William Angerer - Department of Physics and Astronomy - University ...

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From this criteria, we determine the time required to satisfy this inequality. The
count rate is expressed as
(7.15)
where i represents SHG or back. The critical detection time, Td is therefore
(7.16)
where Ide is the duty cycle ratio of the the gate, e.g. Ide = 0.4 for a 400 f.lS gate and
a chopper frequency of 1 kHz. Equation (7.16) illustrates the importance of reducing
the background count to the lowest possible value. For example if the SHG photon
count rate is 100 Hz and the background count rate is 1000 Hz. then Td = 19 s. In
contrast. lowering the background rate to 100 Hz with the same signal count rate
reduces the detection time to 0.27 s. In practice, we were able so far to lower the
background count rate to ",,15 Hz by placing the detection apparatus in a nearly light
tight box and covering the entire apparatus with black cloth.
During a typical measurement of the nonlinear response of carbon nanotubes. \ve
scanned a 10 f.lm x 10 f.lm area of the sample mask at a rate of one pixel per 10 s.
So far we have not been able to unambiguously detect the carbon nanotubes in our
measurements. Using the above analysis, we determined that TSHG < 6 Hz.
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