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Time-Resolved Interferometry of Laser-Produced Plasma
Randall Schur, Jessica McNutt, Igor Jovanovic
The Pennsylvania State University, University Park, PA, 16802, [email protected]
INTRODUCTION
Laser-induced breakdown spectroscopy is a wellknown technique for rapid, in situ analysis of materials. It
is a promising approach for standoff detection of
potentially hazardous or difficult to access nuclear
materials. LIBS employs an intense laser pulse to
generate a plasma on the surface of a target. The emitted
light is then collected and analyzed to determine that
material’s elemental, and in some cases isotopic,
composition. Previous research has shown that the
characteristics of the pulse and the sample’s immediate
environment can directly influence the resolution of the
signal and the effectiveness of the technique [1,2]. To
further study these effects, we have set up a Mach-Zender
interferometer, which uses a femtosecond pulsed laser
probe to study the temporal evolution of the laser-induced
plasma under a variety of conditions using copper and
uranium samples. It is our hope that the use of
interferometry in LIBS will provide us with a better
understanding of how ultra-short laser induced plasmas
form on the surface of various materials. This information
will, in turn, help to improve the resolution and
effectiveness of laser-induced breakdown spectroscopy as
a technique for nuclear material detection.
symmetry, the radial refractive index distribution can be
expressed as a function of the first derivative of the phase
distribution using the inverse Abel equation:
𝑑𝜙(𝑥)
𝜆 𝑅
𝑑𝑥
𝑛(𝑟) − 𝑛0 = − ∫
𝑑𝑥
𝜋 𝑟 √𝑥 2 − 𝑦 2
(1)
with 𝜆 being the wavelength of the probe laser. The
inverse Abel equation can be discretized using a variation
on the Henkel-Fourier method [4]. For sufficiently high
plasma densities, the distribution of heavy particles in the
plasma can be ignored and the refractive index can be
related directly to the electron density by the following
expression [5]:
𝑁𝑒 =
8𝜋𝜀0 𝑚𝑒 𝑐 2
(𝑛(𝜆) − 1)
𝑒02 𝜆2
(2)
By calculating the electron density distribution at
different times, we can develop a picture of plasma
evolution.
DESCRIPTION OF THE ACTUAL WORK
Experimental Setup
A diagram of the LIBS and interferometer setup
is shown in Fig. 1. Our Ti:sapphire laser system has a
pulse duration of ~45 fs and a frequency of 10 Hz. The
laser pulse is split into the LIBS and the interferometer
pulse using an 80/20 beam splitter. The two arms of the
interferometer (shown in green) are separated and
recombined using two 50/50 beam splitters. A delay line
is employed to extend the path length of the
interferometer relative to the LIBS path. This provides the
capability to take ‘snapshots’ of the plasma temporal
evolution in the time span of 6 ns.
Data analysis
The spatial phase distribution of the
reconstructed interferogram can be extracted using a fast
Fourier transform technique [3]. By assuming axial
Fig. 1. Schematic diagram of the experimental setup
combining laser induced breakdown spectroscopy (LIBS)
with interferometry.
FUTURE WORK
The potential of this system to improve upon the
understanding of laser-induced plasma formation is
significant. Future work will include studying the effects
of varying ambient pressures on the plasma density and
LIBS spectra. Additionally, expanding on recent work by
Prasad et. al. [6], we plan to vary the shape of the probe
pulse in an attempt to improve the resolution of the
interferometer. Finally, we will explore plasma formation
on the surface of a recently acquired sample of naturally
enriched uranium, which is of interest for nuclear material
detection.
CONCLUSION
A system has been designed to study the
temporal evolution of laser-induced plasmas using
interferometry
and
laser-induced
breakdown
spectroscopy. LIBS provides information on the
elemental composition of a material with little sample
preparation, and has the potential to become a portable,
standoff detection system. The interferometer provides
further information regarding density and distribution of
the plasma, rather than an indirect analysis of the plasma
via spectral analysis.
Further, the setup can be used to study the effects
of varying environmental parameters and the composition
of the target sample on plasma formation. The results of
these experiments can be used to improve on the
technique of laser induced breakdown spectroscopy.
REFERENCES
1. He X.N., Hu W., Li C.M., Guo L.B., Lu Y.F.;
“Generation of high-temperature and low-density plasmas
for improved spectral resolutions in laser-induced
breakdown spectroscopy.” Optics Express, 19, (2011)
2. Guillermin M., Liebig C., Garrelie F., Stoian R., Loir
A.S., Audouard E.; “Adaptive control of femtosecond
laser ablation plasma emission”, Applied Surface Science,
255, pg. 5163-5166 (2009)
3. Takeda Misuo; Ina Hideki, Kobayashi Seiji; “FourierTransform Method of Fringe-Pattern Analysis For
Computer-Based Topogrophy and Interferometry.”
Journal of the Optical Society of America, 72, pg. 156160 (1982)
4. Alvarez R.;Rodero A; Quintero M.C., “An Abel
Inversion Method for Radially Resolved Measurements in
the Axial Injection Torch.” Spectrochimica Act A Part
B.,57, Pg. 1665 – 1680 (2002).
5.Hongchao Zhang; Jian Lu; Xiaowu Ni, “2D
Reconstruction of Laser Plasma Electron Density with
FFT Method,” Proc. Computer Science and Software
Engineering 2008 International Conference, Wuhan,
Hubei, China, December 12 -14, 2008.
6. Prassad Y. B. S. R., Barnawal S., Naik P.A., Chakera J.
A., Gupta P.D.; “Chirped pulse interferometry for time
resolved density and velocity measurements of laser
produced plasmas,” Journal of Applied Physics, 110,
(2011)