264 research outputs found
Characterization And Application Of Isolated Attosecond Pulses
Tracking and controlling the dynamic evolution of matter under the influence of external fields is among the most fundamental goals of physics. In the microcosm, the motion of electrons follows the laws of quantum mechanics and evolves on the timescale set by the atomic unit of time, 24 attoseconds. While only a few time-dependent quantum mechanical systems can be solved theoretically, recent advances in the generation, characterization, and application of isolated attosecond pulses and few-cycle femtosecond lasers have given experimentalists the necessary tools for dynamic measurements on these systems. However, pioneering studies in attosecond science have so far been limited to the measurement of free electron dynamics, which can in most cases be described approximately using classical mechanics. Novel tools and techniques for studying bound states of matter are therefore desired to test the available theoretical models and to enrich our understanding of the quantum world on as-yet unprecedented timescales. In this work, attosecond transient absorption spectroscopy with ultrabroadband attosecond pulses is presented as a technique for direct measurement of electron dynamics in quantum systems, demonstrating for the first time that the attosecond transient absorption technique allows for state-resolved and simultaneous measurement of bound and continuum state dynamics. The helium atom is the primary target of the presented studies, owing to its accessibility to theoretical modeling with both ab initio simulations and to model systems with reduced dimensionality. In these studies, ultrafast dynamics â on timescales shorter than the laser cycle â are observed in prototypical quantum mechanical processes such as the AC Stark and ponderomotive energy level shifts, Rabi oscillations and electromagnetically-induced absorption iv and transparency, and two-color multi-photon absorption to âdarkâ states of the atom. These features are observed in both bound states and quasi-bound autoionizing states of the atom. Furthermore, dynamic interference oscillations, corresponding to quantum path interferences involving bound and free electronic states of the atom, are observed for the first time in an optical measurement. These first experiments demonstrate the applicability of attosecond transient absorption spectroscopy with ultrabroadband attosecond pulses to the study and control of electron dynamics in quantum mechanical systems with high fidelity and state selectivity. The technique is therefore ideally suited for the study of charge transfer and collective electron motion in more complex systems. The transient absorption studies on atomic bound states require ultrabroadband attosecond pulses â attosecond pulses with large spectral bandwidth compared to their central frequency. This is due to the fact that the bound states in which we are interested lie only 15-25 eV above the ground state, so the central frequency of the pulse should lie in this range. On the other hand, the bandwidth needed to generate an isolated 100 as pulse exceeds 18 eV â comparable to or even larger than the central frequency. However, current methods for characterizing attosecond pulses require that the attosecond pulse spectrum bandwidth is small compared to its central frequency, known as the central momentum approximation. We therefore explore the limits of attosecond pulse characterization using the current technology and propose a novel method for characterizing ultrabroadband attosecond pules, which we term PROOF (phase retrieval by omega oscillation filtering). We demonstrate the PROOF technique with both simulated and experimental data, culminating in the characterization of a world-record-breaking 67 as pulse
The stability of the optical flux variation gradient for 3C120
New - and -band monitoring in 2014 2015 reveals that the Seyfert 1
Galaxy, 3C120, has brightened by a magnitude of , compared to our campaign
that took place in 2009 2010. This allowed us to check for the debated
luminosity and time-dependent color variations claimed for SDSS quasars. For
our 3C120 data, we find that the flux ratio of the variable component in
the bright epoch is indistinguishable from the faint one. We do not find any
color variability on different timescales ranging from about to
days. We suggest that the luminosity and time-dependent color variability is an
artifact caused by analyzing the data in magnitudes instead of fluxes. The flux
variation gradients of both epochs yield consistent estimates of the host
galaxy contribution to our 7.5" aperture. These results confirm that the
optical flux variation gradient method works well for Seyfert galaxies.Comment: 8 pages, 6 figures. Accepted for publication in section 4.
