Stefan Günster

Ultraviolet optical and microstructural properties of MgF2 and LaF3 coatings deposited by ion-beam sputtering and boat and electron-beam evaporation

Single layers of MgF2 and LaF3 were deposited upon superpolished fused-silica and CaF2 substrates by ion-beam sputtering (IBS) as well as by boat and electron beam (e-beam) evaporation and were characterized by a variety of complementary analytical techniques. Besides undergoing photometric and ellipsometric inspection, the samples were investigated at 193 and 633 nm by an optical scatter measurement facility. The structural properties were assessed with atomic-force microscopy, x-ray diffraction, TEM techniques that involved conventional thinning methods for the layers. For measurement of mechanical stress in the coatings, special silicon substrates were coated and analyzed. The dispersion behavior of both deposition materials, which was determined on the basis of various independent photometric measurements and data reduction techniques, is in good agreement with that published in the literature and with the bulk properties of the materials. The refractive indices of the MgF2 coatings ranged from 1.415 to 1.440 for the wavelength of the ArF excimer laser (193 nm) and from 1.435 to 1.465 for the wavelength of the F2 excimer laser (157 nm). For single layers of LaF3 the refractive indices extended from 1.67 to 1.70 at 193 nm to approximately 1.80 at 157 nm. The IBS process achieves the best homogeneity and the lowest surface roughness values (close to 1 nm(rms)) of the processes compared in the joint experiment. In contrast to MgF2 boat and e-beam evaporated coatings, which exhibit tensile mechanical stress ranging from 300 to 400 MPa, IBS coatings exhibit high compressive stress of as much as 910 MPa. A similar tendency was found for coating stress in LaF3 single layers. Experimental results are discussed with respect to the microstructural and compositional properties as well as to the surface topography of the coatings.

Effect of systematic errors in spectral photometric data on the accuracy of determination of optical parameters of dielectric thin films

The determination of optical parameters of thin films from experimental data is a typical task in the field of optical-coating technology. The optical characterization of a single layer deposited on a substrate with known optical parameters is widely used for this purpose. Results of optical characterization are dependent on not only the choice of the thin-film model but also on the quality of experimental data. The theoretical results presented highlight the effect of systematic errors in measurement data on the determination of thin-film parameters. Application of these theoretical results is illustrated by the analysis of experimental data for magnesium fluoride thin films.

Yb:CaF 2 thin-disk laser

Summary form only given. Novel amplifier systems and laser oscillators generating short pulses and high pulse energies are required in numerous applications, not least in material processing. Currently there are significant efforts to develop such systems [1]. Among other Ytterbium-doped laser gain materials, Yb:CaF<;sub>2<;/sub> is an interesting candidate for highpower cw and passively mode-locked operation for the generation of short pulses because it combines a very broad emission spectrum with a high undoped thermal conductivity (~ 9.7 W/(mK)) [2]. The proof-of principle in ultrafast laser operation was already shown with a bulk crystal by demonstrating pulses shorter than 100 fs in a SESAM passively mode-locked laser system [3]. In the present contribution we report on the first investigation of a Yb:CaF<;sub>2<;/sub> thin-disk laser as this concept is ideally suited for power scaling of ultrafast lasers [4]. For the experimental investigation, two crystals from the same boule with different thicknesses were compared. The nominal doping concentration was specified by the supplier to be around 4 %. The first and non-wedged crystal has a thickness of 200 μm whereas that of the second and wedged (~0.1 °) crystal is 250μm. The thin-disk crystals were glued on copper heat sinks and water cooled from the backside with nominal operating temperatures between 15 and 18 °C. A thin-disk pumping module with a total of 24 pump light passes through the laser crystal was used for sufficient absorption of the pump radiation. The crystals were pumped either with a fiber-coupled laser diode with up to 40 W of power centered at a wavelength of 976 nm with a spectral bandwidth of about 10 nm or at high-powers or, with a 1.2 kW pump source exhibiting a spectral bandwidth of 4 nm centered at a wavelength of 976 nm. In this latter case, only one half of the pump power was used to avoid the damage of the crystal.Figure 1 shows the thermal behavior of the crystal which was examined using a thermal-imaging camera to measure the temperature at its front surface in fluorescence and laser operation. It is recognizable that the temperature at the front surface of the disk in fluorescence operation is significantly lower than in laser operation because of the smaller quantum defect of the shorter centre wavelength of the fluorescence emission. The fact that the temperature is not exceeding 100 °C at power densities of around 8 kW/cm2 represents a relevant and extremely promising information for the further development of high-power Yb:CaF2 thin-disk lasers. After this confirmation, high-power laser operation was demonstrated in a multimode resonator. As shown in Figure 2 a maximum output power of 250 W with an optical efficiency of 47 % was obtained with the 200 μm thick laser crystal. In a further experiment the wedged 250 μm thick crystal was characterized in the perspectives of modelocked operation. The pump spot was set to 1 mm. In a fundamental mode V-shaped resonator (M2 <; 1.1) a maximum of 12.9 W of CW output power with an optical efficiency of 34.2 % has been achieved. The successful demonstration of high power capability and fundamental mode operation were the first steps to set-up a passively mode-locked thin-disk laser. This work is currently under progress and will be presented during the talk.The demonstrated results confirm the potential of Yb:CaF2 for high-power thin-disk laser operation and offer the promising prospect of generating short pulse durations in a passively mode-locked thin-disk laser. Future improvements of the crystal quality, a better control of the doping concentration and a better quality of the polishing will lead to further enhancement of the laser performance.

