bluuebunny/crossref_metadata_2025 huggingface dataset:
Characterization of Etched Laminar Multilayer Amplitude Gratings in the Soft X-Ray Region
Authors: Troussel P. , Schirmann D. , Bac S. , Barchewitz R. , Boher P. , Malek C. Kahn
Published: February 2023

The fabrication of lamellar multilayer gratings has permitted a number of results on the performances of these devices.(1)
Interdiffusion Kinetics in Mo/Si Multilayers
Authors: Rosen R.S. , Stearns D.G. , Viliardos M.A. , Kassner M.E. , Vernon S.P.
Published: February 2023

Mo/Si multilayers (ML's) are attractive for x-ray optics due to the relatively high associated reflectivity. However, the reflectivity is known to decrease with the formation of diffuse interlayer regions resulting from interdiffusion at the Mo-Si interfaces [1-2]. Most of the previous studies have reported interdiffusion coefficients [1-5] and effective activation energies [2-4] over a range of temperature assuming that the interdiffusion coefficient is invariant with annealing time. At temperatures of 560-580 °C, hexagonal molybdenum-disilicide (h-MoSi2) grows with a square-root time dependence at the Mo-Si interface [6]. This suggests diffusion-limited growth with a time-invariant interdiffusion coefficient. However, at temperatures of 300-400 °C, the interlayer structure is known to change with annealing time from amorphous to crystalline [1-3], suggesting the possibility that the rate-controlling mechanism(s) for interdiffusion may also change with annealing time and/or temperature. Therefore, it is the objective of this study to perform a series of annealing treatments of Mo/Si ML's at temperatures lower than those previously reported to determine the Mo-Si interdiffusion kinetics as a function of time.
Modeling of Multilayer Mirror Damage by a Short Pulse of X-Rays
Authors: Eder D.C. , MacGowan B. J. , DaSilva L.B. , Mrowka S. , Barbee T.
Published: February 2023

Multilayer mirrors can have a dramatic reduction in reflectivity as a result of large optical and x-ray fluxes incident on the surface of the mirror. We consider the effect of a short pulse of x-rays on the reflectivity of a variety of mirrors including W/C, WC/C, Cr3C2/C, Mo/Si, and MoSi2/Si multilayers. We use the measured x-ray spectrum from a gold target heated by a 0.5 ns laser for the case of no filter between the gold and the mirror and when various low energy filters are used. One tool we use to determine the reduction in reflectivity is a multilayer damage code ROMULS.1 We have modified the code to allow non-normal incidence and for a spectrum of x rays In the figure we show the calculated normal incidence reflectivity of a W/C multilayer mirror at 44.8 Å. The x-ray flux from the gold target is incident at 50° from normal and we show the reflectivity for three values of integrated fluence. The temporal dependence of the x-ray flux used in the calculation is shown. Two of the curves use an x-ray spectrum measured with no filter and one uses the spectrum after passing through a low energy filter. In all cases, the calculated reflectivity is reduced dramatically during the x-ray pulse. The calculated reflectivities with no damage are approximately a factor of four larger than measured. The calculation assumes smooth layers and no diffusion between the layers. We will discuss the reduction of reflectivity as a result of rough surfaces and diffusion between layers. Our modeling results will be compared with recent experiments that measured the reflectivity both temporally and spectrally.
Multilayer Structures Traversed by Electron Beams as Narrow Line X-Ray Sources
Authors: Kaplan A. E. , Law C. T.
Published: February 2023

When an electron beam traverses the interface between two media with different dielectric constants, the so called transition radiation are generation [1]. In a periodic structure with multiple interfaces, the radiation is greatly enhanced around a spatial region in a conical pattern for certain wavelength. In previous research [1,2] structures with the thickness of each layer is much longer than the wavelength are used. As a result, ultra-relativistic electrons with energies 100 Mev to 50 Gev [2] are required. However, with advent of new technologies, periodic multilayer structures can be constructed with a spatial period less then 100 Å. The generation of soft x rays with a low-energy electron beam that passes through such a multilayer structure was recently proposed [3]. In solid-state structures, the losses due to photoabsorption, and electron scattering may be the main obstacles in x-ray generation with these structures. We showed [4] that the energy of the electron beam from ~ 200 Kev to a few Mev is sufficient to compensate for the loss due to electron scattering. We further demonstrated that above the electron energy of ~ 200 Kev for most of the materials radiating soft x-ray frequency range, the photoabsorption is the limiting factor and electron scattering can be neglected. The phtotoabsorption imposes an energy “ceiling” above which further increase in beam energy is rewarded with diminishing radiation. Based on these results, we prescribe the optimal geometric configuration for maximum radiation from multilayer structures composed of certain materials at certain frequency and electron beam energy.
Roughness Evolution in Films and Multilayer Structures
Authors: Lagally M.G.
Published: February 2023

