bluuebunny/crossref_metadata_2025 huggingface dataset:
CO2 Laser-Heated Plasmas for Electron-Collisionally Pumped XUV Laser Studies
Authors: Enright G. D. , Burnett N. H.
Published: February 2023

We describe a series of experiments to characterize plasmas produced by moderate intensity (≈ 2x1013 W/cm2), nanosecond pulse, CO2 laser irradiation of planar targets using both line (150 μm x 7 mm) and spherical (1mm diam.) focussing optics. X-ray line and continuum diagnostics have been used to infer a heat front penetration of about 20 μgm/cm2 at temperatures of approximately 300 eV. We observe only minor differences between targets whose mass is small compared to the hot electron range and much more massive ones, indicating a poor efficiency for hot electron reflexing in thin targets. In the case of Cu targets, observations of L shell x-ray transitions from neon and fluorine-like ions show a similar ionization distribution to 1 μm laser produced plasmas in which gain has recently been demonstrated. The Ne-like Cu XX 3p-3s emission lines have been observed. A comparison of on-axis to off-axis spectra provides evidence for gain on these lines.
Capillary Discharge Plasmas as Extreme Ultraviolet Laser Sources
Authors: Rocca J. J. , Marconi M. C. , Muniz M. Villagran , Beethe D. C.
Published: February 2023

We propose the generation of XUV and soft X-ray laser radiation by electronion recombination in capillary plasmas excited by fast discharges. Highly ionized plasmas are created in a capillary geometry by a high current pulse; subsequently rapid cooling of the plasma occurs by electron heat conduction and radiation. Electron-ion recombination is expected to result in population inversion and gain in 3-2 transitions of hydrogenic ions with wavelengths ranging from the VUV to soft X-rays. We have generated nearly completely ionized helium and lithium capillary plasmas to study the possibility of amplification in the 3-2 hydrogenic lines during plasma recombination. Time resolved XUV spectra of a 500 μm diameter lithium capillary discharge were obtained. The intensity of the LiIII 72.9 nm 3-2 line is observed to increase during the decay of the current pulse. The capillary discharge recombination scheme is expected to advantageously scale to soft X-rays wavelengths. A time dependent collisional-radiative model of the capillary plasmas predicts a gain of 5 cm−1 in the 18.2 nm CVI transition.
Molecular Multiphoton Ionization and the "Universal" Short-pulse ATI Spectrum
Authors: Gibson G. , Freeman R. R. , McIlrath T. J.
Published: February 2023

The ATI electron energy spectra of atoms in the short pulse regime up to intensities of 1014 W/cm2 are now well understood1. Naively applying this understanding of atoms to molecules would lead one to expect molecules to yield a very complex electron energy spectrum due to the multitude of potential curves and vibrational and rotational structure in the molecule. On the contrary, recently obtained ATI electron spectra of nitrogen show a relatively simple structure surprisingly similar to the spectra of atoms (see Fig. 1).
High intensity multiphoton ionization of H2
Authors: Allendorf Sarah W. , Szöke Abraham
Published: February 2023

A tunable, high-intensity picosecond dye laser system has been employed with electron energy analysis to investigate the dynamics of multiphoton ionization, with vibrational levels of the B
1∑u+ and C
1∏u electronic states in H2 as resonant intermediate states. At the intensities studied (0.2 - 6 x 1013 W/cm2), we find evidence for production of molecular ions in various vibrational levels; at the lower intensities the population distribution of final vibrational states varies with wavelength in a manner consistent with resonant enhancement at the three-photon level, followed by ionization into a vibrational level of
H2+ roughly predictable by a Franck-Condon analysis of ionization out of the C state. At higher intensities, there is a shift to increased population of lower vibrational states of
H2+, consistent with an a. c. Stark shift of the correspondingly lower vibrational levels of the C state into resonance with the three-photon energy of the laser. Clear evidence of direct dissociation of the H2 followed by single photon ionization of the excited H atom is observed as well. Above threshold ionization (ATI) of these two processes occurs readily. We also find that dissociative ionization is an increasingly important ionization pathway as the wavelength is increased. Finally, we see evidence of a new ionization pathway, which we tentatively assign to photoionization into a transient bound state created by the avoided crossing of the first repulsive electronic state of
H2+|2σ
u
,n〉, with the single-photon-dressed ground state of
H2+|1sσ
g
,n+1〉.
Multiphoton Spectroscopy Using Tunable VUV Radiation from a Raman-Shifted Excimer Laser
Authors: Faris Gregory W. , Dyer Mark J.
Published: February 2023

Multiphoton spectroscopy has proved to be a valuable technique at visible and uv wavelengths, allowing access to high-lying states and the excitation of transitions not allowed for one photon excitation. Using vuv radiation for multiphoton excitation allows probing even higher states, but high powers are required to obtain reasonable signals. Two-photon excitation with vuv radiation has been performed using tunable excimer laser radiation1 and by Raman-shifting a frequency-doubled dye laser.2
Density Profile Steepening by the Ponderomotive Force of an Intense Picosecond Laser
Authors: Liu X. , Umstadter D. , Coe J.S. , Chien C.Y.
Published: February 2023

The high-density plasmas produced by the interaction of short-pulse lasers with solid targets have been proposed as candidate coherent x-ray sources [1]. Large absorption of laser light and rapid cooling of the plasma—both of which are required for this scheme to work—depend strongly on the evolution of the electron-density profile during the laser pulse. We report experimental and theoretical results indicating that when the quiver energy of the electrons exceeds their thermal energy, the ponderomotive pressure of the laser significantly modifies the density profile.
Creation of relativistic plasmas using ultra-high-intensity laser radiation
Authors: Bardsley J. N. , Penetrante B. M.
Published: February 2023

