From Semiconductor to Metal in a Flash: Observing Ultrafast Laser-Induced Phase Transformations
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ABSTRACT We use a new broadband spectroscopic technique to measure ultrafast changes in the dielectric function of a material over the spectral range 1.5-3.5 eV following intense 70-fs laser excitation. The results reveal the nature of the phase transformations which occur in the material following excitation. We studied the response of GaAs and Si. For GaAs, there are three distinct regimes of behavior as the pump fluence is increased - lattice heating, lattice disordering, and a semiconductor-to-metal transition.
INTRODUCTION Ultrashort pulses incident on a material excite a dense plasma is complete well before significant heating occurs; thus the electrons and the lattice are far from equilibrium. Furthermore, the timescale for the excitation, i.e., the pulse duration, is much shorter than that for electronic or thermal diffusion, so that all of the pulse energy is deposited within the optical absorption depth. These features enable improvements in laser processing of materials and micromachining; femtosecond pulses can cause optical damage or ablation with lower pulse energies, create smaller and more clearly defined damage zones, and produce more consistent features from shot to shot [1]. Optical breakdown with femtosecond pulses has been studied in metals, semiconductors, transparent solids, liquids and biological tissue [1]. Femtosecond pulses also permit observation of the dynamics of these laser-induced transitions. Over the past two decades, many researchers have studied ultrafast dynamics in semiconductors following laser excitation of carrier densities between 1016 and 1021 cm- 3 [2]. The first studies of laser-excited semiconductors at high energy density used reflectivity and second harmonic generation and showed evidence of rapid changes in the material (within a few hundred femtoseconds) [3]. However, the nature of the material changes cannot be extracted from the results of these experiments. Recent measurements of the dielectric constant [4] and second-order susceptibility [5], which are fundamental optical properties of the material, have shown that the observed changes result mainly from modifications to the bandstructure. In this paper, we present time-resolved measurements of the dielectric function E(o) of semiconductors across the spectral range from the near-infrared to the ultraviolet. In contrast to single-frequency optical measurements, spectral data for the dielectric function enables us to identify the nature of the changes occurring in a material as a function of time. The paper is organized as follows. First, we describe the experimental technique used to measure the dielectric function spectra. Then we present the material changes observed in GaAs following intense femtosecond laser excitation, as well as preliminary results for Si. The observations add new understanding of the ultrafast electronic and structural dynamics induced by the laser excitation. However, only some aspects of these interesting dynamics will be discussed in this paper; a fuller account will appear elsewhere
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