Time-Resolved Interferometry of Laser-Produced Plasma Randall Schur, Jessica McNutt, Igor Jovanovic The Pennsylvania State University, University Park, PA, 16802, rbs5102@psu.edu 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)