Multilayer Reflectors Research Articles

Cr/C Reflective Multilayer for Wavelength of 44.8 Å.

The working wavelength of Ni-like, Ta soft X-ray laser is 44.8 Å, just near the "water window." High reflection multilayers are required for this kind of laser in China. In this work, we design and fabricate carbon-based multilayer reflective samples. The Cr/C multilayer was selected from proposed candidates such as Co/C, Ni/C, and CoCr/C material combinations. The period thickness is only 22.6 Å. Cr/C multilayers were deposited by the magnetron sputtering method. Multilayers with bi-layer numbers of 150, 200, 250 and 300 were deposited onto super polished silicon wafers. All multilayers have been characterized by grazing incidence X-ray reflectance (GIXRR). Then, near-normal incidence reflectance measurements were performed at beamline 3W1B, Beijing synchrotron radiation (BSRF). The highest reflectance of 13.2% is achieved with the bi-layer number of 300. Transmission electron microscopy measurements also clearly show the sharp Cr-C interfaces in the multilayer.

Enhancement of light trapping for thin film solar cells

ABSTRACTLight trapping is one of the key challenges for next generation thin film solar cells. In this work, we have identified the distinct light trapping effects for short and long wavelength solar spectrum range, by investigating lighting trapping structures on both sides of Si thin film solar cells. The sub-wavelength photonic front surface by wet etching and multi-layer photonic crystal reflector on the bottom surface are studied in detail for its solar energy absorption characteristics. Our study reveals the drastic difference of the light trapping effects within the solar spectrum wavelength. This work may provide guidance for the efficiency enhancementfor next generation thin film photovoltaic cells.

Model for confined Tamm plasmon devices

It is shown that cavities formed between a multilayer quarter-wave Bragg reflector and a metal mirror which support Tamm plasmons can be modelled by using a hard-mirror approximation including appropriate penetration depths into the mirrors. Results from this model are in excellent agreement with those found by numerical methods. In addition Tamm modes that are laterally confined by the presence of a metallic disc deposited on the Bragg reflector can be described by the effective index model that is commonly used for vertical-cavity surface-emitting lasers (VCSELs). This enables the lateral modes confined by a circular disc to be found from conventional weakly-guiding waveguide theory similar to that used for optical fibres. The resonant wavelengths of these linearly-polarised (LP) guided modes are calculated as functions of disc diameter and other parameters.

Lifetime Stability and Microstructure Properties of Cr/B₄C X-ray Reflective Multilayer Coatings.

This paper demonstrates that highly reflective Cr/B₄C multilayer interference coatings with nanometric layer thicknesses, designed to operate in the soft X-ray photon energy range, have stable reflective performance for a period of 3 years after deposition. The microstructure and chemical composition of layers and interfaces within Cr/B₄C multilayers is also examined, with emphasis on the B₄C-on-Cr interface where a significant diffusion layer is formed and on the oxide in the top B₄C layer. Multiple characterization techniques (X-ray reflectivity at different photon energies, X-ray photoelectron spectroscopy, transmission electron microscopy, electron diffraction and X-ray diffraction) are employed and the results reveal a consistent picture of the Cr/B₄C layer structure.

Silicon nitride and silica quarter-wave stacks for low-thermal-noise mirror coatings

This study investigates a multilayer high reflector with new coating materials for next-generation laser interferometer gravitational wave detectors operated at cryogenic temperatures. We use the plasma-enhanced chemical vapor deposition method to deposit amorphous silicon nitride and silica quarter-wave high-reflector stacks and studied the properties pertinent to the coating thermal noise. Room- and cryogenic-temperature mechanical loss angles of the silicon nitride and silica quarter-wave bilayers are measured using the cantilever ring-down method. We show, for the first time, that the bulk and shear loss angles of the coatings can be obtained from the cantilever ring-down measurement, and we use the bulk and shear losses to calculate the coating thermal noise of silicon nitride and silica high-reflector coatings. The mechanical loss angle of the silicon nitride and silica bilayer is dispersive with a linear weakly positive frequency dependence, and, hence, the coating thermal noise of the high reflectors show a weakly positive frequency dependence in addition to the normal $1/\sqrt{f}$ dependence. The coating thermal noise of the silicon nitride and silica high-reflector stack is compared to the lower limit of the coating thermal noise of the end test mirrors of ET-LF, KAGRA, LIGO Voyager, and the directly measured coating thermal noise of the current coatings of Advanced LIGO. The optical absorption of the silicon nitride and silica high reflector at 1550 nm is 45.9 ppm. Using a multimaterial system composed of seven pairs of ion-beam-sputter deposited $\mathrm{Ti}:{\mathrm{Ta}}_{2}{\mathrm{O}}_{5}$ and silica and nine pairs of silicon nitride and silica on a silicon substrate, the optical absorption can be reduced to 2 ppm, which meets the specification of LIGO Voyager.

