1. GRK-1203 Workshop Oelde Watching a laser pulse at work
- Laser-plasma interaction seen on ultra-short timescales -
Ralph Jung
- Institut für Laser und Plasmaphysik -

2. What kind of work can a laser pulse do?
Heat
Zum Vergleich: 2x10^21 W/cm^2 entspricht…
Ionize
Push
Illuminate

3. The potentially most popular examples for high-intensity laser-plasma interaction
Zum Vergleich: 2x10^21 W/cm^2 entspricht…

4. Summary Watching a laser pulse at work
- Laser-plasma interaction seen on ultra-short timescales -
Task: Observe optically the breakdown of the the material caused by an ultra-short laser pulse.
Zum Vergleich: 2x10^21 W/cm^2 entspricht…
What happens there (ionization) ?
How can you observe the ionization front (in principle) ?
How looks a typical pump-probe experiment (practically) ?
Data: High-resolution photos of the interaction and comparison with 3D-PIC simulations.

5. What happens there? - A typical laser-plasma experiment -
Laser as a „black box“
Focusing
optics
Target volume
Zum Vergleich: 2x10^21 W/cm^2 entspricht…, Limit im Verstärker bis Selbstfokussierung einsetzt: einige GW/cm^2, proportional zur Intensität, nicht zur Fluenz (Energie), typische Streckung x, sweep = linearer Frequenzverlauf, vershciedene Frequenzen erscheinen zu unterschiedlichen Zeiten, das Gitter dispergiert das Spektrum und streckt den Puls dadurch zeitlich
Wavelength: l
Pulse energy: E
Pulse duration: Dt
Focal spot diameter: typically 5-10 mm (FWHM)

6. Modern ultra-short pulse lasers are based on the CPA-technique
~ nJ
~ nJ
Zum Vergleich: 2x10^21 W/cm^2 entspricht…, Limit im Verstärker bis Selbstfokussierung einsetzt: einige GW/cm^2, proportional zur Intensität, nicht zur Fluenz (Energie), typische Streckung x, sweep = linearer Frequenzverlauf, vershciedene Frequenzen erscheinen zu unterschiedlichen Zeiten, das Gitter dispergiert das Spektrum und streckt den Puls dadurch zeitlich
Amplification:
Verstärkung:
Few-Cycle Pulse
5 fs, 100 Gigawatt
Sub-10-fs-Pulse
5 fs, 100 Gigawatt
~ mJ
~ mJ
„Chirped Pulse Amplification“
[Strickland and Mourou, 1985]

7. Modern ultra-short pulse lasers are based on the CPA-technique
~ nJ
~ nJ
Zum Vergleich: 2x10^21 W/cm^2 entspricht…
Amplification:
Verstärkung:
Ultra-high power laser
500 fs, 1 Petawatt
Sub-10-fs-Pulse
5 fs, 100 Gigawatt
~ kJ
~ kJ
„Chirped Pulse Amplification“
[Strickland and Mourou, 1985]

8. What happens there? - A typical laser-plasma experiment -
Laser as a „black box“
Focusing
optics
Target volume
Zum Vergleich: 2x10^21 W/cm^2 entspricht…, Limit im Verstärker bis Selbstfokussierung einsetzt: einige GW/cm^2, proportional zur Intensität, nicht zur Fluenz (Energie), typische Streckung x, sweep = linearer Frequenzverlauf, vershciedene Frequenzen erscheinen zu unterschiedlichen Zeiten, das Gitter dispergiert das Spektrum und streckt den Puls dadurch zeitlich

9. Ionization mechanisms
I = 0 W/cm2
E = 0 V/m
Example: H-Atom
Bohr radius:
The phase of the electric field of the pulse after the interaction with the gas was measured with a SPIDER.
Electric field:
Light intensity:
Electron undisturbed
1965 Rubi Laser: Ionization !

10. Ionization mechanisms
I = W/cm2
E = V/m
Example: H-Atom
Bohr radius:
Perturbation theory
Electric field:
Light intensity:
Multi-Photon Ionization
1965 Rubi Laser: Ionization !

11. Ionization mechanisms
I > W/cm2
E > V/m
Example: H-Atom
Bohr radius:
Also: I < 10^13: Perturbation theory,
Hier: Löse zeitabhängige Schrödinger Gleichung in Single-Active-Electron Approximation (SAE), problematisch, da Wellenpaket wegläuft (continuum-continuum-transitions) bei höher werdenden Intensitäten.
The multiphoton ionization (MPI) of noble gases by strong laser fields can be well described by time dependent quantum mechanical calculations within the single active electron approximation (SAE). -> Sequential ionization
Electric field:
Light intensity:
Above Threshold Ionization
1965 Rubi Laser: Ionization !

12. Ionization mechanisms
I = W/cm2
E = V/m
Example: H-Atom
Bohr radius:
Electric field:
Light intensity:
Tunneling Ionization

13. Ionization mechanisms
I = W/cm2
E = V/m
Example: H-Atom
Bohr radius:
The phase of the electric field of the pulse after the interaction with the gas was measured with a SPIDER.
Electric field:
Light intensity:
Barrier Suppression Ionization

14. Ionization mechanisms
The phase of the electric field of the pulse after the interaction with the gas was measured with a SPIDER.

