Angular-polarization-engineered interferometric second-harmonic generation microscopy

Abstract Conventional second-harmonic generation (SHG) microscopy fundamentally lacks the phase sensitivity, which is essential for characterizing symmetry properties that depend on phase information. Although interferometric SHG (I-SHG) enables phase retrieval through coherent superposition with an external reference beam, standard implementations based on phase-difference modulation face inherent limitations in microscopic systems due to optical aberrations, dispersion artifacts, and polarization constraints. Here, we introduce an angular-polarization-engineered I-SHG microscopy technique for phase-resolved imaging, offering minimal temporal walk-off, uniform phase distribution, and freedom from sample orientation and polarization restrictions. Instead of actively scanning the reference–sample phase difference with external phase-control elements, we tune the polarization-angle-dependent projection of the interference term by rotating the monolayer reference, thereby enabling stable and high-contrast phase-sensitive readout. Specifically, a thin two-dimensional material reference attached to a transparent substrate is placed between the objective and the sample in a sample-facing configuration. This setup enables the aforementioned advantages while maintaining diffraction-limited resolution and compatibility with cryo-vacuum conditions. Demonstrated through discrimination of inversion twin domains (intrinsic π-phase contrast), this approach achieves non-destructive phase-sensitive imaging at micron and sub-micron structural scales. The work establishes a versatile platform for quantitative symmetry metrology in nonlinear optical microscopy.