Extending the color of ultra-thin gold films to blue region via Fabry-Pérot-Cavity-Resonance-Enhanced reflection

Interband-transition-induced absorption of gold films limits its color rendering in the blue part of the visible spectrum. In this work, we investigate extending the color of ultra-thin gold film to the blue region via Fabry-Pérot-Cavity-Resonance-Enhanced reflection. Systematic experiments and simulations were conducted to demonstrate the color variation of the Fabry-Pérot cavities. By choosing the appropriate dielectric thickness, colors covering the whole visible range including blue color were realized. The electronic field and absorbed power distribution were analyzed to understand the origin of the blue color. A multi-layer Fresnel reflection equation was employed to explain the whole color variation of this structure and the theoretical calculations were in agreement with both simulations and experimental results. Our work provides an approach to achieve blue color for gold and may have potential applications in surface decoration and functional optical elements.

The effect of dissipation on defect modes in a one-dimensional photonic crystal

Photonic crystals are artificial structures with periodic dielectric properties, which are divided into one-dimensional, two-dimensional, and three-dimensional crystals. In this study, electromagnetic wave propagation was studied in a one-dimensional photonic crystal. To understand wave propagation in a one-dimensional photonic crystal, which is a multi-layered system, reflection and transmission of TE and TM waves in a thin film, and finally in a multi-layer system was investigated by using the transfer matrix method. Transmission spectrum of symmetric and asymmetric ((Si / Si02)ND (Si02/ Si)N and (Si / Si02)ND (Si / Si02)N)) crystal with defect layers (D) PbSe, Pb 1-xSnxTe and HgxCdxTe was depicted in frequency range of 4–7 THz and a defect mode was observed in band gap. Because these three materials had a complex refractive index, they rendered dissipation and reduced the amount of transmission in the system. Then, temperature dependence behavior of defect modes was studied. The results indicated that with increasing temperature, the height of defect mode decreases. As the temperature rose, decrement of defect mode height in photonic crystal with a Pb xSnxTe defect became much more than two others. Finally, Plots of electrical permittivity was plotted in terms of frequency and temperature for three materials. Only real part of electrical permittivity of Pb 1-xSnxTe remained constant with frequency changing. Also, in all cases, with increasing temperature, amplitude of real part of electrical permittivity decreased and imaginary part increased.

‘It’s called Change’: Regenring, reconfiguring and representation

Abstract This article is built around an art piece, which depicts a particular phenomenon in physics concerning the process of transition and transformation; in other words, change. In itself the art piece is an example of ‘regenring’ in that it re-presents information that has previously been produced in other genres (scientific papers, mathematical formulae) by producing it using a different genre: a mixed media collaged picture displayed on a wall. In keeping with the theme of transformation and change, the article uses four different genres in discussing the piece: account of what is observed, interview with the maker of the piece, reflection-in-action shown in italics and reflection-on-action shown in boxes (the distinction of reflection-in- and reflection-on- action is taken from Schön) and in so doing explores, through these different lenses, how the art piece helps or hinders the unpacking of the science that it represents. It also provides a multilayered reflection on the theme of change itself since every aspect of this article, the topic, the art piece, the physics and the textual activity of how the discussion is represented, revolves around representing and observing through different lenses, regenring and transformation.

Analysis of temporal broadening of isolated attosecond pulses induced by periodic Mo/Si multilayer mirrors reflection

When using a multilayer mirror to reflect an attosecond pulse, the temporal broadening effect always happens, and enlarges the duration of incident attosecond pulse. In this paper, the temporal broadening of an isolated attosecond pulse induced by a periodic Mo/Si multilayer mirror reflection is analyzed. In order to evaluate the effect of pulse broadening quantitatively, two temporal parameters, the pulse broadening coefficient and the pulse reflection coefficient, are used. By studying two temporal parameters of periodic Mo/Si multilayer mirrors with different bi-layers for incident isolated attosecond pulses with various durations, we find that the incident isolated attosecond pulse with smaller duration suffers from more serious pulse broadening and energy loss, when reflected by a periodic Mo/Si multilayer mirror. In addition, a periodic Mo/Si multilayer mirror with more bi-layers causes more serious pulse broadening for an incident isolated attosecond pulse, even though it can provide larger pulse reflection efficiency. Furthermore, by analyzing of the reflectivity and the dispersion of periodic multilayer mirrors in the spectral region of incident pulses, we demonstrate that the spectrum filtering and dispersion inducing by periodic Mo/Si multilayer mirrors are responsible for the pulse broadening of incident isolated attosecond pulses. The study of pulse broadening induced by periodic Mo/Si multilayer mirrors reflection could improve the understanding of the performances of periodic multilayer mirrors for pulse reflection, and provide a guide for high-performance attosecond multilayer mirrors design.

