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Optical Fiber with Two-dimensional Materials Integration for Photonic and Optoelectronic Applications

Jin-hui Chen1, 2 and Fei Xu2

1 Xiamen University, Institute of Electromagnetics and Acoustics, Siming South Road, Xiamen, 361005, China

2 Nanjing University, College of Engineering and Applied Sciences, Xianlin Road, Nanjing, 210023, China

1.1 Introduction

The booming development of long-haul optical communications has been built on the ultralow-loss optical fiber network. Inspired by the seminal study by Kao and Hockham [1] that the optical loss of silica fibers can be significantly mitigated by reducing the impurity concentrations, the invention of low-loss silica fibers (~20 dB/km) marked the debut of modern optical communications [2]. The standard optical fibers are mainly made of silica materials with concentric core and cladding structures. Since the fiber core has a higher refractive index than the encircling cladding, the optical fiber usually confines light field in the core region through the total internal reflection at the interface. With the advances in materials science and manufacturing, state-of-the-art optical fibers show a transmission loss of 0.154 dB/km, which means that the light signal intensity only suffers from half loss after propagating 20 km along the fiber [2]. While the optical fibers simply transmit the light signals in the optical communications system, they have also found extended applications in various fields, such as optical imaging, distributed sensing, fiber lasers, and nonlinear optics.

In the past 20 years, it has witnessed the great progress of fiber optics with the development of fundamental science and advanced technology. From microstructures, the one-dimensional (1D) photonic crystals are implemented to realize the fiber Bragg gratings, and later many other kinds of grating structures, such as long-period gratings and tilted fiber Bragg gratings, are proposed and demonstrated for the optical sensing and signal processing. The photonic crystal fibers (PCFs) are emerged with two-dimensional (2D) structures running along the fiber length. These unique photonic structures overcome the limitations of total internal reflection principle and permit light guidance in a hollow core by a photonic bandgap, which provides a paradigm shift in fiber optics, and has stimulated important technological and scientific applications [3]. For example, PCFs have been implemented for in-line controlling the wavelength, modes, polarization, and orbital angular momentum of the light fields. There is continuous progress in designing the low-loss microstructured fibers for optical communications, and recently developed hollow-core conjoined-tube negative-curvature fiber shows the potential of low loss beyond the standard optical fibers [4].

The photonic integrations with small footprints are always in pursuit of goals for improved performance, power, volume, and cost. By tapering from the bulk fiber/materials, the miniaturized microfibers are demonstrated with many intriguing properties, such as the strong light field confinement, large surface evanescent fields, tunable dispersion, and mechanical configurability. These micro-/nano-scale waveguides bridge the fiber optics and nanotechnology for enhanced light–matter interactions. The mechanical flexibility and strength of microfibers also enable various photonic structures for the proposed sensing and optoelectronic applications [5–7]. On the other hand, the merging of materials and microstructures into a single fiber is proposed and demonstrated to realize optical, electronic, and mechanical functions that can sense and communicate. By sophisticatedly stacking multimaterials and co-drawing the preforms, they yield kilometers of functional fiber devices. While the multifunctional multimaterial fibers show the great potential of lab-on-fiber technology, there are very limited materials for joint integration, considering the interface-energy-driven capillary break-up effect [8]. Moreover, they are still challenging to be connected with the standard optical fiber systems with low loss for the intrinsic mode-field mismatch and splicing difficulty. As a result, there is a rising interest in hybrid integration of functional materials to the structured silica fiber platform by the well-developed post-processing techniques. In the beginning of the twenty-first century, the rise of Dirac fermionic graphene materials has attracted great research interest among many fields from the seminal work of Novoselov et al. [9]. Graphene is demonstrated with many fascinating properties, including the ultrahigh carrier mobility, thermal conductivity, broadband light absorption, and mechanical strength. The fast development of graphene-based 2D materials brings new opportunities in the fiber integration, as shown in Figure 1.1, since they have excellent optical and optoelectronic properties. The strong in-plane mechanical strength of the typical 2D materials with naturally passivated bonds enables the tight attachment to the fiber structures either in the flat or curved surface without the need for lattice match. Besides, the atomic thickness of 2D materials keeps the optical modes almost intact along with the enhanced light–matter interactions. Consequently, 2D materials can be fully complementary to the conventional passive silica fibers and achieve the functions of light emitting, modulating, sensing, and detecting.

Figure 1.1 Schematic of 2D materials integration to optical fiber platform. TMDCs: transition metal dichalcogenides, BP: black phosphorous.

Source: Chen et al. [10] / Springer Nature / CC BY 4.0.

The optical fiber integration with 2D materials for optics and optoelectronics is a highly interdisciplinary research field that includes condensed matter physics, optics, and mechanics. This field is still in its early stages of development, and there have been substantial advances recently. This chapter will first review the development of optical fibers. Second, the basic properties of mainstay 2D materials, i.e. graphene, transition metal dichalcogenides (TMDC), and black phosphorous (BP), and their heterostructures are introduced. Third, the 2D-materials-fiber design strategy and fabrications are described. Fourth, recent progress of fiber integrations for optics and optoelectronics is discussed, mainly in the scope of polarimetric device, light sources, modulators, detectors, nonlinear optics, and fiber-optic sensors. Finally, the challenges and prospects of this field are briefly overlooked.

1.2 Fiber-integrated 2D Materials for Photonics and Optoelectronics

Basically, 2D materials are defined as crystalline materials composing single- or few-layer atoms, most of which are formed by the strong intralayer bonds and the weak interlayer van der Waals force. Benefited from the graphene research, the library of 2D materials is rapidly being increased from the mono-elements to compounds. Different from their three-dimensional (3D) counterparts, 2D materials can strongly interact with light, and their optical responses can be modulated with high flexibilities, such as by electrical gating, optical excitations, chemical doping, and strain gauge [10, 11]. Note that the fiber integration with 2D materials is still limited to a few mature 2D materials, and there is plenty of room to explore in this emerging field. As following, the basic optical and optoelectronic properties of the mainstay 2D materials are discussed.

1.2.1 Basic Properties of 2D Materials

1.2.1.1 Graphene

Monolayer graphene consists of carbon atoms arranged in a 2D honeycomb lattice, and each carbon atom is connected to the three nearest neighbors by a strong σ bond. Consequently, graphene shows an ultrahigh Young’s modulus of ~1.0 TPa and an intrinsic strength of 130 GPa [12]. Besides, the monolayer graphene is stable in oxygen atmosphere at temperatures up to 300 °C, and the multilayered samples can withstand ~500 °C without oxidation [13]. From the tight-binding model, the electronic band structures of graphene manifest the linear Dirac cones in the low-energy regime (Figure 1.2a), which leads to many intriguing electrical and optical properties [14]. For example, graphene has an ambipolar electric field effect with intrinsic carrier mobility reaching 106 cm2/V s. In optics, graphene has linear light absorption of approximately 2.3% per layer in the visible and near-infrared band (Figure 1.2b), defined by the fine structure constant [18]. More importantly, the light absorption can be readily tuned by the external fields such as electric gating, chemical doping, and strain. For nonlinear optics, the typical third-order nonlinearity (χ(3)) of graphene is as large as ~10−19 m2/V2, which is three orders of magnitude larger than that of fused silica. The ultrafast carrier relaxation of graphene has enabled the low-threshold saturable absorber and all-optical switch [19, 20]. In addition, the third harmonic generation (THG), four-wave mixing (FWM), saturable absorption (SA), and Kerr effect of graphene have been observed based on the third-order nonlinearity. It is revealed that the THG and FWM...