arXiv subjects

Yoshito Y. Tanaka

Publications and source records attributed to Yoshito Y. Tanaka.

7 recordsLinked to original sources

arXiv2026

Orbital Optical Chirality as the Origin of Vortex Dichroism

Optical chirality quantifies the geometrical twisting of electromagnetic fields and underlies chiral light-matter interactions, yet its conventional formulation captures only the spin-associated chiral geometry of light. We introduce orbital optical chirality and derive its continuity equation, revealing physical properties distinct from those of angular momentum. For vortex beams, spin and orbital optical chiralities follow the spin and orbital indices, respectively. Applied to a twisted nanorod dimer, orbital optical chirality gives rise to vortex dichroism through quadrupolar hybridized modes, while dipolar modes exhibit only circular dichroism. These results establish a unified framework for optical chirality and provide access to chiral geometries beyond those resolved by spin optical chirality alone.

physics.optics↗

arXiv2026

Structured Optical Fields Reveal Nanoscale Chiral Light-Matter Interactions Governed by Optical Chirality

Optical chirality has been proposed as the local electromagnetic quantity governing chiral light-matter interactions, yet in conventional circularly polarized fields its magnitude is locked to the electric energy density, obscuring its independent role. Here we create a structured optical field in which optical chirality arises spatially in magnitude and sign while the electric energy density remains nearly uniform. A single chiral nanoparticle exhibits a differential response that follows this spatial variation, whereas no modulation is observed for an achiral nanoparticle, providing direct experimental evidence that optical chirality governs nanoscale chiral light-matter interactions. Measurements of wavelength-dependent optical rotation further provide an experimental estimate of the chiral polarizability, predicting a chiral gradient force of approximately 100 fN and a one-dimensional trapping potential exceeding the thermal energy at room temperature under optimized aqueous trapping conditions.

physics.optics↗

arXiv2025

3-dimensional plasmonic nanomotors enabled by independent integration of Optical Pulling and Lateral Forces

Light-matter interactions generally involve momentum exchange between incident photons and the target object giving rise to optical forces and torques. While typically weak, they become significant at the nanoscale, driving intense research interest in the exploitation of photon recoil to drive micro- and nanostructures. While great progress has been attained in controlling transversal degrees of freedom, three-dimensional movement remains challenging, particularly due to the impractical realization of pulling forces that oppose the direction of incident light. Here we theoretically present a novel nanomotor design that enables independent control over both transverse and longitudinal motion. This design exploits coupling between an azimuthally polarized Bessel beam and a dielectric glass cylinder to realistically achieve optical pulling forces. At the same time, asymmetric plasmonic dimers, embedded within the cylinder, provide lateral motion, through asymmetric scattering under plane wave illumination. We further demonstrate that unwanted displacements and rotations can be restrained, even at long illumination times. Our design unlocks a new degree of freedom in motion control, allowing for pulling, pushing, and lateral movement by simply tuning the polarization or switching between plane waves and Bessel beams.

physics.optics↗

arXiv2025

Unveiling orbital optical chirality through multipolar chiral light-matter interaction

Chiral light-matter interactions have traditionally been understood in terms of electric-magnetic dipolar interference driven by light with spin angular momentum. Here, we show that optical chirality can also originate from the orbital angular momentum (OAM) of light, giving rise to higher-order multipolar chiral responses. Using a twisted gold nanorod dimer and tightly focused circularly polarized optical vortex beams carrying spin and orbital angular momenta of the same sign, we measure spectrally and spatially resolved chiral dichroism signals that persist even where spin optical chirality vanishes, revealing a quadrupole-mediated chiral interaction driven by OAM. The spectra reveal clear quadrupole resonances whose spectral profile is strongly modulated by the OAM sign, demonstrating an OAM-driven chiral interaction. Crucially, the signal satisfies optical reciprocity, ruling out artefacts from anisotropy or misalignment and confirming its nature as a true chiral response. Angular momentum dissipation analysis further shows that orbital contributions dominate over spin. These findings establish the existence of a distinct form of optical chirality, referred to as orbital optical chirality, which opens new avenues for probing and controlling multipolar chiral light-matter interactions beyond the dipolar paradigm.

physics.optics↗

arXiv2025

Helical dichroism for hybridized quadrupole plasmon modes in twisted nanorods

Helical dichroism (HD), originating from the interplay between chiral plasmonic structures and left and right vortex light carrying orbital angular momentum (OAM), has attracted significant attention across various disciplines owing to its implications in fundamental physics and applications. However, the precise relationship between HD and the excited plasmon modes remains elusive. Owing to the weak chiroptical response to OAM light, chiral structures have required dimensions larger than the incident light wavelength to obtain observable HD signals, resulting in complex superpositions of higher-order plasmon modes. In this work, we reveal that a simple twisted nanorod dimer with a size smaller than the incident light wavelength, exhibits remarkable HD due to the strong coupling between quadrupole plasmon modes excited in the nanorods, followed by the plasmon hybridization. Positive and negative HD responses were measured at different resonance wavelengths corresponding to two hybridized quadrupole modes, in good agreement with the calculated results. This spectral behavior of the HD is clearly different from that of the circular dichroism (CD) based on spin angular momentum (SAM) of light, indicating that the quadrupole HD arises from the OAM rather than the SAM. These findings pave the way for a deeper understanding of light-matter interactions concerning angular momentum.

physics.optics↗

arXiv2025

Conservation law for angular momentum based on optical field derivatives: Analysis of optical spin-orbit conversion

We present a theoretical framework for analyzing the loss of optical angular momentum (AM), including spin (SAM) and orbital (OAM) components, in light-matter interactions. Conventional SAM and OAM conservation laws rely on transverse field components, neglecting longitudinal fields and limiting applicability to vacuum. Our approach defines optical AM using time derivatives of the electric and magnetic fields, yielding a gauge-invariant formulation that includes both transverse and longitudinal components and explicitly incorporates charge and current densities. This enables a more complete description of AM dissipation in materials. We apply this framework to analyze spin-orbit conversion (SOC) in two scenarios: scattering of circularly polarized (CP) beams by a gold nanoparticle and focusing of CP and linearly polarized optical vortex beams by a lens. The results show that SOC depends on particle size and polarization, with notable OAM loss in larger particles and CP beam focusing. This framework enables the evaluation of previously overlooked SAM and OAM losses, providing a powerful tool for studying systems in which the analysis of AM losses is intrinsically important, such as chiral materials, as well as for designing photonic devices and exploring light-matter interactions at the nanoscale.

physics.optics↗

arXiv2017

Unidirectional control of optically induced spin waves

Unidirectional control of optically induced spin waves in a rare-earth iron garnet crystal is demonstrated. We observed the interference of two spin-wave packets with different initial phases generated by circularly polarized light pulses. This interference results in unidirectional propagation if the spin-wave sources are spaced apart at 1/4 of the wavelength of the spin waves and the initial phase difference is set to pi/2. The propagating direction of the spin wave is switched by the polarization helicity of the light pulses. Moreover, in a numerical simulation, applying more than two spin-wave sources with a suitable polarization and spot shape, arbitrary manipulation of the spin wave by the phased array method was replicated.

physics.app-ph↗