Multilayer Optics Research Articles

The Influence of Surface Reflectivities on Measurement of the Torsional Anchoring Strength of Nematic Liquid Crystals

The polarisation dependent normal incidence transmissivity of a multilayered system encompassing within it an optically uniaxial layer has been analysed in detail using multilayer optics modelling. It is shown that even for this simple case of normal incidence the orientation of the uniaxial sample and two polarisers needed to give a transmission minimum is significantly influenced by the optical phase shifts caused by reflections at the isotropic boundary layers. We highlight the failure of the simplified approach to this problem which uses the Jones matrix, ignoring multiple reflections within the system. Consequently we show that the standard technique of quantifying the boundary twist offset angles by determining with polarisers the minimum points in the normal incidence transmission for a thin nematic liquid crystal cell may well lead to seriously wrong values for the torsional anchoring strength of aligning surfaces.

Helicon plasma deposition of a TiO2/SiO2 multilayer optical filter with graded refractive index profiles

Thirty one layer TiO2/SiO2 optical filters with graded refractive index profiles were fabricated by helicon plasma sputtering at room temperature. Multilayer films were deposited on glass (BK7) and Si (100) substrates simultaneously and sequentially. The measured transmittance spectrum exhibited a reflectance of 99.8% at a central wavelength of 730 nm and high transmittance over the wavelength region outside of the reflected band as a result of the suppression of the sidelobes. The experimental transmittance spectrum corresponded almost completely with that calculated based on the optical multilayer film theory and using the measured refractive indices of TiO2, SiO2 and TiO2–SiO2 composite films. Transmission electron microscopic observations confirmed the expected microstructure of the filter.

Isoréflectance des dépôts optiques en couches minces

With the addition of some particular layers, characterized in this note, a periodic non-absorbing optical multilayer allows us to keep the reflectance (or the transmittance) unchanged whatever the number of sequences in the coating. This invariant property, called isoreflectance (or isotransmittance) can be obtained only for a wavelength located inside the passband of the filter. Isoreflectance stretches the buffer single layer concept to multilayer and can be useful in the design of optical coatings.

Narrowband laser produced extreme ultraviolet sources adapted to silicon/molybdenum multilayer optics

The extreme ultraviolet radiation emitted from a plasma generated by a pulsed Nd:yttrium aluminum garnet laser is investigated around 13 nm wavelength for several low Z elements (lithium, nitrogen, oxygen, fluorine). A narrowband EUV source can be designed by using the narrowband line emission of low Z elements in combination with the broadband reflection characteristic of silicon/molybdenum (Si/Mo) multilayer mirrors. Experimental results are discussed within a theoretical model, which allows a deduction of an optimization criterion for a maximum conversion efficiency. The Lyman-α line of hydrogenlike lithium ions fulfills the demands for high intense, free-standing narrowband emission at the long wavelength side of the silicon absorption L edge.

Characterization of substrates for use in X-ray multilayer optics.

The optical performance of platinum-carbon multilayers deposited onto different substrates has been examined. Specular reflectivity and non-specular diffuse scattering were measured to study the replication of substrate roughness into the multilayer structure. Surface topography was measured before and after deposition using a scanning probe microscope and a mechanical profiler.

High precision large area PLD of X-ray optical multilayers

To realize high quality X-ray optical multilayer stacks on large areas a double-beam PLD source was integrated into a commercial MBE system. Optimization of ablation conditions and film growth regime, resp., for various kinds of homogeneous thin films and multilayer systems has been realized by a reproducible variation of pulse energy and repetition rate of each of the two Nd:YAG-lasers. In addition, the lasers can be independently controlled by a predetermined pulse delay. Thus, plasma parameters of two plumes generated from locally separated origins can be influenced by the pulse delay of the Nd:YAG-lasers, too. The influence of laser parameters and pulse delay on thin film growth is investigated by the deposition of Ni/C layer stacks. Optimum irradiation conditions are zero delay and moderate pulse energies. Multilayer interface roughnesses on the order of σ R≈0.1 nm are deduced from high resolution electron microscopy (HREM)-micrographs. The interface roughness increases with higher pulse energy. For changing the pulse delay from τ=0 ns to τ=2.5 ms, a destruction of the layer stack is observed. Laterally graded Ni/C multilayers showing X-ray optical activity were synthesized with these optimized deposition parameters in the period thickness range from 3 to 5 nm. Average values of thickness gradients typically Δ t/Δ x≈2×10 −8 for 4″ substrate length in maximum and period thickness variations on the order of σ t≈0.1 nm are confirmed by grazing incidence X-ray reflectometry and HREM.

