Nanoscopic control and quantification of enantioselective optical forces

Abstract

Circularly polarized light (CPL) exerts a force of different magnitude on left- and right-handed enantiomers, an effect that could be exploited for chiral resolution of chemical compounds as well as controlled assembly of chiral nanostructures. However, enantioselective optical forces are challenging to control and quantify because their magnitude is extremely small (sub-piconewton) and varies in space with sub-micrometre resolution. Here, we report a technique to both strengthen and visualize these forces, using a chiral atomic force microscope probe coupled to a plasmonic optical tweezer. Illumination of the plasmonic tweezer with CPL exerts a force on the microscope tip that depends on the handedness of the light and the tip. In particular, for a left-handed chiral tip, transverse forces are attractive with left-CPL and repulsive with right-CPL. Additionally, total force differences between opposite-handed specimens exceed 10 pN. The microscope tip can map chiral forces with 2 nm lateral resolution, revealing a distinct spatial distribution of forces for each handedness.

Figure 1. Schematic, microscopic images, and simulations.

a, Schematic illustration of enantioselective force mapping, where circularly polarized light illuminates a coaxial nano-aperture made of gold. b, Scanning electron microscope (SEM) images of grating-flanked coaxial nano-aperture. c, Finite difference time domain (FDTD, Lumerical) simulations of the field distribution on resonance.

Figure 2. Spectroscopically measured optical forces with an achiral tip.

a, Normalized transmission spectra of the grating-flanked coaxial aperture shown in Figure 1b. The dotted curve is from numerical simulation (FDTD, Lumerical); the solid curve is from experimental measurement. The shoulder of the spectrum at longer wavelengths (~825-875 nm) comes from the converging beam interacting with the grating. The blue and red dashed lines highlight wavelengths of 660 nm and 770 nm, respectively. b, Measured optical force when the incident laser is at 770 nm, near the resonance of the coaxial aperture. The laser is manually toggled on and off (black line) to confirm the fidelity of the measurement. c, Measured optical force away from the coaxial resonance at 660 nm. d, Spectroscopic measurement of optical forces at the aperture and away from the aperture, where the on-aperture measurement follows the spectrum of the aperture. The error bars show standard deviations of measured forces at each wavelength. The solid and dashed curves are Lorentzian and polynomial fits, respectively.

Figure 3. Enantioselective optical forces with achiral and chiral tips.

a, SEM images of an achiral silicon tip. b, SEM images of a chiral tip, which is made with focused ion milling (see Methods). “t” denotes thickness of the gold coating; “p” is the period of the spiral. c, Measured optical forces using the achiral tip with left-handed (blue) and right-handed (red) CPL, at 750 nm. d, Measured optical force using the chiral tip with left-handed CPL (blue), and right-handed CPL (red). e, Comparison of the enantio-selectivity in the measured force (difference in the forces with left- and right-handed illumination), with both achiral and chiral tips. f, Measured transverse forces with the chiral tip, with left-handed (blue) and right-handed (red) CPL illumination. In all panels, the void dots are raw data, the solid dots are mean values, and error bars show standard deviations. The solid curves in panels (c), (d), and (f) are fitted with exponential decay equations, while solid curves in panel (e) are fitted with polynomial equations. g, Simulated transverse trapping potentials exerted from the coaxial aperture on a chiral nanoparticle with size comparable to the AFM tip radius. Note that we plot the negative of the trapping potential to emphasize the similar trend as the measurements.

Figure 4. Enantioselective optical force map.

a, A quadrant of an experimental chiral force map on the coaxial nano-aperture with left-CPL illumination. b, Force map with the same chiral tip, but with right-CPL illumination. Both panels (a) and (b) are total forces normalized to the background signal, and plotted in linear scale with ranges indicated in the color bar to the right. c, Simulated optical forces with left-CPL and d, right-CPL illumination on the optical tweezer. The dashed square indicates the quadrant where the optical force map is experimentally measured. e, Normalized measured forces (signal-to-background ratio) in linear scale, along three horizontal locations across the coaxial aperture. The location of each line scan is indicated by the yellow dashed line in the inset, overlaid on the AFM topological map. The white lines in the inset indicate the quadrant of the coaxial aperture where the optical force maps are taken.