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.

Date Published: 25 February 2009
PDF: 8 pages
Proc. SPIE 7183, Multiphoton Microscopy in the Biomedical Sciences IX, 71831K (25 February 2009); doi: 10.1117/12.807607

Published in SPIE Proceedings Vol. 7183:
Multiphoton Microscopy in the Biomedical Sciences IX
Ammasi Periasamy; Peter T. C. So, Editor(s)