Design of an efficient illuminator for partially coherent sources in the extreme ultraviolet.

In this paper, the design of an efficient illuminator for extreme ultraviolet (EUV) applications such as photolithography, metrology, and microscopy is investigated. Illuminators are arrangements of optical components that allow us to tailor optical parameters to a targeted application. For the EUV spectral range, illuminators are commonly realized by an arrangement of several multilayer mirrors. Within this publication, design methods are developed to tailor optical parameters such as the intensity distribution, the spatial coherence, and the spectral bandwidth by using only one multilayer mirror. For the demonstration of the methods, an illuminator is designed for a compact in-lab EUV interference lithography system that is suited for industrial EUV resist qualification and large-area nanopatterning. The designed illuminator increases the wafer-throughput and improves the imaging quality.

Automated lens design for optical systems consisting of stock lenses.

The design of lens systems requires advanced knowledge and the mastery of highly specialized software tools. Furthermore, for the realization of the designed lens systems often custom-made lenses are needed, which are expensive and have lead times of several weeks compared to stock lenses with several days. To shorten realization time, a new approach for the automated design of lens systems consisting of stock lenses is developed. In this work, a multi-step process is described which identifies the most robust stock lens combination fulfilling prior defined requirements. The approach is realized with a computer program that can be used by a non-expert to find the most suited selection of stock lenses for a three-lens system for a set of requirements.

Rigorous approach for unifying wave optics and state of the art rendering techniques.

With the capabilities of diffractive optics there is a rising demand for determining the light interaction of diffractive elements with arbitrary illumination and scenery. Since the structured surfaces' scale lies within the visible wavelengths and below, the light's interaction cannot be simulated with state of the art geometric optic rendering approaches. This paper presents a new model for the inclusion of wave-optical effects into Monte Carlo path rendering concepts. The derived method allows the coupling of a rigorous full-field approach with the concept of backward ray propagation through complex scenes. Therefore, the rendering of arbitrarily structured periodic optical components is now possible for complex sceneries for design, verification and testing purposes. The method's performance is demonstrated by comparing rendering results of complex sceneries including CDs with corresponding photographs.

Freeform optics design for extended sources in paraxial approximation exploiting the expectation maximization algorithm.

Freeform optics generating specific irradiance distributions have been used in various applications for some time now. While most freeform optics design algorithms assume point sources or perfectly collimated light, the search for algorithms for non-idealized light sources with finite spatial as well as angular extent is still ongoing. In this work, such an approach is presented where the resulting irradiance distribution of a freeform optical surface is calculated as a superposition of pinhole images generated by points on the optical surface. To compute the required arrangement of the pinhole images for a prescribed irradiance pattern, the expectation maximization algorithm from statistics is applied. The result is then combined with a ray-targeting approach for finding the shape of the corresponding freeform optical surface. At its current state, the approach is applicable to Gaussian input irradiances, single-sided freeform optics and for the paraxial case. An example freeform optical surface for laser material processing is shown and discussed demonstrating the performance and the limitations of the presented approach.

Characterization of nanoscale gratings by spectroscopic reflectometry in the extreme ultraviolet with a stand-alone setup.

The authors present a study on the dimensional characterization of nanoscale line gratings by spectroscopic reflectometry in the extreme ultraviolet spectral range (5 nm to 20 nm wavelength). The investigated grating parameters include the line height, the line width, the sidewall angle and corner radii. The study demonstrates that the utilization of shorter wavelengths in state-of-the-art optical scatterometry provides a high sensitivity with respect to the geometrical dimensions of nanoscale gratings. Measurable contrasts are demonstrated for dimensional variations in the sub-percent regime, down to one tenth of a nanometer and one tenth of a degree in absolute terms. In an experimental validation of the method, it is shown that reflectance curves can be obtained in a stand-alone setup using the broadband emission of a discharge produced plasma as the source of EUV radiation, demonstrating the potential scalability of the method for industrial uses. Simulated reflectance curves are fit to the experimental curves by variation of the grating parameters using rigorous electromagnetic modeling. The obtained grating parameters are cross-checked by a scanning electron microscopy analysis.

