The possibility of using integrated photonics to scale multiple optical components on a single monolithic chip offers transformative advantages in fields such as communications, computing, bioengineering, and sensing. However, today’s integrated photonic circuits are rudimentary compared to the complexity of modern electronic circuits. Any advancements to efficiently integrate new photonic functionalities bring us closer to replicate the enormous impact of electronic integrated circuits.
Slow light propagation in chip-integrated nanophotonic structures with engineered band dispersion is a highly promising approach for controlling the relative phase of light and for enhancing optical nonlinearities on a chip. A primary goal in this field is to achieve devices with large, approximately constant group index (n_g) over the largest possible bandwidth, thereby enabling multimode and pulsed operation. We present an experimental record high group-index-bandwidth product (GBP) in genetically optimized coupled-cavity-waveguides (CCWs) designed by L3 photonic crystal nanocavities. The resulting designs were realized in SOI buckling-free suspended slabs with CCWs integrating up to 800 coupled nanocavities. The samples were characterized by measuring the CCW transmission, the mode dispersion through Fourier-space imaging, and ng via Mach-Zehnder interferometry. Various nanocavity designs were investigated, with theoretical n_g ranging from 37 to 100. Record high GBP = 0.47 was demonstrated over a bandwidth of 19.5 nm with a homogeneous flat-top transmission profile (variations lower than 10 dB) and losses below 56 dB/ns. Our results open the path towards building enhanced slow-light-based devices such as of slow-light-enhanced spectroscopic interferometers and single-photon buffers.