We present the science and technology roadmap for graphene, related two-dimensional crystals, and hybrid systems, targeting an evolution in technology, that might lead to impacts and benefits reaching into most areas of society. This roadmap was developed within the framework of the European Graphene Flagship and outlines the main targets and research areas as best understood at the start of this ambitious project. We provide an overview of the key aspects of graphene and related materials (GRMs), ranging from fundamental research challenges to a variety of applications in a large number of sectors, highlighting the steps necessary to take GRMs from a state of raw potential to a point where they might revolutionize multiple industries. We also define an extensive list of acronyms in an effort to standardize the nomenclature in this emerging field.

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Science and technology roadmap for graphene, related two-dimensional crystals, and hybrid systems

Light modulation is an essential operation in photonics and optoelectronics. With existing and emerging technologies increasingly demanding compact, efficient, fast and broadband optical modulators, high-performance light modulation solutions are becoming indispensable. The recent realization that 2D layered materials could modulate light with superior performance has prompted intense research and significant advances, paving the way for realistic applications. In this Review, we cover the state of the art of optical modulators based on 2D materials, including graphene, transition metal dichalcogenides and black phosphorus. We discuss recent advances employing hybrid structures, such as 2D heterostructures, plasmonic structures, and silicon and fibre integrated structures. We also take a look at the future perspectives and discuss the potential of yet relatively unexplored mechanisms, such as magneto-optic and acousto-optic modulation.

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Optical modulators with 2D layered materials

Although phosphorene has attracted much attention in electronics and optoelectronics as a new type of two-dimensional material, in-depth investigations and applications have been limited by the current synthesis techniques. Herein, a basic N-methyl-2-pyrrolidone (NMP) liquid exfoliation method is described to produce phosphorene with excellent water stability, controllable size and layer number, as well as in high yield. Phosphorene samples composed of one to four layers exhibit layer-dependent Raman scattering characteristics thus providing a fast and efficient means for the in situ determination of the thickness (layer number) of phosphorene. The linear and nonlinear ultrafast absorption behavior of the as-exfoliated phosphorene is investigated systematically by UV–vis–NIR absorption and Z-scan measurements. By taking advantage of their unique nonlinear absorption, ultrashort pulse generation applicable to optical saturable absorbers is demonstrated. In addition to a unique fabrication technique, our work also reveals the large potential of phosphorene in ultrafast photonics.

From Black Phosphorus to Phosphorene: Basic Solvent Exfoliation, Evolution of Raman Scattering, and Applications to Ultrafast Photonics

Black phosphorous (BP), the most thermodynamically stable allotrope of phosphorus, is a high-mobility layered semiconductor with direct band-gap determined by the number of layers from 0.3 eV (bulk) to 2.0 eV (single layer). Therefore, BP is considered as a natural candidate for broadband optical applications, particularly in the infrared (IR) and mid-IR part of the spectrum. The strong light-matter interaction, narrow direct band-gap, and wide range of tunable optical response make BP as a promising nonlinear optical material, particularly with great potentials for infrared and mid-infrared opto-electronics. Herein, we experimentally verified its broadband and enhanced saturable absorption of multi-layer BP (with a thickness of ~10 nm) by wide-band Z-scan measurement technique, and anticipated that multi-layer BPs could be developed as another new type of two-dimensional saturable absorber with operation bandwidth ranging from the visible (400 nm) towards mid-IR (at least 1930 nm). Our results might suggest that ultra-thin multi-layer BP films could be potentially developed as broadband ultra-fast photonics devices, such as passive Q-switcher, mode-locker, optical switcher etc.

Broadband nonlinear optical response in multi-layer black phosphorus: an emerging infrared and mid-infrared optical material

2D layered materials (2DLMs) are a subject of intense research for a wide variety of applications (e.g., electronics, photonics, and optoelectronics) due to their unique physical properties. Most recently, increasing research efforts on 2DLMs are projected toward the nonlinear optical properties of 2DLMs, which are not only fascinating from the fundamental science point of view but also intriguing for various potential applications. Here, the current state of the art in the field of nonlinear optics based on 2DLMs and their hybrid structures (e.g., mixed-dimensional heterostructures, plasmonic structures, and silicon/fiber integrated structures) is reviewed. Several potential perspectives and possible future research directions of these promising nanomaterials for nonlinear optics are also presented.

