Depth resolved multiphoton microscopy is performed by collecting the fluorescent emission of two-exciton states
in colloidal quantum dots. The biexciton is formed via two sequential resonant absorption events. Due to the
large absorption cross-section and the long lifetime of the intermediate (singly excited) state, unprecedented
low excitation energy and peak powers (down to 105W/cm2) are required to generate this nonlinear response.
Depending on the quantum dot parameters, the effective two-photon cross section can be as large as 1010 GM,
orders of magnitude higher than for nonresonant excitation. The biexciton emission can be differentiated from
that of the singly excited state by utilizing its different transient dynamics. Alternate methods for discrimination
are also discussed. This system is ideal for performing
three-dimensional microscopy using low excitation power.
Moreover, it enables to perform multiphoton imaging even with
near-infrared emitting quantum dots, which
are highly compatible with imaging deep into a scattering tissue. The depth resolution of our microscope is
shown to be equivalent to a standard two-photon microscope. The system also shows slow saturation due to the
contribution of higher (triply and above) excited states to the emitted signal.
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