arXiv subjects

Yuma Kawaguchi

Publications and source records attributed to Yuma Kawaguchi.

4 recordsLinked to original sources

arXiv2026

Freeform Spectrally Stable Topological Photonic Vortex Resonators

Topological concepts have been at the forefront of materials research in recent years, driving a revolution in our understanding of the response of quantum materials and enabling new ways to manipulate light and sound in topological metamaterials. Topological defects and topological boundaries of different dimensions have driven a paradigm shift in photonics, where topological photonic crystals and metamaterials can be engineered to create one-way flow of energy robust to defects or to control such flows with synthetic degrees of freedom along topological domain walls. More recently, topological point singularities encoded into photonic structures have been shown to enable confinement of optical modes with the topologically nontrivial nature of the cavity imprinted into the vorticity of optical far fields. Here we demonstrate that the two latter concepts - domain wall and point singularities - can be unified into an even more powerful tool to enable arbitrarily shaped resonant cavities of any dimension supporting spectrally stable zero-energy modes. We experimentally confirm that such modes, whose existence is guaranteed by topological principles, allow an unprecedented degree of control over the optical field, which appears to have no phase modulation across space, can have any desirable radiation pattern, and enables spectral stability regardless of shape or length.

physics.optics↗

arXiv2025

Topological Pseudospin Hall Effect and Multi-frequency Corner Modes in Kagome-based Lattices

Topological phases and modes, including pseudospin-Hall-selective edge transport and corner states, provide robust control of wave propagation and modal confinement in classical wave platforms. Under a tight-binding framework, we theoretically investigate two lattice designs derived from the kagome lattice. These extended kagome lattices support a series of localized modes, including pseudospin-Hall-like topological edge states and corner modes in different bandgaps and frequencies, which were not only achieved under lower lattice symmetries than Wu-Hu lattices, but also enable more degrees of freedom in topological and localized modes. By introducing two types of extended kagome lattices with different topological properties, multiple interesting phenomena, including newly emerged multiple groups of corner states, parametric tunable pseudospin Hall effect, and type-II corner states without long-range interactions, are found in theoretical models, which are possible and viable to achieve in artificial classical systems such as photonic, acoustic, or electrical circuits.

cond-mat.mes-hall↗

arXiv2020

All-optical nonreciprocity due to valley polarization in transition metal dichalcogenides

Nonreciprocity and nonreciprocal optical devices play a vital role in modern photonic technologies by enforcing one-way propagation of light. Most nonreciprocal devices today are made from a special class of low-loss ferrites that exhibit a magneto-optical response in the presence of an external static magnetic field. While breaking transmission symmetry, ferrites fail to satisfy the need for miniaturization of photonic circuitry due to weak character of nonreciprocal responses at optical wavelengths and are not easy to integrate into on-chip photonic systems. These challenges led to the emergence of magnetic-free approaches relying on breaking time reversal symmetry, e.g. with nonlinear effects modulating optical system in time. Here, we demonstrate an all-optical approach to nonreciprocity based on nonlinear valley-selective response in transition metal dichalcogenides (TMDs). This approach overcomes the limitations of magnetic materials and it does not require an external magnetic field. We provide experimental evidence of photoinduced nonreciprocity in a monolayer WS2 pumped by circularly polarized light. Nonreciprocity stems from valley-selective exciton-exciton interactions, giving rise to nonlinear circular dichroism controlled by circularly polarized pump fields. Our experimental results reveal a significant effect even at room temperature, despite considerable intervalley-scattering, showing potential for practical applications in magnetic-free nonreciprocal platforms. As an example, we propose a device scheme to realize an optical isolator based on a pass-through silicon nitride (SiN) ring resonator integrating the optically biased TMD monolayer.

physics.optics↗

arXiv2019

Integrated collinear refractive index sensor with Ge PIN photodiodes

Refractive index sensing is a highly sensitive and label-free detection method for molecular binding events. Commercial implementations of biosensing concepts based on plasmon resonances typically require significant external instrumentation such as microscopes and spectrometers. Few concepts exist that are based on direct integration of plasmonic nanostructures with optoelectronic devices for on-chip integration. Here, we present a CMOS-compatible refractive index sensor consisting of a Ge heterostructure PIN diode in combination with a plasmonic nanohole array structured directly into the diode Al contact metallization. In our devices, the photocurrent can be used to detect surface refractive index changes under simple top illumination and without the aid of signal amplification circuitry. Our devices exhibit large sensitivities > 1000 nm per refractive index unit in bulk refractive index sensing and could serve as prototypes to leverage the cost-effectiveness of the CMOS platform for ultra-compact, low-cost biosensors.

physics.ins-det↗