Theories of ponderomotive forces, relativistic focusing, and harmonic generation are reviewed, and nonlinear effects are demonstrated by using numerical simulations of plasmas produced by subpicosecond lasers.
Generation of Ultrafast VUV Radiation by Reflection from a Relativistic Ionization Front
Authors: Kapteyn Henry C. , Mumane Margaret M.
Published: February 2023

Herein we propose a method of producing short-wavelength radiation, which has the potential to produce optical pulses shorter than any that have been generated to date. This method is based on the process of upshifting light by reflecting it off a moving ionization front. This process has been considered in detail as a method for upshifting microwave radiation1. We show that by using an intense femtosecond light pulse to generate an ionization front through multiphoton ionization (MPI), it is possible to upshift a single-cycle far-infrared light pulse into the ultraviolet or vacuum ultraviolet. Since the ionization front created by MPI can be very sharp, the reflection process may add little dispersion to the pulse, making it possible to generate nearly single-cycle light pulses in the VUV. In this paper, we demonstrate that the process is experimentally realizable using the current generation of high energy femtosecond (0.1 Joule in 100 fsec) lasers. We discuss several physics issues unique to the generation of short-wavelength light using this technique. More detailed calculations of multiphoton ionization rates are necessary to determine the reflected pulse energy achievable using this technique, as will be discussed.
Recent Progress in Soft X-Ray Laser Research at the Institute of Laser Engineering
Authors: Daido H. , Kato Y. , Azuma H. , Murai K. , Shiraga H. , Yamakawa K. , Togawa T. , Miura E. , Kanabe T. , Takagi M. , Takabe H. , Tanaka K. , Yamanaka M. , Nakai S. , Lewis C. , O'Neill D. , Neely D. , Pert G. , Ramsden S. , Key M.
Published: February 2023

In this paper, we present recent progress in soft x-ray laser research at the Institute of Laser Engineering. Detailed characterization of the explosive mode Balmer-alpha soft x-ray laser has been made. Extension of the Balmer-alpha laser to a larger gain length product and to shorter wavelengths are being tested.
Broadband Diffractive Optics*
Authors: Ceglio N. M. , Hawryluk A. M. , Gaines D. P. , London R. , Seppala L.
Published: February 2023

In many regions of the EM spectrum refractive lenses are not practical, so that diffractive lenses are all that are available for high resolution focussing or imaging. For example, Fresnel structures (eg. zone plates) have demonstrated diffraction limited imaging at soft x-ray wavelengths, but only for narrowband illumination (Δλ <λO/N, where N = # of zones). There have been various attempts to design diffractive lens doublets or triplets to correct for and/or reduce the chromatic aberrations associated with diffractive optics. These approaches generally involve two or more diffractive elements separated by a finite distance. As such, they represent a "lens system" instead of compact, broadband lens, and they suffer from the compound inefficiency of the multiple diffractive elements.
Soft-X-Ray Emission Characteristics of Line-Shaped, Laser-Produced Plasmas
Authors: Chen Shisheng , Wang Xiaofang , Qian Aidi , Xu Zhizhan
Published: February 2023

Line-shaped plasmas produced by laser irradiating solid targets in line focus are of a large aspect ratio. Such plasmas are useful for x-ray laser demonstration and amplification and absorption spectroscopy as well. Soft x rays emitted from line-shaped plasmas can provide accurate information of such plasmas. Here, we report the studies of x-ray radiation transfer and spatial uniformity of line-shaped plasmas with a pinhole transmission grating spectrometer (PTGS), with a spectral resolution of 2-5 Å and a spatial resolution of 60 µm, and a soft-x-ray streak camera, with a time resolution of 30 ps.
Modal Study of Refractive Effects on X-ray Laser Coherence
Authors: Amendt Peter , London Richard A. , Strauss Moshe
Published: February 2023

Current X-ray laser designs rely on amplifying spontaneous emission in a high temperature plasma.1 An important issue involved in the study of X-ray lasers is the degree of transverse spatial coherence necessary for holographic applications.2 Longitudinal coherence appears to be satisfactory, but transverse coherence remains problematic and requires further optimization study.
Plasma Physics at the Atomic Level
Authors: Corkum P. B.
Published: February 2023

High-harmonic generation, multiphoton two-electron ejection and very high energy above-threshold ionization electrons are all described by a quasi-static model. This model of Taser-atom interactions applies concepts from plasma physics to the interaction of strong laser fields with atoms.
High-Order Harmonic Generation by Ultrashort KrF/Ti: Sapphire Lasers
Authors: Watanabe S. , Kondo K. , Nabekawa Y.
Published: February 2023

Development of 100-fs, multiterawatt lasers and its application to high intensity physics have attracted growing interest.
Shortwavelength Radiation Produced by Strong Field Harmonic Generation
Authors: L’Huillier Anne , Salières P. , Balcou Ph. , Lewenstein M. , Wahlström C. G. , Larsson J. , Starczewski T. , Persson A. , Svanberg S. , Ivanov M. Yu.
Published: February 2023

