Stokes vector based second harmonic generation microscopy reveals molecular structure

We developed a four-channel photon counting based Stokes polarimeter for spatial characterization of polarization properties of Second Harmonic (SH) light. In this way, the critical polarization parameters can be obtained concurrently without the need of repeated image acquisition. Various polarization parameters, including the degree of polarization (DOP), the degree of linear polarization (DOLP), and the degree of circular polarization (DOCP), are extracted from the reconstructed 2D Stokes vector based SH images in a pixel-by-pixel manner. The Stokes vector measurements are further extended by varying the polarization states of the incident light and recording the resulting Stokes parameters of the SH signal. In turn this allows the molecular structure and orientation of the samples to be determined. Use of Stokes polarimetry is critical in determination of the full polarization state of light, and enables discrimination of material properties not possible with conventional crossed-polarized detection schemes. The combination of SHG microscopy and Stokes polarimeter hence makes a powerful tool to investigate the structural order of targeted specimens.