Antennas are one of the most impactful inventions in modern technology. From the pioneering work of Heinrich Hertz in the late 19th century, who first demonstrated radio wave transmission, antennas have evolved into indispensable tools for communication, sensing, and space exploration.
Traditional antenna systems capture electromagnetic fields and convert them into electrical signals for further processing. Over time, as wireless standards pushed into higher frequency domains, especially in the mm-wave and sub-THz ranges, the limitations of electronics became evident: noise, distortions, and power inefficiencies restrict system performance.
Plasmonic modulator antennas (PMAs) represent a breakthrough by merging optical and RF domains directly. Instead of converting RF signals into electrical currents and then modulating them onto light, PMAs perform direct electromagnetic-to-optical conversion within the plasmonic slot – a nanoscale structure where electromagnetic fields are locally enhanced by several orders of magnitude.
Schematic of an on-chip plasmonic modulator antenna (PMA).
The principle of operation is based on capturing an incident mm-wave signal via a miniature antenna that couples the field directly into the plasmonic modulator. The key is the plasmonic slot waveguide: a nanogap structure where the RF field is compressed, achieving ultra-high field confinement and overlap with the optical mode.
This geometry enables efficient electro-optic interaction without the need for an intermediate electronic amplifier. In mathematical terms, the conversion efficiency is determined by the overlap integral between the optical mode and the plasmonic-enhanced RF field, normalized by the antenna aperture gain.
Where and represent the optical and RF field distributions in the slot. The result is a direct RF-to-optical conversion with minimal added noise.
The absence of active electronic stages makes PMAs a fully passive solution – a decisive advantage for scaling to frequencies beyond 100 GHz.
Applications of Plasmonic Modulator Antennas
— Wireless Communication (5G/6G):
PMAs provide a path to integrate antennas and modulators for backhaul, fronthaul, and small-cell transceivers operating in the 100–300 GHz range. They allow direct radio-over-fiber (RoF) transmission, removing bottlenecks of electronic up- and down-conversion.
— Space and Free-Space Optical Communication:
Satellite-to-ground and inter-satellite communication links demand terabit-per-second (Tbit/s) capacity. PMAs enable free-space optical (FSO) links by directly translating high-frequency electromagnetic radiation into optical carriers for coherent long-distance transmission.
— Field Sensing:
With their ability to capture wideband electromagnetic fields, PMAs are suited for near-field and far-field sensing in security, industrial, and scientific applications. Their noise-free operation makes them attractive for radio astronomy, where receiver sensitivity is paramount.
— Radio-over-Fiber (RoF):
By bypassing electronics, PMAs deliver ultra-fast and low-distortion RoF transmission, ideal for transporting RF signals to baseband converters in central office locations.
PMAs in Practice
Recent experimental demonstrations highlight their potential:
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Transmission rates exceeding 80 Gbit/s were reported by H. Ibili et al. (ETH Zurich).
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Optical antennas are foreseen for sub-THz-to-optical receivers in future passive RoF systems.
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Free-space optical demonstrations achieved Tbit/s satellite feeder links, pushing beyond classical electronic limits.
PMAs in Practice – From Millimeter Waves to Light
