Nanoscale chiral imaging under complex optical field excitation with controllable oriented chiral dipole moment.

Abstract

Since chirality is a fundamental building block of nature, the identification of the chiral specimen's structure is of great interest, especially in applications involving the modification and utilization of proteins. In this work, by exploiting photoinduced force exerted on an achiral tip placed in the vicinity of a reciprocal chiral sample, a novel technique is proposed to detect the sample's chirality in nanoscale spatial resolution. Under separate excitation of focal field carrying chiral dipole moment with opposite handedness, there is a differential optical force ΔF exerted on the tip apex, which is connected to the enantiomer type and quasi-linearly depends on specific component of the sample's chirality parameter. With the help of time-reversal approach, we prove that the required excitation can be derived by radiation fields from the superposition of parallel electric and magnetic dipoles. Through adjusting the orientation of the chiral dipole moment, all the diagonal components of the sample's chirality can be exclusively retrieved. In addition, the sensitivity of the proposed technique is demonstrated to enantiospecify nanoscale chiral samples with chirality parameter on the order of 0.001. The proposed technique may open new avenue for wide applications in biomedicine, material science and pharmaceutics.