XUV Region Research Articles

Hit detection in serial femtosecond crystallography using X-ray spectroscopy of plasma emission.

Serial femtosecond crystallography is an emerging and promising method for determining protein structures, making use of the ultrafast and bright X-ray pulses from X-ray free-electron lasers. The upcoming X-ray laser sources will produce well above 1000 pulses per second and will pose a new challenge: how to quickly determine successful crystal hits and avoid a high-rate data deluge. Proposed here is a hit-finding scheme based on detecting photons from plasma emission after the sample has been intercepted by the X-ray laser. Plasma emission spectra are simulated for systems exposed to high-intensity femtosecond pulses, for both protein crystals and the liquid carrier systems that are used for sample delivery. The thermal radiation from the glowing plasma gives a strong background in the XUV region that depends on the intensity of the pulse, around the emission lines from light elements (carbon, nitrogen, oxygen). Sample hits can be reliably distinguished from the carrier liquid based on the characteristic emission lines from heavier elements present only in the sample, such as sulfur. For buffer systems with sulfur present, selenomethionine substitution is suggested, where the selenium emission lines could be used both as an indication of a hit and as an aid in phasing and structural reconstruction of the protein.

Observation of an optical vortex beam from a helical undulator in the XUV region.

The observation of an optical vortex beam at 60 nm wavelength, produced as the second-harmonic radiation from a helical undulator, is reported. The helical wavefront of the optical vortex beam was verified by measuring the interference pattern between the vortex beam from a helical undulator and a normal beam from another undulator. Although the interference patterns were slightly blurred owing to the relatively large electron beam emittance, it was possible to observe the interference features thanks to the helical wavefront of the vortex beam. The experimental results were well reproduced by simulation.

Theoretical Study of Laser Emission for C-Like (Ar XIII), (Ti XVII) and (Fe XXI)

Energy levels, transition probability and oscillator strengths have been calculated for the Ar XIII, Ti XVII and Fe XXI. The configurations included in the calculations are 2s2 2p2, 2s2 2p 3l (l = s, p & d) and 4l (l = s, p, d, & f) of C-like Ar XIII, Ti XVII & Fe XXI which has 69 fine structures by using the fully relativistic flexible atomic code (FAC) program. These data are used in the determination of the reduced population and gain coefficients over a wide range of electron densities from (10+18 to 10+23) and at various plasmas temperatures. The results show that the transitions in Ar18+, Ti22+, and Fe26+ ions are the most promising laser emission lines in the XUV and soft X-ray spectral regions.

Nonlinear Attosecond Metrology by Intense Isolated Attosecond Pulses

For the future progress of nonlinear attosecond science, one of the most important issues is the development of high-energy isolated attosecond pulse sources. In this paper, we review our research on the energy-scaling method for isolated attosecond pulse generation, which combines infrared two-color gating and an energy-scaling method of high-order harmonic generation. By establishing the robust energy-scaling method, the maximum pulse energy of the isolated attosecond pulse reaches 1.3 $\mu$ J/pulse in the XUV region, which is almost 100-fold higher than the values ever reported before. The pulse duration of this isolated attosecond pulse is directly measured to be 500 as by the autocorrelation method using the nonlinear interaction of N $_2$ . This nonlinear experiment clearly shows that our developed isolated attosecond pulse has enough pulse energy to make a breakthrough for nonlinear attosecond metrology as well as for attosecond-pump/attosecond-probe experiments investigating electronic processes. In addition, we further extend the high-energy harmonic continuum by using the infrared two-color gating method up to the soft-x-ray region. With the aim to temporally characterize an isolated attosecond pulse in the soft-x-ray region, we demonstrate the first observation of above-threshold ionization of He with two-photon absorption of multiple higher harmonics in the soft-x-ray region.

Generation of ultrashort intense optical pulses at 1.6 μm from a bismuth triborate-based optical parametric chirped pulse amplifier with carrier-envelope phase stabilization

