  Photonic molecules, named by analogy with chemical molecules, are clusters of
closely located electromagnetically interacting microcavities or "photonic
atoms". As two or several microcavities are brought close together, their
optical modes interact, and a rich spectrum of photonic molecule supermodes
emerges, which depends both on geometrical and material properties of
individual cavities and on their mutual interactions. Here, we discuss ways of
controllable manipulation of photonic molecule supermodes, which improve or add
new functionalities to microcavity-based optical components. We present several
optimally-tuned photonic molecule designs for lowering thresholds of
semiconductor microlasers, producing directional light emission, enhancing
sensitivity of microcavity-based bio(chemical)sensors, and optimizing
electromagnetic energy transfer around bends of coupled-cavity waveguides.
Photonic molecules composed of identical microcavities as well as of
microcavities with various degrees of size or material detuning are discussed.
Microwave experiments on scaled photonic molecule structures are currently
under way to confirm our theoretical predictions.