Design of structured YAG:Ce scintillators with enhanced outcoupling for image detection in the extreme ultraviolet.

In this Letter, the authors present a design study on YAG:Ce scintillator plates with a microstructured and coated surface. The goal of the study is to improve the outcoupling efficiency and to optimize the directionality of the scintillation light with respect to indirect image detection in the extreme ultraviolet spectral range (5-50 nm wavelength). In a geometric optical simulation, a gain in outcoupling efficiency by over a factor of 4 is shown while the directionality of the scintillation light is improved with respect to state-of-the-art plane scintillator plates.

Laser cutting of bone tissue under bulk water with a pulsed ps-laser at 532 nm.

Hard-tissue ablation was already investigated for a broad variety of pulsed laser systems, which cover almost the entire range of available wavelengths and pulse parameters. Most effective in hard-tissue ablation are Er:YAG and CO2 lasers, both utilizing the effect of absorption of infrared wavelengths by water and so-called explosive vaporization, when a thin water film or water­air spray is supplied. The typical flow rates and the water layer thicknesses are too low for surgical applications where bleeding occurs and wound flushing is necessary. We studied a 20 W ps-laser with 532 nm wavelength and a pulse energy of 1 mJ to effectively ablate bones that are submerged 14 mm under water. For these laser parameters, the plasma-mediated ablation mechanism is dominant. Simulations based on the blow-off model predict the cut depth and cross-sectional shape of the incision. The model is modified considering the cross section of the Gaussian beam, the incident angle, and reflections. The ablation rate amounts to 0.2 mm3/s, corresponding to an increase by at least 50% of the highest values published so far for ultrashort laser ablation of hard tissue.

Optical set-up for dynamic superposition of three laser beams for structuring and polishing applications.

Structuring by remelting is an innovative approach for structuring metallic surfaces with laser radiation, where no material is removed but reallocated while molten. Based on this remelting principle an innovative structuring technique is investigated, where laser beams are superposed. A melt pool is generated by a cw laser beam with constant feed rate. A pulsed laser is superposed onto the cw laser and evaporates a small amount of molten material and, therefore, generates vapour pressure, which shapes the melt pool surface. The solidification follows this newly shaped surface. For this process a new optical system was designed and built up, which allows the combination of cw and pulsed laser beams.

39.1 µJ picosecond ultraviolet pulses at 355 nm with 1 MHz repeat rate.

Based on our reliable high-power picosecond laser source with high beam qualities, we designed a compact and efficient third harmonic generation scheme by cascading a frequency doubling and a sum frequency generation using LBO as the nonlinear material. A maximum output of 39.1 W with a repeat rate of 1 MHz at 355 nm was obtained, which implied a pulse energy of 39.1 µJ, which was the highest picosecond UV pulse energy with an all-solid-state setup so far. The total conversion efficiency from infrared to UV was up to 46%. And the output UV has excellent beam qualities with an M-square factor less than 1.1.

Deformation of ultra-short laser pulses by optical systems for laser scanners.

Current experiments of processing glass with ultra-short laser pulses (< 1 ps) lead to scan angle depending processing results. This scan angle depending effect is examined by simulations of a common focusing lens for laser scanners. Due to dispersion, focusing lenses may cause pulse deformations and increase the pulse duration in the focal region. If the field angle of the incoming laser beam is variable, the pulse deformation may also vary as a function of the field angle. By ray tracing as well as wave optical simulations we investigate pulse deformations of optical systems for different scan angles.

Limitations of the ray mapping approach in freeform optics design.

It was previously demonstrated by Bäuerle et al. [Opt. Express20, 14477 (2012)] that the computation of ray paths through the optical system (ray mapping) can be used to design multisurface freeform optical elements creating a prescribed irradiance pattern for a zero-étendue source. The procedure outlined there uses the heuristic step of reducing the ray mapping's curl to improve adherence to surface integrability criteria. This Letter formally derives a quantitative estimate for the limitations of this approach in the collimated case and shows the key factors influencing the quality of the final optics.