Structures fabricated from multiple layers of very thin films make up an increasing component of our high-technology base in electronics, photonics, and x-ray optics. In particular, in x-ray optics, quite dissimilar materials are combined to create what are effectively one-dimensional diffraction gratings, in which the layers must have indices of refraction that are as different as possible. But that feature alone is not enough. The interfaces between the different layers affect the quality of the structure as an x-ray reflector, bringing into consideration questions of crystal growth and the kinetic and thermodynamic mechanisms of growth. This paper provides an overview of the most important kinetic mechanisms that control growth and illustrates then with scanning tunneling microscopy (STM) and diffraction measurements.
In-Situ Grazing Incidence X-Ray Diffraction During Sputter Deposition
Authors: Clemens B.M. , Payne A.P. , Hufnagel T.C. , Bain J.A. , Brennan S.
Published: February 2023

Many aspects of thin film growth can be studied most directly using in-situ techniques. These include a host of critical phenomena which are only observed for very thin layers and are no longer available for study if the film is grown to bulk levels. Although sputter deposition is a very widely used deposition technique both in research and industry, in-situ studies of film growth during sputter deposition are rare. This is primarily because the sputtering gas precludes the use of conventional electron diffraction techniques. Photon based diffraction techniques, on the other hand, are immune to the deleterious effects of the argon sputtering gas and offer a powerful method for investigating film structure provided a source of sufficient brightness is used. We recently performed experiments in which grazing incidence x-ray scattering (GIXS)[1] using synchrotron radiation was implemented in-situ to study film growth by sputter deposition . A special UHV deposition chamber was developed with appropriate x-ray windows and precision sample positioning mechanisms needed in performing a diffraction experiment [2]. During the initial test of this new technique and apparatus, we examined two metal/metal systems: gadolinium/cobalt and molybdenum/nickel.
Stability Of Metallic Superlattices
Authors: Turchi P. E. A. , Sluiter M.
Published: February 2023

With the constant progress made in designing and manufacturing materials with composition modulation in one dimension on an atomic scale [1], artificial superlattices have opened up new opportunities to study both theoretically and experimentally a wide range of properties. This new class of materials consist of a sandwich of layers of different species or mixtures thereof, and with the development of atom by atom deposition techniques, composition, layer repeat period, growth orientation and chemical order may be controlled. Such atomic engineering has led to the discovery of new phenomena such as the supermodulus effect [2], giant magneto-resistance effect [3] and also complex phenomena at or near interfaces such as solid state amorphization [4]. Moreover, unusual properties are expected in one dimensional quasiperiodic multilayers [5].
Preparation and Characterization of Platinum-Carbon Multilayers
Authors: Yamashita K. , Lodha G.S. , Suzuki T. , Hatsukade I. , Ohtani M.
Published: February 2023

Development of platinum-carbon(Pt/C) multilayers is aimed at applying to grazing incidence x-ray optical systems such as x-ray telescope, x-ray microscope and x-ray beamline optics of synchrotron radiation.
Spectral Trends in the Status and Limitations of Multilayer Reflectance
Authors: Kortright Jeffrey B.
Published: February 2023

Different mechanisms can limit the reflectance of x-ray multilayers. The status of normal incidence reflectance vs. energy of multilayers will be discussed in the context of these different limiting mechanisms.
Epitaxial Growth with Dissimilar Materials: Basic Concepts
Authors: Zangwill Andrew
Published: February 2023

This talk purports to be a review of the basic concepts of epitaxial growth with a special emphasis on those aspects relevant to the growth of superlattices and multilayers. But the listener is warned that this subject is almost entirely terra incognita from the theoretical point of view when the material pairs differ significantly from one another. Nonetheless, talking points for the presentation include: diffusion on perfect and imperfect surfaces, the role of steps, chemical versus size effects, compound formation, template effects, sources of interface roughening, and kinetic energy effects.
W/C Multilayer Mirrors for 4.47nm X-Ray
Authors: Shao Jianda , Fan Zhengxiu , Yin Gongjie , Yi Kui , Yuan Lixiang
Published: February 2023

W/C multilayers had been investigated by many research workers[1,5]. Reported here is our preliminary results of deposition and characterization of W/C multilayer mirror for 4.47nm x-ray.
X-Ray, HRTEM and AFM Analysis of Thick and Thin Ni/C Multilayers
Authors: Abdali S. , Christensen F.E. , Joensen K.D. , Mogensen K. , Gerward L. , Garnæs J.
Published: February 2023

The evaluation of interface roughness as a function of the film thickness (100-500 layers) have been studied. The combination of X-ray reflectivity, High Resolution Transmission Electron Microscopy (HREM) and Atomic Force Microscopy (AFM) has provided detailed information about the structure of the deposited multilayers. Here we demonstrate the results and a comparison of these methods on three Ni/C multilayers of comparable periods (39Å<d<45Å) on Si single crystal substrates. The HREM investigation, using a cross-section technique, has shown interesting results, in that the Ni layers contained nanocrystalized particles. A diffraction pattern from such a sample revealed superlattice Bragg diffraction of the multilayer structure, the (110) diffraction pattern of the silicon substrate and diffuse rings of the amorphous carbon. No rings were observed for the crystalline Ni. Graded d-spacing multilayers were also investigated. The same preparation technique, used for HREM analysis, was employed for the AFM, not to study the surface of the multilayers but in order to make further characterization of the interface roughness of the multilayers. Comparisons of the results obtained by X-ray reflectivity and high resolution microscopy are reported.
A Stochastic Model for Multilayer Growth
Authors: Stearns D. G.
Published: February 2023

