UC Irvine

We show that the gradient force generated by the near field of a chiral nanoparticle carries information about its chirality. On the basis of this physical phenomenon we propose a new microscopy technique that enables the prediction of spatial features of chirality of nanoscale samples by exploiting the photoinduced optical force exerted on an achiral tip in the vicinity of the test specimen. The tip-sample interactive system is illuminated by structured light to probe both the transverse and longitudinal (with respect to the beam propagation direction) components of the sample's magnetoelectric polarizability as the manifestation of its sense of handedness, i.e., chirality. We specifically prove that although circularly polarized waves are adequate to detect the transverse polarizability components of the sample, they are unable to probe the longitudinal component. To overcome this inadequacy and probe the longitudinal chirality, we propose a judiciously engineered combination of radially and azimuthally polarized beams as optical vortices possessing pure longitudinal electric and magnetic field components along their vortex axis, respectively. The proposed technique may benefit branches of science such as stereochemistry, biomedicine, physical and material science, and pharmaceutics.