pa, respectively.

X-ray diffraction (XRD) (PANalytical Empyrean) is used to characterize the structure information of the coating. A VUV spectrometer (LZH ML 6500) and a UV-VIS-NIR spectrometer (Perkin Elmer Lambda 1050) are employed to measure the transmittance spectra in the ranges of 150–200 nm and 200–1200 nm, respectively. The reflectance spectra in the VIS region are calculated from the transmission data neglecting absorption. The refractive indices and optical bandgaps are estimated using the commercial thin film software (Essential Macleod) and the Tauc equation [24], respectively. The laser-induced temperature rise in the multilayer coating is obtained from the finite-element method (FEM) simulation.

The elemental composition profiles are determined by X-ray photoelectron spectroscopy (XPS, Thermo Scientific K-Alpha) using a monochromatic Al Kα (1486.6 eV) X-ray source. The spectra are recorded after every 20 s of etching with 1 keV Ar⁺ ions.

A 632.8 nm wavelength interferometer (ZYGO Mark III-GPI) is employed to inspect the sample surfaces before (substrates) and 60 days after the deposition (coatings) in a controlled environment with a temperature of 23±1.5°C and relative humidity of 45%±5%. The coating stress is obtained from Stoney’s formula.

The absorption of the coating at 1064 nm is measured by a home-made system based on the surface thermal lensing technique. The interfacial adhesion is characterized by a scratch test using a nano indenter (KLA Tencor). A load is gradually increased from 20 μN to 50 mN for the scratch test.

The 1-on-1 LIDT is tested according to ISO 21254. An s-polarized 2ω Nd:YAG laser with a 7.7 ns pulse width and a p-polarized 1ω Nd:YAG laser with a 12 ns pulse width are used for 532 and 1064 nm LIDT measurements, respectively. The test is performed at an angle of incidence of 45°. The effective beam sizes on the sample surface for 532 and 1064 nm LIDT measurements are approximately 0.32mm2 and 0.072mm2, respectively. Fifteen sites are tested for each energy fluence. The damage morphology is characterized by a focused ion beam scanning electron microscope (FIB-SEM, Carl Zeiss AURIGA CrossBeam). The chemical composition of the damaged area is analyzed by energy dispersive spectroscopy (EDS, Oxford X-Max, 50mm2).

The mixture layer has two advantages over the nanolaminate layers: the deposition process is simpler, and the bandgap is larger when n is close. The mixture is therefore chosen as the high-n material for MDLM coating in this work. This allows one to develop MDLM coatings with excellent optical and LIDT properties.

An FEM simulation is used to investigate the 1064 nm p-polarized laser-induced temperature rise in the two coatings. The extinction coefficients (k) of the HfO2 layer (1.36×10−6) and HfO2−Al2O3 mixture layer (4.52×10−7) are calculated by using Eq. (1) [31], based on the measured absorption, neglecting the absorption of the SiO2 layers: 1+AT=exp(4πkd/λ),(1)where A and T are the absorption and the transmittance of coating, d is the coating thickness (only the layers with absorption are taken into account), and λ is the wavelength. The extinction coefficient of dielectric materials typically increases with the decrease of wavelength; in this work, the mixture layer suggests a lower extinction coefficient over the whole wavelength range of interest than the HfO2 layer.

In summary, we have proposed and experimentally demonstrated a new MDLM coating with mixture layers and sandwich-like-structure interfaces. The proposed MDLM coating shows excellent spectral performance, and an LIDT that is almost twice of that of the TDLM coating at the two wavelengths of interest for the following reasons. First, the mixture layer in the MDLM coating has a larger optical bandgap and a lower absorption, resulting in a smaller temperature rise under the same fluence laser irradiation; second, the sandwich-like-structure interface allows the MDLM coating to exhibit enhanced mechanical properties. We believe that the described concept opens new avenues for improved dichroic mirror coatings and other laser coatings and can benefit many areas of laser technology that rely on high-quality laser coatings.

Acknowledgment. The authors express their appreciation to Prof. Wolfgang Rudolph for the fruitful discussions. The authors thank Ziyuan Xu and Yun Cui for the LIDT and FIB measurements, respectively.