Femtosecond Time Resolved Surface Structural Dynamics of Optically Excited Silicon
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FENTOSECOND TIME RESOLVED SURFACE STRUCTURAL DYNAMICS OF OPTICALLY EXCITED SILICON
C. V. Shank,* R. Yen,** and C. Hirlimann*** *Bell Laboratories, Holmdel, New Jersey 07733 **Bell Laboratories, Murray Hill, New Jersey 07974 ***Universite Pierre et Marie Curie, Paris, France
ABSTRACT Laser processing of materials has raised a number of interesting issues relating to phase transitions and the structure of optically excited semiconductors. In this paper we describe the dynamics of structural changes that take place on a silicon surface following excitation with an intense optical pulse. Second harmonic generation from the silicon surface is used as a tool to measure crystalline order with 90 femtosecond resolution. The three-fold rotational symmetry of the silicon surface is observed to become rotationally isotropic within a picosecond after excitation consistent with a transition from the crystalline to the liquid molten state. INTRODUCTION The properties of Si excited with short intense optical pulses ha been the subject of extensive experimentalll-4]and theoretical discussion.[5] Previously, we reported the use of 90 femtosecond optical pulses to excite 4 and probe the reflectivity of optically excited Si.i ] We observed and time resolved the rapid appearance of an increase in reflectivity of the Si surface following excitation. In the work reported here, we seek to investigate the dynamics of structural changes following excitation with intense pulses. Unlike reflectivity, the process of second-harmonic generation is governed by a tensor quantity that can contain elements of the crystal symmetry. In the experiments described here, we use a 90 fs pulse to optically excite a silicon surface and then measure the second-harmonic generation from a weaker probe pulse that is delayed in time, in order to measure the time evolution of the crystalline order following excitation. Determination of the structural dynamics will assist in elucidating the nature of phase transitions of Si induced by intense optical pulses.fl-3, 6 ]
SURFACE SECOND HARMONIC GENERATION Second-harmonic generation from the surface of a medium with inversion symmetry, such as silicon, was first reported by Bloembergen and Chang.[7] In general, the nonlinear response of a medium with a center of inversion symmetry requires the consideration of higher-order non-local contributions to the susceptibility. For a medium with cubic symmetry we can write the ith component of the nonlinear source polarization as[8,9] pNL(2w)=(&-5[E(w)-V]E i(w)+ýE i[V.1(w)]+(2iw/c)y[t(w)xA(w)] i +E i(W)V iE i(W)
(I) where 6, B, y, and E are frequency-dependent constants. The first three terms give rise to an isotropic contribution to the second-harmonic Mat. Rs. Soc.
Symp.Proc. Vol. 23 (1984)@ Elsevier science Publishing Co.,
Inc.
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polarization and the last term provides an anisotropic contribution which is determined by the crystal symmetry. Recently. second harmonic generation from the surface of silicon has been measuredilOllland described theoretically. Heinz, et
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