Computer-aided assembly of X-ray parabolic cylinder laterally graded multilayer reflector to collimate divergent beam in XRD

In X-ray diffraction, it is essential to irradiate the sample with a parallel beam to avoid severe diffraction pattern aberration. A parabolic cylinder laterally graded multilayer reflector is a good choice for this purpose. However, its assembly is critical. Usually, reflector performance is simulated during design, and the effects of assembly errors are qualitatively computed before assembly. However, for micro-scale assembly processes, the qualitative error results in a time-assuming procedure and an unsatisfactory data. A Computer-aided assembly process, details of a quantitative guide of adjustment, is hereby presented for a precision adjustment. Firstly, a high-efficiency setup to collimate the divergent beam and evaluate the collimation beam is proposed. Secondly, the performance of the reflector used in the experiment is simulated according to the parameters of each unit in the setup. Thirdly, during the adjustment process, each essential adjustment step is analyzed, and the resetting value of each error is quantitatively computed and used to guide the adjustment. Lastly, the assembled reflector is highly effective with a divergent angle of less than 0.004 deg.

Wing scale ultrastructure underlying convergent and divergent iridescent colours in mimetic Heliconius butterflies.

Iridescence is an optical phenomenon whereby colour changes with the illumination and viewing angle. It can be produced by thin film interference or diffraction. Iridescent optical structures are fairly common in nature, but relatively little is known about their production or evolution. Here we describe the structures responsible for producing blue-green iridescent colour in Heliconius butterflies. Overall the wing scale structures of iridescent and non-iridescent Heliconius species are very similar, both having longitudinal ridges joined by cross-ribs. However, iridescent scales have ridges composed of layered lamellae, which act as multilayer reflectors. Differences in brightness between species can be explained by the extent of overlap of the lamellae and their curvature as well as the density of ridges on the scale. Heliconius are well known for their Müllerian mimicry. We find that iridescent structural colour is not closely matched between co-mimetic species. Differences appear less pronounced in models of Heliconius vision than models of avian vision, suggesting that they are not driven by selection to avoid heterospecific courtship by co-mimics. Ridge profiles appear to evolve relatively slowly, being similar between closely related taxa, while ridge density evolves faster and is similar between distantly related co-mimics.

Ab-initio determination of porous silicon refractive index confirmed by infrared transmittance measurements of an omnidirectional multilayer reflector

We report a multiscale design of omnidirectional Bragg mirrors based on dielectric multilayers by combining quantum mechanical and electromagnetic theories. This design begins with the calculation of electric permittivity for each layer using the density functional theory, followed by a transfer matrix study of light propagation along the multilayer reflector. The theoretical results were further validated by fabricating free-standing porous silicon multilayer films obtained through electrochemical etching of p+-type [100]-oriented crystalline Si wafers, alternating two anodic current densities. The measured infrared transmittance spectra confirm the position and width of the photonic bandgap predicted by the multiscale design.

Mask Materials and Designs for Extreme Ultra Violet Lithography

Extreme ultra violet lithography (EUVL) is no longer a future technology but is going to be inserted into mass production of semiconductor devices of 7 nm technology node in 2018. EUVL is an extension of optical lithography using extremely short wavelength (13.5 nm). This short wavelength requires major modifications in the optical systems due to the very strong absorption of EUV light by materials. Refractive optics can no longer be used, and reflective optics is the only solution to transfer image from mask to wafer. This is why we need the multilayer (ML) mirror-based mask as well as an oblique incident angle of light. This paper discusses the principal theory on the EUV mask design and its component materials including ML reflector and EUV absorber. Mask shadowing effect (or mask 3D effect) is explained and its technical solutions like phase shift mask is reviewed. Even though not all the technical issues on EUV mask are handled in this review paper, you will be able to understand the principles determining the performance of EUV masks.

