Extreme Ultraviolet Region Research Articles

Brilliance improvement of laser-produced extreme ultraviolet and soft x-ray plasmas based on pulsed gas jets

Two methods improving the brilliance of laser-induced plasmas emitting in the extreme UV (EUV) and soft x-ray (SXR) regions were investigated, using three different gases (nitrogen, krypton, and xenon) from a pulsed gas jet. Utilizing a newly designed piezoelectric valve, up to almost ten times higher gas pressures were applied, resulting in increased target densities and thus, higher conversion efficiencies of laser energy into EUV and SXR radiation. Secondly, geometrically reducing the angle between the incoming laser beam and the observed plasma emission minimizes reabsorption of the emitted short wavelength radiation. Combining both methods, the source brilliance is increased by a factor of 5 for nitrogen. Furthermore, a compact EUV focusing system for metrological applications is presented utilizing the optimized plasma source. An energy density of 1 mJ/cm2 at wavelength λ = 13.5 nm in the focal spot of an ellipsoidal mirror is achieved with xenon as the target gas being sufficient for material removal of PMMA samples with an ablation rate of 0.05 nm/pulse.

Generation of enhanced even harmonics of fundamental radiation in temporally separated two-color laser fields

High harmonic generation (HHG) produces coherent light in the extreme ultraviolet (XUV) spectral region. We present results on generation of high harmonics (HHs) with a temporally separated two laser pulses of fundamental (ω0) and its second harmonic (2ω0) in argon (Ar), molecular nitrogen (N2) and carbon dioxide (CO2) gases having close ionization potentials. The special feature of this study is that the ω0 and 2ω0 fields are temporary separated, so that the fundamental radiation comes first and the second harmonic radiation arrives to the interaction region with a delay. High harmonic spectra were measured for parallel and perpendicular relative polarizations of the fundamental and the second harmonic fields. With the combined (ω0 + 2ω0) field more than an order of magnitude enhancement of the HHs signal was observed compared to the ω0 or the 2ω0 field alone. An increased conversion efficiency into HHs, notably of the even orders of 2(2m + 1) (m = 1 − 3) was found for the latter case. We also discuss similarities and differences of the HH generation in the three gas media with single and two-color fields.

Editorial

Editorial

Phase-matched nonlinear wave-mixing processes in XUV region with multicolor lasers.

We report here experimental results of perturbative nonlinear optical wave-mixing processes in the extreme ultraviolet region by using two-color and three-color laser fields. Besides the usual odd-harmonic spectrum of high harmonic generation, new spectral components are observed when multiple incommensurate lasers (one driving plus one or two control field) interact with neutral krypton gas. To demonstrate the wave-mixing process underlying such an observation, we first couple the driving field with either the signal or the idler field of an optical parametric amplifier in the gaseous ensemble to generate certain mixing frequencies. The two control fields are then simultaneously combined with the driving field to produce broad and distinguishable mixing peaks that clearly reveal the contribution of each control laser. Finally, the variation of the intensity of the mixing waves with the intensity of each control field, the gas density, and the relative focus position is examined for signatures of phase-matched generation of the mixing fields in this spectral region.

Polarization-state-resolved high-harmonic spectroscopy of solids

Attosecond metrology sensitive to sub-optical-cycle electronic and structural dynamics is opening up new avenues for ultrafast spectroscopy of condensed matter. Using intense lightwaves to precisely control the fast carrier dynamics in crystals holds great promise for next-generation petahertz electronics and devices. The carrier dynamics can produce high-order harmonics of the driving field extending up into the extreme-ultraviolet region. Here, we introduce polarization-state-resolved high-harmonic spectroscopy of solids, which provides deeper insights into both electronic and structural sub-cycle dynamics. Performing high-harmonic generation measurements from silicon and quartz, we demonstrate that the polarization states of the harmonics are not only determined by crystal symmetries, but can be dynamically controlled, as a consequence of the intertwined interband and intraband electronic dynamics. We exploit this symmetry-dynamics duality to efficiently generate coherent circularly polarized harmonics from elliptically polarized pulses. Our experimental results are supported by ab-initio simulations, providing evidence for the microscopic origin of the phenomenon.

Relativistic Hartree-Fock and Dirac-Fock atomic structure and radiative parameter calculations in nine-times ionized xenon (Xe X)

A new set of oscillator strengths and transition probabilities is reported for 92 radiative transitions of nine-times ionized xenon, Xe X, in the extreme ultraviolet region from 110 to 164 Å. They have been obtained by two different theoretical approaches, i.e. the pseudo-relativistic Hartree-Fock (HFR) and the fully relativistic multiconfiguration Dirac-Hartree-Fock (MCDHF) methods, which allowed us to estimate the precision of the final results. The eigenvector compositions are also given for the first time for all the experimentally known energy levels in Xe X.

