1. Generation of short pulses

2. Ultrashort pulse generation
15 fs pulse
Single cycle pulse
Time [fs]
Time [fs]
Wavelength [m]
Wavelength [m]

3. Raman scattering and attosecond pulses
Input two frequencies nearly resonant with a Raman resonance.
At high intensity, the process cascades many times.
Output pulse of second process as input to a third process
Output pulse of third process as input to a fourth process
Output pulse as input to a second process
Etc.
Input pulses 1 0
Raman processes can cascade many times, yielding a series of equally spaced modes
frequency
=1+/- n0
S. E. Harris and A. V. Sokolov PRL 81, 2894

4. Cascaded Raman generation
Dwba = cm-1
A. V. Sokolov et al. PRL 85,
This can be done with nanosecond laser pulses!

5. Experimental demonstration of cascaded Raman scattering
Detuning from 2-photon resonance
2994 cm-1
- 400MHz
+ 100MHz
+ 700MHz
75,000 cm-1 (2.3 x 1015 Hz) of bandwidth has been created!
A. V. Sokolov et al. PRL 85, 562

6. Experimental demonstration of cascaded Raman scattering
The different frequencies are locked
Pulses with 1 fs duration are measured
The spectrum is discrete: the pulses are emitted in a pulse train, separated by the vibrational period.
The main advantage of this process: high efficiency
The main drawback: the carrier frequency is in the visible regime
We cannot produce an isolated pulse.
A. V. Sokolov et al. PRL 85, 562

7. Breaking the femtosecond limit
2001: First observation of an attosecond pulse (650 as)
M. Hentschel et al., Nature 414, (2001)
G. Sansone et al., Science 314, 443 (2006)
2006: (130 as)

8. Our main tool: intense laser pulses
Field Intensity: 1014 –1015 W/cm2
2.7 fs
The force is comparable to the force binding the electrons in the atom or molecule.

9. Attosecond pulse generation process
Re-collision
Acceleration by the electric field
E>100eV
Tunnel ionization
With I~1014 W/cm2
Fundamental frequency

10. Attosecond pulse generation process
Acceleration by the electric field
Tunnel ionization
Optical radiation with attoseconds duration

11. Attosecond pulse generation process
Classical model

12. Attosecond pulse generation process
Classical model

13. Attosecond pulse generation process
Classical model
The return times are determined such that x0(t,t0)=0
Long trajectories
Short trajectories
Ek is the instantaneous frequency of the attosecond pulse

14. Attosecond pulse generation process
Quantum model
The electron’s wavefunction
The induced dipole moment
The dynamics of the free electron is mapped into the optical field

15. Electron wave packet dynamic
Attosecond pulse

16. Electron wave packet dynamic
XUV field:
Husimi reprsentation

17. Attosecond pulse generation process
where only the gas was changed in between.
0,0 0,1 0,2 0,3 0,4 0,5
0,01
0,1 1
H21 Ar N2
normalized signal at H21 el
Attosecond pulse generation process
Classical model
Elliptically polarized light:
The electron is shifted in the lateral direction: the recollision probability reduces significantly

18. Isolating a single attosecond pulse
The multi-cycle regime

19. Isolating a single attosecond pulse
The multi-cycle regime
Femtosecond pulse
20 fs, 800nm
High harmonics
I~1014 W/cm2
H15
23.3eV
H21
32.6eV
H27
41.9eV
H39
60.5eV

20. Attosecond pulse generation process
M. Hentschel et al., Nature 414, (2001)

21. Attosecond pulse generation process
G. Sansone et al., Science 314, 443 (2006)

22. Time resolved measurements in the attosecond regime
Attosecond pulses generation
Measurement

23. How to measure an attosecond pulse?
XUV Autocorrelation
NLO effects: 2-photon absorption
2-photon ionization t
Problems: low XUV flux small sabs
focusing NL
Kobayashi et al., Opt. Lett. 23, 64 (1998)

24. Attosecond streak camera
momentum
Laser field
Photo-electrons
Electron release time
Attosecond pulse
M. Hentschel et al., Nature 414, (2001)

25. Momentum transfer depends on instant of electron release within the wave cycle

26. Mapping time to momentum
change along the EL vector
800-nm laser electric field
Δp(t7)
Δp(t6)
Δp(t5) t1 t2 t3 t4 t5 t6 t7
Δp(t4)
instant of
electron
release
Δp(t3)
Δp(t2)
Δpi
Δp(t1)
Incident X-ray
intensity
-500 as
500 as
Optical-field-driven streak camera
J. Itatani et al., Phys. Rev. Lett. 88, (2002)
M. Kitzler et al., Phys. Rev. Lett. 88, (2002)

27. Full characterization of a sub-fs, ~100-eV XUV pulse
Field-free
spectrum
td = -T0/4
Reconstructed temporal intensity profile and chirp of the xuv excitation pulse:
Time [fs]
Intensity [arb. u.] 1
Instantaneous energy shift [eV] -3 -2 -1 2
-0.4
-0.2
0.0
0.2 t
xuv
= 250as
td = +T0/4
 = 250 attoseconds!!

28. Energy shift of sub-fs electron wave-packet
As we vary the relative delay between the XUV pulse and the 800-nm field, the direction of the emitted electron packet will vary.
+10 eV
-10 eV ΔW tD
dN/dW

29. Attosecond streak camera trace90 80 70
Photoelectron kinetic energy [eV] 60
Goul bild beim 3. mausklick, Titel: Direct measurement of light waves 50 2 4 6 8 10 12 14 16 18 20 22
Delay Dt
[fs]
E. Goulielmakis et al., Science 305, 1267 (2004)

30. RABITT (Reconstruction of Attosecond Beating by Interference of Two-photon Transition)
The different paths interfere with a relative phase of:
Two photon transition
Narrow one photon transition

31. RABITT

32. RABITT (Reconstruction of Attosecond Beating by Interference of Two-photon Transition)
RABBITT takes advantage of the interference of the even-harmonic sidebands created when the XUV pulse interacts with the intense IR laser pulse.

33. RABITT results for a 250-as pulse

34. Time resolved measurements
Can we performed an attosecond pump probe measurement?
The main problem is the low photon flax t
focusing NL
One solution is to use the strong IR field as either the pump or the probe

35. Attosecond streaking spectroscopy
Auger Decay
Core level ionization
Valence level ionization
M. Drescher et al, Nature 419, 803 (2002)

36. Time resolved atomic inner shell spectroscopy

37. Time resolved atomic inner shell spectroscopy

38. Time resolved atomic inner shell spectroscopy

39. Oscillating dipole
The attosecond pulses contains the spatial information of the ground and the free electron wavefunctions.

40. Imaging the ground state
c ~1A
d(t)= a(k) <g|er|eikx-()t>
The free electron act as a probe - the re-collision step maps the ground state wave function to the spectrum

41. Harmonic intensities
Harmonic intensities from N2 at different molecular angles EL

42. Tomographic image reconstruction

43. Reconstructed Molecular Orbital - N2
Reconstructed orbital
Calculated orbital
J. Itatani, et al., Nature 432, 867 (2004).