70 research outputs found
Two-photon coherent control of femtosecond photoassociation
Photoassociation with short laser pulses has been proposed as a technique to
create ultracold ground state molecules. A broad-band excitation seems the
natural choice to drive the series of excitation and deexcitation steps
required to form a molecule in its vibronic ground state from two scattering
atoms. First attempts at femtosecond photoassociation were, however, hampered
by the requirement to eliminate the atomic excitation leading to trap
depletion. On the other hand, molecular levels very close to the atomic
transition are to be excited. The broad bandwidth of a femtosecond laser then
appears to be rather an obstacle. To overcome the ostensible conflict of
driving a narrow transition by a broad-band laser, we suggest a two-photon
photoassociation scheme. In the weak-field regime, a spectral phase pattern can
be employed to eliminate the atomic line. When the excitation is carried out by
more than one photon, different pathways in the field can be interfered
constructively or destructively. In the strong-field regime, a temporal phase
can be applied to control dynamic Stark shifts. The atomic transition is
suppressed by choosing a phase which keeps the levels out of resonance. We
derive analytical solutions for atomic two-photon dark states in both the
weak-field and strong-field regime. Two-photon excitation may thus pave the way
toward coherent control of photoassociation. Ultimately, the success of such a
scheme will depend on the details of the excited electronic states and
transition dipole moments. We explore the possibility of two-photon femtosecond
photoassociation for alkali and alkaline-earth metal dimers and present a
detailed study for the example of calcium
Generation of isolated attosecond pulses in the far field by spatial filtering with an intense few-cycle mid-infrared laser
We report theoretical calculations of high-order harmonic generation (HHG) of
Xe with the inclusion of multi-electron effects and macroscopic propagation of
the fundamental and harmonic fields in an ionizing medium. By using the
time-frequency analysis we show that the reshaping of the fundamental laser
field is responsible for the continuum structure in the HHG spectra. We further
suggest a method for obtaining an isolated attosecond pulse (IAP) by using a
filter centered on axis to select the harmonics in the far field with different
divergence. We also discuss the carrier-envelope-phase dependence of an IAP and
the possibility to optimize the yield of the IAP. With the intense few-cycle
mid-infrared lasers, this offers a possible method for generating isolated
attosecond pulses.Comment: 8 figure
N2 HOMO-1 orbital cross section revealed through high-order-harmonic generation
Citation: Troß, J., Ren, X., Makhija, V., Mondal, S., Kumarappan, V., & Trallero-Herrero, C. A. (2017). N2 HOMO-1 orbital cross section revealed through high-order-harmonic generation. Physical Review A - Atomic, Molecular, and Optical Physics, 95(3). doi:10.1103/PhysRevA.95.033419We measure multi-orbital contributions to high harmonic generation from aligned nitrogen. We show that the change in revival structure in the cutoff harmonics has a counterpart in the angular distribution when a lower-lying orbital contributes to the harmonic yield. This angular distribution is directly observed in the laboratory without any further deconvolution. Because of the high degree of alignment we are able to distinguish angular contributions of the highest occupied molecular orbital 1 (HOMO-1) orbital from angle-dependent spectroscopic features of the HOMO. In particular, we are able to make a direct comparison with the cross section of the HOMO-1 orbital in the extreme ultraviolet region. © 2017 American Physical Society
Seguimiento y evaluación de complicaciones en 1000 implantes consecutivos de dispositivos de estimulación cardiaca en una unidad de arritmias
Las innovaciones tecnológicas logradas en los últimos años nos han proporcionado el diseño de sistemas de estimulación cardíaca, que cada vez se asemejan más al fisiológico normal, adaptándose a la actividad física o al aumento de la demanda metabólica. Todo ello se inicia en 1932 cundo Hyman construye el primer aparato experimental al cual llamó marcapasos cardíaco artificial y que posteriormente se perfeccionó para su utilización en el ser humano, siendo Senning, en Estocolmo (1958) y Chardack en Estados Unidos (1959) los que por primera vez implantaron un marcapasos permanente en un paciente con Síndrome de Stokes Adams1. Desde entonces se crea la necesidad de establecer guías para el implante de marcapasos cardíacos, siendo la “North American Society for Pacing and Electrophysiology”, el “British Pacing and Electrophysilogy Group y la “European Heart Rhythm Association, Task Force”, quien nos proporciona una revisión más detallada de elementos de juicio para su implante2. El presente trabajo de tesis pretende evaluar las complicaciones quirúrgicas, médicas y eléctricas que presentan 1000 pacientes a los que se les implanta un dispositivo de estimulación cardíaca en la Unidad de Arritmias del Hospital Clínico Universitario de Zaragoza, de manera consecutiva a lo largo de 4 años
Generation and control of non-local quantum equivalent extreme ultraviolet photons
We present a high precision, self-referencing, common path XUV interferometer
setup to produce pairs of spatially separated and independently controllable
XUV pulses that are locked in phase and time. The spatial separation is created
by introducing two equal but opposite wavefront tilts or using superpositions
of orbital angular momentum. In our approach, we can independently control the
relative phase/delay of the two optical beams with a resolution of 52 zs (zs =
zeptoseconds). In order to explore the level of entanglement between the
non-local photons, we compare three different beam modes: Bessel-like, and
Gaussian with or without added orbital angular momentum. By reconstructing
interference patterns one or two photons at a time we conclude that the beams
are not entangled, yet each photon in the attosecond pulse train contains
information about the entire spectrum. Our technique generates non-local,
quantum equivalent XUV photons with a temporal jitter of 3 zs, just below the
Compton unit of time of 8 zs. We argue that this new level of temporal
precision will open the door for new dynamical QED tests. We also discuss the
potential impact on other areas, such as imaging, measurements of non-locality,
and molecular quantum tomography.Comment: 11 pages 5 figures and supplemental materials with 12 pages and 7
figure
Measuring the Angle-Dependent Photoionization Cross Section of Nitrogen using High-Harmonic Generation
We exploit the relationship between high harmonic generation (HHG) and the molecular photorecombination dipole to extract the molecular-frame differential photoionization cross section (PICS) in the extreme ultraviolet (XUV) for molecular nitrogen. A shape resonance and a Cooper-type minimum are reflected in the pump-probe time delay measurements of different harmonic orders, where high-order rotational revivals are observed in N₂. We observe the energy- and angle-dependent Cooper minimum and shape resonance directly in the laboratory-frame HHG yield by achieving a high degree of alignment, [SEE FORMULA IN ABSTRACT cos2 θ] 0.8. The interplay between PICS and rotational revivals is confirmed by simulations using the quantitative rescattering theory. Our method of extracting molecular-frame structural information points the way to similar measurements in more complex molecules
Generation of broad XUV continuous high harmonic spectra and isolated attosecond pulses with intense mid-infrared lasers
We present experimental results showing the appearance of a near-continuum in
the high-order harmonic generation (HHG) spectra of atomic and molecular
species as the driving laser intensity of an infrared pulse increases. Detailed
macroscopic simulations reveal that these near-continuum spectra are capable of
producing IAPs in the far field if a proper spatial filter is applied. Further,
our simulations show that the near-continuum spectra and the IAPs are a product
of strong temporal and spatial reshaping (blue shift and defocusing) of the
driving field. This offers a possibility of producing IAPs with a broad range
of photon energy, including plateau harmonics, by mid-IR laser pulses even
without carrier-envelope phase stabilization.Comment: 7 pages, 5 figures, submitted to J.Phys. B (Oct 2011
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