Twisting a monolayer reference lets SHG microscopy read hidden phase information in 2D crystal defects
A post-objective interferometric second-harmonic generation microscope uses a rotatable monolayer MoSe2 reference to reveal phase-opposed MoS2 inversion twins with contrast up to 0.96
image: A rotatable monolayer MoSe2 reference enables angular-polarization-engineered interferometric second-harmonic generation (I-SHG) microscopy. The method converts phase-sensitive interference into high-contrast imaging, allowing inversion twins in monolayer MoS2 to be clearly distinguished.
Credit: Zhao et al., PhotoniX (2026), Beijing Normal University
A new optical microscopy method is helping researchers see an important kind of “hidden” information in atomically thin materials: phase.
Second-harmonic generation (SHG) microscopy is widely used to study crystal symmetry, lattice orientation, domain structures, and interfaces in advanced materials. Because SHG signals arise only in non-centrosymmetric structures, the technique is especially valuable for investigating low-dimensional materials and symmetry-related defects. However, conventional SHG microscopy mainly measures signal intensity, which means that structures with similar brightness but different phase can remain difficult to distinguish.
This challenge is especially relevant for inversion twins in two-dimensional semiconductors such as monolayer MoS2. Adjacent inversion-twin domains can generate nearly identical SHG intensity while differing by a π phase shift. As a result, they are often difficult to identify using conventional SHG imaging alone.
In a paper published in PhotoniX, researchers from Beijing Normal University report an angular-polarization-engineered interferometric second-harmonic generation (I-SHG) microscopy technique that overcomes this limitation. Instead of actively scanning the phase difference between the reference and sample with external phase-control elements, the team rotates a monolayer reference to tune the polarization-angle-dependent projection of the interference term, enabling stable and high-contrast phase-sensitive readout.
The key design feature is a monolayer MoSe2 reference placed behind the objective in a sample-facing configuration. Because the reference is atomically thin, the setup minimizes temporal walk-off while preserving coherence in the interferometric measurement. The geometry also avoids several practical limitations of conventional phase-modulated I-SHG schemes in microscopic systems.
Using this system, the researchers imaged monolayer MoS2 inversion twins with different morphologies and showed that structures difficult to resolve in conventional SHG could be clearly distinguished in I-SHG mode. The measured contrast varied periodically with the reference rotation angle, in agreement with the theoretical model, and reached a maximum value of 0.96.
The work provides a new route for phase-sensitive nonlinear optical microscopy and may be useful for studying symmetry-related microstructures in a broader class of materials, including other two-dimensional systems and complex functional materials.