In the last 30 days

• Open/close Atom gyroscope with disordered arrays of quantum rings

  Daniel J Dayon et al 2010 J. Phys. B: At. Mol. Opt. Phys. 43 115302 Tag this article

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  Atom interferometry is of considerable interest in part because of the ability to interferometrically detect inertial rotations via the Sagnac effect with a potential sensitivity 10 ¹⁰ greater than optical gyroscopes. It has been shown recently that a coherently coupled array of identical interferometers can significantly enhance the sensitivity to rotations due to the appearance of transmission bands as a function of the inertial rotation rate Ω. Here we consider phase coherent transport of atomic matter waves in a chain of ring interferometers with a single occupied transverse mode in the presence of a rotation, Ω, and study the effect of variations in the size of the rings. We show that for randomly sized rings, the entire array functions as a highly sensitive Sagnac interferometer provided the level of random size fluctuations does not exceed a few per cent of the mean size. We also analyse how the use of individual defect states and controlled variations of the sizes in the array can be used to further enhance the sensitivity by creating narrow transmission resonances inside of a zero transmission gap.

• Open/close Photoionization dynamics of excited Ne, Ar, Kr and Xe atoms near threshold

  V L Sukhorukov et al 2012 J. Phys. B: At. Mol. Opt. Phys. 45 092001 Tag this article

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  A review of experimental and theoretical studies of the threshold photoionization of the heavier rare-gas atoms is presented, with particular emphasis on the autoionization resonances in the spectral region between the lowest two ionization thresholds ²P _3/2 and ²P _1/2, accessed from the ground or excited states. Observed trends in the positions, widths and shapes of the autoionization resonances depending on the atomic number, the principal quantum number n, the orbital angular momentum quantum number ℓ and further quantum numbers specifying the fine- and hyperfine-structure levels are summarized and discussed in the light of ab initio and multichannel quantum defect theory calculations. The dependence of the photoionization spectra on the initially prepared neutral state are also discussed, including results on the photoionization cross sections and photoelectron angular distributions of polarized excited states. The effects of various approximations in the theoretical treatment of photoionization in these systems are analysed. The very large diversity of observed phenomena and the numerous anomalies in spectral structures associated with the threshold ionization of the rare-gas atoms can be described in terms of a limited set of interactions and dynamical processes. Examples are provided illustrating characteristic aspects of the photoionization, and sets of recommended parameters describing the energy-level structure and photoionization dynamics of the rare-gas atoms are presented which were extracted in a critical analysis of the very large body of experimental and theoretical data available on these systems in the literature.

• Open/close Review of attosecond resolved measurement and control via carrier–envelope phase tagging with above-threshold ionization

  T Rathje et al 2012 J. Phys. B: At. Mol. Opt. Phys. 45 074003 Tag this article

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  A precise, real-time, single-shot carrier–envelope phase (CEP) tagging technique for few-cycle pulses was developed and combined with cold-target recoil-ion momentum spectroscopy and velocity-map imaging to investigate and control CEP-dependent processes with attosecond resolution. The stability and precision of these new techniques have allowed for the study of intense, few-cycle, laser-matter dynamics with unprecedented detail. Moreover, the same stereo above-threshold ionization (ATI) measurement was expanded to multi-cycle pulses and allows for CEP locking and pulse-length determination. Here we review these techniques and their first applications to waveform characterization and control, non-sequential double ionization of argon, ATI of xenon and electron emission from SiO ₂ nanospheres.

• Open/close High harmonic generation with long-wavelength few-cycle laser pulses

  Bruno E Schmidt et al 2012 J. Phys. B: At. Mol. Opt. Phys. 45 074008 Tag this article

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  We report the extension of hollow-core fibre pulse compression to longer wavelengths. High-energy multi-cycle infrared pulses are generated via optical parametric amplification and subsequently broadened in the fibre. 2.5-cycle pulses at the Signal wavelength (1.4 µm) and 1.6-cycle pulses at the Idler wavelength (1.8 µm) in the sub-millijoule regime have been generated. New compression schemes can be applied at 1.8 µm and beyond. In this manner, 1.6-cycle carrier envelope phase stable pulses were generated by linear propagation in the anomalous dispersion regime of bulk glass which surprisingly enables compression below its third-order dispersion limit. Furthermore, a dispersion-free way of controlling the carrier envelope phase is demonstrated. Moreover, we experimentally confirm the increase in high-harmonic cut-off energy with driving laser wavelength and demonstrate the beneficial effect of few-cycle pulses which enable higher saturation intensities on target compared to multi-cycle pulses. It will be an ideal tool for future synthesis of isolated attosecond pulses in the sub-keV regime. With this laser source, we revealed for the first time multi-electron effects in high harmonic generation in xenon.

