First-principles study of the dynamical and nonlinear optical properties of urea single crystals

Raman and infrared spectra of urea single crystals have been calculated using the density
functional theory. This allowed us to assign the remaining experimental low-frequency phonon
modes. Then, we have determined the sign of the second-harmonic nonlinear optical susceptibility
coefficient in urea to be negative, clarifying a long debate in the literature. Finally, we computed
for the first time the electro-optic coefficients of urea. We found that the electronic and ionic
contributions have a similar order of magnitude and an opposite sign, yielding a smaller value
than that expected, and necessitating further experimental clarifications.