Varied Line Spacing Research Articles

Soft X-ray spectrometers based on aperiodic reflection gratings and their application

This paper is concerned with the history, properties, development, application, and prospects of soft X-ray (2 – 300 Å) VLS spectrometers, i.e., spectrometers with reflection diffraction gratings whose spacing varies monotonically across the aperture according to a prescribed law (so-called Varied Line-Space (VLS) gratings). An important feature of grazing-incidence VLS spectrometers is that the spectrum is formed on a nearly flat surface perpendicular (or slightly inclined) to the diffracted beams, making them perfectly compatible with modern CCD detectors. VLS spectrometers are employed for the spectroscopy of laboratory and astrophysical plasmas, including the diagnostics of relativistic laser-produced plasmas, for measuring the linewidth of an X-ray laser, for recording the high-order harmonics of laser radiation, and for recording the emission of fast electric discharges and other laboratory X-ray sources. Instruments with VLS gratings are employed to advantage in reflectometry/metrology, X-ray fluorescence analysis, and microscopy with the use of synchrotron, free-electron laser, and laser-produced plasma radiation, as well as in SXR emission spectroscopy, combined with an electron microscope (SXES). Recent years have seen the active development of VLS spectrometers dedicated to the investigation of the electronic structure of different materials and molecules by resonant inelastic X-ray scattering (RIXS) spectroscopy with synchrotron radiation. Among recent trends is the development of VLS gratings with a multilayer reflective coating and extension of the operating spectral range towards ‘tender’ X-rays (ℏω ∼ 1.5 – 6 keV), some projects aiming to achieve a resolving power λ/δ λ ∼ 105 in the region ℏω ∼ 1 keV.

Angular sensing system based on Y-type twin-core fiber and reflective varied-line spacing grating fabricated by electron beam lithography

A varied-line spacing grating (VLSG) inscribed by electron beam lithography was proposed and used for an experimental demonstration of angular sensing. The designed VLSG central groove density was 655 L/mm, and it had an aluminized surface to reflect light. A Y-type twin-core fiber of 600 μm diameter cores connected with a collimator lens was used to input and collect reflected light from the VLSG. The Y-type fiber input and output paths were connected to a white light source and an optical spectrum analyzer, respectively. The fabricated quasi-continuous VLSG size was 70 × 30 mm, with a fixed 23° input angle. The reflected wavelength changed from 917.3 to 1656.9 nm as the incident light spot moved to different grating positions, with a displacement sensitivity of 14.812 nm/mm. As the input angle was adjusted from 23° to 35° in 1° increments, the spot location on the VLSG changed simultaneously, and the reflection wavelength peak positions changed from 917.3 to 1584.5 nm, with a sensitivity of 57.544 nm/° and linearity of 0.993.

Flat-field VLS spectrometer for a wavelength range of 50 – 275 Å

A flat-field VLS spectrograph for a wavelength range ∼50 – 275 Å with an average linear dispersion of 0.18 mm Å−1, which makes use of a grazing-incidence varied line-space (VLS) grating, was calculated and implemented (a spectrograph of Harada class). To fabricate the VLS grating by interference lithography technique, an algorithm was developed for calculating the writing configuration with an auxiliary aberrating mirror (the solution of the inverse problem of interference lithography). The spectrograph was put to a test and line spectra were recorded from the laser-produced plasma of lithium fluoride and teflon targets, which were excited by a focused laser beam (0.5 J, 8 ns, 1.06 μm). A resolving power λ/d l = 800 was demonstrated in a wavelength region of 135 Å.

Switchable pupil expansion propagation using orthogonal superposition varied-line-spacing H-PDLC gratings in a holographic waveguide system.

