Jintai Fan

Ultrafast Saturable Absorption of Two-Dimensional MoS2 Nanosheets

Employing high-yield production of layered materials by liquid-phase exfoliation, molybdenum disulfide (MoS2) dispersions with large populations of single and few layers were prepared. Electron microscopy verified the high quality of the two-dimensional MoS2 nanostructures. Atomic force microscopy analysis revealed that ~39% of the MoS2 flakes had thicknesses of less than 5 nm. Linewidth and frequency difference of the E(1)2g and A1g Raman modes confirmed the effective reduction of flake thicknesses from the bulk MoS2 to the dispersions. Ultrafast nonlinear optical (NLO) properties were investigated using an open-aperture Z-scan technique. All experiments were performed using 100 fs pulses at 800 nm from a mode-locked Ti:sapphire laser. The MoS2 nanosheets exhibited significant saturable absorption (SA) for the femtosecond pulses, resulting in the third-order NLO susceptibility Imχ((3)) ~ 10(-15) esu, figure of merit ~10(-15) esu cm, and free-carrier absorption cross section ~10(-17) cm(2). Induced free carrier density and the relaxation time were estimated to be ~10(16) cm(-3) and ~30 fs, respectively. At the same excitation condition, the MoS2 dispersions show better SA response than the graphene dispersions.

Optical Limiting and Theoretical Modelling of Layered Transition Metal Dichalcogenide Nanosheets.

Nonlinear optical property of transition metal dichalcogenide (TMDC) nanosheet dispersions, including MoS2, MoSe2, WS2, and WSe2, was performed by using Z-scan technique with ns pulsed laser at 1064 nm and 532 nm. The results demonstrate that the TMDC dispersions exhibit significant optical limiting response at 1064 nm due to nonlinear scattering, in contrast to the combined effect of both saturable absorption and nonlinear scattering at 532 nm. Selenium compounds show better optical limiting performance than that of the sulfides in the near infrared. A liquid dispersion system based theoretical modelling is proposed to estimate the number density of the nanosheet dispersions, the relationship between incident laser fluence and the size of the laser generated micro-bubbles, and hence the Mie scattering-induced broadband optical limiting behavior in the TMDC dispersions.

Intense photoluminescence at 2.7 μm in transparent Er 3+ :CaF 2 -fluorophosphate glass microcomposite

Er3+ doped CaF2-fluorophosphate (FP) glass microcomposites were produced by heat-treating the mixture of Er3+:CaF2 precipitate and FP glass powder above the melting temperature of the FP glass. The appearance of CaF2 crystallites in the resulting composites was confirmed by x ray diffraction. Despite the fact that the average diameter of the crystallites was around 10 μm as revealed by the micromorphology study, a transparent composite was obtained by matching the refractive index of FP glass to that of CaF2. Intense IR fluorescence at around 2.7 μm was observed in the composite, implying the composite would be a promising candidate for IR lasers and amplifiers.

Linearly Polarized 1180-nm Raman Fiber Laser Mode Locked by Graphene

In this paper, we demonstrate a linearly polarized 1180-nm passively mode-locked Raman fiber laser using graphene-based saturable absorber (SA). The pump source is a linearly polarized ytterbium-doped fiber laser at 1120 nm. Stable nanosecond mode-locked pulses with a repetition rate of 0.4 MHz are generated. The combination of Raman technology and the graphene-based ultrawide bandwidth SA offer a prospect of real wavelength-versatile mode-locked laser source. The mode-locked laser operating at 1180 nm is suitable for frequency doubling to yellow light after amplification.

Micro-structure of grain boundary in post-annealed Sinter plus HIPed Nd:Lu3Al5O12 ceramics

This paper demonstrates that we can further eliminate the remaining micro pores by hot isostatic press (HIP) approach. The porosity of Nd:LuAG laser ceramics sharply declined from 7 × 10−6 to 5.60 × 10−7 after additional HIP. Annealing is one of the key steps to optimize the quality of vacuum sintered ceramics, such as make the transmittance rise from 79.98% to 81.02% at 1200nm. However, the trend is in the opposite direction for the HIPed Nd:LuAG: fallen from 82.65% to 25.50% at 1200nm. The microstructure of Nd:LuAG studied by microscope, SEM and TEM shows that a large number of pores and a few second phases like Lu2SiO5 appeared along grain boundaries in post-annealed HIPed Nd:LuAG. The gas composition was confirmed as argon by GCMS analysis. By further optimizing the annealing process of HIPed ceramics we found that vacuum annealed at 1600 °C for 10h and oxygen re-annealed at 1200 °C for 30h can reduce pores and make the ceramic quality better, the transmittance can reach 83.05% at 1200nm, and the porosity dropped to 3.2 × 10−7.

