Fig. 4: Polarization dependence of the FWM process.
From: Phonon-enhanced nonlinearities in hexagonal boron nitride
a Pump and probe polarization dependence of total FWM emission (normalized). White dashed lines indicate ZZ axes and are included as a guide for the eye and to emphasize the 60o periodicity. b Linecuts along the ZZ and AC axes from (a). Solid black lines are fit from Eq. 1, with \(\alpha\) and \(\beta\) as fit parameters. c TDDFT-computed harmonic generation spectra for pump and probe pulses co-polarized along the ZZ (blue) and AC (red) directions. The TO (E1u) phonon present along the ZZ axes leads to the greatest nonlinearity.
