Reflective multilayer coatings for the far uv region

Extreme ultraviolet metalens by vacuum guiding

Extreme ultraviolet (EUV) radiation is a key technology for material science, attosecond metrology, and lithography. Here, we experimentally demonstrate metasurfaces as a superior way to focus EUV light. These devices exploit the fact that holes in a silicon membrane have a considerably larger refractive index than the surrounding material and efficiently vacuum-guide light with a wavelength of ~50 nanometers. This allows the transmission phase at the nanoscale to be controlled by the hole diameter. We fabricated an EUV metalens with a 10-millimeter focal length that supports numerical apertures of up to 0.05 and used it to focus ultrashort EUV light bursts generated by high-harmonic generation down to a 0.7-micrometer waist. Our approach introduces the vast light-shaping possibilities provided by dielectric metasurfaces to a spectral regime that lacks materials for transmissive optics.

Influence of seed layers on optical properties of aluminum in the UV range

The potential of titanium and copper seed layers to enhance the optical properties of aluminum films for ultra-violet (UV) applications is analyzed. The seed layers significantly influence the initial layer growth of aluminum films. For the titanium-seeded aluminum, a surface roughness of 0.34 nm was observed. UV spectral reflectance measurements showed an average higher reflectivity of 4.8% for wavelengths from 120 nm to 200 nm for the aluminum film grown on the titanium seed layer. Furthermore, the titanium-seeded aluminum coatings were stable at an elevated temperature of 225°C and showed no increase in surface roughness or pinholes.

Enhancement of THz resonance using a multilayer slab waveguide for a guided-mode resonance filter

We propose a multilayer slab waveguide (SWG) to enhance the resonance of the transmittance with a guided-mode resonance (GMR) filter. The resonance characteristics of the GMR filter were studied in three types according to the method of attaching the grating film to a SWG, which consists of 25 µm thick polyethylene terephthalate (PET) film layers separated by 25 µm air layers. The resonance depth with the multilayer SWG was improved over that of the monolayer SWG because the refractive index and absorption of the multilayer SWG were reduced. However, because resonance with a high Q-factor in the monolayer SWG has a large attenuation loss due to material absorption, the resonance enhancement was insufficient even for the multilayer SWG. We were able to make the resonance depth up to 5.2 times larger than the monolayer SWG in the TE_1,1 mode using a five-layer SWG. We verified the enhancements with the multilayer SWG using a finite-difference frequency-domain (FDFD) simulation.

Attosecond Pulse Shaping by Multilayer Mirrors

The emerging research field of attosecond science allows for the temporal investigation of one of the fastest dynamics in nature: electron dynamics in matter. These dynamics are responsible for chemical and biological processes, and the ability to understand and control them opens a new door of fundamental science, with the possibility to influence all lives if medical issues can thereby be addressed. Multilayer optics are key elements in attosecond experiments; they are used to tailor attosecond pulses with well-defined characteristics to facilitate detailed and accurate insight into processes, e.g., photoemission, Auger decay, or (core-) excitons. Based on the investigations and research efforts from the past several years, multilayer mirrors today are routinely used optical elements in attosecond beamlines. As a consequence, the generation of ultrashort pulses, combined with their dispersion control, has proceeded from the femtosecond range in the visible/infrared spectra to the attosecond range, covering the extreme ultraviolet and soft X-ray photon range up to the water window. This article reviews our work on multilayer optics over the past several years, as well as the impact from other research groups, to reflect on the scientific background of their nowadays routine use in attosecond physics.

Extended asymmetric-cut multilayer X-ray gratings

The fabrication and characterization of a large-area high-dispersion blazed grating for soft X-rays based on an asymmetric-cut multilayer structure is reported. An asymmetric-cut multilayer structure acts as a perfect blazed grating of high efficiency that exhibits a single diffracted order, as described by dynamical diffraction throughout the depth of the layered structure. The maximum number of grating periods created by cutting a multilayer deposited on a flat substrate is equal to the number of layers deposited, which limits the size of the grating. The size limitation was overcome by depositing the multilayer onto a substrate which itself is a coarse blazed grating and then polish it flat to reveal the uniformly spaced layers of the multilayer. The number of deposited layers required is such that the multilayer thickness exceeds the step height of the substrate structure. The method is demonstrated by fabricating a 27,060 line pairs per mm blazed grating (36.95 nm period) that is repeated every 3,200 periods by the 120-μm period substrate structure. This preparation technique also relaxes the requirements on stress control and interface roughness of the multilayer film. The dispersion and efficiency of the grating is demonstrated for soft X-rays of 13.2 nm wavelength.