Study of Relativistic excitation energies and transition data of X–ray and EUV transitions in Be-like ions

In the present work, fine structure energies, transition wavelengths, transition rates, oscillator strengths and line strengths have been presented for lowest 40 levels of Be-like Fe, Os to Hg ions by using GRASP2K. We have analyzed the effects of core-valence (CV) correlations in excitation energies as a function of active set. We accomplish that effects of core-valence correlations are necessary to attain accuracy and convergence in excitation energies. We have compared our presented excitation energies of Be-like Fe with available theoretical and experimental energies and found good agreement. But due to unavailability of data for Be-like Os to Hg ions for comparison, we have calculated excitation energies from flexible atomic code (FAC). We show that magnetic multipole transitions (M2) have effective contribution in lifetime of 2s3p 3P2o and 2p3d 3P2o. We demonstrate that transitions of Be-like ions among lowest 40 levels mainly occurs in Hard X-ray (HXR), Soft X-ray(SXR) and Extreme Ultraviolet (EUV) regions.

Fast and easy fabrication methodology of Fresnel zone plates for the extreme ultraviolet and soft x-ray regions.

Zone plate design and efficient methods for the fabrication of zone plates for extreme ultraviolet (EUV) and soft x-ray applications in a newly developed scanning reflection microscope are presented. Based on e-beam lithography, three types of transmission zone plates with focal lengths between 6 and 15mm are reported: (i)phase-shifting zone plates made by 190nm thick PMMA rings on Si3N4 membranes, (ii)absorbing zone plates made by 75nm thick Au ring structures on Si3N4, and (iii)freestanding Au rings of 50nm thickness and increased transmission in the EUV range. Experiments at the DELTA synchrotron facility reveal a minimum spot size and resulting spatial resolution of 9±3 μm, which is the theoretical limit resulting from the synchrotron beam parameters at 60eV photon energy. Images of a Ti/Si chessboard test pattern are recorded exploiting the energy dependence of the element-specific reflectance.

Ptychographic amplitude and phase reconstruction of bichromatic vortex beams.

We experimentally demonstrate that ptychographic coherent diffractive imaging can be used to simultaneously characterize the amplitude and phase of bichromatic orbital angular momenta-shaped vortex beams, which consist of a fundamental field, together with its copropagating second-harmonic field. In contrast to most other orbital angular momentum characterization methods, this approach solves for the complex field of a hyperspectral beam. This technique can also be used to characterize other phase-structured illumination beams, and, in the future, will be able to be extended to other complex fields in the extreme ultraviolet or X-ray spectral regions, as well as to matter waves.

Refined Ptychographic Reconstruction of Attosecond Pulses

Advanced applications of attosecond pulses require the implementation of experimental techniques for a complete and accurate characterization of the pulse temporal characteristics. The method of choice is the frequency resolved optical gating for the complete reconstruction of attosecond bursts (FROG-CRAB), which requires the development of suitable reconstruction algorithms. In the last few years, various numerical techniques have been proposed and implemented, characterized by different levels of accuracy, robustness, and computational load. Many of them are based on the central momentum approximation (CMA), which may pose severe limits in the reconstruction accuracy. Alternative techniques have been successfully developed, based on the implementation of reconstruction algorithms which do not rely on this approximation, such as the Volkov-transform generalized projection algorithm (VTGPA). The main drawback is a notable increase of the computational load. We propose a new method, called refined iterative ptychographic engine (rePIE), which combines the advantages of a robust algorithm based on CMA, characterized by a fast convergence, with the accuracy of advanced algorithms not based on such approximation. The main idea is to perform a first fast iterative ptychographic engine (ePIE) reconstruction and then refine the result with just a few iterations of the VTGPA in order to correct for the error introduced by the CMA. We analyse the accuracy of the novel reconstruction method by comparing the residual error (i.e., the difference between the reconstructed and the simulated original spectrograms) when VTGPA, ePIE, and rePIE reconstructions are employed. We show that the rePIE approach is particularly useful in the case of short attosecond pulses characterized by a broad spectrum in the vacuum-ultraviolet (VUV)–extreme-ultraviolet (XUV) region.

Boris Peter Stoicheff. 1 June 1924—15 April 2010

Boris Stoicheff was a pioneer in the use of high-resolution Raman spectroscopy at the National Research Council of Canada to elucidate the structural properties of molecules. He was also one of the first scientists to apply lasers to spectroscopy, investigating spontaneous and stimulated Raman and Brillouin scattering in liquids and solids at the University of Toronto. He later extended the range of tunable coherent sources into the vacuum ultraviolet (VUV) and extreme ultraviolet (XUV) regions down to 80 nm, allowing investigations of electronic states and their lifetimes for rare gas dimers. He authored 190 research and review articles as well as a biography of his postdoctoral mentor and later his senior colleague, Gerhard Herzberg. He used his keen insight, warm personality and strong personal skills to serve the scientific community in numerous administrative roles, including terms as president of the Canadian Association of Physicists and the Optical Society of America. His 24 PhD graduates and more than 20 postdoctoral fellows and visitors, all of whom benefitted from his strong mentoring skills and high standards, have gone on to prominent positions in academia, government and industry.

