bluuebunny/crossref_metadata_2025 huggingface dataset:
Phase Error Correction by Shear Averaging
Authors: Fienup J.R.
Published: February 2023

Synthetic aperture radar (SAR) [1,2] (and other imaging systems) measures the complex Fourier transform (called the signal history or phase history) of the scene being imaged, but it often suffers from one-dimensional (1-D) phase errors due to unknown system platform motion, target motion, system phase instabilities, and propagation through atmospheric turbulence. If uncorrected, these phase errors can cause severe blurring or smearing of the imagery. Phase errors (the residual phase errors remaining after correction for the measured motion) can be corrected by digital phase-error correction (sometimes called autofocus) methods, the most widely used being "sub-aperture processing" and "prominent-point processing." The disadvantage of sub-aperture processing, which is analogous to the concept of a Hartmann sensor in optics, is that it only works for low-order (up to about fourth-order) polynomial-type phase errors. The disadvantage of prominent-point processing, which deconvolves an image based on an estimate of the impulse response of the system, is that it requires the presence of an identifiable, isolated, strong point-like reflector in the scene being imaged. Furthermore, both of these two processing methods require extensive computations.
Superresolution with an Opacity Constraint
Authors: Paxman R.G.
Published: February 2023

The term superresolution can be defined as the use of a priori information to achieve resolution superior to the diffraction-limited resolution. In this paper we explore the use of a novel type of prior knowledge in the context of 3-D imaging. We wish to exploit knowledge that the object being imaged is confined to a 2-D manifold (surface) embedded in a 3-D space. Such prior knowledge is valid when the object is opaque to the illuminating radiation so that only secondary sources that lie in the outer surface of the object contribute to the reflected field.
Image Motion Recovery Using the Method of Total Least Squares
Authors: Chu C. Henry , Delp Edward J.
Published: February 2023

Recovering image motion in the form of a velocity field is particularly useful in image analysis when the signal-to-noise ratio is low in individual frames of an image sequence. The velocity vector of a point on an image plane is the position change of the projection of an object point due to a change of viewing angle between the sensor and the object point. A change in the viewing angle can be caused by the velocity of the sensor, or the movement of the object point in the scene, or both. The velocity vector is thus an approximation to the velocity of the image point.
Coding and Quantization of Digital Holograms with Iterative Fourier Transform Algorithms
Authors: Wyrowski F.
Published: February 2023

The basic problem in digital Fourier holography is the calculation and fabrication of a thin diffractive structure - the digital hologram - which possesses a special Fraunhofer diffraction pattern. We call the diffracted light distribution signal and indicate it with f(x). Because f(x) is the Fraunhofer spectrum of the hologram transmittance F(u), we have the Fourier relationship Computer calculation of this equation results in a sampled version of F(u). In general the calculated data set is complex valued and analogous, i.e. the fabricated diffractive structure has to control the amplitude and the phase of the illuminating wave in an analogous manner. The fabrication of such a structure is very difficult.
Quantitative Comparisons of Choices of Prior Information in Image Reconstruction
Authors: Gooley T.A. , Barrett H.H. , Barth M. , Denny J.L.
Published: February 2023

Medical image reconstruction is fraught with problems that are a result of noisy and incomplete data. The incomplete data give rise to null functions that are associated with the imaging operator, thus yielding an infinite number of solutions that fit the data equally well. Noise in the data often will lead to very rough reconstructions, which can be inconsistent with previous experience. The use of prior information can sometimes be introduced to help alleviate the aforementioned problems. If one knows that an object (or class of objects) possesses certain characteristics, then the reconstructions should possess the same characteristics.
High Resolution Image Recovery from Image Plane Arrays Using Convex Projections
Authors: Stark Henry , Oskoui Peyma
Published: February 2023

In remote sensing and astronomy, images are often reconstructed from data obtained by fixed or scanning detector arrays superimposed on the image field [1]. Since detectors are typically much larger than the blur spot of the imaging optics, a naive reconstruction of the image from the detector data would produce an image of lower resolution than that furnished by the imaging optics. The reduction of detector size to match the blur spot of the imaging optics may not be technically feasible, and even if it were, the associated signal-to-noise ratio of the detector output might be too low to be useful.
An Analysis of Regularized Linear Image Recovery
Authors: Galatsanos Nikolas P. , Katsaggelos Aggelos K.
Published: February 2023

In many applications that necessitate image recovery the imaging system can be modeled as a convolution of the original image with an ill-posed operator and then adding noise. We assume that the M×M original image is blurred by a space-invariant convolution operator and independent identically distributed (i.i.d.) zero mean additive noise described by the model where H is a M2×M2 Toeplitz matrix modeling the space-invariant blur, the vectors g, f, and n are M2×1 lexicographic orders of the degraded observed image, the original image and the noise, respectively.
Aperiodic Grating for TE02 to TE01 Conversion in a Highly Overmoded Circular Waveguide
Authors: Ul Haq Tanveer , Webb Kevin J. , Gallagher Neal C.
Published: February 2023

