Ultra-wide-band structural slow light

Abstract

The ability of using integrated photonics to scale multiple optical components on a single monolithic chip offers key advantages to create miniature light-controlling chips. Numerous scaled optical components have been already demonstrated. However, present integrated photonic circuits are still rudimentary compared to the complexity of today's electronic circuits. Slow light propagation in nanostructured materials is a key component for realizing chip-integrated photonic devices controlling the relative phase of light and enhancing optical nonlinearities. We present an experimental record high group-index-bandwidth product (GBP) of 0.47 over a 17.7 nm bandwidth in genetically optimized coupled-cavity-waveguides (CCWs) formed by L3 photonic crystal cavities. Our structures were realized in silicon-on-insulator slabs integrating up to 800 coupled cavities, and characterized by transmission, Fourier-space imaging of mode dispersion, and Mach-Zehnder interferometry.

Figure 1

(a) Schematic of the CCW unit cell. The radii of the colored holes are modified (Δr_1,2,3) with respect to the PC bulk holes. The blue holes are also shifted outward (Δx). (b) GME-simulated band structure and (c) corresponding n_g in normalized frequency units (a/λ = ωa/2πc). The solid line corresponds to the guided mode in a representation where the period of the structure is assumed to be L_y. The dashed line indicates the folded band, in an equivalent representation where instead the period is taken as the elementary cell, of length 2L_y, containing the two staggered cavities. The operational bandwidth Δf is marked by the gray region [in panels (b and c)]. The pink region in (c) indicates the region where n_g deviates from a mean value (〈n_g〉 (by less than ±10%. (d) SEM top view image of a 50-CCW. Red arrows indicate the light input and output.

Figure 2

(a) Normalized transmission spectrum of CCWs made up of 50, 100, 200, 400, and 800 PCCs, respectively. (b) Optical image of an 800-CCW when light is propagating from left to right.

Figure 3

(a) Photonic band structure of a CCW made up of 50 PCC_S (50-CCW) measured by FSI overlaid with the GME simulation (white dashed line). (b) Red crosses: experimental n_g calculated from data in (a). Blue continuous curve: fit of the experimental n_g using the TB model. Black dashed curve: prediction of the GME simulation.

Figure 4

(a) Schematic of the optical fiber-based Mach-Zehnder interferometer set up. Each CCW was singly coupled to a left and right on-chip waveguide system (WG). (b) Calculated n_g when a 800-CCW was present in arm-A. (Inset) Change in the relative phase, Δφ(ω) − Δφ(ω₀), with respect to ω₀ = 2πc/(1560.54 nm), as obtained from the MZ fringes when in arm-A there was: no CCW (blue), a 400-CCW (red), and a 800-CCW (black). The slope of the curves decreased when longer CCWs were inserted as the arm length difference decreased.