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Large-bandwidth pulses produced by cutting-edge X-ray free-electron lasers (FELs) are of great importance in research fields like material science and biology. In this paper, a new method to generate high-power ultrashort FEL pulses with tunable spectral bandwidth with spectral coherence using a dielectric-lined waveguide without interfering operation of linacs is proposed. By exploiting the passive and dephasingless wakefield at terahertz frequency excited by the beam, stable energy modulation can be achieved in the electron beam and large-bandwidth high-intensity soft X-ray radiation can be generated. Three-dimensional start-to-end simulations have been carried out and the results show that coherent radiation pulses with duration of a few femtoseconds and bandwidths ranging from 1.01% to 2.16% can be achieved by changing the undulator taper profile. [ABSTRACT FROM AUTHOR]

With fast advances in enhancing the focusing/imaging resolution of Fresnel zone plate lenses toward sub-10 nm, low diffraction efficiency in connection with their rectangular zone shape still remains a big issue in both soft and hard X-ray microscopy. In hard X-ray optics, encouraging progress has recently been reported in our earlier attempts of high focusing efficiency by 3D kinoform shaped metallic zone plates, formed by greyscale electron beam lithography. This paper addresses our efforts towards high focusing/imaging efficiency by developing a novel dielectric kinoform zone plate lens for soft X-rays. The effects of the zone materials and zone shapes on the focusing/imaging quality were first theoretically investigated by a modified thin-grating-approximation method, revealing superior efficiencies of dielectric kinoform zone plates over rectangular ones in metals. Optical characterizations of replicated dielectric kinoform zone plates by greyscale electron beam lithography demonstrate a focusing efficiency of 15.5% with a resolution of 110 nm in the water window of X-rays. Apart from high efficiency, the novel kinoform zone plate lenses developed in this work exhibit significant advantages over conventional zone plates, i.e. simplified process, low cost and no need for a beamstop. [ABSTRACT FROM AUTHOR]

The interaction of two subsequent ultra-short sub-milli-Joule laser pulses with a thin water flow results in an emission of a strong single-cycle THz pulse associated with enhanced soft X-ray emission. In this paper, a chain of processes produced in this interaction is analyzed and compared with other THz generation studies. It is demonstrated that the enhanced THz and X-ray emissions are produced by an energetic electron beam accelerated in the interaction of a main laser pulse with liquid water ejected from the surface by the pre-pulse. This scheme thus provides an efficient laser energy conversion in a THz pulse, avoiding laser self-focusing and filamentation in air. [ABSTRACT FROM AUTHOR]

A phase-merging enhanced harmonic generation free-electron laser (FEL) was proposed to increase the harmonic conversion efficiency of seeded FELs and promote the radiation wavelength towards the X-ray spectral region. However, this requires a specially designed transverse gradient undulator (TGU) as the modulator to couple the transverse and longitudinal phase space of the electron beam. In this paper, the generation of the phase-merging effect is explored using the natural field gradient of a normal planar undulator. In this method, a vertical dispersion on the electron beam is introduced and then the dispersed beam travels through a normal modulator in a vertical off-axis orbit where the vertical field gradient is selected properly in terms of the vertical dispersion strength and modulation amplitude. The phase-merging effect will be generated after passing through the dispersive chicane. Theoretical analysis and numerical simulations for a seeded soft X-ray FEL based on parameters of the Shanghai Soft X-ray FEL project are presented. Compared with a TGU modulator, using the natural gradient of a normal planar modulator has the distinct advantage that the gradient can be conveniently tuned in quite a large range by adjusting the beam orbit offset. [ABSTRACT FROM AUTHOR]

The Paul Scherrer Institute is implementing laser-based seeding in the soft X-ray beamline (Athos) of its free-electron laser, SwissFEL, to enhance the temporal and spectral properties of the delivered photon pulses. This technique requires, among other components, two identical modulators for coupling the electron beam with an external laser with a wavelength range between 260 and 1600 nm. The design, magnetic measurements results, alignment, operation and also details of the novel and exotic magnetic configuration of the prototype are described. [ABSTRACT FROM AUTHOR]