Single–atom harmonic generation spectra fall off at an energy equal to the ionization energy plus about three times the oscillation energy of a free electron in the field [1]. We have explored experimentally this cutoff law using a short pulse Ti: Sapphire laser. We show how this law is modified by the effect of propagation in the nonlinear medium. Moreover, the generation of high order harmonics from Xe+ and Ar+ ions is demonstrated.
Propagation of Intense, Ultrashort Laser Pulses in Plasmas
Authors: Sullivan A. , Gordon S. , Hamster H. , Falcone R. W. , Nathel H.
Published: February 2023

We have searched for self-channeling of 140 fs, 2.3 TW laser pulses in plasmas and observe a null result in experiments using nitrogen and argon. However, we observe an increase in the Rayleigh range of a factor of 3 in 100 torr of ionized helium.at lower than predicted laser power.
Self-Focusing of Intense Subpicosecond Laser Pulses in a Low Pressure Gas
Authors: Liu X. , Umstadter D.
Published: February 2023

Multiple foci formation in a gas cell with a high peak power, subpicosecond laser is reported. It results from the combined effects of nonlinear refractive index and defocusing by the laser-generated plasma.
Optimization of Self-Channeling of Relativistically Intense Laser Beams in Underdense Plasmas
Authors: Borisov A. B. , Shi X. , Shiyaev O. B. , McPherson A. , Boyer K. , Rhodes C. K. , Korobkin V. V. , Karpov V.
Published: February 2023

Self-channeling of high power subpicosecond ultraviolet (248 nm) laser pulses has been observed experimentally in various high pressure gases. The propagation zone was observed to be approximately 2 µm in diameter and 3 mm in length, a distance of about 100 Rayleigh ranges. Theoretical modeling of relativistic and charge- displacement self-channeling of laser pulses in plasmas with our experimental parameters yield results which are in qualitative agreement with the experimental observations.
Plasma-Induced Self-Phase Modulation of a Subpicosecond Excimer Laser Pulse
Authors: Le Blanc S. P. , Sauerbrey R. , Rae S. C. , Burnett K.
Published: February 2023

Plasma-induced spectral blueshifting in rare gases has been investigated using a subpicosecond KrF excimer laser focused to a peak intensity in the region of 1014 W/cm2 (adiabaticity parameter in the range 8 < γ < 10). Blueshifts of up to 2 nm have been observed, and the blueshifted spectrum shows an interference-like oscillatory structure. By comparing experimental results with numerical simulations, the blueshifted spectra are shown to be the result of plasma-induced self-phase modulation, and can be modeled qualitatively by assuming tunneling ionization and one-dimensional pulse propagation. Under the condition of weak ionization, pump-probe experiments and theoretical results show that field ionization occurs over a time equal to about half the pulse width. A two-dimensional pulse propagation model indicates that spatial defocusing causes spectral blueshifting to be spatially dependent.
Dynamics of Dense Plasmas with Significant Quiver Motion
Authors: Umstadter D. , Liu X. , Workman J. , Korn G.
Published: February 2023

Conditions are found under which the expansion dynamics of a short-scalelength laser-driven plasma are controlled primarily by the ponderomotive force of the high-intensity laserdriver. It is postulated that this occurs when the laser intensity is such that the average quiver energy of electrons oscillating in the laser field becomes comparable to their thermal energy. To test this hypothesis, a pump-probe experiment was performed to measure the expansion velocity with 250-fs time resolution. The experimental results are found to confirm this prediction as well as the predictions of a 1-D computer model that includes the hydrodynamic fluid coupled to the wave Also discussed are the ramifications of these results on the production of the high-density and temperature plasmas that are required for the generation of bright short-pulse x-rays.
Resonance Absorption in a Laser-Produced Plasma Generated by a High-Intensity Ultrashort Pulse
Authors: Sauerbrey R. , van Wonterghem B. , Bergmann J. , Teubner U. , Schäfer F. P.
Published: February 2023

Plasmas were generated by 400 fs Kr F laser puises at intensities of ~ 1017 W cm‒2 on aluminum and carbon targets. Reflectivity and x-ray emission were measured as a function of laser polarization, angle of incidence, and intensity. For the same absorbed intensity, p-polarized laser light is up to a factor of five more efficient in generating x-rays (> 0.5 KeV) than s-polarized light. These results show the importance of resonance absorption for short scale length plasmas. I.
Strong Kα Emission in Picosecond Laser-Plasma Interactions
Authors: Soom B. , Chen H. , Fisher Y. , Meyerhofer D. D.
Published: February 2023

Kα emission is observed from the interaction of a
picosecond, 1.05-μm laser pulse at 4 × 1015
W/cm2 with a silicon target coated with various thicknesses
of aluminum. Strong Kα emission requires a p-polarized,
high-intensity-contrast laser pulse. At 7.1 Å, the Kα yield
is 1.5 × 108 photon/sr with a duration of emission of ~8 ps
and a source area of 1.6 × 10‒6 cm2.
The radiance is ~3 × 109 W/cm2 sr. Monte-Carlo
simulations have suggested that under these conditions Kα
emission is a promising candidate to obtain an efficient, ultrashort
x-ray source at ~1.5 Å.
Germanium Soft X-Ray Laser in a Wall-Confined Laser Produced Plasma
Authors: Daido H. , Kodama R. , Murai K. , Yuan G. , Schulz M. S. , Takagi M. , Kato Y. , Nakai S. , Neely D. , MacPhee A. , Lewis C. L. S.
Published: February 2023

An intense soft x-ray laser (wavelength : mainly 23.6 nm) was produced in a wall-confined germanium plasma pumped by a line focused laser having an energy of 1.3 kJ, with a pulse duration of 1 ns and a wavelength of 1 µm.
Soft X-Ray Amplification of the Lyman-α Transition of the Hydrogen-Like Lithium by Optical-Field-Induced Ionization
Authors: Nagata Yutaka , Midorikawa Katsumi , Obara Minoru , Tashiro Hideo , Toyoda Koichi
Published: February 2023