Periodic gratings have frequently been used for conversion of modes in highly overmoded circular waveguides [1, 2, 3]. These gratings are formed by periodically varying the waveguide radius resulting in a rippled wall structure and are usually analyzed by coupled mode theory. Very high efficiencies have been reported for these gratings but their lengths remain large compared to the waveguide transverse dimension. Various techniques have been implemented to optimize the length of these gratings [3, 4, 5], but the overall conversion length remains limited by the grating period, δ = 2π/|β
m
– β
n
|, where β
m
and β
n
are the propagation constants for the input and the output modes. The smallest conversion length reported for a TE02 to TE01 mode converter at 60 GHz is equal to one grating period [4]. This converter was designed for a highly overmoded waveguide with a diameter of 2.771 cm using the coupled mode equations. The efficiency reported for this converter is 97.6%.
Noise in Frequency-Resolved-Optical-Gating Measurements of Ultrashort Laser Pulses
Authors: Fittinghoff David N. , DeLong Kenneth W. , Trebino Rick , Ladera Celso L.
Published: February 2023

Frequency-resolved optical gating[1, 2] (FROG) is a technique that uses a phase-retrieval algorithm to obtain the intensity, I(t), and phase, ϕ(t), from a measured spectrogram of the pulse. Previous simulations have shown that, for noise-free data, the algorithm retrieves the correct intensity and phase for all pulses attempted, including those with complex intensity and phase structure. In practice, however, noise is present in actual FROG traces, and here we discuss the effects of noise on FROG and image-processing techniques to improve the retrieval.
Direct Ultrashort-Pulse Retrieval Using Frequency-Resolved Optical Gating and a Computational Neural Network
Authors: Ladera Celso L. , DeLong Kenneth W. , Trebino Rick , Fittinghoff David N.
Published: February 2023

Frequency-Resolved Optical Gating (FROG) is a method for measuring the time-dependent intensity and phase of an ultrashort laser pulse. In FROG a nonlinear autocorrelation signal is frequency-resolved by a spectrometer to produce a "FROG trace", which is a type of spectrogram of the pulse [1]. The FROG trace, a two-dimensional image (intensity vs. frequency and delay) is then input into a phase-retrieval-based iterative algorithm [2], that determines the intensity and phase of the laser pulse. Although the FROG algorithm performs well, it requires a minute or more to converge for complex pulse shapes. It is therefore desirable in many situations to have a direct (i.e., non-iterative) computational method capable of quickly inverting the highly non-linear and complex function that relates the ultrashort pulse intensity and phase to its experimental FROG trace. In this work, we show that computational neural networks can directly obtain the intensity and phase of a pulse from its FROG trace in less than one second, independent of the pulse shape. Our demonstration using a serial personal computer is a proof of this principle, utilizing a set of pulses defined by only five parameters. Because neural networks now take advantage of very simple, fast, and powerful parallel-processing hardware, however, future waveform recovery, even in the general case of arbitrary pulses, could be nearly instantaneous.
Holographic Reconstruction for Macromolecular Structure Completion in X-Ray Crystallography by Iterative Applications of Linear Programming
Authors: Wild D. L. , Chen X. , Saldin D. K.
Published: February 2023

The problem of the recovery of some unknown part of a crystal structure from a knowledge of another part of the structure and X-ray diffraction intensities is a familiar problem in crystallography. Amongst the techniques developed for tackling this problem is the difference Fourier method. Another one, recently proposed by Szöke [1], is based on an analogy with holography [2].
Comparison of shift-and-add & bispectrum image reconstruction methods for astronomy in the near-infrared
Authors: Klückers V.A. , Longmore A.J. , Dainty J.C.
Published: February 2023

It is well known that atmospheric turbulence limits the resolution available to ground based astronomical observations to 0.5-1.0 arcseconds in the infrared. The advent of speckle interferometry in the 1970’s [1] has allowed the recovery of diffraction limited Fourier modulus information of astronomical objects of interest to be attempted routinely. A number of methods have since been proposed to obtain diffraction limited Fourier phase information, and thus image recovery. In the visible, where D/r
o
is large, it is now generally accepted that phase recovery from the average image bispectrum (or equivalently the triple correlation) appears to be the most successful [2] [3] [4].
Simulation Validation of Phase-Diverse Speckle Imaging
Authors: Paxman Richard G. , Seldin John H.
Published: February 2023

Astronomers have long known that the resolution in ground-based astronomy is usually limited by aberrations introduced by the atmosphere. Over the years, researchers have developed a variety of clever pre- and post-detection approaches for correcting these effects, each with its own merits and regimes of operation. These approaches include stellar speckle imaging, deconvolution from wavefront sensing, and adaptive optics. We have been investigating a novel data-collection and processing approach for combating the effects of atmospheric seeing called phase-diverse speckle imaging.
Blind deconvolution and phase retrieval using point zeros
Authors: Chen Pi-tung , Fiddy M.A. , Greenaway A. H. , Pommet Drew A.
Published: February 2023

The Fourier transform of a signal or image of compact support is an entire function of exponential type which can be represented in terms of their zeros by means of a product of factors each encoding a zero of the function. In more than one dimensional problems, an entire function is generally not factorizable into an infinite product of terms, but is irreducible, [1,2] from which it follows that there is a unique phase to be associated with its power spectrum, in principle.
Ocean acoustic tomography
Authors: Cornuelle Bruce D.
Published: February 2023

