It is well understood that circularly polarized light possesses optical chirality. This optical chirality can engage with material chirality, leading to optical activity, the underlying physics of chiroptical spectroscopy. Optical vortex beams with azimuthal phase possess helical wavefronts that are chiral. Under spatial confinement (e.g. tight focusing), the polarization state of the input beam and the geometrical wavefront chirality are not independent of one another, and their interplay significantly influences the optical chirality density of the beam. Here we highlight how the state of input beam polarization, e.g. linear, elliptical, and circular, influences the optical chirality of vortex beams. We show that the spatial distributions of the optical chirality density are acutely dependent on the polarization of the input beam, alongside other characteristics like the topological charge and radial index. Most striking is that even an unpolarized optical vortex beam possesses non-zero optical chirality density.
In this work we highlight enantioselective optical gradient forces acting on Rayleigh-seized chiral particles present in 3D structured (non-paraxial) optical vortex tweezing systems. One discriminatory force originates from the circular polarization of the light and is similar to previous chiral optical gradient trapping forces. Much more remarkable is the other which is independent of the input beam’s polarization state - even occurring for unpolarized light – and is not present in 2D structured light nor propagating plane waves. This latter chiral sorting mechanism allows for the enantioselective trapping of chiral particles into distinct rings in the transverse plane through conservative radial forces.
Unlike single photon absorption, the rate of multiphoton absorption processes is dependent on the polarization state of the exciting optical field. Multiphoton absorption spectroscopy is predominantly carried out with an exciting light source which is structured and polarized in a 2D sense in the transverse (x,y) plane, but homogeneous along the direction of propagation (z). Here we study two-photon absorption (TPA) with tightly focused optical vortices, where the spatial confinement generates significant longitudinal components of the electromagnetic fields in the direction of propagation, producing a 3D structured optical field at the focal plane. We discover that the additional polarization degree of freedom in the z direction for 3D structured light produces novel results in TPA compared to the preceding paraxial source excitation.
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