Soft-x ray amplification has been observed on the Lyman-α transition of hydrogen-like lithium ions by optical-field-induced ionization, using a subpicosecond KrF laser pulse (0.5ps,50 mJ) focused at 1017 W/cm2. A small signal gain coefficient of 20 cm‒1 was obtained at 13.5 nm. Roles of a preformed plasma for the realization of an optical-field-induced ionization x-ray laser are discussed.
Electron Emission from Metal Surfaces Excited by Intense Femtosecond Pulses
Authors: Wang X. Y. , Riffe D. M. , Downer M. C. , Erskine J. L. , Fisher D. L. , Tajima T. , Rashed M. , Maziar C. M. , More R. M.
Published: February 2023

Electron emission from a metal surface excited by an intense visible laser pulse can be attributed to thermionic emission, thermally assisted photoemission, or quantum excitation channels (multiphoton photoemission). With intense sub-picosecond laser excitation, thermionic emission due to highly non-equilibrium electron heating can be the dominant process because of the small electronic heat capacity and brief uncoupling between the electron and lattice, as first predicted by Anisimov et al1. Experimental observations of such transient thermionic emission could potentially serve as a diagnostic of peak electron temperature Te. However, their interpre­tation is rendered difficult because the large electron emission rate introduces tremendous space-charge fields at the surface, which substantially suppress the electron yield, while simultaneously distorting the detected electron energy distribution from the initial thermal distribution. In order to understand quantitatively the electron yield and energy distribution in the intense femtosecond pulse regime and to develop a useful diagnostic technique, we present a combined experimental, analytical theory, and particle-simulation investigation of femtosecond pulse induced electron emission from Al, Ag and W surfaces. Both the analytical theory and simulation work incorporate the strong space-charge fields present in the experiment.
Photo-Ionized Inner-Shell X-Ray Lasers
Authors: Eder D. C. , Strobel G. L. , London R. A. , Rosen M. D. , Falcone R. W. , Gordon S. P.
Published: February 2023

Intense ultrashort optical lasers can be used to generate x rays that can photo-ionize inner-shell electrons in a range of elements. For a sufficiently intense and short pulse of x rays, lasing can occur in the filling of the inner-shell hole [1]. In these x-ray laser schemes, the very fast Auger decay of the upper laser level requires a very intense source of x-rays and the filling of the lower laser level requires a very fast rise time for the x rays. Inner-shell schemes are of particular interest because of their potential for lasing at short wavelengths. Collisional x-ray laser schemes using Ni-like ions have demonstrated lasing down to 35 Å (Ni-like Au) but cannot be extended below 20 Å (Ni-like U) [2]. Photoionization of the K-shell offers the possibility of x-ray lasing in the wavelength range from 15 down to 5 Å for elements from Ne to K.
Soft X-Ray Holography Using an X-Ray Laser at 23.2/23.6 nm and 19.6 nm
Authors: Schulz M. S. , Daido H. , Murai K. , Kato Y. , Kodama R. , Yuan G. , Nakai S. , Iwasaki H. , Yoshinobu T. , Shinohara K. , Kodama I. , Honda T. , Neely D. , Slark G.
Published: February 2023

An x-ray laser at wavelengths of 23.2/23.6 nm and 19.6 nm was used to carry out in-line Gabor holography. Holograms of simple structures, such as thin tungsten and glass fibers and standard electron microscope test specimens, using two types of recording media, PMMA photoresist and Ilford Q-Plates, were obtained. The PMMA samples were initially examined using a differential interference microscope and subsequently using an atomic force microscope (AFM). Also attempted was Fourier Transform (FT) holography using two zone plates (one as the focusing device and the other as the object); the criteria for FT holography are discussed.
Specifying Optical Fabrication Tolerances for Soft X-ray Projection Lithography Systems
Authors: Harvey James E.
Published: February 2023

The rapidly emerging technology of soft X-ray projection microlithography requires very stringent tolerances upon the residual surface errors inherent with any optical fabrication process. The scattering effects of these optical fabrication errors can severely degrade system performance. These optical fabrication errors must therefore be controlled over the entire range of relevant spatial frequencies, including "mid" spatial frequency surface irregularities that bridge the gap between the traditional "figure" and "finish"errors. The surface power spectral density (PSD) function thus becomes the natural quantity to monitor during the optical fabrication process.
Analysis of Thermally Induced Distortion of Optics for Soft X-ray Projection Lithography
Authors: Watson R. D. , Stulen R. H.
Published: February 2023

The development of lens systems for projection x-ray lithography presents significant challenges associated with the fabrication and testing of ultra precise optical surfaces. Once assembled, these projection lenses must further be dimensionally stable to tolerances determined by the wavelength of the soft x-rays used for illumination, typically between 100Å and 300Å. Lens systems capable of printing over large areas will contain a number of mirrors with reflectivities in the range of 60±10%. For these systems, the first element will be subjected to a significant incident x-ray flux, of which ~40% will be absorbed. This absorbed power causes heating which in turn will cause a distortion of the optical surface. The intent of this study has been to examine the magnitude of these distortions under a variety of conditions.
Design & development of an integrated laser-plasma soft-X-ray projection system
Authors: Bijkerk F. , Voorma H.-J. , Pulk E.J. , Louis E. , van der Wiel M.J. , Verhoeven J. , van der Drift E. , Romijn H.
Published: February 2023