Extragalactic astronomy of Astronomy and Astrophysics v2: Language-Editor
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High Mass Star Formation. II. The Mass Function of Submillimeter Clumps in M17
We have mapped an approximately 5.5 by 5.5 pc portion of the M17 massive
star-forming region in both 850 and 450 micron dust continuum emission using
the Submillimeter Common-User Bolometer Array (SCUBA) on the James Clerk
Maxwell Telescope (JCMT). The maps reveal more than 100 dusty clumps with
deconvolved linear sizes of 0.05--0.2 pc and masses of 0.8--120 solar masses,
most of which are not associated with known mid-infrared point sources. Fitting
the clump mass function with a double power law gives a mean power law exponent
of alpha_high = -2.4 +/- 0.3 for the high-mass power law, consistent with the
exponent of the Salpeter stellar mass function. We show that a lognormal clump
mass distribution with a peak at about 4 solar masses produces as good a fit to
the clump mass function as does a double power law. This 4 solar mass peak mass
is well above the peak masses of both the stellar initial mass function and the
mass function of clumps in low-mass star-forming regions. Despite the
difference in intrinsic mass scale, the shape of the M17 clump mass function
appears to be consistent with the shape of the core mass function in low-mass
star-forming regions. Thus, we suggest that the clump mass function in
high-mass star-forming regions may be a scaled-up version of that in low-mass
regions, instead of its extension to higher masses.Comment: 33 pages, 6 figures, 3 tables. Accepted for publication in the
Astrophysical Journa
Attosecond transient absorption spectroscopy of molecular hydrogen
We extend attosecond transient absorption spectroscopy (ATAS) to the study of hydrogen molecules, demonstrating the potential of the technique to resolve-simultaneously and with state resolution-both the electronic and nuclear dynamic
Reconstruction of an excited-state molecular wave packet with attosecond transient absorption spectroscopy
Attosecond science promises to allow new forms of quantum control in which a broadband isolated attosecond pulse excites a molecular wave packet consisting of a coherent superposition of multiple excited electronic states. This electronic excitation triggers nuclear motion on the molecular manifold of potential energy surfaces and can result in permanent rearrangement of the constituent atoms. Here, we demonstrate attosecond transient absorption spectroscopy (ATAS) as a viable probe of the electronic and nuclear dynamics initiated in excited states of a neutral molecule by a broadband vacuum ultraviolet pulse. Owing to the high spectral and temporal resolution of ATAS, we are able to reconstruct the time evolution of a vibrational wave packet within the excited BâČÎŁu1+ electronic state of H2 via the laser-perturbed transient absorption spectrumThis material is based on work supported by the DARPA PULSE program through a grant from AMRDEC under Award No. W31P4Q1310017; the Army Research Office under Awards No. W911NF-11-1-0297, No. WN911NF-14- 1-0383, and No. FA9550-16-1-0013; the Air Force Office of Scientific Research under Awards No. FA9550-15-1-0037 and No. FA9550-16-1-0149; the National Science Foundation under Award No. PHY-1506345, an Advanced Grant of the European Research Council XCHEM 290853; and European Grants MC-ITN CORINF, the European COST Action XLIC CM1204, the MINECO Project No. FIS2013-42002-R; and the ERA-Chemistry Project PIM2010EEC-0075
Ellipticity dependence of 400 nm-driven high harmonic generation
We studied the dependence of high harmonic generation efficiency on the ellipticity of 400 nm driving laser pulses at 7.7 x 10(14) W/cm(2) and compared it with the 800 nm driving laser under the same conditions. The measured decrease of high harmonic yield with the ellipticity of the 400 nm laser is similar to 1.5 times slower that of the 800 nm, which agrees well with theoretical predictions based on a semi-classical model. The results indicate that it is feasible to use the generalized double optical gating with 400 nm lasers for extracting single attosecond pulses with high efficiency
Mechanism of quasi-phase-matching in a dual-gas multijet array
This is the published version, also available here: http://dx.doi.org/10.1103/PhysRevA.86.021802.Quasi-phase-matching in a dual gas (Ar-H2) multijet array has recently been demonstrated to be a promising way to enhance the yield of high-order harmonics (HH). Here, we investigate the HH produced individually from these two gases under identical conditions. Our results indicate that the quasi-phase-matching results from the much lower recombination cross section of H2 as compared to that of Ar in the energy range of interest, rather than from full ionization of H2 by the driving laser as proposed previously
Characterizing ultrabroadband attosecond lasers
Recent progress in sub-laser-cycle gating of high-order harmonic generation promises to push the limits on optical pulse durations below the atomic unit of time, 24 as, which corresponds to a bandwidth broader than 75 eV. However, the available techniques for attosecond pulse measurement are valid only for narrow-bandwidth spectra, due to one of the key approximations made in the phase retrieval. Here we report a new technique for characterizing attosecond pulses, whereby the spectral phase of the attosecond pulse is extracted from the oscillation component with the dressing laser frequency in the photoelectron spectrogram. This technique, termed PROOF (Phase Retrieval by Omega Oscillation Filtering), can be applied to characterizing attosecond pulses with ultrabroad bandwidths
Sub-cycle Oscillations in Virtual States Brought to Light
Understanding and controlling the dynamic evolution of electrons in matter is among the most fundamental goals of attosecond science. While the most exotic behaviors can be found in complex systems, fast electron dynamics can be studied at the fundamental level in atomic systems, using moderately intense (less than or similar to 10(13) W/cm(2)) lasers to control the electronic structure in proof-of-principle experiments. Here, we probe the transient changes in the absorption of an isolated attosecond extreme ultraviolet (XUV) pulse by helium atoms in the presence of a delayed, few-cycle near infrared (NIR) laser pulse, which uncovers absorption structures corresponding to laser-induced virtual intermediate states in the two-color two-photon (XUV+NIR) and three-photon (XUV+NIR+NIR) absorption process. These previously unobserved absorption structures are modulated on half-cycle (similar to 1.3 fs) and quarter-cycle (similar to 0.6 fs) timescales, resulting from quantum optical interference in the laser-driven atom
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