E-beam Coatings Research Articles

Investigation of UV, ns-laser damage resistance of hafnia films produced by electron beam evaporation and ion beam sputtering deposition methods

Laser-induced damage in coating materials with a high index of refraction, such as hafnia, limits the performance of high power and high energy laser systems. Understanding the underlying physics responsible for laser damage holds the key for developing damage-resistant optical films. Previous studies have reported a substantial difference in laser damage onset for hafnia films produced by different deposition methods, yet the underlying mechanisms for the observed difference remain elusive. We combined laser damage testing with analytical characterizations and theoretical simulations to investigate the response of hafnia films produced by electron (e-) beam evaporation vs ion beam sputtering (IBS) methods upon UV ns-laser exposure. We found that e-beam produced hafnia films were overall more damage resistant; in addition, we observed a polarization anisotropy associated with the onset of damage in the e-beam films, while this effect was absent in the latter films. The observed differences can be attributed to the stark contrast in the pressure inside the pores inherent in both films. The high pressure inside the IBS-induced nanobubbles has been shown to reduce the threshold for laser-induced plasma breakdown leading to film damage. The polarization effects in the e-beam coatings can be related to the asymmetric electric field intensification induced by the columnar void structure. Our findings provide a fundamental basis for developing strategies to produce laser damage-resistant coatings for UV pulsed laser applications.

Strategy to improve the long-term stability of low-stress e-beam coatings

The performance of the electron-beam (e-beam) coatings can be easily affected by the environmental humidity due to their porous nature. We propose a shell layer strategy deposited by plasma ion-assisted deposition to cover the top surface and sidewall of the e-beam coating. The long-term optical and mechanical stability of the prepared multilayer coatings with and without the shell layer are compared. Our results show that the shell layer strategy can effectively isolate the underlying e-beam coating from the ambient humidity, thereby significantly improving the long-term stability and mechanical properties of the e-beam coating. Meantime, the shell layer strategy of isolating ambient humidity avoids the increase in compressive stress caused by water absorption of the e-beam coating, thereby keeping the entire coating stress at a low level. The proposed shell layer strategy can be used to address the time- and environment-dependent instability issues of the e-beam coatings for high-power laser applications.

Effects of water adsorption on properties of electron-beam HfO2/SiO2 high-reflection coatings

The moisture-induced instability of electron-beam (e-beam) coating can even result in the performance instability of a large laser system. A dense plasma-ion-assisted-deposition SiO2 capping layer was employed to isolate the top surface of the coating from ambient humidity and extend the originally rapid water adsorption process, allowing a deeper understanding of the instability of e-beam coatings caused by moisture. Spectral measurements suggested that four water adsorption steps, including filling, stabilization, swelling and refilling, led to the overall redshift and short-term fluctuations in spectra. The stress and Fourier transform infrared analysis revealed that water adsorption and ice-like/ice structure water was responsible for the increasing compressive stress evolution at the first stage and subsequent transition from compressive to tensile stress evolution, respectively. A model based on the finite element method was proposed to calculate the water vapor transmission rate of e-beam coatings. Our study on the underlying mechanism of moisture-induced instability of e-beam coating is helpful to ensure the reliability and stability of large laser systems.

Ten-year summary of the Boulder Damage Symposium annual thin film laser damage competition

The thin film damage competition series at the Boulder Damage Symposium provides an opportunity to observe general trends in laser damage behavior between different coating types (high reflector, antireflector, polarizer, and Fabry–Perot filter), wavelength ranges (193 to 1064 nm), and pulse length ranges (40 fs to 18 ns). Additionally, the impact of deposition process, coating material, cleaning process, and layer count can be studied within a single year or more broadly across the history of this competition. Although there are instances where participants attempted to isolate a single variable to better understand its impact on laser resistance, this series of competitions isolates the variable of the damage testing service and protocol for a wide variety of participants to enable the observation of general trends. In total, 275 samples from 58 different participants have been tested at four different laser damage testing facilities over the last 10 years. Hafnia was clearly the best high refractive index material except for ultraviolet (UV) applications, although a wide range of high refractive index materials performed well. The best deposition process varied significantly between the different competitions. The best deposition process was dependent on the coating type, wavelength, and pulse duration. For 1064-nm coatings with nanosecond scale pulse lengths, e-beam coatings tended to be the best performers. For short-pulse length NIR mirrors and nanosecond pulse length UV mirrors, densified coating processes, which all involved sputtering of the target material, were the best performers. For UV antireflector (AR) coatings and excimer mirrors, both tested at nanosecond pulse lengths, they tended to favor very low energetic deposition methods yielding soft coatings, such as sol gel dip coating for the AR and resistive heating of fluorides for the excimer mirrors. Finally, cleaning method and layer count had a less obvious correlation with laser resistance over the history of this thin film damage competition series.

Environmental stability investigation on electron-beam deposited coatings with dense capping layer

The properties of coatings deposited by electron-beam (e-beam) technique can be easily influenced by environmental humidity, causing spectrum shift, residual stress evolution, and wave front errors. HfO2 / SiO2 multilayer coatings with different overcoat layer deposition parameters have been prepared. The optical spectrum shifts induced by atmosphere-vacuum effect are investigated by a spectrometer. The laser resistance is studied and their damage morphologies are characterized by a scanning electron microscope. The surface morphologies and the global mechanical stresses of the films are analyzed by an atomic force microscope and zygo interferometer, respectively. The experimental results demonstrate that by introducing a dense capping SiO2 layer employed with plasma ion assisted deposition, considerable environmental stability of e-beam coatings can be improved due to retarded water vapor transport. A relatively smaller grain size can be obtained as well. Moreover, the laser-induced damage threshold shows no significant difference.

Thermal conductivity of e-beam coatings

We present the results of the study on the thermal conductivity of different thin film materials produced by conventional thermal evaporation. The main features of the thermal pulse method employed for the measurement of the thermal conductivity are described. Thermal conductivity can be measured by determining the traveling time of a thermal wave propagating trough the film. A pump laser beam is directed onto a sample consisting of a thin transparent test layer and a totally absorbing substrate for the laser wavelength. As a consequence of the laser pulse, a temperature profile builds up at the substrate–film interface. A thermal pulse starts to diffuse from the substrate–film interface to the surface of the layer. Therefore, the temperature rise at the surface of the test layer starts with a time delay with respect to the laser pulse. The time delay depends on the propagation time of the thermal wave through the layer and is related to the thermal conductivity and the thickness of the layer. Measurements are evaluated by calculations based on the finite difference method. The results show that the analyzed thin films have lower thermal conductivity than the corresponding materials in bulk form.

The use of ion beam sputtered optical coatings as protective overcoats

Diamond turned aluminium mirrors are being used more often in forward looking infrared (FLIR) imaging systems (operating in the 8–12 μm region) because they provide high optical performance at low cost. The original high reflectivity of these surfaces must be maintained through military standard humidity/temperature cycling tests. We have developed ion beam sputtered Y2O3 as a protective coating that meets all optical and environmental requirements. Data are presented that show that ion beam sputtered optical coatings are much less permeable to water than electron-beam coatings while retaining many of the desirable features of e-beam coatings. The deposition process and the properties of the coatings will be presented. Thin protective coatings of Y2O3 that exhibit very little angular dependence in reflectivity on aluminum have been prepared by ion beam sputtering.