Ma Hong-Liang

Size-dependent optical properties and carriers dynamics in CdSe/ZnS quantum dots

Size-dependence of optical properties and energy relaxation in CdSe/ZnS quantum dots (QDs) were investigated by two-colour femtosecond (fs) pump-probe (400/800 nm) and picosecond time-resolved photoluminescence (ps TRPL) experiments. Pump-probe measurement results show that there are two components for the excited carriers relaxation, the fast one with a time constant of several ps arises from the Auger-type recombination, which shows almost particle size-independence. The slow relaxation component with a time constant of several decades of ns can be clearly determined with ps TRPL spectroscopy in which the slow relaxation process shows strong particle size-dependence. The decay time constants increase from 21 to 34 ns with the decrease of particle size from 3.2 to 2.1 nm. The room-temperature decay lifetime is due to the thermal mixing of bright and dark excitons, and the size-dependence of slow relaxation process can be explained very well in terms of simple three-level model.

Influence of citric acid content on magnetic properties of BaFe12O19 powder prepared by sol-gel auto-combustion method

BaFe12O19 powders with nanocrystalline sizes were produced by sol-gel auto-combustion method. The precursors were prepared under the molar ratios of citric acid to the metal nitrate of 0.5, 1.0 and 1.5. Appropriate ethylene diamine (C2H8N2) was added in order to adjust pH of 7. The ions distribution of citric acid at different pH explains the effect of citric acid in the starting solution. The XRD patterns of the as-burnt powders and annealing powders show different phases for different citric acid content. In addition, the lattice constants (a, c) derived from X-ray diffraction pattern were changed from 0.58881 nm to 0.58997 nm and 2.32057 nm to 2.32296 nm respectively. The data from VSM indicated that the powder with high citric acid content took on good magnetic properties. Pure single BaFe12O19 of the specific maximum magnetization M(1 T) ≈ 49.73 Am2/kg, the specific remanent magnetization Mr ≈ 30.77 Am2/kg and the coercive force Hc ≈ 467 kA/m was produced when the molar ratios of citric acid to the metal nitrate was 1.5.

The effect of spherical aberration on temperature distribution inside glass by irradiation of a high repetition rate femtosecond pulse laser

In this paper, we study the effect of spherical aberrations on the light intensity and the temperature distribution in the focal region in a 250-kHz femtosecond laser irradiated Ag+-doped borosilicate glass. When a focused beam goes through an interface between air and glass, spherical aberration will result in the separation of the focal point and then cause a clear change of the light intensity distribution along the incident direction. That phenomenon will further influence the longitudinal cross-section temperature distribution in glass. Here we use Ag nanoparticle formation as a marker for establishing temperature distribution and we find that the formation of nanoparticle shows a strong dependence on the temperature field and the detailed precipitation process is also discussed.

Hyperfine structure of singly ionized lanthanum and praseodymium

Hyperfine structure spectra of singly ionized lanthanum and praseodymium have been measured by collinear fast-ion-beam laser spectroscopy. All the spectral lines were resolved and the magnetic dipole and electric quadruple coupling constants of the metastable levels and excited levels were determined. Our results are in agreement with the published data within the experimental uncertainty. For praseodymium ions, the accuracy of the magnetic dipole coupling constants are improved by one order of magnitude compared with other published data and the electric quadruple coupling constants are reported for the first time.

Changes of the Nuclear Charge Distribution of Nd from Optical Isotope Shifts

The isotope shifts and hyperfine structures of seven optical transitions for all seven stable isotopes of Nd II were measured by using collinear fast-ion-beam laser spectroscopy. The nuclear parameter λ was obtained from the measured optical isotope shifts for all seven stable isotopes with improved accuracy. The λ values were analysed by using the Fermi distribution for the nuclear charge density. The values of δr2, δr4 and δr6 were determined.

Femtosecond pulse laser-induced self-organized nanostructures on the surface of ZnO crystal

This paper reports self-organized nanostructures observed on the surface of ZnO crystal after irradiation by a focused beam of a femtosecond Ti:sapphire laser with a repetition rate of 250 kHz. For a linearly polarized femtosecond laser, the periodic nanograting structure on the ablation crater surface was promoted. The period of self-organization structures is about 180 nm. The grating orientation is adjusted by the laser polarization direction. A long range Bragg-like grating is formed by moving the sample at a speed of 10 mu m/s. For a circularly polarized laser beam, uniform spherical nanoparticles were formed as a result of Coulomb explosion during the interaction of near-infrared laser with ZnO crystal.

Measurement of hyperfine structure and isotope shifts in the 580.56nm line of 142−145,146,148,150Nd+

Isotope shifts and hyperfine spectrum of singly ionized neodymium ion was measured by collinear fast-ion-beam laser spectroscopy. The hyperfine A constants and B constants are obtained for the (23230)o9/2 level and 4f45d 6K9/2 level, respectively. The optical isotope shifts between seven isotopes in the 580.56 nm of 142−145,146,148,150Nd+ line are determined. The configuration admixtures for (23230)o9/2 level were quantitatively analysed to be 4f46p, 4f35d2 and 4f35d6p with mixing coefficients of 67%, 5% and 28%, respectively.

Space-Selective Precipitation of Ba2TiSi2O8 Crystals in Sm3+-Doped BaO-TiO2-SiO2 Glass by Femtosecond Laser Irradiation

The ferroelectric crystal Ba2TiSi2O8 with high second-order optical nonlinearity is precipitated in Sm3+-doped BaO-TiO2-SiO2 glass by a focused 800nm, 250kHz and 150 fs femtosecond laser irradiation. No apparent blue and red emissions are observed at the beginning, while strong blue emission due to second harmonic generation and red emission due to the f-f transitions of Sm3+ are observed near the focal point of the laser beam after irradiation for 25s. Micro-Raman spectra confirm that Ba2TiSi2O8 crystalline dots and lines are formed after laser irradiation. The mechanism of the phenomenon is discussed.

Phase transformation and nanograting structure on TiO2 rutile single crystal induced by infrared femtosecond laser

In this paper, Titanium dioxide (TiO2) rutile single crystal was irradiated by infrared femtosecond laser pulses with repetition rate of 250 kHz. For a P-polarized femtosecond laser, the periodic nanograting structure on the ablation crater surface was formed. The periodicity is much less than the laser wavelength. The direction of nanograting alignment depends on the polarization laser beam. Micro–Raman spectra show that the intensity of Eg Raman vibrating mode of rutile phase increases and that of A1g Raman vibrating mode decreases apparently within the ablation crater. With the increase of irradiation time and laser average power, the Raman vibrating modes of anatase phase emerged. Rutile phase of TiO2 single crystal is partly transformed into anatase phase.

Two-photon absorption coefficient dichroism in II-VI semiconductor crystals

Considering two beams propagate in semiconductor crystal, this paper discusses the polarization dependence of pump beam-induced intensity attenuation of probe beam due to two-photon absorption (TPA). Numerical calculation and experimental measurement demonstrate that TPA coefficient is polarization dependent. For homogeneous materials, probe beam attenuation arises from the imaginary part of diagonal and off-diagonal components of third-order nonlinear susceptibilities.