Elliptical polarized undulators (EPUs) are broadly used in the soft X‐ray energy range. They have the advantage of providing photons with both varied energy and polarization through adjustments to the value of the gap and/or shift magnet arrays in an undulator. Yet these adjustments may create a disturbance on the stability of the electron beam in a storage ring. To correct such a disturbance, it is necessary to establish a feed‐forward table of key nodes in the gap‐shift‐defined two‐dimensional parameter space. Such a table can only be scanned during machine‐study time. For a free‐walking mode, whereby an undulator is allowed to manoeuvre in the whole gap‐shift space, all the key nodes need to be scanned at the expense of a large amount of machine‐study time. This will greatly delay the employment of a full‐polarization capable undulator (especially circularly polarized). By analyzing data‐collecting patterns of user experiments, this paper defines a reduced set of key nodes in gap‐shift parameter space, with the number of key nodes to be scanned for feed‐forwarding scaled down to one‐third of the original; and introduces a new walking mode for EPUs: confined‐walking mode, whereby the undulator is manoeuvred only within the reduced set of key nodes. Such a mode is firstly realized on the EPUs at the DREAMLINE beamline at Shanghai Synchrotron Radiation Facility (SSRF). Under confined‐walking mode, the undulator movements are stable and there is no obvious disturbance to the electron beam with the feed‐forward system in operation. Successful experiments have been carried out using the circularly polarized light obtained via the new walking mode. This mode is expected to be applied to future EPUs at SSRF with the increasing requirements for various polarization modes. [ABSTRACT FROM AUTHOR]

Attosecond soft X-ray pulses are of great importance for the study of ultrafast electronic phenomena. In this paper, a feasible method is proposed to generate isolated fully coherent attosecond soft X-ray free electron laser via optical frequency beating. Two optical lasers with the opposite frequency chirps are used to induce a gradient frequency energy modulation, which helps to generate a gradually varied spacing electron pulse train. Subsequently, the undulator sections with electron beam delay lines are used to amplify the target ultra-short radiation. Numerical start-to-end simulations have been performed and the results demonstrate that an isolated soft X-ray pulse with the peak power of 330 GW and pulse duration of 620 as can be achieved by the proposed technique. [ABSTRACT FROM AUTHOR]

A microbunching instability (MBI) driven by beam collective effects can deteriorate electron beam quality in advanced x-ray free electron lasers. In this paper, we demonstrate a scheme to suppress microbunching instability by adjusting the compression ratio of two-stage bunch compressor in linear accelerator of the Shanghai soft X-ray Free-Electron Laser User Facility (SXFEL-UF). The calculation of the microbunching instability gain and the mechanism of MBI suppression scheme can be revealed by the theoretical model in this paper. Numerical simulations are presented to verify the analytical results. Furthermore, benefit by the high temporal resolution of transverse deflecting structure (TDS), beam experiments for direct analysis of MBI were carried out at SXFEL-UF. Theoretical calculations as well as simulation and consistent experimental results illustrate that the suppression of microbunching instability can be achieved by increasing the first stage compression ratio of the linac in the SXFEL-UF. The feasible theme provides a promising beam optimization method for future X-FEL facilities. [ABSTRACT FROM AUTHOR]

Laser–plasma accelerators (LPAs) have great potential to realize a compact X-ray free-electron laser (FEL), which is limited by the beam properties currently. Two-color high-intensity X-ray FEL provides a powerful tool for probing ultrafast dynamic systems. In this paper, we present a simple and feasible method to generate a two-color X-ray FEL pulse based on an LPA beam. In this scheme, time-dependent mismatch along the bunch is generated and manipulated by the designed lattice system, enabling FEL lasing at different wavelength within two undulator sections. The time separation between the two pulses can be precisely adjusted by varying the time-delay chicane. Numerical simulations show that two-color soft X-ray FELs with gigawatt-level peak power and femtosecond duration can be generated, which confirm the validity and feasibility of the scheme. [ABSTRACT FROM AUTHOR]

The Korea fourth-generation storage ring (Korea-4GSR) has a low electron beam emittance of 60 p.m.-rad, and provides a photon beam that is 100 times brighter and 100 times more coherent than the third-generation Pohang Light Source (PLS-II). However, the new source imposes a higher heat load on beamline optics. In particular, the first mirror (M1), which receives the broad-bandwidth white beam will be distorted by the thermal load from X-ray beam and the heat bump will degrade the beam quality. For the thermal analysis of M1, a method is developed to solve the thermal deformation with temperature gradient and absorbed power density. To implement the method in the analysis of heat load effect on M1, a benchmark study was performed on beamline 8A at PLS-II. In this paper, we describe the thermal analysis of M1 in Korea-4GSR after verifying our simulation method in a benchmark study. [ABSTRACT FROM AUTHOR]

