Schwarzschild Objective Research Articles

Computational Imaging at the Infrared Beamline of the Australian Synchrotron Using the Lucy–Richardson–Rosen Algorithm

The Fourier transform infrared microspectroscopy (FTIRm) system of the Australian Synchrotron has a unique optical configuration with a peculiar beam profile consisting of two parallel lines. The beam is tightly focused using a 36× Schwarzschild objective to a point on the sample and the sample is scanned pixel by pixel to record an image of a single plane using a single pixel mercury cadmium telluride detector. A computational stitching procedure is used to obtain a 2D image of the sample. However, if the imaging condition is not satisfied, then the recorded object’s information is distorted. Unlike commonly observed blurring, the case with a Schwarzschild objective is unique, with a donut like intensity distribution with three distinct lobes. Consequently, commonly used deblurring methods are not efficient for image reconstruction. In this study, we have applied a recently developed computational reconstruction method called the Lucy–Richardson–Rosen algorithm (LRRA) in the online FTIRm system for the first time. The method involves two steps: training step and imaging step. In the training step, the point spread function (PSF) library is recorded by temporal summation of intensity patterns obtained by scanning the pinhole in the x-y directions across the path of the beam using the single pixel detector along the z direction. In the imaging step, the process is repeated for a complicated object along only a single plane. This new technique is named coded aperture scanning holography. Different types of samples, such as two pinholes; a number 3 USAF object; a cross shaped object on a barium fluoride substrate; and a silk sample are used for the demonstration of both image recovery and 3D imaging applications.

Fast fabrication of diffractive patterns on glass by excimer laser ablation

ArF excimer laser ablation is used for the periodic patterning of glass surfaces. Projection of phase gratings provides the parallel generation of high resolution line patterns with structure periods in the µm-range. A fast and efficient process is possible, if laser pulse repetition and work piece motion are synchronized, so that multiple pulses can be applied to each work piece position, while continuously moving the work piece. A writing speed of 0.1 cm2/s is demonstrated and the scalability to 1 cm2/s is presented. Sub-micron periods are obtained by using Schwarzschild objectives. The process is applied for high contrast diffractive marking of glass.

Система визуализации плазменного факела бетатронного источника рентгеновского излучения

The paper describes the design of a microscope for studying a betatron radiation source based on the PEARL femtosecond laser complex in the SXR and EUV wavelength range. The main optical element of the microscope is a spherical Schwarzschild objective a x5 magnification. The device allows to study the size and spatial structure of the interaction area of laser radiation with matter, at a selected wavelength in the EUV or SXR range with a resolution of δx = 2.75 microns. The operation wavelength (=13.5 nm) is set by multilayer X-ray mirrors. Thin-film absorption filters are used to suppress the background component of the signal.

Imaging system of a plasma torch of a Betatron X-ray source

The paper describes the design of a microscope for studying a betatron radiation source based on the PEARL femtosecond laser complex in the SXR and EUV wavelength range. The main optical element of the microscope is a spherical Schwarzschild objective a x5 magnification. The device allows to study the size and spatial structure of the interaction area of laser radiation with matter, at a selected wavelength in the EUV or SXR range with a resolution of delta x=2.75 μm. The operation wavelength (λ=13.5 nm) is set by multilayer X-ray mirrors. Thin-film absorption filters are used to suppress the background component of the signal. Keywords: SXR and EUV radiation, betatron radiation, imaging x-ray optics, SXR microscope.

Projection Objective For an EUV-Lithographic Workbench

A two-mirror projection Schwarzschild objective (reduction ratio ×5), formed by two aspheric mirrors and developed as part of upgrading a lithographic workbench with the operating wavelength of 13.4 nm, is proposed. The measurement procedure and results of correction of the shape of the aspheric mirrors and aberrations of the objective lens in the assembled version are described in detail. As a result of several corrections of local errors by an ion beam, a root-mean-square deviation of the shape of the primary mirror of 0.8 nm is achieved. The objective-lens aberrations were equal to 5.4 nm. Simulation of the resolving power of the objective lens, taking into account the measured aberrations, shows that 200-nm-wide strips are resolved with a contrast of 32%, 100 nm—23%, and 30 nm—15%. The reflectivities of the multilayer Mo/Si mirrors with an anti-stress Cr/Y sublayer were 63–65% depending on the coordinate on the mirror. The reasons why it was not able to correct the aberration of the objective lens to the level of 1 nm necessary for achievement of the diffraction limit of resolution are discussed.

