Biomedical Engineering Reference
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Fig. 11.11 Gated photon counting technique. Modulated laser excitation (250 Hz) energy above
WL (532 nm) and corresponding charging behavior of the optically generated electric field F o ( t )
schematically shown in the top figure .( a ) Gated photon counts measured by the single photon
counting module (SPCM) for different delay times (DL) with a fixed gate width ( W ). ( b )Sample
spectra for an example delay times and gate width shown
that no optically generated field is created by this probe laser. Using this two-color
approach we are able to continuously monitor the local electric field of QDM while
modulating this field with the high energy laser.
For the detection we used the same PL setup as discussed earlier in this chapter
while replacing the CCD with the avalanche photodiode single photon counting
module (SPCM) attached to an SR-400 photon counter. To isolate only the photons
within a given time interval, a gated photon counting technique was used to
discriminate against photons that arrive outside of the time interval of interest. The
process by which gated photon counting was carried out is schematically illustrated
in Fig. 11.11 . For example, at delay DL0 the SPCM registers only the photons
which occur within the gate width of W . To monitor the energy of the indirect PL
emission for a given gate delay the spectrometer is scanned while equal numbers
of gate periods are counted. By then increasing the gate delay and repeating the
spectrometer scan the entire time decay of F o could be measured. An example
spectrum for a specific gate width, W
s, is shown in Fig. 11.11 bandfits
to additional spectra showing the decay of the field are plotted as a function of
delay time (DL). The experiment was performed with different gate widths when
determining the decay time constant in order to eliminate any possible influence of
the gate width. There was no measurable difference when varying the gate width
=
100
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