1. Mirela Cerchez, ILPP, HHU, Düsseldorf Meeting GRK1203, Bad Breisig, 11th October 2007 Absorption of sub-10 fs laser pulses in overdense solid targets Mirela Cerchez, Ralph Jung, Jens Osterholz, Toma Toncian, and Oswald Willi Institute of Laser and Plasma Physics, Heinrich-Heine University Düsseldorf, Germany Harmut Ruhl Institute for Theoretical Physics, Ruhr University, Bochum, Germany Peter Mulser Theoretical Quantum Electronics, Technical University, Darmstadt, Germany

2. Mirela Cerchez, ILPP, HHU, Düsseldorf Meeting GRK1203, Bad Breisig, 11th October 2007 Outline Motivation Experimental set-up and conditions Data evaluations and significant dependences Simulations: kinetic approach (2-D PIC simulations by PSC code) collisions influences (MULTI-fs hydrodynamic code) Conclusions and further plans

3. Mirela Cerchez, ILPP, HHU, Düsseldorf Meeting GRK1203, Bad Breisig, 11th October 2007 Outline Motivation Experimental set-up and conditions Data evaluations and significant dependences Simulations: kinetic approach (2-D PIC simulations by PSC code) collisions influences MULTI-fs hydrodynamic code Conclusions and further plans

4. Mirela Cerchez, ILPP, HHU, Düsseldorf Meeting GRK1203, Bad Breisig, 11th October 2007 Experimental investigations of dense plasma at ILPP Dense plasma conditions and parameters Experimental investigations of dense plasma emission spectra : pressure ionization (*), resonance lines satellites features, spectral lines broadening and shift, opacity effects High-contrast laser pulse, sub-10 fs, I laser = (1-5)*10 16 W/cm 2 Solid target (*) J. Osterholz et al., Phys. Rev Lett. 96, 085002 (2006)

5. Mirela Cerchez, ILPP, HHU, Düsseldorf Meeting GRK1203, Bad Breisig, 11th October 2007 Experimental investigations of dense plasma at ILPP Dense plasma conditions and parameters Experimental investigations of dense plasma emission spectra : pressure ionization (*), resonance lines satellites features, spectral lines broadening and shift, opacity effects High-contrast laser pulse, sub-10 fs, I laser = (1-5)*10 16 W/cm 2 Solid target (*) J. Osterholz et al., Phys. Rev Lett. 96, 085002 (2006) Heating mechanisms in steep plasma density gradients Experimental investigations and studies on absorbed laser fraction energy by solid targets Spectroscopic investigations on emission of plasmas produced by laser on coated targets

6. Mirela Cerchez, ILPP, HHU, Düsseldorf Meeting GRK1203, Bad Breisig, 11th October 2007 Outline Motivation Experimental set-up and conditions Data evaluations and significant dependences Simulations: kinetic approach (2-D PIC simulations by PSC code) collisions influences MULTI-fs hydrodynamic code Conclusions and further plans

7. Mirela Cerchez, ILPP, HHU, Düsseldorf Meeting GRK1203, Bad Breisig, 11th October 2007 Experimental set-up Oscilloscope Retro-focus diagnostic Ulbricht sphere HR @ 800nm /22.5° mirror OA parabola f/3 HR @ 800nm /45° mirror ~ 100 µJ, @790 nm, 8 fs laser pulses Al target Optic bundle photodiode filtered Z

8. Mirela Cerchez, ILPP, HHU, Düsseldorf Meeting GRK1203, Bad Breisig, 11th October 2007 Experimental set-up Laser pulses: 100 -120 µJ in the target chamber; Sub-10 fs pulse duration; Diameter focal spot ~3 µm (FWHM); Average intensity : (4 - 5)۰10 16 W/cm 2 p and s – polarization; Energy fluctuations less than 5%; Target : mirror-flat aluminium layers deposited on planar silicon substrates. Oscilloscope Retro-focus diagnostic Ulbricht sphere HR @ 800nm /22.5° mirror OA parabola f/3 HR @ 800nm /45° mirror ~ 100 µJ, @790 nm, 8 fs laser pulses Al target Optic bundle photodiode filtered Z

