Sub-terahertz plasmonic lithium niobate phase modulator

Ultrahigh-bandwidth, high-stability, and ultra-compact electro-optic (EO) modulators are the key components for next-generation high-speed optical communications and computing systems. Surface plasmon polaritons (SPPs) have attracted much attention for reducing the device size and accelerating the device response owing to their nanoscale field confinement below the diffraction limit. However, existing plasmonic EO modulators still have challenges in either bandwidth or stability due to the frequency-dependent permittivity or their limited thermal and photochemical stability. These limitations lead to a modulation efficiency drop-off at low frequencies due to the decrease in dielectric constant and obstruct reliable long-term operation. Here, by integrating SPPs with the lithium niobate (LN) platform, we develop an EO phase modulator with ultrahigh bandwidth, high thermal stability, and an ultra-compact footprint. This plasmonic LN phase modulator exhibits a measured ultra-flat bandwidth exceeding 110 GHz with an ultralow half-wave voltage length product of 0.081 V cm and a compact modulation length of 15 μm. Further theoretical analysis validates a calculated 3-dB bandwidth of 184 GHz and indicates a potential 3-dB bandwidth beyond 760 GHz through optimizing the electrode pad size. Moreover, under a thermal shock of 260 °C, the variation of the EO modulation response is below 0.2 dB, evidencing excellent thermal reliability. This study offers a new solution for photonic integrated circuits that demand ultrahigh bandwidth, long-term reliability, and extreme integration density, with potential applications in next-generation high-speed optical communications.