To solve the problem of small field of view in traditional holographic waveguides, this paper proposes a waveguide-coupling technique using orthogonal superposition varied-line-spacing (VLS) gratings. These gratings expand the x and y directions and orthogonally superimpose to achieve transmission in a waveguide and enlarge the pupil field of view. At the optical coupling input end and the coupling output end of a waveguide, one-dimensional VLS gratings are used to realize horizontal expansion of the image. Then a vertical VLS grating is orthogonally superimposed at the exit end to realize vertical expansion of the image. Finally, the waveguide transmission and expansion of the image are completed. In the experiment, holographic polymer dispersed liquid crystal one-dimensional VLS gratings in the x and y directions are fabricated and coupled with a waveguide. An image source with a diameter of 0.5cm is waveguide-transferred and coupled out, and then passed through the vertical grating. Amplification is performed to obtain an expanded image of a diameter of 2.28cm. In this study, the diffraction characteristics of the grating used to realize pupil expansion in the holographic waveguide system are analyzed and simulated. It is calculated that the diffraction efficiency of the VLS gratings can reach 80% or more in the 532nm band. Additionally, the characteristics of an electronically controlled switch are studied. Experimental results show that the method can be used for expanding the field of view and can be applied to waveguide systems for image transmission and expansion.

Scanning spectrometer/monochromator for a wavelength range of 50 – 330 Å

A flat-field scanning spectrometer/monochromator of the Hettrick – Underwood type is implemented for a wavelength range λ ≈ 50 – 330 Å. The optical arrangement of the spectrograph comprises a focusing spherical (R = 6000 mm) mirror, mounted at a grazing angle of 8.34 °, and a grazing-incidence plane varied line-space (VLS) grating operating for a constant deflection angle of 16.68 °. The entrance and output slits of the instrument are immobile, and the focal distance varies only slightly over its operating spectral range. The short-wavelength boundary of the operating range is determined by the spectral source brightness and the reflection coefficients of the grating and the mirror rather than by defocusing. Laser-produced plasma spectra excited by a focused laser beam (0.5 J, 8 ns, 1.06 μm) are recorded. A spectral resolving power λ/δλ = 1300 is demonstrated at a wavelength of 182 Å. The configuration of the instrument of this type is suited to soft X-ray reflectometry and metrology with the use of laser-plasma and synchrotron radiation sources, and is perfectly compatible with modern CCD detectors.

Grating metrology for X-ray and V-UV synchrotron beamlines at SOLEIL.

Diffraction gratings are key elements of soft X-ray synchrotron beamlines. Besides wavelength dispersion, specific parameters can be tailored to adjust the energy dependent efficiency and focusing, and to correct wavefront aberrations. As key elements of a beamline, any departure from the design values can severely reduce the overall performance. On the other hand, known non-conformities can often be corrected by slight adjustment of the alignment parameters. A careful and accurate metrology is therefore required before installation on the beamline. After presenting what variable line spacing gratings, variable groove depth gratings, and alternate multilayer gratings are, the use of the SOLEIL long trace profiler for the measurement of groove density variation along the surface and of the atomic force microscope for the groove geometry and roughness characterizations will be discussed. A few examples of grating metrology will be presented and analyzed with the help of optical simulations.

一种新型keV波段超高分辨光栅光谱仪设计

This paper presents a novel grating spectrometer design that combines a convexly cylindrical mirror with a concavely cylindrical varied line spacing grating to achieve an extreme resolving power of 33000-70000 in the photon energy range of 0.6-1.5 keV for a Gaussian light source with effective meridional size of 30 μm (root mean square) at an upstream initial object distance of 10 m. This type of spectrometer can be used to diagnose the radiation spectrum of a soft X-ray free electron laser and properly measure the spectral distribution and the detailed structure of the self-amplified spontaneous emission mode due to its extremely high resolving power. The overall optical aberrations of the system are well analyzed and compensated, providing an excellent flat field at the detector domain throughout the entire spectral range. A machine learning support vector machine algorithm is introduced into explore the optimal system to deliver the desired resolving performance. In addition, to enhance the spectral intensity and detection efficiency simultaneously, a sagittal confinement focusing mirror is introduced into the optical path of the spectrometer to reduce the astigmatism in spectral imaging. © 2019, Chinese Lasers Press. All right reserved.

A soft x-ray-ultraviolet (SUV) beamline and diffractometer for resonant elastic scattering and ultraviolet-vacuum ultraviolet reflectance at the Singapore synchrotron light source.

