[show abstract][hide abstract] ABSTRACT: We investigated systematically the modification of the two-photon-pumped random lasing (TPPRL) behavior of ZnO nanorods through femtosecond (fs) laser ablation. The excitation wavelength dependence of the TPPRL behavior of ZnO nanorods was measured and the threshold of TPPRL before and after fs laser ablation was compared. With increasing excitation wavelength, a rapid increase in the threshold of TPPRL was observed at ∼720 nm and it offers us the opportunity to significantly modify the two-photon luminescence from ZnO nanorods by fs laser ablation. The excitation wavelength dependence of the threshold was attributed to the wavelength dependence of the mean free path of photons in ZnO nanorods. When the fs laser ablation was carried out at 720 nm, it was found that the improvement in TPPRL could be achieved at excitation wavelengths longer than 620 nm while the TPPRL behavior became deteriorated for excitation wavelengths shorter than 620 nm. If the ablation and characterization wavelengths were chosen to be the same, then no obvious change was found for wavelengths shorter than 680 nm. However, a significant improvement in TPPRL could still be observed at 720 nm. The effects of ablation time and ablation method on the TPPRL behavior were also studied. It was revealed that the reduction in the threshold and the enhancement in the internal quantum efficiency could be realized by increasing ablation time and using continuous ablation. The results presented in this paper will be useful for the modification of the TPPRL behavior of random lasing media.
Journal of Applied Physics 09/2012; 112(6). · 2.21 Impact Factor
[show abstract][hide abstract] ABSTRACT: We propose and demonstrate a method to achieve large effective Soret
coefficient in colloids by suitably mixing two different particles, e.g.,
silica beads and Fe3O4 nanoparticles. It is shown that the thermophoretic
motion of Fe3O4 nanoparticles out of the heating region results in a large
nonequlibrium depletion force for silica beads. Consequently, silica beads are
driven quickly to the heating region, forming a three-dimensional crystal with
few defects and dislocations. The binding of silica beads is so tight that a
colloidal photonic crystal can be achieved after the complete evaporation of
solvent, water. Thus, for fabrication of defect free colloidal PCs, periodic
structures for molecular sieves, among others, the proposed technique could be
a low cost alternative. In addition as we use biocompatible materials, this
technique could be a tool for biophysics studies where the potential of large
effective Soret coefficient could be useful.
[show abstract][hide abstract] ABSTRACT: The optical trapping and manipulation of magnetic holes (MHs) dispersed in a magnetic fluid is systematically investigated. It is found that the gradient force, which tends to attract MHs to the beam center, can be completely counteracted by the repulsive force between MHs induced by a magnetic field. As a result, a depletion region is created at the laser beam spot for a sufficiently strong magnetic field. This phenomenon can be easily observed for large MHs with a diameter of 11 μ m . However, it does not appear for MHs with a smaller diameter of 4.3 μ m . It is revealed that the enhancement in the concentration of magnetic nanoparticles in the laser spot region as well as the clustering of these nanoparticles leads to a much stronger interaction between MHs when a magnetic field is applied. Consequently, the magnetic field strength necessary to create the depletion region is significantly reduced. We also find that the trapping behavior of MHs depends strongly on the thickness of the sample cells. For thin sample cells in which only one layer (or a two-dimensional distribution) of MHs is allowed, we can observe the creation of depletion region. In sharp contrast, MHs can be stably trapped at the center of the laser beam in thick sample cells even if a strong magnetic field is imposed. This phenomenon can be explained by the existence of a gradient in magnetic field strength along the direction perpendicular to the sample cells. Apart from individual MHs, we also investigate the movement of MH chains under the scattering force of the laser beam. It is observed that MH chains always move along the direction parallel to the magnetic field. This behavior can be easily understood when the anisotropy in viscosity caused by the applied magnetic field is considered.
Journal of Applied Physics 06/2010; · 2.21 Impact Factor
[show abstract][hide abstract] ABSTRACT: This paper systematically investigates the response of colloidal liquids containing magnetic holes of different volume densities to magnetic field by conventional transmission measurements. It finds that the enhancement in the transmission of such a colloidal liquid under a magnetic field exhibits a strong dependence on the volume density of magnetic holes. A linear increase in the maximum enhancement factor is observed when the volume density of magnetic holes is below a critical level at which a maximum enhancement factor of ~150 is achieved in the near infrared region. Once the volume density of magnetic holes exceeds the critical level, a sharp drop of the maximum enhancement factor to ~2 is observed. After that, the maximum enhancement factor increases gradually till a large volume density of ~9%. By monitoring the arrangement of magnetic holes under a magnetic field, it reveals that the colloidal liquids can be classified into three different phases, i.e., the gas-like, liquid-like and solid-like phases, depending on the volume density of magnetic holes. The response behaviour of colloidal liquids to magnetic field is determined by the interaction between magnetic holes which is governed mainly by their volume density. A phase transition, which is manifested in the dramatic reduction in the maximum enhancement factor, is clearly observed between the liquid-like and solid-like phases. The optical switching operations for colloidal liquids in different phases are compared and the underlying physical mechanisms are discussed.