Reliable determination of wavelength dependence of thin film refractive index

Depending on the choice of thin film models and measurement data used for the characterization analysis one can obtain essentially different characterization results. It is especially difficult to reliably determine refractive index wavelength dependencies in the case of low accuracy measurement data. We consider possible approaches aimed to improve a stability of refractive index determination. The ways of the verification of characterization results are also discussed. Practical examples used to illustrate the proposed approaches are connected with the most difficult case of the determination of the refractive indices of fluoride films in the VUV spectral region.

Frequency tripling mirror.

A frequency tripling mirror (FTM) is designed, fabricated and demonstrated. The mirror consists of an aperiodic sequence of metal oxide layers on a fused silica substrate tailored to produce the third harmonic in reflection. An optimized 25-layer structure is predicted to increase the reflected TH by more than five orders of magnitude compared to a single hafnia layer, which is a result of global compensation of the phase mismatch of TH and fundamental, field enhancement and design favoring reflection. Single pulse conversion efficiencies approaching one percent have been observed with the 25-layer stack for fundamental wavelengths in the near infrared and 55 fs pulse duration. The FTM is scalable to higher conversion, larger bandwidths and other wavelength regions making it an attractive novel nonlinear optical component based on optical interference coatings.

Storage ring free-electron lasing at 176 nm--dielectric mirror development for vacuum ultraviolet free-electron lasers

Mirrors for storage ring free-electron lasers in the vacuum ultraviolet must provide adequate reflectivity and resistance against synchrotron radiation. The free-electron laser system at ELETTRA (Trieste, Italy) is targeted to lase in the spectral range between 155 and 200 nm. It was demonstrated that dense oxide multilayer coatings allow lasing down to 189.9 nm. However, pure oxide systems show significant absorption at lower wavelengths and cannot be employed below 189.9 nm. Fluoride stacks can be deposited down to 130 nm with high reflection values above 95%, but their resistance against the harsh synchrotron environment is poor. They rapidly degrade; lasing cannot be realized with this mirror approach. For the range between 170 and 190 nm, hybrid systems--combining fluoride and oxide materials--have been manufactured. With appropriate deposition procedures, mirrors achieve reflectance values up to 99% and an adequate radiation resistance simultaneously. A mirror based on a conventional fluoride stack protected by a dense silicon dioxide protection layer was deposited and successfully employed for free-electron lasing at 176.4 nm.

Dual wavelength laser-induced damage threshold measurements of alumina/silica and hafnia/silica ultraviolet antireflective coatings.

An approach for the measurement of the laser-induced damage threshold with two wavelengths combined was made while testing antireflective coatings for the wavelengths 266 and 532 nm. Samples were made of Al2O3/SiO2 and HfO2/SiO2 ion beam sputtered films. The results show that adding radiation of a second wavelength might lead to a significant reduction of the threshold. The damage morphology of single and dual wavelength tests is very similar and does not suggest an altered damage mechanism. Further investigations indicated that the dual wavelength threshold is a function of the temporal delay of the two pulses.

Design and realization of advanced multi-index systems.

Practically all thin film systems for normal incidence can be realized using only two-layer materials. But for oblique incidence, polarization effects occur, designs may become complex, and polarization control is difficult or impossible to achieve. Here multi-index or gradient designs offer additional degrees of freedom, and can simplify or even enable challenging designs. Such gradient thin film stacks can be designed ab initio without any start or index profile approximations using a new design software developed by Carl Zeiss. With this software, a rugate omnidirectional AR coating was calculated and transferred to three different multi-index systems. All three examples were realized using ion beam sputter technology, and characterized at Laser Zentrum Hannover. Here we present comparative measurements of the optical performance together with femtosecond laser-induced damage threshold measurements.

Toward resistant vacuum-ultraviolet coatings for free-electron lasers down to 150 nm

Research and development are currently trying to run a storage ring free-electron laser down to 150 nm with robust optics. Vacuum-ultraviolet fluoride optics with protected oxide layers and enhanced metallic mirrors are investigated.

Role of HfO2/SiO2 thin-film interfaces in near-ultraviolet absorption and pulsed laser damage

Abstract. The role of thin-film interfaces in the near-ultraviolet (near-UV) absorption and pulsed laser-induced damage was studied for ion-beam-sputtered and electron-beam-evaporated coatings comprised from HfO2 and SiO2 thin-film pairs. To separate contributions from the bulk of the film and from interfacial areas, absorption and damage threshold measurements were performed for a one-wave (355-nm wavelength) thick, HfO2 single-layer film and for a film containing seven narrow HfO2 layers separated by SiO2 layers. The seven-layer film was designed to have a total optical thickness of HfO2 layers, equal to one wave at 355 nm and an E-field peak and average intensity similar to a single-layer HfO2 film. Absorption in both types of films was measured using laser calorimetry and photothermal heterodyne imaging. The results showed a small contribution to total absorption from thin-film interfaces as compared to HfO2 film material. The relevance of obtained absorption data to coating near-UV, nanosecond-pulse laser damage was verified by measuring the damage threshold and characterizing damage morphology. The results of this study revealed a higher damage resistance in the seven-layer coating as compared to the single-layer HfO2 film in both sputtered and evaporated coatings. The results are explained through the similarity of interfacial film structure with structure formed during the codeposition of HfO2 and SiO2 materials.