A solid-state photomultiplier (SSPM) includes an optical isolation structure therein. The SSPM includes a substrate and an epitaxial diode layer positioned on the substrate. A plurality of avalanche photodiodes (APDs) are fabricated on the epitaxial diode layer and the optical isolation structure is positioned about the plurality of APDs to separate each of the plurality of APDs from adjacent APDs. The optical isolation structure contains at least one of a light absorbing material and a light reflecting material deposited therein to reduce optical crosstalk and dark count rate in the SSPM.