Bi-plasma interactions on femtosecond time-scales

1. Presented by Emily Sprague PULSE Institute, Aaron Lindenberg, Dan Daranciang, & HaidanWen Bi-plasma interactions on femtosecond time-scales

2. Overview • Background • Plasma Filamentation • THz generation • Experimental Setup • Results • Conclusions • Future Work Courtesy of http://en.wikipedia.org/wiki/Plasma_%28physics%29

3. Ultrafast pulses are on the order of the femtosecond (10 − 15 second) • Created by mode-locked oscillators • Ti:sapphire oscillators • wavelengths of 680 nm to 1130 nm • Optimization • Minimal chirp • Large bandwidth • Used to generate plasma

4. Plasma is formed through a process called photoionization • Photons from an external source are absorbed by a gas, emitting electrons • Because of abundance of charge carriers, interacts with itself and surrounding EM fields • Used in THz generation Courtesy of http://www.isibrno.cz/omitec/index.php?action=libs.html

5. THz radiation are E&M waves with frequencies of ~ 1012 Hz • Could potentially replace x-rays as a form of non-ionizing radiation • Applications in medical imaging, material science studies, and atomic spectroscopy • 5 types of plasma-based generation methods Courtesy of http://www.stanford.edu/group/lindenberg/research.html

6. AC-bias method produces a transverse polarization without use of electrodes • Superposition of fundamental and second-harmonic pulse fields • Optimization • Relative phase shift • Exact temporal overlap • Polarization Courtesy of M.D. Thomson, M. Kreß, T. Loffler, and H.G. Roskos. Laser & Photon. Rev. 1, No. 4, 349–368 (2007)

7. Studying multiple plasmas could lead to production of more efficient THz radiation • Ti:sapphire laser • 50 fs 800 nm pulse • Mirrors • Lenses • f=100 mm (beam 2) • f=200 mm (beam 1) • Beam splitter • Controls polarization • beam 1: p-polarized • beam 2: s-polarized • Delay Stage • Controls path length and relative delay between arrival of plasmas

8. Polarization studies • s-p polarized • Beam 2 vertically polarized • Beam 1 horizontally polarized • s-s polarized • Beam 1 and beam 2 vertically polarized • p-p polarized • Beam 1 and beam 2 horizontally polarized

9. Time delay studies • Before time-zero: no plasma interaction • Time zero: both plasmas arrive and interfere • After time-zero: secondary fluorescence

10. Camera images (from above) of bi-plasma overlap Before time zero Time Zero: two plasmas arrive simultaneously After time zero

11. Origin of dramatic enhancement at time zero is not understood

12. Results (cont’d)

13. Trends

14. Conclusions • Peak intensity and point of decay consistently occuredat the same time values • Decay time was constant across all polarizations (~50 steps) • All power levels and polarization sets experienced a full decay back to the starting intensities • No valuable data was obtained below a power of 250 mW • Peak intensity was always strongest for s-p polarizations and weakest for p-p polarizations

15. Conclusions (cont’d) • Slope of the decay decreased with decreasing power in stationary arm • Peak and decay ratios increased with decreasing power in the stationary arm • Results are reproducible • Spike at time zero is dramatic and still not understood by scientific community

16. Future Work • Time dependent spectral studies of plasma • Analysis of wavelengths of plasma fluorescence • Resolve between scatter or enhanced tunneling ionzation • Better camera resolution Courtesy of http://opticsclub.engineering.ucdavis.edu/

18. Thank you!