Tunable Fabry-Perot interferometer from ferroelectric polymer based on surface energy modification

Surface energy modification was utilized in the fabrication of hollow transmission Fabry-Perot interferometer (FPI) for the first time. Polydimethylsiloxane (PDMS) was used to modify the surface energy of substrate for the self-assembly of poly(vinylidenefluoride-trifluoroethylene) [P(VDF-TrFE)] 70/30 mol% copolymer film on given areas, which is simple and low destructive for the photoelectric device. A strain of 7.12% under a field of 22.3 MV/m was observed from the copolymer film, which led to the FPI with a tunable range of 54 nm at wavelength of 604 nm.

Comparison of Mg-based multilayers for solar He II radiation at 30.4 nm wavelength

Mg-based multilayers, including SiC/Mg, Co/Mg, B(4)C/Mg, and Si/Mg, are investigated for solar imaging and a He II calibration lamp at a 30.4 nm wavelength. These multilayers were fabricated by a magnetron sputtering method and characterized by x-ray reflection. The reflectivities of these multilayers were measured by synchrotron radiation. Near-normal-incidence reflectivities of Co/Mg and SiC/Mg multilayer mirrors are as high as 40.3% and 44.6%, respectively, while those of B(4)C/Mg and Si/Mg mirrors are too low for application. The measured results suggest that SiC/Mg, Co/Mg multilayers are promising for a 30.4 nm wavelength.

Widely tunable reflection-type Fabry-Perot interferometer based on relaxor ferroelectric poly(vinylidenefluoride-chlorotrifluoroethylene-trifluoroethylene)

A reflection-type Fabry-Perot interferometer (FPI) with a large tunability has been demonstrated on relaxor ferroelectric poly(vinylidenefluoride-chlorotrifluoroethylene-trifluoroethylene) [P(VDFCTFE-TrFE)] 78.9/13.9/7.2 mol% with a thickness of 9.2 microm. The optical path length of the FPI is modulated by the electrostrictive strain of the terpolymer under electric field, where the low-loss distributed Bragg reflector and aluminium film are employed as the mirrors in the FPI. A positive strain of 20% has been achieved in the terpolymer film under a field of 30 MV/m, which leads to the FPI with a tunable range of more than 200 nm at wavelengths around 680 nm.

Reflection filter with high reflectivity and narrow bandwidth

A kind of reflection filter with a narrow bandwidth and high reflectivity is designed as being composed of an all-dielectric Fabry-Perot filter, a high-reflectivity Ag film, and especially an ultrathin metallic film, n approximately = k. Exemplary structures and some formulas concerning spectral-reflection performances, such as general reflectivity, maximum reflectivity, and the halfwidth of the reflection band, are given. Experiments show the validity of this design.

Modeling the image quality of enhanced reflectance x-ray multilayers as a surface power spectral density filter function

Residual surface roughness over the entire range of relevant spatial frequencies must be specified and controlled in many high-performance optical systems. This is particularly true for enhanced reflectance multilayers if both high reflectance and high spatial resolution are desired. If we assume that the interfaces making up a multilayer coating are uncorrelated at high spatial frequencies (microroughness) and perfectly correlated at low spatial and midspatial frequencies, then the multilayer can be thought of as a surface power spectral density (PSD) filter function. Multilayer coatings thus behave as a low-pass spatial frequency filter acting on the substrate PSD, with the exact location and shape of this cutoff being material and process dependent. This concept allows us to apply conventional linear systems techniques to the evaluation of image quality and to the derivation of optical fabrication tolerances for applications utilizing enhanced reflectance x-ray multilayers.

Ultraviolet narrow-band rejection filters composed of multiple metal and dielectric layers

Band rejection filters with a bandwidth as narrow as <5% that operate in the UV region can be constructed from multiple thin metal films and insulating layers. A physical model is derived to illustrate the dependence of the wavelength of maximum reflectance and rejection bandwidth on the optical constants of metal and dielectrics, on the thickness, and on the number of repetitive layers. This design prevails over conventional dielectric-dielectric coatings because of its development and simplicity in the fabricating process and its applicability to large spectral ranges.

