Monolithic Terahertz Topological Mach-Zehnder Interferometer.

The pursuit of on-chip electromagnetic wave control for high throughput communication, spectroscopy, and quantum computing underlies the motivation for terahertz photonic integrated circuitry. Recent breakthroughs in topological photonics have enabled the development of chips that harness topologically resilient interface modes to achieve area and performance efficiency. However, the demonstration of a compact, monolithic topological Mach-Zehnder interferometer (MZI), remains a critical gap. In this work, a terahertz topological MZI is presented, exhibiting six interferometric fringes across a 17 GHz bandwidth, with splitter, combiner, and arms integrated on a single chip. The interferometric fringes exhibiting high on-off contrast with extinction ratios over 20 dB are achieved by utilizing tailored unit cells that facilitate interface-dependent out-of-plane radiation losses. Active tuning of the MZI response is also demonstrated through photoexcitation of the MZI arms. The presented approach is the first step toward realizing topological photonic modulators that leverage the phase degree of freedom for photonic integrated communication and quantum information processing.