4.8 Nonlinear Response of Indium Tin Oxide Thin Film Near Bulk Plasma Frequency
119
(a)
(b)
Fig. 4.25 a Schematic illustration of ITO thin film structure used by Alam et al., b permittivity of
the film obtained by ellipsometry
Fig. 4.26 Transient
response according to
two-temperature model
4.8.1 Hot Electron Phenomenon
The extraordinary nonlinear behavior of ITO is based on hot electron phenomenon
and can be explained by the two-temperature model of laser heating. When an intense
beam of ultrashort laser pulses hits a small particle or a small region of the sample,
electrons absorb some part of the energy and rise to very high temperatures, much
higher than the rest of the lattice, so these electrons are called hot electrons. Hot electrons are not in thermal equilibrium with the lattice. They stay in high-temperature
state for a short duration and obey an altered Fermi–Dirac distribution [227]. The
modified Fermi–Dirac distribution results in modified values of permittivity and susceptibilities. They lose the energy within 300–500 fs by electron–phonon interaction
and the thermal equilibrium is achieved. Figure 4.26 shows how the electron temperature elevates with the growth of femtosecond laser pulse intensity. Even after the
decay of the laser pulse, the electrons stay in for a short period of time and gradually
lose energy to finally de-excite down to the lattice temperature and achieve thermal
equilibrium. The red curve represents the input laser pulse, the blue one indicates the
temperature variation of electrons with time, while the green one shows the same for
the lattice. We take a pause to mention here that the graphs shown in Figs. 4.25b and
119
(a)
(b)
Fig. 4.25 a Schematic illustration of ITO thin film structure used by Alam et al., b permittivity of
the film obtained by ellipsometry
Fig. 4.26 Transient
response according to
two-temperature model
4.8.1 Hot Electron Phenomenon
The extraordinary nonlinear behavior of ITO is based on hot electron phenomenon
and can be explained by the two-temperature model of laser heating. When an intense
beam of ultrashort laser pulses hits a small particle or a small region of the sample,
electrons absorb some part of the energy and rise to very high temperatures, much
higher than the rest of the lattice, so these electrons are called hot electrons. Hot electrons are not in thermal equilibrium with the lattice. They stay in high-temperature
state for a short duration and obey an altered Fermi–Dirac distribution [227]. The
modified Fermi–Dirac distribution results in modified values of permittivity and susceptibilities. They lose the energy within 300–500 fs by electron–phonon interaction
and the thermal equilibrium is achieved. Figure 4.26 shows how the electron temperature elevates with the growth of femtosecond laser pulse intensity. Even after the
decay of the laser pulse, the electrons stay in for a short period of time and gradually
lose energy to finally de-excite down to the lattice temperature and achieve thermal
equilibrium. The red curve represents the input laser pulse, the blue one indicates the
temperature variation of electrons with time, while the green one shows the same for
the lattice. We take a pause to mention here that the graphs shown in Figs. 4.25b and