Shaped Femtosecond Laser Pulse Spectroscopy for Nuclear Forensics
PI: I. Jovanovic, Penn State University
December 2011 Report
Over the past month the effect of the use of chirped pulses on the laser induced
plasmas on the surface of a copper penny were explored by means of spectral analysis.
For these experiments, the pulse shaper (Dazzler) was used to introduce the desired chirp.
Figs. 1 and 2 show the obtained preliminary results of the effect of chirp on the 521.8 nm
Cu peak. As the chirp varies from negative to positive, an initial increase and then a slight
decrease in peak intensity has been observed on both sides of the transform limited pulse.
These results have confirmed those seen in the literature.1 Future work will include
further exploration of the effects of varying chirp and more complex chirp structure (such
as asymmetric chirp, for example), and the threshold intensity and various energy levels
on LIBS spectra.
In addition to this, the optics table was rearranged to make room for filament
creation and the addition of a Mach-Zehnder interferometer for plasma diagnostics. Fig. 3
shows the newly designed setup for the LIBS beam path as well as that for the new
interferometer. They have been designed to be of equal length and a rail will be used to
adjust the interferometer beam path to allow information to be collected at different times
after the plasma formation. Work during the next few weeks will include the setup of
optics for the interferometer, as they arrive, and the development of a Labview interface
to acquire and analyze the interference patterns generated by the interferometer on a CCD
in real time. As previously discussed, the table has been rearranged to allow for formation
of filaments in air. Because the index of refraction in a medium is related to the intensity
of the beam, non-linear, self-focusing effects occur as the intensity of the beam passes a
certain threshold in air. During the past several weeks, it has been shown that our laser
can be used to successfully generate filaments in air over a ~cm distances. Future work
will explore the conditions necessary for creating and maintaining a filament, production
of longer filaments, and its effects on laser induced plasmas.
Gunaratne, Tissa, and Marcos Dantus. "Influence of Bandwidth and Phase Shaping on Laser Induced
Breakdown Spectroscopy with Ultrashort Laser Pulses." Chemical Physics Letters 423 (2006): 197201. Www.sciencedirect.com. Elsevier, 3 Apr. 2006. Web. 1 Dec. 2011.
1
Figure 1: The effect of chirped pulses on the LIBS spectra of 25%
Cu penny. The dispersion is purely quadratic and given in the units
of fs2.
Figure 2: LIBS 521.8 nm peak intensity as a function of seond order
dispersion for the 521.8 nm Cu peak.
Figure 3: New beam path design incorporating the MachZehnder interferometer into LIBS measurement.