Polarized electron acceleration in beam-driven plasma wakefield based on density down-ramp injection.

We investigate the precession of electron spins during beam-driven plasma-wakefield acceleration based on density down-ramp injection by means of full three-dimensional (3D) particle-in-cell (PIC) simulations. A relativistic electron beam generated via, e.g., laser wakefield acceleration, serves as the driving source. It traverses the prepolarized gas target and accelerates polarized electrons via the excited wakefield. We derive the criteria for the driving beam parameters and the limitation on the injected beam flux to preserve a high degree of polarization for the accelerated electrons, which are confirmed by our 3D PIC simulations and single-particle modeling. The electron-beam driver is free of the prepulse issue associated with a laser driver, thus eliminating possible depolarization of the prepolarized gas due to ionization by the prepulse. These results provide guidance for future experiments towards generating a source of polarized electrons based on wakefield acceleration.

Fabrication of high-Q lithium niobate microresonators using femtosecond laser micromachining.

We report on fabrication of high-Q lithium niobate (LN) whispering-gallery-mode (WGM) microresonators suspended on silica pedestals by femtosecond laser direct writing followed by focused ion beam (FIB) milling. The micrometer-scale (diameter ~82 µm) LN resonator possesses a Q factor of ~2.5 × 10(5) around 1550 nm wavelength. The combination of femtosecond laser direct writing with FIB enables high-efficiency, high-precision nanofabrication of high-Q crystalline microresonators.

Diffuse reflection inside a hexagonal nanocavity.

Geometrical diffuse reflection is a common optical phenomenon that occurs when a reflecting surface has roughness of order of hundreds of micrometres. Light rays thus reflect uniformly in all directions with each ray obeying Snell's law. Of interest is knowing what happens when light reflects off surfaces with roughness of nanometres. Here, by introducing nanoscaled roughness on the hexagonal faces of ZnO nanocavities, we observe luminescent profiles with flowery patterns, replacing the usual whispering gallery modes. The unique profile for these nanocavities is attributed to wave diffuse reflection, which occurs when the features on the reflecting surfaces are typically nanometre-sized. Light with wavelengths of similar scale "sees" these nano-perturbations, and undergoes scattering rather than geometrical diffuse reflection. These findings could benefit the fields of nanoscale topography and nanoscopic uniform lighting by using wave diffuse reflection.

Great blue-shift of luminescence of ZnO nanoparticle array constructed from ZnO quantum dots.

ZnO nanoparticle array has been fabricated on the Si substrate by a simple thermal chemical vapor transport and condensation without any metal catalysts. This ZnO nanoparticles array is constructed from ZnO quantum dots (QDs), and half-embedded in the amorphous silicon oxide layer on the surface of the Si substrate. The cathodoluminescence measurements showed that there is a pronounced blue-shift of luminescence comparable to those of the bulk counterpart, which is suggested to originate from ZnO QDs with small size where the quantum confinement effect can work well. The fabrication mechanism of the ZnO nanoparticle array constructed from ZnO QDs was proposed, in which the immiscible-like interaction between ZnO nuclei and Si surface play a key role in the ZnO QDs cluster formation. These investigations showed the fabricated nanostructure has potential applications in ultraviolet emitters.