Photonic crystal nanocavity with a Q-factor of ~9 million.

We have investigated absorption losses due to surface water adsorbed on the surface of silicon heterostructure nanocavities with quality (Q) factors of several million. Measurements performed while changing the ambient humidity that the nanocavity is exposed to show that the Q value depends linearly on humidity. We also reveal that chemical treatment to change the degree of hydrophilicity of the surface results in a drastic increase of Q; we have obtained an experimental value of 9 million, which represents a new record for a heterostructure nanocavity. We analytically determine the absolute value of absorption loss by exploiting the degree of fluctuation of Q values between different samples.