Two-dimensional (2D) materials are attractive building blocks for future photonic and optoelectronic devices at the nanoscale, because they allow achieving new degrees of both performance and functionality — a combination unachievable by most conventional, three-dimensional materials.

2D materials are crystalline materials consisting of a single layer or a few layers of atoms. Since the isolation of graphene in 2004 [1], a tremendous amount of research has been directed at synthesizing graphene, understanding its unique physical properties, and exploring its applications in electronics, photonics, and optoelectronics. Six years later, new excitements were initiated as 2D materials with large band gaps were identified [2]. Typical examples are transition metal dichalcogenides (TMDs) and black phosphorus. Such 2D materials open up new physics and applications - two-dimensional semiconductor physics and devices. Quantum physics in two-dimensional semiconductors leads to both superior performance and functionalities of devices, which could not be found in their three-dimensional counterparts. In this feature issue, we aim at graphene, TMDs, black phosphorus and others.

The feature issue starts with an invited review paper by Wang et al. [3] on the photoresponses of TMDs. In this paper, both fundamental physical mechanisms and device performance are reviewed. The authors also discuss future challenges and opportunities in developing TMDs based photodetection devices. It is followed by an article by Liu et al. [4], in which high responsivity of unfocused laser and white light is reported of using graphene-based photodetectors.

As for applications of lasers and nonlinear optics, there are four articles (Zhang et al. [5], Guo et al. [6], Chu et al. [7], and Li et al. [8]), focusing on exploiting TMDs or black phosphorus as saturable absorbers for Q-switched or mode-locked lasing operations. In addition, it should be mentioned that Kowalczyk et al. [9] have demonstrated a mode-locked lasing operation with a topological insulator system, Sb₂Te₃, which possesses a similar electronic band structure to graphene. And, Day et al. [10] have achieved microcavity–enhanced second harmonic generation in monolayer MoS₂; and Kleinert et al. [11] have presented a graphene-based electro-absorption modulator.

In the materials fabrication and characterization, Marchena et al. [12] have reported low-temperature direct growth of graphene patterns on flexible glass substrates catalysed by a sacrificial ultrathin Ni film. Chakraborty et al. [13] and Snure et al. [14] have presented their studies on localized emission from defects in MoSe₂ layers and probing phonon/electrical anisotropy in black phosphorus, respectively. Miao et al. [15] have characterized the nonlinear optical properties of bismuth telluride nanosheets with Z-scans. It is also interesting to note that atomic layer deposited films can assist the growth of porous 6H-SiC, as reported by Lu et al. [16].

In conclusion, this feature issue of Optical Materials Express gives a topical and highly interesting overview of the state-of-the-art in the emerging research field of 2D materials and offers many possibilities for future research. The field spans the entire spectrum from basic material fabrications, material characterizations, to 2D-materials-based lasers and optoelectronic devices.

We express our gratitude to all reviewers and authors for their efforts in improving the manuscripts during the review process. We also thank Alexandra Boltasseva, Editor-in-Chief of Optical Materials Express, for supporting this feature issue and the OSA journal staff for their excellent support during the review and production.