Multi-stage generation of extreme ultraviolet dispersive waves by tapering gas-filled hollow-core anti-resonant fibers.

In this work, we numerically investigate an experimentally feasible design of a tapered Ne-filled hollow-core anti-resonant fiber and we report multi-stage generation of dispersive waves (DWs) in the range 90-120 nm, well into the extreme ultraviolet (UV) region. The simulations assume a 800 nm pump pulse with 30 fs 10 µJ pulse energy, launched into a 9 bar Ne-filled fiber with a 34 µm initial core diameter that is then tapered to a 10 µm core diameter. The simulations were performed using a new model that provides a realistic description of both loss and dispersion of the resonant and anti-resonant spectral bands of the fiber, and also importantly includes the material loss of silica in the UV. We show that by first generating solitons that emit DWs in the far-UV region in the pre-taper section, optimization of the following taper structure can allow re-collision with the solitons and further up-conversion of the far-UV DWs to the extreme-UV with energies up to 190 nJ in the 90-120 nm range. This process provides a new way to generate light in the extreme-UV spectral range using relatively low gas pressure.

Observation of the 1S\u20132P Lyman-\u03b1 transition in antihydrogen

In 1906, Theodore Lyman discovered his eponymous series of transitions in the extreme-ultraviolet region of the atomic hydrogen spectrum1,2. The patterns in the hydrogen spectrum helped to establish the emerging theory of quantum mechanics, which we now know governs the world at the atomic scale. Since then, studies involving the Lyman-α line—the 1S–2P transition at a wavelength of 121.6 nanometres—have played an important part in physics and astronomy, as one of the most fundamental atomic transitions in the Universe. For example, this transition has long been used by astronomers studying the intergalactic medium and testing cosmological models via the so-called ‘Lyman-α forest’3 of absorption lines at different redshifts. Here we report the observation of the Lyman-α transition in the antihydrogen atom, the antimatter counterpart of hydrogen. Using narrow-line-width, nanosecond-pulsed laser radiation, the 1S–2P transition was excited in magnetically trapped antihydrogen. The transition frequency at a field of 1.033 tesla was determined to be 2,466,051.7 ± 0.12 gigahertz (1σ uncertainty) and agrees with the prediction for hydrogen to a precision of 5 × 10−8. Comparisons of the properties of antihydrogen with those of its well-studied matter equivalent allow precision tests of fundamental symmetries between matter and antimatter. Alongside the ground-state hyperfine4,5 and 1S–2S transitions6,7 recently observed in antihydrogen, the Lyman-α transition will permit laser cooling of antihydrogen8,9, thus providing a cold and dense sample of anti-atoms for precision spectroscopy and gravity measurements10. In addition to the observation of this fundamental transition, this work represents both a decisive technological step towards laser cooling of antihydrogen, and the extension of antimatter spectroscopy to quantum states possessing orbital angular momentum.

Switching of the harmonic order in free-electron lasers by controlling the density modulation of an electron bunch.

It was demonstrated that harmonic order in free-electron laser (FEL) oscillations could be switched by adjusting the dispersive gap of the optical klystron ETLOK-III in the storage ring NIJI-IV. The effective gains for the fundamental and third-harmonic FEL oscillations were evaluated and it was confirmed that the FEL oscillated at the order of the harmonic with the higher effective gain. The ratio between the effective gain for the fundamental and that for the third harmonic was controlled by the dispersive gap. It was also demonstrated that a spectral measurement of the FEL-based Compton scattering X-ray beam was effective for directly observing the switching of the harmonic order. These results contribute to the development of higher-harmonic FEL oscillations suppressing the fundamental FEL oscillation in the extreme ultraviolet and X-ray regions.

Phase-matched four-wave mixing in the extreme ultraviolet region

We report here a detailed study of the four-wave mixing process in the extreme ultraviolet region around 30 nm by using two collinear incommensurate frequency laser pulses. The experimental results reveal evidence of the coherent accumulation of the wave-mixing fields and low-order (third-order and fifth-order) nonlinear response of an argon medium. The dependence of the intensities of the mixing fields on the intensity of a weak control field, on the argon pressure, and on the interaction length is analyzed to show that the four-wave mixing fields in this spectral range are generated under the phase-matched condition.

