Individual colloidal quantum dots can be optically trapped and manipulated by a single infrared laser beam
operated at low laser powers.1, 2 If the absorption spectrum and the emission wavelength of the trapping laser
are appropriately chosen, the trapping laser light can act as a source for two-photon excitation of the trapped
quantum dot. This eliminates the need for an additional excitation laser in experiments where individual quantum
dots are used both as force transducers and for visualization of the system. To use quantum dots as handles for
quantitative optical force transduction, it is crucial to perform a precise force calibration. Here, we present an
Allan variance analysis3 of individual optically trapped quantum dots and show that the optimal measurement
time for experiments involving individual quantum dots is on the order of 0.3 seconds. Due to their small size
and strong illumination, quantum dots are optimal for single molecule assays where, optimally, the presence of
the tracer particle should not dominate the dynamics of the system. As an example, we investigated the thermal fluctuations of a DNA tether using an individual colloidal quantum dot as marker, this being the smallest tracer for tethered particle method reported.
One limitation on the performance of optical traps is the noise inherently present in every setup. Therefore,
it is the desire of most experimentalists to minimize and possibly eliminate noise from their optical trapping
experiments. A step in this direction is to quantify the actual noise in the system and to evaluate how much each
particular component contributes to the overall noise. For this purpose we present Allan variance analysis as a
straightforward method. In particular, it allows for judging the impact of drift which gives rise to low-frequency
noise, which is extremely difficult to pinpoint by other methods. We show how to determine the optimal sampling
time for calibration, the optimal number of data points for a desired experiment, and we provide measurements
of how much accuracy is gained by acquiring additional data points. Allan variances of both micrometer-sized
spheres and asymmetric nanometer-sized rods are considered.
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