We describe a few-cycle intense optical parametric chirped pulse amplifier (OPCPA) in the IR that is based on bismuth triborate (BiB3O6, BIBO) crystals. Two Ti:sapphire chirped pulse amplification systems are used to generate seed and pump pulses for the OPCPA. Carrier-envelope phase (CEP)-stabilized seed pulses in the IR are produced by difference frequency mixing of white light spanning from the visible to near IR range. The seed pulses, which have a nearly one octave-spanning spectrum around 1.6 μm, are temporally stretched in an acousto-optic programmable dispersive filter and amplified up to 550 μJ in two BIBO-based parametric amplifiers without losing their bandwidth. After compression in a fused silica block, we obtained 9.2 fs optical pulses with a repetition rate of 1 kHz, which comprise less than two optical cycles at 1.6 μm. These optical pulses were used to generate extreme ultraviolet (XUV) high harmonics in krypton. We succeeded in measuring the CEP dependence of the XUV radiation near the silicon L edge around 100 eV, verifying the passive stabilization of the CEP of the sub-two cycle IR pulses. The XUV spectra consist of two half-cycle cutoffs and show a clear transition from a modulated structure to a continuum near the cutoff. The continuum-like spectra as well as their CEP dependence indicate that they originate in a single recombination burst, showing that few-cycle IR OPCPA light sources are capable of generating attosecond pulses in the XUV region.

Laser driven structured quantum rings

In this work we study harmonic emission from structured quantum rings (SQRs). In SQRs, electrons trapped in two-dimensional structures are further confined by an external potential composed of N scattering centers arranged on a circle. We build a suitable one-dimensional model Hamiltonian describing this class of systems and analytically solve the associated Schödinger equation. We find that the solution can be expressed in terms of Mathieu functions and focus on the specific case of N = 6. By exactly solving the time-dependent Schödinger equation, we then show how the harmonic response to linearly polarized lasers strongly depends on the ring physical parameters. The results illustrate how the additional degrees of freedom introduced by these parameters provide important handles to control the emitted spectrum that in some cases extends into the XUV region.

Population Inversion and X-Ray Laser Gain by Electron Impact Excitation of Ni-Like Tin

Excitation energies and transition probabilities for the 1s22s22p63s23p63d10, and the 1s22s22p63s23p63d94l (l= s, p, d, and f) configurations in nickel-like tin have been used to calculate the reduced population of 99 fine structure levels over a wide range of electron densities and at different electron plasma temperatures. The gain coefficients for those transitions with positive population inversion factors have been calculated as a function of electron density. This study is concerned with the transitions which produce positive gain at XUV and X-ray spectral regions.

Suppression of driving laser in high harmonic generation with a microchannel plate.

Separating the infrared driving laser from the extreme ultraviolet (XUV) pulses after high-order harmonic generation has been a long-standing difficulty. In this Letter, we propose and demonstrate that the driving laser can be blocked by simply installing a microchannel plate (MCP) into the beam line. In addition to its high damage threshold, the MCP filter also transmits photons over the entire XUV region. This paves the way for attosecond pulse generation with unprecedented bandwidth.

Quantified H I morphology - VI. The morphology of extended discs in UV and H I

Extended ultraviolet (XUV) discs have been found in a substantial fraction of late-type – S0, spiral and irregular – galaxies. Similarly, most late-type spirals have an extended gas disc, observable in the 21-cm radio line (H i). The morphology of galaxies can be quantified well using a series of scale-invariant parameters; concentration-asymmetry-smoothness (CAS), Gini, M20, and GM parameters. In this series of papers, we apply these to H i column density maps to identify mergers and interactions, lopsidedness and now XUV discs. In this paper, we compare the quantified morphology and effective radius (R50) of the Westerbork observations of neutral Hydrogen in Irregular and SPiral galaxies Project (WHISP) H i maps to those of far- and near-ultraviolet images obtained with GALEX, to explore how close the morphology and scales of H i and UV in these discs correlate. We find that XUV discs do not stand out by their effective radii in UV or H i. However, the concentration index in far-ultraviolet (FUV) appears to select some XUV discs. And known XUV discs can be identified via a criterion using asymmetry and M20; 80 per cent of XUV discs are included but with 55 per cent contamination. This translates into 61 candidate XUV disc out of our 266 galaxies, 23 per cent consistent with previous findings. Otherwise, the UV and H i morphology parameters do not appear closely related. Our motivation is to identify XUV discs and their origin. We consider three scenarios; tidal features from major mergers, the typical extended H i disc is a photo-dissociation product of the XUV regions and both H i and UV features originate in cold flows fueling the main galaxy. We define extended H i and UV discs based on their concentration (CHI > 5 and CFUV > 4 respectively), but that these two subsamples never overlap in the WHISP sample. This appears to discount a simple photo-dissociation origin of the outer H i disc. Previously, we identified the morphology space occupied by ongoing major mergers. Known XUV discs rarely reside in the merger-dominated part of H i morphology space but those that do are type 1. The exceptions, XUV discs in ongoing mergers, include the previously identified UGC 4862 and UGC 7081, 7651, and 7853. This suggests cold flows as the origin for the XUV complexes and their surrounding H i structures.