Ocean acoustic tomography (OAT) is a method proposed by Munk and Wunsch (1979) to observe the ocean using acoustic transmissions. Although the ocean is nearly opaque to most electromagnetic radiation, acoustic waves propagate well, and low frequencies (less than 100 Hz) can propagate usefully for more than 10 megameters (Munk, Worcester, and Wunsch, 1995). The term "tomography" was chosen to highlight the analogy to medical computed axial tomography (CAT), although because ocean instrumentation and access are expensive, the number of data available in OAT has been far smaller than in medical applications. For this reason, the reconstruction of the unknown field depends critically on utilizing all information, including prior guesses of first and second moment statistics of the data and unknowns and, in some cases, dynamical models of the ocean.
Effect of the “∇D” Term in Optical Diffusion Imaging
Authors: Ye J. C. , Millane R. P. , Webb K. J. , Downar T. J.
Published: February 2023

Optical diffusion imaging in highly scattering media such as tissue, as an alternative to X-ray tomography, presents significantly lower health risks, and has successfully demonstrated its potential in biomedical applications. Although current reconstruction algorithms have been applied with some success, there are a number of opportunities for improving both the accuracy of the reconstructions and the speed of convergence. Most studies of frequency-resolved diffusion imaging involve casting the diffusion as a Helmholtz equation, which involves the approximation that ∇D = 0. We describe here an analysis of the effects of this approximation on both the generation of synthetic data for simulations, and the effect on reconstructions.
A Comparison of Microwave Inverse Scattering and Imaging Techniques
Authors: LaHaie Ivan J.
Published: February 2023

Microwave imaging and inverse scattering, although closely related disciplines, have nonethe-less evolved separately over the last few years because of the differences in their respective applications. While both seek to characterize a scattering object (the “target”) from measurements of its scattered field, the methodology used historically to achieve this end differs. Although there is no formal delineation between the two, microwave imaging techniques typically construct a spatial distribution of a field or source-like quantity, such as current, reflectivity, or scattering centers, which is linearly related to the scattered field, but which is often nonliteral and difficult to interpret. Inverse scattering methods, on the other hand, generate reconstructions of the target’s intrinsic characteristics, namely shape and materials properties, which are nonlinearly related to the measured data, and hence require significant computational resources for their implementation. For these reasons, inverse scattering is considered more rigorous and quantitative, while imaging is generally applicable to a wider class of targets, particularly those which are electrically large and complex.
Image recovery from far-field data at 10GHz
Authors: Fiddy M. A. , McGahan R. V. , Morales-Porras A. E. , Morris J. B.
Published: February 2023

Methods for diffraction tomography, which are both numerically feasible and mathematically rigorous, have required that the scattering object only interact weakly with the incident field1,2. The approximations used include the first-order Born and the Rytov methods, which are rarely appropriate in practice, thus limiting their usefulness. More general methods or "exact" inversion procedures have proved extremely difficult, if not impossible, to implement. Recent developments have been made which do extend the domain of validity of the Born and Rytov approximations to some extent2 these are based on the distorted- wave Born or Rytov approximations. These methods assume that the strongly scattering component of the object is known, and that an unknown perturbation to this satisfies the Born or Rytov approximation. Thus, some a priori information about the scatterer must be acquired and which represents a (strongly scattering) background, against which small fluctuations in permittivity are imaged.
Phase Retrieval for Multiple Undersampled Polychromatic Images
Authors: Fienup J.R.
Published: February 2023

We previously developed gradient-search phase-retrieval algorithms and used them to determine the aberrations of the Hubble Space Telescope [1-4]. We were not able to apply these algorithms to all the available data because (i) our algorithms [3-4] required that the images of point objects be of narrow spectral bandwidth, limiting us to using images recorded through narrowband optical filters, and (ii) we required that the measured data in the focal plane be Nyquist sampled for the optical fields, limiting us to the longer wavelengths and the Planetary Camera.
New findings on the Zeros of Fourier Integrals
Authors: Noushin A. J. , Fiddy M. A.
Published: February 2023

There has been considerable interest over the years in the so-called Fourier phase retrieval problem. Applications abound and it still remains a difficult problem. Based on the analytic properties of bandlimited functions, it is well known that the 1D phase retrieval problem generally has no unique solution. The lack of uniqueness arises from the existence of complex zeros located off the real axis, i.e. in the complex plane. These analytic properties also suggest that in 2D or higher dimensional problems there is a unique solution1. The question is how to find this “unique” solution, especially when only noisy sampled power spectral data are available. Indeed, the meaning of uniqueness needs to be redefined under these circumstances.
Design of Photonic Bandgap Structures
Authors: Yablonovich Eli
Published: February 2023

We will pursue the rather appealing analogy1-4 between the behavior of electromagnetic waves in artificial, 3-dimensionally periodic, dielectric structures, and the rather more familiar behavior of electron waves in natural crystals.
LD Pumped Tunable Single Frequency Polarized Nd:YVO4 Laser
Authors: Huo Yujing , He Shufang
Published: February 2023

Many deffercnt diode-pumped single-frequency solid-atate lasers have been demonstrated in recent years. The LD pumped tunable single frequency polarized laser is of greet importance for precision measurements. A LD pumped 1.06μm single frequency Nd:YAG micro-chip laser with 120GHz of tunable frequency range and 0.5 mW of output power has been reported. For some important applications the higher laser power and polarized laser beam are needed. We have reported the LD pumped CW and pulsed micro-chip Nd:YVO4 laser with high polarized output power. Here we report the LD pumped tunable single frequency Nd:YVO4 laser with higher output power and polarized beam.
Dark Soliton Stripes, Vortices and Soliton-induced Waveguides Formed in Bulk Photorefractive Media
Authors: Chen Zhigang , Segev Mordechai
Published: February 2023

