PicoQuant GmbH HydraHarp 400 Software V. 3.0.0.1
2.3. Photon Counting Detectors
2.3.1. Photomultiplier Tube (PMT)
A PMT consists of a light–sensitive photo cathode that generates electrons when exposed to light. These
electrons are directed onto a charged electrode called dynode. The collision of the electrons with the dynode
produces additional electrons. Since each electron that strikes the dynode causes several electrons to be
emitted, there is a multiplication effect. After further amplification by multiple dynodes, the electrons are
collected at the anode of the PMT and output as a current. The current is directly proportional to the intensity of
light striking the photo cathode. Because of the multiplicative effect of the dynode chain, the PMT is a photo
electron amplifier of high sensitivity and remarkably low noise. The high voltage driving the tube may be varied
to change the sensitivity of the PMT. Current PMTs have a wide dynamic range, i.e. they can also measure
relatively high levels of light. They are furthermore very fast, so that rapid successive events can be reliably
monitored. One photon on the photo–cathode can produce a short output pulse containing millions of
photoelectrons. PMTs can therefore be used as single photon detectors. In photon counting mode, individual
photons that strike the photo cathode of the PMT are registered. Each photon event gives rise to an electrical
pulse at the output. The number of pulses, or counts per second, is proportional to the light impinging upon the
PMT. As the number of photon events increase at higher light levels, it will become difficult to differentiate
between individual pulses and the photon counting detector will become non–linear. This usually occurs at
1..10 Mcps, dependent on the model. Similarly, in TCSPC applications, individual photon pulses may merge
into one as the count rate increases. This leads to pulse pile–up and distortions of the collected histograms.
The timing uncertainty between photon arrival and electrical output (transit time spread) is usually small
enough to permit time–resolved photon counting at a sub–nanosecond scale. In single photon counting mode
the tube is typically operated at a constant high voltage where the PMT is most sensitive.
PMTs usually operate between the blue and red regions of the visible spectrum, with greatest quantum
efficiency in the blue–green region, depending upon photo–cathode materials. Typical quantum efficiencies are
about 25 %. For spectroscopy experiments in the ultraviolet / visible / near infrared region of the spectrum, a
PMT is very well suited.
Because of noise from various sources in the tube, the output of the PMT may contain pulses that are not
related to the light input. These are referred to as dark counts. The detection system can to some extent reject
these spurious pulses by means of electronic discriminator circuitry. This discrimination is based on the
probability that some of the noise generated pulses (those from the dynodes) exhibit lower signal levels than
pulses from a true photon event. Thermal emission from the cathode that undergoes the full amplification
process can usually not be suppressed this way. In this case cooling of the detector is more helpful.
2.3.2. Micro Channel Plate PMT (MCP)
A Micro Channel Plate PMT consists of an array of glass capillaries (5–25 µm inner diameter) that are coated
on the inside with a electron–emissive material. The capillaries are biased at a high voltage. Like in the PMT,
an electron that strikes the inside wall of one of the capillaries creates an avalanche of secondary electrons.
This cascading effect creates a gain of 10
3
to 10
6
and produces a current pulse at the output. Due to the narrow
and well defined electron path inside the capillaries, the transit time spread of the output pulses is much
reduced compared to a normal PMT. The timing jitter of MCPs is therefore sufficiently small to perform time –
resolved photon counting on a picosecond scale, usually outperforming PMTs. Good but also expensive MCPs
can achieve timing uncertainties as low as 25 ps. Microchannel plates are in this respect the best match for the
HydraHarp 400 but they are limited in permitted count rate and provide lower sensitivity towards the red end of
the spectrum compared to suitably optimized SPADs.
2.3.3. Single Photon Avalanche Photo Diode (SPAD)
Avalanche Photo Diodes (APDs) are semiconductor devices, usually restricted to operation in the visible to
infrared part of the spectrum. Generally, APDs may be used for ultra–low light detection (optical powers <
1 pW), and can also be used as photon–counters in the so–called "Geiger" mode (biased slightly above the
breakdown voltage). In the case of the latter, a single photon may trigger an avalanche of about 10
8
carriers. In
this mode the device can be used as a detector for photon counting with very accurate timing of the photon
arrival. In this context they are also referred to as Single Photon Avalanche Photo Diodes (SPAD). Selected
and small devices may achieve timing accuracies down to 50 ps, but small devices are usually hard to align
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