Third and fifth order nonlinear susceptibilities in thin HfO2 layers.

Third harmonic generation (THG) from dielectric layers is investigated. By forming a thin gradient of HfO2 with continuously increasing thickness, we are able to study this process in detail. This technique allows us to elucidate the influence of the substrate and to quantify the layered materials third χ(3)(3ω: ω, ω, ω) and even fifth order χ(5)(3ω: ω, ω, ω, ω, - ω) nonlinear susceptibility at the fundamental wavelength of 1030 nm. This is to the best of our knowledge the first measurement of the fifth order nonlinear susceptibility in thin dielectric layers.

Reduction of nanoparticles in optical thin films through ion etching.

Contaminating particles in optical thin films can lead to the formation of nodules and reduction of laser induced damage threshold (LIDT). This work investigates the suitability of ion etching of the substrates to reduce the impact of nanoparticles. Initial investigations suggest that ion etching can remove nanoparticles from the sample surface; however, doing so introduces texturing to the surface of the substrate. This texturing leads to an increase in optical scattering loss, though LIDT measurements show no significant reduction in the durability of the substrate.

Ultrafast switching with nonlinear optics in thin films.

We demonstrate a novel, to the best of our knowledge, concept for an all-optical switch based on the optical Kerr effect in optical interference coatings. The utilization of the internal intensity enhancement in thin film coatings as well as the integration of highly nonlinear materials enable a novel approach for self-induced optical switching. The paper gives insight into the design of the layer stack, suitable materials, and the characterization of the switching behavior of the manufactured components. A modulation depth of 30% could be achieved, which prepares the way for later applications in mode locking.

Group delay dispersion monitoring for computational manufacturing of dispersive mirrors.

We present a computational manufacturing program for monitoring group delay dispersion (GDD). Two kinds of dispersive mirrors computational manufactured by GDD, broadband, and time monitoring simulator are compared. The results revealed the particular advantages of GDD monitoring in dispersive mirror deposition simulations. The self-compensation effect of GDD monitoring is discussed. GDD monitoring can improve the precision of layer termination techniques, it may become a possible approach to manufacture other optical coatings.

Quantizing nanolaminates as versatile materials for optical interference coatings.

In this paper, the theoretical foundation of quantizing nanolaminates is explained, and the dependence of the optical band gap on quantum-well thickness is demonstrated. The production is investigated by applying molecular dynamics growth simulation and by correlating the results with layers deposited by ion beam sputtering and atomic layer deposition. The properties of manufactured nanolaminates are then compared to the theoretical behavior, and good agreement is found.

Correlation of structural and optical properties using virtual materials analysis.

Thin film growth of ${\textrm{TiO}}_2$TiO2 by physical vapor deposition processes is simulated in the Virtual Coater framework resulting in virtual thin films. The simulations are carried out for artificial, simplified deposition conditions as well as for conditions representing a real coating process. The study focuses on porous films which exhibit a significant anisotropy regarding the atomistic structure and consequently, to the index of refraction. A method how to determine the effective anisotropic index of refraction of virtual thin films by the effective medium theory is developed. The simulation applies both, classical molecular dynamics as well as kinetic Monte Carlo calculations, and finally the properties of the virtual films are compared to experimentally grown films especially analyzing the birefringence in the evaluation.

Enhancement of the damage resistance of ultra-fast optics by novel design approaches.

Dielectric components are essential for laser applications. Chirped mirrors are applied to compress the temporal pulse broadening crucial in the femtosecond regime. However, the design sensitivity and the electric field distribution of chirped mirrors is complex often resulting in low laser induced damage resistances. An approach is presented to increase the damage resistance of pulse compressing mirrors up to 190% in the NIR spectral range. Layers with critical high field intensity of a binary mirror design are substituted by ternary composites and quantized nanolaminates, respectively. The deposition process is improved by an in situ technique monitoring the phase of reflectance.

Tunable optical properties of amorphous Tantala layers in a quantizing structure.

Plasma deposition techniques like ion-beam-sputtering (IBS) are state of the art to manufacture high quality optical components for laser applications. Besides the well optimized process and monitoring systems, the coating material selection is integral to achieve optimum optical performances. Applying the IBS technology, an approach is presented to create novel materials by the direct application of binary oxides in a quantizing structure. By reducing the physical thickness of the high refractive index material to a few nm, within a classical high-low index stack, the electron confinement can be changed. Optical characterizations of the manufactured samples with decreasing quantum well thicknesses result in an increasing blue shift of the absorption gap and offer a method to approximate the effective mass of the high refractive index material in conjunction with theoretical models. Laser-induced damage threshold tests of coating samples prepared with different well thicknesses indicate an increase of the measured threshold values with optical gap energy.

Spatial separation effects in a guiding procedure in a modified ion-beam-sputtering process.

