  We study the coupling between photonic molecules and waveguides in photonic
crystal slab structures using finite-difference time-domain method and coupled
mode theory. In a photonic molecule with two cavities, the coupling of cavity
modes results in two super-modes with symmetric and anti-symmetric field
distributions. When two super-modes are excited simultaneously, the energy of
electric field oscillates between the two cavities. To excite and probe the
energy oscillation, we integrate photonic molecule with two photonic crystal
waveguides. In coupled structure, we find that the quality factors of two
super-modes might be different because of different field distributions of
super-modes. After optimizing the radii of air holes between two cavities of
photonic molecule, nearly equal quality factors of two super-modes are
achieved, and coupling strengths between the waveguide modes and two
super-modes are almost the same. In this case, complete energy oscillations
between two cavities can be obtained with a pumping source in one waveguide,
which can be read out by another waveguide. Finally, we demonstrate that the
designed structure can be used for ultrafast optical switching with a time
scale of a few picoseconds.
