Fangteng Zhang

Efficient spectral conversion from visible to near-infrared in transparent glass ceramics containing Ce3+–Yb3+ codoped Y3Al5O12 nanocrystals

Transparent glass ceramics containing Ce3+–Yb3+ codoped Y3Al5O12 nanocrystals were prepared, and their microstructures were characterized by X-ray diffraction and transmission electron microscopy. Intense near-infrared emission at around 1000 nm, attributed to the 2F5/2 → 2F7/2 transition of Yb3+, was observed upon excitation at 452 nm. Efficient energy transfer from Ce3+ ions to Yb3+ ions was confirmed by the luminescence spectrum and fluorescent lifetime measurements; the mechanism was investigated and demonstrated to be a single-photon process rather than a two-photon process. Greatly enhanced near-infrared emission was achieved from the glass ceramics excited by simulated sunlight from 400 to 800 nm compared with that from as-prepared glass. These results demonstrate that the glass ceramics are promising materials for spectral conversion from visible sunlight to near-infrared emission and may have potential applications as spectral convertors to enhance the photoelectric conversion efficiency of c-Si solar cells.

Ultrafast saturable absorption in topological insulator Bi 2 SeTe 2 nanosheets

Topological insulator (TI) Bi2SeTe2 nanosheets with very regular hexagonal morphology were synthesized by a hydrothermal route. Open aperture (OA) z-scan method was performed to measure the saturable absorption (SA) characteristics of the as-prepared TI Bi2SeTe2 nanosheets. The measured modulation depth, saturation intensity and nonsaturable loss of the sample were 61.9%, 4.46 GW/cm2 and 4.5% respectively. An ultrafast intraband scattering time of ~50 fs was obtained through simulating the SA curve, which indicates the TI Bi2SeTe2 nanosheets may be a good candidate for mode-locking material.

Space-selective precipitation of ZnO crystals in glass by using high repetition rate femtosecond laser irradiation

We report on three-dimensional (3D) precipitation of ZnO crystals inside a silicate glass by a 500 kHz femtosecond pulse laser. The precipitation and distribution of ZnO crystals in glass are confirmed and analyzed by Raman spectra and Raman mapping. Mirco- luminescence is observed in the laser modified region when excited by femtosecond pulse laser or Xenon lamp. The effect of laser average power on the precipitation of the ZnO crystals has also been investigated. The possibility of 3D optical data storage using the observed phenomena is demonstrated.

Influence of laser-induced air breakdown on femtosecond laser ablation of aluminum

We investigated the influence of laser-induced air breakdown on the femtosecond laser ablation of aluminum target using time-resolved pump-probe shadowgraphic imaging method. The early-stage plasma expanding dynamics and subsequent expanding behaviors of shockwaves and material ejection plume were analyzed through shadowgraphs recorded at different time delays. The dominated mechanisms were clarified at different stages during femtosecond laser pulses ablating aluminum, which provide very valuable information for ultrashort laser ablation of metals.

Evolution of polarization dependent microstructures induced by high repetition rate femtosecond laser irradiation in glass

We report the observation of an anomalous polarization dependent process in an isotropic glass induced by long time stationary irradiation of a high repetition rate near-infrared femtosecond laser. Two distinctive types of polarization dependent microstructures were induced at different irradiation stages. At early stage (a few seconds), a dumbbell-shaped structure elongated perpendicularly to the laser polarization formed at the top of the modified region, which was later erased by further irradiation. At later stage (above 30 s), bubbles filled with O2 formed by the irradiation, which were distributed along the laser polarization at a distance far beyond the radius of the laser beam. Based on a simple modeling of light reflection on boundaries, a thermal accumulation process was proposed to explain the formation and evolution of the dumbbell-shaped microstructure. The possible factors responsible for polarization dependent distribution of bubbles are discussed, which needs further systematic investigations. The results may be helpful in the development of femtosecond laser microprocessing for various applications.

Grating-assisted generation of regular two-dimensional multicolored arrays in a tellurite glass.

A grating structure was inscribed in a tellurite glass after irradiation with high-repetition rate femtosecond laser pulses. High diffraction efficiency was obtained due to the large refractive index change, which was caused by the precipitation of Te crystals in the laser modified region. Two-dimensional multicolored arrays were generated by cascaded four-wave mixing (CFWM) together with the prefabricated grating structure, which showed much more superior than those induced by beam breakup.