Eberhard Welsch

High-efficiency dielectric reflection gratings: design, fabrication, and analysis

We report on reflection gratings produced entirely of dielectric materials. This gives the opportunity to enhance the laser damage threshold over that occurring in conventional metal gratings used for chirped-pulse-amplification, high-power lasers. The design of the system combines a dielectric mirror and a well-defined corrugated top layer to obtain optimum results. The rules that have to be considered for the design optimization are described. We optimized the parameters of a dielectric grating with a binary structure and theoretically obtained 100% reflectivity for the -1 order in the Littrow mounting for a 45 degrees angle of incidence. Subsequently we fabricated gratings by structuring a low-refractive-index top layer of a multilayer stack with electron-beam lithography. The multilayer system was fabricated by conventional sputtering techniques onto a flat fused-silica substrate. The parameters of the device were measured and controlled by light scatterometer equipment. We measured 97% diffraction efficiency in the -1 order and damage thresholds of 4.4 and 0.18 J/cm(2) with 5-ns and 1-ps laser pulses, respectively, at a wavelength of 532 nm in working conditions.

Photothermal measurements on optical thin films

An overview of the application of the photothermal technique for optical as well as thermophysical characterizations of thin films is given. The peculiarities of this technique are discussed in some detail, and selected important results are pointed out. Emphasis is placed on the influence of both residual absorption and randomly distributed inhomogeneities in thin films on their laser-damage resistance.

Measuring the absolute absorptance of optical laser components.

The precise determination of the absolute absorptance of a laser component is of high scientific and commercial importance. Our intention is to demonstrate that laser calorimetry can be a reliable and sensitive characterization tool for this purpose. Furthermore, the limitations of laser calorimetry are discussed and suggestions for possible revisions of the ISO 11551 (International Organization for Standardization, Geneva, Switzerland) standard are made. Finally, laser calorimetry is compared with photothermal deflection methods with respect to their practicability in different fields of laser optic characterization.

Investigation of the absorption induced damage in ultraviolet dielectric thin films

The interaction of UV laser radiation with optical coatings is investigated by a pulsed two-probe-beam photothermal technique. UV laser damage resistance studies on LaF3/MgF2, Al2O3/SiO2 , HfO2/SiO2 multilayer stacks are performed at ? = 248 nm and t520 ns. By investigating the relationship of the number of high-low (HL) pairs and the substrate material, optical and thermal coating properties are shown to be responsible for UV single-shot laser damage. The damage threshold of selected samples is influenced by the deposition technique. The influence of the bandgap energy of typical UV thin film oxide materials on the damage is investigated. Furthermore, multishot damage measurements on LaF3/MgF2 high-reflection multilayer coatings reveal the accumulation of laser energy in the predamage range and lead to an increase in absorption.

Femtosecond laser damage of a high reflecting mirror

Abstract Multiple pulse investigations of 130-fs Ti:sapphire laser-induced damage of a high reflecting mirror consisting of alternating λ/4-layers of Ta 2 O 5 and SiO 2 and a single 500-nm Ta 2 O 5 film were performed. In both cases, fused silica served as the substrate. For a fixed number of 1000 laser pulses per spot, a decrease in the damage threshold fluence of the mirror by a factor of two was observed by changing the repetition rate from 10 Hz to 1 kHz. A single 500-nm Ta 2 O 5 film shows higher damage resistance compared to the mirror. The mirror and the Ta 2 O 5 film samples were partially coated with a 300-nm-thick aluminium layer. The aluminium coating does not influence the damage threshold of the dielectrics underneath.

Photothermal Surface Deformation Technique—a Goal for Nondestructive Evaluation in Thin-film Optics

Abstract Using a localized excitation source such as an appropriately focused laser beam to illuminate a sample under investigation, the photohermoelastic material response leads, necessarily, to a localized surface deformation. A suitable theoretical calculation has been performed showing the characteristic dependence of this absorption-induced surface deformation on optical and thermophysical parameters as well as on experimental conditions. For frequently existing, experimentally realized limiting cases, useful analytical expressions have been derived interpreting both depth and laterally resolved photothermal measurements of material parameters of thin-film-on-bulk optics. Thus, the photothermal surface deformation (PTD) technique is suggested as an alternative, contactless method in the field of nondestructive material evaluation (NDE).

Structure-related bulk losses in ZrO2 optical thin films

Abstract Results of measurements of bulk scattering as well as absorption losses of evaporated ZrO 2 single-layer films are presented. A special layer design was used to eliminate the losses originating from the film interfaces. The scattering and absorption measurements are performed at λ = 515 nm by means of total integrated scattering and photoacoustic techniques, respectively. Transmission electron micrographs of C-Pt replicas of cross-sections and electron diffraction studies reveal the correlations between bulk losses and morphology. The compositional depth profiles of the films were investigated by secondary neutral mass spectrometry. The observed absorption can be explained by contaminations homogeneously distributed throughout the film thickness. The results are discussed with respect to different deposition conditions and post-deposition annealing.

Pulsed top-hat beam thermal-lens measurement for ultraviolet dielectric coatings

A mode-mismatched surface thermal-lens technique with pulsed top-hat beam excitation and a near-field detection scheme are developed to measure in situ the thermoelastic response of ultraviolet dielectric coatings to excimer-laser (193- or 248-nm) irradiation. The thermal-lens technique is demonstrated to be not only convenient for accurate determination of the laser-induced damage threshold (LIDT) but also sensitive to measurement of the thermoelastic response of dielectric coatings irradiated with fluence far below the LIDT, and hence is shown to be appropriate for time-resolved predamage investigation. A minimum detectable surface displacement of approximately 0.002 nm is achieved with a simple experimental configuration. Nonlinear absorption as well as the nonlinear effect in laser conditioning of a LaF(3)/MgF (2) highly reflective dielectric coating are observed for what is believed to be the first time.

Separation of optical thin-film and substrate absorption by means of photothermal surface deformation technique

The photothermal surface deformation (PTD) technique is shown to be feasible to separate absorption losses of single‐layer film and substrate absorption from each other by a completely contactless detection of the thermoelastic sample response only appropriately varying the focus radius of the heating beam. This offers a method for a depth‐resolved distinction of absorptive regions within a layered structure by the help of lateral resolved PTD absorption measurements. Additionally, both the sensitivity and feasibility of the apparatus is demonstrated by selected results at 10.6 μm.

In situ measurement on ultraviolet dielectric components by a pulsed top-hat beam thermal lens

A simple and sensitive mode-mismatched thermal lens (TL) technique with a pulsed top-hat beam excitation and a near-field detection scheme is developed to measure in situ the thermoelastic and the thermooptical responses of ultraviolet (UV) dielectric coatings as well as bulk materials under excimer laser (193- or 248-nm) irradiations. Owing to its high sensitivity, the TL technique can be used for measurements at fluences far below the laser-induced damage threshold (LIDT). We report on the measurement of both linear and nonlinear absorption of the UV dielectric coatings and bulk materials as well as the investigation of time-resolved predamage phenomena, such as laser conditioning of highly reflective dielectric coatings and irradiation-induced changes of a coatings various properties. The pulsed TL technique is also a convenient technique for accurate measurement of the LIDT of dielectric coatings and for distinguishing different damage mechanisms: thermal-stress-induced damage or melting-induced damage.