On-chip, inverse-designed active wavelength division multiplexer at THz frequencies

The development of photonic integrated components for the terahertz region has become an active and growing research field. Despite the numerous applications in this spectral range, hardware design still faces several challenges. We demonstrate an on-chip, active wavelength division multiplexer (WDM) operating at THz frequencies (> 1 THz). The WDM architecture is based on an inverse design topology optimization applied to an active quantum cascade heterostructure embedded in a double metal cavity and planarized with a polymer. Such an approach enables the fabrication of a strongly subwavelength device, with a normalized volume of V/λ³ ≃ 0.5. The WDM input is coupled to an integrated THz quantum cascade laser (QCL) frequency comb, providing three 330 GHz broadband output ports, ranging from 2.2 THz-3.2 THz, with a maximum crosstalk of -6 dB. The three ports are outcoupled via integrated broadband patch array antennas enabling surface emission. Such a device can also operate as a stand-alone element, unlocking advanced on-chip signal processing in the THz range.