Ionization penalty in nonlinear optical bioimaging.

The noninvasiveness of nonlinear optical imaging techniques is quantified in terms of the number of free electron generated in the laser-tissue interaction region per photon emitted into the nonlinear optical signal. For a broad variety of biomarker dyes and bioactivity reporter proteins, this ratio is shown to approach a critical value of unity for field intensities above 1 TW/cm2. Closed-form analytical expressions for the ionization penalty function and the critical pulse repetition rate are derived for few-cycle laser pulses.