Zhanshan Wang

Preparation of spherical silica particles by Stöber process with high concentration of tetra-ethyl-orthosilicate.

In this paper, Stöber process with high concentration of tetra-ethyl-orthosilicate (TEOS) up to 1.24 M is used to prepare monodisperse and uniform-size silica particles. The reactions are carried out at [TEOS]=0.22-1.24 M, low concentrations of ammonia ([NH(3)]=0.81[TEOS]), and [H(2)O]=6.25[TEOS] in isopropanol. The solids content in the resulting suspension achieves a maximum value of 7.45% at 1.24 M TEOS. Various-sized particles in the range of 30-1000 nm are synthesized. The influences of TEOS, NH(3), and H(2)O on the size and size distribution of the particles are discussed. A modified monomer addition model combined with aggregation model is proposed to analyze the formation mechanism of silica particles.

An ultrathin twist-structure polarization transformer based on fish-scale metallic wires

This study theoretically and experimentally investigates the transmission properties of a metamaterial slab comprised of two layers of metallic fish-scale structure arrays and a sandwiched dielectric layer. Calculations show that the asymmetric transmission can be tuned by varying the slab thickness, due to near-field interlayer coupling. The spatial evolution of the electric field indicates that the twist structure functions as a perfect polarization transformer at certain frequencies. Measured transmission spectra are in good agreement with calculated results when material dissipation is considered.This study theoretically and experimentally investigates the transmission properties of a metamaterial slab comprised of two layers of metallic fish-scale structure arrays and a sandwiched dielectric layer. Calculations show that the asymmetric transmission can be tuned by varying the slab thickness, due to evanescent interlayer coupling. The spatial evolution of the local field inside the structure indicates that the slab functions as a perfect polarization transformer at certain frequencies in the manner of a waveguide twist. Measured transmission spectra are in good agreement with calculated results when material dissipation is considered.

Multiple channeled phenomena in heterostructures with defects mode

Transmission studies for multiple heterostructures inserted with defects are presented. The results show that the enlargement of the forbidden band and the realization of multiple channeled filtering can be reached simultaneously with these configurations. The frequency, frequency interval, and number of channels can be tuned by adjusting geometric and physical parameters of the defects. Sidebands of the forbidden band can be substantially restricted as compared with that of periodic structures. Experimental fabrication of the designed configurations is carried out with the heterostructures containing TiO2/SiO2 multilayer defects, and the results agree with the calculation very well.

Guided-mode resonance Brewster filters with multiple channels

In this letter, a type of guided-mode resonance (GMR) reflection filter incorporating multimode resonance and the Brewster effect is presented. Long-range, low sidebands, and multiple channels are found to be obtainable for a single-layer GMR reflection filter with a TM-polarized plane wave incident at the Brewster angle. Detailed characteristics are calculated using rigorous coupled-wave analysis. In our calculation, for TM-polarized incident wave, we find it sufficiently accurate to calculate the effect index by using the zero-order permittivity e0,TM of the effective media theory, and narrowband multichannel reflectance can be achieved at the Brewster angle by tuning the grating depth.

Integrated optical filter arrays fabricated by using the combinatorial etching technique

A combinatorial etching technique is developed to fabricate integrated narrow bandpass filters on a single substrate. It is highly efficient for fabrication of integrated filter arrays in optical regions. A monolithic filter array has been fabricated by using the technique with a two-step deposition process. The filter contains 32 elements in the near-infrared region. The relative full width at half-maximum (FWHM) sigmalambda/lambda of the filter elements is less than 0.2%. Such a narrow bandpass filter array can be utilized in many optical applications.

Electrical tunable random laser emission from a liquid-crystal infiltrated disordered planar microcavity

Here we report electric-field-induced, wavelength-tunable random lasing action in a liquid-crystal infiltrated disordered planar microcavity. With the increasing of external voltage from 0 V to 30 V, the emission wavelength of the random laser was continuously tuned from 648 nm to 597 nm. It was also tuned back to 648 nm again by reducing the voltage back to 0 V.

Liquid-crystal-based tunable high-Q directional random laser from a planar random microcavity.

Temperature-tunable directional laser emission from a dye-doped liquid-crystal-based planar random cavity laser is presented. The optically pumped nematic liquid crystal infiltrated planar random microcavity produces an ultranarrow linewidth (0.03nm, corresponding to Q>20,000), highly directional (1.4° divergence angle) laser emission. By increasing the temperature from 27°C to 34°C, the wavelength of an emitted polarized laser can be tuned between 605.8 and 608.5 (ordinary light) and 631.3 and 624.9 nm (extraordinary light). A simulation result from the transfer matrix method that matches the experimental results well is also presented.

Systematic study of the mirror effect in a poly-Si subwavelength periodic membrane

By using the equivalent eigenvalue equation of a waveguide grating, the mirror effect (Deltalambda/lambda>15%) with very high reflectivity (R>99%) based on guided-mode resonance (GMR) effects in a poly-Si subwavelength periodic membrane is obtained, and the reflection performance of the poly-Si subwavelength periodic membrane is systematically studied. It is shown that the equivalent eigenvalue equation of a waveguide grating can provide a solid starting point for designing the broadband grating with very high reflectivity. The physical mechanisms of broadband reflection of the strongly modulated waveguide grating structures are investigated theoretically and the important role of multiple GMRs for a broad reflection band is discussed in detail. By using the overlap of a resonance pair in which leaky waveguide modes TE0 and TE1 are excited by the strong first diffraction order, enhanced reflection occurs and a flat reflection band with high reflectivity can be achieved by adding a poly-Si thin film under the grating. The grating period, the grating thickness, and the layer thickness do not change the mirror effect except for the incident angle and the filling factor. A flat band with high reflectivity centered near 1.55 microm is designed to demonstrate this concept.

Optimisation of depth-graded multilayer coatings for broadband reflectivity in the soft X-ray and EUV regions

Abstract A systematic method which allows the optimum thickness of each layer in a depth-graded multilayer coating to be determined is described. This enables specific reflectivity responses over broad wavelength bands in the soft X-ray and EUV regions to be calculated. The method is applied to the design of some depth-graded molybdenum/silicon multilayers for the wavelength range 13–19 nm, with average normal incidence reflectivities of about 13% in this range, but it is generally applicable for other material pairs and wavelength ranges. In addition, the effects of layer thickness errors on the performance of depth-graded multilayers can be simulated. The model gives better results than those based on power law variation of the layer thicknesses.

Optical nonlinearity enhancement in heterostructures with thick metallic film and truncated photonic crystals

It is hard to use bulk metals with high nonlinear susceptibilities since they are nearly opaque. Metal-dielectric photonic crystals can be transparent, but the electric field at each metal layer is still low. We theoretically studied the nonlinear response of heterostructures composed of truncated all-dielectric photonic crystals and thick metallic films. Because of the localized interface modes, both transmittance and the electric field in the metal are enhanced greatly. Compared to metal-dielectric photonic crystals, the critical intensity of threshold for bistability in the heterostructures with the same thickness of metal can be reduced by nearly 2 orders of magnitude.