Efficiency enhancement of a two-beam free-electron laser using a nonlinearly tapered wiggler

A nonlinear and non-averaged model of a two-beam free-electron laser (FEL) wiggler that is tapered nonlinearly in the absence of slippage is presented. The two beams are assumed to have different energies, and the fundamental resonance of the higher energy beam is at the third harmonic of the lower energy beam. By using Maxwell's equations and the full Lorentz force equation of motion for the electron beams, coupled differential equations are derived and solved numerically by the fourth-order Runge—Kutta method. The amplitude of the wiggler field is assumed to decrease nonlinearly when the saturation of the third harmonic occurs. By simulation, the optimum starting point of the tapering and the slopes for reducing the wiggler amplitude are found. This technique can be applied to substantially improve the efficiency of the two-beam FEL in the XUV and X-ray regions. The effect of tapering on the dynamical stability of the fast electron beam is also studied.

THE SPACE DENSITY OF EXTENDED ULTRAVIOLET (XUV) DISKS IN THE LOCAL UNIVERSE AND IMPLICATIONS FOR GAS ACCRETION ONTO GALAXIES

We present results of the first unbiased search for extended UV (XUV)-disk galaxies undertaken to determine the space density of such galaxies. Our sample contains 561 local (0.001 < z < 0.05) galaxies that lie in the intersection of available GALEX deep imaging (exposure time > 1.5 x 10^4 s) and SDSS DR7 footprints. We explore modifications to the standard classification scheme for our sample that includes both disk- and bulge-dominated galaxies. Visual classification of each galaxy in the sample reveals an XUV-disk frequency of up to 20% for the most nearby portion of our sample. On average over the entire sample (out to z=0.05) the frequency ranges from a hard limit of 4% to 14%. The GALEX imaging allows us to detect XUV-disks beyond 100 Mpc. The XUV regions around XUV-disk galaxies are consistently bluer than the main bodies. We find a surprisingly high frequency of XUV emission around luminous red (NUV-r > 5) and green valley (3 < NUV-r < 5) galaxies. The XUV-disk space density in the local universe is > 1.5-4.2 x 10^-3 Mpc^-3. Using the XUV emission as an indicator of recent gas accretion, we estimate that the cold gas accretion rate onto these galaxies is > 1.7-4.6 x 10^-3 Msun Mpc^-3 yr^-1. The number of XUV-disks in the green valley and the estimated accretion rate onto such galaxies points to the intriguing possibility that 7%-18% of galaxies in this population are transitioning away from the red sequence.

Dependence of the Generation Efficiency of High-Order Sum and Difference Frequencies in the Extreme Ultraviolet Region on the Wavelength of an Added Tunable Laser Field

We investigate the dependence of the generation efficiency of sum and difference frequencies in the extreme ultraviolet (xuv) region on the wavelength of an added tunable laser field. The wavelength of the added field ranges from 600 to 1500 nm. The generation efficiency of sum and difference frequencies is dramatically enhanced when the wavelength of the added field is longer than that of the fundamental field for pure harmonics. The discussions are held to the added field with perturbative intensity first, and they are further extended to that with nonperturbative intensity.

Two-color ionization of hydrogen by short intense pulses

Photoelectron energy spectra resulting by the interaction of hydrogen with two short pulses having carrier frequencies, respectively, in the range of the infrared and XUV regions have been calculated. The effects of the pulse duration and timing of the X-ray pulse on the photoelectron energy spectra are discussed. Analysis of the spectra obtained for very long pulses show that certain features may be explained in terms of quantum interferences in the time domain. It is found that, depending on the duration of the X-ray pulse, ripples in the energy spectra separated by the infrared photon energy may appear. Moreover, the temporal shape of the low frequency radiation field may be inferred by the breadth of the photoelectron energy spectra.

Flare location on the solar disk: Modeling the thermosphere and ionosphere response

Solar flare enhancements to the soft X‐ray (XUV) and extreme ultraviolet (EUV) spectral irradiance depend on the location of the flare on the solar disk. Most emission lines in the XUV region (∼0.1 to ∼25 nm) are optically thin and are weakly dependent on the location of the flare, but in the EUV region (∼25 to ∼120 nm), many important lines and continua are optically thick, so enhancements are relatively smaller for flares located near the solar limb, due to absorption by the solar atmosphere. The flare irradiance spectral model (FISM) was used to illustrate these location effects, assuming two X17 flares that are identical except that one occurs near disk center and the other near the limb. FISM spectra of these two flares were used as solar input to the National Center for Atmospheric Research (NCAR) thermosphere‐ionosphere‐mesosphere electrodynamics general circulation model (TIME‐GCM) to investigate the ionosphere/thermosphere (I/T) response. Model simulations showed that in the E region ionosphere, where XUV dominates ionization, flare location does not affect I/T response. However, flare‐driven changes in the F region ionosphere, total electron content (TEC), and neutral density in the upper thermosphere, are 2–3 times stronger for a disk‐center flare than for a limb flare, due to the importance of EUV enhancement. Flare location did not affect the timing of the ionospheric response, but the thermospheric response was ∼20 min faster for the disk‐center flare. Model simulations of I/T responses to an X17 flare on 28 October 2003 were consistent with measurements of TEC and neutral density changes.