Photorefractive spatial solitons [1-3] are promising for applications in all-optical switching, optical interconnects and the development of novel photonic devices. We report on study of steady-state dark photorefractive screening-solitons, formed when a laser beam containing a dark notch propagates through a bulk strontium barium niobate crystal biased by an electric field. A dark photorefractive soliton, although generated with a weak beam, induces a waveguide in the bulk of the crystal that guides other strong beams of longer wavelengths. Fundamental, Y-Junction and multiple dark photorefractive solitons are observed, so are the waveguides they induce that can guide other beams into multiple channels [3]. A dark photorefractive soliton can also couple with another dark or bright soliton, forming interesting coupled spatial soliton pairs that consist of self-guided propagation of two beams in the photorefractive materials [4]. Recently, we have demonstrated steady-state self-trapping of circular and elliptical optical vortices (in both transverse dimensions) due to two distinctly different nonlinear mechanisms: the bulk photovoltaic effect in an unbiased LiNbO3 crystal, and the photorefractive screening effect in a biased SBN crystal [5].
Pulsed laser operation of Yb doped KY(WO4)2, and KGd(WO4)2
Authors: Kuleshov N.V. , Lagatsky A.A. , Podlipensky A.V. , Mikhailov V.P. , Heumann E. , Huber G.
Published: February 2023

Recently, efficient cw laser action has been demonstrated in two new Yb3+-doped crystals KY(WO4)2 (KYW) and KGd(WO4)2 (KGW) under Ti-sapphire and diode laser pumping [1,2], In this paper laser performance of these tungstates is reported under pulsed Ti-sapphire laser pumping.
Enhancement of Conversion Efficiency of Intracavity-Frequency-Doubled Nd:YVO4 Laser by Cross-Polarized Longitudinal Modes
Authors: Hsieh H. S. , Hsu C. S. , Huang S. L. , Kao F. J.
Published: February 2023

Significant amount of effort has been made in the development of diode laser pumped green light Nd:YVO4 through intracavity frequency doubling in recent years. The sum frequency generation from longitudinal modes often cause the so called 'green problem' even at low power operation [1]. Nonetheless, it has been shown that higher frequency conversion efficiency can be made possible by utilizing the interplay between second harmonic and sum frequency generation to produce periodic pulse oscillation [2]. In this paper, we are showing that under cross-polarized two-mode oscillation (CPTMO), the convergent efficiency can be 50 % higher than that of parallel polarized two-mode oscillation (PPTMO).
Frequency Conversion of High Peak and High Average Power Lasers*
Authors: Ebbers Chris A.
Published: February 2023

Mercury1 is an advanced diode pumped laser facility under development at Lawrence Livermore National Laboratory as a 10% scale prototype of a single beamlet of a future inertial confinement fusion driver. The goals of this Yb:S-FAP based laser are 1 kW average power (100 J/pulse, 10 Hz) with a pulsewidth near 10 ns and a 10% wall-plug efficiency. High conversion efficiency (>50%) to the second harmonic (523.5) at the full aperture (3 cm x 5 cm) of the source is desired. We compare the potential of the available frequency conversion materials in the context of the peak and average power constraints using the ‘threshold power’ and ‘thermally limited power’ models developed at LLNL.
A New Kind of Nonlinear Optical Phenomenon
Authors: Huo Yujing , Wang Pu , He Shufang , Lu Baosheng , Wei Zibiao
Published: February 2023

For the first time in the world, we found that there are two frequency-doubled laser beams simultaneously in the same self-frequency-doubled laser with rectilinear resonant cavity. One of the beams is the normal frequency-doubled laser beam propagates along the axis of the cavity, and the other is abnormal frequency-doubled laser beam. They are simultaneously generated in the same nonlinear crystal and are of the same frequency but propagate in two deferent directions orthogonal to each other.
Regimes of Surface Roughness Measurable with Light Scattering
Authors: Vorburger T.V. , Lettieri T.R. , Marx Egon
Published: February 2023

We have performed several experiments1,2,3 on the scattering of light from moderately rough surfaces in order to address the questions: how rough a surface can be usefully measured by light scattering? and what parameters of the surface roughness can be reliably measured? We obtained optical scattering results with an angular resolved scattering system called DALLAS4 (detector array for lasser light angular scattering) which uses a HeNe laser (λ=0.6328 μm) as the source of light. Measurements were performed on a number of surfaces with roughness (σ) ranging from about 0.02 μm to about 1 μm. The results were analyzed by different methods depending on the rms roughness and compared with the results of stylus measurements.
Comparison between laser stylus and contact stylus measurements on rough surfaces
Authors: Mattsson Lars , Wågberg Pia
Published: February 2023

The introduction of submicron sensitive laser stylus profilometers1,2 based on the compact disk (CD) laser pick-up head and operating by an auto focusing technique, has opened up a new area for non-contact surface roughness measurement in the critical micron range. As many machined surfaces, paper surfaces and painted coatings have surface structures in this roughness range the field for applications is very wide. Careful investigations are therefore needed to show how the laser stylus technique compares with the common technique of contact stylus measurements. This paper, which is related to a thorough study on surface roughness of paper and ceramic materials,3 presents some of the special features observed for the laser stylus instrument.
Light Scattering from Photofabricated Surfaces with non-Gaussian Statistical Properties
Authors: Méndez E. R. , Espinosa R. , Aguilar J. F. , Escamilla H. M. , Gu Z. H.
Published: February 2023