High resolution irradiance tailoring using multiple freeform surfaces.

More and more lighting applications require the design of dedicated optics to achieve a given radiant intensity or irradiance distribution. Freeform optics has the advantage of providing such a functionality with a compact design. It was previously demonstrated in [Bäuerle et al., Opt. Exp. 20, 14477-14485 (2012)] that the up-front computation of the light path through the optical system (ray mapping) provides a satisfactory approximation to the problem, and allows the design of multiple freeform surfaces in transmission or in reflection. This article presents one natural extension of this work by introducing an efficient optimization procedure based on the physics of the system. The procedure allows the design of multiple freeform surfaces and can render high resolution irradiance patterns, as demonstrated by several examples, in particular by a lens made of two freeform surfaces projecting a high resolution logo (530 × 160 pixels).

Using adaptive weighted least squares approximation for coupling thermal and optical simulation.

Thermo-optical simulation is a mandatory enhancement of classical ray tracing, since nowadays many fields in the branch of optical technology have to deal with thermal effects. This paper discusses an approach for coupling the finite element method (FEM) and ray tracing simulation by processing finite element (FE) data using scattered data approximation techniques, particularly with an adaptive weighted least squares approximation algorithm in two dimensions. The validation of the implemented interface is being conducted by comparing approximated data to analytical functions. Finally, FEM data are being processed by the developed algorithm to demonstrate the applicability on appropriate problems.

Algorithm for irradiance tailoring using multiple freeform optical surfaces.

The design of freeform lenses and reflectors allows to achieve non-radially symmetric irradiance distributions whilst keeping the optical system compact. In the case of a point-like source, such as an LED, it is often desired to capture a wide angle of source light in order to increase optical efficiency. This generally results in strongly curved optics, requiring both lens surfaces to contribute to the total ray refraction, and thereby minimising Fresnel losses. In this article, we report on a new design algorithm for multiple freeform optical surfaces based on the theory of optimal mass transport that adresses these requirements and give an example of its application to a problem in general lighting.

Diode-pumped efficient slab laser with two Nd:YLF crystals and second-harmonic generation by slab LBO.

We demonstrate a diode-pumped electro-optical Q-switched slab laser with a high optical efficiency, high pulse energy, and short pulse width with two Nd:YLF crystals inside one resonator. The single compact slab resonator can generate a 1D top-hat beam at both the far field and the near field. With a slab-geometry-design lithium triborate (LBO) crystal, efficient critical phase-matching second-harmonic generation for a 1D top-hat beam with multiple transverse modes is achieved.

Serial cerebrospinal fluid tryptophan and 5-hydroxy indoleacetic acid concentrations in healthy human subjects.

The role of the serotonergic system in the pathogenesis of behavioral disorders such as depression, alcoholism, obsessive-compulsive disorder, and violence is not completely understood. Measurement of the concentration of neurotransmitters and their metabolites in cerebrospinal fluid (CSF) is considered among the most valid, albeit indirect, methods of assessing central nervous system function in man. However, most studies in humans have measured lumbar CSF concentrations only at single time points, thus not taking into account rhythmic or episodic variations in levels of neurotransmitters, precursors, or metabolites. We have continuously sampled lumbar CSF via subarachnoid catheter in 12 healthy volunteers, aged 20-65 years. One ml (every 10 min) CSF samples were collected at a rate of 0.1ml/min for 24-hour (h), and the levels of tryptophan (TRP) and 5-hydroxy indoleacetic acid (5-HIAA) were measured. Variability across all 12 subjects was significantly greater (P < 0.0001) than the variability seen in repeated analysis of a reference CSF sample for both 5-HIAA (32.0% vs 7.9%) and TRP (25.4% vs 7.0%), confirming the presence of significant biological variability during the 24-hr period examined. This variability could not be explained solely by meal related effects. Cosinor analysis of the 24-hr TRP concentrations from all subjects revealed a significant diurnal pattern in CSF TRP levels, whereas the 5-HIAA data were less consistent. These studies indicate that long-term serial CSF sampling reveals diurnal and biological variability not evident in studies based on single CSF samples.