The Q factor and electric field pattern (radiation pattern) for a cavity made from a donor-type point defect 4 as is illustrated in FIG. 1 were simulated by the FDTD method. The simulation parameters were configured by selecting silicon for the slab 1; and setting approximately 1.55 .mu.m, which is generally used in optical communications, for the wavelength .lamda.; 0.42 .mu.m for the lattice constant a; 0.6a for the slab 1 thickness; and 0.29a for the predetermined sectional radius of the through-holes 2.