# Previous Michelson Postdoctoral Prize Lectureships

## Tue Oct 16, 2012 , 11:30 a.m. in Miller Room

Kin Fai Mak, Cornell

#### Optics with Dirac electrons

Optical spectroscopy provides an excellent means of understanding the
distinctive properties of electrons in the two-dimensional system of
graphene. Within the simplest picture, one has a zero-gap semiconductor
with direct transitions between the well-known conical bands. This picture
gives rise to a predicted frequency-independent absorption of pi alpha =
2.3 %, where alpha is the fine-structure constant. We will demonstrate that
this relation is indeed satisfied in an appropriate spectral range in the
near infrared, but that at higher photon energies electron-hole
interactions significantly modify this result through the formation of
saddle-point excitons. Optical spectroscopy also permits a detailed
analysis of how the linear bands of graphene, corresponding to massless
Dirac Fermions, are modified to yield massive electrons through interlayer
interactions in bilayer and few-layer graphene sheets. The observation of
a tunable band gap in bilayer and trilayer graphene will also be discussed.

Host: Harsh Mathur