Er3+-doped transparent glass ceramics containing micron-sized SrF2 crystals for 2.7 μm emissions

Er3+-doped transparent glass ceramics containing micron-sized SrF2 crystals were obtained by direct liquid-phase sintering of a mixture of SrF2 powders and precursor glass powders at 820 °C for 15 min. The appearance and microstructural evolution of the SrF2 crystals in the resulting glass ceramics were investigated using X-ray diffraction, field-emission scanning electron microscopy and transmission microscopy. The SrF2 crystals are ~15 μm in size and are uniformly distributed throughout the fluorophosphate glass matrix. The glass ceramics achieve an average transmittance of 75% in the visible region and more than 85% in the near-IR region. The high transmittance of the glass ceramics results from matching the refractive index of the SrF2 with that of the precursor glass. Energy dispersive spectroscopy, photoluminescence spectra, and photoluminescence lifetimes verified the incorporation of Er3+ into the micron-sized SrF2 crystals. Intense 2.7 μm emissions due to the 4I11/2 → 4I13/2 transition were observed upon excitation at 980 nm using a laser diode. The maximum value of the emission cross section of Er3+ around 2.7 μm is more than 1.2 × 10−20 cm2, which indicates the potential of using transparent glass ceramics containing micron-sized SrF2 crystals for efficient 2.7 μm lasers and amplifiers.

Europium doped transparent glass ceramics containing CaF2 micron-sized crystals: structural and optical characterization

Transparent glass ceramics containing CaF2 micron-sized crystals were prepared using a modified liquid-phase sintering route. The morphology and microstructural evolution of the CaF2 crystals before and after liquid-phase sintering were examined using X-ray diffraction, field-emission scanning electron microscopy and transmission microscopy. After sintering at 800 °C for 10 min, the CaF2 crystals were approximately 20 μm in size and were uniformly distributed throughout a precursor glass prepared using melt-quenching method. The glass ceramic containing CaF2 micron-sized crystals prepared using liquid-phase sintering achieved the highest in-line transmittance of 76% in the visible region and 92% in the near-IR region. High near-IR transmittance of glass ceramics was attributed to the matching of the refractive index of the precursor glass with that of CaF2 in the near-IR region. The luminescence properties and local environment of Eu3+ in the precursor glass and glass ceramic were investigated using photoluminescence spectra, photoluminescence lifetimes and phonon sideband spectroscopy, and the results determined that some of the Eu3+ ions in the precursor glass incorporate into CaF2 micron-sized crystals after sintering and enter C4v site symmetry.

Effects of annealing on Cr-sensitized Nd:LuAG laser ceramics

This paper demonstrates that annealing can greatly improve the quality of Cr-sensitized Nd:LuAG (Cr,Nd:LuAG) ceramics. Additionally, the pumping efficiency of Nd:LuAG ceramics can be improved by co-doping with Cr3+ ions. The two broad and strong absorption bands around 600 nm (4A2- 4T2) and 440 nm (4A2- 4T1) caused by Cr3+ ions act as effective sensitizers to absorb and convert UV and visible light from a flash lamp. Furthermore, a near-infrared absorption band appears in the in-line transmittance spectra of Cr,Nd:LuAG ceramics. The XPS spectra show that this band is caused by Cr2+ ions. The effect of annealing is studied, and it is found that the near-infrared absorption could be eliminated by annealing in air at 1450 °C for 25 h. Furthermore, the effective fluorescence lifetime of Nd3+ ions in Cr co-doped LuAG ceramics increases from 211.28 μs to 553.80 μs when pumped at 440 nm.

Synthesis of Yb:Lu3Al5O12 nano powders with the reverse strike co-precipitation method: influence of decomposition of NH4HCO3

Abstract Yb:Lu 3 Al 5 O 12 (Yb:LuAG) nano powders were synthesized with the co-precipitation method from cation (Lu 3+ , Yb 3+ , Al 3+ ) solution of different concentrations using NH 4 HCO 3 as the precipitant. The influence of cation concentration on the morphology and purity of the calcined powders was studied. Contrary to the conventional suggestion that recommends thin solution to obtain loosely agglomerated nano powders, results showed that thin cation (Lu+Yb+Al) concentration ≤0.08 mol/L led to formation of Al 2 O 3 and LuAP impurities in the obtained nano powders. By means of Tg/(DTA), X-ray diffraction (XRD), scanning electron microscopy (SEM), the cause was revealed to be re-dissolution of Lu at high pH value that resulted from the decomposition of NH 4 HCO 3 . (Lu+Yb+Al) concentration of ∼0.24 mol/L was recommended to achieve pure Yb:LuAG nano powders. Transparent Yb:LuAG ceramics were fabricated with the prepared Yb:LuAG nano powders. The transmittance around 800 nm was 77.5%.

Preparation of LuAG Powders with Single Phase and Good Dispersion for Transparent Ceramics Using Co-Precipitation Method

The synthesis of pure and well dispersed lutetium aluminum garnet (LuAG) powder is crucial and important for the preparation of LuAG transparent ceramics. In this paper, high purity and well dispersed LuAG powders have been synthesized via co-precipitation method with lutetium nitrate and aluminum nitrate as raw materials. Ammonium hydrogen carbonate (AHC) was used as the precipitant. The influence of aging time, pH value, and dripping speed on the prepared LuAG powders were investigated. It showed that long aging duration (>15 h) with high terminal pH value (>7.80) resulted in segregation of rhombus Lu precipitate and Al precipitate. By decreasing the initial pH value or accelerating the dripping speed, rhombus Lu precipitate was eliminated and pure LuAG nano powders were synthesized. High quality LuAG transparent ceramics with transmission >75% at 1064 nm were fabricated using these well dispersed nano LuAG powders.