Optical Characterizations And Time Resolved Spectroscopy
Ultrafast laser pump-and-probe experiments can be used to probe carrier dynamics
in electronic materials. The transient reflectivity and/or transmission
measurement is an important application of the mode locked laser. Such lasers
have been used to determine the carrier diffusion, recombination, and relaxation
time in semiconductors in the past. In this experiment, a femtosecond laser beam
is split into two beams as shown in the figure. One beam that is relatively
strong is used to heat up electrons to a much higher temperature than the
lattice. The pulse can excite valence-band electrons into the conduction band.
The other beam is time delayed with respect to the pump beam and measures the
reflectance change of the sample as it cools. The relaxation of excited
electrons and/or the redistribution of the excess energy initially contained in
these excited carriers is monitored with this time-resolved probe. Theoretical
calculations suggest that both carrier-phonon and carrier-carrier scattering
contribute to the dephasing. The latter also produces a well-defined carrier
temperature on a time scale of approximately 100 to 300 fs. Electron-phonon
scattering also thermalizes the excited carriers with the lattice within a few
hundred femtoseconds. On longer time scales, carrier recombination processes
operate to transfer the excited electrons back to the valence band. At the same
time that these local dynamical processes take place, bipolar diffusion and/or
band-bending induced transport of the excited carriers may occur.
From the transient reflectivity measurements data, one can obtain quantitative information on carrier momentum relaxation and carrier-phonon energy relaxation as qualitatively, the phase-sensitive-detected probe-beam signal is proportional to ΔR/R.
Figure 1: Femtosecond pump-and-probe experimental setup at PI’s lab.
Achievements
Femtosecond pump and probe experimental setup
We have recently completed
this experimental system (Figure 1). The project was partially funded by NSF.
The base of the system is a 10fs Ti sapphire laser. We plan to use the system to
study electron energy relaxation time in TE nanocomposite samples. We can also
consider the effect of interfacial potential barriers on electron cooling,
aiming to extract potential barrier heights from the experimental data, which is
a key parameter affecting the transport properties of the nanocomposite
materials. Across a GB potential since hot electrons (with energies larger than
the Fermi energy, Ef) are thermionically emitted above the
barrier, electron-electron and electron-phonon interactions try to restore the
quasi Fermi distributions by absorbing heat from the lattice, thus cooling the
emitter. The laser pulse excitation of electron gas leads to electron
thermionic emission across the GB potential, hence cooling of the lattice.
Consequently, the lattice-temperature contribution to the reflectivity
variations can be deduced from experimental measurements of
, where n is the
refractive index and TL is
the lattice temperature.
By coating surfaces with metals, we can also extract thermal conductivity and
potentially frequency dependent phonon relaxation time.
Time Resolved Coherent Anti-Stock Raman Spectroscopic (CARS) System
We have recently developed a unique time
resolved coherent anti-stock Raman spectroscopic (CARS) system at Oklahoma
State University in collaboration with EKSPLA Optics (Figure 2). The system is
unique as it combines three laser beams to study the transient response of the
CARS signal. The system has a temporal resolution of 4 ps, which can provide
sufficiently high time and spectral resolutions (< 10 cm-1) for many
practical applications of Raman spectroscopy. The laser wavelength is tunable
over the broad spectral range of 740-4000 cm-1, which is another
unique feature of this system. In addition, a two photon excitation fluorescent
and a second harmonic generation (SHG) microscopes are integrated with the CARS
microscope.