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

2. Overview Background Experimental Setup Results Conclusions Future Work
Plasma Filamentation
THz generation
Experimental Setup
Results
Conclusions
Future Work
Courtesy of

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
Simplest example is photoelectric effect
Courtesy of

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

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 s-s polarized p-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 occurred at 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 scattering or enhanced tunneling ionization
Better camera resolution
Courtesy of

17. Thank you!