Single-Photon Detectors from UV to IR

Designed and manufactured by Becker & Hickl: bh offer a wide range of detectors including photomultiplier tubes (PMC), hybrid photo detectors (HPM), single-photon avalanche diodes (SPAD) for the IR range as well as multi-channel- and multi-spectral detectors. Time rersolution for the fastest detectors is in the sub-20-ps range (full width at half-maximum of the IRF). The detection range covers the wavelengths from UV to the IR, with time resolution spanning from smaller than 20 ps to a few 100 ps. The detectors are suitable for fluorescence-decay recording by classic TCSPC, anti-bunching, FCS, FLIM, PLIM and other applications. bh guarantee that their TCSPC modules work with any photon counting detector, including superconducting NbN detectors,  and reach the shortest possible IRF width. Please see The bh TCSPC Handbook, chapter 'Detectors for TCSPC'. Choose your detector now:

Overview on Single-Photon Detectors

Table of Contents:

• Detectors for Time-Correlated Single Photon Counting
• Photomultiplier Tubes (PMTs)
• MCP PMTs
• Hybrid Detectors
• Single-Photon Avalanche Photodiodes (SPADs)
• Silicon Photomultipliers (SIPMs)
• Superconducting Single-Photon Detectors (SSPDs)

Detectors for Time-Correlated Single Photon Counting

Next to the TCSPC device itself, the detector is the most critical part of a TCSPC or TCSPC FLIM system. It has to have enough gain to generate an electrical pulse for a single detected photon, the detection probability should be as high as possible, the electrical pulse amplitude should be well above the noise level, the transit-time jitter should be on the order of a few 100 ps or less, and the amplitude jitter of the pulses should by as small as possible. There are also a few other parameters, such as background count rate, spectral sensitivity, size of the active area, and afterpulsing probability. All these parameters have to be considered when selecting a detector for a particular application. Commonly used detectors for TCSPC and TCSPC FLIM are described below.

Photomultiplier Tubes (PMTs)

Photomultiplier tubes, or PMTs, are around since to 40s of the last century. A photomultiplier tube (PMT) is a vacuum device which contains a photocathode, a number of  dynodes (amplifying stages) and an anode which delivers the output signal. The principle is shown in the figure below.

The operating voltage builds up an electrical field that accelerates the electrons from the photocathode to the first dynode D1, further to the next dynodes, and from D8 to the anode. When a photoelectron hits D1 it releases several secondary electrons. The same happens for the electrons emitted by D1 when they hit D2. The overall gain reaches values of 10⁶ to 10⁸. The secondary emission at the dynodes is very fast, therefore the secondary electrons resulting from one photoelectron arrive at the anode within a few ns. Due to the high gain and the short response a single photoelectron yields an easily detectable current pulse at the anode. The transit time spread, i.e. the time jitter between the photon detection and the output pulse, can be as small as 120 ps. Despite the fact that the PMT is the oldest of all single-photon detectors it is by far not outdated. The main advantage of a PMT is that can be made with an extremely large active area. PMTs are used in experiments where the light cannot be concentrated on a small area. Examples are diffuse optical imaging applications or particle detection in nuclear physics. Also the optical detectors of the large neutrino detection facilities are PMTs.

MCP PMTs

A similar gain effect as in the conventional PMTs is achieved in the Microchannel-Plate PMT, see figure below. The MCP PMT use channels with a resistive coating. A high voltage is applied along the channels. The walls of the channels work as secondary emission targets. A microchannel plate contains a large number of channels with a diameter of 3 to 15 µm. Two or three channel plates can be arranged in series to obtain a high gain. Due to the small distance between the photocathode and the first MCP and the small size of the channels the transit time spread of the photoelectrons can be as short as 25 ps or less. Tests with the bh TCSPC modules have produced IRFs of less than 20 ps FWHM.

The gain of an MCP PMT is on the order of 10⁶. A simple high-bandwidth amplifier is sufficient to record the single-photon pulses by a TCSPC module.

Hybrid Detectors

The basic principle of a Hybrid PMT (shortly ‘Hybrid Detector’) is shown in the figure below. The photoelectrons emitted by a photocathode are accelerated by a strong electrical field and injected directly into a silicon avalanche diode. The device has two amplification mechanisms. When an accelerated photoelectron hits the avalanche diode it generates a large number of electron-hole pairs in the silicon. These carriers are then further amplified by the linear gain of the avalanche diode. (Please note that the avalanche diode does not work in the break-down mode.)