• Open/close Attosecond physics in photoemission from a metal nanotip

  M Krüger et al 2012 J. Phys. B: At. Mol. Opt. Phys. 45 074006 Tag this article

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  Nanometre-scale metal tips irradiated by femtosecond laser pulses represent ultrafast electron sources. The combination of the laser pulse and the tip offers the possibility of extending attosecond science from atomic or molecular gases to surfaces of solid nanoemitters. We first review this emerging research field focusing on electron rescattering at sharp metal tips. In particular, we investigate the carrier–envelope phase effects that reveal attosecond emission dynamics. Furthermore, we present detailed theory models that support this interpretation.

• Open/close Circularly polarized attosecond pulses from molecular high-order harmonic generation by ultrashort intense bichromatic circularly and linearly polarized laser pulses

  Kai-Jun Yuan and André D Bandrauk 2012 J. Phys. B: At. Mol. Opt. Phys. 45 074001 Tag this article

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  We describe the generation of high-order elliptically and circularly polarized harmonic spectra in an aligned H ⁺ ₂ molecule ion by a combination of two-colour ultrashort intense laser fields from numerical solutions of the corresponding time-dependent Schrödinger equation (TDSE). In intense bichromatic circularly and linearly or circularly polarized laser pulses with intensity I ₀ and angular frequencies ω ₀ and 2ω ₀, it is found that maximum molecular high-order harmonic generation (MHOHG) energies are functions of the molecular internuclear distance. Based on a classical model of laser-induced electron collisions with neighbouring ions, the optimal values of the pulse relative carrier envelope phase , the molecular internuclear distance R and the angle of molecular alignment to the laser polarization axis are obtained for efficiently producing MHOHG spectra with the maximum harmonic energy I _p + 13.5 U _p, where I _p is the ionization potential of the molecule and U _p = I ₀/4 m _eω ² ₀ is the ponderomotive energy of the continuum electron at intensity I ₀ and frequency ω ₀ of the laser pulse. The results have been confirmed from corresponding TDSE nonperturbative numerical simulations. The polarization property of the generated harmonics is also presented. The mechanism of MHOHG is further characterized with a Gabor time frequency analysis. It is confirmed that a single collision trajectory of the continuum electron with neighbouring ions dominates in the MHOHG processes. The high efficiency of the proposed MHOHG scheme provides a possible source for production of elliptically and/or circularly polarized attosecond extreme ultraviolet pulses. Circularly polarized attosecond pulses can also be generated by using intense ultrashort circularly polarized laser pulses in combination with static electric fields of comparable intensity for H ⁺ ₂ at equilibrium. A time frequency analysis also confirms the role of single recollisions as the dominant mechanism of the generation of circularly polarized harmonics.

• Open/close Photoelectron kinetic and angular distributions for the ionization of aligned molecules using a HHG source

  Arnaud Rouzée et al 2012 J. Phys. B: At. Mol. Opt. Phys. 45 074016 Tag this article

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  We present an experimental and theoretical investigation of the angular distributions of electrons ejected in aligned molecules by extreme ultra-violet ionization using a high harmonic generation (HHG) source. Impulsive alignment in O ₂, N ₂ and CO molecules was achieved using a near-IR laser pulse and the photoelectron angular distribution after ionization by a fs harmonic comb composed of harmonic H11 to H29 (17.5–46 eV) was recorded at the maximum of both alignment and anti-alignment. The experiment reveals signatures that are specific for the electronic orbitals that are ionized as well as the onset of the influence of the molecular structure and is well reproduced by theoretical calculations based on the multichannel Schwinger configuration interaction method.

