Bragg grating filter for hybrid near- and mid-infrared silicon membrane waveguides

The mid-infrared (MIR) wavelength range contains the vibrational and rotational resonances of a wide number of chemical and biological substances. This enabled absorption spectrometers operating in the MIR to achieve... [ view full abstract ]

The mid-infrared (MIR) wavelength range contains the vibrational and rotational resonances of a wide number of chemical and biological substances. This enabled absorption spectrometers operating in the MIR to achieve outstanding specificity and sensitivity. The unique versatility and precision of MIR absorption spectrometers makes them an ideal solution for applications aiming multi-target detection in complex mixtures, such as non-invasive medical diagnostics, food quality control and air pollution monitoring. Unfortunately, the cost and complexity of current bulky implementations limit their use to laboratory environments. The large transparency window of silicon, covering the 1.1 - 8 µm wavelength range, makes it a promising material for the implementation of on-chip MIR photonic circuits with the potential for large-volume and low-cost production. Furthermore, such platform would open new opportunities for the realization of ultra-wideband photonic circuits that combine the near-infrared (NIR) and MIR wavelength ranges. However, the implementation of the silicon-on-insulator (SOI) platform in MIR is restricted by the absorption of buried oxide layer for wavelengths above 4 µm. A promising solution is to locally remove the buried oxide layer to form silicon membrane waveguides that can cover the full transparency window of silicon, yet benefiting from the high quality materials and mature silicon fabrication processes. Still, implementing Si membrane waveguides that exhibit single-mode operation at both NIR and MIR wavelengths is a challenge. Due to the high index contrast between Si and air cladding, conventional strip waveguides with cross-sections large enough to guide a MIR mode are multi-mode in the NIR range. Here we present, for the first time, an integrated Bragg grating waveguide filter for hybrid NIR and MIR operation. A waveguide cross- section of 0.5 µm by 1.1 µm allows single-mode propagation in the MIR, while an original waveguide corrugation approach is used to radiate out the higher order modes in the NIR infrared, yielding effective single-mode NIR operation. Exploiting this concept, we demonstrate a waveguide Bragg grating filter implemented on a single-etch suspended silicon waveguide, yielding a 4 nm wide and 40 dB deep spectral notch.