The majority of work in the theory of thin film growth has been directed at studying two extreme regimes of the problem. On the one hand, there have been numerous investigations of heteroepitaxial growth of thin films under near-equilibrium conditions. At the other extreme, there has been interest in the morphology of films grown under highly nonequilibrium conditions where the surface mobility of the adatoms is severely limited, resulting in strong shadowing effects and nonlinear columnar growth modes. It has been found, however, that the best conditions for producing x-ray multilayer structures correspond to an intermediate regime, that being far from equilibrium but under sufficiently energetic conditions to promote layer formation. Although of considerable technical importance, there has been little theoretical attention paid to this intermediate regime. Experimentally, this growth regime is realized by bombarding the surface during deposition with a primary or secondary flux of energetic particles, such as is achieved in ion-assisted evaporation or sputtering at low pressures. Under these conditions the deposited layers are usually amorphous or nanocrystalline and the root-mean-square (rms) roughness of the interfaces can be maintained to < 5 Å throughout the multilayer, a condition required for good x-ray optical performance. In this paper we present a simple linearized, stochastic model for the growth of multilayer structures in this moderately energetic regime.
Beamsplitting and Polarization Properties of Cr/C Transmission Multilayers close to the Carbon K-edge
Authors: Schäfers F. , Furuzawa A. , Yamashita K. , Watanabe M. , Underwood J. H.
Published: February 2023

The availability of circularly polarized synchrotron radiation in the soft x-ray region has in recent years stimulated many new experiments in this energy region. Essential for all experiments is the exact knowledge of the degree of circular polarization. So far only a few attempts were made for a complete determination of the full Stokes vector describing the state of polarization in this energy region. Due to the lack of suitable phase shifting elements conventional reflection optics and multilayer reflection optics can be used only up to approximately 100 eV /1-3/. Multilayers used in transmission were shown to provide a significant phase shift between the s- and the p-polarized components even above 100 eV /4,5/. At 100 eV a perfect quarter wave plate of Mo/Si has been reported /6-9/.
X-ray Supermirrors: Novel multilayer structures for broad band hard X-ray applications
Authors: Joensen K. D. , Gorenstein P. , Høghøj P. , Christensen F. E.
Published: February 2023

With the increased availability of hard X-ray (≥ 20 keV) synchrotron sources and a concurrent wish for hard X-ray telescopes. the need for hard X-ray optics has become considerable. For some applications crystals and periodic multilayers are desirable; for others the bandwidth of these may be too narrow. A wider bandwidth can be obtained by using total reflection from heavy elements. This approach. however, is impractical for harder X-rays since it is limited by the inverse proportionality between the critical angle of total reflection and the energy. One solution to this broad-band reflection problem may be the supermirror: a multilayer structure where the bi-layer thickness is gradually changed from top to bottom. In such a structure radiation with different wavelengths will have the criteria for constructive interference fulfilled at different depths in the multilayer. If the bi-layer thicknesses are changed sufficiently slowly. simultaneous diffraction of a continuum of X-ray energies can be obtained. The multilayered supermirror was originally designed for neutron applications1 such as increasing the throughput of guide-tubes. For neutrons, where absorption is extremely low for most materials. reflectivities up to95 can be obtained in a continuous band up to 3 times the critical angle or energy2. For X-rays. the absorption is considerably larger and this strongly influences the performance of the X-ray supermirror.
Determination of X-ray indices of refraction by interferometry
Authors: Svatos J. , Joyeux D. , Polack F. , Phalippou D.
Published: February 2023

As well known, the propagation of an electromagnetic field inside matter is described by the complex index of refraction n = 1-δ+iβ, where δ and β account respectively for phase retardation and absorption. δ and β are related by the Kramers-Kronig relationships, which theoretically, permits calculating δ(λ) from experimental determination of β(λ) [1]. Practically, however, the method gives questionable results near absorption edges, mainly because only discrete absorption data can be obtained in a finite wavelength range and some assumptions on the material structure must be introduced. More direct methods exist, based on reflectivity measurements [2-5]. Again, assumptions on the material structure must be introduced, which makes the results questionable near absorption edges. In all cases, the influence of β significantly limits the accuracy of the determination of δ. In addition, it is often found that the index values depends strongly on the conditions of deposition of materials. Such methods, which often make use of composite stacks, are therefore not well adapted.
Recent Studies of the Morphology and Soft X-Ray Optical Properties of Mo/Si Multilayers
Authors: Vernon S. P. , Stearns D. G. , Cerjan C.
Published: February 2023

Several laboratories are aggressively pursuing the development of soft x-ray projection lithography (SXPL) technology. The enabling technology for SXPL is the ability to produce high quality, multilayer reflectors in the soft x-ray spectral region. Mo/Si multilayers are of particular interest for SXPL since reflectivities as large as 66% have been obtained at normal incidence at 13 nm. The SXPL systems currently under development contain multiple reflective elements (typically 8:3 condensers, a reflective mask, and a four element imaging system). Marked increase in the system throughput results from slight improvements in multilayer reflectance: for an 8 element system, a 6% increase in multilayer reflectance yields a 50% increase throughput. Therefore, there is considerable interest in developing higher reflectance Mo/Si multilayer coatings.
Epitaxial Growth of Be(0001) on Ge(111) and Ge(111) on Be(0001)
Authors: Eickmann James , Slaughter J. M. , Falco Charles M.
Published: February 2023

