Third Harmonic Generation (THG), as a third-order nonlinear optical process, strongly depends on tuning the excitation power density—i.e., a modest increase of excitation laser power results in a substantial increase in THG signal intensity, which is beneficial for bioimaging.

The authors of the paper, published in Biomedical Optics Express, used a unique technique for activation of microbubbles from the nanodroplets, which consist of a lipid-stabilized perfluorocarbon (PFC) core with embedded light-absorbing dye molecules. Under excitation by the laser wavelength matched with the absorption band of an embedded dye, a liquid-to-gas transition of the PFC core occurs. The interface between the aqueous surrounding and the PFC gas leads to a sharp local refractive index mismatch, providing favorable conditions for THG. This activation of the microbubbles, controlled in time and space, could allow the authors to develop efficient nanocarriers for theranostics (personalized medicine), enabling spatiotemporally controllable drug delivery, release, and efficient monitoring of these processes.

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