An Ultra‐Efficient Integrated Plasmonic Lithium Niobate Electro‐Optic Mach‐Zehnder Modulator

Electro‐optic (EO) Mach‐Zehnder modulators (MZMs) featuring high modulation efficiency and wide bandwidth are essential for large‑capacity optical communication systems. To date, thin‐film lithium niobate (TFLN) MZMs have emerged as a promising solution owing to the exceptional EO bandwidth and compactness, and thus significantly reduced capacitance. However, integrated TFLN MZMs are still several millimeters to centimeters in path length due to the limited modulation efficiency determined by the suboptimal confinement of the electric and optical fields. This limitation hinders large‐scale integration for parallelization or multiplexing and prevents the economically efficient co‐integration with compact electronics. Here, this challenge is overcome by merging sub‐wavelength plasmonic slot waveguides with the TFLN platform to form a strong field confinement below the diffraction limit of optics, and enhance EO overlap and light‐matter interactions. A record‐high modulation efficiency of 0.070 V cm is demonstrated with an ultra‐short length of 15 µm. This plasmonic TFLN MZM exhibits a 3‐dB EO bandwidth exceeding 110 GHz that allows for transmitting 110 Gbaud binary phase‐shift keying signals with a bit error ratio of 2.5 × 10 −5 . This demonstrated plasmonic TFLN MZM provides a promising solution to future ultra‐high‐speed and large‐scale photonic integrated systems for optical interconnections, optical computing, and optical sensing functions.