Second Harmonic Generation Microscopy (SHG Microscopy)

In second-harmonic generation (SHG) microscopy, a focused pulsed laser beam excites the sample, inducing a second-order nonlinear polarization in non-centrosymmetric regions of the material. This interaction generates photons at twice the optical frequency of the excitation light. The emitted SHG signal is collected through the microscope optics and spectrally separated from the fundamental excitation. By scanning the laser focus across the sample, a spatially resolved SHG intensity map is formed. The signal strength depends on crystal symmetry, orientation, and local electric field distribution, providing contrast based on structural rather than absorptive properties.

SHG microscopy data can be analyzed using SymPhoTime 64 and snAPI, as well as QuCoa for advanced photon-counting data processing and visualization.

Reliable SHG microscopy requires a pulsed laser source with sufficient peak power to drive nonlinear optical processes, typically operating in the picosecond or femtosecond regime. A scanning or confocal microscope provides high spatial resolution and optical sectioning capability. Efficient spectral filtering is essential to isolate the second-harmonic signal from the fundamental excitation light, while photon-counting detectors enhance sensitivity for weak SHG signals. Stable beam alignment, polarization control, and precise scanning electronics are critical for reproducible and quantitative SHG imaging in materials research.