An avalanche photodiode includes a cathode region and an anode region. A lateral insulating region including a barrier region and an insulating region surrounds the anode region. The cathode region forms a planar optical guide within a core of the cathode region, the guide being configured to guide photons generated during avalanche. The barrier region has a thickness extending through the planar optical guide to surround the core and prevent propagation of the photons beyond the barrier region. The core forms an electrical-confinement region for minority carriers generated within the core.