Together with one of its variants, the recently proposed phase-merging enhanced harmonic generation (PEHG) free-electron laser (FEL) is systematically studied in this paper. Different from a standard high-gain harmonic generation scheme, a transverse gradient undulator is employed to introduce a phase-merging effect into the transversely dispersed electron beam in PEHG. The analytical theory of the phase-merging effect and the physical mechanism behind the phenomenon are presented. Using a representative set of beam parameters, intensive start-to-end simulations for soft x-ray FEL generation are given to illustrate the performance of PEHG. Moreover, some practical issues that may affect the performance of PEHG are also discussed. [ABSTRACT FROM AUTHOR]

Wire X-pinches (WXPs) have been studied comprehensively as fast (∼ 1 ns pulse width), small (∼ 1 μ m) x-ray sources, created by twisting two or more fine wires into an "X" to produce a localized region of extreme magnetic pressure at the cross-point. Recently, two alternatives to the traditional WXP have arisen: the hybrid X-pinch (HXP), composed of two conical electrodes bridged by a thin wire or capillary, and the laser-cut foil X-pinch (LCXP), cut from a thin foil using a laser. We present a comparison of copper wire, hybrid, and laser-cut foil X-pinches on a single experimental platform: UC San Diego's ∼ 200 kA, 150 ns rise time GenASIS driver. All configurations produced 1–2 ns pulse width, ≤ 5 μ m soft x-ray (Cu L-shell, ∼ 1 keV) sources (resolutions diagnostically limited) with comparable fluxes. WXP results varied with linear mass and wire count, but consistently showed separate pinch and electron-beam-driven sources. LCXPs produced the brightest (∼ 1 MW), smallest (≤ 5 μ m) Cu K-shell sources, and spectroscopic data showed both H-like Cu K α lines indicative of source temperatures ≥ 2 keV, and cold K α (∼ 8050 eV) characteristic of electron beam generated sources, which were not separately resolved on other diagnostics (within 1–2 ns and ≤ 200 μ m). HXPs produced minimal K-shell emission and reliably single, bright, and small L-shell sources after modifications to shape the early current pulse through them. Benefits and drawbacks for each configuration are discussed to provide potential X-pinch users with the information required to choose the configuration best suited to their needs. [ABSTRACT FROM AUTHOR]

Carbon/Phenolic (C/P), a typical anisotropic material, is an important component of aerospace and often used to protect the thermodynamic effects of strong X-ray radiation. In this paper, we establish the anisotropic elastic-plastic constitutive model, which is embedded in the in-house code "RAMA" to simulate a two-dimensional thermal shock wave induced by X-ray. Then, we compare the numerical simulation results with the thermal shock wave stress generated by the same strong current electron beam via experiment to verify the correctness of the numerical simulation. Subsequently, we discuss and analyze the rules of thermal shock wave propagation in C/P material by further numerical simulation. The results reveal that the thermal shock wave represents different shapes and mechanisms by the radiation of 1 keV and 3 keV X-rays. The vaporization recoil phenomenon appears as a compression wave under 1 keV X-ray irradiation, and X-ray penetration is caused by thermal deformation under 3 keV X-ray irradiation. The thermal shock wave propagation exhibits two-dimensional characteristics, the energy deposition of 1 keV and 3 keV both decays exponentially, the energy deposition of 1 keV-peak soft X-ray is high, and the deposition depth is shallow, while the energy deposition of 3 keV-peak hard X-ray is low, and the deposition depth is deep. RAMA can successfully realize two-dimensional orthotropic elastoplastic constitutive relation, the corresponding program was designed and checked, and the calculation results for inspection are consistent with the theory. This study has great significance in the evaluation of anisotropic material protection under the radiation of intense X-rays. [ABSTRACT FROM AUTHOR]

In its startup configuration the SASE3 beamline of the European XFEL provides only soft X-ray radiation, linearly polarized in the horizontal plane. In order to enhance capabilities of this beamline an afterburner scheme is proposed. It will be used as a coherent radiator using the micro-bunched electron beam of the linear SASE3 system. Radiation with variable polarization, which covers the full SASE3 wavelength range can be generated. For the radiator a new type of undulator design called Apple-X will be used. In this paper the design is described and magnet parameters, which are compatible with the SASE3 afterburner are determined using RADIA simulations. The end structure of such a device is optimized for minimum 1st field integrals. [ABSTRACT FROM AUTHOR]

