Qu Shi-Liang

Superposition of orbital angular momentum of photons by a combined computer-generated hologram fabricated in silica glass with femtosecond laser pulses

This paper introduces a novel method to realize the superposition of orbital angular momentum of photons by combined computer-generated hologram (CCGH) fabricated in silica glass with femtosecond laser pulses. Firstly, the two computer-generated holograms (CGH) of optical vortex were obtained and combined as a CCGH according to the design. Then the CCGH was directly written inside glass by femtosecond laser pulses induced microexplosion without any pre-or post-treatment of the material. The vortex beams with different vortex topological charges (including new topological charges) have been restructured using a collimated He-Ne laser beam incidence to the CCGH normally. A theoretical and experimental explanation has been presented for the generations of the new topological charges.

Size-dependent nonlinear absorption and refraction of Ag nanoparticles excited by femtosecond lasers

Silver (Ag) nanoparticles with different average sizes are prepared, and the nonlinear absorption and refraction of these nanoparticles are investigated with femtosecond laser pulses at 800 nm. The smallest Ag nanoparticles show insignificant nonlinear absorption, whereas the larger ones show saturable absorption. By considering the previously reported positive nonlinear absorption of 9 nm Ag nanoparticles, the nonlinear absorptions of Ag nanoparticles are found to be size-dependent. All these nonlinear absorptions can be compatibly explained from the viewpoints of electronic transitions, energy bands and electronic structures in the conduction band of Ag nanoparticles. The nonlinear refraction is attributed to the effect of hot electrons arising from the intraband transition in the s—p conduction band of Ag nanoparticles.

Novel metallo-phthalocyanine with high performances for ns optical limiting in solution

Abstract The optical limiting performances of metallo-phthalocyanines of (C12H25O)8PcNi and (C12H25O)8PcPb were investigated in toluene, in which the optical limiting threshold and clamped value of the (C12H25O)8PcPb were much lower than those of C60 at same conditions in toluene and the optical limiting threshold and clamped value of the (C12H25O)8PcNi were also comparable to the C60 for 8 ns pulses at 532 nm. The photophysical parameters were calculated according to a multiple levels model, which indicated that the reverse saturable absorption in the excited triplet state is responsible for the optical limiting mechanism for (C12H25O)8PcNi and (C12H25O)8PcPb with improved stability.

Third-order optical nonlinearities and optical limiting of octa-octyloxy phthalocyanine free base in toluene

The third-order nonlinear optical properties of the octa-octyloxy phthalocyanine free base were studied by Z-scan using 35-ps laser pulses at 532 nm in toluene. The nonlinear refractive index or the refractive cross section, and the excited singlet state absorption cross section were determined. The optical limiting performances at both nanosecond and picosecond time scales have been investigated. The results suggest that the octa-octyloxy phthalocyanine free base is promising for optical limiting materials.

Slow-rise and fast-drop current feature of ultraviolet response spectra for ZnO-nanowire film modulated by water molecules

This study describes the fabrication of ZnO-nanowire films by electro-chemical anodization of Zn foil. The ZnO films are characterized by field emission scanning electron microscopy, X-ray diffraction patterns, and transmission electron microscopy, respectively. The ultraviolet (UV) photo-response properties of the surface-contacted ZnO film are studied through the current evolution processes under different relative humidities. Unlike the usually observed current spectra of the ZnO films, the drop time is shorter than the rise time. The photo-conductivity gain G and the response time τ are both increased with the increase of the applied bias. The photo-conductivity gain G is lowered with the increase of the environmental humidity, while the response time τ is increased. These results can be explained by considering three different surface processes: 1) the electron-hole (e—p) pair generation by the UV light illumination, 2) the following surface O−2 species desorption, and 3) the photo-catalytic hydrolysis of water molecules adsorbed on the ZnO surface. The slow-rise and fast-drop current feature is suggested to originate from the sponge-like structure of the ZnO nanowires.

Rotational motions of optically trapped microscopic particles by a vortex femtosecond laser

The rotational motions of the optically trapped microscopic particles by the vortex femtosecond laser beam are investigated in this paper. Black particles can be trapped and rotated by a vortex femtosecond laser beam very effectively because the vortex beam carries orbital angular momentum due to the helical wave-front structure in assoication with the central phase singularity. Trapped black particles rotate in the vortex beam due to the absorption of the angular momentum transferred from the vortex beam. The rotating directions of the trapped particles can be modulated by reversing the topological charge of the optical vortex in the vortex femtosecond beam. And the rotating speeds of the trapped microscopic particles greatly depend on the topological charges of the vortex tweezer and the used pulse energies.

Enhanced Excited-State Optical Nonlinearities in a Novel Metallophthalocyanine Compound (C12H25O)8PcPb

Nonlinear absorption in a novel metallophthalocynine compound (C12H25O)8PcPb was investigated by the Z-scan technique with the irradiation of 8 ns laser pulses at the wavelength of 532 nm. The large optical limiting response was observed. The nonlinear absorption cross section was obtained by the simulation with a simplified rate equation model in which excited triplet-triplet state absorption is dominant. The excited-state absorption cross section and its effective ratio to the ground-state absorption cross section are larger than those of C60. The attenuation factor for (C12H25O)8PcPb is 2.5 times larger than that of C60, which is induced by the heavy atom lead interposition.

Excited state optical nonlinearity of inorganic metal cluster WSe4(AgPPh3)3Cl with picosecond pulses

Abstract The excited state absorption and refraction of a novel cubane-like shaped metal cluster WSe 4 (AgPPh 3 ) 3 Cl in DMF solution were studied by using the Z-scan technique with laser pulses of 40 ps pulse-width at the wavelength of 532 nm. The compound possesses a self-focusing performance. The positive nonlinear refraction is attributed to population transitions among singlet states. The cluster displayed a strong excited state absorption, which induced asymmetry in the Z-scan results.

The effects of heat treatment on microfluidic devices fabricated in silica glass by femtosecond lasers

We fabricated complex microfluidic devices in silica glass by water-assisted femtosecond laser ablation and subsequent heat treatment. The experimental results show that after heat treatment, the diameter of the microchannels is significantly reduced and the internal surface roughness is improved. The diameters of the fabricated microchannels can be modulated by changing the annealing temperature and the annealing time. During annealing, the temperature affects the diameter and shape of the protrusions in microfluidic devices very strongly, and these changes are mainly caused by uniform expansion and the action of surface tension.

Measurements of femtosecond temporal speckle field of a random medium

The femtosecond temporal speckle field of a random medium is studied theoretically and experimentally. Femtosecond temporal speckle arises from the interference of multiple randomly scattered electric fields. The femtosecond temporal speckle field is measured with a cross-correlation frequency-resolved optical gating method. The spatial average of the speckle field yields a smooth transmitted profile. The speckle field is a circular complex Gaussian variable because the scattered light beams from different trajectories have no correlation with each other. The field and the intensity profiles of individual speckle spots fluctuate randomly in time. The ensemble average of the temporal intensity profiles converges, thereby yielding the photon travel time probability distribution function.