Chemical microanalysis by selected-area ESCA using an electron energy filter in a photoemission microscope

We present a new and simple device for microspectroscopy being independent of the mode of electron-excitation. Micro-X-ray photoelectron spectroscopy, electron-induced Auger-spectroscopy, as well as local energy-loss spectroscopy were used to investigate metal-adsorption on silicon. This new approach employs a non-imaging electron energy analyser attached to a new-generation photoemission electron microscope with integral microarea selector. Photoelectron microspectroscopy was performed using the direct beam of an undulator (U2 at BESSY) being monochromatised and focused by means of multilayer optics at hv = 95 eV. Similarly, local Auger-electron and EELS spectra have been taken using a simple electron gun for the excitation. The chemical compositions of inhomogenities in thin layers of indium on silicon and the local state of oxidation of a structured Pt Co multilayer have been determined.

248 nm laser interaction studies on LaF3/MgF2 optical coatings by cross-sectional transmission electron microscopy

LaF3/MgF2 high-reflectance optical multilayer coatings were investigated by cross-sectional transmission electron microscopy (XTEM) concentrating on the evolution of the film structure during growth and due to laser irradiation. The irradiated samples show enhanced texture compared to the as deposited films, indicating that grain growth took place due to the laser irradiation. The waviness of the interfaces between the single quarter-wave layers in the multilayer system increases with increasing distance from the substrate. This effect may result in increased scatter losses, especially in the ultraviolet spectral region.

Mutual coherence and conical pattern of sources optimally excited within multilayer optics

We calculate the power density provided by a set of sources that radiate simultaneously and interfere within a multilayer planar microcavity. This power is shown to depend on interaction of currents, that is, on mutual coherence of sources. Numerical calculation is performed for a single-layer and a double-cavity Fabry–Perot filter, including free-space and modal powers. Elimination of modal power carried by guided modes is discussed, as well as optimal excitation of sources by means of multidielectric resonances. The resulting pattern is shown to be conical in free space and controlled by the cavity poles. Experiments are given concerning absorption and scattering measurements at each multidielectric resonance of the cavity. The applications concern light scattering and roughness-induced absorption within multilayers, as well as spontaneous emission in classical microcavities.

Electromagnetic power provided by sources within multilayer optics: free-space and modal patterns

An electromagnetic theory is presented that makes possible the development of a complete energy balance within an arbitrary multilayer microcavity that supports different kinds of classical optical sources. The theory is based on a single Fourier spectrum of waves and is valid for transparent or dissipative stacks, with no use of modal methods. We show how the power provided by the cavity is converted into Poynting flux and absorption. Free-space and guided-mode patterns are calculated for single layers, mirrors, and narrow-band filters. The modal pattern is shown to be strongly dependent on the cavity poles. Discretization of the high-frequency energy into a set of guided modes is introduced as an asymptotic limit of the problem when absorption vanishes to zero. The applications concern defect-induced absorption in optical multilayers or guided-mode coupling through microirregularities in a stack, as well as spontaneous emission in classical microcavities.

Speckle in coherent x-ray reflectivity from Si(111) wafers

We report the observation of x-ray speckle in the reflected beam from Si(111) wafers illuminated at grazing incidence. An intense coherent 8-keV x-ray beam was prepared using a wiggler source and multilayer monochromator optics. We demonstrate that the speckle patterns are specific to the region of the sample that is illuminated. From the trade-off between surface sensitivity and signal as a function of perpendicular momentum transfer, we infer that the speckle is due to the surface morphology on a micrometer length scale. We document and explain the evolution of the speckle patterns from nearly specular at low ${q}_{z}$ to highly structured at larger ${q}_{z}.$

Extreme ultraviolet probing of laser imprint in a thin foil using an x-ray laser backlighter

For direct drive inertial confinement fusion, a capsule is imploded by directly illuminating the surface with laser light. Beam smoothing and uniformity of illumination affect the seeding of instabilities at the ablation front. We have developed a technique for studying the imprint of a laser beam on a thin foil using an x-ray laser as an extreme ultraviolet (XUV) backlighter. We use multilayer XUV optics to relay the x-ray laser onto the directly driven foil, and then to image the foil modulation onto a charged coupled device camera. This technique allows us to measure small fractional variations in the foil thickness. We have measured the modulation due to imprint from a low intensity 0.35 μm drive beam incident on a 3 μm Si foil using an yttrium x-ray laser on Nova. We present results from a similar technique to measure the imprinted modulation due to a low intensity 0.53 μm drive beam incident on a 2 μm Al foil using a germanium x-ray laser at the Vulcan facility.

BESSY Bragg-Fresnel multilayer beam monitors

X-ray optical systems based on Bragg-Fresnel multilayer components imaging an electron beam in a storage ring with μm resolution are presented. Design concepts are compared to alternative methods, and the aberrations and limits of Bragg-Fresnel multilayer optics are discussed. Experimental results of imaging the BESSY I source with sub-10-μm resolution are presented, and the development of a compact Bragg-Fresnel multilayer telescope as a BESSY II standard beam monitor is described.

