Principal Investigator Nuh Gedik
A technique our lab has been researching to gain exclusive sensitivity to the surface of a material is optical second harmonic geneartion (SHG). In general, the electrical polarization of a material Pi(w) has a dominant component linear in the driving optical field Ej(w) as well as weaker components proportional to higher powers of Ej(w), where ω is the optical frequency and the indices run through three spatial coordinates. Components that contain two powers of Ej(w) are responsible for SHG. For electric dipole processes, the polarization Pi = χ(2)ijkEjEk is obtained from a third rank susceptibility tensor χ(2)ijk that vanishes under inversion symmetry. Therefore dipole induced SHG is forbidden in bulk crystals with inversion symmetry and is only allowed at surfaces or interfaces where inversion symmetry is necessarily broken. By measuring the reflected output as the sample is rotated about its surface normal axis, we obtain patterns like those shown below for Bi2Se3, which reveal the symmetry of the surface electronic polarizability.
The ultrafast pulsed nature of the laser light used to perform SHG naturally lends itself to the study of ultrafast dynamics on the surface of materials. In these types of experiment, a pulse of laser light (pump) is first impinged onto the sample in order to create a non-equilibrium electron distribution. A second time delayed pulse (probe) is then used to monitor the temporal evolution of the SHG signal. The relaxation dynamics of the non-equilibrium distribution can be used to understand the microscopic energy loss mechanisms of the surface electrons.