Optical antennas have received considerable attention in recent years due to their unique ability to convert localized energy to freely propagating radiation and vice versa. Sidelobe level (SLL) is one of the most crucial parameters in antenna design. A low SLL is beneficial to minimize the antenna interference with other optical components. Here a plasmonic optical leaky-wave antenna with low SLL is reported. Shifting spatial frequency by periodically modulating the electric-field amplitude in a plasmonic gap waveguide enables a free-space coupled wave out of the antenna. At the same time, precise control of the aperture fields by the modulation depth allows for reducing SLL. Simulation results indicate that the proposed design can achieve a high directivity of 15.8 dB and a low SLL of -20dB at the wavelength of 1550 nm. A low SLL below -15dB is experimentally demonstrated within the wavelength range from 1527 to 1570 nm. In addition, the low-SLL property is further verified by comparing it with a uniformly modulated antenna. By modulating the guided waves in the plasmonic gap waveguide in different forms, the aperture fields can be flexibly arranged to achieve arbitrary wavefront shaping. It bridges the gap between guided and free-space waves and empowers plasmonic integrated devices to control free-space light, thus enabling various free-space functions.

Millimeter-wave (mmWave) band (30–300 GHz) is an emerging spectrum range for wireless communication, short-range radar, and sensor applications. mmWave-optic modulators that could efficiently convert mmWave signals into the optical domain are crucial components for long-haul transmission of mmWave signals through optical networks. At these ultrahigh frequencies, however, the modulation performances are highly sensitive to the transmission line loss as well as the velocity- and impedance-matching conditions, while precise measurements and modeling of these parameters are often non-trivial. Here we present a systematic investigation of the mmWave-optic modulation performances of thin-film lithium niobate modulators through theoretical modeling, electrical verifications, and electro-optic measurements at frequencies up to 325 GHz. Based on our experimentally verified model, we demonstrate thin-film lithium niobate mmWave-optic modulators with a measured 3-dB electro-optic bandwidth of 170 GHz and a 6-dB bandwidth of 295 GHz. The device also shows a low RF half-wave voltage of 7.3 V measured at an ultrahigh modulation frequency of 250 GHz. This work provides a comprehensive guideline for the design and characterization of mmWave-optic modulators and paves the way toward future integrated mmWave photonic systems for beyond-5G communication and radar applications.