15. Ionization mechanisms
Barrier Suppression Ionization
The phase of the electric field of the pulse after the interaction with the gas was measured with a SPIDER.
Augst et. al. PRL 63 (20), 2212 (1989)

16. Ionization mechanisms
Keldysh ionization rates (1965), extended form by Ammosov, Delone an Krainov (1986)
ADK-Theory
For hydrogen like atoms,
Influence of pulse duration:
Pulse duration may be relevant !
Take also level depletion into account
Nagaya et. al. J. Phys. Soc. of Japan, 75 (2006)

17. What happens there? - A typical laser-plasma experiment -
Laser as a „black box“
Focusing
optics
Target volume
Zum Vergleich: 2x10^21 W/cm^2 entspricht…, Limit im Verstärker bis Selbstfokussierung einsetzt: einige GW/cm^2, proportional zur Intensität, nicht zur Fluenz (Energie), typische Streckung x, sweep = linearer Frequenzverlauf, vershciedene Frequenzen erscheinen zu unterschiedlichen Zeiten, das Gitter dispergiert das Spektrum und streckt den Puls dadurch zeitlich e- + + e- e- +

18. Optical properties of the plasma channel created
Way of a beam path through the plasma:
Zum Vergleich: 2x10^21 W/cm^2 entspricht… s

19. Hence: Diagnose the plasma with a second, well timed laser pulse
Typical setup of a „pump-probe“ experiment
Zum Vergleich: 2x10^21 W/cm^2 entspricht…
Image the plasma channel onto a camera
Optical probing : shadowgraphy, interferometry, polarimetry etc.
Temporal resolution given by probe pulse duration

20. The sub-10-fs laser system used here
Central wavelength: l0 = 800 nm
Pulse duration (fwhm): t = 8 fs
Pulse energy: E = 150 mJ
Intensity in focus: I = W/cm2
+ Hollow-fibre + compressor
Pulse consists only of few optical cycles
V/m
Electric field
Pre-pulse level so low that no measureable pre-plasma produced
3 mm
High ADK-ionization rates expected
Time or space

21. Gastarget
The pulse ionizes the gas and an optically transparent plasma is produced x x
Pulsed gas jet:
Supersonic expansion, Mach 3.3 with up to particles/cm3 (~10 bar)
Laser pulse z z
10 fs are a slap of light of just 3 microns in length
Focus diamater about 5 microns
Gases:
Helium, Neon, Argon, Nitrogen

22. Geometrie des „pump-probe“ Experiments
Geometry of „pump-probe“ experiment x
(Kalorimetrie/ Spektroskopie)
Pulsed gas jet:
Supersonic expansion, Mach 3.3 with up to particles/cm3 (~10 bar)
„Pump“- pulse z y
Gases:
Helium, Neon, Argon, Nitrogen
„Probe“- pulse
(opt. imaging / shadowgrams / interferograms)

23. Focused shadowgraphy
Explain focused shadowgraphy

24. Focused shadowgraphy

25. Focused shadowgraphy

26. Focused shadowgraphy

27. Focused shadowgraphy

28. Focused shadowgraphy

29. A „Pump-Probe“- experiment using an ultra-short optical probe pulse
BBO 40 microns in thickness, 10 x microscope objective, High magnification 44 x,
High resolution of better than 1 micron
elliptical illuminating spot of about 50 µm x 150 µm (both FWHM).
group velocity dispersion effects in BBO (D2 = 37,0 fs2/mm) is of the order of less than 0.1%.
View inside
Target Chamber

30. Typische Aufnahmen des Plasmakanals
High-resolution images of the ionization front using „focused shadowgraphy“
Example: Helium at a neutral density of n0 = 2 – cm-3
 = 0 fs
 = +30 fs
Zeitauflösung ~ 10 fs
(Ortsauflösung 1 mm)

31. From the shadowgrams the electron density within the channel can be obtaind
Plasma
Plasma
Neutral gas
Neutralgas
Feld ionization
Feldionisation

32. 3D-Particle-In-Cell simulation of the ionization front

33. 3D-Particle-In-Cell simulation of the ionization front
Simulated electron density produced
by pump-pulse

34. 3D-Particle-In-Cell simulation of the ionization front

35. 3D-Particle-In-Cell simulation of the ionization front
Simulation of deflection

36. 3D-Particle-In-Cell simulation of field-ionization within the gas
Plasma-Simulation-Code (PSC) (Hartmut Ruhl)
Electron density at t = 50,0 fs
Elektronendichte t = 66,7 fs
Electron density at t = 66,7 fs
He2+
He+ .
1019
cm-3
Example: Helium at a neutral density of n0 = cm-3

37. 3D-Particle-In-Cell simulation of field-ionization within the gas
Plasma-Simulation-Code (PSC) (Hartmut Ruhl)
<Z> = 1,5
<Z> = 1,5
Electron density at t = 50,0 fs
Electron density at t = 66,7 fs .
1019
cm-3
Example: Helium at a neutral density of n0 = cm-3

38. Plasma-Simulation-Code (PSC) (Hartmut Ruhl)
3D-PIC simulation of the interaction between plasma channel and probe pulse
Plasma-Simulation-Code (PSC) (Hartmut Ruhl)
Electron density at t = 50,0 fs
Shadowgram at t = 74,7 fs .
1019
cm-3
Example: Helium at a neutral density of n0 = cm-3

39. Summary What happens there (ionization) ?
Task: Observe optically the breakdown of the the material caused by an ultra-short laser pulse.
What happens there (ionization) ?
Ionization mechanism (Multiphoton, Tunneling, BSI, ADK-Rates)
Zum Vergleich: 2x10^21 W/cm^2 entspricht…
How can you observe the ionization front (in principle) ?
Optical properties of a laser-produced plasma, „Pump-Probe“-technique, trajectories of probe-puls rays
How looks a typical pump-probe experiment (practically) ?
Experimental setup, some pictures
High-resolution photos of the interaction and comparison with 3D-PIC simulations.
Time-resolved shadowgrams, PIC results