We report on design and development activities of an integrated soft-X-ray projection system, consisting of all-reflective multilayer optics and a repetitive laser plasma soft-X-ray source.
Soft X-ray Projection Imaging Using a Laser Plasma Source
Authors: Tichenor D. A. , Kubiak G. D. , Malinowski M. E. , Stulen R. H. , Haney S. J. , Berger K. W. , Brown L. A. , Freeman R. R. , Mansfield W. M. , Wood O. R. , Tennant D. M. , Bjorkholm J. E. , Jewell T. E. , White D. L. , Windt D. L. , Waskiewicz W. K.
Published: February 2023

Previous experiments by AT&T Bell Laboratories [1] have demonstrated the feasibility of using a soft x-ray projection system to produce feature sizes down to 0.05 microns in a tri-level resist. Their experiments used a Schwarzschild objective and 140Å radiation from an undulator at the National Synchrotron Light Source. We describe here a similar imaging system with the exception that it is based upon illumination using a high-fluence laser plasma source (LPS) of soft x-rays instead of a synchrotron radiation source. The use of a laser plasma point source offers significant advantages of size and cost over synchrotron sources but with some tradeoffs. The intent of the experimental program described in this paper is to determine the viability of the LPS for future implementation of soft x-ray projection lithography (SXPL).
Simulation of Soft -X-Ray Production by Laser Irradiation
Authors: Cerjan C. , Rosen M. D.
Published: February 2023

An extensive series of one- and two-dimensional LASNEX simulations have been performed to establish the parameter range for efficient conversion of laser light into soft-x-rays in spectral regions of interest to x-ray lithography. In particular, the calculations assumed that 1.06 µm and .53 µm laser light of varying intensity was deposited on stainless steel and Tantalum slab targets. The subsequent emission of radiation in several spectral regions, between 10-14 Å and at 124 Å, was monitored as the high-temperature plasma evolved, and the overall conversion efficiency was calculated after the emission ended.
Characteristics of Sub-100-ps Laser Irradiation of Cylindrical Cavities
Authors: Balmer J.E. , Soom B. , Ellenberger U. , Weber R.
Published: February 2023

Laser irradiation of cylindrical cavities has attracted considerable interest in the context of soft x-ray laser experiments because the confined plasma may act as an x-ray waveguide.1) Waveguiding requires a concave, radially symmetric density profile and thus uniform plasma ablation inside the cavity. In the case of transverse irradiation presented in this work, the uniformity of plasma ablation is primarily determined by the mechanisms which redistribute the energy from the initial location of plasma generation by the incident laser. Among the mechanisms to be considered are x-radiation, hot electrons, and laser light reflected at the primary spot. Experimentally, evidence was found that for short-pulse, long-wavelength, high-irradiance conditions, the redistribution of energy is dominated by reflected laser light.
Differential Phase Contrast Imaging in the Scanning Transmission X-ray Microscope
Authors: Palmer J R , Morrison G R
Published: February 2023

In the scanning transmission x-ray microscope (STXM) the specimen is scanned in a raster by an x-ray probe formed with a Fresnel zone plate. To achieve near diffraction limited resolution it is necessary to have a coherent source, even when forming an incoherent brightfield image by measuring the x-ray intensity transmitted by the object. This has so far been the only imaging mode used in the STXM and for hydrated biological specimens is well suited to soft x-ray wavelengths within the “water window” (2‧33 to 4‧36 nm) where carbon absorbs much more strongly than water. However, by the use of phase contrast rather than amplitude contrast, it is possible to form images at wavelengths where the absorption is low, resulting in lower radiation dose for the same level of contrast. Calculations made by Howells [1] and Rudolph and Schmahl [2] have demonstrated very clearly the advantages of phase contrast imaging at wavelengths outside the water window.
Soft X-ray Optic For An Efficient Laser Plasma Spectrometer
Authors: Davey S. C. , Freeman R.R. , McIlrath T.J. , vanWoerkom L.D. , Waskiewicz W.K. , Lucatorto T.
Published: February 2023

A soft x-ray figured optic has been incorporated into a spectrometer with a multi-channel detector to enhance signal levels from short pulse laser produced plasmas.
Soft X-ray Projection Lithography Technology*
Authors: Ceglio N. M. , Hawryluk A. M. , Steams D. G. , Gaines D. P. , Rosen R. , Vernon S.
Published: February 2023

Recent analyses of critical system issues in Soft X-ray Projection Lithography are discussed. An overall system design provides detailed requirements (specifications) for x-ray mirror reflectivity, source power and parameters, resist sensitivity, etc. A cost analysis for SXPL systems leads to important requirements for x-ray mirror technology. An analysis of power loading limitations on precision SXPL imaging optics has important implications for source power and x-ray mirror reflectivity requirements.
Effect of amplitude and phase dispersion on images in multilayer coated soft x-ray projection systems
Authors: Jewell Tanya E.
Published: February 2023

Multilayer soft x-ray coatings on curved optical surfaces may affect the quality of the image in the imaging system due to variation in phase and reflectance across the aperture. Because the reflectance stopband for these coatings is narrow, these variations tend to be large. The multilayer coating reflectance and phase variations depend on both wavelength and incidence angle of the radiation going through the imaging system. Figure 1 shows these dependencies for a multilayer coating containing 20 bilayers of Mo/Si. As can be seen from this figure for a given wavelength the phase change due to incidence angle variation can be significant.
Lithography - "The technology to he used is ????"
Authors: Hill Robert W.
Published: February 2023