In the present state of the art, ion beam sputtering is used to produce low-loss dielectric optics. During the manufacturing of a dielectric layer stack, the deposition material must be changed, which requires rapid mechanical movement of vacuum components. These mechanical components can be regarded as a risk factor for contamination during the coating process, which limits the quality of high-end laser components. To minimize the particle contamination, we present a novel deposition concept that does not require movable components to change the coating material during the coating process. A magnetic field guiding technique has been developed, which enables the tuning of the refractive index in the layer structure by sputtering mixtures with varying compositions of two materials using a single-ion source. The versatility of this new concept is demonstrated for a high-reflection mirror.

Pulse characterization by THG d-scan in absorbing nonlinear media.

We report on few-cycle pulse characterization based on third harmonic generation dispersion scan (THG d-scan) measurements using thin films of different TiO(2)-SiO(2) compositions as nonlinear media. By changing the TiO(2) concentration in the thin film the band gap and therefore the position of the absorption edge were varied. The retrieved pulse durations from different nonlinear media agree within 5%, and the reconstructed pulse shapes prove to be immune against the absorption edges as well. The reason is the robust retrieval algorithm which takes the influence of wavelength dependent nonlinearity into account by a spectral weight function.

Calculation of optical and electronic properties of modeled titanium dioxide films of different densities.

The electronic and optical properties of TiO2 atomic structures representing simulated thin films have been investigated using density functional theory. Suitable model parameters and system sizes have been identified in advance by validation of the results with experimental data. Dependencies of the electronic band gap and the refractive index have been calculated as a function of film density. The results of the performed calculations have been compared to characterized optical properties of titania single layers deposited using different coating techniques. The modeled dependencies are consistent with experimental observations, and absolute magnitudes of simulated values are in agreement with measured optical data.

Investigations in the guiding efficiency in a modified ion beam sputtering process.

Ion beam sputtering (IBS) is an established deposition process used in the production of optical coatings. In this study, a modification of the IBS process, based on additional electromagnetic fields, is examined in an effort to improve the technology. The reported experiments reveal the underlying effects of electromagnetic fields on the distribution of the coating material sputtered from the target. An increase in local deposition rate is observed and discussed in the context of the interaction between the introduced fields and the species in the target area. First approaches toward an optimization of the observed bunching effect on the deposition material are illustrated.

Role of two-photon absorption in Ta2O5 thin films in nanosecond laser-induced damage.

Laser-induced damage in the nanosecond domain has been connected to the heating and breakdown of local defects within the thin film and the various interfaces. Within the femtosecond regime, the damaging events can be traced back to multiphoton-based excitation into the conduction band. When critical electron density is exceeded, an optical breakdown will occur. In this Letter we report on evidence that two-photon absorption also significantly triggers laser-induced damage in Ta(2)O(5) thin films at 532 nm and 8 ns pulse duration. For experimental verification, single layers of Ta(2)O(5)/SiO(2) mixtures have been analyzed.

Femtosecond laser damage resistance of oxide and mixture oxide optical coatings.

We report on the laser damage resistance of ion beam-sputtered oxide materials (Al2O3, Nb2O5, HfO2, SiO2, Ta2O5, ZrO2) and mixtures of Al2O3-SiO2, Nb2O5-SiO2, HfO2-SiO2, Ta2O5-SiO2, and ZrO2-SiO2, irradiated by single 500 fs pulses at 1030 nm. Laser-induced damage threshold (LIDT), refractive index, and bandgaps of the single-layer coatings are measured. For pure oxide materials a linear evolution of the LIDT with bandgap is observed. The results are in accordance with our simulations based on photo-ionization and avalanche-ionization. In the case of mixtures, however, deviations from the previous behaviors are evidenced. The evolution of the LIDT as a function of the refractive index is analyzed, and an empirical description of the relation between refractive index and LIDT is proposed.

Calculations and experimental demonstration of multi-photon absorption governing fs laser-induced damage in titania.

Laser damage phenomena are governed by a number of different effects for the respective operation modes and pulse durations. In the ultra short pulse regime the electronic structure in the dielectric coating and the substrate material set the prerequisite for the achieved laser damage threshold of an optical component. Theoretical considerations have been done to assess the impact of contributing ionization phenomena in order to find a valid description for laser-induced damage in the femtosecond (fs) domain. Subsequently, a special set of sample has been designed to verify these considerations via ISO certified laser damage testing. Examining the theoretical and experimental data reveals the importance of multi-photon absorption for the optical breakdown. For titania, the influence of multi-photon absorption has been clearly shown by a quantized wavelength characteristic of the laser damage threshold.

Highly efficient transmission gratings in fused silica for chirped-pulse amplification systems.

We report on highly efficient transmission gratings in fused silica with a grating period of 800 nm generated by electron-beam lithography. At a wavelength of 1060 nm, 95% diffraction efficiency is achieved under Littrow conditions. The damage threshold, extremely enhanced compared with conventional gold-coated diffraction gratings, makes these gratings the key elements in high average power (>100 W) femtosecond fiber chirped-pulse amplification systems.