  The focusing of electron flow in a symmetric p-n junction (PNJ) of graphene
ribbon with different chiralities is studied. Considering the PNJ with the
sharp interface, in a armchair ribbon, the electron flow emitting from $(-L,0)$
in n-region can always be focused perfectly at $(L,0)$ in p-region in the whole
Dirac fermion regime, i.e. in whole regime $E_0<t$ where $E_0$ is the distance
between Dirac-point energy and Fermi energy and $t$ is the nearest hopping
energy. For the bipolar ribbon with zigzag edge, however, the incoming electron
flow in n-region is perfectly converged in p-region only in a very low energy
regime with $E_0<0.05t$. Moreover, for a smooth PNJ, electrons are
backscattered near PNJ, which weakens the focusing effect. But the focusing
pattern still remains the same as that of the sharp PNJ. In addition, quantum
oscillation in charge density occurs due to the interference between forward
and backward scattering. Finally, in the presence of weak perpendicular
magnetic field, charge carriers are deflected in opposite directions in the
p-region and n-region. As a result, the focusing effect is smeared. The lower
energy $E_0$, the easier the focusing effect is destroyed. For the high energy
$E_0$ (e.g. $E_0=0.9t$), however, the focusing effect can still survive in a
moderate magnetic field on order of one Tesla.