Several desirable characteristics make single crystal beryllium films and Be-Ge heterostructures of interest in various areas. Because of its very low absorption at soft x-ray wavelengths,1 Be is attractive to the x-ray optics community for use as a spacer material in multilayer mirrors. Such mirrors would be of use in x-ray microscopy,2,3 lithography,4,5 and astronomy.6 Be is also potentially useful in infrared (IR) optics because of its high IR reflectance and good thermal properties.7 In addition to these optical properties, the electrical and electronic properties of Be thin films could also be utilized in other areas, such as electromagnetic shielding in space applications,8 studies of superconductivity,9,10,11,12,13,14,15 and hot-electron transistor technology.16
Characterization of interface structure in multilayers using specular x-ray reflectance
Authors: Fullerton Eric E.
Published: February 2023

X-ray reflectivity measurements are commonly used to characterize the structure and morphology of surfaces, thin films and multilayers.1-3 In most reflectivity measurements the scattering vector q is much smaller than the characteristic length scale for diffraction =2n/d, where d is the crystal lattice spacing. Therefore, the reflectivity measurements are insensitive to the crystal structure of the layers; the scattered intensity for multilayers occurs predominantly from the interfaces. It is this sensitivity to the interfacial structure that makes reflectivity measurements complementary to diffraction4 for characterizing multilayers. Nb/Si multilayers provides an instructive case because extremely smooth, as will as conformal and cumulative roughness cases can be realized by adjusting the sputtering conditions.5 We will discuss the quantitative analysis of the specular reflectivity from a series of Nb/Si multilayers sputtered at various Ar pressures. The multilayers were grown by dc magnetron sputtering onto sapphire substrates at ambient temperatures. The target-substrate distance was fixed at 9 cm and the Argon sputtering pressure was varied from 3 to 15 mTorr. This spans the range from high kinetic energy to nearly thermalized deposited atoms. The expected pressure for thermalization is ≈9 mTorr.
Glancing-Incidence X-ray Analysis of multilayer structures
Authors: de Boer D. K. G. , Leenaers A. J. G. , van den Hoogenhof W.W.
Published: February 2023

Glancing-incidence x-ray analysis (GIXA) is a combination of x-ray reflectivity and angle-dependent x-ray fluorescence (AD-XRF) in one instrument [1]. This method enables to obtain the compositional depth profile of layered materials, exploiting x-ray standing waves (XSW) [2]. With the same set-up, also diffuse scattering can be measured, yielding information on the lateral extent of interface roughness [3, 4, 5]. It is well known that specular reflectivity and AD-XRF depend on the root-mean square (r.m.s.) value of the interface roughness. As we will discuss below, they are affected by its lateral extent as well. First, however, we give some examples to show how XSW effects can be exploited for depth profiling of multilayers.
Trends in the X-Ray Diffraction of Multilayers
Authors: Tarrio C. , Deslattes R. D. , Caticha A. , Pedulla J.
Published: February 2023

Understanding the morphology of multilayer structures (MLS's) is an important step toward the preparation of high-quality multilayer mirrors for use in the extreme ultraviolet. To gain information about the structure, the x-ray reflectivity is measured at grazing angles. These data are then analyzed using any of a number of models to gain information about such characteristics as d spacing, spacing uniformity, heavy element fill factor (Γ), and diffusion boundary widths. Several models have been used with varying degrees of success to describe the x-ray diffraction of multilayers. The matrix method, in which the reflectivity is explicitly calculated at each interface, is exact for abrupt interfaces, but to include diffusion it is necessary to divide each boundary into many layers, which slows the calculations considerably. Traditional x-ray diffraction models allow for rapid calculations, but make approximations that are not generally applicable to multilayers. The kinematical model, in which only one reflection is allowed within the sample, is only accurate for low reflectivities, and thus is not valid near the Bragg peaks of the MLS. The dynamical model allows for multiple reflections, but is not accurate away from Bragg peaks at grazing incidence.
Characterization of Interface Structure in Multilayers Using Diffuse X-Ray Scattering
Authors: Sinha S. K.
Published: February 2023

The scattering of radiation (e.g., X-rays or neutrons) by multilayer films containing interface roughness is discussed. As is by now well known, such interface roughness usually possesses a degree of conformality (i.e., correlations between different interfaces), which is known to produce structure in the diffuse scattering which mimics that in the specular reflectivity [1-6]. There are more subtle effects when either the incident or scattered wave vector is close to the condition for specular Bragg reflection from the multilayer, which have also been noted [7]. Such effects can be treated within the framework of the Distorted Wave Born Approximation (DWBA) [8-10], which can also be thought of as including “double-scattering” (i.e., Bragg + diffuse scattering) processes. Interesting effects in the diffuse scattering (i.e., sharp minima or edges) manifest themselves in the line shapes observed for rocking curves at these positions, and these can be qualitatively explained within the framework of the dynamical theory of diffraction by considering the effects of correlated interface roughness. Such correlated roughness can produce both constructive and destructive interference in the dynamically scattered waves. The theory is illustrated with Synchrotron X-ray diffuse scattering studies from several Nb/Si multilayer samples [11].
Thin-Film Multilayer X-Ray Waveguide--A New Element of X-Ray Technic
Authors: Konstantinov Ya. A. , Dudchik Yu. I. , Komarov F. F.
Published: February 2023