A new beamline and a six-circle UHV diffractometer have been constructed at the Singapore Synchrotron Light Source with a broad energy coverage from 3.5 to 1500 eV. The beamline is optimized for ultraviolet-vacuum-ultraviolet optical reflectivity and resonant soft X-ray scattering with medium energy resolution over a broad energy range, achieved by using a self-focusing monochromator consisting of a plane mirror and three variable line spacing gratings. The unique character of the diffractometer comprises 4-circles in the vertical plane and 2-circles in the horizontal plane. Thirteen motions are available inside the UHV chamber with a base pressure of 1 × 10-9 mbar. Two sample holders working independently over a temperature range of 37 K-400 K are controlled by a closed-cycle cryostat, while the bottom holder inside a high field compact pulsed magnet is available for measurements requiring a magnetic field.

X-ray spectroscopy with variable line spacing based on reflection zone plate optics.

X-ray spectroscopy is a method, ideally suited for investigating the electronic structure of matter, which has been enabled by the rapid developments in light sources and instruments. The x-ray fluorescence lines of life-relevant elements such as carbon, nitrogen, and oxygen are located in the soft x-ray regime and call for suitable spectrometer devices. In this Letter, we present a high-resolution spectrum of liquid water, recorded with a soft x-ray spectrometer based on a reflection zone plate (RZP) design. The RZP-based spectrometer with meridional variation of line space density from 2953 to 3757l/mm offers extremely high detection efficiency and, at the same time, medium energy resolution. We can reproduce the well-known splitting of liquid water in the lone pair regime with 10s acquisition time.

Non-homogeneous composite GMR structure to realize increased filtering range.

A non-homogeneous composite guided-mode resonant (GMR) filter structure is proposed that avoids the multi-mode resonance effect and increases resonant wavelength tuning range. The composite filter structure is engineered using a combination of a varied-line-spacing (VLS) grating layer with a wedge-shaped waveguide layer. The grating is fabricated by holographic interference lithography (IL), while the wedge-shaped layer is fabricated using masked ion beam etching (MIBE) technology. The resonant wavelength has been observed to vary as a function of the spatial position on the structure. In the fabricated structure, over a length of 30 mm, the grating period increment is measured to be 149.2 nm, whereas the increment of the waveguide film thickness is approximately 100 nm. Experimental results show that a primary reflectance peak is achieved spanning a wavelength range of 805.8-1119.0 nm. The device is designed using the rigorous coupled-wave analysis (RCWA) method, and the proposed device is toward the practical application of GMR filters.

High-resolution stigmatic spectrograph for a wavelength range of 125–30 nm

We describe a broadband (12.5-30 nm) extreme ultraviolet (XUV) spectrograph, which is stigmatic throughout its operating range. The instrument employs a near-normal-incidence aperiodic Mo/Si multilayer mirror and a grazing-incidence plane varied line-space (VLS) grating. Strict stigmatism is fulfilled simultaneously at two wavelengths and the condition of practical stigmatism is fulfilled over two octaves in wavelength. The vertically space-resolved line spectra of multiple charge ions from laser plasma were recorded to demonstrate a spectral resolving power of 103 and a spatial resolution of ~26 μm, both figures corresponding to two detector pixels. The electron density was evaluated from the Stark broadening of the Balmer line Hβ (135 Å) of C VI in the plasma excited by 0.5 J, 8 ns, 1.06 μm pulses.

Study of Higher Diffraction Order Contribution in a Flat Field Grating Spectrograph Using a High-Order Harmonic Source.

An experimental study has been carried out to characterize and investigate the performance of a flat-field grating spectrograph (FFGS) for higher diffraction order in the wavelength range of 80-170 Å. An aberration-corrected, mechanically ruled spherical grating with variable line spacing was used as a dispersive element in the spectrograph. The study was carried out using high-order harmonic radiation generated by an interaction of Ti:sapphire laser pulses with inert gas-filled cells. It was observed that the fraction of photons diffracted in the second diffraction order to that of the first order is as high as ∼ 65% at ∼ 80 Å wavelength, which reduces to ∼ 15% at ∼ 150 Å, whereas for the third diffraction order, the fraction was substantially lower, ∼ 15% at ∼ 100 Å. The observed results match well with reflectivity calculated using REFLEC software and also with the measurement carried out using INDUS-1 reflectivity beamline at RRCAT, Indore. The use of a high-order harmonic source for the study is advantageous, as the higher diffraction order contribution can be estimated from a single spectral image, which can be recorded in a single laser shot. The study will be useful for estimation of conversion efficiency-photon flux of a source using a variable line spaced (VLS) grating based spectrograph/monochromator.

