Nanoscopic observation of chiro-optical phenomena is an intriguing scientific topic but have large difficulties in the measurements, and hence the physics is still largely unexplored. So far, the chiro-optical physics have not been well unveiled; conventionally, those phenomena have been investigated only through indirect information from the macroscopic far-field optical measurements and/or predictions by theoretical simulations. To fully understand and to utilize the full potential of chiro-optical systems, the local in-situ observation of the effects from individual components is essential, as the macroscopic chiro-optical effect is not straightforward for the analysis of the microscopic phenomena. In the present study, we attempted the imaging of the chiro-optical forces confined in the nanospaces based on a photoinduced force microscopy. On the right-handed gammadion structure, the strong optical gradient force appeared at the edges of the structure under illumination of LCP light at 785 nm. The left-handed gammadion gave the similar result under RCP illumination. This research paves the way for clarifying the physics of nanoscale chiro-optics and application of that.
The chiro-optical effects are measured through the spectroscopic methods typified by optical rotation (OR) and circular dichroism
(CD). The chiro-optical effect can also appear as the motion of the chiral particles illuminated by the circularly polarized light. When a
chiral nanoparticle is optically trapped using a circularly polarized laser beam, the circular polarization (CP) dependent gradient force is
expected to be induced on the particle. We investigated the CP-dependent gradient force for the three-dimensionally chiral
nanoparticles. The experimental result showed that the gradient force depended on the handedness of CP (left- or right-handed) of the
trapping light as well as on the handedness of the particle chirality. The extended aspect of the chiral optical force obtained here can
give us novel methodologies for the researches of chirality sensing, manipulation, separation, enantio-selective biological reaction.
Optical trapping of nanoparticles is realized by optical gradient force originated from the intensity gradient of light with a focused beam. It is expected that the gradient force depending on the circular polarization (CP) acts on particles with chiral structures. Here, we investigate the CP-dependent gradient force on the chiral gold nanoparticles. We found that the amplitude (dispersion of the position of the Brownian motion) depends on the handedness of the incident light in both cases of D- and L-form particles. Based on the results on the gradient force for the chiral particles, it is expected that chiral nanomaterials can be handled by the circularly polarized light.
In photoinduced force microscopy (PiFM), amplitude modulation techniques, such as direct mode, and heterodyne amplitude modulation techniques have been used to detect the photoinduced force. These amplitude modulation techniques are affected by other forces because the resonance frequency of a cantilever shifts and non-conservative force damp the cantilever motion. Here, we investigate and propose the heterodyne frequency modulation technique (heterodyne-FM) for reduction of the influence of the other forces and photothermal force. Heterodyne-FM PiFM enabled the acquisition of PiFM images and force spectra without those artifacts. Using the heterodyne-FM technique, we succeed to visualize and evaluate photoinduced force between a tip and a quantum dot on gold surface.
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