The use of nanometer scale multilayers structures for optics of hard X-radiation is discussed.
Computer Simulation and Field Visualization in Diffraction Optics
Authors: Popov A. V. , Kopylov Yu. V. , Vinogradov A. V.
Published: February 2023

We describe numerical modeling of the wave fields produced by complex quasioptic elements. The parabolic equation method and exact transparency boundary conditions are used in the computational algorithm. The computer code provides global field visualization using color graphics.
Determining the vertical correlation of interfacial roughness in multilayer films using x-ray scattering*
Authors: Phang Y. H. , Savage D. E. , Zhang Z. Y. , Lagally M. G.
Published: February 2023

Multilayers have many interesting and potentially useful properties (mechanical, electronic, magnetic, and optical). They have drawn considerable scientific and technological interest. Their performance is frequently closely related to their structural perfection. Interfacial roughness is an imperfection that is always present in multilayers. The sources of interfacial roughness can be considered broadly as induced by either the film formation itself or by interdiffusion between species during or after the completion of the interface. Interfacial roughness contains information about the underlying atomic mechanisms during growth. An accurate description of the interfacial roughness is crucial not only in understanding the influence of roughness on the performance of multilayers, but also in elucidating the atomic processes that occur during growth.
Diffuse x-ray scattering measurements from multilayers
Authors: Schlatmann R. , Shindler J. D. , Verhoeven J.
Published: February 2023

We have measured diffuse x-ray scattering from three multilayer samples both at low and high angles (up to 2θ = 15 deg). The line shape of the diffuse scattering yields information on the lateral spatial frequency distribution of the roughness of the interfaces. However, at low angle those line shapes are stongly disturbed by dynamical effects. At high angle this is not the case, and therefore we are able to use the line shapes to extract roughness profiles of multilayers as a function of the constituting materials and deposition conditions. The samples used all consisted of 10 periods of roughly 6 nm and were deposited by electron beam evaporation at room temperature. The first two samples were Mo/Si. One of those had each of its layers bombarded with 300 eV Kr ions, while the other sample was not ion bombarded. The third sample was a non ion bombarded WRe/ C multilayer.
Nonlinear stage of the decomposition in multilayer structures.
Authors: Gorshkov K. A. , Nefedov I. M. , Fraerman A. A. , Shereshevsky I. A.
Published: February 2023

Interdiffusion in multilayer structures is a subject of much current interest [1]. Special interest is interdiffusion in stratifying solid solution.
Evolution of Roughness with Polishing at Fused Silica Surfaces
Authors: Kortright J. B.
Published: February 2023

Roughness limits x-ray multilayer specular reflectance and contributes to scattering. Roughness is generally more important for multilayer mirrors than for total reflection mirrors, because multilayers operate at higher momentum transfer than total reflection mirrors, where the effects of roughness are more severe. As the multilayer period decreases, the effects of roughness increase, so that roughness is generally expected to play a significant role in limiting reflectance in the small d-spacing limit. Roughness in multilayers can originate from microstructural defects within the multilayer, or from propagation of roughness from the substrate into the multilayer. In any case, complete understanding of the roughness in multilayers cannot be obtained without first understanding the roughness spectrum ofthe substrates on which the multilayers are grown.
Imaging Microchannel Plate Detector Systems
Authors: Timothy J. Gethyn
Published: February 2023

The microchannel plate (MCP) is a high-gain electron multiplier which can also provide a high-resolution imaging capability. A very wide range of imaging detector systems based on the MCP have been developed for use at all wavelengths from the x-ray region to the near infrared. The ability of the MCP to operate stably in an open-structure configuration under high vacuum makes it particularly useful for studies at extreme ultraviolet (EUV) and soft x-ray wavelengths. Sealed detector tubes in which the photocathode material is deposited directly on, or is mounted in proximity-focus with the front face of the MCP are widely used at ultraviolet and visible wavelengths. The intrinsic simplicity and rugged nature of these MCP detectors makes them particularly suitable for use in instruments for studies in space.
Miniature, Solid-State Photodetectors for Photon Correlation Spectroscopy and Laser Anemometry
Authors: Brown R G W , Ridley K D
Published: February 2023

The purpose of our investigations was to characterise cooled silicon avalanche photodiodes (APD) and assess their suitability as photon counting detectors for photon correlation laser anemometry and spectroscopy.
Astronomical Speckle Interferometry and Speckle Masking at Low Light Levels*
Authors: Weigelt G.
Published: February 2023

