Guanjun You

Femtosecond pulse excited two-photon photoluminescence and second harmonic generation in ZnO nanowires

With femtosecond pulse excitation at the wavelength near 800nm, we observed three upconverted emission peaks from ZnO nanowires. The ultraviolet peak at about 385nm and the green emission band centered at about 515nm were attributed to the near band edge emission and defect level emission, respectively, while the intermediate peak was assigned to the second harmonic generation. From the quadratic dependence of the emission intensity on the excitation pulse energy, it is confirmed that the 385nm photoluminescence was mainly generated via two-photon absorption in ZnO nanowires under very intense light-matter interaction.

Multiphoton route to ZnO nanowire lasers

With intense femtosecond laser excitation, multiphoton absorption-induced stimulated emission and laser emission in ZnO bulk crystal and nanowires have been demonstrated at room temperature. UV-stimulated emission peaks appeared in both bulk crystal and nanowires when the excitation exceeded certain thresholds, and a sharp lasing peak with a linewidth of ~0.5 nm was observed from ZnO nanowires. The emission properties were attributed to the band-edge emission of the recombination of carriers excited by two- and three-photon absorption processes in the wide-bandgap semiconductor.

Excited-state dynamics of magnesium phthalocyanine thin film

Abstract Ultrafast excitation relaxation dynamics of magnesium phthalocyanine (MgPc) thin film were investigated by femtosecond pump-probe spectroscopy. The excited state dynamics of MgPcs Q-band were found to be strongly dependent on the probe wavelength. Ultrafast exciton–exciton annihilation with a t −1/2 time dependence of the excited-state population was observed.

Observation of two-photon-induced photoluminescence in ZnO microtubes

The micrometer-sized ZnO tubes were fabricated by a hydrothermal method and characterized by scanning electron microscopy, high resolution transmission microscopy and micro-Raman spectroscopy. Photoluminescence (PL) emission from ZnO microtubes excited by light in the ultraviolet range from xenon lamp was measured. By using nanosecond laser beam at 527nm to excite ZnO microtube, we observed two-photon induced PL emission with an intense peak at 3.15eV and a weak peak at 2.65eV, which was confirmed by the quadratic dependence of the emission intensity on the excitation power.

Linear and ultrafast nonlinear optical response of Ag:Bi2O3 composite films

Silver/bismuth oxide composite thin films with Ag concentration varying from 13.2% to 59.3% were prepared by cosputtering in a multitarget magnetron sputtering system. The absorption spectra of these thin films were measured, and the absorption peaks due to surface plasmon resonance of Ag particles showed a redshift behavior with increasing Ag concentration. By using the femtosecond time-resolved optical Kerr effect technique under 800 nm wavelength, the nonlinear optical properties of these films were investigated. The Kerr signals showed an ultrafast rise, suggesting that the main physical mechanism involved in the optical nonlinearity of Ag:Bi2O3 thin films on the femtosecond scale was the pure electronic contributions. The maximum value of the third-order nonlinear optical susceptibility of Ag:Bi2O3 thin films was 4.1×10−10 esu, and occurred at a Ag concentration of about 35.7%.

Annealing effect of linear and nonlinear optical properties of Ag:Bi2O3 nanocomposite films.

Silver nanoparticle/bismuth oxide composite films were deposited by cosputtering and annealed at different temperatures. The X-ray diffraction results demonstrate that the silver and bismuth oxide were well crystallized after 600 degrees C thermal annealing. The linear absorption peaks show a red-shift behavior while increasing annealing temperature. The annealing effect of the third-order nonlinear optical susceptibilities and ultrafast nonlinear optical response of the silver nanoparticles/bismuth oxide composite thin films are investigated using the femtosecond time-resolved optical Kerr effect technique under 800 nm wavelength. The maximum value of chi(3) of Ag:Bi2O3 thin films is 2.1X10 esu, which occurs at an annealing temperature of 600 degrees C. The ultrafast optical response spectra demonstrate the temperature dependence of decay process time.

Multi-photon excitation UV emission by femtosecond pulses and nonlinearity in ZnO single crystal

Optical nonlinearities and multi-photon ultraviolet (UV) excitation in ZnO single crystal were investigated by the z-scan method utilizing an intense femtosecond (fs) laser at different wavelengths. Based on the analysis of the experimental results at different excitation wavelengths near 800 nm, nonlinear optical effects that may emerge under an intense field are attributed to be responsible for the efficient two-photon absorption process under detuned excitation.

Defect dynamics and spectral observation of twinning in single crystalline LaAlO3 under subbandgap excitation

We have investigated the photoluminescence and ultrafast dynamics of LaAlO3 crystal. The photoluminescence consists of a broad spectrum and two sharp peaks, which arise from various defect levels within the bandgap. A doublet splitting of roughly 6 nm is seen in these two sharp peaks. An Al displacement of 0.09 A in a sublattice, which is possible because of twinning, is adequate to explain the spectral splitting. Femtosecond pump probe experiments reveal further that many of these defect levels have a few picosecond decay times while the lowest defect states have decay times longer than nanosecond to the valence band.

Heat treatment effect on the ultrafast dynamics and nonlinear optical properties of Ag : Si3N4 nanocermets

Ag : Si3N4 nanocermets were fabricated by the magnetron cosputtering method and were thermally treated at different temperatures. The off-resonant third-order nonlinear susceptibilities were determined to be of the order of 10−10 esu at 800 nm by the optical Kerr effect experiment. Further time-resolved pump–probe researches prove that heat treatment is a feasible way to improve the performance of ultrafast response for this material. The research results demonstrate that in addition to the common use in microelectronics, the Si3N4 nanocermets are promising in future applications in optical communication and photoelectric fields.

Synthesis and optical properties of PbS/ZnO composite films

Pure ZnO films and PbS/ZnO nanocomposite films of optical quality were prepared with a sol–gel method. The particle size, optical absorption, and linear refractive index of the PbS/ZnO films were measured by TEM (transmission electron microscopy), UV-Vis absorption, and ellipsometry techniques, respectively. For high Pb/Zn ratios, PbS particles of size 5–8 nm are observed in TEM, and a large refractive nonlinearity of 10−7 esu with a saturable absorption was measured in picosecond Z-scan experiments.