In this paper we present in a first part the latest results of our group which are in relation with the study of DNA damages inflicted by low energy electrons (0-20 eV) in ultra-high vacuum as well as in air under atmospheric conditions. A short description of the drop-casting technique we developed to produce thin and nanometre-scaled DNA layers onto graphite sheets is given. We provide the absolute cross-section for loss of supercoiled topology of plasmid DNA complexed with 1,3-diaminopropane (Dap) in the vacuum under 10 eV electron impact and suggest a specific pathway for the dissociation of the transient negative ion formed by resonant capture of such a low energy electron (LEE) by the DNA's phosphate group when complexed to Dap. Well-gauged DNA-Dap layers with various nanometre-scaled thicknesses are used to evaluate the effective attenuation length of secondary photo-LEEs in the energy range (0–20 eV). The values of 11–16 nm for DNA kept under atmospheric conditions are in good agreement with the rare literature data available and which are stemming from computer simulations. In a second part, we describe the method we have developed in order to expose liquid samples of plasmid DNA to ultra-soft X-rays (Al Kα line at 1.5 keV) under hydroxyl radical scavenging conditions. We provide an experimentally determined percentage of indirect effects in aqueous medium kept under standard conditions of 94.7±2.1% indirect effects; in satisfactory agreement with the data published by others (i.e. 97.7%) relative to gamma irradiation of frozen solutions (Tomita et al., 1995). [ABSTRACT FROM AUTHOR]

The X-ray pulse originating from high altitude nuclear detonation (HAND) is mainly soft X-ray and its intensity is high enough to gasify the penetrated material and then lead to the severe thermo-mechanical deformation of unpenetrated material from the gasified blow-off effect. This effect cannot be directly reproduced in a lab for the lack of the X-ray source like HAND. At present, the low-energy relativistic electron beams resulting from an electron accelerator are usually used to approximately reproduce this effect, but the difference in the energy-deposited profile in materials between the electron and X-ray cannot be eliminated. In this paper, the symmetric linear least squares method was used to optimize the electron spectrum, and the general Monte Carlo N-Particle Transport Code calculations showed the optimized spectrum can produce the same energy-deposited profile in aluminum, copper, and tantalum with the soft X-rays like 1 keV or 3 keV spectrums. This indicates that it is possible to simulate the severe thermo-mechanical deformation resulting from HAND using the optimized electron spectrums. [ABSTRACT FROM AUTHOR]

In this study, we implement attosecond afterburners in an ultrashort soft X-ray free-electron laser (FEL) to improve the performance of generating attosecond pulses. In this scheme, the FEL pulse produced in the normal radiator section is dumped while the well bunched electron beam is reserved and reused in downstream afterburners. Subsequently, radiation in the afterburners gains rapidly as the bunching factor in the current spike is large, making the radiation pulse much shorter and cleaner than that from a normal radiator. Multi-shot simulations are carried out to demonstrate the performance and stability of the proposed technique. [ABSTRACT FROM AUTHOR]

X-ray free-electron lasers (XFELs) are powerful tools to explore and study nature for achieving remarkable advances. Generally, seeded FELs are ideal sources for supplying fully coherent soft x-ray pulses. Benefiting from the high-frequency up-conversion efficiency, the cascading configuration with echo-enabled harmonic generation (EEHG) and high-gain harmonic generation (HGHG) holds promising prospects for generating fully coherent radiation at 1 nm wavelength. In this paper, we design and optimize EEHG-HGHG configuration using parameters of Shanghai High-Repetition-Rate XFEL and Extreme Light Facility. In addition, we systematically analyze the effect of relative timing jitter on the output FEL performance based on various start-to-end electron beams. The intensive numerical simulations show that the cascaded EEHG-HGHG scheme can achieve 1 nm FEL pulses with peak power up to 15 GW. Further sensitivity analysis indicates that the relative timing jitter between the electron beam and seed laser has a significant impact on the FEL performance. The RMS timing jitter of 3 fs can lead to the final output pulse energy fluctuations of 29%. [ABSTRACT FROM AUTHOR]