Design method for an aspheric Schwarzschild objective with a low obscuration and an infinite conjugate distance

This paper proposes a design method for an aspheric Schwarzschild objective with a low obscuration and infinite conjugate distance. The method takes the obscuration ratio as a priority condition, establishes the relationship between magnification and image distance of the infinite reflector objective, and solves the expressions of aspheric curvature radius and aspheric coefficient. It overcomes the constraint of concentric sphere in the conventional design method and effectively reduces the obscuration ratio. For different structural parameters of the Schwarzschild objective, the model can be used to obtain an initial structure whose imaging quality is close to the diffraction limit.

Sample Volume Reduction Using the Schwarzschild Objective for a Circular Dichroism Spectrophotometer and an Application to the Structural Analysis of Lysine-36 Trimethylated Histone H3 Protein.

With the increasing interest in scarce proteins, reducing the sample volume for circular dichroism (CD) spectroscopy has become desirable. Demagnification of the incident beam size is required to reduce the sample volume for CD spectroscopy detecting transmitted light passed through the sample. In this study, the beam size was demagnified using a focal mirror, and small-capacity sample cells were developed in an attempt to reduce the sample volume. The original beam size was 6 × 6 mm2; we successfully converged it to a size of 25 × 25 μm2 using the Schwarzschild objective (SO). The new sample cell and SO allowed the required sample volume to be reduced to 1/10 (15 → 1.5 μL), when using a 15 μm path length cell. By adopting a smaller sample cell, further sample reduction could be achieved. By using the SO system, the secondary structural contents of the lysine-36 trimethylated histone H3 protein were analyzed. The trimethylation induced the increment of helix structures and decrement of unordered structures. These structural alterations may play a role in regulating cellular function(s), such as DNA damage repair processes.

A table-top EUV focusing optical system with high energy density using a modified Schwarzschild objective and a laser-plasma light source.

For investigating extreme ultraviolet (EUV) damage on optics, a table-top EUV focusing optical system was developed in the laboratory. Based on a modified Schwarzschild objective with a large numerical aperture and a laser-plasma light source, this optical system can generate a focusing spot with the maximum energy density of 2.27 J/cm2 at the focal plane of the objective at the wavelength of 13.5 nm. The structures and the characterized properties of this optical system are presented in this paper. For testing the performance of this setup, single-shot EUV damage tests were carried out on an optical substrate of CaF2 and a gold thin film. The damage thresholds and morphologies of these samples were given and discussed with former research studies.

Four-mirror flat-field anastigmat aplanat telescopes with simple closed-form solutions

A four-mirror anastigmat aplanat telescope can be constructed using a Gregorian-form Mersenne afocal telescope as a feeder for a concentric spherical Schwarzschild objective. A flat image field can be obtained by adjusting the ratio of the radii of the mirrors. With the tertiary at the image of the aperture stop (the primary) the primary's conic K1 can be varied, using the tertiary's conic K3 to cancel the primary's spherical aberration without affecting the system's coma or astigmatism. For a spherical primary, the tertiary is an oblate ellipsoid (K3>0). The minimum obscuration ratio is 44.7% (20% by area).

A sensitive EUV Schwarzschild microscope for plasma studies with sub-micrometer resolution

We present an extreme ultraviolet (EUV) microscope using a Schwarzschild objective which is optimized for single-shot sub-micrometer imaging of laser-plasma targets. The microscope has been designed and constructed for imaging the scattering from an EUV-heated solid-density hydrogen jet. Imaging of a cryogenic hydrogen target was demonstrated using single pulses of the free-electron laser in Hamburg (FLASH) free-electron laser at a wavelength of 13.5 nm. In a single exposure, we observe a hydrogen jet with ice fragments with a spatial resolution in the sub-micrometer range. In situ EUV imaging is expected to enable novel experimental capabilities for warm dense matter studies of micrometer-sized samples in laser-plasma experiments.

