Third harmonic imaging contrast from tubular structures in the presence of index discontinuity

Accurate interpretation of third harmonic generation (THG) microscopy images in terms of sample optical properties and microstructure is generally hampered by the presence of excitation field distortions resulting from sample heterogeneity. Numerical methods that account for these artifacts need to be established. In this work, we experimentally and numerically analyze the THG contrast obtained from stretched hollow glass pipettes embedded in different liquids. We also characterize the nonlinear optical properties of 2,2'-thiodiethanol (TDE), a water-soluble index-matching medium. We find that index discontinuity not only changes the level and modulation amplitude of polarization resolved THG signals, but can even change the polarization direction producing maximum THG near interfaces. We then show that a finite-difference time-domain (FDTD) modeling strategy can accurately account for contrast observed in optically heterogeneous samples, whereas reference Fourier-based numerical approaches are accurate only in the absence of index mismatch. This work opens perspectives for interpreting THG microscopy images of tubular objects and other geometries.

Automated summary

This study experimentally and numerically analyzed the contrast of third harmonic generation (THG) microscopy images from stretched hollow glass pipettes in different liquids. The nonlinear optical properties of 2,2'-thiodiethanol (TDE) were also characterized. The presence of index discontinuity was found to affect the THG signals, including the level, modulation amplitude, and polarization direction. The finite-difference time-domain (FDTD) modeling strategy was shown to accurately account for contrast in optically heterogeneous samples, while reference Fourier-based numerical approaches were only accurate in the absence of index mismatch. This work provides insights for interpreting THG microscopy images of tubular objects and other geometries.