LCLS in—photon out: fluorescence measurement of neon using soft x-rays

We measured the fluorescence photon yield of neon upon soft x-ray ionization (∼1200 eV) from the x-ray free-electron laser at Linac Coherent Light Source, and demonstrated the usage of a grazing incidence spectrometer with a variable line spacing grating to perform x-ray fluorescence spectroscopy on a gas phase system. Our measurements also allowed us to estimate the focal size of the beam from the theoretical description developed, in terms of the rate equation approximation accounting for photoionization shake off of neutral neon and double auger decay of single core holes.

Absolute, high-accuracy characterization of a variable line spacing grating for the European XFEL soft X-ray monochromator.

The European XFEL is a free-electron laser (FEL) with a superconducting linear accelerator and three beamlines (SASE1, SASE2, and SASE3) covering the energy range from 250 eV to 24 keV. The SASE3 beamline is dedicated to the soft x-ray range (0.25-3 keV) and is designed to operate in both monochromatic and non-monochromatic mode. A variable line spacing - plane grating (VLS-PG) monochromator is placed along the beam transport system for the monochromatic mode. The VLS parameters of the grating profile are challenging from a manufacturing and measuring perspective, especially at the desired length of 530 mm. A shorter grating of 150 mm has been procured to allow early operation of the facility. We describe the characterization method that was used to assess the VLS parameters of the grating using Fizeau interferometry. The method is intrinsically absolute and limited only by the quality of the test grating and the noise level. Further measurements using a white-light-interferometry profilometer are also reported. We discuss the possibility of extending the method to the future 530 mm long grating.

Design of the new soft X-ray beamline for in situ analysis of energy materials at National Synchrotron Radiation Laboratory

A new bending magnet based soft X-ray beamline for in situ analysis of energy materials at National Synchrotron Radiation Laboratory will be constructed and the optical design is presented. This beamline will mainly focus on nanomaterials and their applications in energy conversion, storage, and catalysis studies. A varied-line-spacing plane grating monochromator is employed to cover the photon energy region of 60–1000 eV by two plane varied line spacing gratings with central groove densities of 840 l/mm and 1400 l/mm respectively. The designed energy resolution power E∕ΔE is about 1300∼3000. The calculated flux is higher than 1×1010 phs/s under 300 mA ring beam current for the whole covered photon energy. This beamline will provide a powerful and efficiency instrument in the future for users doing research on nanomaterials, energy materials and catalytic science.

Large area nanoimprint enables ultra-precise x-ray diffraction gratings.

A process for fabrication of ultra-precise diffraction gratings for high resolution x-ray spectroscopy was developed. A grating pattern with constant or variable line spacing (VLS) is recorded on a quartz plate by use of e-beam lithography with nanometer scale accuracy of the groove placement. The pattern is transferred to a massive grating blank by large area nanoimprint followed by dry or/and wet etching for groove shaping. High fidelity of the nanoimprint transfer step was confirmed by differential wavefront measurements. Successful implementation of the suggested fabrication approach was demonstrated by fabrication of a lamellar 900 lines/mm VLS grating for a soft x-ray fluorescence spectrometer.

Polymeric spatial resolution test patterns for mass spectrometry imaging using nano-thermal analysis with atomic force microscopy.

