The relativistic nature of charge carriers in graphene is expected to lead to an angle- dependent transmission through a potential barrier, where Klein tunneling involves annihilation of an electron and a hole at the edges of the barrier. The signatures of Klein tunneling have been observed in gated graphene devices, but the angle dependence of the transmission probability has not been directly observed. Here we show measurements of the angle-dependent transmission through quasi-ballistic graphene heterojunctions with straight and angled leads, in which the barrier height is controlled by a shared gate electrode. Using a balanced differential measurement technique, we isolate the angle-dependent contribution to the resistance from other angle-insensitive, gate-dependent and device-dependent effects. We find large oscillations in the transmission as a function of the barrier height in the case of Klein tunneling at a 45 deg angle, as compared to normal incidence. Our results are consistent with the model that predicts oscillations of the transmission probability due to interference of chiral carriers in a ballistic barrier. The observed angle dependence is the key element behind focusing of electrons and the realization of a Veselago lens in graphene.
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