Inverse design of mid-infrared diamond waveguide beam splitter

Diamond is a supreme material for mid-infrared (MIR) integrated photonics as it has a transparency window up to 20 µm that covers the entire fingerprint region. However, its relatively low refractive index poses a challenge in designing an MIR diamond functional device with both small footprint and high transmission efficiency. Here we propose and demonstrate the inverse design of an MIR diamond waveguide beam splitter operating at the wavelength of 15 µm with a small footprint of ∼15 µm × ∼15 µm and a total transmission efficiency above 95%. Our work paves a new avenue for the design of compact and high-efficiency MIR diamond photonic devices.