This invention describes a method of third harmonic-generation (THG) multiphoton microscopy for interface profilometry with depth resolution at the nanometer scale. While the axial resolution of the system is on the scale of micrometers, this invention can precisely define the peak of a THG depth response curve, or how intensity varies as a function of depth within a sample, at each point on the surface of the material it is imaging. Background: When applied to interface profilometry, third harmonic-generation (THG) microscopy offers a non-invasive way to measure and map interfaces within transparent or semi-transparent materials. THG signals are generated at boundaries where there is a discontinuity in the refractive index, meaning this method can precisely identify and image internal interfaces without physical contact. In contrast, traditional profilometry methods, such as white-light interferometry or stylus profilometry, struggle with buried interfaces and require surface contact. These methods are limited by sample roughness or reflectivity. THG microscopy, however, is not limited in these ways and enables 3D profiling deep within samples. Applications:
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