As the spatial resolution of mass spectrometry imaging technologies has begun to reach into the nanometer regime, finding readily available or easily made resolution reference materials has become particularly challenging for molecular imaging purposes. This paper describes the fabrication, characterization and use of vertical line array polymeric spatial resolution test patterns for nano-thermal analysis/atomic force microscopy/mass spectrometry chemical imaging. Test patterns of varied line width (0.7 or 1.0μm) and spacing (0.7 or 1.0μm) were created in an ~1-μm-thick poly(methyl methacrylate) thin film using electron beam lithography. The patterns were characterized by scanning electron microscopy, energy-dispersive X-ray spectroscopy, atomic force microscopy topography and nano-thermal analysis/mass spectrometry imaging. The efficacy of these polymeric test patterns for the advancement of chemical imaging techniques was illustrated by their use to judge the spatial resolution improvement achieved by heating the ionization interface of the current instrument platform. The spatial resolution of the mass spectral chemical images was estimated to be 1.4μm, based on the ability to statistically distinguish 0.7-μm-wide lines separated by 0.7-μm-wide spacings in those images when the interface cross was heated to 200°C. This work illustrates that e-beam lithography is a viable method to create spatial resolution test patterns in a thin film of high molecular weight polymer to allow unbiased judgment of intra-laboratory advancement and/or inter-laboratory comparison of instrument advances in nano-thermal analysis/atomic force microscopy/mass spectrometry chemical imaging. Published in 2017. This article is a U.S. Government work and is in the public domain in the USA.

Tunable Filter With Varied-Line-Spacing Grating Fabricated Using Holographic Recording

In this letter, we experimentally demonstrate a compact and tunable guided-mode resonance (GMR) filter incorporates a varied-line-spacing (VLS) grating layer that is fabricated using holographic interference lithography. The reflectance wavelength varies as a function of the spatial position on the structure. In an interference exposure system, a cylindrical lens is placed between the collimating mirror and the sample to achieve a VLS grating at different positions on the photo-resist film. The grating period increment is approximately 108 nm over a length of 22 mm, which results in a primary reflectance peak that spans 762.1-908.3 nm. The device is designed using the rigorous coupled-wave analysis method, and the proposed device is directed toward the practical application of GMR tunable filters.

High-resolution compact spectrometer based on a custom-printed varied-line-spacing concave blazed grating.

We present the modeling, design and characterization of a compact spectrometer, achieving a resolution better than 1.5 nm throughout the visible spectrum (360-825 nm). The key component in the spectrometer is a custom-printed varied-line-space (VLS) concave blazed grating, where the groove density linearly decreases from the center of the grating (530 g/mm) at a rate of 0.58 nm/mm to the edge (528 g/mm). Parametric models have been established to deterministically link the system performance with the VLS grating design parameters, e.g., groove density, line-space varying rate, and to minimize the system footprint. Simulations have been performed in ZEMAX to confirm the results, indicating a 15% enhancement in system resolution versus common constant line-space (CLS) gratings. Next, the VLS concave blazed grating is fabricated via our vacuum nanoimprinting system, where a polydimethylsiloxane (PDMS) stamp is non-uniformly expanded to form the varied-line-spacing pattern from a planar commercial grating master (600 g/mm) for precision imprinting. The concave blazed grating is measured to have an absolute diffraction efficiency of 43%, higher than typical holographic gratings (~30%) used in the commercial compact spectrometers. The completed compact spectrometer contains only one optical component, i.e., the VLS concave grating, as well as an entrance slit and linear photodetector array, achieving a footprint of 11 × 11 × 3 cm3, which makes it the most compact and resolving (1.46 nm) spectrometer of its kind.

Highly efficient soft X-ray spectrometer based on a reflection zone plate for resonant inelastic X-ray scattering measurements.

We present a newly designed compact and flexible soft X-ray spectrometer for resonant inelastic X-ray scattering (RIXS) studies within an energy range from 380 eV to 410 eV, which would include the K alpha emission lines of vital elements like nitrogen. We utilized an off-axis reflection zone plate (RZP) as the wavelength selective element with a maximum line density of 10000 l/mm. A higher energy resolution over a broader range of ± 15 eV around the designed energy was achieved by displacing the RZP. Additionally, for the first time, an actual optical side effect, the so-called comatic aberration was exploited to increase the energy resolution. First results show a resolving power in the order of 1300 for photon energy of 395 eV, which is comparable to a commercial varied line spacing grating (VLS).