45.5X Infinity Corrected Schwarzschild Microscope Objective Lens Design

In this article, the design of a 45.5X (numerical aperture (NA) =0.5) infinity corrected, or infinite conjugate, Schwarzschild reflective microscope objective lens is discussed. Fast Fourier transform modulation transfer function (FFT MTF= 568.4 lines/mm at 50% contrast for the on-axis field-of-view), root-mean-square wavefront error (RMS WFE= 0.024 waves at 700 nm), point spread function (PSF, Strehl ratio= 0.972), encircled energy (0.88 µm spot radius at 80% fraction of enclosed energy), optical path difference (OPD=-0.644 waves) and Seidel coefficients calculated with Zemax® are provided to show that the design is diffraction-limited and aberration-free. Furthermore, formulas expressing the relationship between the parameters of the two spherical mirrors and the Schwarzschild objective lens focal length are given. In addition, tolerance and sensitivity analysis for the Schwarzschild objective lens, two spherical mirrors indicate that tilting the concave mirror (or secondary mirror) has a higher impact on the modulation transfer function values than tilts introduced by the convex mirror (or primary mirror). Finally, the performed tolerance and sensitivity analysis on the lens design suggests that decentering any of the mirrors by the same distance has the same effect on the modulation transfer function values.

Wide-Spectrum Microscope with a Long Working Distance Aspherical Objective Based on Obscuration Constraint.

We present an approach for an initial configuration design based on obscuration constraint and on-axis Taylor series expansion to realize the design of long working distance microscope (numerical aperture (NA) = 0.13 and working distance (WD) = 525 mm) with a low obscuration aspherical Schwarzschild objective in wide-spectrum imaging (λ = 400–900 nm). Experiments of the testing on the resolution target and inspection on United States Air Force (USAF) resolution chart and a line charge-coupled device (CCD) (pixel size of 14 μm × 56 μm) with different wavelength light sources (λ = 480 nm, 550 nm, 660 nm, 850 nm) were implemented to verify the validity of the proposed method.

X-ray optical system for imaging laser plumes with a spatial resolution of up to

We consider an X-ray optical system which permits obtaining laser plume images at a wavelength of with a resolution of up to . The X-ray optical system comprises an X-ray Schwarzschild objective made up of two aspherical multilayer mirrors, a scintillator (YAG : Ce ceramics), which converts X-rays to the visible radiation, and a visible-optical system, which images the scintillator surface onto a CCD camera. The spatial resolution of the system is limited by the resolution of the optical system and the magnification of the X-ray objective and is as high as . The effect of Schwarzschild objective mirror shapes on the spatial resolution is analysed. The profile of concave mirror aspherisation is considered, which provides the attainment of the diffraction-limited quality of the objective. Data are given for the quantum efficiency of the system at a wavelength of . We describe the experimental test bench intended for studying the developed X-ray optical system and outline the first experimental data which illustrate its efficiency. Owing to the natural division into the ‘X-ray’ and ‘visible’ parts, the optical system under discussion permits an easy change of the magnification and the field of view without realigning the X-ray optical elements. The wavelength may be varied in a range between 3 and by changing the multilayer mirrors.

Miniature Schwarzschild objective as a micro-optical component free of main aberrations: concept, design, and first realization with silicon-glass micromachining.

This paper presents the conception of a new micro-optical component fabricated within the wafer-level approach: a micromachined reflective objective, the so-called micro-Schwarzschild objective, characterized by superior optical performances than widespread microlenses. The system, made of two vertically integrated mirrors, works in transmission similarly as microlenses. While the specific geometric configuration of the two-mirrors allows elimination of most common optical aberrations, the reflective architecture provides inherent achromaticity. This paper presents in detail the optical design and analyzes fabrication tolerances. It also describes a fabrication flow chart based on silicon micromachining done at the wafer level that could allow production of thousands of such micro-optical devices within a single fabrication run. The realized prototype employs the two-step KOH etching process to generate the micromirror pairs followed by glass reflow for the secondary mirror generation and selective metallic deposition. Despite an insufficient mirror quality attributed to this specific silicon etching technique and highlighted by the reflective configuration, the objective fabrication in terms of alignment, bonding, and coating is shown as feasible.

