Global optimization of ultrahigh-Q H0 photonic crystal nanocavity in silicon

The design of photonic crystal cavities has always presented a considerable challenge over the past two decades. In this work, a global optimization technique has been used to maximize the Q / V ratio of a simple H0 type nanocavity. The optimization is carried out by allowing for shifts of several neighbouring holes along the x-and y-axes. The objective function of the evolutionary optimization was the cavity Q-factor, while reasonable restrictions were imposed on the magnitudes of the shifts in order to limit the variations in modal volume. The fabricated devices were coupled using a side-coupled scheme involving a photonic crystal W1 waveguide and the Q-factors measured. A maximum value of 418´000 was measured along with a modal volume (computed) of 0.34(λ/n)³ enabling an extremely high Q /V ratio of 1.2 million, which has also resulted in the onset of nonlinear optical effects including optical bistability at μW input powers.