Zhaolin Li

Band gap and wave guiding effect in a quasiperiodic photonic crystal

A two-dimensional octagonal quasiperiodic photonic crystal composed of alumina cylinders is prepared. The transmission spectra of the quasicrystal are measured in the microwave region for the TM wave. We find that the position and width of the band gap do not depend on the incident direction, while the band structure can appear for quite a small piece of the quasicrystal. Two types of waveguide, a straight guide and a bending guide with two sharp 90° corners, are fabricated by removing three rows of cylinders. The measured transmittances show that the guiding efficiency for both waveguides is high.

More direct evidence of the fcc arrangement for artificial opal

We present a scanning electron microscopy (SEM) investigation of artificial opals prepared by the sedimentation of suspension with sub-micrometer silica spheres. An analysis of the SEM images confirms that the fcc stacking is the most stable phase for the artificial crystals. Optical diffraction and transmission spectrum indicate a good alignment of the top (111) crystal plane. We also show that defects and dislocations of opal crystals are caused mainly by the irregular spheres. Slow sedimentation and uniform size of silica spheres may be the key factors for growing high quality opal crystals.

Broadband optical reflector—an application of light localization in one dimension

A broad‐band optical reflector based on the localization of light for one‐dimensional disordered systems is proposed and demonstrated both theoretically and experimentally. The binary disordered multilayer reflector consists of alternating random layers of two coating materials. Its high reflection range is much wider than that of standard λ/4 stacks and can be obtained by a conventional coating method and ordinary materials. This kind of reflector will have significant applications in laser technology and optical equipment.

Three-dimensional ordered patterns by light interference.

Based on numerical simulation it is found that three-dimensional periodic patterns with micrometer or submicrometer lattice constants, such as those with simple cubic, face-centered cubic, body-centered cubic, or body-centered tetragonal symmetry, can be formed by use of multiple laser beam interference. Changes in the number and angle of the incident beams result in different spatial patterns, whereas variation of the incident wavelength affects only the lattice constant of the patterns. Face-centered cubic and body-centered tetragonal patterns have been observed experimentally. Our results, combined with the optical trapping of dielectric particles, might provide a promising approach to the realization of a new kind of material-a photonic crystal.

NARROW-BANDWIDTH EMISSION FROM A SUSPENSION OF DYE AND SCATTERERS

We studied the light emission from a laser pumped suspension of Rh640 dye and micron-size scatterers, TiO2 and SrTiO3. Two different excitation geometries, point area and line area, were tried for examining the relation between the gain length and the properties of the emission. The results show that the spectral width of light emission relates strongly to the concentration of dye and the gain length in the excited suspension. The existence of scatterers is an important factor for enhancing the gain length. The influences of the surface shape and the size of the scatterers seem to be insensitive.

Planar optical lattice of TiO 2 particles.

We present an experimental study of the formation of two-dimensional optical lattices of titanium dioxide particles. The titanium dioxide particles, with an average size of 0.24 μm and a high refractive index of 2.6–2.9 in the visible region, were trapped in the antinodes of a standing-wave field. The shortest lattice constant reached so far is 0.8 μm. A corresponding diffraction pattern of the lattice was observed. This is an effort to create a photonic crystal with a lattice constant extended to visible and near-infrared wavelength scales.

Kinetics of microtubule-AtMAP65-1 bond studied with dual-optical tweezers

The unbinding force of microtubule-AtMAP65-1 bond is measured with dual-optical tweezers. Force histograms reveal quantized force distributions under physiological condition and with 100 mM NaCl treatment. Based on Bell-Evans-model of multiple bonds, histograms are fitted and kinetic parameters are obtained. Under the force loading rate of 9pN/s, the most probable unbinding forces for the single bond are 16.8 +/- 0.8 and 14.7 +/- 0.8 pN under the physiological and treatment conditions, respectively. Position of transition state of the bond is given to be Delta x = 2.1 +/- 0.1 nm both with and without treatment of NaCl. Intrinsic dissociation rate constants k(0) are 0.0010 +/- 0.0002 and 0.0020 +/- 0.0005 s(-1) corresponding to without and with NaCl treatment, which means that 100 mM NaCl shortens the natural lifetime of the bond by 50%. In addition, the activation barrier also changes by 3.4% after treatment with NaCl, which indicates that NaCl makes the dissociation of the bond easier.

MEAN FREE-PATH OF A PHOTON IN A GOLD SUSPENSION

Abstract The possibility of light localization in a gold suspension is discussed in terms of Mie scattering theory. The results show that at the longer wavelength side of the Mie resonance, namely at 0.9-1 μm, the Ioffe-Regel criterion is able to be met, while the absorption length can be much longer than the elastic mean free path of a photon.

Collisional pooling of metastable Zn atoms

As the metastable state 4p 3P of Zn atoms is excited by a laser, a series of sensitive fluorescence from higher levels appears. Because of its longer lifetime the collisional effect can be identified. The results show that the pressure of filled argon gas affects the peak intensities and pulse duration of these fluorescences. A rate equation model is proposed and the agreement between theory and experiment is qualitatively established. The pooling process is found to be spin related.

Microcavities composed of point defects and waveguides in photonic crystals

A novel two-dimensional photonic crystal (PC) microcavity composed of a point defect, where a point emitter is placed, and a line defect (waveguide) was proposed and studied numerically. The results show that the light radiated from the point defect can be coupled into the waveguide with a more than 97% coupling efficiency under certain conditions. Because the PC waveguide can guide light around a sharp corner with a high efficiency, the radiation can be directed to any area in a PC by using our design. The microcavity quality factor and coupling efficiency can be adjusted through changing the space and the number of cylinders between the point defect and the waveguide, and the oscillation frequency can be tuned by changing the mode volume of the point defect. This may be valuable for the design of microwave integrated devices and all-optical integrated circuits.