Transcript Beyond Carbon K-Edge Harmonic Emission

Beyond Carbon K-Edge
Harmonic Emission Using a
Spatial and Temporal
*
Synthesized Laser Field
Muhammed Sayrac
Phys-689 Modern Atomic Physics
Spring-2016
*PRL
110, 053001 (2013)
Motivation
 Numerical simulations of HHG in helium using a temporally synthesized and spatially nonhomogeneous strong
laser field.
 The goal of this study is to extend the cutoff harmonic far beyond the usual semi classical limit by using
temporal and spatial laser field.
 This laser field has been proven capable of generating coherent extreme ultraviolet photons beyond the carbon
K edge (284eV, 4.37nm), an energy region of high interest as it can be used to initiate inner-shell dynamics by
using 800nm pulses with synthesis fields.
 The new approach we propose involves combining the two techniques to controllably shape the final laser field
both in time and in space.
*PRL
110, 053001 (2013)
https://books.google.com/books?id=clAANTKBStcC&pg=PA2&lpg=PA2&dq=carbon+absorption+edge+in+wavelength&sour
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%20absorption%20edge%20in%20wavelength&f=false
Introduction
X-ray absorption spectroscopy is a very powerful technique for the probing of the local chemical environment
of molecules and to explore ultrafast inner shell charge dynamics in molecular systems.
3-step model
Ecutofff = Ip+3.17Up
Up~λ2
 One way to extend HHG cutoff is use longer wavelength as it is well known that the HHG cutoff scales as λ2.
 The generation efficiency of the harmonic photons decreases with increasing laser wavelength according to a
λ-5.5 power law.
*PRL
110, 053001 (2013), Nature Photonics 5, 640–641 (2011)
Method
Two 4-cycle pulses at 800nm are delayed in time for performing the temporal synthesis.
For the simulation total number of cycle
(N)=4 and ϕ=0 are considered.
The potential between the atom and the laser pulse is modified
in order to treat the spatially nonhomogeneous fields.
where Vl is the laser atom interaction, E is the laser field, the β is the strength of the nonhomogeneity.
This parameters are adjusted in such a way that the laser ionized electron feels only a linear variation of the laser
field when in the continuum.
*PRL
110, 053001 (2013)
Results
 The TDSE is solved in order to calculate the harmonic spectra while employing double pulse nonhomogeneous driving
laser field.
The harmonic spectrum obtained in helium for β=0.002.
Then the cutoff is extended up to
12.5Up that is greater than 1 keV.
 The decrease beyond 650eV can be explained that two trajectories contribute to the harmonic yield, inducing structures
in the corresponding harmonic spectrum.
 Toward the cutoff energy the excursion time of these trajectories increases, resulting in a harmonic yield drop due to
the spreading of the electronic wave packet. *PRL 110, 053001 (2013)
Results (cont.)
 The direct effect is that the amount of recombination event decreases as β increases.
 For β=0.002 the short and long trajectories recombine almost simultaneously, meaning the laser field forces to
electron ionized at different times to recombine around the same time.
*PRL 110, 053001 (2013)
Results
(cont.)
 t : 1.25-2.25 long trajectories correspond t >2.5 optical cycle, and short trajectories are for the t <2.5 optical cycle.
i
r
r
 The long trajectories are modified both by the spatial nonhomogeneity and the temporal double-pulse configuration.
 In the homogeneous case (β=0) with ionization times ti around 1.25 and 1.75 optical cycles merge into unique trajectories.
 The trajectory with ti ~1.75 now has its ionization times greater than half an optical cycle that get smaller while β increases.
As a result, the time spent by the electron excursion in the continuum increases.
 The electric field strength at the ionization time for short trajectories being greater than for long trajectories, and considering
that the ionization rate is a nonlinear function of this electric field, long trajectories are then less efficient than the short ones.
 Also short trajectories are almost independent of β and get noticeably different only for really high values of β.
*PRL
110, 053001 (2013)
Results (cont.)
The time-frequency analysis of the calculated dipole (from the 3D-TDSE) corresponding to the case of a nonhomogeneous
laser field using a wavelet analysis.
β=0.002
 The brown lines are the calculated classical re-scattering energies.
 The classical calculations confirms that the mechanism of the generation of this 12.5Up cutoff extension exhibiting a nice
continuum
 This is the consequence of trajectory selection and consequences of employing the combination of temporally and
spatially synthesized laser field.
*PRL
110, 053001 (2013)
Conclusion
 Two identical few cycle pulses delayed in time together with a weak spatial nonhomogeneity are used for
extending HHG cutoff.
 The main effect of this two identical pulses on the HHG is a considerable extension of the cutoff energy up to
12.5Up.
 Trajectories are highly selected while using a laser field that consist of a combination of the double pulse
temporal synthesis and the spatial nonhomogeneity.
 This approach provides the generation of a coherent attosecond light source at energies beyond the carbon K
edge directly from an 800 nm laser system.
*PRL
110, 053001 (2013)
Thank you