The scattering of light from randomly rough surfaces has attracted a great deal of attention in recent years. Despite all this work, several aspects of the problem remain poorly understood. More specifically, analytical progress is difficult in situations when multiple scattering effects are important, when the statistics of the surface are non-Gaussian, and when the surface profile is multiscale (fractal-like). For a number of years, it has been clear that more systematic studies of the problem are required. In particular, for experimental work, it is important to be able to fabricate surfaces with known statistical properties in a more or less controlled way.
Is The Opposition Effect of Saturn’s Rings Caused by Coherent Backscattering?
Authors: Muinonen Karri
Published: February 2023

Theoretical studies by Shkuratov (1988 and 1991), Muinonen (1989 and 1990), and Muinonen et al. (1991) have indicated the importance of coherent (or enhanced) backscattering for interpreting observations of the regoliths of atmosphereless solar system bodies. My work concentrated on scattering by two dipole particles (dipole ↔ dipole), scattering by a dipole particle close to an interface (dipole ↔ surface), and second-order external reflection from two spheres large compared to the wavelength (surface ↔ surface), and I showed that the coherent backscattering peak is unequivocally accompanied by a reversal of linear polarization. The calculations indicate the task of modeling: for all particle and surface roughness sizes, and for all volume densities, studies of electromagnetic scattering by single particles, their rough surface structures, and scattering by volumes consisting of such particles need to be carried out.
Very Near Specular Scatter Measurements
Authors: Stover John C. , Bernt Marvin L.
Published: February 2023

Near specular scatter from imaging optics can be the key factor
limiting resolution. This is especially true for large diameter space
based telescopes that do not have to contend with atmospheric
distortion and are capable (in principle) of resolving very tight
diffraction limited points. This paper discusses a laboratory
technique to measure, and/or calculate, the near specular BRDF of such
optics so that manufacturing techniques of low scatter mirrors can be
optimized and final product can be checked. The technique was
conceived in response to a government lab interested in measuring to
"a few arc seconds from specular."
Options and Issues for XUV FEL Oscillators
Authors: Madey John M. J. , Deacon D.A.G. , LaSala John
Published: February 2023

Recent developments in FEL physics, accelerator, and optical technology point the way towards the development of a powerful and flexible new class of XUV lasers.
The Optical Klystron
Authors: Elleaume P. , Billardon M. , Ortega J.M. , Lapierre Y. , Bazin C. , Marilleau J. , Bergher M. , Couperie M. E. , Velghe M. , Petroff Y.
Published: February 2023

An optical klystron is a magnetic device consisting of two undulators separated by a "dispersive section". It can be used for free electron laser experiments (1) (with an optical cavity) or for harmonic generation of coherent radiation in the ultraviolet and the vacuum ultraviolet with the help of an external laser (2).
Transition Radiation as a Coherent Soft X-ray Source
Authors: Moran M. J. , Dahling B. A. , Piestrup M. A. , Berman B. L. , Kephart J. O.
Published: February 2023

A series of experiments using 54 MeV electrons from the Lawrence Livermore National Laboratory electron-positron linear accelerator has measured the spectral and angular distributions of soft x-ray transition radiation generated by targets of thin low-Z foils. Early results demonstrated the spatial coherence of x-rays generated at the two surfaces of single foils.1,2
X-Ray Microscopy Experiments with Synchrotron Radiation.
Authors: Schmahl G.
Published: February 2023

During the last years an X-ray microscope has been developed at the University of Göttingen based on the development of condenser zone plates and high resolution micro zone plates as X-ray lenses. The X-ray optical and mechanical set up of the system, which has been established at the BESSY storage ring in Berlin, is described in /1/. X-ray microscopy experiments of biological specimens with a resolution of 50 nm and exposure times of a few seconds have been performed using this microscope with synchrotron radiation of the storage ring in the wavelength region between 2.3 nm and 4.5 nm. As an example spores of the moss Dawsonia superba have been imaged /2/. In addition, measurements have been made to determine the lethal radiation dose for these spores.
Prospects for Soft X-Ray Lasing Driven by High-Power Optical Lasers
Authors: Rosen M. D.
Published: February 2023

We review the recent progress of experiments at the 2-beam target chamber of the Nova laser system at LLNL, and present prospects for further progress on that system as well as at the 10-beam Nova target chamber.
Spectroscopy of Collision-Free Multiphoton Processes in Intense Ultraviolet Laser Fields
Authors: Johann U. , Luk T. S. , McIntyre I. , McPherson A. , Schwarzenbach A. P. , Rhodes C. K.
Published: February 2023

The availability of two intense UV-light sources operating at 193 nm (~ 5 psec, 1015 W/cm2) and 248 nm (< 0.5 psec, ~ 1016 W/cm2), respectively, has allowed comparative studies of the wavelength, pulse rise time and peak intensity dependence of the atom-field interaction.
Symmetries and Asymmetries in Above-Threshold Ionization
Authors: Bucksbaum P. H. , Bashkansky M. , Schumacher D. W.
Published: February 2023

Angular distributions of atomic electrons photoionized above threshold by intense elliptically polarized light contain strong asymmetries with respect to reflection about either principal polarization axis.1 These observations, which contradict Keldysh-Faisal-Reiss theory predictions,2,3,4 point to the significant role of the long range electrostatic potential of the ion in shaping the wave function of the outgoing electron. The experimental evidence for this phenomenon is reviewed for helium, krypton, and xenon photoelectron spectra, as a function of laser intensity, polarization, and electron energy.
X-Ray Holography: X-Ray Interactions and Their Effects
Authors: London Richard A. , Trebes James E. , Rosen Mordecai D.
Published: February 2023

We summarize a theoretical study of the interactions of x rays with a biological sample during the creation of a hologram. The choice of an optimal wavelength for x-ray holography is discussed, based on a description of scattering by objects within an aqueous environment.

