Far-field investigation of slow-light propagating below the light cone in planar photonic structures

The propagation of light in photonic structures is governed by the dispersion curve omega(k) relating the frequency omega to the spatial wave vector k. Engineering the slope of the dispersion curve allows to implement innovative concepts such as slow light in coupled resonators, with surface plasmons or in photonic crystal structures. In this context, some important regions of the dispersion curves lie outside of the light cone, implying that the associated modes do not radiate into free space and as a result propagate with minimal optical losses. In this work, we demonstrate that far-field optical experiments can be used to accurately extract the dispersion curve of a slow mode propagating below the light cone in a planar nanophotonic waveguide. The combination of an end-fire set-up working in the 1.5 mum range, generally used to measure the transmission through the waveguide, along with a high numerical aperture Fourier space imaging set-up and an integrated linear probe grating (LPG), allows a fine spectral analysis of each mode of the photonic structure. As an example, we measured the dispersion curve of a specially designed single defect photonic crystal (PhC) waveguide (SW1). The PhC waveguide has been engineered to operate in a slow-light regime over a large frequency bandwidth, by changing the diameter of the first and second row of holes in a conventional W1 PhC waveguide.