31 research outputs found
Purely excitonic lasing in ZnO microcrystals: Temperature-induced transition between exciton-exciton and exciton-electron scattering
Since the seminal observation of room-temperature laser emission from ZnO thin films and nanowires, numerous attempts have been carried out for detailed understanding of the lasing mechanism in ZnO. In spite of the extensive efforts performed over the last decades, the origin of optical gain at room temperature is still a matter of considerable discussion. In this work, we show that a ZnO film consisting of well-packed micrometer-sized ZnO crystals exhibits purely excitonic lasing at room temperature without showing any symptoms of electron-hole plasma emission, even under optical excitation more than 25 times above the excitonic lasing threshold. The lasing mechanism is shifted from the exciton-exciton scattering to the exciton-electron scattering with increasing temperature from 3 to 150 K. The exciton-electron scattering process continues to exist with further increasing temperature from 150 to 300 K. Thus, we present distinct experimental evidence that the room-temperature excitonic lasing is achieved not by exciton-exciton scattering, as has been generally believed, but by exciton-electron scattering. We also argue that the long carrier diffusion length and the low optical loss nature of the micrometer-sized ZnO crystals, as compared to those of ZnO nanostructures, plays a key role in showing room-temperature excitonic lasing
Formation of x-ray Newton’s rings from nano-scale spallation shells of metals in laser ablation
The initial stages of the femtosecond (fs) laser ablation process of gold, platinum, and tungsten were observed by single-shot soft x-ray imaging technique. The formation and evolution of soft x-ray Newton’s rings (NRs) were found for the first time. The soft x-ray NRs are caused by the interference between the bulk ablated surface and nanometer-scale thin spallation layer; they originate from the metal surface at pump energy fluence of around 1 J/cm2 and work as a flying soft x-ray beam splitter
Mechanism of enhanced optical second-harmonic generation in the conducting pyrochlore-type PbIrO oxide compound
The structural, electronic, and optical properties of pyrochlore-type
PbIrOO', which is a metal without spatial inversion
symmetry at room temperature, were investigated. Structural analysis revealed
that the structural distortion relevant to the breakdown of the inversion
symmetry is dominated by the Pb-O' network but is very small in the Ir-O
network. At the same time, gigantic second-harmonic generation signals were
observed, which can only occur if the local environment of the Ir 5
electrons features broken inversion symmetry. First-principles electronic
structure calculations reveal that the underlying mechanism for this phenomenon
is the induction of the noncentrosymmetricity in the Ir 5 bands by the
strong hybridization with O' 2 orbitals. Our results stimulate theoretical
study of inversion-broken iridates, where exotic quantum states such as a
topological insulator and Dirac semimetal are anticipated
Relaxation processes from charge-transfer excited states of organic radical 1,3,5-trithia-2,4,6-triazapentalenyl crystals studied by ultrafast luminescence spectroscopy
Ultrafast dynamics of lattice relaxation of excitons in quasi-one-dimensional halogen-bridged platinum complexes
Spin canting in nonlinear terahertz magnon dynamics revealed by magnetorefractive probing in orthoferrite
We excite the spin precession in rare-earth orthoferrite YFeO3 by the
magnetic field of intense terahertz pulse and probe its dynamics by transient
absorption change in the near infrared. The observed waveforms contain
quasi-ferromagnetic-mode magnon oscillation and its second harmonics with a
comparably strong amplitude. The result can be explained by dielectric function
derived from magnetorefractive Hamiltonian. We reveal that the strong second
harmonic signal microscopically originates from novel dynamics of the
quasi-ferromagnetic mode magnon at nonlinear regime, wherein spin canting angle
periodically oscillates.Comment: Main text: 11 pages including abstract, 4 figures and reference list.
Supplemental Material: 11 pages including 4 figures and reference lis