Jingya Wang

Effect of erbium concentration on spectroscopic properties and 2.79 μm laser performance of Er:CaF 2 crystals

The effect of erbium concentration on spectroscopic properties of Er:CaF2 crystals was investigated. Two highest absorption cross-sections (σabs) at 967 nm were achieved in the 4at.% and 8at.% Er:CaF2 samples with the value of 0.22 × 10−20 cm2 and 0.23 × 10−20 cm2, respectively. And the 4at.% and 8at.% Er:CaF2 samples also had the highest emission cross-section (σem) at 2727 nm with the value about 0.67 × 10−20 cm2. Lifetime of 4I13/2 decreased faster than that of 4I11/2 with the increase of erbium concentration. Under laser diode (LD) pumping, the continuous-wave (CW) laser operations around 2.79 μm were demonstrated in the 4at.%, 8at.% and 11at.% Er:CaF2 samples. And the 4at.% Er:CaF2 sample had the best laser performance with a maximum output power of 0.282 W and a slop efficiency of 13.9%.

2.8 μm passively Q-switched Er:CaF 2 diode-pumped laser

A mid-infrared Er doped CaF2 crystal was successfully grown by the bridgeman method. Efficiently continuous wave and Q-switched laser operations were demonstrated at 2.8 μm with a 4 at.% Er doped CaF2 crystal end-pumped by a fiber-coupled 974 nm diode laser. The continuous wave output power of 295 mW was obtained in a compact linear cavity. A stable 2.8 μm passively Q-switched Er:CaF2 laser was also demonstrated with a graphene saturable absorber. Under an absorbed pump power of 2.353 W, an average output power of 172 mW was generated with a pulse duration of 1.324 μs and a repetition rate of 62.70 kHz, corresponding to the single pulse energy of 2.74 μJ and the peak power of 2.07 W, respectively. The high-quality Er:CaF2 crystal and the monolayer graphene are an ideal combination to directly obtain a near 3 μm mid-infrared region pulse laser.

Highly efficient dual-wavelength mid-infrared CW Laser in diode end-pumped Er:SrF2 single crystals.

The spectral properties and laser performance of Er:SrF2 single crystals were investigated and compared with Er:CaF2. Er:SrF2 crystals have larger absorption cross-sections at the pumping wavelength, larger mid-infrared stimulated emission cross-sections and much longer fluorescence lifetimes of the upper laser level (Er3+:4I11/2 level) than those of Er:CaF2 crystals. Dual-wavelength continuous-wave (CW) lasers around 2.8 μm were demonstrated in both 4at.% and 10at.% Er:SrF2 single crystals under 972 nm laser diode (LD) end pumping. The laser wavelengths are 2789.3 nm and 2791.8 nm in the former, and 2786.4 nm and 2790.7 nm in the latter, respectively. The best laser performance has been demonstrated in lightly doped 4at.% Er:SrF2 with a low threshold of 0.100 W, a high slope efficiency of 22.0%, an maximum output power of 0.483 W.

Diode-pumped femtosecond mode-locked Nd, Y-codoped CaF2 laser

A passively mode-locked femtosecond laser based on an Nd, Y-codoped CaF2 disordered crystal was demonstrated. The Y3+-codoping in Nd : CaF2 markedly suppressed the quenching effect and improved the fluorescence quantum efficiency and emission spectra. With a fiber-coupled laser diode as the pump source, the continuous wave tuning range covering from 1042 to 1076 nm was realized, while the mode-locked operation generated 264 fs pulses with an average output power of 180 mW at a repetition rate of 85 MHz. The experimental results show that the Nd, Y-codoped CaF2 disordered crystal has potential in a new generation diode-pumped high repetition rate chirped pulse amplifier.

Diurnal and seasonal variations in photosynthetic characteristics of switchgrass in semiarid region on the Loess Plateau of China

In order to use rationally switchgrass (Panicum virgatum L.) introduced in a large scale in semiarid regions on the Loess Plateau of China, we investigated and compared soil water storage dynamics, diurnal and seasonal changes in leaf photosynthetic characteristics, and biomass production of switchgrass grown under three different row spacing (20, 40, and 60 cm). Results indicated that photosynthetic parameters showed a pronounced seasonality. Diurnal course of net photosynthetic rate (PN) was bimodal, showing obvious midday depression, which was mainly due to stomatal limitation in May and June, by nonstomatal limitation in August, and both stomatal and nonstomatal factors in September. Generally, PN, stomatal conductance, instantaneous water-use efficiency, light-saturated net photosynthetic rate, saturation irradiance, and compensation irradiance increased with increasing row spacing. Plant height, leaf width, and a relative growth rate of biomass accumulation were significantly higher at the row spacing of 60 cm, while 20 cm spacing showed significantly higher aboveground biomass production and the biomass water-use efficiency. All these confirmed that soil water is the key limiting factor influencing switchgrass photosynthesis, and suggested that the wide row plantation (i.e., 60 cm) was more beneficial to switchgrass growth, while narrow spacing was in favor of improving switchgrass productivity and water-use efficiency.