Important for TCSPC, the high acceleration voltage between the photocathode and the APD results in low transit time spread. With an acceleration voltage of 8 kV the transit-time spread of the electron time-of-flight is less than 20 ps. With a TTS of the amplification system this low, the real IRF obtained with TCSPC noticeably depends on the cathode material. With GaAsP and GaAs cathodes the IRF width is between 80 ps an about 200 ps. With classic bi-alkali and multi-alkali cathodes the IRF width is below 20 ps FWHM.

Hybrid PMTs combine large area, fast response, and high detection efficiency. However, the most significant advantage of the hybrid PMT is that it is free of afterpulsing. Afterpulsing is the major source of counting background in high-repetition-rate TCSPC applications, and a known problem in fluorescence correlation measurements. Background has a detrimental effect on the accuracy of fluorescence lifetime determination. Afterpulsing in FCS results in a false peak at correlation times shorter than a few µs. Hybrid detectors are the best detectors for fluorescence decay, Molecular Imaging, Photon Correlation, and Single-Molecule experiments.

Single-Photon Avalanche Photodiodes (SPADs)

Single-Photon Avalanche Photodiodes, or SPADs, are semiconductor devices. Absorption of a photon inside the device creates an electron-hole pair. Of course, the electrical signal created by a single pair of carriers is far too small to be detected. SPADs therefore use an avalanche effect. A voltage applied over the device accelerates the electrons an holes. These take up enough energy to create more electron-hole pairs. As a result, an avalanche of electron-hole pairs develops. The avalanche causes an easily detectable pulse of current through the device, and a breakdown of the voltage over the device similar to the operation of a Geiger counter. The mode of operation is therefore also called ‘Breakdown Mode’ or ‘Geiger Mode’. To avoid total breakdown with destruction of the device, the avalanche is ‘quenched’ by temporarily reducing the voltage across the diode, see figure below.

The transit time spread of a SPAD depends on the size of the detector and the depth of the active volume. Standard devices deliver a TTS on the order of 50 ps to about 200 ps. The active areas are in the range of 50 to 200 micrometers in diameter. Experiments with bh TCSPC modules have shown that fast SPADs can reach a transit-time spread of less than 10 ps FWHM. However, this comes at the price of a very small active area. Diameters of only 10 micrometers are not unusual for ultra-fast SPADs.

Silicon Photomultipliers (SIPMs)

Silicon Photomultipliers (SIPMs) are arrays of thousands on SPADs integrated on a single silicon chip. The SPADs are passively quenched by individual series resistors, and their outputs connected into a single signal output. Active areas range from 1 mm² to more than 4 cm². A test result for a SIPM of 0.5 cm² is shown below.

The FWHM of the detector tested was 270 ps. This is not extremely fast but well within the range where TCSPC applications appear possible. However, the dark count rates of SIPMs are extremely high, and do not sufficiently decrease by cooling. The pulse amplitude jitter is high, and setting the right combination of detector gain and CFD parameters in the TCSPC module is critical. Although SIPMs are more and more praised as the non plus ultra in photon detection their performance in TCSPC applications has, so far, been disappointing. For details please see bh TCSPC Handbook, 10th edition, chapter ‘Detectors for TCSPC’.

Superconducting Single-Photon Detectors (SSPDs)

Superconducting single photon detectors (SSPDs) are made of an ultra small and ultra-thin meander or a single nanowire of niobium nitride on an isolating substrate, see figure below. The device is cooled down to a temperature below 3 Kelvin so that the NbN structure becomes superconducting. A DC current of a few µA is sent through the structure. When a photon is absorbed somewhere within the meander superconductivity is temporarily lost. The voltage across the device temporarily increases, and the corresponding voltage pulse indicates the detection of a photon. The pulse amplitude into a load of 50 Ω is on the order of 0.5 to 1  mV. Therefore, a high-gain ultra-wideband amplifier is necessary to increase the amplitude to a level that triggers the CFD of a TCSPC module.

The time resolution obtained from SSPDs can be in the sub-10ps range. SSPDs are sensitive from the visible spectral range up to almost 10 µm. The dark count rate is on the order of a few pulses per second. The problem of the SSPD in spectroscopy applications is the extremely small active area. Typically, the active area is no larger than a few micrometers.

bh TCSPC devices have obtained IRFs of 17.8 ps FWHM with a detector of SCONTEL, Moscow, and 4.4 ps FWHM with a single-nanowire detector from JPL, California.