Ion distribution in dry polyelectrolyte multilayers: a neutron reflectometry study.

Ultrathin films of complexed polycation poly(diallyldimethylammonium), PDADMA, and polyanion poly(styrenesulfonate), PSS, were prepared on silicon wafers using the layer-by-layer adsorption technique. When terminated with PDADMA, all films had excess PDADMA, which was balanced by counterions. Neutron reflectivity of these as-made multilayers was compared with measurements on multilayers which had been further processed to ensure 1 : 1 stoichiometry of PDADMA and PSS. The compositions of all films, including polymers and counterions, were determined experimentally rather than by fitting, reducing the number of fit parameters required to model the reflectivity. For each sample, acetate, either protiated, CH3COO-, or deuterated, CD3COO-, served as the counterion. All films were maintained dry under vacuum. Scattering length density profiles were constrained to fit reflectivity data from samples having either counterion. The best fits were obtained with uniform counterion concentrations, even for stoichiometric samples that had been exposed to PDADMA for ca. 5 minutes, showing that surprisingly fast and complete transport of excess cationic charge occurs throughout the multilayer during its construction.

Structural broadband absorbing metamaterial based on three-dimensional printing technology

In order to verify the feasibility of three-dimensional (3D) printing technology in preparing the metamaterial absorbers with complex structure, a three-layer broadband absorbing metamaterial is designed and fabricated by 3D printing technology. The surface layer and middle layer of the metamaterial are composed of periodic arrays with different unit dimensions and the bottom layer of a slab structure. The optimized thickness of the metamaterial is 4.7 mm. A composite absorbent which consists of carbonyl iron powder and nylon is used to fabricate the absorber. In experiment, the obtained absorber is vertically irradiated by an electromagnetic (EM) wave. Two strong absorption peaks at 5.3 GHz and 14.1 GHz are achieved, with the reflection losses of -15.1 dB and -12.5 dB, respectively. The superposition of the two absorption peaks results in a reflection loss below -10 dB in a range from 4 to 18 GHz. The effective EM parameters of the surface layer and the middle layer are calculated by the S parameter inversion method. An effective model of the three-layer structure absorber is proposed and its reflectivity is calculated by using a multilayer structure reflectivity formula. The calculated reflectivity agrees well with the measured one. The absorbing and resonance mechanisms of the two absorption peaks are investigated by analyzing the dynamic distributions of power density loss, electric field and magnetic field. It can be clearly confirmed that the reflection losses at 5.3 GHz and 14.1 GHz are primarily concentrated on the bottom layer and surface layer, and the broadband absorption performance can be derived from the superposition of broadband absorptions of the three absorbing layers. Meanwhile, the strong electric coupling effect between the adjacent units in the surface layer is demonstrated by analyzing the electric-field distributions, which indicates that the strong reflection loss at 14.1 GHz is mainly caused by the electric response. The multiple scattering effects among the three layers are also considered according to the magnetic field distributions at two resonance frequencies. It is shown that there are two magnetic responses at 5.3 GHz and 14.1 GHz, respectively, and the multiple scattering contributes to increasing the EM wave propagation distance and enhancing the power loss. The designed absorbing metamaterials in this paper achieve good broadband absorption performances, particularly in the low frequency band. Combined with 3D printing rapid technology, a promising route to constructing 3D absorbing metamaterials with complex structures is proposed, which would be of great significance and broad practical prospect.

High Q reflection filter using a gradient-index membrane with a grating surface

High Q reflection filter using a gradient-index (GI) membrane with a grating surface is proposed. The thickness of GI membrane is very small comparing with the traditional multilayer reflection filter or the GI reflection filter, and the GI membrane can also break the restriction of the resonant excitation condition of the conventional guided-mode resonance (GMR) filter. High Q filtering features can be maintained even on the high-index substrate. The grating thickness of the GI membrane filter can be used to select the resonance wavelength with different quality factors (QFs), the reflection peak is blue-shifted, and the QF is decreased from 554.4 to 207.8 as the grating thickness is increased from 50 nm to 150 nm. The gradient coefficient of the GI membrane filter can be used to tailor the number of the reflection channels. The resonant excitations of high order waveguide modes confined in the GI membrane are responsible for the high Q filtering properties with multiple channels.

