
Theoretical Framework forSHG Microscopy 13
1.3.3 Inuence of Tight Focusing on the Local Polarization: Beyond the
Paraxial Approximation
Under conditions of tight focusing, the local elds experienced within the confocal volume are gener-
ally distinct from those introduced in the far eld. e reasons for this eect are depicted qualitatively
in Figure 1.4. In brief, the high angles of incidence arising when using high NA objectives introduce
eld components in all three directions even if starting with linearly polarized light. e most com-
mon strategies developed for modeling the local elds generally consider the focusing of a Gaussian