The atmosphere of the earth restricts the resolution of conventional astrophotography to about 1 arcsec. We will describe various speckle methods which can overcome image degradation caused by the atmosphere and telescope aberrations. These speckle methods yield diffraction-limited resolution, for example 0.03 arcsec for a 3.6m telescope. All methods can be applied to faint astronomical objects. In this case each speckle interferogram consists of a small number of photon events.
Statistical Limits of Fourier Transform Imaging in the γ-ray Energy Range
Authors: Hurford Gordon J. , Megeath S. Thomas , Palmer David , Prince Thomas A.
Published: February 2023

Recent advances in imaging techniques and position-sensitive γ-ray detectors have made feasible hard x-ray and γ-ray telescopes with arc-second resolution [1]. Above an energy of 100 keV, past instrumentation has been limited to a typical angular resolution of a few degrees. A γ-ray imaging device with 1 arc-second resolution would be a dramatic improvement over conventional, non-imaging instrumentation and have substantial new capabilities for observation of astrophysical γ-ray sources. The arc-second γ-ray imager is based on the Fourier transform imaging technique [2]. We briefly describe Fourier transform imaging and its application to hard x-ray and γ-ray imaging. This description is followed by an analysis of Fourier transform imaging in the statistics limited regime. Computer simulations and laboratory demonstrations of practical γ-ray imaging systems are presented.
Quantum Noise Simulations for a Synthetic Aperture X-Ray Telescope
Authors: Mertz L.N. , Nakano G.H. , Kilner J.R.
Published: February 2023

Quantum noise is examined for an imaging system where the photons are detected in the Fourier domain as opposed to the more familiar image domain.
Quantum Limits in Gamma-Ray Imaging
Authors: Barrett H. H.
Published: February 2023

Gamma-ray and X-ray imaging systems are required in many fields, including astronomy, industrial radiography, nuclear medicine and reactor safety research. Because hard X rays and gamma rays cannot be efficiently reflected or refracted, rather crude imaging systems are required, often with the result that very few photons are detected. Quantum noise is thus almost always the limiting factor in system performance. Nevertheless, there is considerable debate in the literature about how to specify and quantify these quantum limits. Widely differing conclusions are reached by different authors on questions such as: When is a pinhole preferable to a coded aperture in imaging a particular object? It is the goal of this paper to show that these discrepancies disappear when one carefully specifies the nature of the imaging system, the object or class of objects to be studied, the nature of any a priori information about the object, and the purpose of the imaging procedure. When the problem to be solved is thus carefully stated, general procedures can be given for quantifying the quantum limits to system performance.
Low Contrast Detection Limits of X-ray Screen-Film Imaging Systems
Authors: Motz J. W. , Danos M.
Published: February 2023

The input signal to an x-ray imaging system is given1 by the quantity, α |X1-X2|, where α is the area of the spatial resolution element and X1 and X2 are the X-ray exposures at points 1 and 2 in the image plane. Accordingly for a signal with a given magnitude, there is a tradeoff in the values of α and |X1-X2| such that a signal with high spatial resolution (small α) and high'contrast (X2≪X1) may be quantitatively equivalent to a signal with low spatial resolution (large α) and low contrast (X2≲X1).
Dead Time Correction of Photon Correlation Functions
Authors: Schätzel Klaus
Published: February 2023

While dead time corrections of single interval photon counting statistics received extensive attention in the literature (some references are given in [1,2]), similar calculations for even the simplest case of double interval statistics - photon correlation functions - were sparse and covered lowest order approximations [3] or rather special cases of clipped correlograms [4] only. The rapidly growing use of photon correlation techniques for velocimetry and, especially, for particle sizing or diffusion measurements in chemistry and biology, however, calls for a proper treatment of dead time effects, in order to increase the dynamic range of quasi elastic light scattering experiments towards higher count rates [5].
Estimation Under Quantum-Limited Conditions*
Authors: Snyder Donald L. , Miller Michael I.
Published: February 2023

The estimation of a signal from quantum-limited measurements of it is a problem arising in diverse scientific fields, including radionuclide imaging, electron-microscopic autoradiography, gamma-ray astronomy, optical communication, and low light-level photography. As the measurements become more quantum limited, it becomes increasingly important to use accurate mathematical models for the signal, noise, and measurement instrumentation and to use estimation approaches that are dictated by such models. In this paper, we describe a general random point­process model which accurately represents the characteristics of quantum-limited measurements in many situations. We then derive a method for realizing desired estimates within this model.
Quantum Statistics Basis for Maximum Entropy Restoration
Authors: Soffer Bernard H. , Kikuchi Ryoichi
Published: February 2023

