Graphene photonics and optoelectronics

611 Electrons propagating through the bidimensional structure of graphene have a linear relation between energy and momentum, and thus behave as massless Dirac fermions1–3. Consequently, graphene exhibits electronic properties for a two-dimensional (2D) gas of charged particles described by the relativistic Dirac equation, rather than the non-relativistic Schrödinger equation with an eff ective mass1,2, with carriers mimicking particles with zero mass and an eff ective ‘speed of light’ of around 106 m s–1. Graphene exhibits a variety of transport phenomena that are characteristic of 2D Dirac fermions, such as specifi c integer and fractional quantum Hall eff ects4,5, a ‘minimum’ conductivity of ~4e2/h even when the carrier concentration tends to zero1, and Shubnikov–de Haas oscillations with a π phase shift due to Berry’s phase1. Mobilities (μ) of up to 106 cm2 V–1 s–1 are observed in suspended samples. Th is, combined with near-ballistic transport at room temperature, makes graphene a potential material for nanoelectronics6,7, particularly for high-frequency applications8. Graphene also shows remarkable optical properties. For example, it can be optically visualized, despite being only a single atom thick9,10. Its transmittance (T) can be expressed in terms of the fi ne-structure constant11. Th e linear dispersion of the Dirac electrons makes broadband applications possible. Saturable absorption is observed as a consequence of Pauli blocking12,13, and nonequilibrium carriers result in hot luminescence14–17. Chemical and physical treatments can also lead to luminescence18–21. Th ese properties make it an ideal photonic and optoelectronic material.