Fabrication of 250-nm-hole arrays in glass and fused silica by UV laser ablation

Parallel nanohole drilling in glass using an ArF excimer laser (193nm) is demonstrated. For the first time, hole arrays with 500nm pitch and individual holes with 250nm diameter and more than 100nm depth are fabricated by phase mask imaging using a Schwarzschild objective. Holes in soda lime glass are drilled by direct ablation; fused silica is processed by depositing a SiOx-film on SiO2, patterning the SiOx by ablation, and finally oxidizing the remaining SiOx to SiO2. Thermally induced ordered dewetting of noble metal films deposited on such templates may be used for the fabrication of plasmonic devices.

Sub-micrometer resolution proximity X-ray microscope with digital image registration.

A compact laboratory proximity soft X-ray microscope providing submicrometer spatial resolution and digital image registration is described. The microscope consists of a laser-plasma soft X-ray radiation source, a Schwarzschild objective to illuminate the test sample, and a two-coordinate detector for image registration. Radiation, which passes through the sample under study, generates an absorption image on the front surface of the detector. Optical ceramic YAG:Ce was used to convert the X-rays into visible light. An image was transferred from the scintillator to a charge-coupled device camera with a Mitutoyo Plan Apo series lens. The detector's design allows the use of lenses with numerical apertures of NA = 0.14, 0.28, and 0.55 without changing the dimensions and arrangement of the elements of the device. This design allows one to change the magnification, spatial resolution, and field of view of the X-ray microscope. A spatial resolution better than 0.7 μm and an energy conversion efficiency of the X-ray radiation with a wavelength of 13.5 nm into visible light collected by the detector of 7.2% were achieved with the largest aperture lens.

Long working distance microscope with a low obscuration aspherical Schwarzschild objective.

For fine stitching of multiwindow wide-spectrum detectors, an aspherical Schwarzschild objective with an effective working distance of 525 mm has been realized for chromatic aberration-free imaging in 400-900 nm wavelength range and with a numerical aperture of 0.13. A theoretical approach for analytical design of an initial configuration has been modeled using an on-axis Taylor series expansion, and the beam obscuration ratio (OR) has been controlled down to 4%. In comparison against the theoretical spherical-based Schwarzschild objective, 77.5% of the OR has been reduced for improved contrast.

Spatial characterization of the focus produced by an EUV Schwarzschild objective

Schwarzschild objectives are used in the EUV spectral range because of their large aperture, high mechanical stability and excellent achromaticity. The large aperture results in a small, theoretically diffraction limited focus diameter with ideal values of below 200 nm with the current configuration. We employed a zone plate with matched numerical aperture (0.19) to image the focus onto an X-ray CCD camera. Emission from high harmonic generation and a liquid-jet laser-plasma were used as light sources. Images at magnifications of about 150-fold were acquired and focus diameters of 300 nm (FWHM) were observed.

High spatial resolution ZnO scintillator for an in situ imaging device in EUV region

A single shot image of a ZnO crystal excited by the EUV laser of Kansai Photon Science Institute was captured. The evaluated EUV beam waist radii from the ZnO emission pattern along the horizontal and vertical axes are 5.0 and 4.7μm, respectively. The expected focal spot size of EUV laser and the spatial resolution of the magnifier (including the Schwarzschild objectives and lenses) are however 1 and 4μm, respectively. The discrepancy on the spatial resolutions is attributed to exciton diffusion. We estimated the ZnO exciton diffusion length from the effective decay time which is shortened by exciton–exciton collision quenching and which is dependence on excitation energy density. Our results indicate that the short lifetime of ZnO is required to improve the spatial resolution.

Polarization sensitive ultrafast mid-IR pump probe micro-spectrometer with diffraction limited spatial resolution

A setup of ultrafast transient infrared IR spectrometer is described in this paper that employed Schwarzschild objectives to focus the probe beam to a diffraction limited spot. Thus measurements were performed with very high spatial resolution in the mid-IR spectral region. Furthermore, modulating the polarization of the probe light enabled detecting transient dichroism of the sample. These capabilities of the setup were applied to study transient absorption of Photosystem II core complex and to image an organized film of methylene blue chloride dye. Moreover, a study of noise sources in a pump probe measurement is presented. The predicted noise level of the current setup was 8.25 μOD in 10(4) acquisitions and compared very well with the experimental observation of 9.6 μOD.