Atomic layer deposition (ALD) has been widely used in Micro-electronics due to its self-terminating process. The ALD films have precise thickness and nodular-free structure, which are desirable properties for high power laser applications. ALD Al2O3 and HfO2 single layers were prepared through ALD process. The layer properties relevant to high power laser industry were studied and compared with IBS Al2O3 and HfO2 single layers, respectively. Anti-reflection coatings for different harmonics of Nd: YAG laser were prepared through ALD and IBS respectively. ALD single layer films show comparable refractive indices and band gap energies with IBS films. ALD films have tensile stress, in contrast to the compressive stress of IBS films. ALD single layer films have absorption as small as several ppm at fundamental wavelength of Nd: YAG laser, and higher LIDT than IBS films. The spectra of anti-reflection coatings indicate precise thickness control of ALD by counting the cycles. ALD coatings show better laser resistance than IBS anti-reflection coatings at 1ω-4ω harmonics of Nd: YAG lasers. The LIDT of both ALD and IBS coatings are limited by the HfO2 layers. Also in ALD films, defects are the main cause for ns laser damage. The study in this text indicates the high versatility of ALD films for applications in high-power coatings.
An established method for precise determination of optical absorption is the so called laser calorimetry. According to ISO 115511 laser calorimetry is preferred to other photothermal test methods, because of its capability to deliver absolute calibration. Many optical materials have low heat conductivity, which can affect the calibration significantly. The timeand spatial dependent temperature profile in a sample of materials with low heat conductivity requires accurate temperature measurement strategies to determine material-independent and absolutely calibrated absorption values. For thin cylindrical samples, ISO 11551 provides a strategy to compensate heat conductivity effects. The optimal temperature sensor position, where accordingly calibrated measurement results2 can be obtained, is simply based on the symmetric sample geometry. For thick geometries an additional temperature distribution along propagation direction of the heating beam must be considered. The current version of ISO 11551 does not provide a sophisticated solution for this problem, because the heating scheme of a sample is usually unknown. Therefore, a reliable calibration procedure can only be applied to samples of well-known absorption properties of surfaces and bulk material. Utilizing such kind of specifically prepared reference samples in combination with Finite Element Method (FEM) calculations, a general measurement and data evaluation concept based on laser calorimetry is presented, that allows deriving absolutely calibrated absorption measurement results for rectangular sample geometries.
Atomic layer deposition (ALD) has been widely studied in Micro-electronics due to its self-terminating property. ALD also grows film coatings with precise thickness and nodular-free structure, which are desirable properties for high power coatings. The depositing process was studied to produce uniform, stable and economic Al2O3 single layers. The layer properties relevant to high power laser industry were studied and compared with IBS Al2O3 single layers. ALD Al2O3 showed a stable growth of 0.104 nm/cycle, band gap energy of 6.5 eV and tensile stress of about 480 MPa. It also showed a low absorption at wavelength 1064 nm within several ppm, and LIDT above 30 J/cm2. These properties are superior to the reference IBS Al2O3 single layers and indicate a high versatility of ALD Al2O3 for high power coatings.
Rugate and high/low quarter-wave stacks high reflector coatings for 1064 nm have been prepared with Ta 2 O 5 and SiO 2 by an ion-beam sputtering technique. A laser-induced damage experiment of the samples has been conducted at 1064 nm with pulse duration of 5 ns [full width at half maximum (FWHM)]. These two samples’ damages both initiate at defects and show almost the same damage threshold within the experimental error. Nevertheless, the damage morphology on rugate is less severe at higher fluences. The thermal shock wave induced by a nanosecond pulsed laser is considered to be the main cause of catastrophic damage.
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