The FERMI@Elettra free-electron-laser source for coherent x-ray physics: photon properties, beam transport system and applications

FERMI@Elettra comprises two free electron lasers (FELs) that will generate short pulses (τ∼25–200 fs) of highly coherent radiation in the XUV and soft x-ray region. The use of external laser seeding together with a harmonic upshift scheme to obtain short wavelengths will give FERMI@Elettra the capability of producing high-quality, longitudinally coherent photon pulses. This capability, together with the possibilities of temporal synchronization to external lasers and control of the output photon polarization, will open up new experimental opportunities that are not possible with currently available FELs. Here, we report on the predicted radiation coherence properties and important configuration details of the photon beam transport system. We discuss the several experimental stations that will be available during initial operations in 2011, and we give a scientific perspective on possible experiments that can exploit the critical parameters of this new light source.

Limitations and possible improvements of DLC dielectric response model based on parameterization of density of states

Several extensions of previously published PDOS model of DLC dielectric response are tested on a typical DLC film on Si. The extended models are applied to the processing of optical measurements, namely UV/VIS/NIR ellipsometry and spectrophotometry as well as FTIR transmittance spectroscopy. The tested extensions of PDOS approach, that originally took into account only π → π ⁎ and σ → σ ⁎ interband transitions, are (i) the inclusion of π → σ ⁎ and σ → π ⁎ transitions, (ii) the addition of transitions from σ to bands above σ ⁎ , (iii) the removal of the assumption that the bands are symmetrical with respect to the Fermi level. However, it is found that these extensions are not needed for the processing of optical data in the measured range, although they could become important in the VUV and XUV regions. It is shown that the PDOS models can be successfully combined with models of phonon absorption in IR region. The application of the combined model in the wide spectral range allows correct calculation of absorption peaks because the interference and dispersion effects are taken into account.

Short wavelength regenerative amplifier free electron lasers

In this paper we discuss extending the operating wavelength range of tunable Regenerative Amplifier FELs to shorter wavelengths than current design proposals, notably into the XUV regions of the spectrum and beyond where the reflectivity of broadband optics is very low. Simulation studies are presented which demonstrate the development of good temporal coherence in generic systems with a broadband radiation feedback of less than one part in ten thousand.

Efficient generation of high-order sum and difference frequencies in the xuv region by combining a weak longer-wavelength field

We demonstrate the efficient generation of sum and difference frequencies in the extreme-ultraviolet wavelength region both experimentally and theoretically. The importance of the wavelength of the weak combined field is emphasized.

Nonlinear Interaction of Attosecond XUV Pulses with Atoms and Molecules

We review nonlinear multiphoton processes induced by intense attosecond XUV pulses. Phase-matched high harmonics by a loosely focusing geometry produce high focused intensity sufficient to induce nonlinear optical phenomena in the XUV region. Such nonlinear phenomena not only have been attracting much attention in the field of quantum electronics, but also are of great importance for direct measurement of temporal shapes of the XUV pulses. Nonresonant two photon processes in atoms and molecules in the XUV region are employed for the direct observation of a train of attosecond pulses by means of autocorrelation. A train of attosecond pulses is characterized by energy-resolved autocorrelation with photoelectrons produced by two-photon above threshold ionization in Ar. Molecular Coulomb explosion induced by two-photon double in N2 is demonstrated for the first time and also is found to be useful for efficient measuring a train of attosecond pulses

Generation of extreme ultraviolet continuum radiation driven by a sub-10-fs two-color field

We have proposed and demonstrated a novel approach for generating high-energy extreme-ultraviolet (XUV) continuum radiation. When a two-color laser field consisting of a sub-10-fs fundamental and its parallel-polarized second harmonic was applied to high-order harmonic generation in argon, a continuum spectrum centered at 30 nm was successfully obtained with an energy as high as 10 nJ. This broadband emission indicates the possibility of generating intense single attosecond pulses in the XUV region.