We are pleased to introduce the feature issue on novel optical and photonic devices based on 2D materials. 2D materials with crystalline structure consist of a single layer or a few layers of atoms, showing some unique optical, electronic, thermal and mechanical properties. The emergence of these 2D materials not only offers unique insights on light–matter interaction at the atomic layer level, but also provides unprecedented opportunities to fabricate photonic devices based on 2D materials. 2D materials have remarkably served as high-speed optical detectors/modulators, all-optical information processing, ultrafast lasers generation, second or third harmonic generation, etc. Their applications have recently extended into photo-thermal therapy, photo-acoustic imaging for medical treatment and photonic/plasmonic integrated circuits. The intention of this feature issue is to provide new insights and research progress in the optic and photonic devices of 2D materials.

This issue combined 7 research articles, covering a wide range of the recent advances in photonics based on 2D materials. The first article from the six research groups of USA and China investigated and gave the design criteria and optimization guidelines towards high-performance graphene-integrated thermo-optic switches based on the chalcogenide glass-on-graphene platform, and the authors obtained a record thermo-optic switching efficiency of 10 nm/mW via judicious engineering of the overlap between optical mode and thermal profile [1]. The second article from Shanghai Jiaotong University demonstrated a 523 MHz fundamental repetition rate WS₂-based mode-locked fiber laser, which was further used to high-quality microwave photonic application of frequency synthesis obtained with a tuning range up to 10 GHz and an extinction ratio greater than 60 dB in the whole range [2]. The third article is from Chinese Academy of Sciences [3]. This contribution proposed narrowband perfect absorbers combined with graphene for broadband mid-IR light modulation. Such perfect absorbers can show >90% large-intensity modulation depth in 5-6 µm wavelength, and phase modulation application with a tuning range of 180° [3]. The fourth article developed a high-quality MoS₂/Graphene heterostructure based saturable absorbers, and further demonstrated erbium-doped mode-locked fiber lasers from anomalous, near-zero to normal cavity dispersion [4]. The authors reveal that the MoS₂/Graphene heterostructure can be well applied to generate conventional soliton, stretched soliton and dissipative soliton [4]. The fifth article designed and fabricated an optical modulator of a metasurface constructed by an arrayed nano-ridge-aperture with a central nano-cylinder [5]. The last two articles reported two new-type nanomaterial Q-switches (i.e. Hafnium diselenide [6], silver nanowires [7]), and successfully realized the passively Q-switched fiber lasers, respectively. The authors obtained the pulse energy as high as 567 nJ in the telecommunication waveband (1561 nm) [6].

In conclusion, this feature issue provides a forum to discuss various 2D-material photonic devices, including graphene-integrated thermo-optic switch, mid-infrared light modulation, new 2D-material saturable absorbers for ultrafast lasers and microwave photonic application. Although this feature issue only provides a glimpse of the vast field of 2D-material optical and photonic devices, we hope that it can offer a useful reference that will stimulate your further research and applications in this exciting field.