Enantio-specific Detection of Chirality at Nanoscale Using Photo-induced Force

Abstract

We propose a novel high resolution microscopy technique for enantio-specific detection of chiral samples down to sub-100 nm size, based on force measurement. We delve into the differential photo-induced optical force F  exerted on an achiral probe in the vicinity of a chiral sample, when left and right circularly polarized beams separately excite the sample-probe interactive system. We analytically prove that F  is entangled with the enantiomer type of the sample enabling enantio-specific detection of chiral inclusions. Moreover, we demonstrate that F  is linearly dependent on both the chiral response of the sample and the electric response of the tip and is inversely related to the quartic power of probe-sample distance. We provide physical insight into the transfer of optical activity from the chiral sample to the achiral tip based on a rigorous analytical approach. We support our theoretical achievements by several numerical examples, highlighting the potential application of the derived analytic properties. Lastly, we demonstrate the sensitivity of our method to enantiospecify nanoscale chiral samples with chirality parameter in the order of 0.01, and discuss how this could be further improved.

In pursuit of photo-induced magnetic and chiral microscopy

“…However, as we increase the amplitude of the chirality parameter of the sample, the differential force becomes more obvious, in the order of piconewtons ( Fig. 5b) which is measurable with the present force microscopes [48,58]. Note also the sign of the differential force depends on the sign (i.e., the handedness) of the chiral parameter k of the sample material.…”

“…⟩ are the exerted time-average force on the tip for the two excitation scenarios with RCP and LCP incident beams, respectively. It has been shown in [48] that this differential time-average force linearly depends on the electric polarizability of the tip a ee t , magneto-electric polarizability of the sample a em s and the intensity of the beam |E 0 | 2 , i.e.,…”

“…We put particular emphasis on a Si nanodisk structure, and discuss PiFM measurements of the electric field distribution under APB illumination. Simultaneously, we propose a novel method and a roadmap that will enable chiral force microscopy with nanoscale resolution based on the interaction of a chiral sample with an achiral (plasmonic) scanning tip under structured light illumination [48][49][50][51][52][53][54][55][56][57]. Lastly, we discuss difficulties and future developments of this exciting area of research.…”

“…(a) Schematic of the photo-induced chiral force microscopy (PiCFM) concept, enabling the detection of chirality of a sample based on differential force measurement. (b) Photo-induced differential force exerted on the tip (represented by the yellow nanosphere), from two incident scenarios of RCP and LCP light versus chirality parameter of the sample[48]. Differential time-average force can be positive or negative, depending on the chiral parameter k of the material in the sample (represented by the blue sphere).…”