Laser Components Compares Single Photon Detector Technologies

Laser Components Germany GmbH has published a comparison of two primary technologies used for detecting single photons: photomultiplier tubes (PMT) and single photon avalanche diodes (SPAD).

Single photon counting is crucial for advancements across various scientific and technological fields, including astronomy, particle sizing, disease diagnosis, medical analysis, imaging methods, and quantum cryptography. In biomedical research, particularly with confocal and STED microscopy, single photon counting has enabled significant progress in analyzing the structure and function of biological molecules.

Photomultiplier tubes (PMT) typically consist of a photocathode and a secondary electron multiplier within an evacuated glass tube. When photons strike the photocathode, they emit electrons, which are then accelerated and strike subsequent electrodes called dynodes. Each dynode releases multiple secondary electrons, creating an electron avalanche. This cascade is collected at the anode, generating an electrical pulse that can be counted.

Single Photon Avalanche Diodes (SPAD) operate as semiconductor diodes biased above their breakdown voltage. A single photo-emitted electron can initiate an avalanche of approximately 10^8 charge carriers. Specialized circuitry (passive or active quenching) ensures the diode quickly returns to its ready state after each detection, allowing individual photon pulses to be counted with short dead times, achieving count rates up to 10 MHz.

Laser Components highlights the higher quantum efficiency and broader spectral sensitivity (300 nm to NIR) of SPADs. Their "VLoK APD" is noted for achieving low dark counts (< 10 c/s) with high quantum efficiency (> 80% at 670 nm). For shorter wavelengths, where PMTs have traditionally been preferred, Laser Components now offers the "COUNTblue" module, providing over 50% quantum efficiency at 405 nm.