Cyclotron Resonance in Bilayer Graphene

We present the first measurements of cyclotron resonance of electrons and holes in bilayer graphene. In magnetic fields up to $B=18\mathrm{T}$, we observe four distinct intraband transitions in both the conduction and valence bands. The transition energies are roughly linear in $B$ between the lowest Landau levels, whereas they follow $\sqrt{B}$ for the higher transitions. This highly unusual behavior represents a change from a parabolic to a linear energy dispersion. The density of states derived from our data generally agrees with the existing lowest order tight binding calculation for bilayer graphene. However, in comparing data to theory, a single set of fitting parameters fails to describe the experimental results.