Single layer antireflection coatings (SLAR) consisting of nanoporous silica (NP SiO 2 ) films are developed by selective chemical etching of atomic layer deposited (ALD) Al 2 O 3 :SiO 2 composite films. The reflective index of the final NP SiO 2 film is finely adjusted from 1.132 to 1.400 at 600 nm wavelength by applying an appropriate ratio in the composite. To meet the requirements of the SLAR coatings from the deep UV (DUV) to the near IR (NIR) spectral range, the film thickness is controlled with nanometer precision by the ALD process. The SLAR are simultaneously applied on both sides of flat or highly curved substrates. Transmittance values above 99.4% are achieved even at a wavelength of 193 nm on fused silica substrates. Various characterization methods demonstrate the advantages of these SLAR with regard to impurities, optical losses, laser induced damage threshold (LIDT) properties, and surface super-hydrophilicity. The absorption losses at 193 nm wavelength as determined by laser induced deflection measurements amount to approximately 200 ppm, and to approximately 2 ppm at a wavelength of 1064 nm, while the scattering losses are around 30 ppm at 532 nm wavelength for quarter-wave layers. The LIDT values at 1064 nm are in the range of 93 J cm À2 being close to the values measured on the uncoated substrate.

Samples composted of chemically synthesized Au nanoparticles (NPs) (16.0±2.0 nm) embedded within a planar silica film are used as model system to investigate the evolution of a second phase under irradiation when the temperature and the ion stopping power are changed. Samples are irradiated with 4 MeV Au2+ ions and 4 MeV Br2+ ions for temperature ranging from 30 °C up to 800 °C and for fluences up to 8×1016 cm−2. We show that at room temperature the complete dissolution of the NPs leads to the formation of smaller precipitates with a narrower size distribution, i.e., 2.0±0.3 nm. However, when the temperature is increased and/or the nuclear stopping power is decreased, a reduction in the dissolution rate was observed. This leads to the formation of a bimodal size distribution. Finally, the evolution of the density of the precipitates with the temperature is discussed in term of the thermal stability of the irradiation-induced defects within the silica matrix.

In this work, we present our results about the thermal crystallization of ion beam sputtered hafnia on 0001 SiO2 substrates and its effect on the laser-induced damage threshold (LIDT). The crystallization process was studied using in-situ X-ray diffractometry. We determined an activation energy for crystallization of 2.6 ± 0.5 eV. It was found that the growth of the crystallites follows a two-dimensional growth mode. This, in combination with the high activation energy, leads to an apparent layer thickness-dependent crystallization temperature. LIDT measurements @355 nm on thermally treated 3 quarter-wave thick hafnia layers show a decrement of the 0% LIDT for 1 h @773 K treatment. Thermal treatment for 5 h leads to a significant increment of the LIDT values.

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Substrate cleaning prior to coating has a strong influence on the performance of the optical component. Exemplary, none or inadequate cleaning reduces the resistance against laser irradiation drastically. Especially in laser components coated with anti-reflective layers, the interface between substrate and coating is one of the most limiting factors. This study investigates different precision cleaning processes and their influence on the laser resistance of ion-beam sputtered anti-reflective coatings. Therefore, a SiO2/Ta2O5 multilayer anti-reflective coating for a wavelength of 1064 nm and a normal angle of incidence was deposited onto high-quality fused silica substrates. Prior to deposition, the substrates were cleaned with various cleaning processes using different solutions and ultrasonic frequencies. To characterize the cleaned surface quality, the surfaces were analyzed with respect to root-mean-square (RMS) roughness and particle density. Laser damage was measured using a 1064 nm ns-pulsed laser test bench. It was found that an alcoholic pre-clean is recommendable to prevent laser damage caused by organic films remaining from the polishing process. The applied ultrasonic frequencies strongly influenced the particle density down to the sub-micrometer range and in consequence, the laser-induced damage threshold (LIDT). Ultrasonic cleaning at excessive power levels can reduce laser resistance.

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The optimization of millimeter wave Gunn oscillators comprises two avenues, namely the use of heterojunctions for hot electron injection and multidomain operation Gunn diodes. A combination of these two techniques is proposed resulting in multiple hot electron launchers spaced throughout the diode. The results presented here suggest that this configuration proves to be a very promising option which warrants further investigation.

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