There is great indecision in the semiconductor industry as to what the lithography technology will be for the back half of the 1990's (1995-2000). When applied against an idealized process requirements model, all technologies have problems which make the choice extremely expensive and difficult. Technologies considered are: optical extensions, e beam technologies, ion beam technologies, and x-ray technologies. This paper discusses the main technologies with a description of the advantages and disadvantages.
Design Considerations for a "Front End" Illumination System for Soft X-ray Projection Lithography
Authors: Ceglio N. M.
Published: February 2023

We define the "front-end" or illumination system to include all those components necessary to produce the x-rays, and to transport them to and appropriately illuminate the mask. We will provide a systematic discussion of the constraints that limit the design of an illumination system for soft x-ray projection lithography.
A Continuous Emission Source Covering the 50 to 300 Angstrom Band
Authors: Bowyer Stuart
Published: February 2023

Because of the growing importance of extreme ultraviolet radiation, there is considerable interest in high intensity laboratory sources for this spectral range. A variety of sources have been proposed for these wavelengths, and many of these are described in the classic book of Samson (1967). Most of these sources have substantial limitations, especially those that are intended for use below 1200 Å. We (Paresce et al. 1971) and others have developed continuous discharge sources which are stable and maintenance-free and which provide a large number of intense lines at wavelengths down to ~300 Å. Soft X-ray sources of the type developed and refined by Henke (1975) are capable of producing substantial amounts of soft X-ray radiation. However, these sources have severe limitations for use at wavelengths longer than ~50 Å. For example, much of the radiation produced is continuum radiation with rather low flux at any specific line. In addition, the flux is peaked at shorter wavelengths. When dispersed by a grazing incidence monochromator, wavelengths at first order and many higher orders emerge from the exit slit, rather than a true monochromatic flux which is usually desired. Essentially the only sources available that can provide reasonable flux levels at wavelengths between 50 and 300 Å are capacitive discharge sources with attendant problems of severe electromagnetic interference, and plasma discharge sources.
Contamination Studies of XUV Reflecting Surfaces for Projection Lithography*
Authors: Newnam Brian E. , Scott Marion L.
Published: February 2023

The results of carbon and oxide contamination experiments with Al, Si, Rh, and Ag films and surfaces quantify certain film growth rates and parameter dependences.
Atomic Layer Growth of Tungsten and Boron for Multilayer X-ray Optics
Authors: Shurtleff J.K. , Allred D.D. , Perkins R.T. , Knight L.V. , Thorne J.M. , Phillips J.D.
Published: February 2023

We are investigating atomic layer growth which can produce crystalline superlattices or amorphous multilayers with smaller periods and better uniformity than currently available.
Condenser Optics, Partial Coherence and Imaging for Soft X-Ray Projection Lithographapy
Authors: Sommargren Gary E. , Seppala Lynn G.
Published: February 2023

In this paper we have concentrated on the design and implications of condenser systems for an all reflective soft x-ray projection system. The optical system for a soft x-ray projection lithography system has five main components: the source, condenser (illumination system), mask, imaging system and wafer.
Controlling short wavelength x-ray multilayer period variation on focussing optics
Authors: Kortright J.B. , Nguyen K. , Denham P. , Windt D.L.
Published: February 2023

The variation of multilayer period, or d-spacing, across the reflecting surfaces of soft x-ray normal-incidence focussing optics is of primary importance in the optical performance of these systems. Focussing necessarily implies a specific optimal variation of period across the surfaces, and the multilayer bandpass sets the tolerance scale for acceptable deviations from this ideal variation.1,2 Mo/Si multilayers for use at wavelengths above 12.4 nm have relatively broad bandpasses, easing these tolerances. Multilayers for use at shorter wavelengths have significantly narrower bandpasses, thus placing significantly greater demands on the control of the period variation. Other design considerations, such as higher magnification systems and larger optics, also place more stringent demands on the control of multilayer period.
Structural Modification of Mo-Si X-Ray Multilayer Mirrors: Ion-Assisted Sputter Deposition
Authors: Vernon S. P. , Stearns D. G. , Rosen R. S.
Published: February 2023

The structural properties and soft x-ray normal incidence reflectivity of Mo-Si multilayers fabricated using ion-assisted dc magnetron sputter deposition are reported.
An Undulator Facility for Precision Optical Testing
Authors: Attwood David
Published: February 2023

The development of reflective optics for projection soft x-ray lithography will require new tools, previously available only at longer wavelengths. Among these is a test facility for large numerical aperture interferometry at short wavelengths. As with visible light interferometry this is most effectively done with coherent radiation. In this paper we discuss the spatial and temporal coherence properties of undulator radiation, wavelength tuning range, design tradeoffs involving harmonic content and thermal loading, and challenges associated with at-wavelength interferometry of complex surfaces. Preliminary plans for such a facility at Berkeley's Advanced Light Source will be discussed. This work is supported by DARPA, AFOSR, and DOE/BES.
Schwarzchild microscope for carbon Kα radiation
Authors: Murakami Katsuhiko , Nakamura Hiroshi , Oshino Tetsuya , Ohtani Masayuki , Nagata Hiroshi
Published: February 2023

With the development of x-ray multilayers, Schwarzchild optics can be applied to x-ray imaging such as microscopic imaging1 and projection lithography2,3.
A High Efficiency Condenser for Laser Plasma Sources and Ringfield Masks
Authors: White D. L.
Published: February 2023