Maximum Entropy (ME) estimation has been applied in various forms with various names to a wide variety of problems ranging from the depths of seismic spectral analysis, sonar and radar beam forming and filter formation, to astronomical imaging and beyond to economics. The particular techniques and theoretical points of view differ greatly among the several disciplines. Our very general interpretation, which includes these others as special cases, is based on two considerations. Any image, measured as signal, pattern or spectrum, whatever it represents, is necessarily a degraded version of the true object because real measurement systems have limited spatial and temporal bandwidth. The samples are finite and perhaps undersampled. Furthermore noise cannot be ignored. Therefore, many different possible object patterns can produce the same measured image pattern. One way to resolve this ambiguity is to apply the ME method. In our interpretation, a probability is assigned to every possible object pattern and the most probable pattern is chosen as the estimated or restored object. Patterns are assigned probabilities based on the physics and statistics of the immediate problem. The entropy is understood to mean the logarithm of the probability, following Boltzmann. So, to find a maximum of the entropy is to find a maximum of the probability, subject to the measured image data constraints and any a priori bias. No new "principle of ME" or appeal to information theory is needed to justify the method, though they may enrich our understanding. Sometimes misunderstandings have arisen in the use of the information theoretic entropy of Shannon, –f log f, and it has been used inappropriately. These considerations have been developed at length,1 so only a brief summary will be given here. We develop the idea of ME in an analogy to the well known statistical mechanical principle of the minimization of free energy, and derive some useful benefits in the consideration of fluctuations or noise. The degree of confidence in the ME estimate is derived and some examples of the ME method are given showing super-resolution.
Statistical Aspects of Low-Dose Electron Microscopy.
Authors: Ferwerda H.A. , Slump C.H.
Published: February 2023

The bombardment by electrons of (biological) objects in the electron microscope leads to radiation damage. Accordingly we have to minimize the number of electrons impinging on the object in order to get a reliable image. The image gets a very noisy appearance (shot noise) and the results derived from these images become stochastic quantities of which we would like to obtain the probability distributions. For this program to be carried out we have to know the relation between the image and the object. A schematic of the imaging process is depicted in Fig. 1. The image plane is supposed to be covered by N2 ideal electron counters, where N2 is the number of degrees of freedom (Shannon number) of the image.
Noise Properties of Superlattice Avalanche Photodiodes in Quantum-Limited Detection
Authors: Saleh Bahaa E. A. , Teich Malvin C.
Published: February 2023

Optical receivers are either operated in the photon-counting mode or in the photoelectric-current mode. An ideal photon-counting detector measures the number of photons received within the counting time. Its performance is governed by the photon statistics. For Poisson statistics, the signal-to-noise ratio (SNR) equals the mean number of photons per counting time,
SNR=n¯. For a detector with quantum efficiency η,
 SNR =ηn¯. The performance of detectors that measure the photoelectric current is limited by a combination of photon statistics and circuit noise. To eliminate circuit noise, devices with internal gain, such as the photomultiplier tube and the avalanche photodetector (APD) are used. Unfortunately, the gain mechanism itself introduces noise. Photon-counting receivers discriminate against gain noise by using a nonlinear filtering circuit that counts the electric pulse associated with each photoevent as a single count, regardless of its height. Thus they eliminate both circuit noise and gain noise and, ideally, achieve
 SNR =ηn¯.
Restoration Techniques for Quantum-Limited Images
Authors: Rabbani Majid
Published: February 2023

A review of the techniques used for the restoration of quantum-limited images is presented. The restoration is based on an image model degraded by Poisson noise and linear blur.
Speckle Masking and Speckle Spectroscopy in Astronomy
Authors: Weigelt Gerd
Published: February 2023

The atmosphere of the earth restricts the resolution of conventional astronomical imaging to about 1 arcsec. Much higher resolution can be obtained by speckle methods. The Knox-Thompson method and the speckle masking method (bispectrum or triple correlation processing) yield diffraction-limited images in spite of image degradation by the atmosphere and by telescope aberrations. For example, with a 3.6-m telescope a resolution of 0.03” can be obtained at a wavelength of 400 nm The limiting magnitude is about 18. We will discuss the theory and applications of speckle masking. High-resolution images and simultaneously the spectrum of each resolution element can be obtained by objective prism speckle spectroscopy and projection speckle spectroscopy methods. Finally, we will discuss the application of speckle masking to coherent arrays of telescopes. For example, observations with the 4x8-m ESO VLT can yield the fantastic angular resolution of about 2milli-arcsec.
Photon-Limited Performance of Three Stellar Speckle Image Reconstruction Algorithms
Authors: Aitken G.J.M. , Johnson R. , Meng J.
Published: February 2023

Atmospheric turbulence limits the resolution of conventional astronomical observations through large ground-based telescopes to about 1". Recently, sub-arcsecond resolution has been achieved by estimating the complex spatial spectrum of a stellar object from a sequence of short-exposure images called specklegrams. Speckle interferometry (SI) gives the diffraction-limited modulus [1]. Phase-measuring processes are the Knox-Thompson (KT) [2], the closely related Phase-Gradient (PG) [3] and the Triple-Correlation (TC) [4] algorithms. Because of the narrow bandwidth, Δλ ≈ 10 nm, and short-exposure times, tE ≈ 10 ms, the specklegrams of most stellar objects of interest contain less than 103 photons. Consequently, it is advantageous to record the specklegrams as a list of photon addresses.
An Approach to Bias Removal in Photon-Limited Speckle Imaging
Authors: Bates R. H. T.
Published: February 2023

Because the spatial power spectrum of an object can be estimated from photon-limited speckle images of the object, and because any shift-and-add image is ideally a convolution of the true image with some (unknown) psf, and because blind deconvolution of a single blurred image is always feasible in principle, self-calibrating bias-removal schemes are possible for photon-limited shift-and-add images, as will be demonstrated theoretically in this paper.
The transition from photonic band gaps to microcavities, a one-dimensional unified model
Authors: Houdré R. , Stanley R.P. , Oesterle U. , Ilegems M. , Weisbuch C.
Published: February 2023

