Thin-film lithium niobate (TFLN) in the form of x- or z-cut lithium-niobate-on-insulator (LNOI) has recently popped up as a very promising and novel platform for developing integrated optoelectronic (nano)devices and exploring fundamental research. Here, we investigate the coherent interaction length $l_{c}$ of optical second-harmonic (SH) microscopy in such samples, that are purposely prepared into a wedge shape, in order to elegantly tune the geometrical confinement from bulk thicknesses down to $approx$ 50 nm. SH microscopy is a very powerful and non-invasive tool for the investigation of structural properties in the biological and solid-state sciences, especially also for visualizing and analyzing ferroelectric domains and domain walls. However, unlike bulk LN, SH microscopy in TFLN is largely affected by interfacial reflections and resonant enhancement that both rely on film thickness and substrate material. In this paper we show that the dominant SHG contribution measured in back-reflection, is the co-propagating phase-matched SH signal and textit{not} the counter-propagating SH portion as is the case for bulk LN samples. Moreover, $l_{c}$ dramatically depends also on the incident pump laser wavelength (sample dispersion) but even more on the numerical aperture of the focussing objective in use. These experimental findings on x- and z-cut TFLN are excellently backed up by our advanced numerical simulations.
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