Morphological changes in roots of Bothriochloa ischaemum intercropped with Lespedeza davurica following phosphorus application and water stress

Root morphological characteristics are important parameters for the evaluation of plant adaptation to stress environment. In this study, a pot experiment was conducted to investigate changes in the root morphology of Bothriochloa ischaemum intercropped with Lespedeza davurica under three soil water regimes and two phosphorus (P) fertilizer treatments. Results showed that root biomass (RB) per B. ischaemum plant decreased as its proportion increased in the mixtures. There were no significant differences in root:shoot ratio (RSR) among the mixture ratios, and P application did have consistent effects on the RSR. Bothriochloa ischaemum tended to have smaller root surface area (RSA), root average diameter (RAD), specific root length (SRL), and specific root area (SRA) under water stress conditions. There was a negative linear relationship between RB and RSA under each water and P treatment. Negative and positive linear relationships were found between RB and TRL (total root length), TRL and RSA, respectively, except under severe water stress without P application. P application decreased the RAD and increased the SRA and SRL of B. ischaemum under water stress. All these suggest two apparent response mechanisms for B. ischaemum under water stress and P application: an increase in length of small diameter roots and decrease in root weight density.

Femtosecond diode-pumped mode-locked neodymium lasers

Fluoride-type crystals (CaF2, SrF2) doped with neodymium Nd3+ and codoped with buffer ions for breaking clusters of active ions and increasing fluorescence efficiency, present interesting alternative as laser active media for the diode-pumped mode-locked lasers. In comparison with widely used materials as Nd:YAG or Nd:YVO4, they have broad emission spectra as well as longer fluorescence lifetime, in comparison with Nd:glass, SrF2 and CaF2 have better thermal conductivity. In spite of the fact, that this thermal conductivity decreases with Nd3+ doping concentration, these crystals are alternative for the Nd:glass in subpicosecond mode-locked laser systems. In this paper we review the basic results reported recently on these active materials and in the second part we present our results achieved in low power diode pumped passively mode locked lasers with Nd,La:CaF2 and Nd,Y:SrF2 crystals. The pulses as short as 258 fs at wavelength of 1057 nm were obtained in the first case, while 5 ps long pulses at 1065 nm were generated from the second laser system.

Optical Spectra Properties and Continuous-Wave Laser Performance of Tm,Y:CaF2 Single Crystals

3 at.% Tm, at.% Y:CaF2 crystals ( , 0.5, 1, 2, and 3) were grown by the vertical Bridgman method and investigated. Codoping Y3+ ions can manipulate the local structure of Tm3+ ions in the CaF2 crystal and then improve the spectroscopic properties. Compared with 3 at.% Tm:CaF2, 3 at.% Tm, 3 at.% Y:CaF2 crystal has several advantages. Firstly, the absorption cross section is improved from 0.35 × 10−20 cm−2 to 0.45 × 10−20 cm−2 at 767 nm, and the fluorescence intensity had elevated 3.4 times. Secondly, the linewidth of the fluorescence spectrum and lifetime also increased from 164 nm to 191 nm and from 6.16 ms to 8.15 ms at room temperature, respectively. Furthermore, quantum efficiency improved from 58.2% to 80.3%. The maximum laser output power of 583 mW and slope efficiency of 25.3% were achieved in 3 at.% Tm, 3 at.% Y:CaF2 crystal under 790 nm diode pumping.

Modulated visible spectra properties of Pr:Ca1-xRxF2+x(R=Y, La, Gd) crystals

The spectroscopic properties of the 1.0 at.%Pr:Ca0.97R0.02F2.03(R=Y, La, Gd) crystals are investigated. X-diffraction and room temperature absorption spectra have been registered and analyzed. The emission spectra and decay curves of the crystals were obtained at room temperature. The photoluminescence intensity in the visible region is significantly enhanced by co-doping R3+ ions in Pr:CaF2 crystal. The different effects among the R3+ (Y3+, La3+ and Gd3+) regulating ions on the crystals were observed and compared. Pr:Ca0.97La0.02F2.03 and Pr:Ca0.97Y0.02F2.03 crystals have substantially strong emission at blue and orange region, while the Pr:Ca0.97Gd0.02F2.03 crystal is more suitable for the red emission emitting.

Effects of Gd3+ on the photoluminescence properties of Nd3+-doped SrF2 crystal

The effect of doping concentration of Nd3+ and co-doping Y3+ on the spectroscopic properties are investigated systematically. Due to the particular clustering effect, the quench effect was demonstrated in lightly doped NdxGd0.03Sr0.97-xF2.03+x (x=0.0005,0.0015,0.0065,0.01) crystals. For a 3% Gd:SrF2 crystal, the fluorescence lifetime at 1054 nm decrease from 380.9 to 159.8 μs by doping Nd3+ from 0.15 at.% to 1 %at.%, while the emission cross section decreases to 4.12 × 10−20cm2 at 1054 nm. However, the absorb cross section were increased when the concentration of Nd3+ increase from 0.5 % to 0.65 %. Thus, there is an optimum doping concentration of Nd3+. According to the research, the optimum doping concentration of Nd3+ is 0.15 % in 3% Gd:SrF2 crystals.