Interface profilometry with third harmonic generation microscopy

We present the application of third harmonic generation (THG) multiphoton microscopy (MPM) for interface profilometry with depth resolution down to the nanometer scale. This is achieved by acquiring a z-stack of images, resulting in a THG depth response curve for each point. Despite the diffraction limited axial resolution of the system being on the scale of micrometers, the peak of the THG depth response curve can be defined precisely at each point on the surface. Because the THG signal can be found at any interface with differing third order nonlinear responses, this technique allows for characterization of sub-surface layers or interfaces. As a result, more information can be obtained compared to other profilometry techniques. In this work, we present the description of this surface profilometry technique and demonstrate its capability using real samples such as MEMS devices and semiconductor chips.