Deep Red Photoluminescence from Cr3+ in Fluorine-Doped Lithium Aluminate Host Material.

Deep red phosphors have attracted much attention for their applications in lighting, medical diagnosis, health monitoring, agriculture, etc. A new phosphor host material based on fluorine-doped lithium aluminate (ALFO) was proposed and deep red emission from Cr3+ in this host material was demonstrated. Cr3+ in ALFO was excited by blue (~410 nm) and green (~570 nm) rays and covered the deep red to near-infrared region from 650 nm to 900 nm with peaks around 700 nm. ALFO was a fluorine-doped form of the spinel-type compound LiAl5O8 with slightly Li-richer compositions. The composition depended on the preparation conditions, and the contents of Li and F tended to decrease with preparation temperature, such as Al4.69Li1.31F0.28O7.55 at 1100 °C, Al4.73Li1.27F0.17O7.65 at 1200 °C, and Al4.83Li1.17F0.10O7.78 at 1300 °C. The Rietveld analysis revealed that ALFO and LiAl5O8 were isostructural with respect to the spinel-type lattice and in a disorder-order relationship in the arrangement of Li+ and Al3+. The emission peak of Cr3+ in LiAl5O8 resided at 716 nm, while Cr3+ in ALFO showed a rather broad doublet peak with the tops at 708 nm and 716 nm when prepared at 1200 °C. The broad emission peak indicated that the local environment around Cr3+ in ALFO was distorted, which was also supported by electron spin resonance spectra, suggesting that the local environment around Cr3+ in ALFO was more inhomogeneous than expected from the diffraction-based structural analysis. It was demonstrated that even a small amount of dopant (in this case fluorine) could affect the local environment around luminescent centers, and thus the luminescence properties.

Diabatic and adiabatic transitions between Floquet states imprinted in coherent exciton emission in monolayer WSe2.

Floquet engineering is a promising way of controlling quantum system with photon-dressed states on an ultrafast time scale. So far, the energy structure of Floquet states in solids has been intensively investigated. However, the dynamical aspects of the photon-dressed states under ultrashort pulse have not been explored yet. Their dynamics become highly sensitive to the driving field transients, and thus, understanding them is crucial for ultrafast manipulation of a quantum state. Here, we observed the coherent exciton emission in monolayer WSe2 at room temperature at the appropriate photon energy and the field strength of the driving light pulse using high-harmonic spectroscopy. Together with numerical calculations, our measurements revealed that the coherent exciton emission spectrum reflects the diabatic and adiabatic dynamics of Floquet states of excitons. Our results provide a previosuly unexplored approach to Floquet engineering and lead to control of quantum materials through pulse shaping of the driving field.

Anomalous Temperature Dependence of High-Harmonic Generation in Mott Insulators.

We reveal the crucial effect of strong spin-charge coupling on high-harmonic generation (HHG) in Mott insulators. In a system with antiferromagnetic correlations, the HHG signal is drastically enhanced with decreasing temperature, even though the gap increases and the production of charge carriers is suppressed. This anomalous behavior, which has also been observed in recent HHG experiments on Ca_{2}RuO_{4}, originates from a cooperative effect between the spin-charge coupling and the thermal ensemble, as well as the strongly temperature-dependent coherence between charge carriers. We argue that the peculiar temperature dependence of HHG is a generic feature of Mott insulators, which can be controlled via the Coulomb interaction and dimensionality of the system. Our results demonstrate that correlations between different degrees of freedom, which are a characteristic feature of strongly correlated solids, have significant and nontrivial effects on nonlinear optical responses.

Crystal structure of silver carbonate iodide Ag10(CO3)3I4.

The title silver carbonate iodide, Ag10(CO3)3I4, deca-silver(I) tris-(carbonate) tetra-iodide, was recently reported as a precursor of the new superionic conductor Ag17(CO3)3I11. Ag10(CO3)3I4, was prepared by heating a stoichiometric powder mixture of AgI and Ag2CO3 at 430 K. A single-crystal suitable for X-ray diffraction analysis was obtained by slow cooling of a melt with an AgI-rich composition down from 453 K. Ag10(CO3)3I4 exhibits a layered crystal structure packed along [10], in which Ag atoms are inter-calated between the layers of hexa-gonally close-packed I atoms, and CO3 groups. Up to now, Cs3Pb2(CO3)3I is the only other compound containing carbonate groups and iodide ions registered in the Inorganic Crystal Structure Database.

Superionic Ag+ Conductor Ag17(CO3)3I11.

A new superionic Ag+ conductor with a nominal composition Ag17(CO3)3I11 (11AgI-3Ag2CO3) was found in the AgI-Ag2CO3 system. The conductor, which was formed at temperatures from 100 to 170 °C, is a metastable phase that gradually decomposes into AgI and Ag10(CO3)I4 over a period of a few weeks at room temperature (RT). A Ag+ ionic conductivity of 0.16 S/cm was measured at RT, and an activation energy of 0.33 eV was evaluated in the temperature range from -9 to 19 °C. Single-crystal X-ray analysis revealed that Ag17(CO3)3I11 crystallized in a rhombohedral unit cell with hexagonal parameters of a = 15.8831(6) Å and c = 30.0730(13) Å at -183 °C and space group R3c. The Ag atoms were distributed over 53 sites in the asymmetry unit, with a maximum occupancy of 0.33(8). The continuous distribution of the partially occupied Ag sites was associated with the conduction paths of the Ag+ ions in the structure.

Control of High-Harmonic Generation by Tuning the Electronic Structure and Carrier Injection.

High-harmonic generation (HHG), which is the generation of light with multiple optical harmonics, is an unconventional nonlinear optical phenomenon beyond the perturbation regime. HHG, which was initially observed in gaseous media, has recently been demonstrated in solid-state materials. Determining how to control such extreme nonlinear optical phenomena is a challenging subject. Here, we demonstrate the control of HHG through tuning the electronic structure and carrier injection using single-walled carbon nanotubes (SWCNTs). We reveal systematic changes in the high-harmonic spectra of SWCNTs with a series of electronic structures ranging from a metal structure to a semiconductor structure. We demonstrate enhancement or reduction of harmonic generation by more than 1 order of magnitude by tuning the electron and hole injection into the semiconductor SWCNTs through electrolyte gating. These results open a path toward the control of HHG in the context of field-effect transistor devices.

Interband resonant high-harmonic generation by valley polarized electron-hole pairs.

High-harmonic generation in solids is a unique tool to investigate the electron dynamics in strong light fields. The systematic study in monolayer materials is required to deepen the insight into the fundamental mechanism of high-harmonic generation. Here we demonstrated nonperturbative high harmonics up to 18th order in monolayer transition metal dichalcogenides. We found the enhancement in the even-order high harmonics which is attributed to the resonance to the band nesting energy. The symmetry analysis shows that the valley polarization and anisotropic band structure lead to polarization of the high-harmonic radiation. The calculation based on the three-step model in solids revealed that the electron-hole polarization driven to the band nesting region should contribute to the high harmonic radiation, where the electrons and holes generated at neighboring lattice sites are taken into account. Our findings open the way for attosecond science with monolayer materials having widely tunable electronic structures.