9. Mirela Cerchez, ILPP, HHU, Düsseldorf Meeting GRK1203, Bad Breisig, 11th October 2007 Experimental set-up Laser pulses: 100 -120 µJ in the target chamber Sub-10 fs pulse duration Diameter focal spot ~3 µm (FWHM) Average intensity : (4 - 5)۰10 16 W/cm 2 p and s – polarization Energy fluctuations less than 5% Target : mirror-flat aluminium layers deposited on planar silicon substrates. Oscilloscope Retro-focus diagnostic Ulbricht sphere HR @ 800nm /22.5° mirror OA parabola f/3 HR @ 800nm /45° mirror ~ 100 µJ, @790 nm, 8 fs laser pulses Al target Optic bundle photodiode filtered Z

10. Mirela Cerchez, ILPP, HHU, Düsseldorf Meeting GRK1203, Bad Breisig, 11th October 2007 Experimental set-up Laser pulses: 100 -120 µJ in the target chamber Sub-10 fs pulse duration Diameter focal spot ~3 µm (FWHM) Average intensity : (4 - 5)۰10 16 W/cm 2 p and s – polarization Energy fluctuations less than 5% Target : mirror-flat aluminium layers deposited on planar silicon substrates. Oscilloscope Retro-focus diagnostic Ulbricht sphere HR @ 800nm /22.5° mirror OA parabola f/3 HR @ 800nm /45° mirror ~ 100 µJ, @790 nm, 8 fs laser pulses Al target Optic bundle photodiode filtered Z ~ E reflected

11. Mirela Cerchez, ILPP, HHU, Düsseldorf Meeting GRK1203, Bad Breisig, 11th October 2007 Experimental set-up Laser pulses: 100 -120 µJ in the target chamber Sub-10 fs pulse duration Diameter focal spot ~3 µm (FWHM) Average intensity : (4 - 5)۰10 16 W/cm 2 p and s – polarization Energy fluctuations less than 5% Target : mirror-flat aluminium layers deposited on planar silicon substrates. Oscilloscope Retro-focus diagnostic Ulbricht sphere HR @ 800nm /22.5° mirror OA parabola f/3 HR @ 800nm /45° mirror ~ 100 µJ, @790 nm, 8 fs laser pulses Al target Optic bundle photodiode filtered Z ~ E reflected A= 1 - R = 1 - E reflected /E incident

12. Mirela Cerchez, ILPP, HHU, Düsseldorf Meeting GRK1203, Bad Breisig, 11th October 2007 Outline Motivation Experimental set-up and conditions Data evaluations and significant dependences Simulations: kinetic approach (2-D PIC simulations by PSC code) collisions influences MULTI-fs hydrodynamic code Conclusions and further plans

13. Mirela Cerchez, ILPP, HHU, Düsseldorf Meeting GRK1203, Bad Breisig, 11th October 2007 Experimental tasks Laser pulses energy absorption and its significant dependences laser polarization angular dependence laser intensity dependence angle of the optimum laser absorption absorption dependence on the plasma profile,

14. Mirela Cerchez, ILPP, HHU, Düsseldorf Meeting GRK1203, Bad Breisig, 11th October 2007 Experimental results Experimental investigations of the angular dependence of the laser energy absorption for both p- and s- polarization; over wide range of incidence angles.

15. Mirela Cerchez, ILPP, HHU, Düsseldorf Meeting GRK1203, Bad Breisig, 11th October 2007 Experimental results Experimental investigations of the angular dependence of the laser energy absorption for both p- and s- polarization; over wide range of incidence angles. large preponderance of p- absorption on s- absorption maximum absorption value : ~ 77% at 80° angle of the maximum absorption: ≥ 80°

16. Mirela Cerchez, ILPP, HHU, Düsseldorf Meeting GRK1203, Bad Breisig, 11th October 2007 Experimental results Experimental investigations of the laser energy absorption as a function of laser intensity for both p- and s- polarization; over 4 orders of magnitude of the laser intensity (5x10 12 W/cm 2 – 5x10 16 W/cm 2 ).

