The whole range of 2D materials, from graphene to transition metal dichalcogenides (TMDCs), are gaining more widespread use at both an academic and commercial level. Additionally, new applications are always coming to light, and in this article, we look at how 2D materials can be used to construct waveguides.
What are 2D Materials?
2D materials are atomically thin materials that (in theory) consist of a single layer of atoms arranged (almost always) in a planar array. The most known 2D material is graphene, but many others exist, including materials that are only composed of a single element, but others which contain multiple elements.
2D materials are usually a single layer of standard bulk material (e.g., graphene from graphite) and can exhibit many quantum effects; because aside from being 2D dimensional in the atomic spatial arrangement, the electrons in 2D materials are also confined to two-dimensions. 2D materials are well renowned for possessing excellent electronic, physical and optical properties, although the exact effects do vary from material to material.
A sound wave (Image credit: natrot/Shutterstock)
What is a Waveguide?
As the name suggests, a waveguide is a device that guides waves. Many types of waves can be guided including optical, sound, electromagnetic and microwaves. The structure of a waveguide can vary drastically depending on the type of wave which needs to be transmitted. The most basic waveguides come in the form of hollow conductive metal pipes for transporting radio waves and increases in complexity all the way to optical fibers that guide electromagnetic waves for optical communication applications. To show how varied a wave guide can be, one of the oldest and most fundamental waveguides is a piece of string to transmit sound waves between two cans or cups.
Waves usually propagate in all directions and are bound by the inverse square law, which is when the wave propagates away from the source, its intensity is inversely proportional to the square of the distance from the source. The primary function of a waveguide is to confine a wave and propagate it in a single given direction. This is to ensure that the power of the wave is not lost in transport. The walls of the waveguide also confine the wave to a specific area, so that it doesn’t propagate away from the intended direction.
The geometry of a waveguide is also related to its function. There are three common waveguide geometries. These are slab, fiber and strip waveguides. Slab waveguides confine the energy to one transverse dimension, whereas fiber and strip waveguides transmit in two transverse directions. The frequency of the transmitted wavelength also plays a big part in the shape of the waveguide, and the width of the waveguide needs to be of the same order of magnitude as the transmitted/guided wave.
How 2D Materials are Used to Construct Waveguides
The most common construct for 2D materials is in optical waveguides. Many of the complex waveguides require materials with particular transport properties. 2D materials have some of the best charge carrier transport properties known to science, and many change transmit charge carriers in two directions (although there are 2D materials that are uni-directional).
Additionally, for optical waveguides, 2D materials possess some excellent optical and electrical properties over 3D bulk materials, that enables the construction of efficient waveguides. Unlike bulk materials, 2D materials can be easily integrated into waveguides because the planar and functionalization-free surface means that there are no dangling covalent bonds to interact with the surrounding materials in the waveguide.
It is also possible to create vertical heterostructures without the lattice mismatch that you often find with 3-dimensional materials. This is because the individual layers that have different lattice constants are only weakly bonded by van der Waals forces.
2D materials can also interact strongly with light, making them excellent for optical waveguides. In a lot of cases, the Fermi level of the 2D material can also be tuned to help modulate the light in the waveguide.
Many 2D-based waveguides out there use either graphene, black phosphorus or TMDCs. These materials are often used in optical waveguides as the sandwiched material between silicon waveguides, by being grown onto silicon photonic layers, coupling 2D materials with waveguides, and are integrated into metal slot-line waveguides, surface plasmonic metal waveguides and dielectric waveguides. Additionally, optical waveguides with incorporated 2D materials are known to produce technologies in the THz frequency region.
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