Graphene optics:

Graphene is a novel two dimensional material with extraordinary electrical and mechanical properties. It also has promising optical and optoelectronic properties: their low energy transitions can be continuously tuned by a gate voltage and fundamentally different behaviors are expected for single- and double- layered graphene. We developed sensitive infrared spectroscopy using a microscopy setup with a femtosecond tunable light source to probe these effects.

In Fig. 1 we show such unusual gate tunable optical transitions in graphene. Fig. 1a is the gated graphene device and Fig. 1b is the change of optical reflection from graphene as a function of electrical gating. The gate-tunable optical transition opens a new dimension in graphene spectroscopy and allows for an optical determination of the graphene electronic structure (Fig. 2).

(a)	(b)
Fig. 1: (a) Gated graphene device with laser excitation. (b) Gate-induced change of graphene reflecitivy as a function of electrical gate voltage.

Fig. 2. A 2D plot of graphene reflectivity change versus gate voltage and probing photon energy. The modulation maximum (red line) yields the Fermi velocity of the linear band. F. Wang et al. Science 320, 206 (2008)

Using infrared spectroscopy with ultrafast excitation and probe, we will be able to investigate the electron-electron and electron-phonon interactions in this unique material system. Our technique can also open the door for observing and controlling the pseudo-spin physics and quantum behavior of the electrons in gated graphene devices.