The problem of the motion resulting from the absorption of x rays during a short exposure is described. The possibility of using very short exposures in order to capture the image before motion can compromise the resolution is explored.

The impact of these calculation on the question of the feasibility of using an x-ray laser for holography of biological structures is discussed.
Recent Advances and Prospects of Bragg-Fresnel Optics
Authors: Aristov V. V.
Published: February 2023

Diffraction optics, i.e. Fresnel zone plates, holograms and diffraction gratings, has a limited application in the optical wavelength range. A decrease in wavelength to 100 nm and less changes conventional relationships between different types of optical elements owing to the change in the radiation-substance interaction since absorption starts to play a more important role as compared to refraction. Therefore at 1 nm≲λ≲100nm focusing elements may be fabricated only from thin membranes di stinguished by an abrupt change of either absorption (amplitude zone plates) or transmitted wave phase (phase zone plates).
Effectiveness of Metal-Coated Diffraction Gratings at Grazing and Near Normal Incidence as Harmonic Scrubbers
Authors: Wantuck P. J. , Appert Q. D. , Tong K. , Pistoresi D. J.
Published: February 2023

High power free electron lasers (PEL) can produce significant amounts of harmonic radiation. These harmonic components represent a substantial damage threat to the optical elements comprising the laser resonator. In this paper we discuss the merits of using blind gratings in both the paraboloid and grazing incidence hyperboloid mirrors of a 1.06-micron FEL under construction by the Boeing Aerospace Company. Grating designs developed for use at grazing incidence are not very efficient for dispersing the harmonics. However, blind gratings positioned in the paraboloid exhibit better filtering capability.
Coplanar vacuum photodiode for measurement of short-wavelength picosecond pulses
Authors: Bokor J. , Johnson A. M. , Simpson W. M. , Storz R. H.
Published: February 2023

A vacuum photodiode is the most elementary photoelectric detector. Such a device is sensitive to photon energies which exceed the photocathode work function in accord with the classical photoelectric effect. For vacuum ultraviolet and soft X-ray radiation, photoelectric quantum yields in excess of 50% can be reached.1 With a carefully designed external circuit, the response time of such a detector is limited by the diode capacitance. The present state-of-the-art risetime in a commercial biplanar vacuum photodiode2 is 60 psec.
Vacuum Ultraviolet Laser Spectroscopy of Molecular Autoionizing States
Authors: Milburn D. , Hart D. J. , Hepburn J. W.
Published: February 2023

This paper describes a new apparatus which uses coherent vacuum ultraviolet light to photoionize molecules in a supersonic molecular beam, and energy analyzes the resulting photoelectrons by time of flight spectroscopy. The broadly tunable coherent VUV is generated by four wave sum mixing in Hg. Using this apparatus, we have studied the dynamics of autoionization for two different systems: spin-orbit autoionization in HI, and vibrational autoionization in NO. In both cases, our recent results have pointed out dramatic new features of the dynamics in these molecules.
Coherent Excitations of Nuclei in Crystals by Synchrotron Radiation Pulses
Authors: Trammell G. T. , Hannon J. P.
Published: February 2023

Synchrotron pulses will excite low lying nuclear levels, and in crystals, will create nuclear exciton states which are spatially coherent superpositions of the various excited state hyperfine levels of all the nuclei in the crystal. The subsequent radiative decay is radically affected by coherence, exhibiting both a speed-up due to "coherent enhancement", and a quantum beat modulation of the decay rate which gives a periodic speed-up/slow-down of the rate for photon decay into the coherent channels. For a nuclear exciton in an antiferromagnet, the quantum beat modulation will result in an interesting "temporal pendulösung" effect.
Measurement of the Quenching of Spontaneous Emission Coefficients in Laser-Produced Plasmas
Authors: Chung Y. , Hirose H. , Suckewer S.
Published: February 2023

The quenching of Einstein A-coefficients was observed by measuring the branching ratio of visible and VUV line intensities for CIV, CIII and NV ions.
Soft X-Ray Amplification in a Magnetically Confined Recombining H-like and Li-like Plasma
Authors: Kim D. , Skinner C. H. , Wouters A. , Valeo E. , Voorhees D. , Suckewer S.
Published: February 2023

A series of experiments with H-like carbon has been performed using a Nd/Glass laser ( maximum output energy is 100 J in 3 nsecs ) in a line focus configuration. The maximum gain observed was 8.1 cm−1 using 25 J laser energy. This work is an initial step toward the development of a soft x-ray amplifier for the presently operating C VI 182 Å soft x-ray laser in a CO2 laser produced plasma confined in a magnetic field.

