Mapping electric field components of superchiral field with photo-induced force.

Circular dichroism (CD) of materials, difference in absorbance of left- and right-circularly polarized light, is a standard measure of chirality. Detection of the chirality for individual molecules is a frontier in analytical chemistry and optical science. The usage of a superchiral electromagnetic field near metallic structure is one promising way because it boosts the molecular far-field CD signal. However, it is still elusive as to how such a field actually interacts with the molecules. The cause is that the distribution of the electric field vector is unclear in the vicinity of the metal surface. In particular, it is difficult to directly measure the localized field, e.g., using aperture-type scanning near-field optical microscope. Here, we calculate the three-dimensional (3D) electric field vector, including the longitudinal field, and reveal the whole figure of the near-field CD on a two-dimensional (2D) plane just above the metal surface. Moreover, we propose a method to measure the near-field CD of the whole superchiral field by photo-induced force microscopy (PiFM), where the optical force distribution is mapped in a scanning 2D plane. We numerically demonstrate that, although the presence of the metallic probe tip affects the 3D electric field distribution, the PiFM is sufficiently capable to evaluate the superchiral field. Unveiling the whole figure of near-field is significantly beneficial in obtaining rich information of single molecules with multiple orientations and in analyzing the boosted far-field CD signals.

Optical Imaging of a Single Molecule with Subnanometer Resolution by Photoinduced Force Microscopy.

Visualizing the optical response of individual molecules is a long-standing goal in catalysis, molecular nanotechnology, and biotechnology. The molecular response is dominated not only by the electronic states in their isolated environment but also by neighboring molecules and the substrate. Information about the transfer of energy and charge in real environments is essential for the design of the desired molecular functions. However, visualizing these factors with spatial resolution beyond the molecular scale has been challenging. Here, by combining photoinduced force microscopy and Kelvin probe force microscopy, we have mapped the photoinduced force in a pentacene bilayer with a spatial resolution of 0.6 nm and observed its "multipole excitation". We identified the excitation as the result of energy and charge transfer between the molecules and to the Ag substrate. These findings can be achieved only by combining microscopy techniques to simultaneously visualize the optical response of the molecules and the charge transfer between the neighboring environments. Our approach and findings provide insights into designing molecular functions by considering the optical response at each step of layering molecules.

Enantioselective optical trapping of single chiral molecules in the superchiral field vicinity of metal nanostructures.

In this study, we theoretically analyzed the optical force acting on single chiral molecules in the plasmon field induced by metallic nanostructures. Using the extended discrete dipole approximation, we quantitatively examined the optical response of single chiral molecules in the localized plasmon by numerically analyzing the internal polarization structure of the molecules obtained from quantum chemical calculations, without phenomenological treatment. We evaluated the chiral gradient force due to the optical chirality gradient of the superchiral field near the metallic nanostructures for chiral molecules. Our calculation method can be used to evaluate the molecular-orientation dependence and rotational torque by considering the chiral spatial structure inside the molecules. We theoretically showed that the superchiral field induced by chiral plasmonic nanostructures can be used to selectively optically capture the enantiomers of a single chiral molecule.

Near-field circular dichroism of single molecules.

Near-field images of molecules provide information about their excited orbitals, giving rise to photonic and chemical functions. Such information is crucial to the elucidation of the full potential of molecules as components in functional materials and devices at the nanoscale. However, direct imaging inside single molecules with a complex structure in the near-field is still challenging because it requires in situ observation at a higher resolution than the molecular scale. Here, using a proven theoretical method that has demonstrated sub-nanoscale resolution based on photoinduced force microscopy (PiFM) experiment [Nat. Commun.12, 3865 (2021)10.1038/s41467-021-24136-2], we propose an approach to obtaining the near-field imaging with spatial patterns of electronic transitions of single molecules. We use an extended discrete dipole approximation method that incorporates microscopic nonlocal optical response of molecules and demonstrate that PiFM can visualize circular-dichroism signal patterns at sub-nanometer scale for both optically allowed and forbidden transitions. The result will open the possibility for the direct observation of complex spatial patterns of electronic transitions in a single molecule, providing insight into the optical function of single molecules and helping realize new functional materials and devices.

Optical gradient force on chiral particles.

When a chiral nanoparticle is optically trapped using a circularly polarized laser beam, a circular polarization (CP)-dependent gradient force can be induced on the particle. We investigated the CP-dependent gradient force exerted on three-dimensional chiral nanoparticles. The experimental results showed that the gradient force depended on the handedness of the CP of the trapping light and the particle chirality. The analysis revealed that the spectral features of the CP handedness-dependent gradient force are influenced not only by the real part of the refractive index but also by the electromagnetic field perturbed by the chiral particle resonant with the incident light. This is in sharp contrast to the well-known behavior of the gradient force, which is governed by the real part of the refractive index. The extended aspect of the chiral optical force obtained here can provide novel methodologies on chirality sensing, manipulation, separation, enantioselective biological reactions, and other fields.

Single-molecule laser nanospectroscopy with micro-electron volt energy resolution.

Ways to characterize and control excited states at the single-molecule and atomic levels are needed to exploit excitation-triggered energy-conversion processes. Here, we present a single-molecule spectroscopic method with micro-electron volt energy and submolecular-spatial resolution using laser driving of nanocavity plasmons to induce molecular luminescence in scanning tunneling microscopy. This tunable and monochromatic nanoprobe allows state-selective characterization of the energy levels and linewidths of individual electronic and vibrational quantum states of a single molecule. Moreover, we demonstrate that the energy levels of the states can be finely tuned by using the Stark effect and plasmon-exciton coupling in the tunneling junction. Our technique and findings open a route to the creation of designed energy-converting functions by using tuned energy levels of molecular systems.

Optical force mapping at the single-nanometre scale.

Three-dimensional (3D) information of the optical response in the nanometre scale is important in the field of nanophotonics science. Using photoinduced force microscopy (PiFM), we can visualize the nano-scale optical field using the optical gradient force between the tip and sample. Here, we demonstrate 3D photoinduced force field visualization around a quantum dot in the single-nanometre spatial resolution with heterodyne frequency modulation technique, using which, the effect of the photothermal expansion of the tip and sample in the ultra-high vacuum condition can be avoided. The obtained 3D mapping shows the spatially localized photoinduced interaction potential and force field vectors in the single nano-scale for composite quantum dots with photocatalytic activity. Furthermore, the spatial resolution of PiFM imaging achieved is ~0.7 nm. The single-nanometer scale photoinduced field visualization is crucial for applications such as photo catalysts, optical functional devices, and optical manipulation.

Theoretical analysis of optically selective imaging in photoinduced force microscopy.

We present a theoretical study on the measurement of photoinduced force microscopy (PiFM) for composite molecular systems. Using discrete dipole approximation, we calculate the self-consistent response electric field of the entire system, including the PiFM tip, substrate, and composite molecules. We demonstrate a higher sensitivity for PiFM measurement on resonant molecules than the previously obtained tip-sample distance dependency, z-4, owing to multifold enhancement of the localized electric field induced at the tip-substrate nanogap and molecular polarization. The enhanced localized electric field in PiFM allows high-resolution observation of forbidden optical electronic transitions in dimer molecules. We investigate the wavelength dependence of PiFM for dimer molecules, obtaining images at incident light wavelengths corresponding to the allowed and forbidden transitions. We reveal that these PiFM images drastically change with the frequency-dependent spatial structures of the localized electric field vectors and resolve different types of nanoparticles beyond the resolution for the optically allowed transitions. This study demonstrates that PiFM yields multifaceted information based on microscopic interactions between nanomaterials and light.