Semyon Papernov

Near-ultraviolet absorption and nanosecond-pulse-laser damage in HfO2 monolayers studied by submicrometer-resolution photothermal heterodyne imaging and atomic force microscopy

Localized absorption in hafnium dioxide used as a high-index component in multilayer coatings for near-ultraviolet, nanosecond-pulse-laser applications is directly linked to laser-induced damage. The nature of the absorbing species and their physical properties remains unknown because of their extremely small sizes. Previous experimental evidence provided by the atomic force microscopy mapping of damage morphology points to a few-nanometer scale of these absorbers. This work demonstrates the submicrometer-resolution mapping of 355-nm absorption in HfO2 monolayers using a recently developed photothermal heterodyne imaging technique. The comparison of absorption maps with the atomic force microscopy investigation of pulsed-laser-induced damage morphology allows one to better estimate the spatial distribution of nanoscale absorbing defects in hafnia thin films. Possible defect-formation mechanisms are discussed.

Stress compensation in hafnia/silica optical coatings by inclusion of alumina layers

Hafnium dioxide films deposited using electron-beam evaporation tend to exhibit high tensile stresses, particularly when deposited on low-thermal-expansion substrates for use in a low-relative-humidity environment. Hafnia has been shown to be a critical material, however, for use in high-peak-power laser coatings, providing exceptional deposition control and laser-damage resistance. To correct for tensile thin-film stresses in hafnia/silica multilayer coatings, alumina compensation layers were incorporated in the multilayer design. Determination of the stresses resulting from alumina layers in different coating designs has led to the realization of the influence of water diffusion and the diffusion-barrier properties of alumina that must be considered. The inclusion of alumina layers in a hafnia/silica multilayer provides the ability to produce low-compressive-stress, high-laser-damage-threshold coatings.

Laser-induced damage of photo-thermo-refractive glasses for optical holographic element writing

Bulk and surface-damage thresholds of photo-thermo- refractive glasses used for hologram writing were measured after different stages of development. Values obtained are compared with threshold from widely used glasses, such as BK7 or fused silica. Polished glass samples were exposed to 325-nm He-Cd laser radiation and thermodeveloped at 520 degrees C. Nd-glass, nanosecond laser pulses were used to irradiate the bulk and surfaces of glasses. It was found that laser-induced damage thresholds of photo-thermo- refractive glasses are within a factor of 2-4 of those for fused silica or BK7 glass. This indicates that phase element made from these glasses are promising candidates for use in high-power laser systems. Possible mechanisms of laser- induced damage are discussed.

Improving the performance of high-laser-damage-threshold, multilayer dielectric pulse-compression gratings through low-temperature chemical cleaning

A low-temperature chemical cleaning approach has been developed to improve the performance of multilayer dielectric pulse-compressor gratings for use in the OMEGA EP laser system. X-ray photoelectron spectroscopy results guided the selection of targeted cleaning steps to strip specific families of manufacturing residues without damaging the gratings fragile 3D profile. Grating coupons that were cleaned using the optimized method consistently met OMEGA EP requirements on diffraction efficiency and 1054 nm laser-damage resistance at 10 ps. The disappearance of laser-conditioning effects for the highest-damage-threshold samples suggests a transition from a contamination-driven laser-damage mechanism to defect-driven damage for well-cleaned components.

Large-aperture plasma-assisted deposition of inertial confinement fusion laser coatings

Plasma-assisted electron-beam evaporation leads to changes in the crystallinity, density, and stresses of thin films. A dual-source plasma system provides stress control of large-aperture, high-fluence coatings used in vacuum for substrates 1m in aperture.

Thin-film polarizers for the OMEGA EP laser system

Thin-film polarizers are essential components of large laser systems such as OMEGA EP and the NIF because of the need to switch the beam out of the primary laser cavity (in conjunction with a plasma-electrode Pockels cell) as well as providing a well-defined linear polarization for frequency conversion and protecting the system from back-reflected light. The design and fabrication of polarizers for pulse-compressed laser systems is especially challenging because of the spectral bandwidth necessary for chirped-pulse amplification. The design requirements for a polarizer on the OMEGA EP Laser System include a Tp greater than 98% over a spectral range of 1053±4 nm while maintaining a contrast ratio (Tp/Ts) of greater than 200:1 (500:1 goal) over the same range. An allowance must be made for the uniformity of the film deposition such that the specifications are met over the aperture of the component while allowing for some tolerance of angular misalignment. Production results for hafnia/silica designs will be shown, illustrating high transmission and contrast over an extended wavelength/angular range suitable for the 8 nm spectral bandwidth of OMEGA EP. Difficulties in production will also be illustrated, as well as the methods being implemented to overcome these challenges. A key challenge continues to be the fabrication of such a coating suitable for use on fused-silica substrates in a dry environment. Laser-damage thresholds for 1-ns and 10-ps pulse widths will be discussed.

Optimization of laser-damage resistance of evaporated hafnia films at 351nm

A systematic study was undertaken to improve the laser-damage resistance of multilayer high-reflector coatings for use at 351 nm on the OMEGA EP Laser System. A series of hafnium dioxide monolayer films deposited by electron-beam evaporation with varying deposition rates and oxygen backfill pressures were studied using transmission electron microscopy (TEM), x-ray diffraction (XRD), and refractive index modeling. These exhibit microstructural changes for sufficiently slow deposition rates and high oxygen backfill pressures, resulting in an absence of crystalline inclusions and a lower refractive index. Hafnia monolayers exhibited laser-damage resistance as high as 12 J/cm2 at 351 nm with a 0.5-ns pulse. This process was utilized in the fabrication of reduced electric-field-type multilayer high-reflector coatings. Measured laser-damage thresholds as high as 16.63 J/cm2 were achieved under identical test conditions, an exceptional improvement relative to historical damage thresholds of the order of 3 to 5 J/cm2.

Using colloidal gold nanoparticles for studies of laser interaction with defects in thin films

A model thin-film system based on SiO2 coating with artificially introduced gold nanoparticles was investigated for the mechanism of 351-nm, pulsed-laser-radiation interaction with well-characterized nanoabsorbers. Damage morphology, represented by craters, provides strong evidence of the important role of the melting and vaporization processes. Measured crater volumes and numerical estimates based on them suggest that crater formation cannot proceed through laser-energy absorption confined within the particle. It instead starts in the particle and then, due to energy transfer, spreads out to the surrounding matrix during the laser pulse.

AFM-mapped nanoscale absorber-driven laser damage in UV high-reflector multilayers

Submicrometer lateral-size craters that develop independently of the presence of micron-scale growth nodules and whose number density follows the intensity profile of the full laser beam are the dominant laser-damage features in 351-nm high-reflector production coatings on the University of Rochesters 60-beam OMEGA laser. Coupled with the observation that the smallest measured damage craters allow for initiating absorber sizes not larger than 10 nm, the experimental evidence points toward randomly distributed nano-cluster absorbers as the sources for energy transfer from the optical field to the porous film medium.

Liquid-Crystal Materials for High Peak-Power Laser Applications

Abstract This is a brief review of materials and device-development issues related to the application of liquid-crystal technology to multikilojoule, high peak-power lasers.