160 research outputs found
Near-threshold high-order harmonic spectroscopy with aligned molecules
We study high-order harmonic generation in aligned molecules close to the
ionization threshold. Two distinct contributions to the harmonic signal are
observed, which show very different responses to molecular alignment and
ellipticity of the driving field. We perform a classical electron trajectory
analysis, taking into account the significant influence of the Coulomb
potential on the strong-field-driven electron dynamics. The two contributions
are related to primary ionization and excitation processes, offering a deeper
understanding of the origin of high harmonics near the ionization threshold.
This work shows that high harmonic spectroscopy can be extended to the
near-threshold spectral range, which is in general spectroscopically rich.Comment: 4 pages, 4 figure
Attosecond time-resolved photoelectron holography
Ultrafast strong-field physics provides insight into quantum phenomena that evolve on an attosecond time scale, the most fundamental of which is quantum tunneling. The tunneling process initiates a range of strong field phenomena such as high harmonic generation (HHG), laser-induced electron diffraction, double ionization and photoelectron holography—all evolving during a fraction of the optical cycle. Here we apply attosecond photoelectron holography as a method to resolve the temporal properties of the tunneling process. Adding a weak second harmonic (SH) field to a strong fundamental laser field enables us to reconstruct the ionization times of photoelectrons that play a role in the formation of a photoelectron hologram with attosecond precision. We decouple the contributions of the two arms of the hologram and resolve the subtle differences in their ionization times, separated by only a few tens of attoseconds
Theory of selective excitation in Stimulated Raman Scattering
A semiclassical model is used to investigate the possibility of selectively
exciting one of two closely spaced, uncoupled Raman transitions. The duration
of the intense pump pulse that creates the Raman coherence is shorter than the
vibrational period of a molecule (impulsive regime of interaction). Pulse
shapes are found that provide either enhancement or suppression of particular
vibrational excitations.Comment: RevTeX4,10 pages, 5 figures, submitted to Phys.Rev.
When does an electron exit a tunneling barrier?
We probe the dynamics of tunnel ionization via high harmonic generation. We characterize the ionization dynamics in helium atoms, and apply our approach to resolve subtle differences in ionization from different orbitals of a CO 2 molecule
Observation of coherent transients in ultrashort chirped excitation of an undamped two-level system
The effects of Coherent excitation of a two level system with a linearly
chirped pulse are studied theoretically and experimentally (in Rb (5s - 5p)) in
the low field regime. The Coherent Transients are measured directly on the
excited state population on an ultrashort time scale. A sharp step corresponds
to the passage through resonance. It is followed by oscillations resulting from
interferences between off-resonant and resonant contributions. We finally show
the equivalence between this experiment and Fresnel diffraction by a sharp
edge.Comment: 4 pages, 4 figures, to appear in PR
Transform-limited pulses are not optimal for resonant multiphoton transitions
Maximizing nonlinear light-matter interactions is a primary motive for
compressing laser pulses to achieve ultrashort transform limited pulses. Here
we show how, by appropriately shaping the pulses, resonant multiphoton
transitions can be enhanced significantly beyond the level achieved by
maximizing the pulse's peak intensity. We demonstrate the counterintuitive
nature of this effect with an experiment in a resonant two-photon absorption,
in which, by selectively removing certain spectral bands, the peak intensity of
the pulse is reduced by a factor of 40, yet the absorption rate is doubled.
Furthermore, by suitably designing the spectral phase of the pulse, we increase
the absorption rate by a factor of 7.Comment: 4 pages, 3 figure
Noise auto-correlation spectroscopy with coherent Raman scattering
Ultrafast lasers have become one of the most powerful tools in coherent
nonlinear optical spectroscopy. Short pulses enable direct observation of fast
molecular dynamics, whereas broad spectral bandwidth offers ways of controlling
nonlinear optical processes by means of quantum interferences. Special care is
usually taken to preserve the coherence of laser pulses as it determines the
accuracy of a spectroscopic measurement. Here we present a new approach to
coherent Raman spectroscopy based on deliberately introduced noise, which
increases the spectral resolution, robustness and efficiency. We probe laser
induced molecular vibrations using a broadband laser pulse with intentionally
randomized amplitude and phase. The vibrational resonances result in and are
identified through the appearance of intensity correlations in the noisy
spectrum of coherently scattered photons. Spectral resolution is neither
limited by the pulse bandwidth, nor sensitive to the quality of the temporal
and spectral profile of the pulses. This is particularly attractive for the
applications in microscopy, biological imaging and remote sensing, where
dispersion and scattering properties of the medium often undermine the
applicability of ultrafast lasers. The proposed method combines the efficiency
and resolution of a coherent process with the robustness of incoherent light.
As we demonstrate here, it can be implemented by simply destroying the
coherence of a laser pulse, and without any elaborate temporal scanning or
spectral shaping commonly required by the frequency-resolved spectroscopic
methods with ultrashort pulses.Comment: To appear in Nature Physic
Integrated liquid-core optical fibers --- ultra-efficient nonlinear liquid photonics
We have developed a novel integrated platform for liquid photonics based on
liquid core optical fiber (LCOF). The platform is created by fusion splicing
liquid core optical fiber to standard single-mode optical fiber making it fully
integrated and practical - a major challenge that has greatly hindered progress
in liquid-photonic applications. As an example, we report here the realization
of ultralow threshold Raman generation using an integrated CS2 filled LCOF
pumped with sub-nanosecond pulses at 1064nm and 532nm. The measured energy
threshold for the Stokes generation is ~ 1nJ, about three orders of magnitude
lower than previously reported values in the literature for hydrogen gas. The
integrated LCOF platform opens up new possibilities for ultralow power
nonlinear optics such as efficient white light generation for displays, mid-IR
generation, slow light generation, parametric amplification, all-optical
switching and wavelength conversion using liquids that have orders of magnitude
larger optical nonlinearities compared with silica glass.Comment: 4 pages, 3 figure
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Multi-channel electronic and vibrational dynamics in polyatomic resonant high-order harmonic generation
High-order harmonic generation in polyatomic molecules generally involves multiple channels of ionization. Their relative contribution can be strongly influenced by the presence of resonances, whose assignment remains a major challenge for high-harmonic spectroscopy. Here we present a multi-modal approach for the investigation of unaligned polyatomic molecules, using SF6 as an example. We combine methods from extreme-ultraviolet spectroscopy, above-threshold ionization and attosecond metrology. Fragment-resolved above-threshold ionization measurements reveal that strong-field ionization opens at least three channels. A shape resonance in one of them is found to dominate the signal in the 20-26 eV range. This resonance induces a phase jump in the harmonic emission, a switch in the polarization state and different dynamical responses to molecular vibrations. This study demonstrates a method for extending high-harmonic spectroscopy to polyatomic molecules, where complex attosecond dynamics are expected
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