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Summary

   The research group at the Institute for Molecular Science successfully observed the left and right handedness of material structures at the nanoscale, by illuminating chiral gold nanostructures with circularly polarized light and detecting the optical force acting on a probe near the nanostructures. This result demonstrated that it is possible to analyze the chiral structure of matter at the nanoscale using light.

   Chirality describes the property of a material structure not being superimposable onto its mirror image. Since the left and right hands, which are mirror images of each other, do not coincide (they are not the same), they are chiral. Chiral objects can be distinguished to right- or left-handedness. Many substances that constitute life are chiral, and often only one of either the right- or left-handedness naturally exists. Also, in new functional materials, their chiral nature often plays an important role for the functions. One characteristic of such chiral materials is to exhibit different responses to right- and left-circularly polarized light (Figure 1), known as the chiro-optical effect. However, observation of the chiro-optical effect at the nanoscale, occurring near a chiral substance, had not been realized until now.

Figure 1: Response to circularly polarized light (chiro-optical effect) of a chiral material (lactic acid). The response of D-lactic acid to right-circularly polarized light and that of L-lactic acid to left-circularly polarized light are equivalent. The response of L-form to right-circularly polarized light and that of D-form to left-circularly polarized light are also equivalent.

   In this study, the chiro-optical effect at the nanoscale was observed by using Photo-induced Force Microscopy under optical force mode (OF-PiFM), which detects the optical force exerted on the tip near the illuminated object. Although it was theoretically considered that the chiro-optical effect at the nanoscale could be observed using OF-PiFM, no actual observations had been reported. The research group successfully observed the chiro-optical effect at the nanoscale by detecting the optical force induced on the probe near the chiral gold nanostructure illuminated with right- and left-circularly polarized light using OF-PiFM (Figure 2).

Figure 2: Measurement concept diagram of this study.

As a sample to verify the effectiveness of this method, the research group used a gammadion-shaped gold nanostructures (Figure 3).

Figure 3: Scanning electron microscope image of the chiral gold nanostructure. (The nanostructures may be viewed as potentially offensive; however, the organization of the nanostructures was not intentional but was used for purely research purposes.)

As a result of imaging the gammadion structures with OF-PiFM, different images were obtained when illuminating with right-circularly polarized light compared to left-circularly polarized light (Figure 4).

Figure 4: Optical force images with right and left circularly polarized light illumination near the chiral gold nanostructure.

These results clarify that local right- or left-handedness at the nanoscale can be distinguished and observed using OF-PiFM with circularly polarized light.

Information of the paper

Journal Name: Nano Letters
Title: “Nanoscopic Observation of Chiro-Optical Force”
Authors: Junsuke Yamanishi, Hyo-Yong Ahn, and Hiromi Okamoto
Article Publication Date: 2023/10/04(online)
DOI: 10.1021/acs.nanolett.3c02534

Financial Supports

   This research was conducted with the support of various grants, including the Grant-in-Aid for Scientific Research (JP16H06505, JP21H04641, JP21K18884, JP22H05135, etc.).

Contact

【On the research】

YAMANISHI, Junsuke
Res. Assist. Prof., Center for Mesoscopic Sciences, Institute for Molecular Science
TEL: 0564-55-7322
E-mail: yamanishi_at_ims.ac.jp
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OKAMOTO, Hiromi
Professor, Center for Mesoscopic Sciences, Institute for Molecular Science
TEL: 0564-55-7320
E-mail: aho_at_ims.ac.jp
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【media contact】

Institute for Molecular Science, NINS
E-mail: press_at_ims.ac.jp
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