A laser produced plasma x-ray source has a 150 to 350 microns in diameter, while the mask for a ringfield camera is an arc 1 to 5 mm wide and 50 to 150 mm long. This is a gross mismatch - the strip is too small in the scanning direction and too large in the transverse direction.
Cost of Ownership for Soft X-ray Projection Lithography
Authors: Arnold William H. , Early Kathleen
Published: February 2023

This paper explores the projected cost of ownership for a soft x-ray projection lithography process. The analysis includes the cost of capital equipment (exposure tool and resist processing), resist materials, and masks, as well as the economics associated with tool productivity and utilization. There is a particular focus on the impact of the cost of the masks, as it applies to different IC product types and their volumes. Also studied are the tradeoff of system throughput with required resist dose and the costs associated with multilayer resist systems.
Soft X-ray Conversions Efficiencies from Laser-Produced Plasmas for Soft X-ray Projection Lithography Sources
Authors: Spitzer R. C. , Kauffman R. L. , Orzechowski T. , Phillion D. W. , Cerjan C.
Published: February 2023

Soft x-ray projection lithography (SXPL) systems are designed to operate with narrow bandwidths around 130Å, where the highest reflectivity mirrors have been demonstrated1. Several different sources are under consideration. One possibility is the laser-produced plasma, in which Intense visible laser light incident on a solid metal surface in vacuum generates an x-ray emitting plasma. An important question is whether sufficient x-ray radiation is generated at 130Å. Although much work has been performed at the very high laser intensities used in fusion applications, no experiments existed at the pulse widths and intensities necessary for SXPL systems. In this work a comprehensive soft x-ray database for laser plasma lithographic sources was developed. We determined the absolute conversion efficiency from laser light into x-rays for various target materials, laser wavelengths {1.064μm & 532nm),pulse lengths (7.5-30ns), incident intensities{1x109W/cm2 - 1x1013W/cm), and spot sizes(15μm-2mm).
Manufacture and Characterization of Holographic Diffraction Gratings for Soft X-Rays
Authors: von Brenndorff Alexander Irtel
Published: February 2023

In this paper, a manufacturing process for laminar gratings is
described and the effects of variations of this process on the surface
roughness are studied using a STM.
Laser Plasma Source Targets for Soft X-ray Projection Lithography: Production and Mitigation of Debris*
Authors: Kubiak Glenn D. , Berger Kurt W. , Haney Steven J.
Published: February 2023

The high cost and complexity of commercial illumination systems envisioned for soft x-ray projection lithography (SXPL) will require that their minimum replacement period is likely to be 6-12 months. For a laser plasma soft x-ray source, this corresponds to approximately 109 laser pulses. Given that a deposited film just 100 Å thick of a typical target element (Au, W, Ta, etc.) will reduce the reflectance of a multilayer mirror by an order of magnitude, it is clear that the contamination rate of condenser elements from plasma source debris must approach zero. In the present Sandia laser plasma source SXPL illumination systems, reflectance values of multilayer-coated plasma-facing optical elements are reduced by ~50% within 106 laser pulses. Clearly, large improvements in target debris production and/or mitigation are needed.
The Investigation of Discharge-laser-driven Plasmas as Sources for Soft X-ray Projection Lithography*
Authors: Rockett Paul D. , Hunter John A. , Olson Richard E. , Sweatt William C. , Kubiak Glenn D. , Berger Kurt W. , Schmitt Randal L. , Shields Harry , Powers Michael , Windt David L.
Published: February 2023

Following two years of work to understand the conversion of excimer laser light into soft x-rays, we have moved onto implementing code predictions to improve conversion, and to studying the optical characteristics of these sources.
"Residual Stresses in Mo/Si Multilayers and their Response to Thermal Annealing"
Authors: Koike J. , Kassner M. E. , Weber F. J. , Stearns D. G. , Rosen R. S. , Vernon S. P.
Published: February 2023

The thermal stability of sputter-deposited Mo/Si multilayers was investigated by annealing studies at relatively low temperatures for various times. Two distinct stages of thermally-activated Mo/Si interlayer growth were found: a relatively rapid initial "surge" of approximately one monolayer, followed by a (slower) secondary "steady-state" growth that is diffusion controlled. The interdiffusion coefficients for the interlayer formed during deposition of Mo on Si are substantially higher than those of the interlayer formed during deposition of Si on Mo. It was also observed that significant residual tensile elastic strains develop within the Mo layer (normal to the multilayer) as a result of annealing. Explanations for these observed behaviors are presented.
Tarnishing and Restoration of Mo/Si Multilayer X-ray Mirrors
Authors: Underwood J. H. , Gullikson E. M. , Nguyen Khanh
Published: February 2023

Multilayer x-ray mirrors of molybdenum and silicon operating at normal incidence at wavelengths just longward of the Si LII,III absorption edges, are a key component in the development of soft x-ray projection lithography. In this application high reflectivity is essential. Because the achievement and maintenance of high reflectivity surfaces is vital to the success of soft x-ray projection lithography, it is important to know if Mo/Si multilayers show "aging" effects, e.g. decreasing reflectivity with time. Since, in a series of measurements of such mirrors at x-ray wavelengths, we had found evidence of such decay, a series of systematic experiments was carried out.
Soft-x-ray projection lithography using two arrays of phase zone plates
Authors: Hector Scott D. , Smith Henry I.
Published: February 2023

