OPTICAL PROPERTIES OF GRAPHENE

Source: azooptics.com

We discussed that graphene has many unique properties. Let's discuss the optical properties here. Unlike graphite, graphene is actually very transparent. It absorbs 2.3% of the light incident per layer (one atom thick) over a wide range of wavelengths. A few hundred graphene layers are sufficient to make it opaque as the absorbance decreases linearly with increasing thickness. The absorbance can also be tuned by doping it with other materials making it promising for photonic applications.

However, graphene absorbs a lot of light considering its thickness, unlike other materials such as diamonds. This is because graphene has three covalent bonds, which allow free electrons to roam about on its surface. These free electrons can absorb a photon and get excited. However, there are no free electrons in the case of diamonds, where almost all the photons pass through them. Thickness is the primary factor that is responsible for graphene's transparency.

Graphene can also absorb photons due to its zero bandgap structure. Every semiconductor has a conduction band and valence band, where the difference between the two bands is called the bandgap of the material. Unlike single atoms, where the energy of electrons is discrete, electrons in graphene have continuous plots with respect to momentum. Dirac point is the point where these two bands (conduction band and valence band) meet.

Source: Quora

However, graphene is a 2D material with electrons having momentum in both x and y directions. So, these Dirac points are effectively intersection points of two cones known as Dirac cones.

Source: Quora

Graphene absorbs a considerable amount of light owing to the presence of these Dirac cones. Since the two bands are aligned, electrons can easily absorb energy from the photons and get excited to the conduction band.

Source: Quora

Graphene also has effective EM radiation shielding capabilities that can be tuned according to the amount of doping in the material. These optical properties of graphene have made it a promising candidate in photonic applications such as cameras, photodetectors, microscopy, imaging, etc.

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