Near-Infrared Spectroscopic Cathodoluminescence Imaging Polarimetry on Silicon Photonic Crystal Waveguides

We measure polarization- and wavelength resolved spectra and spatial emission intensity distributions from silicon photonic crystal waveguides in the near-infrared spectral range using spectroscopic cathodoluminescence imaging polarimetry. A 30 keV electron beam, incident along the surface normal of the sample, acts as an ultrabroadband and deeply subwavelength excitation source. For photonic crystal waveguides with a broad range of design parameters, we observe a dominant emission intensity distribution that is strongly confined to the waveguide. For a period of 420 nm and a hole radius of 120 nm, this occurs at a wavelength of 1425 nm. The polarization-resolved measurements demonstrate that this feature is fully linearly polarized along the waveguide axis. Comparing the modal pattern and polarization to calculations of the electric field profiles confirms that we measure the odd TE waveguide mode of the system. This result demonstrates that the electron beam can couple to modes dominated by in-plane field components in addition to the more commonly observed modes dominated by out-of-plane field components. From the emission directionality, we conclude that we sample a leaky portion of the odd waveguide mode.