17. Mirela Cerchez, ILPP, HHU, Düsseldorf Meeting GRK1203, Bad Breisig, 11th October 2007 Experimental results Experimental investigations of the laser energy absorption as a function of laser intensity for both p- and s- polarization over 4 orders of magnitude of the laser intensity (5x10 12 W/cm 2 – 5x10 16 W/cm 2 )

18. Mirela Cerchez, ILPP, HHU, Düsseldorf Meeting GRK1203, Bad Breisig, 11th October 2007 Experimental results Experimental investigations of the laser energy absorption as a function of laser intensity for both p- and s- polarization over 4 orders of magnitude of the laser intensity (5x10 12 W/cm 2 – 5x10 16 W/cm 2 )

19. Mirela Cerchez, ILPP, HHU, Düsseldorf Meeting GRK1203, Bad Breisig, 11th October 2007 Outline Motivation Experimental set-up and conditions Data evaluations and significant dependences Simulations: kinetic approach (2-D PIC simulations by PSC code) collisions influences (MULTI-fs hydrodynamic code) Conclusions and further plans

20. Mirela Cerchez, ILPP, HHU, Düsseldorf Meeting GRK1203, Bad Breisig, 11th October 2007 2-D PIC simulation by Plasma Simulation Code (*) 20 µm 40 cells/µm cell number n n (cm 3 ) cell number f/f 0 E z (y) 2 (*) H. Ruhl, in Introduction to Computational Methods in Many Particle Body Physics (Rinton Press, Paramus, New Jersey, 2006)

21. Mirela Cerchez, ILPP, HHU, Düsseldorf Meeting GRK1203, Bad Breisig, 11th October 2007 20 µm 40 cells/µm cell number n n (cm 3 ) cell number f/f 0 E z (y) 2 In the centre of the simulation box was placed a rectangular aluminium target of 15 µm x 1 µm. The electron density profile at the target boundary is given by: Various density scale length L of 2 nm, 10 nm, 20 nm, 200 nm and 500 nm were used. (*) H. Ruhl, in Introduction to Computational Methods in Many Particle Body Physics (Rinton Press, Paramus, New Jersey, 2006) 2-D PIC simulation by Plasma Simulation Code (*)

22. Mirela Cerchez, ILPP, HHU, Düsseldorf Meeting GRK1203, Bad Breisig, 11th October 2007 20 µm 40 cells/µm cell number n n (cm 3 ) cell number f/f 0 E z (y) 2 In the centre of the simulation box was placed a rectangular aluminium target of 15 µm x 1 µm. The electron density profile at the target boundary is given by: Various density scale length L of 2 nm, 10 nm, 20 nm, 200 nm and 500 nm were used. Gaussian laser pulse with a duration of 10 fs (FWHM) with a focus diameter of 1.5 µm (FWHM) focused under an angle of 18°. In the focus, the laser pulse achieved an intensity 2۰10 16 W/cm 2. The target was rotated in steps of 10° around the position of maximum gradient where the pulse focus is localized. (*) H. Ruhl, in Introduction to Computational Methods in Many Particle Body Physics (Rinton Press, Paramus, New Jersey, 2006) 2-D PIC simulation by Plasma Simulation Code (*)

23. Mirela Cerchez, ILPP, HHU, Düsseldorf Meeting GRK1203, Bad Breisig, 11th October 2007 Simulation results and remarks The fraction of absorption of pulse energy calculated by analyzing the Poynting flux of the pulse before and after being reflected by the target surface. Binary collisions were included in the PSC and the simulations were performed for p- and s-polarization of the incident laser pulse. The absorption profiles of p and s- polarization were subtracted from each other in order to determine the collisionless contribution in p-polarization case.

24. Mirela Cerchez, ILPP, HHU, Düsseldorf Meeting GRK1203, Bad Breisig, 11th October 2007 The fraction of absorption of pulse energy calculated by analyzing the Poynting flux of the pulse before and after being reflected by the target surface. Binary collisions were included in the PSC and the simulations were performed for p- and s-polarization of the incident laser pulse. The absorption profiles of p and s- polarization were subtracted from each other in order to determine the collisionless contribution in p-polarization case. Simulation results and remarks

25. Mirela Cerchez, ILPP, HHU, Düsseldorf Meeting GRK1203, Bad Breisig, 11th October 2007 The fraction of absorption of pulse energy calculated by analyzing the Poynting flux of the pulse before and after being reflected by the target surface. Binary collisions were included in the PSC and the simulations were performed for p- and s- polarization of the incident laser pulse. The absorption profiles of p and s- polarization were subtracted from each other in order to remove the effect of collisions. Simulation results and remarks The computation and experimental results at average intensity of 2۰10 16 W/cm 2 are in well agreement for steeper profiles in the range of L = 10 - 20 nm.