We present a scheme to generate synchronised THz and soft X-ray radiation pulses by using a free-electron laser oscillator driven by a high repetition rate (of order 10–100 MHz) energy recovery linac. The backward THz radiation in the oscillator cavity interacts with a successive electron bunch, thus producing few 10 5 soft/hard X-ray photons per shot (namely 10 12 – 10 13 photons/s) via Thomson/Compton back-scattering, synchronised with the mJ-class THz pulse within the temporal jitter of electron beams accelerated in the superconducting cavities of the linac (less than 100 fs). Detailed simulations have been performed in order to assess the capability of the scheme for typical wavelengths of interest, between 10 and 50 μ m for the TeraHertz radiation and 0.5–3 nm for the X-rays. [ABSTRACT FROM AUTHOR]

Cavity Beam Position Monitor (CBPM) adopting a resonant cavity structure and using the characteristic modes excited by the electron beam to measure the beam parameters, has the advantage of high position resolution and is widely used in Free-electron Laser (FEL) facilities. The wake field of different modes carries different bunch information, the amplitude and phase of the signals of different modes can be extracted through the signal processing method to obtain the characteristic parameters of the source bunch. In the application of bunch charge and position measurement, the accurate amplitude extraction method for cavity BPM signal is the primary issue to be considered when designing the data acquisition and processing system. In this paper, through theoretical analysis and numerical simulation, it is proved that the optimal algorithm of amplitude extraction for CBPM exists, and the dependence between the data processing window size and the decay time of the cavity BPM under the optimal design is given. In addition, the relationship between the optimized amplitude extraction uncertainty and the noise-to-signal ratio, sampling rate of data acquisition and processing system, and the decay time of the cavity BPM is also proposed, which can provide clear guidance for the design and optimization of the CBPM system. Accordingly, the CBPM system in Shanghai Soft X-ray FEL (SXFEL) facility is optimized, and the resolution improved from 273 nm to 177 nm at the bunch charge of 500 pC. [ABSTRACT FROM AUTHOR]

High resolution Fresnel zone plates for nanoscale 3D imaging of materials by both soft and hard X-ray are growingly needed in broad applications in nanoscience and technology. When the outmost zone-width approaches 30 nm or even below, electro-deposition of gold into narrow trenches replicated by electron beam lithography in resist becomes increasingly difficult. Pulse electroplating instead of conventional direct-current one has been attempted in this work to overcome the problem in the au deposition in narrow trenches. Careful study of the au deposition rate as the function of adjustment parameters including the anodic bias, the pulse frequency and the deposition time has been systematically carried out. The optimized window of the parameters is adopted in the fabrication of 30 nm zone plates with the aspect ratio from 3:1 up to 7:1 and the duty cycle close to 0.5. Imaging of in-house made Siemens stars with 30 nm as the narrowest line-width has been successfully demonstrated in soft X-ray (706 eV). The origin of electroforming of well-structured au zones by pulse electroplating is discussed This paper reports our latest progress in 30 nm Fresnel zone plates by e-beam lithography and well controlled pulse voltage electroplating. When the outmost zone-width approaches 30 nm or even below, electro-deposition of gold into narrow trenches replicated by electron beam lithography in resist becomes increasingly difficult. Pulse electroplating instead of conventional direct-current one has been attempted in this work to overcome the problem in the Au deposition in narrow trenches The optimized window of the parameters is adopted in the fabrication of 30 nm zone plates with the aspect ratio from 3:1 up to 7:1 and the duty cycle close to 0.5. Imaging of in-house made Siemens stars with 30 nm as the narrowest line-width has been successfully demonstrated in soft X-ray (706 eV). Unlabelled Image [ABSTRACT FROM AUTHOR]

A fabrication method comprising near‐field holography (NFH) with an electron beam lithography (EBL)‐written phase mask was developed to fabricate soft X‐ray varied‐line‐spacing gratings (VLSGs). An EBL‐written phase mask with an area of 52 mm × 30 mm and a central line density greater than 3000 lines mm−1 was used. The introduction of the EBL‐written phase mask substantially simplified the NFH optics for pattern transfer. The characterization of the groove density distribution and diffraction efficiency of the fabricated VLSGs indicates that the EBL–NFH method is feasible and promising for achieving high‐accuracy groove density distributions with corresponding image properties. Vertical stray light is suppressed in the soft X‐ray spectral range. [ABSTRACT FROM AUTHOR]

