Strong Group Delay Dispersion in 3D Photonic Band Gap Crystals and Planar Microcavities

It is well-known that nanophotonic structures give rise to intricate dispersion relations that strongly differ from those in homogeneous media, for instance, the appearance of band structures. Such band structures reveal forbidden gaps where wave vectors become complex and the slope – the group velocity – is strongly modified to reveal slow light, and a strong alteration of the group velocity dispersion, also known as chirp in optics and effective mass in solid state physics. Control thereof is crucial for applications (in comms and telecom) and for fundamental reasons namely as an alternative probe to local density of states and localization.
Therefore, we have developed an interferometric optical reflectivity microscope to observe the phase sensitive reflectivity of nanophotonic structures with high spatial and spectral resolution over broad frequency ranges from 4000 to 13300 cm-1 (750 to 2500 nm). From the frequency resolved phase we get the group (reflection) delay. On planar microcavities made from GaAs-AlAs, we observe a stopband centred at 1331 nm with a relative bandwidth of 16 %, and large group delays of -750 ± 40 fs at the stopband edges. Analytic transfer matrix theory agrees very well with the experiments, with an RMS difference of only 1.5 %-points.
On the 3D direct woodpile photonic band gap crystal we observe a very broad stopband in the {001} crystal direction that includes the 3D band gap, with high reflectivity. At the band gap edges, we observe for the first time the clear phase jumps resembling those expected from theory, corresponding to a group delay of -76 ± 10 fs at the red band gap edge. Remarkably, this large negative delay is not band-edge slow light, but a Fabry-Pérot resonance. Above the stopband we find a wide region of large positive delay, up to 44 ± 6 fs, which is photonic slow light. Since our structures have obvious 3D properties, yet are thin and highly dispersive, we propose to call them volume metasurfaces. These structures serve as photonic devices to engineer dispersion. Current studies include the development of EM-wave modelling to interpret the large group delay, and studies of structures with lateral structural shifts to obtain extra resonances.