White Paper: Silicon Photomultiplier Detectors For Low Light Detection

By SensL

Low light detectors and multidetector imagers enable a wide range of applications ranging from cell imaging, biodiagnostic instrumentation, semiconductor wafer inspection, particle counting, nuclear medicine, radiation detection, and quantum cryptography, to low light imaging applications such as security cameras and light detection and ranging LIDAR systems. These application areas rely on the ability of the sensor to detect light levels, which range from the single photon (less than an attowatt) to millions of photons (picowatts to nanowatts) per second incident on the detector surface. The detector that has enabled these applications has traditionally been the photomultiplier tube, or PMT.

The PMT is a vacuum tube detector that uses a coated photocathode to convert incident photons into photoelectrons. The photoelectrons are then accelerated by the high electric field and their interaction with multiple metal dynodes in the PMT cause a shower of electrons, which are collected at the anode of the detector. The electrons are then read out as a current that indicates the level of light incident on the photocathode surface. Depending on the configuration of the PMT and the voltages that are applied, the PMT can operate in two specific modes of operation: single photon counting and analog or linear mode.

In single photon counting the goal for the detector is to provide sufficient amplification to the signal that a single photon incident onto the detector surface causes a measurable signal at the output of the detector. A PMT performs this function through the use of high voltage power supplies on the order of 1,000 to 2,000 volts and the use of external discriminator circuits to convert the electric charge signal into a digital signal. By photon counting, the signal is binary and is either "on" or "off". This allows the number of incident photons to be counted and the arrival time of the photon determined by the timing of the output pulse of the detector. This enables a wide range of measurement techniques such as fluorescence lifetime imaging and laser range finding which rely on knowing precisely the arrival time of the photon. Single photon counting techniques are ideal for low signal fluxes of less than one million photons per second. Above that rate photon arrival time is hard to discriminate since the possibility that two or more photons can arrive at the same time is increased. Beyond these signal levels it is required to use analog or linear mode of operation to determine the signal level.