Biomedical Engineering Reference
In-Depth Information
(a)
(b)
Output
pulse
Output
pulse
D2
D4
D6
D8
Avalanche
Photo-
cathode
Photon
D1
D3
D5
D7
(c)
Channels stage
(d)
Multi-
anode
Output
pulses
Detail of the channels stage
Output
pulse
A1
A2
A3
A4
A5
Photo-
cathode
Photo-
cathode
Anode
FIgurE 2.15
Sketch of various detectors: (a) Photomultiplier tube; (b) avalanche photodiode; (c) microchannel
plate; (d) photomultiplier array. (From Becker, W. and Bergmann, A.
Tutorial—Becker & Hickl GmbH.
Available at:
http://www.becker-hickl.com/literature.htm.)
the electrons from the cathode to the next dynodes and to the anode. In each dynode, the number
of photoelectrons is amplified and the total gain reaches values of 10
6
-10
8
. A wide variety of pho-
tocathode and dynode geometries has been developed. The sensitivity of the PMT and its spectral
response depend on the photocathode type, while the dynode geometry affects the overall gain
and the temporal response of the detector.
•
Avalanche photodiodes (APDs):
The APD consists in a photodiode driven with a voltage close to
or slightly above the breakdown voltage. In this way, when a photon is detected, the generated
electron−hole pair creates an avalanche that can be detected. The quantum efficiency is extremely
high in the green range of the spectrum (up to 0.7), but it drops in the blue region (0.2-0.4) (Cova
et al., 1982). The sensitivity and the quantum efficiency are related by the following expression:
S
hc
e
QE
=
(2.7)
λ
where
QE
is the quantum efficiency,
S
the sensitivity,
h
the Planck constant,
c
the speed of light in
vacuum,
e
the electron charge, and λ the wavelength.
•
Microchannel plates (MCPs):
An MCP is a detector with a working principle similar to the con-
ventional PMT, but the photoelectron amplification is achieved in a channel with a conductive
coating instead of in a multiple dynode stage, as depicted in Figure 2.15c. MCPs are the fastest
detectors currently available. Moreover, the MCPs technique allows building position-sensitive
detectors and image intensifiers. The sensitivity is comparable to PMTs.
•
CCDs and PMT arrays:
These array detectors are mostly used in combination with digital holo-
graphic equipment (CCD) and with multifocal setups (both CCD and PMT arrays). In general,
the sensitivity and the noise level of a common CCD sensor are not suitable for the detection of
weak SHG signals, so that an intensifying system is required to amplify the signal originated by
detected photons. Very often, cooled intensified CCDs or electron-multiplying CCDs (EMCCDs)
are used in both digital holographic and multifocal systems in non-de-scan mode. On the oppo-
site, a PMT array (Figure 2.15d), although it does not provide enough resolution to be used with
digital holographic equipment, can be used in MMM in de-scan mode (Kim et al., 2007, Martini
et al., 2007). It offers the advantage of a very high sensitivity and fast response but can be used
only in backward detection. A forward detection is possible as well, but it requires a second
scanning head for de-scanning SHG light synchronously with the scanning of the beam focus.
