Lasers, Spectrographs, and Detectors - Emerging Raman Applications and Techniques in Biomedical and Pharmaceutical Fields - page 19

limitations make a deep-depleted back-thinned CCD more susceptible to dark
noise (requiring extra cooling), the increase in sensitivity for 785 nm excitation
systems can be as much as a factor of 5-8 over a standard front-illumination
system.
Further enhancement of the signal can be obtained using an EMCCD (elec-
tron multiplying CCD) [50]. The operation of the EMCCD is analogous to an
avalanche photodiode in that the signal from each pixel is amplified by mul-
tiple avalanche gain events. The construction of the EMCCD is the same as a
'regular' CCD (with options for similar back-thinned deep-depleted configu-
rations), but with an added avalanche gain register between the shift register
and the amplifier. This gain register may be comprised of several hundred
elements, each at a potential of 50 V which has the effect of amplifying the

signal from each pixel proportionate to the number of photoelectrons present.
Because the gain in the signal before reading the pixel electron is large, while
the number of counts due to read noise is constant (and small), the contribu-
tion of read noise to the signal becomes negligible. Naturally, dark electrons
are also amplified, so EMCCDs require significant cooling -

∼

100 ◦ Cisnot
uncommon - to make full use of their gain advantage. The advantages of the
high gain are most fully realized for very low light level measurements, and
single photon counting measurements can be performed using this detector.
However, the principal advantage of the EM approach is that the CCD can be
read very rapidly, typically on the order of milliseconds; other techniques to
maximize gain and minimize read noise tend to slow down the operation of the
chip significantly. This advantage is important for high-duty cycle applications
such as imaging [51].
For very high speed applications, where gating on the nanosecond timescale
is important [52, 53], intensified CCDs (ICCD) are also available. The ICCD
is a CCD coupled to an image intensifier; this is a similar scheme to that
used in the days before CCDs, when photodiode detectors were coupled with
image intensifiers to overcome their noise limitations. The image intensifier
is essentially a multichannel photomultiplier tube. The Raman photon hits
the photocathode, and the resulting photoelectrons are accelerated through
a micro-channel plate (MCP). The MCP consists of multiple fine tubes, each
at high potential. The electrons passing through the tube cause an electron
cascade, just like a PMT; the resulting electrons impinge on a phosphor screen
and are converted back into photons for the CCD to detect. The resulting sys-
tem is complicated, expensive, and has a limited lifetime. However, the image
intensifier can be gated very rapidly; by changing the potential on the pho-
tocathode with respect to the MCP, the image intensifier becomes a shutter,
blocking all light from reaching the phosphor screen. While the CCD itself is
still limited by how fast the data can be polled off the chip to the readout
amplifier, the very fast shutter allows discrimination of very fast events. In
addition, the selection of appropriate photocathode material allows detection
of deep UV photons using a silicon detector.

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