Multilayer optics for harmonic control of angiography beamline sources

In recent work, multilayers with band-tailored optics for dual energy digital subtraction angiography (DDSA) applications have been designed and tested at SSRL. Control of various multilayer parameters, including period grading, ratio of high-to-low Z material thickness, number of layers, etc., was used to produce reflectors with bandwidths ranging from 0.6%–10% and efficiencies in the 30%–95% range. In this paper, we consider the control of multilayer bandshapes and the implementation of double-reflection multilayer configurations to further control the first harmonic (33 keV) bandwidth and to suppress or eliminate the 66 keV and 99 keV harmonics present on angiography beamlines driven by wiggler or micropole undulator sources.

Measurement of 0.35 microm laser imprint in a thin Si foil using an x-ray laser backlighter.

Imprinted modulations in optical depth of a thin Si foil due to 0.35 \ensuremath{\mu}m laser irradiation at about $3\ifmmode\times\else\texttimes\fi{}{10}^{12}$ W/c${\mathrm{m}}^{2}$ have been measured at shock breakout. These measurements were made by high resolution face-on radiography using a gain saturated yttrium x-ray laser backlighter operating at a wavelength of 15.5 nm with a multilayer optics imaging system. The imprinted modulation due to a static speckle pattern and a 0.29 and a 0.33 THz spectral dispersion smoothed speckle pattern were studied. Comparison of the imprinted modulation is made with simulations.

X-ray laser radiography of perturbations due to imprint of laser speckle in 0.35 μm laser irradiation of a thin Si foil

A gain saturated yttrium x-ray laser operating at a wavelength of 15.5 nm has been used as an extreme ultraviolet probe to measure optical depth modulations in a thin Si foil by face-on radiography. The high brightness of this Ne-like x-ray laser allows probing of a sample with a high opacity. This technique is sensitive to very small modulations in the optical depth of the foil, corresponding to thickness variations of a few 10 nm of cold material. This technique is used to measure the effect of direct drive laser imprint on a thin Si foil by face-on radiography using multilayer optics to image the foil with 26× magnification. We have recorded modulations in a thin Si foil that was irradiated by a 400 ps, 0.35 μm beam at an intensity of about 3×1012 W/cm2.

Diffraction at 8‐keV x rays of Bragg–Fresnel optics

The basic elements of Bragg-Fresnel multilayer optics are multilayer gratings. In this paper, the fabrication process for the simplest Bragg-Fresnel multilayer optics is given; the diffraction measurements at 8-keV x rays of multilayer gratings are presented; and the measurement results in the diffraction spectrum are analyzed and discussed in detail. (C) 1996 Society of Photo-Optical Instrumentation Engineers.

Study of the reflectivity in the XUV domain of normal-incidence multilayer mirrors

The development of multilayer optics has profound implications for soft-X-ray/UV astronomy, since it allows to extend the use of normal-incidence telescopes to cover the XUV region where lines are formed at greatly different temperatures (105–107 K). A multilayer mirror consists of alternating thin layers of suitable materials deposited on a substrate and its performance depends not only on the optical properties of the materials but also on the design of the multilayer. In this study we have computed the reflectivity of multilayer mirrors to select both the materials and the multilayers design to achieve the best performance in the wavelength range from 30 to 350 A. Our calculations show that high theoretical reflectivities, from 0.2 to 0.8 and relatively narrow bandpasses, from ∼ 1A to ∼30 A, can be obtained, in the wavelength range from 30 to 350 A, by a suitable choice of the materials and of the multilayer design.

Multilayer x-ray mirrors

The angular, dispersive, polarization and energy features of multilayer x-ray optics are considered within the framework of unified theoretical approach. The ultimate optical parameters of multilayer structures are investigated, and the possibilities of their application to certain physical problems are discussed. Practical recommendations and techniques for fabrication of x-ray interference mirrors are considered.

High-resolution imaging and spectroscopy of dense plasmas using multilayer x-ray optics

Instruments incorporating normal-incidence mirrors and gratings have been used to record images and spectra in the extreme u.v. and soft x-ray regions. The multilayer coatings were optimized to reflect wavebands of interest for the diagnosis of dense plasmas. Images at wavelengths of 33.8 and 130 Å were recorded with spatial resolution of 5 μm. Using normal-incidence multilayer gratings operating in the wavelength region 130–300 Å, spectra with resolution of 14,000 were recorded which allows the use of line profiles to determine Doppler plasma motion an Stark densities. Because of the high throughput of the multilayer optics, the instruments can be located at large distances from the plasmas, and the optics were undamaged by the intense radiation and debris fluxes from laser-produced plasmas. The absolute radiation flux was determined at wavelengths of 33.8 and 130 Å.