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.

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.

 Figure 2: Schematic diagram of the time resolved CARS system. A two-photon excitation fluorescent and a Second Harmonic Generation (SHG) microscope are also integrated to this system.

Figure 2: Schematic diagram of the time resolved CARS system. A two-photon excitation fluorescent and a Second Harmonic Generation (SHG) microscope are also integrated to this system.