Rapid modulation of left- and right-handed optical vortices for precise measurements of helical dichroism.

Helical dichroism (HD), which is defined as the difference in optical absorption between chiral pairs of lights involving left-handed (LH) and right-handed (RH) optical vortices (OVs) carrying orbital angular momentum (OAM), is a promising way to characterize chiral materials. In the current major methods of OV generation using spatial light modulators (SLMs), the speed of OAM switching is typically as slow as 100 Hz, which is comparable to low-frequency noise, making precise chiral detection difficult. Here, we theoretically propose and experimentally demonstrate a rapid modulation of the LH and RH OVs at around 50 kHz. This modulation is achieved through a rapid modulation of circularly polarized lights carrying spin angular momentum (SAM), combined with a SAM-OAM conversion technique. We establish a theory not only for rapid OV modulation but also for HD measurements using the modulated OVs. We experimentally verify the theory using helical phase holograms drawn on a SLM as a pseudo-HD active sample. Our work addresses the limitations of current methods and offers a new avenue for precise HD measurements, paving the way for the development of sensitive chiral-optical spectroscopy techniques.

Circular dichroism of pseudo-two-dimensional metal nanostructures: Rotational symmetry and reciprocity.

Circular dichroism (CD) spectra for pseudo-two-dimensional chiral nanomaterials were systematically investigated and analyzed in relation to the rotational symmetry of the nanomaterials. Theoretically, an ideal two-dimensional chiral matter is CD inactive for light incident normal to the plane if it possesses threefold or higher rotational symmetry. If the matter has two- or onefold rotational symmetry, it should exhibit CD activity, and the CD signal measured from the back side of the matter is expected to be inverted from that measured from the front side. For pseudo-two-dimensional chiral gold nanostructures fabricated on glass substrates using electron beam lithography, matter with fourfold rotational symmetry is found to be CD active, even when special care is taken to ensure that the optical environments for the front and back sides of the sample are equivalent. In this case, the CD signal measured from the back side is found to be almost exactly the same as that measured from the front side. It is revealed that the observed chiro-optical behavior arises from three-dimensional chiral characteristics due to differences in the surface shape between the front and back sides of the structures. For matter that is two- or onefold rotationally symmetric, the CD signal measured from the back side is not coincident with that from the front side. For certain wavelength regions, the CD signals measured from the front side and back side are observed to be similar, while at other wavelengths, the inverted component of the CD signals is found to dominate. The observed CD spectral behavior for reciprocal optical measurement configurations is considered to be determined by a balance between the in-plane isotropic and anisotropic components of the chiral permittivity.

Near-Field Probing of Optical Superchirality with Plasmonic Circularly Polarized Luminescence for Enhanced Bio-Detection.

Nanophotonic platforms in theory uniquely enable < femtomoles of chiral biological and pharmaceutical molecules to be detected, through the highly localized changes in the chiral asymmetries of the near fields that they induce. However, current chiral nanophotonic based strategies are intrinsically limited because they rely on far field optical measurements that are sensitive to a much larger near field volume, than that influenced by the chiral molecules. Consequently, they depend on detecting small changes in far field optical response restricting detection sensitivities. Here, we exploit an intriguing phenomenon, plasmonic circularly polarized luminescence (PCPL), which is an incisive local probe of near field chirality. This allows the chiral detection of monolayer quantities of a de novo designed peptide, which is not achieved with a far field response. Our work demonstrates that by leveraging the capabilities of nanophotonic platforms with the near field sensitivity of PCPL, optimal biomolecular detection performance can be achieved, opening new avenues for nanometrology.

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.

Nanoscale chiral surface relief of azo-polymers with nearfield OAM light.

An optical vortex with orbital angular momentum (OAM) can be used to induce microscale chiral structures in various materials. Such chiral structures enable the generation of a nearfield vortex, i.e. nearfield OAM light on a sub-wavelength scale, thereby leading to further nanoscale mass-transport. We report on the formation of a nanoscale chiral surface relief in azo-polymers due to nearfield OAM light. The resulting nanoscale chiral relief exhibits a diameter of ca. 400 nm, which corresponds to less than 1/5-1/6th of the original chiral structure (ca. 2.1 µm). Such a nanoscale chiral surface relief is established by the simple irradiation of uniform visible plane-wave light with an intensity of <500 mW/cm2.

Optical Activity Governed by Local Chiral Structures in Two-Dimensional Curved Metallic Nanostructures.

Chiral nanostructures show macroscopic optical activity. Local optical activity and its handedness are not uniform in the nanostructure, and are spatially distributed depending on the shape of the nanostructure. In this study we fabricated curved chain nanostructures made of gold by connecting linearly two or more arc structures in a two-dimensional plane. Spatial features of local optical activity in the chain structures were evaluated with near-field circular dichroism (CD) imaging, and analyzed with the aid of classical electromagnetic simulation. The electromagnetic simulation predicted that local optical activity appears at inflection points where arc structures are connected. The handedness of the local optical activity was dependent on the handedness of the local chirality at the inflection point. Chiral chain structures have odd inflection points and the local optical activity distributed symmetrically with respect to structural centers. In contrast, achiral chain structures have even inflection points and showed antisymmetric distribution. In the near-field CD images of fabricated chain nanostructures, the symmetric and antisymmetric distributions of local CD were observed for chiral and achiral chain structures, respectively, consistent with the simulated results. The handedness of the local optical activity was found to be determined by the handedness of the inflection point, for the fabricated chain structures having two or more inflection points. The local optical activity was thus governed primarily by the local chirality of the inflection points for the gold chain structures. The total effect of all the inflection points in the chain structure is considered to be a predominant factor that determines the macroscopic optical activity. Chirality 28:540-544, 2016. © 2016 Wiley Periodicals, Inc.