1. Laboratoire d’Optique Appliquée
Palaiseau – FRANCE
Complete characterization of ionization
induced self-compression
(17/09 – EAAC)
D. Guénot, B. Beaurepaire, A. Vernier, A. Lifschitz, J. Faure
UMR 7639
FemtoElec

2. Laser wakefield acceleration for electron diffraction
Generation of 5 MeV, <10 fs, 1 kHz electron bunch R lp
Blowout regime:
Laser pulse has to be resonant with plasma wave:
R≈λp/2
For a 5 mJ laser pulse, scaling laws give:
t = 5 fs W0 = 2.5 µm
I = 8x1018 W/cm² ne = 7x1019 cm-3
Emax = 10 MeV
Now: 20fs pulse keV electrons
Lu et al., PRSTAB 10, (2007)
O. Lundh et al., Nature Phys., 7, (2011).
Talk B. Beaurepaire 1

3. Motivation How to produce short intense pulse?
Ti-sapphire laser limited to 20fs duration necessity to post-compress pulse
New frequencies generated by non linear variation of refractive index
A. Couairon, A Mysyrowicz, Phys. Rep. 441, 47 (2007).
M. Nisoli, etal, New J. Phys. 10, (2008).
Z. H. He, etal, PRL, 113, (2014). 2

4. Motivation Self phase modulation: How to produce short intense pulse?
n = n0 +n2*I(t)
 Energy limited by ionization, complex setup
I(t)
A. Couairon, A Mysyrowicz, Phys. Rep. 441, 47 (2007).
M. Nisoli, etal, New J. Phys. 10, (2008).
Z. H. He, etal, PRL, 113, (2014). 3

5. Motivation Laser wakefield compression:
How to produce short intense pulse?
Laser wakefield compression:
 High intensity, few studies
I(t) ρe
A. Couairon, A Mysyrowicz, Phys. Rep. 441, 47 (2007).
M. Nisoli, etal, New J. Phys. 10, (2008).
Z. H. He, etal, PRL, 113, (2014). 3

6. Motivation Ionization in capillaries:
How to produce short intense pulse?
Ionization in capillaries:
 low transmission, complex setup
I(t) ρe
A. Couairon, A Mysyrowicz, Phys. Rep. 441, 47 (2007).
M. Nisoli, etal, New J. Phys. 10, (2008).
Z. H. He, etal, PRL, 113, (2014). 3

7. Motivation Ionization in gas jet Simplicity (1 gas jet)
High transmission (No coupling)
But: No guiding structure poor beam quality
What compression can we achieve while maintaining a homogenous beam?
Need for characterizing the process 4

8. Experimental setup 1. Interaction 5
B. Beaurepaire, etal, PRX, 5, (2015)

9. Experimental setup 2. Plasma imaging Imaging of the plasma
Capillary position 5

10. Experimental setup
3. Diagnostics
From interaction
chamber 6

11. Experimental setup
3. Diagnostics
λ/2 (nm)
From interaction
chamber 6

12. Experimental setup
3. Diagnostics
From interaction
chamber 6

13. Experimental setup 3. Diagnostics α (arb. unit.) From interaction
chamber
λ(nm) 6

14. On axis results How much can we compress the pulse?
We can double the spectrum and compress the pulse from 20 to 12 fs
N2, 8bar cm-3
λ/2 (nm)
How much can we compress the pulse?
Is the beam homogenous? 7

15. Results
1. Methods 8

16. Results 1. Homogeneity study z Laser
Results
1. Homogeneity study
Variation of the homogeneity with the position of the capillary
S/S0
Capillary position (µm)
Best homogeneity when the capillary is 100µm after the focus 8

17. Results Excellent refocusability!!! 2. Refocusability α (arb. unit.)9

18. Results
3. Spatial homogeneity
Laser z 10

19. Results 4. Temporal homogeneity From interaction chamber 7 Step of 4mm7
Step of 4mm
From interaction
chamber 11

20. Results FROG scan homogenous along the beam! 4. Temporal homogeneity
FROG trace 1 2 3 4 5 6 7
λ/2(nm)
τ (fs)
FROG scan homogenous along the beam! 12

21. Results The width is doubled: Δω=50 to 95nm (rms)7
Results
4. Temporal homogeneity
Spectrum
Phase
8 bar
No gas 7 6 5 4 3 2
The width is doubled: Δω=50 to 95nm (rms)
The spectrum and phase are homogenous along the beam
A chirp remains (plasma) 13

22. Results Small chirp remaining recompress sub 10fs7
Results
4. Temporal homogeneity
FWMH
Temporal duration
FTL
Measured
FTL
Measured
Transverse position
Small chirp remaining recompress sub 10fs
Homogenous duration refocus pulse 14

23. Towards higher compression
How much can we compress the pulse?
Using tighter focusing (W0 = 2µm) 15

24. Results 4. Towards higher compression z Laser
4 bar cm-3, Δω = 95nm
S/S0
Capillary position (µm)
Best homogeneity when the capillary is before the focus 16

25. Results At best the bandwidth can be doubled
4. Towards higher compression
Increasing pressure
S/S0
Bandwidth (nm)
At best the bandwidth can be doubled 17
Z. H. He, etal, PRL, 113, (2014).

26. Conclusion and perspective
New compression scheme for 1kHz, 4mJ, 20fs
Optimal capillary position depends on pressure and focal length
Homogeneous beam, can be refocused
Limited to 12fs (7.5 FTL) 18

27. Appendix 19

28. Results 1. Homogeneity study No gas 8 bar λ (nm) λ (nm) Spatial chirp!
The reference beam has a spatial chirp!!!
The ionization induced compression reduce this chirp 9

29. Methods horizontal axis Projection horizontal axis (center) Projection7

30. Methods 7