  We present a systematic study of the Raman spectra of optical phonons in
graphene monolayers under tunable uniaxial tensile stress. Both the G and 2D
bands exhibit significant red shifts. The G band splits into two distinct
sub-bands (G+, G-) because of the strain-induced symmetry breaking. Raman
scattering from the G+ and G- bands shows a distinctive polarization dependence
that reflects the angle between the axis of the stress and the underlying
graphene crystal axes. Polarized Raman spectroscopy therefore constitutes a
purely optical method for the determination of the crystallographic orientation
of graphene.
