544 Examples of Ultrafast Processes in Matter
11.4. ULTRAFAST PROCESSES IN
SOLID-STATE MATERIALS
11.4.1. Excitation Across the Band Gap
Femtosecond techniques made it possible to resolve fundamental interaction
mechanisms and times in solids at room temperature. These processes are of
tremendous importance. For instance, they determine physical limits for speed
and miniaturization in semiconductor devices. Figure 11.7 illustrates essential
processes in semiconductors, following optical excitation above the band gap.
For a comprehensive review of ultrafast processes in semiconductors probed by
laser pulses, see Shah [3].
An ultrashort light pulse of frequency ω
creates electron–hole pairs in states
above the band gap. Their mean excess energy is E = (ω
−ω
gap
), and their
initial energy distribution resembles the excitation spectrum. With large excited
carrier densities, mainly carrier–carrier scattering leads to a thermalization within
the -valley without changing the mean carrier energy. This means that some car-
riers scatter out of their initial states, so that the distribution of occupied states
becomes broader. Such processes are generally associated with momentum relax-
ation and are responsible for the dephasing of the polarization. Corresponding
T
2
times can be measured by means of photon echo experiments as described in
the previous chapter. The temperature that can be attributed to the thermalized
Lattice
CB
VB
Phonon
excitation
Thermalization
Intervalley
scattering
Excitation
Exciton
E
b
E
gap
Figure 11.7 Simplified diagram of ultrafast processes occurring after above band gap excitation in
semiconductors.
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