26. Mirela Cerchez, ILPP, HHU, Düsseldorf Meeting GRK1203, Bad Breisig, 11th October 2007 The fraction of absorption of pulse energy calculated by analyzing the Poynting flux of the pulse before and after being reflected by the target surface. Binary collisions were included in the PSC and the simulations were performed for p- and s- polarization of the incident laser pulse. The absorption profiles of p and s- polarization were subtracted from each other in order to remove the effect of collisions. Simulation results and remarks The computation and experimental results at average intensity of 2۰10 16 W/cm 2 are in well agreement for steeper profiles in the range of L = 10 - 20 nm.

27. Mirela Cerchez, ILPP, HHU, Düsseldorf Meeting GRK1203, Bad Breisig, 11th October 2007 1-D hydrodynamic simulation by MULTI-fs code (**) (**) K. Eidmann et al, Phys. Rev. E 14, 1202 (2000) Hydrodynamic simulations have been carried out using the MULTI-fs code which offer the opportunity to test the influence of the electron-ion collisions in our experimental conditions. MULTI-fs can account for collisional ionization and thermal heat conductivity over a wide range of temperatures from cold solid to ideal plasma. Electron density spatial distribution → L = 2 - 5 nm

28. Mirela Cerchez, ILPP, HHU, Düsseldorf Meeting GRK1203, Bad Breisig, 11th October 2007 1-D hydrodynamic simulation by MULTI-fs code (**) (**) K. Eidmann et al, Phys. Rev. E 14, 1202 (2000) Hydrodynamic simulations have been carried out using the MULTI- fs code which offer the opportunity to test the influence of the electron-ion collisions in our experimental conditions. MULTI-fs can account for collisional ionization and thermal heat conductivity over a wide range of temperatures from cold solid to ideal plasma. Electron density spatial distribution → L = 2 - 5 nm

29. Mirela Cerchez, ILPP, HHU, Düsseldorf Meeting GRK1203, Bad Breisig, 11th October 2007 1-D hydrodynamic simulation by MULTI-fs code (**) (**) K. Eidmann et al, Phys. Rev. E 14, 1202 (2000) Hydrodynamic simulations have been carried out using the MULTI- fs code which offer the opportunity to test the influence of the electron-ion collisions in our experimental conditions. MULTI-fs can account for collisional ionization and thermal heat conductivity over a wide range of temperatures from cold solid to ideal plasma. Electron density spatial distribution → L = 2 - 5 nm The calculated angular dependence of the absorption is in very good agreement with the experimental data for p- and s -polarization, indicating a maximum absorption of 78% at 80°. The results indicate a steep plasma profile of L ~ 2 - 5 nm. The collisional and collisionless contributions are very well reproduced by the simulation results

30. Mirela Cerchez, ILPP, HHU, Düsseldorf Meeting GRK1203, Bad Breisig, 11th October 2007 Conclusions and further plans The computational results are in very good agreement with the experiment assuming a profile of L/λ ~ 1%; The simulations confirm the absorption of p-polarization up to 80° and indicate that the interaction of the ultrashort laser pulses with the target takes place close to the solid density state of matter; Classical model of linear resonance absorption is not valid for these steep profiles. In the non-linear regime of RA, a favorable candidate to explain the experimental observation is the model of anharmonic resonance absorption; In our intensity range, contribution of other collisionless absorption mechanisms like: skin layer absorption, sheath layer inverse bremsstrahlung do not exceed 5 - 10%. Vacuum heating particular characteristics have been not identified, and the contribution of the effect was not possible to distinguished in the experiment. In order to get a better understanding of the collisions contribution during absorption process of extremely short laser pulses, similar analysis are planed for another conductor, Au (high Z). Preliminary results are very promising.

31. Mirela Cerchez, ILPP, HHU, Düsseldorf Meeting GRK1203, Bad Breisig, 11th October 2007