External pressure effect on the electronic, optical and thermoelectric properties of the CdY2Ch4 (Ch = S, Se) spinel compounds: Via modified Becke–Johnson (mBJ) exchange potential

We report the results of the FP-APW + lo calculations of the structural, electronic, optical and thermo-electric properties of the CdY2Ch4 (Ch = S, Se) compounds under hydrostatic pressure effect. The calculated ground state properties agree well with the available experimental data. The investigated materials are semiconductors, with direct band gaps (Γ- Γ) of approximately 2.18 eV, 1.53 eV for CdY2S4 and CdY2Se4, respectively, using GGA-TB-mBJ approximations. Under external pressure up to 20 GPa, both compounds show a linear decrease of energy gaps against pressure. Moreover, our calculated optical spectra over photon energies ranging from 0 to 20 eV suggest that CdY2Ch4 (Ch = S, Se) spinel compounds express a strong response in the energy range between the visible light and extreme UV regions, highlighting their possible application in the optoelectronic devices. Furthermore, the calculated thermoelectric properties indicate that both the CdY2S4 and CdY2Se4 compounds show interesting thermoelectric properties.

High resolution XUV Fourier transform holography on a table top

Today, coherent imaging techniques provide the highest resolution in the extreme ultraviolet (XUV) and X-ray regions. Fourier transform holography (FTH) is particularly unique, providing robust and straightforward image reconstruction at the same time. Here, we combine two important advances: First, our experiment is based on a table-top light source which is compact, scalable and highly accessible. Second, we demonstrate the highest resolution ever achieved with FTH at any light source (34 nm) by utilizing a high photon flux source and cutting-edge nanofabrication technology. The performance, versatility and reliability of our approach allows imaging of complex wavelength-scale structures, including wave guiding effects within these structures, and resolving embedded nanoscale features, which are invisible for electron microscopes. Our work represents an important step towards real-world applications and a broad use of XUV imaging in many areas of science and technology. Even nanoscale studies of ultra-fast dynamics are within reach.

Element Selectivity in Second-Harmonic Generation of GaFeO3 by a Soft-X-Ray Free-Electron Laser

Nonlinear optical frequency conversion has been challenged to move down to the extreme ultraviolet and x-ray region. However, the extremely low signals have allowed researchers to only perform transmission experiments of the gas phase or ultrathin films. Here, we report second harmonic generation (SHG) of the reflected beam of a soft x-ray free-electron laser from a solid, which is enhanced by the resonant effect. The observation revealed that the double resonance condition can be met by absorption edges for transition metal oxides in the soft x-ray range, and this suggests that the resonant SHG technique can be applicable to a wide range of materials. We discuss the possibility of element-selective SHG spectroscopy measurements in the soft x-ray range.

Coherent control schemes for the photoionization of neon and helium in the Extreme Ultraviolet spectral region

The seeded Free-Electron Laser (FEL) FERMI is the first source of short-wavelength light possessing the full coherence of optical lasers, together with the extreme power available from FELs. FERMI provides longitudinally coherent radiation in the Extreme Ultraviolet and soft x-ray spectral regions, and therefore opens up wide new fields of investigation in physics. We first propose experiments exploiting this property to provide coherent control of the photoionization of neon and helium, carry out numerical calculations to find optimum experimental parameters, and then describe how these experiments may be realized. The approach uses bichromatic illumination of a target and measurement of the products of the interaction, analogous to previous Brumer-Shapiro-type experiments in the optical spectral range. We describe operational schemes for the FERMI FEL, and simulate the conditions necessary to produce light at the fundamental and second or third harmonic frequencies, and to control the phase with respect to the fundamental. We conclude that a quantitative description of the phenomena is extremely challenging for present state-of-the-art theoretical and computational methods, and further development is necessary. Furthermore, the intensity available may already be excessive for the experiments proposed on helium. Perspectives for further development are discussed.

Sub-50-as isolated extreme ultraviolet continua generated by 1.6-cycle near-infrared pulse combined with double optical gating scheme

We demonstrate the generation of ultrabroad bandwidth attosecond continua extending to sub-50-as duration in the extreme ultraviolet (EUV) region based on a 1.6-cycle Ti:sapphire laser pulse. The combination of the amplitude gating scheme with a sub-two-cycle driver pulse and the double optical gating scheme achieves the continuum generation with a bandwidth of 70 eV at the full width at half maximum near the peak photon energy of 140 eV, which supports a Fourier-transform-limited pulse duration as short as 32 as. The carrier-envelope-phase (CEP) dependence of the attosecond continua shows a single-peak structure originating from the half-cycle cut-off at appropriate CEP values, which strongly indicates the generation of a single burst of an isolated attosecond pulse. Our approach suggests a possibility for isolated sub-50-as pulse generation in the EUV region by compensating for the intrinsic attosecond chirp with a Zr filter.