The possibility of lasing action at 187.2 Å in Li-like Mg X, 154.7 Å in Li-like Al XI, and 129 Å in Li-like Si XII in a magnetically confined recombing plasma has been also investigated. A CO2 laser was focused onto a target assembly and the resulting plasma confined in a solenoidal magnet field up to 9 Tesla. Impurities such as iron or titanium were introduced to increase the cooling in the recombination phase to produce more favorable conditions for high gain. Gain-length products of 1 ~ 2 at 187.2 Å in Mg X, 3 ~ 4 at 154.7 Å in Al XI, and 1 ~ 2 at 129 Å in Si XII were observed. To better understand plasma dynamics, a one-dimensional hydro-atomic code has been developed to simulate the experiment. The comparison between experiments and computer simulation will be discussed.
Gain Measurement on a 18.2-nm Carbon Recombination Laser Produced by an Intense CO2 Laser
Authors: Daido H. , Miura E. , Kitagawa Y. , Nishihara K. , Kato Y. , Yamanaka C. , Nakai S.
Published: February 2023

We propose a cylinder-type target as a carbon recombination laser medium made of 30-μm thick parylene (C8H8) whose diameter and length are 3mm and 3mm-12mm, respectively. The high power CO2 laser (500J in 50ns) irradiates a 2500-Å thick parylene foil attached to the end of the cylinder. Then the plasma from the thin foil and the cylinder wall produced by the scattered light expands and is confined in the cylinder, producing an almost uniform plasma without magnetic field. The plasma length of more than 1 cm was observed with x-ray pinhole camera. We measured the gain of CVI 3d-2p line (18.2 nm) using the axial and the transverse spectrometers. The maximum gain length product of 2.4 at cylinder length of 12 mm was obtained.
Continuous Anti-Stokes Raman Laser Operation
Authors: Feitisch A. , Wellegehausen B.
Published: February 2023

The non parametric anti-Stokes Raman process provides a possibility for the efficient upconversion of infrared or visible laser radiation into the uv and vuv spectral range. Especially for cw operation the active materials in discharge excited gas laser systems may be used. A first cw system has been operated in an argon ion laser, where 648.3 nm radiation of a dye laser could be converted into 437.5 nm radiation, with a conversion efficiency of more than 20 % and a maximum output power of 150 mW.
Studies of the Interaction of Molecules and Solids with Intense Subpicosecond 248 NM Radiation
Authors: Rhodes C. K.
Published: February 2023

Advances in femtosecond lasers are extending the exploration of multiphoton interactions well into the strong-field regime, the physical situation in which the external field is greater than an atomic unit
(e/a02). The performance projected for ultraviolet rare gas halogen technology is currently being realized1−10 and new near-infrared solid state systems, such as Ti:Al2O3, are under vigorous development. Both technologies11 should reach a field strength of
∼100 e/a02 with instruments that produce an output energy of ~ 1J.
Intense Field Non-Resonant Multiphoton Absorption of Diatomic Molecules
Authors: Yang B. , Saeed M. , Breeden T. , DiMauro L.F.
Published: February 2023

Recently, schematic theoretical1,2 and experimental3 investigations have begun to examine questions concerning the high field behavior of molecules. Obviously these studies become inherently more complex due to the richness of molecular structure (internal degrees of freedom) but do raise some interesting and unique aspects concerning the role and interplay of ionization and dissociation. The special problem presented by the study of molecular species is the disentanglement of the field from the structural effects. We have begun a schematic investigation of homo- and hetero-nuclear diatomics in an intense field. In this paper we will report results for hydrogen, oxygen, and chlorine molecules.
Towards Extreme VUV by Harmonic Generation in Strong Fields
Authors: Lompré L.A. , L’Huillier A. , Mainfray G.
Published: February 2023

Most of experiments on harmonic generation in a gaseous medium use short incident wavelengths in order to obtain the shortest generated wavelengths1−4. McPherson et al have reported up to the 17th harmonic in Ne using a 248 nm (KrF) excimer laser4 at an intensity of 1016 W/cm2. This radiation corresponds to the emission of a photon around 15 nm, or a photon energy of 85 eV. It has been demonstrated5,6 that high-order harmonics can be produced in a rare gas medium with laser photon energies more than 10 times smaller than the ionization energy.
Spectral Blueshifts of Femtosecond Pulses in Atmospheric Density Kr and Xe Plasmas
Authors: Wood Wm. M. , Downer M. C.
Published: February 2023

