A subwavelength structured multimode interference coupler for the 3-4 micrometers mid-infrared band

The mid-infrared is attracting increasing attention since many molecules, including potentially hazardous gases such as methane and carbon dioxide, exhibit very specific absorption spectra in this wavelength region. Integrated silicon photonics circuits are envisioned to enable compact and low-cost measurement solutions for these molecules. Multimode interference couplers (MMIs) are basic building blocks for photonic circuits and a broad operational bandwidth is key if flexible operation is to be achieved, e.g. to detect different gases. Here we overcome the bandwidth limitations found in classical MMIs by segmenting the multimode region at a sub-wavelength pitch to engineer its refractive index and dispersion. We achieve less than 0:5 dB imbalance and excess loss in the complete 3 - 4 µm wavelength range. The sub-wavelength MMI not only exhibits nearly threefold improvement in bandwidth, but is also about three times shorter than the conventional device