(1) Most obviously the secondary photons can travel directly to a neighboring cell (direct optical crosstalk),
(2) A secondary photon is able to generate an electron-hole-pair close to a neighboring cell. The carriers can diffuse to the microcell and cause their discharge (delayed optical crosstalk),
(3a, b) Secondary photons, being reflected at one of the various interfaces like e.g. package boundary or SiPM backside, can reach a neighboring cell via an indirect path (indirect optical cross talk).
The optical crosstalk mechanism according to (1) can be suppressed by a deep optical trench isolation between the individual microcells. In case the trench is deep enough, it helps also to suppress the crosstalk according to mechanism (2) by covering a larger solid angle.