High-numerical aperture extreme ultraviolet scanner for 8-nm lithography and beyond

Current extreme ultraviolet (EUV) projection lithography systems exploit a projection lens with a numerical aperture (NA) of 0.33. It is expected that these will be used in mass production in the 2018/2019 timeframe. By then, the most difficult layers at the 7-nm logic and the mid-10-nm DRAM nodes will be exposed. These systems are a more economical alternative to multiple-exposure by 193 argon fluoride immersion scanners. To enable cost-effective shrink by EUV lithography down to 8-nm half pitch, a considerably larger NA is needed. As a result of the increased NA, the incidence angles of the light rays at the mask increase significantly. Consequently, the shadowing and the variation of the multilayer reflectivity deteriorate the aerial image contrast to unacceptably low values at the current 4× magnification. The only solution to reduce the angular range at the mask is to increase the magnification. Simulations show that the magnification has to be doubled to 8× to overcome the shadowing effects. Assuming that the mask infrastructure will not change the mask form factor, this would inevitably lead to a field size that is a quarter of the field size of the current 0.33-NA step and scan systems and reduce the throughput (TPT) of the high-NA scanner to a value below 100 wafers per hour unless additional measures are taken. This paper presents an anamorphic step and scan system capable of printing fields that are half the field size of the current full field. The anamorphic system has the potential to achieve a TPT in excess of 150 wafers per hour by increasing the transmission of the optics, as well as increasing the acceleration of the wafer stage and mask stage. This makes it an economically viable lithography solution.

Subcutaneous veins depth measurement using diffuse reflectance images.

Intravenous (IV) procedures are often difficult due to the poor visualization of subcutaneous veins. Because existing vein locators lack the ability to assess depth, and also because mis-punctures and poor vascular access remain problematic, we propose an imaging system that employs diffuse reflectance images at three isosbestic wavelengths to measure both the depth and thickness of subcutaneous veins. This paper describes the proposed system as well as proof-of-principle experimental demonstrations. We initially introduce the working principle and structure of the system. All measurements were based on the Monte Carlo (MC) method and accomplished by referring an optical density (OD) ratio to a multi-layer diffuse reflectance model. Results were all validated by comparative ultrasound measurements. Experimental trials included 11 volunteers who were subjected to both ultrasound measurements and the proposed optical process to validate the system's applicability. However, the unreliability of the "thickness" measurement of the vein may be due to the fact that the veins have collapsible walls - so excess pressure by the transducer will give a false thickness.

Design of polymer multilayer heterostructure broadband reflector for the near-infrared using genetic algorithm

A broadband reflector for the near-infrared (NIR) was designed using a multilayer heterostructure consisting of several quarter-wave stacks (QWSs), which were composed of polymethyl methacrylate (PMMA) and polyethylene terephthalate (PET) films. Taking the solar power density as the target and using the Bragg wavelength of each QWS as the variable, the genetic algorithm was applied to look for the optimal multilayer heterostructure for broadband NIR reflection. As high as 99.46% total energy reflectivity in the short-wavelength NIR region (780 to 1100 nm) and 89.56% total energy transmissivity in the visible light region (380 to 780 nm) were realized by a multilayer heterostructure consisting of six quarter-wave PMMA/PET stacks, which can be easily fabricated based on the micronano multilayer coextrusion technology. The designed structure possesses good stability, and its total energy reflectivity is not sensitive to the incident angle of light. The proposed broadband NIR reflector can be applied to buildings as energy-saving films.

Thermal and temporal stability of V/Sc multilayer mirror for the soft X-ray water window region

V/Sc multilayer has been developed as a promising candidate for the water window region working near both the Sc-L edge (3.11 nm) and the V-L edge (2.42 nm). To meet the application requirements, its thermal and temporal stability were studied. A series of V/Sc multilayers with a d-spacing ranging from 1.59 to 1.62 nm and a number of bilayers of 100 and 400 was fabricated. Different samples were annealed at temperatures from 100 to 300 °C. Grazing incidence X-ray reflectivity and soft X-ray (SXR) reflectivity measurements showed that the multilayer structure and reflectivity remain mostly unchanged at 100 °C. An obvious deterioration started at 200 °C, while the SXR reflectivity drops close to zero at 250 °C. The multilayer structure completely broke down at 300 °C. Both X-ray diffraction and transmission electron microscopy measurements demonstrated a severe crystallization occurring at 250 °C. Energy dispersive X-ray spectroscopy measurements have further confirmed a significant elemental mixture occurring after 250 °C annealing. The severe intermixing and polycrystallization resulted in the degradation of the multilayer structure. On the other hand, the V/Sc multilayer is relatively stable after half year storage in air environment showing a slight decrease of SXR reflectivity from 18.4% to 16.5%.