Extreme ultraviolet performance of a multilayer coated high density toroidal grating

The performance of a multilayer coated diffraction grating has been evaluated at EUV wavelengths both in terms of absolute efficiency and spectral resolution. The application of a ten-layer Ir/Si multilayer coating to a 3600-lines/mm blazed toroidal replica grating produced a factor of 9 enhancement in peak efficiency near the design wavelength approximately 30 nm in first order, without degrading its excellent quasistigmatic spectral resolution. The measured EUV efficiency peaked at 3.3% and was improved over the full spectral range between 25 and 35 nm compared with the premultilayer replica which had a standard gold coating. In addition, the grating's spectral resolution of >5000 was maintained.

Performance of multilayer coated diffraction gratings in the EUV

Performance of multilayer coated diffraction gratings has been evaluated in the extreme UV. The application of multilayer coatings to two blazed gratings has produced a significant enhancement in grating efficiency in the 300-A spectral region in first order.

Refractive index of amorphous carbon near its K-edge

The refractive index of the amorphous carbon layers inside multilayer soft x-ray mirrors is derived in the lambda = 42-58-A wavelength range by measuring the shift in the Bragg angle caused by refraction. Reflectivity curves are measured with a reflectometer behind a zone plate monochromator at the National Synchrotron Light Source. The monochromator consists only of a freestanding zone plate of gold and an exit slit and is free of any of the contamination problems often found in monochromators that contain mirrors.

Comparative efficiency of natural crystals and multilayers as dispersing devices in the copper L(2,3) range

Recent sputtering techniques have been used to produce layered synthetic microstructures (LSMs) as dispersing devices for varied applications in x-ray optics and spectroscopy. These analyzers, specially suited for synchrotron radiation, have been mounted in a two-parallel crystal monochromator. In this paper we show the first experimental results obtained with beryl crystals and multilayers for analyzing x-ray spectral distributions transmitted through screens or reflected on mirrors of copper near the L(2) and L(3) absorption edges. The significance of these findings is discussed in terms of comparison with natural and synthetic crystals and of designing a useful dispersing device for x-ray spectroscopy.

Layered synthetic microstructures as Bragg diffractors for X rays and extreme ultraviolet: theory and predicted performance

Recent developments in thin film technology have made possible the construction of multilayered thin film structures that act as efficient Bragg diffractors for x rays and extreme ultraviolet (EUV) radiation. These structures (which we term layered synthetic microstructures or LSMs) are analogous to multilayer interference filters for the visible spectral region and have important potential applications in many areas of x-ray/EUV instrumentation. In this paper the theory of x-ray diffraction by periodic structures is applied to LSMs, and approximate formulas for estimating their performance are presented. A more complete computation scheme based on optical multilayer theory is described, and it is shown that, by adjusting the refractive indices and thicknesses of the component layers, the diffracting properties may be tailored to specific applications. Finally, it is shown how the theory may be modified to take account of imperfections in the LSM structure and to compute the properties of nonperiodic structures.

Bragg reflection concentrators for hard x-ray astronomy

The use of parabolic concentrators with mosaic Bragg crystals as reflectors to the problems of hard x-ray source and cyclotron line detection is discussed. The primary application of the instrument is to detect the broad continuum of hard x-ray sources at energies above ~5 keV where normal grazing incidence concentrators become inefficient. The sensitivity of the instrument is calculated for pyrolytic graphite and lithium fluoride reflectors. Sensitivity for sources with continuum fluxes 10(-3) Crab in the >20-keV energy range is possible for observing times of 10(4) to 10(5) sec.

Stigmatic performance of an EUV spectrograph with a single toroidal grating

The stigmatic performance of conventionally ruled toroidal gratings used near normal incidence is explored in detail. A single-grating spectrograph, particularly suitable for observing the dynamics of the solar corona in the 520-630-A wavelength range, is described. The arrangement uses a 3600-line/mm grating with a horizontal radius of curvature of 2 m that generates images with a blur of less than 20 microm over an area 2.6 mm high x 80 mm wide, while over an area 6 mm x 80 mm the blur does not exceed 40 microm. In terms of wavelength intervals these blurs correspond to 28 mA and 56 mA, and the width of the area covers a spectral range of 110 A. If the spectrograph is equipped with a 20-microm wide entrance slit, and if this slit is placed in the focal plane of a telescope with 4-m focal length, spatial resolution elements of 1 x 1 (sec of arc)(2) and 1 x 2 (sec of arc)(2) over a slit height of 2 min of arc and 5 min of arc, respectively, result.