  Periodical corrugation in dielectric slab transfers the two waveguide modes
at zero Bloch wave number into a leaky resonant mode and a symmetry protected
bound states in the continuum (BIC) with small frequency detune. The leaky
resonant mode can be directly excited by weak linearly polarized normally
incident optical field. In the presence of Kerr nonlinearity, the BIC can be
indirectly excited by an optical bistable response. Two types of bistable
operations are considered. For the first type, the intensity of the incident
field gradually increases to exceed a critical value, and then decreases to
zero. For the second type, the intensity is fixed, while the linear
polarization angle of the incident field gradually increases to exceed a
critical value, and then decreases to 0$^{o}$. Theoretically, the indirectly
excited BIC can store the optical energy without loss, even though the
intensity of the incident field decreases to zero. Incidence of an optical
field with double frequency or orthogonal linear polarization can erase the
stored optical field by destroying the BIC. The proposed optical system could
function as optical storage and switching device.
