Qian Shi-Xiong

Nonlinear Optical Properties and Ultrafast Dynamics of Undoped and Doped Bulk SiC

Ultrafast third-order nonlinear optical response of bulk 6H-SiC undoped and doped with different nitrogen concentrations are investigated utilizing femtosecond Z-scan and optical Kerr effect (OKE) techniques at the wavelength of 800nm. The Z-scan measurement shows that the third-order nonlinear optical susceptibilities of the doped samples are improved in comparison to the intrinsic sample. The OKE results additionally reveal that the instantaneous nonlinear optical response of the samples can be ascribed to the distortion of the electron cloud. The ultrafast transient spectroscopic measurements with the one-color and two-color pump-probe techniques demonstrate that the ultrafast recovery process in subpicosecond domain is induced by two-photon absorption process, while the slow relaxation component reflects the carrier dynamics of the excited electrons.

Steady-state bright-dark vector spatial solitons in biased photorefractive-photovoltaic crystals

We show that bright–dark vector solitons are possible in biased photorefractive–photovoltaic crystals under steady-state conditions, which result from both the bulk photovoltaic effect and the spatially nonuniform screening of the external bias field. The analytical solutions of these vector solitons can be obtained in the case of |σ|≪1, where σ is the parameter controlling the intensities of the two optical beams. In the limit of −1<σ⩽1, these vector solitons can also be determined by use of simple numerical integration procedures. When the bulk photovoltaic effect is neglectable, these vector solitons are bright–dark vector screening solitons studied previously in the |σ|≪1 regime, and predict bright–dark vector screening solitons in the −1<σ⩽1 regime. When the external bias field is absent, these vector solitons predict bright–dark vector photovoltaic solitons in closed and open circuits.

Off-Resonant Third-Order Optical Nonlinearity of an Ag:TiO2 Composite Film

Using the femtosecond time-resolved optical Kerr effect technique, we investigate the off-resonant nonlinear optical response of an Ag:TiO2 composite film prepared by a vacuum magnetron sputtering method. The third-order nonlinear optical susceptibility of the composite film with silver nanoparticle size of about 30 nm is estimated to be 1.9×10−10 esu at the incident laser wavelength of 800 nm. When the photon energy of the incident beam is lower than that for surface plasmon or the interband transition of silver nanoparticles, the observed third-order optical nonlinearity is attributed to the intraband transition of the free electrons. Based on the linear limit of the electric field within micro-spherical model, we assign this large optical nonlinearity to the local field enhancement of the third-order nonlinearity.

Ablation of GaN Using a Femtosecond Laser

We study the pulsed laser ablation of wurtzite gallium nitride (GaN) films grown on sapphire, using the femtosecond laser beam at a central wavelength of 800 nm as the source for the high-speed ablation of GaN films. By measuring the backscattered Raman spectrum of ablated samples, the dependence of the ablation depth on laser fluence with one pulse was obtained. The threshold laser fluence for the ablation of GaN films was determined to be about 0.25 J/cm2. Laser ablation depth increases with the increasing laser fluence until the amount of removed material is not further increased. The ablated surface was investigated by an optical surface interference profile meter.

Optical Limiting Properties of Two Soluble Polymer/Multi-Walled Carbon Nanotube Composites

Two soluble polymer grafted multi-walled carbon nanotubes (MWNTs), including poly(N-vinylcarbazole)-MWNTs and poly(methyl methacrylate)-MWNTs, are synthesized. Their nonlinear optical properties and optical limiting (OL) performances are investigated by z-scan method with 527nm nanosecond laser pulses. These grafted MWNTs dissolved in chloroform show much better optical limiting performance than those of MWNTs and C60 in toluene solution. Nonlinear absorption and nonlinear scattering mechanism are taken into consideration for explaining the observed results. The comparison of the experimental results shows that nonlinear absorption is the dominant mechanism for OL performance of these new samples.

Time-Resolved Fluorescence Study of C60 Solution

Using a picosecond excitation source and a time-resolved streak camera detection system, the time-resolved fluorescence spectra of purified C60 have been recorded from toluene solution at room temperature. A fluorescence peak at about 730 nm was detected and its lifetime was directly determined to be 1.1±0.1 ns. Two fluorescence peaks obtained from the mixture of C60 and C70 at 676 and 696 nm were mainly related to C70 content.

Nonlinear Optical Properties of Indium Phthalocyanine Axially Grafted Polystyrene Thin Film

Ultrafast dynamics and third-order nonlinearity of thin films of tert-butyl peripherally-substituted indium phthalocyanine axially grafted polystyrene (tBu4PcIn–PS) are investigated by femtosecond optical-Kerr-effect (OKE) and z-scan experiments. The fastest component (<200 fs) in the OKE traces of the film is related to the electron cloud distortion, where the phthalocyanine-polymer interaction may enhance this contribution. The z-scan measurement also indicates that this interaction might enhance the third-order optical nonlinearity. The measurements shows that the magnitudes of χ(3) of these films are in order of 10−11 esu.

Photoluminescence Study of C70 Film Manufactured by Physical Jet Deposition

We have fabricated C70 film by physical jet deposition (PJD) technique and measured the absorption and photoluminescence (PL) spectra of this kind C70 film. In comparison with the deposited C70 film in high vacuum, the absorption and integrated PL spectra are similar, but there is a big difference in the time-resolved PL. The C70 film made by PJD shows a very fast PL decay of less than 20 ps which should result from the fast relaxation process of C70 molecule in the excited state S1 of this new kind film.

Optical Performance and Nonlinear Scattering of Soluble Polystyrene Grafted Multi-Walled Carbon Nanotubes

Three soluble polystyrene grafted multi-walled carbon nanotube (MWNT) samples are synthesized, and their optical performance and nonlinear scattering properties are investigated by z-scan method using nanosecond pulses of 532 nm from a frequency-doubled Q-switched Nd:YLF laser. Analysis of the experimental results shows that other than nonlinear scattering, nonlinear absorption plays a major role in optical limiting performance of these stable and well-dispersed suspensions. These new synthesized materials which can be better dispersed in common organic solvents than MWNT itself can be considered as potential sources for further optical applications.

Femtosecond Time-Resolved Absorption of Oxyhaemoglobin Photolysis in Living Erythrocytes

By using the pump–probe technique, the femtosecond (fs) time-resolved absorption dynamics of oxyhaemoglobin (oxy Hb) in living erythrocytes was studied for the first time to our knowledge. The changing course of erythrocyte absorption at 400 nm was found to be composed of three exponential decay components with the time constants of 200±28 fs, 3.60±0.14 ps and 215±21 ps respectively, which are consistent with those of oxy Hb water solution measured in the same way, indicating that the fast process (less than 1 ns) of photo-induced response in erythrocytes was as the same as that in oxy Hb aqueous solution. These three decay components were assigned to the transitions from excited states of unligated haemoglobin (Hb*I, Hb*II) and fast geminate recombination of O2 and Hb, respectively.