Physics 598OS Optical Spectroscopy (Fall 06) Clegg/Chao/Liu
Lab 5: Quantum Dots – Report Questions
Answer the following questions for your lab writeup (this is in lieu of the questions in the
original lab handouts). Experiment I: Qdot Blinking
Data Analysis
The time-consuming part of the data analysis has already been done. The figure below
demonstrates how this works. The measured intensity vs time for a single q-dot is shown by the
fluctuating data points. You see repeated transitions from a high intensity to a low intensity and
back. The high intensity corresponds to a bright state when the q-dot is emitting photons. The
low intensity corresponds to a dark state when the q-dot does not emit photons (the non-zero
value is due to background).
Analysis of the time trace proceeds as follows. First, a threshold value of the intensity is selected
for each trace. This is shown by the dotted, horizontal line in the figure. For intensities greater
than this, the q-dot is considered on (bright) and for intensities below the threshold, it is
considered off (dark). The intensity data is then digitized to values of 0 and 1 based on this
criteria. This is shown by the thick black lines above and below the time trace. Finally, to
determine the lifetimes, the average values of the on-time and off-time for each trace are
calculated. This is analogous to determining the average length of the black lines at 1.0 to get τon
and the black lines at 0.0 to get τoff.
0.0
0.2
0.4
0.6
0.8
1.0
1.2
020
Measured Intensity Digitized Data
threshold
value
ON
(bright)
OFF
(dark)
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