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

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.

Owing to their atomic layer thickness, strong light–material interaction, high nonlinearity, broadband optical response, fast relaxation, controllable optoelectronic properties, and high compatibility with other photonic structures, 2D materials, including graphene, transition metal dichalcogenides and black phosphorus, have been attracting increasing attention for photonic applications. By tuning the carrier density via electrical or optical means that modifies their physical properties (e.g., Fermi level or nonlinear absorption), optical response of the 2D materials can be instantly changed, making them versatile nanostructures for optical modulation. Here, up-to-date 2D material-based optical modulation in three categories is reviewed: free-space, fiber-based, and on-chip configurations. By analysing cons and pros of different modulation approaches from material and mechanism aspects, the challenges faced by using these materials for device applications are presented. In addition, thermal effects (e.g., laser induced damage) in 2D materials, which are critical to practical applications, are also discussed. Finally, the outlook for future opportunities of these 2D materials for optical modulation is given.

2D Materials for Optical Modulation: Challenges and Opportunities.

Active modulation of electromagnetically induced transparency analogue in terahertz hybrid metal-graphene metamaterials

Few-layer black phosphorus (BP), as the most alluring graphene analogue owing to its similar structure as graphene and thickness dependent direct band-gap, has now triggered a new wave of research on two-dimensional (2D) materials based photonics and optoelectronics However, a major obstacle of practical applications for few-layer BPs comes from their instabilities of laser-induced optical damage Herein, we demonstrate that, few-layer BPs, fabricated through the liquid exfoliation approach, can be developed as a new and practical saturable absorber (SA) by depositing few-layer BPs with microfiber The saturable absorption property of few-layer BPs had been verified through an open-aperture z-scan measurement at the telecommunication band and the microfiber-based BP device had been found to show a saturable average power of ~45 mW and a modulation depth of 109%, which is further confirmed through a balanced twin detection measurement By further integrating this optical SA device into an erbium-doped fiber laser, it was found that it can deliver the mode-locked pulse with duration down to 940 fs with central wavelength tunable from 1532 nm to 1570 nm The prevention of BP from oxidation through the 'lateral interaction scheme' owing to this microfiber-based few-layer BP SA device might partially mitigate the optical damage problem of BP Our results not only demonstrate that black phosphorus might be another promising SA material for ultrafast photonics, but also provide a practical solution to solve the optical damage problem of black phosphorus by assembling with waveguide structures such as microfiber

Microfiber-based few-layer black phosphorus saturable absorber for ultra-fast fiber laser

Graphene is an optical material of unusual characteristics because of its linearly dispersive conduction and valence bands and the strong interband transitions. It allows broadband light-matter interactions with ultrafast responses and can be readily pasted to surfaces of functional structures for photonic and optoelectronic applications. Recently, graphene-based optical modulators have been demonstrated with electrical tuning of the Fermi level of graphene. Their operation bandwidth, however, was limited to about 1 GHz by the response of the driving electrical circuit. Clearly, this can be improved by an all-optical approach. Here, we show that a graphene-clad microfiber all-optical modulator can achieve a modulation depth of 38% and a response time of ∼2.2 ps, limited only by the intrinsic carrier relaxation time of graphene. This modulator is compatible with current high-speed fiber-optic communication networks and may open the door to meet future demand of ultrafast optical signal processing.

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Ultrafast All-Optical Graphene Modulator

We demonstrate all fiber Fabry-Perot resonators (F-P resonators) based on Sagnac loop mirrors assembled with tellurite microfibers. As-assembled F-P resonators, with dimensions of hundreds of micrometers, show clear resonant responses with typical quality factor of about 5700, free spectral range of about 1 nm, and a maximum extinction ratio of 18 dB. The reflectivity of the loop mirror and the effective cavity length of the F-P resonator can be tuned by micromanipulation under an optical microscope. F-P resonators demonstrated here show advantages of small size, good resonance, easy fabrication, tunability, and easy integration with fiber systems.

All-fiber Fabry–Perot resonators based on microfiber Sagnac loop mirrors

Endface reflectivities (ERs) of optical nanowires are investigated using three-dimensional finite-difference time-domain simulations Typical ERs of both free-standing and substrate-supported silica, tellurite, PMMA and semiconductor nanowires or nanofibers are obtained Unlike in conventional waveguides such as optical fibers, ERs of nanowires are usually considerably lower when operated in single mode Dependences of ER on the diameter and the refractive index of the nanowire, and the wavelength of the guided light are also investigated These results are helpful for estimating and understanding ERs in optical nanowires with diameters close to or smaller than the wavelengths of the light, and may offer valuable references for practical applications such as nanowire or nanofiber-based resonators and lasers

Endface reflectivities of optical nanowires

The synthesis of optical quality β-barium borate microwires and nanowires (MNWs) is reported using an organic-free hydrothermal method with BaCl2·6H2O, NaOH, and H3BO3 as source materials, and assisted with post-annealing. As-synthesized MNWs, with diameters ranging from 500 nm to 2 μm and lengths up to several hundred micrometers, show good optical-waveguiding capabilities. Based on evanescent coupling between a single BBO MNW waveguide and a fiber taper, propagation losses of 0.30 dB μm−1 (at 532 nm) and 0.21 dB μm−1 (at 671 nm) are evaluated, respectively. An evident second-harmonic generation (SHG) signal at 532 nm with a measured conversion efficiency of about 8.4% is observed when excited by waveguided 1064 nm, picosecond laser pulses within a BBO MNW with a length of the order of 100 μm. The dependence of the SHG conversion efficiency on the MNW diameter is also investigated. These results show a much-higher SHG efficiency for BBO single-crystal MNWs compared with bulk crystal, which suggests potential applications in future micro-/nanoscale nonlinear optical applications such as optical modulation and frequency conversion.

Zhifang Hu

Papers

Ultrafast All-Optical Graphene Modulator

All-fiber Fabry–Perot resonators based on microfiber Sagnac loop mirrors

Endface reflectivities of optical nanowires

Synthesis of Optical‐Quality Single‐Crystal β‐BaB2O4 Microwires and Nanowires

High-sensitivity microfiber strain and force sensors