At feature sizes below 70nm, the mask-to-sample gap in proximity x-ray
lithography must be less than 5μm, which, although this is not
a problem in research, it may be undesirable in manufacturing. Thus,
for sub-70nm features, one is persuaded to consider the feasibility of
x-ray projection systems.
Generation of Intense, Tunable Ultrashort Pulses in the Ultraviolet Using a Single Excimer Pump Laser
Authors: Szatmári S. , Schäfer F.P.
Published: February 2023

The cost and complexity of experimental arrangements for the generation of ultrashort, high-power, tunable excimer laser pulses is considerable (1,2). In this paper we describe a simple method and experimental arrangement for the generation of ultrashort excimer laser pulses of GW peak power with <5 % amplified spontaneous emission (ASE) pedestal, using only a single commercial excimer laser for the generation of a single ultrashort dye laser pulse at twice the excimer laser wavelength and amplification of the frequency-dobuled dye laser pulse.
Travelling Wave Pumped Ultrashort Pulse Distributed Feedback Dye Laser
Authors: Müller Alexander , Szabó G. , Rácz B. , Nikolaus B. , Bor Zs.
Published: February 2023

Distributed feedback dye lasers (DFDLs) are convenient sources of wavelength tunable transform limited picosecond laser pulses /1/. Single pulses are obtained by proper choice of the pumping power /2,3/. As we have shown earlier, the duration of the pulses from a standing wave DFDL can be as short as 2 ps when pumped by 16 ps long pulses /4/. The shortest pulse in this case is limited by the transit time of light through the DFDL. It should, however, be possible to overcome this limitation by travelling wave pumping. We have recently introduced this concept experimentally and demonstrated that considerable shortening of a generated ASE pulse with respect to the pump pulse can be achieved in this way /5/.
The Development of the FEL as a Laboratory Radiation Source
Authors: Robinson Kem Edward , Madey John M. J.
Published: February 2023

The results obtained from operational free-electron lasers [FEL], in particular the Stanford single-pass FEL and the ACO storage ring FEL, have provided strong proof-of-principle for FEL theory. Subsequent development of FEL’s will now be in the direction of laboratory radiation sources.
Picosecond two-color photon echoes in doped molecular solids
Authors: Wiersma Douwe A. , Weitekamp D.P. , Duppen Koos
Published: February 2023

The three pulse stimulated echo, long known in magnetic resonance (1), has more recently been demonstrated on optical transitions(2). An interesting way of looking at the stimulated-echo-formation relies on the fact that the first two excitation pulses create an "ordered" population as a function of detuning .This ordered population can also be looked upon as a " grating " in frequency space from which a probe pulse can "reflect" an echo. Since a grating in only one of the levels suffices, a corollary is that the echo may be stimulated at some entirely different frequency ω2 than the one ω1, at which the grating was prepared. We report the observation of such a connected two-color stimulated echo(C2CSE) in a solid and use it to confirm the relaxation out of the 747 cm−1 vibrational state of the S1 manifold of pentacene in naphthalene. It has also been realized (3) that after redistribution of the initially excited population grating, that the echo may be stimulated from any state into which the grating has relaxed. We report here the first observation of such a relaxed two-color stimulated echo(R2CSE) and note that this echo can be seen as the optical analogue of the cross-relaxation peak in a two-dimensional nmr spectrum. The rise and fall of this relaxed echo can be used as a probe for the vibrational deactivation pathway(s) subsequent to excitation of various vibrational levels in the S1 manifold. Prior to discussing briefly some of the results we note that from an analysis of this echo effect it is clear that a necessary prerequiste to observation of such two-color photon echoes is that the inhomogeneous broadening on the selected transitions is strongly correlated. Furthermore we note that in these echo effects interference with stimulated emission and super -fluorescence must be avoided by using small-angle excitation pulses. Some of the results are shown in Fig.1. In (A) the decay of the C2CSE is presented with the solid line being a fit to a population decay parameter of the 747 cm-1 level of 33 psec. In (B) the rise of the R2CSE is shown where the probe wavelength is at the pure electronic transition. The dashed line is the expected echo-intensity rise if the initially excited vibration would directly decay into the vibrationless state. The mismatch between the dashed curve and the experimental one shows that intermediate levels in the relaxation pathway must exist. The solid line in (B) is a fit to a model where one intermediate bottleneck level is assumed with a lifetime of 16±2 psec. Other examples will be presented at the meeting.
Femtosecond Dynamics of Nonequilibrium Correlated Electron-Hole Pair Distributions in Room-Temperature GaAs Multiple Quantum Well Structures
Authors: Knox W. H. , Fork R. L. , Downer M. C. , Miller D. A. B. , Chemla D. S. , Shank C. V. , Gossard A. C. , Wiegmann W.
Published: February 2023

The GaAs multiple quantum well structure (MQWS) is a new and unique nonlinear optical material [1]. The reduced dimensionality of the electron-hole system in the ultra-thin (100 Å) layers gives rise to clear excitonic absorption peaks which are observed even at room temperature [2]. The GaAs MQWS may form the basis for future high speed optoelectronic devices operating at room temperature.
Intramolecular Electronic and Vibrational Redistribution and Chemical Transformation in Isolated Large Molecules — S1 Benzene
Authors: Yoshihara K. , Sumitani M. , O'Connor D.V. , Takagi Y. , Nakashima N.
Published: February 2023

With the advent of powerful and ultrashort optical pulses and with the progress of associated detecting techniques, many phenomena in the field of molecular dynamics become directly accessible to study. One of the important area which came out very recently is the direct time-resolved measurement of the intramolecular energy redistribution and chemical transformation in the isolated molecule condition [1 - 4].