Stokes vector based polarization resolved second harmonic microscopy of starch granules

Abstract

We report on the measurement and analysis of the polarization state of second harmonic signals generated by starch granules, using a four-channel photon counting based Stokes-polarimeter. Various polarization parameters, such as the degree of polarization (DOP), the degree of linear polarization (DOLP), the degree of circular polarization (DOCP), and anisotropy are extracted from the 2D second harmonic Stokes images of starch granules. The concentric shell structure of a starch granule forms a natural photonic crystal structure. By integration over all the solid angle, it will allow very similar SHG quantum efficiency regardless of the angle or the states of incident polarization. Given type I phase matching and the concentric shell structure of a starch granule, one can easily infer the polarization states of the input beam from the resulting SH micrograph.

Investigating starch gelatinization through Stokes vector resolved second harmonic generation microscopy

“…It is noted that the strong SH signal arises due to the fraction of different handed amylopectin molecules being unequal, in turn producing a non-centrosymmetric structure18. The effect of the molecular arrangement of the concentric shell like structure of starch granules is observed in the SH intensity pattern and in various polarization components1217. Figure 2 compares the SH signal (S 0 ), Stoke vector (S 3 ) and DOCP of dry and hydrated starch granules at room temperature.…”

“…The molecular events responsible for this transition are not certain, but changes of shape and size of the granules, absorption of water and swelling, crystallite melting, and leaching of amylose (amylopectin) from the granules are observed3637. Second-order nonlinear effects are, in particular, very sensitive to the structural symmetry of the samples; such that the anisotropic and concentric shell structure of starch granules gives rise to a unique polarization dependent behavior of the SH signal712. The break up and partial dissolution of amylopectin structure upon heating in the presence of water causes the degradation of the SH signal.…”

“…The basic principles of four channel based Stokes polarimetry is discussed in detail in Mazumder et al 1112…”

“…Therefore, using Stokes matrix based formalism, crucial physical parameters of the SH light can be inferred from the measured Stokes parameters111213, including DOP, DOLP and DOCP. These include the degree of polarization (DOP), degree of linear polarization (DOLP), degree of circular polarization (DOCP), as defined respectively by the following relations:…”

“…Stokes vector based SHG microscopy resolves the polarization states of the SH signal and allows the deduction of the molecular organization of collagen, skeletal muscle, starch etc 111213…”