arXiv subjects

Takaaki V. Joya

Publications and source records attributed to Takaaki V. Joya.

4 recordsLinked to original sources

arXiv2026

Up/down-conversion of infrared light by few-layer graphene polytypes

Optical nonlinearity of materials with broken inversion symmetry enables two-photon processes where an incoming pair of photons can generate an up-converted photon with the combined frequency, or a high-energy photon can be split into a correlated pair of down-converted photons. Here, we identify few-layer graphene films that offer up/down-conversion capability in the infrared spectral range. For mixed stacking tetralayers (ABCB), which are non-centrosymmetric 2D crystals, we find highly efficient nearly resonant up/down-conversion of [$ω_1,ω_2$] photon pairs into/from a photon with $0.7$eV$<Ω=ω_1+ω_2<1.1$eV. We also note that a pronounced second-order nonlinearity in the spectral range of $0.7$eV$<Ω<0.9$eV can be promoted in rhombohedral tri- and tetralayers by asymmetrical encapsulation. Potentially, for fibers coated with few-layer graphene, this opens the door for ``in fiber" production of photon pairs with correlated polarizations.

cond-mat.mes-hall↗

arXiv2026

Second-harmonic generation in twisted double bilayer graphene: Double-resonant enhancement from moiré flat bands

We theoretically investigate second-harmonic generation (SHG) in twisted double bilayer graphene (TDBG) with AB--AB and AB--BA stacking configurations using a perturbative approach based on an effective continuum Hamiltonian. We present a systematic analysis of the SHG response as a function of twist angle, vertical bias voltage, Fermi energy, and stacking configuration. We find that the SHG signal is strongly enhanced at small twist angles due to the emergence of moiré flat bands and the associated increase in the joint density of states. Beyond this conventional enhancement mechanism, we demonstrate that the reduced bandwidth enables a pronounced double-resonant process, in which optical transitions at both $ω$ and $2ω$ are simultaneously satisfied over extended regions of the moiré Brillouin zone. This mechanism leads to a substantial amplification of the SHG response, analogous to the enhancement observed in systems with discrete energy levels, but realised here in a tuneable moiré band structure. Furthermore, we show that the AB--AB and AB--BA configurations exhibit a systematic $π$ phase shift in the SHG response at large bias voltages, reflecting their distinct symmetry and electronic structure. Our results identify double-resonance effects as a generic mechanism for enhancing SHG in moiré systems and provide a unified framework for understanding and controlling nonlinear optical responses in tuneable flat-band materials.

cond-mat.mes-hall↗

arXiv2026

Resonant Structure of Second Harmonic Generation in Multilayer Graphene Polytypes

Second harmonic generation (SHG) is a powerful optical tool for identifying non-centrosymmetric crystalline structures. Here, we analyze SHG in multilayer graphenes (MLG), with a focus on its dependence on the stacking order, encapsulation environment and biasing which break inversion symmetry in multilayers, as well as the SHG sensitivity to the electron-hole asymmetry in the MLG spectra and doping. In particular, we identify stacking-order-dependent resonant features in the SHG spectra for trilayers and tetralayers, suggesting that infra-red range SHG offers a non-invasive characterization method for distinguishing between MLG polytypes, as well as optical identification of crystallographic direction in MLG films.

cond-mat.mes-hall↗

arXiv2025

Shift current response in twisted double bilayer graphenes

We calculate the shift current response in twisted double bilayer graphenes (TDBG) by applying the perturbative approach to the effective continuum Hamiltonian. We have performed a systematic study of the shift current in AB-AB and AB-BA stacked TDBG, where we have investigated the dependence of the signal on the twist angle, the vertical bias voltage and the Fermi level. The numerical analyses demonstrate that the signal is greatly enhanced as the twist angle is reduced. Notably, we also found that there is a systematic sign reversal of the signal in the two stacking configurations below the charge neutrality point for large bias voltages. We qualitatively explain the origin of this sign reversal by studying the shift current response in AB-stacked bilayer graphene.

cond-mat.mes-hall↗