Anti-reflection subwavelength gratings for waveguide facets in the mid-infrared

Mid-infrared (IR) spectroscopy is a universal way to identify chemical and biological substances. Indeed, several molecules, such as carbon dioxide or ammoniac, have their fingerprint region in the mid-infrared wavelength range. Silicon (Si) photonics pave the way for making on-chip midinfrared spectroscopy. Silicon germanium (SiGe) photonic circuits are especially promising thanks to the transparency of Ge in the mid-IR. Generally speaking, edge coupling from air to to standard SiGe-based waveguides is expected to suffer facet reflection of around 33 %, due to the high index contrast between Ge and air. This is responsible for Fabry Perot oscillations that can be detrimental during device operation. In this work, a graded index SiGe platform is used and an anti-reflective subwavelength (SWG) grating for waveguide facets in the mid-infrared is demonstrated. Waveguides with SWG-structured facets are fabricated and measured. Reflectivity down to 6 % with an average 3 dB transmission increase in TM mode and 1 dB transmission gain increase in TE mode are obtained at 8 μm wavelength for a 3 μm-long SWG structured facet. This result paves the way for several applications, such as Kerr effect or self-injection locking in the mid-infrared.