  Individual colloidal quantum dots can be optically trapped and manipulated by
a single infrared laser beam operated at low laser powers. 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 analysis 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.
