Optical Properties of Picosecond Laser Irradiated Graphite

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OPTICAL PROPERTIES OF PICOSECOND LASER IRRADIATED GRAPHITE A.M.MALVEZZI,G.REVERBERI AND N. BLOEMBERGEN Division of Applied Sciences, Harvard University, 02138, USA

Cambridge,

MA

ABSTRACT We have employed pump-and-probe techniques coupled to ellipsometry to measure the transient complex index of refraction at 1.064 jim of higly oriented pyrolitic graphite (HOPG) interacting with 20 ps, .532 gm laser pulses. When the laser pump fluence exceeds the threshold value for melting, measurements indicate a substantial decrease of both real and imaginary parts of the index of refraction, thus confirming that molten graphite becomes less metallic. Measurements provide also direct evidence of the insensitivity of our picosecond results to evaporation from the irradiated surface. INTRODUCTION High temperature phases of laser heated materials have recently been studied with several techniques. In particular, post irradiation diagnostics of nanosecond irradiated samples with optical and electron microscopy, Raman scattering, Rutherford backscattering etc. have determined the existence of a liquid state at high temperature in layered compounds such as graphite [1,2] . In our laboratory, instead, picosecond time resolved optical techniques have been used to investigate directly Highly Oriented Pyrolitic Graphite (HOPG) samples during the irradiation with 0.53 im, 20 ps laser pulses. The advantage of this ultrashort excitation is that high surface temperatures are obtained with minimum irradiation energy, while optical probing at short times avoids effects connected with the evaporation from the sample surface [3]. Using time resolved pump and probe techniques, we have previously reported a net decrease of normal incidence reflectivity for some selected wavelengths in a range extending from the IR to the UV, when the laser excitation exceeded a given threshold fluence level [4,5] . The observations were consistent with a decrease, at high temperature, of the complex index of refraction and thus of the dielectric function of HOPG, indicating a less metallic behavior of the material. In order to evaluate quantitatively the transient complex index of refraction nt = n + i k , two simultaneous measurements must be performed on the sample under identical conditions. Since HOPG is highly absorbing, the easiest way is to measure the reflectivity of the sample at two different (orthogonal) polarizations at various angles of incidence (6] . This paper reports on the application of this ellipsometric technique to the case of graphite. The complex index of refraction of HOPG above the melting point has been deduced experimentally for a wavelength of 1.06 jm. The qualitative results previously obtained [4] are confirmed.

Met. Res. Soc. Symp. Proc. Vol. 100. 91988 Materials Research Society

484

EXPERIMENTAL METHOD Our experimental setup consisted of a modified version of the usual pump and probe scheme. HOPG samples, with the c-axis perpendicular to the surface, were irradiated with 0.53 jm, 20 ps laser pulses. A second pulse at 1.06 jm,

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