Visualization of ferroelectric domains in boracite using emission of terahertz radiation

We report on the emission of terahertz radiation by irradiation of femtosecond laser pulses in non-centrosymmetric paraelectric and ferroelectric phases of Co${}_3$B${}_7$O${}_{13}$I boracite. The Generation of the terahertz waves in both phases is caused by optical rectification via a second-order nonlinear optical effect. In the ferroelectric phase, we successfully visualized ferroelectric domains by analyzing the polarization state of the terahertz wave radiated from the crystal. In a large area of the crystal ($\sim$ 500 $\times$ 500 $\mu$m${}^2$), the observed polarization vector of the radiated terahertz wave was tilted from directions of spontaneous polarization, i.e., [100]${\rm cub}$,[010]${\rm cub}$,and[001]$_{\rm cub}$ in cubic setting, which can be explained by the presence of a ferroelectric 90${}^{\circ }$ domain wall of the (101)${}_{\mathrm{c}\mathrm{u}\mathrm{b}}$ plane.