  We demonstrate a new method to detect ferroelectric domains in inside and
surface regions of organic ferroelectrics by mapping out two orthogonally
polarized terahertz waves radiated from the crystal upon the irradiation of
near-infrared femtosecond laser pulses. We used polarization dependence of the
effective depths radiating the terahertz waves, which originate from the
optical anisotropy in the terahertz frequency region. This allows us to
distinguish ferroelectric domains in the inside and surface regions of the
crystals. We applied this method to a room-temperature organic supramolecular
ferroelectric crystal, 1:1 salt of 5,5'-dimethyl-2,2'-bipyridine and deuterated
iodanilic acid. A single domain covering almost all the area of an as-grown
crystal ($\sim$600 $\mu$m $\times$ 800 $\mu$m) is discerned in the inside
region, while complicated multi-domain in size of $\sim$ 200 $\mu$m is observed
in the surface region. By applying external electric field along the 2c-b axis
(ferroelectric polarization direction), the polarization switching proceeds
with successive propagations of uncharged (neutral) and quasi-one-dimensional
180$^\circ$ domain walls (DWs) along the b-axis ($\perp$ 2c-b axis). This
results in the formation of another uncharged and two-dimensional 180$\circ$ DW
parallel to the (100) plane, which covers all the area of the crystal. We
discuss the usefulness of the present terahertz radiation imaging technique and
ferroelectric DW dynamics in terms of anisotropic stacking of hydrogen-bonded
chains.