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Nonlinear Optics with 2D Layered Materials.

The nonlinear optical property of few-layered MoS2 nanoplatelets synthesized by the hydrothermal exfoliation method was investigated from the visible to the near-infrared band using lasers. Both open-aperture Z-scan and balanced-detector measurement techniques were used to demonstrate the broadband saturable absorption property of few-layered MoS2. To explore its potential applications in ultrafast photonics, we fabricated a passive mode locker for ytterbium-doped fibre laser by depositing few-layered MoS2 onto the end facet of optical fiber by means of an optical trapping approach. Our laser experiment shows that few-layer MoS2-based mode locker allows for the generation of stable mode-locked laser pulse, centered at 1054.3 nm, with a 3-dB spectral bandwidth of 2.7 nm and a pulse duration of 800 ps. Our finding suggests that few-layered MoS2 nanoplatelets can be useful nonlinear optical material for laser photonics devices, such as passive laser mode locker, Q-switcher, optical limiter, optical switcher and so on.

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Molybdenum disulfide (MoS_2) as a broadband saturable absorber for ultra-fast photonics

The paper proposes a new approach that enables the structure analysis and reconstruction of a rough surface where the height of inhomogeneities (from the depression to the upper point) varies within the spread about 20 nm. For the surface diagnostics, carbon nanoparticles are used, which serve as sensitive probes of the local surface height. A single nanoparticle can be positioned at a desirable point of the studied surface with the help of an optical tweezer employing the He-Ne laser radiation. Then the particle is illuminated by the strongly focused exciting beam of 405 nm wavelength, with the waist plane precisely fixed at a certain distance from the surface base plane. The particle’s luminescence response (in the yellow-green spectral range) strongly depends on the distance between the exciting beam waist and the particle, thus indicating the local height of the surface. After scanning the surface area and the consecutive interpolation, the surface “vertical” landscape can be reconstructed with a high accuracy: the numerical simulation shows that the RMS surface roughness is restored with an accuracy of 6.9% while the landscape itself is reconstructed with the mean error 7.7%.

Fluorescence Record Diagnostics of 3D Rough-Surface Landscapes With Nano-Scale Inhomogeneities

Self-generated magnetic fields in single-mode ablative Rayleigh–Taylor instability (ARTI) relevant to the acceleration phase of inertial confinement fusion (ICF) implosions are studied via two dimensional simulations. In ARTI, [Formula: see text] T magnetic fields can be generated via the Biermann battery source without considering the Nernst effect. The Nernst effect significantly compresses the magnetic field against the electron temperature gradient and amplifies the peak value by more than three times. A scaling law for the magnetic flux is obtained, and it well predicts the evolution of the magnetic field from linear to deeply nonlinear phases of ARTI. The self-generated magnetic field reduces the ablation near the spike and reduces the width of bubbles by magnetizing the electron heat flows, which results in higher magnitude vorticity inside the bubble and enhances the nonlinear ARTI bubble penetration velocity for short-wavelength modes. The bubble velocity boosting due to self-generated magnetic field indicates the larger impact of the short-wavelength ARTI modes on ICF implosion performance than previously expected.

Self-generated magnetic field in ablative Rayleigh–Taylor instability

The review describes general principles and characteristics of inhomogeneously polarized paraxial optical fields and, especially, the polarization singularities (PSs). Main parameters of the optical vector waves are discussed, with the emphasis on the physical relevance and topological distinctiveness of the PSs. Special features of the stochastic vector fields are considered in the context of the PSs’ genericity and structural stability. A detailed attention is paid to interrelations between the PSs and the phase singularities of scalar fields formed by the orthogonal polarization projections of the total field, and their derivatives (complex Stokes fields, phase-difference fields, etc.). On this base, the practical approaches are discussed for the experimental PS identification and characterization. A particular examination of the internal energy flows associated with the PSs, and accompanying distributions of the optical momentum and angular momentum, reveals meaningful dynamical features of PSs and supplies additional physically transparent and informative means for their studies and characterization.

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J. Zheng

Papers

Molybdenum disulfide (MoS_2) as a broadband saturable absorber for ultra-fast photonics

Fluorescence Record Diagnostics of 3D Rough-Surface Landscapes With Nano-Scale Inhomogeneities

Self-generated magnetic field in ablative Rayleigh–Taylor instability

Polarization singularities: Topological and dynamical aspects