Two photon luminescence from quantum dots using broad and narrowband ultrafast laser pulses



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Nonlinear optical microscopy (NLOM) offers many advantages when imaging intact biological samples. By using ultrafast lasers in the near infrared and two photon excitation (TPE), signal production is limited to the focal volume and provides an excellent means for rendering thin, microscopic images from within the sample. Exogenous fluorophores/lumiphores may be used as efficient contrast agents to tag specific targets and provide enhanced signal. The efficiency of the TPE process in these contrast agents is broadly assumed to vary inversely with the laser pulsewidth, ?. In this work, we investigate the TPE efficiency of transform limited broadband (~133nm, ~10fs) and narrowband (~11nm, ~170fs) pulses in the generation of twophoton luminescence from semiconductor nanocrystals or quantum dots (QD?s) both theoretically and experimentally. Compared to standard organic dyes, QD?s possess a relatively broad, uniform spectral response that enables better use of the full bandwidth from the broadband laser. Theoretical calculations including both degenerate and non-degenerate TPE indicate a rolloff from the 1/? behavior as the pulses? spectral bandwidth becomes broader than the absorption spectra of the QD?s. Experimentally measured enhancement in luminescence intensity while using a broadband pulse is compared with the simulated enhancement in two-photon luminescence. A combination of increased understanding of the excitation processes in NLOM and proper selection of contrast agents will help in advancing the role of broadband ultrafast lasers in NLOM.