Bi-plasma interactions on femtosecond time-scalesPresented by Emily SpraguePULSE Institute, Aaron Lindenberg, Dan Daranciang, & Haidan Wen

Overview Background

▪ Plasma Filamentation▪ THz generation

Experimental Setup

Results

Conclusions

Future WorkCourtesy of http://en.wikipedia.org/wiki/Plasma_%28physics%29

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

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

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

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)

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

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

Time delay studies

Before time-zero: no plasma interaction

Time zero: both plasmas arrive and interfere

After time-zero: secondary fluorescence

Camera images (from above) of bi-plasma overlap

Time Zero: two plasmas arrive simultaneously

Before time zero

After time zero

Origin of dramatic enhancement at time zero is not understood

Results (cont’d)

Trends

200 250 300 350 400 4500

100200300400500600700

P-P Polarized

Delay Arm Power (mW)

Inte

nsity

Rat

io

250 300 350 400 450 500 550 60005

101520253035

S-S Polarized

Delay Arm Power (mW)

Inte

nsity

Rat

io

250 300 350 400 450 5000

100200300400500600700

S-P Polarized

Delay Arm Power (mW)

Inte

nsity

Rat

io

Conclusions Peak intensity and point of decay consistently occured

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

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

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/

Thank you!