Single Photon Detector

A single photon detector or single photon counter is a detector capable of capturing and recording individual photons. This is especially useful for measurements like time-resolved spectroscopy, fluorescence lifetime measurements and microscopy, quantum photonics and even some particle physics detectors (e.g. in Cherenkov radiation). They are also vital components in Time of Flight (ToF) systems, including Light Detection and Ranging (LiDAR).

Practically, fluorescence lifetime systems measure the time between a prompt signal (such as an excitation laser pulse) and the resulting fluorescence emission. Single photon detectors, combined with other electronics, convert incoming photon signals into a digital output. These detectors are often used for timing measurements. 

These detectors provide a signal which act as a time stamp for the fluorescence. Combined with a pulsed light source (which keep the fluorophore in an excited state) and a range of other electronic component (to process this signal), these systems build a fluorescence decay curve one photon at a time. Each time measurement is plotted on a histogram to form a fluorescence decay curve for a particular material.

Photomultiplier tubes (PMTs) are often the standard choices for single photon detectors but other alternatives, such as single photon avalanche diodes (SPADs), could offer cheaper and faster response times. Having said this, SPADs struggle to compete in terms of active area. A large active area is important in SPDs, due to the relatively low signal intensity. One of the main benefits of PMTs is their large active area. 

Key Parameters of Single Photon Detectors

Rise Time

Rise time of a single photon detector represents how quickly the incoming pulse rises from 10% to 90% of its intensity. This quantifies how quickly a detector will respond after a photon hits it, which is a critical quantity for accurate timing measurements. The best LED pulsed light sources have pulse widths of 100s of picoseconds, so any compatible detector should have a rise time on this scale to measure signals from these light sources without further peak broadening. 

Dead Time

Dead time is another important factor for a single photon detector. Dead time quantifies the time it takes for the system to be recover from incoming photon and to be ready for another incoming signal. This determines the repetition rate of a detector. There are several components that contribute to a detectors dead time. For example, one contributing factors is that the system can't detect more than one event per excitation period. However, as pulse to detection ratio is less 1 photon detected per 50 excitation pulses, this is not usually a limiting factor. 

Often more important factors are the detector and conversion dead time pile up. This represents the time it takes for the detector or any converting electronics to process a signal. 

Any systematic time taken for a photon to be processed can be accounted as a delay time that can be corrected. The main issue for detector dead time is the transit time spread. The distribution of transit times as a photon passes through the detector becomes the uncertainty in the timing measurements of the detector.

Dark Count Rate

Another important metric is a detector's dark count rate. This represents the number of signals that are detected when there are no actual signals. In other words, this is the constant baseline noise of the system. This value is measured in counts per second (cps) or Hz.

Photomultiplier Tubes

A device commonly used in single photon detectors are photomultiplier tubes (PMTs). Photomultiplier tubes use certain electrode materials, such Berylium Oxide and Magnesium Oxide as dynodes. With a dynode material, an incoming photon triggers the release of multiple electrons, which can each trigger the release of more electrons and so forth. This means they can easily amplify an incoming photon into a significant electrical signal. In order for this continual amplification, dynodes are often arranged as a series of plates or in a tube structure. 

 Microchannel Plate (MCP) PMTs are a common form of PMTs used in time-correlated single photon counting. In photomulitpliers, electrons are guided down narrow channels by an applied electric field. These tubes coated with dynode materials, so incoming photons create a uniform electrical signal. The other type of PMT used are dynode chain PMTs, which use individual dynodes (discrete metal plates) to multiply electrons.

Dynode chain PMTs bulkier and less efficient than MCP PMTs. As MCP PMTs have very small channels, the electrons are all directed down the similar path lengths so have the same transit time. This leads to a lower TTS compared to dynode chain PMTs. MCP PMTs have 10 times shorter pulse width than other PMTs. . However dynode chain PMTs are less costly than MCPs and are suitable for many single photon detector applications. MCP PMTs also work over a smaller intensity range than dynode PMTs. 

Single Photon Avalanche Diodes

Single Photon Avalanche Diodes (SPADs) have also been established as good detectors for high resolution timing systems. They are inexpensive and have a faster response time alternative photomultiplier tubes.

SPAD systems use photodiodes in reverse bias, who's breakdown voltage is below the energy of a single photon. A single photon creates an electron-hole pair, triggering an avalanche of secondary electron movement, amplifying single photon signals. Often implemented in complementary-metal-oxide semiconductor architectures, these detectors can achieve rise times of picoseconds, so are capable of measuring fluorescence lifetime on the order of nanoseconds.

However, SPADs need to restore initial voltage after any incident, so their dead time is on the order of 10s of nanoseconds. One issue with using avalanche photodiodes is their small active area. Detectors need as large an areas as possible as it is difficult to focus fluorescence to a small area and the available area of photodiodes is significantly less that PMTs.

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