Science and technologies based on terahertz frequency electromagnetic radiation (100 GHz–30 THz) have developed rapidly over the last 30 years. For most of the 20th Century, terahertz radiation, then referred to as sub-millimeter wave or far-infrared radiation, was mainly utilized by astronomers and some spectroscopists. Following the development of laser based terahertz time-domain spectroscopy in the 1980s and 1990s the field of THz science and technology expanded rapidly, to the extent that it now touches many areas from fundamental science to 'real world' applications. For example THz radiation is being used to optimize materials for new solar cells, and may also be a key technology for the next generation of airport security scanners. While the field was emerging it was possible to keep track of all new developments, however now the field has grown so much that it is increasingly difficult to follow the diverse range of new discoveries and applications that are appearing. At this point in time, when the field of THz science and technology is moving from an emerging to a more established and interdisciplinary field, it is apt to present a roadmap to help identify the breadth and future directions of the field. The aim of this roadmap is to present a snapshot of the present state of THz science and technology in 2017, and provide an opinion on the challenges and opportunities that the future holds. To be able to achieve this aim, we have invited a group of international experts to write 18 sections that cover most of the key areas of THz science and technology. We hope that The 2017 Roadmap on THz science and technology will prove to be a useful resource by providing a wide ranging introduction to the capabilities of THz radiation for those outside or just entering the field as well as providing perspective and breadth for those who are well established. We also feel that this review should serve as a useful guide for government and funding agencies.

The 2016 terahertz science and technology roadmap

Silicon (Si) photonics have established as leading technologies in addressing the rapidly increasing demands of huge data transfer in optical communication systems with compact footprints, small power consumption, and ultradense bandwidth, which are driven by the next generation supercomputers and big data era. Particularly, Si photonics will penetrate into optical communication links at an ever‐small scale, namely chip‐to‐chip and on‐chip. However, the nanostructures made of Si with an indirect bandgap are not ideal candidates for on‐chip light sources, modulators, and photodetectors, which most‐frequently require optical materials with direct bandgap. Thanks to the advent of graphene, transition metal dichalcogenides and other two‐dimensional (2D) materials, which can facilitate extraordinary progresses in improving device performance at the ultrathin scale, their integration with Si photonics furnishes a heterogeneous platform to construct fully functional and highly integrated photonic communication systems. In this work, the current advancements in the on‐chip applications of Si photonics‐2D materials heterostructures, inclusive of all essential chip‐scale modules and integrated circuits, as well as the future prospective and challenges are reviewed and discussed. The present study sets out to objectively measure the feasibility of the hybrid integration between Si photonics and 2D materials in on‐chip optical communications and the advanced applications beyond.

https://www.onlinelibrary.wiley.com/doi/pdf/10.1002/lpor.202000239

Hybrid/Integrated Silicon Photonics Based on 2D Materials in Optical Communication Nanosystems

A metasurface is one of the most effectual platforms for the manipulation of complex optical fields. One of the current challenges in the field is to develop active or reconfigurable functionalities to extend its operation band which is limited by its intrinsic resonant nature. Here we demonstrate a kind of active Fano-resonant asymmetric metasurface in the near-infrared (NIR) region with heterostructures made of a layer of asymmetric split-ring resonators and a thin layer of phase-change material (PCM). In the asymmetric metasurface, significant tunability in the frequency, Q-factor and strength of the Fano resonance are all achieved by precisely controlling the phase transition of the contained PCM Ge2Sb2Te5 (GST), together with changing the geometric asymmetry of the split-ring resonators. Moreover, we provide a complete transition process of the optical properties for GST and an optimized modulation on the active Fano-resonant metasurface. Our approach to dynamically control a Fano-resonant metasurface paves the way to realizing various active photonic meta-devices involving PCM.

Realization of a near-infrared active Fano-resonant asymmetric metasurface by precisely controlling the phase transition of Ge2Sb2Te5.

Abstract Optical modulation technique plays a crucial role in photonics technologies, and there is an ever-increasing demand for broadband and ultrafast optical modulation in the era of artificial intelligence. All-optical modulation is known to be able to operate in an ultrafast way and has a broadband response, showing great potential in applications for ultrafast information processing and photonic computing. Two-dimensional (2D) materials with exotic optoelectronic properties bring tremendous new opportunities for all-optical modulators with excellent performance, which have attracted lots of attention recently. In this review, we cover the state-of-art all-optical modulation based on 2D materials, including graphene, transitional metal dichalcogenides, phosphorus, and other novel 2D materials. We present the operations mechanism of different types of all-optical modulators with various configurations, such as fiber-integrated and free-space ones. We also discuss the challenges and opportunities faced by all-optical modulation, as well as offer some future perspectives for the development of all-optical modulation based on 2D materials.

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All-optical modulation with 2D layered materials: status and prospects

High Mobility 3D Dirac Semimetal (Cd 3 As 2 ) for Ultrafast Photoactive Terahertz Photonics

Ultrafast terahertz transmission/group delay switching in photoactive WSe2-functionalized metaphotonic devices

Ultrafast Terahertz Frequency and Phase Tuning by All-Optical Molecularization of Metasurfaces

Ultrafast Frequency Shift of Electromagnetically Induced Transparency in Terahertz Metaphotonic Devices

Metamaterials play an important role in the modulation of amplitude and group delay in the terahertz (THz) regime on account of their optical properties, which are rare in natural materials. Here an ultrafast anisotropic switch of the plasmon-induced transparency (PIT) effect is experimentally and numerically demonstrated by metamaterial devices composed of two pairs of planar split-ring resonators and a pair of closed-ring resonators. By integration with a germanium (Ge) film, a recovery time of 3 ps and a decay constant of 785 fs are realized in the metadevice. Stimulated by the exterior optical pump, the PIT windows at different frequencies are switched off with an excellent property of slow light for vertical and horizontal THz polarizations, realizing an astonishing modulation depth as high as 99.06%. This work provides a new platform for ultrafast anisotropic metadevices tunable for amplitude and group delay.

Ultrafast polarization-dependent all-optical switching of germanium-based metaphotonic devices

Terahertz Metamaterials: Ultrafast Terahertz Frequency and Phase Tuning by All‐Optical Molecularization of Metasurfaces (Advanced Optical Materials 22/2019)

Hao Sun

Papers

Ultrafast terahertz transmission/group delay switching in photoactive WSe2-functionalized metaphotonic devices

Ultrafast Terahertz Frequency and Phase Tuning by All-Optical Molecularization of Metasurfaces

Ultrafast Frequency Shift of Electromagnetically Induced Transparency in Terahertz Metaphotonic Devices

Ultrafast polarization-dependent all-optical switching of germanium-based metaphotonic devices

Terahertz Metamaterials: Ultrafast Terahertz Frequency and Phase Tuning by All‐Optical Molecularization of Metasurfaces (Advanced Optical Materials 22/2019)