Rotational Anisotropy Second Harmonics Generation Spectroscopy
The setup for Rotational Anisotropy Second Harmonics Generation (RA-SHG) opens the route to determine the symmetry of a crystalline solid, including that of its ordered electronic phases. This is achieved by exploiting nonlinear optical generation, that is connected to both the symmetry of the lattice and of the possible ordered electronic phases. However, a key requirement is that the crystal is non-centrosymmetric. By acquiring the second harmonics as a function of the azimuthal angle of the compound under scrutiny, the symmetry of the second order susceptibility tensor χ2 is determined, which is linked to the symmetry of the lattice and of the electronic properties.
The setup exploits the Coherent RegA laser system, hence sample excitation is performed at 1.55 eV, while the faint second harmonics at 3.1 eV is detected by a photomultiplier tube. Both the polarization of the fundamental (incoming) and second harmonics (outcoming) beams can be controlled automatically, while the measurements is performed by sample rotation in its ab plane. The setup only operates at room temperature.
The figure shows the pattern of the second harmonics of a GaAs (001) crystal, for PS and PP geometries. Here XY indicates the polarization of the incoming (X) and outcoming (Y) beam respectively. All combinations can be set. The polar plot shows lobes at 90°, in agreement with the C4v symmetry of the zincblende crystal lattice structure of GaAs.
