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1. Ultrafast modulation of terahertz waves using on-chip dual-layer near-field coupling

   https://doi.org/10.1364/OPTICA.469461  

   Zhang, Yaxin; Ding, Kesen; Zeng, Hongxin; Kou, Wei; Zhou, Tianchi; Zhou, Hongji; Gong, Sen; Zhang, Ting; Wang, Lan; Liang, Shixiong; et al (November 2022, Optica)

   As a key potential component of future sixth-generation (6G) communication systems, terahertz (THz) technology has received much attention in recent years. However, a lack of effective high-speed direct modulation of THz waves has limited the development of THz communication technology. Currently, most high-speed modulators are based on photonic systems that can modulate electromagnetic waves with high speed using sophisticated optoelectronic conversion techniques. Yet, they usually suffer from low conversion efficiency of light to the THz range, resulting in low output power of the modulated THz waves. Here, we describe a guided-wave modulator for THz signals whose performance nearly matches that of existing in-line fiber-optic modulators. Our results demonstrate a maximum modulation depth greater than 20 dB (99%) and a maximum sinusoidal modulation speed of more than 30 GHz, with an insertion loss around 7 dB. We demonstrate the capabilities of this modulator in a point-to-point communication link with a 25 Gbit/s modulation speed. Our modulator design, based on near-field coupling of a THz transmission line to a single resonant meta-element, represents a powerful improvement for on-chip integrated high-performance THz devices. 

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2. Graphene metasurfaces for terahertz wavefront shaping and light emission [Invited]

   https://doi.org/10.1364/OME.473110  

   Li, Yuyu; Krisshnamurthi, Mathan_Ramaswamy; Luo, Weijun; Swan, Anna_K; Ling, Xi; Paiella, Roberto (November 2022, Optical Materials Express)

   Graphene is a promising materials platform for metasurface flat optics at terahertz wavelengths, with the important advantage of active tunability. Here we review recent work aimed at the development of tunable graphene metasurfaces for THz wavefront shaping (including beam-steering metamirrors and metalenses) and light emission. Various design strategies for the constituent meta-units are presented, ranging from metallic phase-shifting elements combined with a nearby graphene sheet for active tuning to graphene plasmonic resonators providing the required phase control or radiation mechanism. The key challenge in the development of these devices, related to the limited radiative coupling of graphene plasmonic excitations, is discussed in detail together with recently proposed solutions. The resulting metasurface technology can be expected to have a far-reaching impact on a wide range of device applications for THz imaging, sensing, and future wireless communications. 

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3. Programmable plasmonic phase modulation of free-space wavefronts at gigahertz rates

   Alexei Smolyaninov, Abdelkrim El (February 2019, Nature photonics)

   Space-variant control of optical wavefronts is important for many applications in photonics, such as the generation of structured light beams, and is achieved with spatial light modulators. Commercial devices, at present, are based on liquid-crystal and digital micromirror technologies and are typically limited to kilohertz switching speeds. To realize significantly higher operating speeds, new technologies and approaches are necessary. Here we demonstrate two-dimensional control of free-space optical fields at a wavelength of 1,550 nm at a 1 GHz modulation speed using a programmable plasmonic phase modulator based on near-field interactions between surface plasmons and materials with an electrooptic response. High χ(2) and χ(3) dielectric thin films of either aluminium nitride or silicon-rich silicon nitride are used as an active modulation layer in a surface plasmon resonance configuration to realize programmable space-variant control of optical wavefronts in a 4 × 4 pixel array at high speed. 

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4. Integrated Planar Antenna with High Field Enhancement for On-Chip Electro-Optical Modulator Design

   Abe Akhiyat, Mustafa Karabiyik (July 2020, 2020 IEEE International Symposium on Antennas and Propagation and USNC-URSI Radio Science Meeting)

   null (Ed.)

   On chip antennas with integrated electro-optic modulators (EOMs) are attractive for millimeter wave applications. Such EOMs are placed at the antenna feed and enable the mixing of the incoming millimeter wave (mm-wave) signal with an optical carrier. As such, the mm-wave capture is turned into an optical signal that can be easily processed for imaging using standard infrared cameras. Our proposed on-chip antenna with the EOM modulator operated at 94 GHz or 77 GHz, with the antenna tuned at that frequency. For high modulation efficiency, it is critical that the modulator region is as small as possible with the optical waveguide leading to the EOM, also being well-matched. Notably, the enhanced field at the antenna feed leads to higher modulation efficiency and sensitivity. Therefore, an optimum antenna design must hasve as large as possible field strength at the feed. 

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5. 3D‐Printed Photonic Crystal Sub‐Terahertz Leaky‐Wave Antenna

   https://doi.org/10.1002/admt.202300698  

   Guerboukha, Hichem; Sakaki, Masoud; Shrestha, Rabi; Li, Jingwen; Kölbel, Johanna; Benson, Niels; Mittleman, Daniel M. (March 2024, Advanced Materials Technologies)

   Abstract Wireless systems are facing increasing pressure due to the growing demand for data transmission. One potential solution to this problem is to shift communication frequencies toward the terahertz (THz) spectrum. However, this requires the development of new components that can efficiently process signals at these high frequencies and transmit them via highly directional beams. In this study, a novel approach is proposed to achieving efficient THz signal processing by combining two existing technologies: photonic crystals and leaky‐wave antennas. Incorporating a 2D photonic crystal inside a leaky‐wave waveguide allows to manipulate the wave vector of the guided wave in unique ways, which in turn impacts the far‐field radiation pattern emitted through the leaky‐wave aperture. The device fabrication uses 3D printing of alumina and allows for convenient and scalable manufacturing. Through numerical simulations and experiments, free‐space data transmission at rates of few hundred Mbps at a carrier frequency of 101.2 GHz is demonstrated. The findings illustrate the feasibility of photonic crystal‐based leaky‐wave antennas and lay the groundwork for the development of compact and high‐performance components for THz wireless communication systems. 

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