Spontaneous emission can be controlled by reducing the number of allowed photon modes. One method suggested by Yablonovitch [1] and others [2], is to use three-dimensional periodic structures to create a Photonic Band Gap (PBG). Impurity states within the band gap can localise light and radically alter the emission characteristics of resonant luminescent centres. A second complementary approach followed by Yamamato [3] and Yokoyama [4] concentrates on microcavity structures, which quantize the photon modes by size effects.
Measurement of modulation saturation intensity in strain-balanced, undefected InGaAs/GaAsP modulators operating at 1.064 μm
Authors: Goossen K. W. , Cunningham J.E. , Santos M.B. , Jan W. Y.
Published: February 2023

The Nd:YAG laser has been considered as a possible light source for optical computing systems based on p-i(MQW)-n modulators, because of its ability to provide high power with high spectral and spatial quality. However, since this laser operates at 1.064 μm, the GaAs/AlGaAs material system cannot be used. Since the GaAs substrate is a better candidate for large-scale integration than InP, much attention has been focused on InGaAs/GaAs MQWs for this application.1-4 However, since the MQW must be at least 1 μm thick for useful surface-normal modulation, strain-relief is bound to occur in this system. This relaxation of the lattice results in dislocations which propagate upward resulting in a striated surface,3 causing diffraction of the light beams. In addition, the defects make the integration of GaAs transistors problematic.
Terahertz Electromagnetic Transients As Probes of a 2-Dimensional Electron Gas
Authors: Walecki W.J. , Some D. , Kozlov V.G. , Nurmikko A.V.
Published: February 2023

We demonstrate here that it is experimentally practical to employ the real­time THz probes to access an equilibrium electron gas in a semiconductor heterostructure at a low temperature and magnetic field, and study the response of a 2D electron gas to picosecond transient electric fields in an n-type MQW GaAs/GaAlAs structure. The experimental arrangement is shown in the upper trace of Figure 1. The collimated beam passes a free standing wire grid polarized and is subsequently focused inside a superconducting magnet onto a sample (with a spot size of approximately 6 mm). The crossed polarizer (power) extinction ratio was about 4x10-4. The average power in the THz beam was measured by a bolometer to be approximately 1 nW, corresponding to an average flux of 104-105 photons/cm2 per pulse incident of the samples.
GaInAs/InP Long-Wavelength Quantum-Wires and Boxes: Fabrication Technology and Lasers
Authors: Arai Shigehisa
Published: February 2023

Improvements in the regrowth process of a low-pressure OMVPE and the
usage of p-type InP substrate enabled to obtain a room temperature CW
operation of GaInAs/InP quantum-wire laser. Approaches to high
performance lasers consisting of low-dimensional quantum-well
structures will be presented.
Temperature Dependence of Electron and Hole Emission Rates from a Single Quantum Well with Asymmetric Barriers
Authors: Bambha R. , Snelling M.J. , Li-Kam-Wa P. , Miller A. , Cavailles J.A. , Miller D.A.B. , Cunningham J.E.
Published: February 2023

In recent years, much attention has been focused on carrier emission rates and cross-well motion in semiconductor quantum well structures because the efficiency and response speed of many MQW optoelectronic devices depend on the dynamics of vertical carrier transport processes. Thermionic emission is believed to be the principal mechanism for the escape of the carriers from each individual quantum well. In most cases, measurements of the transport dynamics have been carried out in structures that contain multiple quantum wells. However in multiple well structures, effects such as resonant tunneling and carrier retrapping make it difficult to unambiguously separate the effects from one another. In recent measurements on single quantum wells with asymmetric barriers, we monitored electron and hole sweep-out rates simultaneously as a function of electric field(1), and showed that the carrier emission from the wells is thermionic in nature, but this preliminary analysis did not give very good quantitative agreement with the conventional thermionic emission model(2). In this paper, we will present a study of the emission for both carrier types as a function of temperature and electric field in order to fully evaluate the applicability of the thermionic emission model to the process of carrier escape in a single quantum well.
Analog Differential Self-linearized Quantum Well Self-Electrooptic Effect Modulator
Authors: De Souza E.A. , Carraresi L. , Boyd G.D. , Miller D.A.B.
Published: February 2023

We demonstrate a new mode of operation for quantum well self-electrooptic-effect devices (SEEDs) using their analog features, and our results shows that this device is potentially useful for analog systems. This device was recently proposed as a new analog SEED circuit operating with differential pairs of light beams[1]. It uses the difference in two light beam powers to represent an analog value. Normally, processing such bipolar values is difficult with optics. This new circuit allows many different analog functions to be performed, including addition, subtraction, and differentiation of images, correlation, and optically controlled bipolar matrix-vector multiplication[2]. In general, such operations operate on and generate both positive and negative values; this circuit allows full use of such values in analog systems, and, since it was fabricated using the symmetric SEED array process[4], it is compatible with being fabricated in large two-dimensional arrays. We demonstrate the linear performance of this circuit from 50 nW to 3 mW optical powers.