Active Inverse-Designed WDM Integrated with Surface Emitting THz Quantum Cascade Laser

Over the past decades there has been growing interest in photonic integration for THz systems for telecommunications, imaging and sensing [1], [2]. We recently developed an integrated THz photonic platform [3] leveraging a double metal waveguide configuration for integration of several active and passive THz photonic components, from broadband coherent sources also acting as frequency combs [4], [5], to ultrafast detectors [6]. Here, we present a compact integrated wavelength division multiplexer (WDM), an essential component to fully exploit the highly coherent comb sources, allowing signal manipulation and routing directly on-chip. A remarkable property of our WDM is that it is an active device with gain, allowing for simultaneous spectral selection and amplification on the same footprint. The device design was obtained with an inverse design algorithm for topology optimization.