• Open/close Excitation energies, radiative and autoionization rates, dielectronic satellite lines, and dielectronic recombination rates for excited states of Yb-like W

  U I Safronova et al 2012 J. Phys. B: At. Mol. Opt. Phys. 45 085001 Tag this article

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  Energy levels, radiative transition probabilities and autoionization rates for [Cd]4f ¹⁴5p ⁶5 l′ nl, [Cd]4f ¹⁴5p ⁶6 l″ nl, [Cd]4f ¹⁴5p ⁵5d ² nl, [Cd]4f ¹⁴5p ⁵5d6 l″ nl, [Cd]4f ¹³5p ⁶5d ² nl and [Cd]4f ¹³5p ⁶5d6 l″ nl ( l′ = d, f, g, l″ = s, p, d, l = s, p, d, f, g and n = 5–7) states of Yb-like tungsten (W ^{4 +}) are calculated using the relativistic many-body perturbation theory method (RMBPT code), the multiconfiguration relativistic Hebrew University–Lawrence Livermore Atomic Code (HULLAC code) and the Hartree–Fock relativistic method (COWAN code). Branching ratios relative to the [Cd]4f ¹⁴5p ⁶5d, [Cd]4f ¹⁴5p ⁶6s and [Cd]4f ¹⁴5p ⁶6p thresholds in Tm-like tungsten and intensity factors are calculated for satellite lines, and dielectronic recombination (DR) rate coefficients are determined for the singly excited, as well as non-autoionizing core-excited states in Yb-like tungsten. Contributions from the autoionizing doubly excited states [Cd]4f ¹⁴5p ⁶5f nl, [Cd]4f ¹⁴5p ⁶6 l″ nl and core-excited [Cd]4f ¹⁴5p ⁵5d ² nl, [Cd]4f ¹⁴5p ⁵5d6 l″ nl, [Cd]4f ¹³5p ⁶5d ² nl, [Cd]4f ¹³5p ⁶5d6 l″ nl states (with n up to 100), which are particulary important for calculating the total DR rates, are estimated. Synthetic dielectronic satellite spectra from Yb-like W are simulated in a broad spectral range from 200 to 1400 Å. These calculations provide recommended values critically evaluated for their accuracy for a number of W ^{4 +} properties useful for a variety of applications including for fusion applications.

• Open/close Path-selective investigation of intense laser-pulse-induced fragmentation dynamics in triply charged 1,3-butadiene

  Li Zhang et al 2012 J. Phys. B: At. Mol. Opt. Phys. 45 085603 Tag this article

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  We experimentally studied proton ejection in the three-body fragmentation of triply charged 1,3-butadiene molecules prepared by intense ultrashort laser pulses using coincidence momentum imaging. The break-up dynamics along the four possible paths that a final set of three fragments can be reached is investigated for the three different fragmentation channels that are analysed. It is found that for each channel the two dominant paths are (i) proton ejection from the triply charged ion and (ii) a sequential path, where the proton is ejected from the doubly charged ion during the second fragmentation step. Based on the measured three-body momentum correlations and accompanying numerical simulations, we discuss whether the fragmentation dynamics, where the proton is ejected from the triply charged ion, proceeds concertedly or sequentially. We also investigate the dependence of the fragmentation dynamics on the intensity and polarization state of the laser pulse.

• Open/close Optical waveform synthesizer and its application to high-harmonic generation

  Shu-Wei Huang et al 2012 J. Phys. B: At. Mol. Opt. Phys. 45 074009 Tag this article

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  Over the last decade, the control of atomic-scale electronic motion by optical fields strong enough to mitigate the atomic Coulomb potential has broken tremendous new ground with the advent of phase-controlled high-energy few-cycle pulse sources. Further investigation and control of these physical processes, including high-harmonic generation, ask for the capability of waveform shaping on sub-cycle time scales, which requires a fully phase-controlled multiple-octave-spanning spectrum. In this paper, we present a light source that enables sub-cycle waveform shaping with a two-octave-spanning spectrum and 15 µJ pulse energy based on coherent synthesis of pulses with different spectra, or wavelength multiplexing. The synthesized pulse has its shortest high-field transient lasting only 0.8 cycles (amplitude FWHM) of the centroid frequency. The benefit of the approach lies in its modular design and scalability in both bandwidth and pulse energy. Full phase control allows for the synthesis of any optical waveform supported by the amplified spectrum. A numerical study shows the uniqueness of the light source for direct isolated soft-x-ray pulse generation based on high-harmonic generation, greatly reducing and eventually even eliminating the need for gating techniques or spectral filtering. The demonstrated system is the prototype of a class of novel optical tools for attosecond control of strong-field physics experiments.