Shixiong Qian

Photochromic thiophene oligomers based on bisthienylethene: syntheses, photochromic and two-photon properties

A series of novel, functional symmetric and unsymmetric photochromic thiophene oligomers based on bisthienylethenes (BTEs) were synthesized via control of the reaction condition with a Suzuki coupling method. Their optical, photochromic quantum yields, two-photon and electrochemical properties in THF as well as in poly(methylmethacrylate) (PMMA) films were measured and discussed. The fluorescence emission can be tuned reversibly by alternating irradiation with UV and visible light. These BTEs could be used as nanowires, fluorescent switch and electrochemical switch.

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%.

Femtosecond nonlinear optical response of metallophthalocyanine films

The third-order nonlinear optical properties of copper phthalocyanine (CuPc) and magnesium phthalocyanine (MgPc) films were investigated by femtosecond optical Kerr effect (OKE) and pump-probe techniques. For both CuPc and MgPc, the third-order nonlinear optical susceptibility was enhanced at least by one order of magnitude in comparison with that of the metal-free Pc, and the enhancement effect arises from different mechanisms for both films. From pump-probe measurement, quite different transmittance and decay behavior between CuPc and MgPc films were observed. The CuPc films show an ultrafast decay, while the decay behavior of MgPc film is complicated with several temporal components, which could be explained by the relaxation of singlet excited state.

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.

Enhanced photovoltaic response of PVK/C60 composite films

Abstract We report the photovoltaic response of composite films formed with polymer PVK and fullerene C 60 by using the physical jet deposition technique. The steady and transient photovoltage (PV) data were obtained for five kinds of photocells formed between ITO and aluminum electrodes. We found that the PV signal of PVK/C 60 two-layer composite structure is enhanced by five orders of magnitude compared to that of single layer of pure PVK or its mixed (with C 60 ) films. The experiments demonstrated that the significant enhancement of PV signal of two-layer composite films is attributed to the efficient charge separation at the interface of PVK/C 60 which results from photoinduced electron transfer from PVK to C 60 . The transient PV response of a two-layer PVK/C 60 film shows that the photoinduced electron transfer between PVK and C 60 molecules is a very fast mechanism of charge separation.

Multiphoton absorption induced amplified spontaneous emission from biocatalyst-synthesized ZnO nanorods

Multiphoton absorption-induced photoluminescence and amplified spontaneous emission (ASE) have been observed from ZnO nanorod arrays upon near-infrared excitation (λexc∼800nm). A low threshold of ∼12mJ∕cm2 was demonstrated for the ASE process. The ultraviolet emission can be attributed to the recombination of carriers coexcited by the processes of three-photon and off-resonant two-photon absorption, which was confirmed by the excitation wavelength-dependent experiments. Additionally, it has been observed that the processes of ASE and second harmonic generation in ZnO nanorods appear to enhance each other when the excitation wavelength approaches the resonant wavelength for two-photon absorption.Multiphoton absorption-induced photoluminescence and amplified spontaneous emission (ASE) have been observed from ZnO nanorod arrays upon near-infrared excitation (λexc∼800nm). A low threshold of ∼12mJ∕cm2 was demonstrated for the ASE process. The ultraviolet emission can be attributed to the recombination of carriers coexcited by the processes of three-photon and off-resonant two-photon absorption, which was confirmed by the excitation wavelength-dependent experiments. Additionally, it has been observed that the processes of ASE and second harmonic generation in ZnO nanorods appear to enhance each other when the excitation wavelength approaches the resonant wavelength for two-photon absorption.