Collisionless multiphoton ionization of atmospheric density gases with intense femtosecond pulses can produce a cold, dense plasma of highly stripped ions, thus providing a promising medium for XUV recombination lasers[1]. We have shown in several recent papers [2-4] that rapid ionization and the subsequent development of of plasma temperature in atmospheric density plasmas cause macroscopic refractive index changes which in turn induce characteristic spectral shifts on the ionizing pulse, or a time delayed probing pulse. Theoretical analysis of detailed features of these shifts in the lighter rare gases (He, Ne, Ax) has revealed several key aspects of femtosecond ionization: 1) an ab initio model of spectral shifts based on fundamental strong field ionization theory (e.g. Keldysh, Ammosov [5,6]); 2) the occurence of anomalously large blue shifts at higher pressures and intensities, when the ionization rate is significantly enhanced by impact-ionizing collisions of the quivering electrons [4]; and 3) evidence for low plasma temperatures, based on red shifts induced on a time-delayed probe [4], In this paper we present new results with Kr and Xe gases, which reveal a distinct "doublehumped" feature in the blue shifted single pulse spectra not seen in the lighter noble gases. A quantitative model is presented that attributes this effect to two characteristic rates at which multiple stepwise ionization occurs: a "fast" rate corresponding to each ionization stage during the early part of the pulse (e.g. Krn+ -> Krn+1)+) that produces the more shifted peak; and a "slow" rate which corresponds to the primarily impact ionization during the peak and latter parts of the pulse. The dwell time following the saturation of each ionization level in the early part of the pulse also contributes to a significant broadening of the bluer shifted part of the spectrum. This two-humped effect is more pronounced in Kr and Xe because of the lower ionization potentials, and therefore greater ionization rates; in addition, the larger number of levels ionized in these heavier noble gases cause a greater broadening of the further-shifted peak. Details of the double-humped blue shifted spectra depend on the role of impact ionizing collisions in the strong field, and also on the presence of chirp on the incident pulse.
Transport in ultra-dense plasmas produced by 1 ps laser pulse
Authors: Chaker M. , Kieffer J.C. , Pépin H. , Chien C.Y. , Coe S. , Mourou G.
Published: February 2023

The recent developement of picosecond laser technology1 has made possible the production of extremely-bright laser sources which are used to produce ultra-dense plasmas2. In the present work we describe the first experimental study of transport in ultrashort scalelength plasmas (L/λ « 0,1).
Four Wave Difference Frequency Generation at 124nm for High Resolution Photoabsorption Studies of O2
Authors: Baldwin K.G.H. , Gibson S.T. , Lewis B.R. , Carver J.H. , McIlrath T.J.
Published: February 2023

Previous VUV generation techniques used to carry out high resolution absorption and photodissociation spectroscopy of molecular oxygen have relied on stimulated anti-Stokes Raman scattering to produce tunable VUV radiation(1,2). However, such studies are limited in practice by low conversion efficiencies to wavelengths >1500A in the Schumann-Runge band and continuum region.
Soft x-ray spectra from laser plasma effectively heated by a pulse train laser
Authors: Hirose Hideo , Hara Tamio , Ando Kozo , Negishi Fumiko , Yashiro Hidehiko , Aoyagi Yoshinobu
Published: February 2023

Considerable progress has been made in the development of soft x-ray laser since 19851-2). Until now a powerful laser of single or double pulses has often been used to produce high density plasma. Recently we observed ASE (amplified spontaneous emission) in some lines of Al X and Al XI in a recombining plasma produced by a very low power laser of only 2 J which was composed of a train of many low power pulses separated by about 350 picosecond3). To understand more clearly the effect of multipulse irradiation on the production of laser plasma and x-ray emissions including ASE, in this paper we constructed a pulse train forming system and investigated the emitted soft x-ray spectra for various pulse trains. Our final goal is to develop a table top x-ray laser in the water window wavelength.
On the feasibility of X-ray nonlinear resonant effects in plasma
Authors: Shkolnikov P. L. , Kaplan A. E.
Published: February 2023

Recent developments in x-ray laser research resulted in the experimental observation of laser amplification at many wavelengths in soft x-ray domain [1] ranging from λ = 280 Å to λ = 42 Å. There is no doubts that within next few years powerful sources of coherent x-ray laser radiation at those wavelengths will also be available. This sets a stage for the research on the interaction of intense coherent x-ray radiation with a matter, in particular on x-ray nonlinear optics. The first obvious choice of the environment and frequency for the nonlinear effects to occur and to be experimentally observed, is the same plasma of ionized atoms that gives rise to the laser action itself, with the nonlinear interaction occurring at the frequency of lasing transition. The nonlinear effects in such a situation are essentially expected to be the same as in any other resonantly-enhanced nonlinear interaction of light with matter in a visible optical domain and could approximately be evaluated using basically two (or three) level model. In order to demonstrate the feasibility of this kind of nonlinear effects both in active and passive x-ray laser medium, we present here evaluations of the characteristic parameters pertinent to the resonant nonlinearity: the saturation driving intensity and nonlinear change of refractive index. We use simple and yet realistic model based on the neonlike Mo XXXIII plasma; observation of soft X-ray amplification in such a system has recently been reported [2]. We show that the third order nonlinear effects, such as saturation of absorption (or gain) and nonlinear refractive index can be observed in the Mo XXXIII amplifier (laser) radiation propagating in such a plasma in the cases of both "hot" laser plasma in the core of the laser system, and "warm" plasma surrounding the core and having considerably lower electron density Ne and temperature Te. Following [3], consider the "warm" plasma with the density Ne sufficiently low such that collisional rates of the quantum levels of interest can be neglected compared with radiative ones.
High Intensity Above-Threshold Ionization of He and He Ion
Authors: Mohideen U. , Sher M. H. , Aumiller G. D. , Wood O. R. , Freeman R. R. , Bokor J. , Tom H. W. K. , Bucksbaum P. H.
Published: February 2023

This is the first high resolution measurement of the electron energy spectrum from high intensity Above- Threshold Ionization of He and He ion at intensities upto 2.5×1016 W/cm2. The ATI electron energy spectrum from the ionization of He upto an intensity of 7×1015 W/cm2 with both linearly and circularly polarized light was done and compared to the existing models. The disagreements with the models are discussed.