High Intensity Picosecond Photoexitation of Semiconductors
In the past half-decade, studies of the optical properties of high-density electron-hole plasmas, generated in undoped semicon ductors by the direct absorption of intense, ultrashort pulses from mode-locked lasers, have provided direct information concerning ultrafast electronic processes (the reader is referred to a previous chapter by Smirl, which reviews these studies, and to the Bibliography accompanying this paper). Generally, early experimental studies in this area employed mode-locked pulses, obtained from a Nd-glass laser, as an excitation source to generate the electron-hole plasma. This source produces optical pulses that are approximately 10 psec in duration and that often have peak powers in excess of 108 watts at a wavelength of 1.06 μm. These pulses when focused on the surface of a thin semiconductor sample can produce a measured irradiance of 10-2J/cm2. Direct absorption of such an optical pulse can create carier densities exceeding 1020cm-3. Germanium was chosen as a candidate for study, in many of these early investigations, primarily because it is a readily-available, well-characterized semiconductor whose bandgap energy is comparable to but less than the energy of a photon at a wavelength of 1.06 μm (1.17 eV).
KeywordsCarrier Density Excitation Pulse Optical Pulse Probe Transmission Probe Pulse
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