A portable high resolution soft x-ray and extreme ultraviolet (EUV) spectrometer has been developed for spectroscopic research at the Shanghai Electron Beam Ion Trap (EBIT) laboratory. A unique way of aligning the grazing incidence spectrometer using the zero order of the grating is introduced. This method is realized by extending the range of the movement of the CCD detector to cover the zero order. The alignment can be done in a few minutes, thus leading to a portable spectrometer. The high vacuum needed to be compatible with the EBITs is reached by mounting most of the translation and rotation stages outside the chamber. Only one high vacuum compatible linear guide is mounted inside the chamber. This is to ensure the convenient interchange of the gratings needed to enable wavelength coverage of the whole range of 10 to 500 Å. Spectra recorded with one of our low energy EBITs shows that a resolving power of above 800 can be achieved. In the slitless configuration used in this work, we found the resolving power to be limited by the width of the EBIT plasma. When mounted on the Shanghai EBIT which is a high energy EBIT and has a narrower EBIT plasma width, the estimated resolving power will be around 1400 at 221.15 Å. [ABSTRACT FROM AUTHOR]

A conceptual design for a soft x-ray free-electron laser (FEL) using a short-pulsed, high energy near infrared laser undulator and a low-emittance modest-energy (∼170 MeV) electron beam is described. This low-cost design uses the laser undulator beam in a nearly copropagating fashion with respect to the electron beam, instead of the traditional 'head-on' fashion. The nearly copropagating geometry reduces the Doppler shift of scattered radiation to yield soft, rather than hard x-rays. To increase the FEL gain a sheared laser pulse from a Ti : sapphire or other broadband laser is used to extend the otherwise short interaction time of the nearly copropagating laser undulator beam with a relativistic electron beam. [ABSTRACT FROM AUTHOR]

Design of a clean, high-brightness light source is presented for extreme ultraviolet/soft x-ray (EUV/SXR) lithography research and mask inspection. Basic characteristics of classical laser-Compton scattering are reviewed, and the laser and electron beam parameters at relatively low energy (EUV to SXR) photon generation are optimized. Recent achievements in each component technology are evaluated on a continuous wave (CW)-operated electron linac and energy recovery linac system, based on superconducting technologies at a 1.3 GHz operation frequency, 10 kW average power, short pulse CO2 laser, and optical super cavity with a 600-enhancement- factor at 10.6 ^m wavelength. Combining both the CW electron beam and short pulse CO2 laser with super-cavity enhancement, 1 mW/2% b.w. flux and 30 kW/mm²/sr/2% b.w. brightness laser-Compton source is designed at 6.7-nm wavelength. The technological gap in the present component technologies are discussed, as well as any further required developments. [ABSTRACT FROM AUTHOR]

Future observatories utilizing reflection grating spectrometers for extreme ultraviolet (EUV) and soft X-ray (SXR) spectroscopy require high-fidelity gratings with both blazed groove facets and custom groove layouts that are often fanned or feature a slight curvature. While fabrication procedures centering on wet anisotropic etching in monocrystalline silicon produce highly efficient blazed gratings, the precision of a nonparallel groove layout is limited by the cubic structure of the silicon crystal. This motivates the pursuit of alternative techniques to grating manufacture, namely thermally activated selective topography equilibration (TASTE), which uses gray-scale electron-beam lithography to pattern multilevel structures in resist followed by an optimized polymer thermal reflow to smooth the 3D patterns into continuous surface relief profiles. Using TASTE, a mold for a reflection grating with a periodicity of 400 nm and grooves resembling an asymmetric sawtooth was patterned in 130 nm thick poly(methyl methacrylate) resist on a silicon substrate over a 50 mm by 7.5 mm area. This structure was coated with 15 nm of gold by electron-beam physical vapor deposition using titanium as an adhesion layer and then tested for EUV and SXR diffraction efficiency at beamline 6.3.2 of the Advanced Light Source synchrotron facility. Results demonstrate a quasi-blaze response characteristic of a 27◦ blaze angle with groove facets smooth to 1.5 nm rms. Absolute peak-order efficiency ranges from 75% to 25%, while total relative efficiency measures ⪆90% across the measured bandpass of 15.5 nm > λ > 1.55 nm. [ABSTRACT FROM AUTHOR]