Phonon Modes of Organic Electro-Optic Molecular Crystals for Terahertz Photonics

We investigate theoretically and
experimentally the phonon modes
of organic electro-optic crystals for THz-wave photonics. THz phonon
modes significantly influence the characteristics of THz-wave generation
and detection through the THz absorption and refractive-index dispersion.
For a model as THz generation material, electro-optic PNP (2-(N-prolinol)-5-nitropyridine) molecular crystal containing
a relatively small numbers of atoms in the unit cell is chosen for
phonon-mode calculation. THz phonon modes with their frequencies and
intensities are calculated by periodic density functional theory,
and the resulting spectrum exhibits a good agreement with the experimental
absorption spectrum measured by THz time-domain spectroscopy. THz-wave
generation experiment is performed with PNP as THz generator and the
organic electro-optic OH1 (2-(3-(4-hydroxystyryl)-5,5-dimethylcyclohex-2-enylidene)­malononitrile)
crystal as THz detector. Considering the phase matching condition
and THz absorption, which are both influenced by the THz phonon modes
of THz generation and detection materials, the calculated THz generation
spectrum also matches well with the experimental THz spectrum obtained
by optical rectification at the pump wavelength of 1300 nm. Therefore,
the phonon mode identification is very important for optimizing and
designing new THz source and detection materials as well as for THz
spectroscopy.