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Description:
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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 , τ .
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 /τ 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 . |