Subfemtosecond determination of transmission delay times for a dielectric mirror (photonic band gap) as a function of the angle of incidence

Using a two-photon interference technique, we measure the delay for single-photon wave packets to be transmitted through a multilayer dielectric mirror, which functions as a ``photonic band-gap'' medium. By varying the angle of incidence, we are able to confirm the behavior predicted by the group delay (stationary-phase approximation), including a variation of the delay time from superluminal to subluminal as the band edge is tuned toward the wavelength of our photons. The agreement with theory is better than 0.5 fs (less than one-quarter of an optical period) except at large angles of incidence. The source of the remaining discrepancy is not yet fully understood.