A hydrogen bond (H-bond) is one of the most fundamental and important noncovalent interactions in chemistry, biology, physics, and all other molecular sciences. Especially, the dynamics of a proton or a hydrogen atom in the H-bond has attracted increasing attention, because it plays a crucial role in (bio)chemical reactions and some physical properties, such as dielectricity and proton conductivity. Here we report unprecedented H-bond-dynamics-based switching of electrical conductivity and magnetism in a H-bonded purely organic conductor crystal, κ-D3(Cat-EDT-TTF)2 (abbreviated as κ-D). This novel crystal κ-D, a deuterated analogue of κ-H3(Cat-EDT-TTF)2 (abbreviated as κ-H), is composed only of a H-bonded molecular unit, in which two crystallographically equivalent catechol-fused ethylenedithiotetrathiafulvalene (Cat-EDT-TTF) skeletons with a +0.5 charge are linked by a symmetric anionic [O···D···O](-1)-type strong H-bond. Although the deuterated and parent hydrogen systems, κ-D and κ-H, are isostructural paramagnetic semiconductors with a dimer-Mott-type electronic structure at room temperature (space group: C2/c), only κ-D undergoes a phase transition at 185 K, to change to a nonmagnetic insulator with a charge-ordered electronic structure (space group: P1). The X-ray crystal structure analysis demonstrates that this dramatic switching of the electronic structure and physical properties originates from deuterium transfer or displacement within the H-bond accompanied by electron transfer between the Cat-EDT-TTF π-systems, proving that the H-bonded deuterium dynamics and the conducting TTF π-electron are cooperatively coupled. Furthermore, the reason why this unique phase transition occurs only in κ-D is qualitatively discussed in terms of the H/D isotope effect on the H-bond geometry and potential energy curve.

We demonstrate two-dimensional photonic crystals of silicon carbide (SiC)-a wide bandgap semiconductor and one of the hardest materials-at near-infrared wavelengths. Although the refractive index of SiC is lower than that of a conventional semiconductor such as GaAs or Si, we show theoretically that a wide photonic bandgap, a broadband waveguide, and a high-quality nanocavity comparable to those of previous photonic crystals can be obtained in SiC photonic crystals. We also develop a process for fabricating SiC-based photonic crystals that experimentally show a photonic bandgap of 200 nm, a waveguide with a 40-nm bandwidth, and a nanocavity with a high quality factor of 4,500. This demonstration should stimulate further development of resilient and stable photonics at high power and high temperature analogous to SiC power electronics.

We demonstrate second-harmonic generation (SHG) in a silicon-carbide (SiC)-based heterostructure photonic crystal nanocavity by using a pulsed laser. We observe SHG light radiated from the SiC nanocavity and estimate the conversion efficiency in the cavity to be 2.59×10(-5) (=0.15  W(-1)) at an average input power of 0.17 mW. The near-field patterns and polarization characteristics of the SHG light are investigated experimentally and theoretically, and the results are in qualitatively good agreement.

Strongly enhanced quantum fluctuations often lead to a rich variety of quantum-disordered states. Developing approaches to enhance quantum fluctuations may open paths to realize even more fascinating quantum states. Here, we demonstrate that a coupling of localized spins with the zero-point motion of hydrogen atoms, that is, proton fluctuations in a hydrogen-bonded organic Mott insulator provides a different class of quantum spin liquids (QSLs). We find that divergent dielectric behavior associated with the approach to hydrogen-bond order is suppressed by the quantum proton fluctuations, resulting in a quantum paraelectric (QPE) state. Furthermore, our thermal-transport measurements reveal that a QSL state with gapless spin excitations rapidly emerges upon entering the QPE state. These findings indicate that the quantum proton fluctuations give rise to a QSL—a quantum-disordered state of magnetic and electric dipoles—through the coupling between the electron and proton degrees of freedom.

To realize nanophotonic devices that operate in both the infrared and visible wavelength ranges on a single wafer, we investigated the optical characteristics of silicon carbide (SiC)-based photonic crystal nanocavities. By fabricating nanocavities with lattice constants ranging from 150 to 600 nm, we experimentally demonstrated resonant wavelengths of individual cavities ranging from 550 to 1450 nm on a single SiC wafer. Furthermore, this ultra-broadband operation reveals the material dispersion of the thin SiC wafer, which is estimated as nSiC = 2.34 + 9.18 × 104/λ2, over the wide range of aforementioned wavelengths.

Chemical one-electron oxidation of [Pt(II)(3,5-DTBCat)(DTBbpy)] (1) (3,5-DTBCat = 3,5-di-tert-butyl catecholato, DTBbpy = 4,4'-di-tert-butyl-2,2'-bipyridine) afforded the corresponding paramagnetic complex, [Pt(II)(3,5-DTBSQ)(DTBbpy)](+)PF6(-) (1(PF6)), as a result of ligand-centred oxidation. While complex 1 is relatively stable in common organic solvents, the present study demonstrates that cationic [1](+) is subject to an unprecedented solvent-induced disproportionation in highly polar solvents such as DMSO and DMF, which results in the formation of 1 and a free benzoquinone.

We show that a SiC photonic crystal cannot only inhibit two photon absorption completely, but also suppress higher-order multiple photon absorption significantly at telecommunication wavelengths, compared to conventional Si-based photonic crystal nanocavities. Resonant spectra of a SiC nanocavity maintain a Lorentzian profile even at input energies 100 times higher than what can be applied to a Si nanocavity without causing nonlinear effects. Theoretical fitting of the results indicates that the four photon absorption coefficient in the SiC nanocavity is less than 2.0 × 10(-5) cm(5)/GW(3). These results will contribute to the development of high-power applications of SiC nanocavities such as harmonic generation, parametric down conversion, and Raman amplification.

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