arXiv subjects

Keisuke Shinokita

Publications and source records attributed to Keisuke Shinokita.

6 recordsLinked to original sources

arXiv2026

Ultrafast Nano-Imaging and Optical Control of Hyperbolic Phonon Polaritons at hBN/WS$_2$ Heterojunctions

Manipulating nanoscale light-matter interactions on ultrafast time scales is indispensable for future polaritonic devices. Hyperbolic phonon polaritons (HPhPs) in van der Waals materials enable deep subwavelength confinement of electromagnetic fields in the infrared region and long-distance propagation of polaritonic waves. However, achieving ultrafast imaging and optical control of HPhPs remains a major challenge. Here, we demonstrate the direct observation of transient modulation of HPhPs induced by local photocarrier generation in WS$_2$/hBN heterostructures using ultrafast infrared scanning near-field optical microscopy. We implement grating-based spectral filtering of broadband near-field scattering to simultaneously achieve nanoscale and femtosecond spatiotemporal resolution together with fine spectral selectivity. This ultrafast nano-imaging technique reveals that photocarriers in WS$_2$ modulate the polaritonic field amplitudes and wavelengths of HPhPs in hBN. Theoretical simulations corroborate that these changes arise from photoinduced changes in WS$_2$ dielectric properties. This approach offers a versatile platform for exploring ultrafast polaritonic dynamics at the nanoscale.

physics.optics↗

arXiv2025

Lattice-guided growth of dense arrays of aligned transition metal dichalcogenide nanoribbons with high catalytic reactivity

Transition metal dichalcogenides (TMDs) exhibit unique properties and potential applications when reduced to one-dimensional (1D) nanoribbons (NRs), owing to quantum confinement and high edge densities. However, effective growth methods for self-aligned TMD NRs are still lacking. We demonstrate a versatile approach for lattice-guided growth of dense, aligned MoS2 NR arrays via chemical vapor deposition (CVD) on anisotropic sapphire substrates, without tailored surface steps. This method enables the synthesis of NRs with widths below 10 nm and longitudinal axis parallel to the zigzag direction, being also extensible to the growth of WS2 NRs and MoS2-WS2 hetero-nanoribbons. Growth is influenced by both substrate and CVD temperature, indicating the role of anisotropic precursor diffusion and substrate interaction. The 1D nature of the NRs was asserted by the observation of Coulomb blockade at low temperature. Pronounced catalytic activity was observed at the edges of the NRs, indicating their promise for efficient catalysis.

cond-mat.mes-hall↗

arXiv2024

Autonomous robotic mechanical exfoliation of two-dimensional semiconductors combined with Bayesian optimization

Simple mechanical exfoliation of layered materials is the most frequently employed method for producing high-quality monolayers of two-dimensional semiconducting materials. However, the mechanical exfoliation by human hands is a microscopically sophisticated process with a large number of microscopic parameters, which requires significant operator efforts and limits the reproducibility in achieving high-quality and large-area monolayer semiconducting materials. Herein, we have proposed a new strategy for the mechanical exfoliation by combining a developed robotic system and Bayesian optimization. We have demonstrated that it is possible to explore the optimized experimental conditions among a large number of parameter combinations for mechanical exfoliation in a relatively small number of experimental trials. Moreover, the entire mechanical exfoliation process from preparation to detection of monolayer semiconductors was performed by the developed autonomous robotic system. The optimized experimental condition was determined through only 30 trials of mechanical exfoliation experiments, representing 2.5% of all experimental parameter conditions. As a result, the critical parameters for the efficient fabrication of large-area monolayer WSe$_2$ were elucidated.

cond-mat.mtrl-sci↗

arXiv2023

Quantum coherence and interference of a single moiré exciton in nano-fabricated twisted semiconductor heterobilayers

Moiré potential acts as periodic quantum confinement for optically generated exciton, generating spatially ordered zero-dimensional quantum system. However, broad emission spectrum arising from inhomogeneity among moiré potential hinders the exploration of the intrinsic properties of moiré exciton. In this study, we have demonstrated a new method to realize the optical observation of quantum coherence and interference of a single moiré exciton in twisted semiconducting heterobilayer beyond the diffraction limit of light. A significant single and sharp photoluminescence peak from a single moiré exciton has been demonstrated after nano-fabrication. We present the longer duration of quantum coherence of a single moiré exciton, which reaches beyond 10 ps and the accelerated decoherence process with elevating temperature and excitation power density. Moreover, the quantum interference has revealed the coupling between moiré excitons in different moiré potential minima. The observed quantum coherence and interference of moiré exciton will facilitate potential application toward quantum technologies based on moiré quantum systems.

physics.optics↗

arXiv2023

Dynamics of moire trion and its valley polarization in microfabricated WSe2/MoSe2 heterobilayer

The moire potential, induced by stacking two monolayer semiconductors with slightly different lattice mismatches, acts as periodic quantum confinement for optically generated excitons, resulting in spatially ordered zero-dimensional quantum systems. However, there are limitations to exploring intrinsic optical properties of moire excitons due to ensemble averaged and broadened emissions from many peaks caused by the inhomogeneity of the moire potential. In this study, we proposed a microfabrication technique based on focused Ga+ ion beams, which enables us to control the number of peaks originating from the moire potential and thus explore unknown moire optical characteristics of WSe2/MoSe2 heterobilayers. By taking advantage of this approach, we reveal emissions from a single moire exciton and charged moire exciton (trion) under electrostatic doping conditions. We show the momentum dark moire trion state above the bright trion state with a splitting energy of approximately 4 meV and clarify that the dynamics are determined by the initial trion population in the bright state. Furthermore, the degree of negative circularly polarized emissions and their valley dynamics of moire trions are dominated by a very long valley relaxation process lasting ~700 ns. Our findings on microfabricated heterobilayers could be viewed as an extension of our groundbreaking efforts in the field of quantum optics application using moire superlattices.

physics.optics↗

arXiv2020

Moiré exciton dynamics and moiré exciton-phonon interaction in a WSe$_2$/MoSe$_2$ heterobilayer

Moiré patterns with angular mismatch in van der Waals heterostructures composed of atomically thin semiconducting materials are a fascinating platform to engineer the optically generated excitonic properties towards novel quantum phenomena. The moiré pattern as a periodic trap potential can give rise to spatially ordered zero-dimensional (0D) exciton ensembles, which offers the possibility for dense coherent quantum emitters and quantum simulation of many-body physics. The intriguing moiré exciton properties are affected by their dynamics and exciton-phonon interaction. However, the moiré exciton dynamics and the interaction between the moiré exciton and phonon are still elusive. Here, we report the moiré exciton and phonon interaction in a twisted WSe$_2$/MoSe$_2$ heterobilayer based on near-resonant photoluminescence excitation spectroscopy. We observed the selective excitation of the ground state of the moiré exciton at phonon resonance. The otherwise negligible small absorption below the continuum state is a hallmark of the density of states of a 0D-like system. In addition, the excitation power dependence of the PL spectra reveals the dynamics of moiré exciton ensembles between different potential minima with discrete energy levels via the resonant phonon scattering process. The results presented here of the moiré exciton dynamics under suppressed phonon interaction could pave a new way for the exploration of novel quantum phenomena of the moiré exciton towards potential applications in quantum optics.

cond-mat.mes-hall↗