Heyun Wu

Preparation and characterization of tungsten oxide thin films with high electrochromic performance

Tungsten oxide thin films were prepared by depositing WO3 onto glass substrates coated with ITO using reactive evaporation process at ambient temperature and 200°C respectively. The thin films were grown at different deposition rate. Chronoamperometry was carried out and spectral measurements were performed in situ. Results showed that the thin films prepared at low deposition rates possess higher coloration efficiency (CE), and the thin films grown at ambient temperature have high CE than those grown at 200°C. The origin of the differences in coloration efficiency of the thin films were analyzed and discussed based on the electrochromic mechanism of amorphous tungsten oxide films. The samples morphology was characterized by atom force microscopy (AFM).

Wavelength tunable properties of the perpendicular dual-grating guided-mode resonance filters

Perpendicular dual-grating (PDG) guided-mode resonance filters were constructed by placing two identical one-dimensional waveguide gratings close to and their grooves perpendicular to each other with a nano air gap between them. Multilayer waveguide theory was used to estimate the split of the resonant reflection peaks corresponding to the TE and TM modes, and the rigorous coupled wave analysis (RCWA) was used to investigate the resonant wavelength, the linewidth of the resonant peaks, and electric field intensity distribution in the filter structures. The filters present identical spectral characteristics for normally incident wave with arbitrary polarization. The TM01 and the TM01 modes are found displaying the greatest wavelength shift for the air gap variation between 0 and 100 nm, and 100 nm and 1000 nm，respectively. The coupling between the TE and TM modes is much greater in the g/w/a/w/g structure than that in the w/g/a/g/w structure, since there is no space between the two waveguide layers in the former. The resonant peaks of the TM01 mode for the one-dimensional PDG g/w/a/w/g structure exhibit narrower width compared with those for the two-dimensional g/w/a/w/g structure. In addtion, the horizontal shift between the two gratings does not influence the measured spectra, although it will certainly have great effect on the resonant peak width if the measurement were carried out by the guided-mode filters where the two gratings are two dimensional.

Multimode resonance bandpass filter with twin wideband channels

A bandpass filter with twin wideband channels in a single-layer guided-mode resonance grating is presented. Strong refractive-index modulation is used to support the excitation of multimode resonances TE1,0, TE1,1, TE2,0, TE1,2, and TE2,1, which are excited by the first and second diffraction orders, relate asymmetrical line shapes and broad low-transmission bands, where TE is the transverse electric. Taking advantage of narrow linewidth and sharp edge line shape in the spectra of TE2,v (v is the mode), a bandpass filter with form factors of 0.61 and 0.7 for long- and short-wave channels is presented to demonstrate this concept.

The influence of micron-sized nodules on the electric-field districution in thin-film polarizers

The influence of micron-sized nodules on the electric-field enhancement in the HfO2/SiO2 thin-film polarizers with non-quarter- wave layers at 1053nm and 56° is studied using the finite-difference time-domain electromagnetic modeling. The theoretical results show that the electric-field enhancements in HfO2 material are greater at s polarization than those at ppolarization. Nodular defect originating from the large, shallow seed leads to the highest electric-field enhancement while that containing the small, deep seed leads to the lowest electric-field enhancement. The TFP coating designed with the electric-field peaks located in the SiO2 layers has no obvious advantage in decreasing the laser-induced damage than that designed with the electric-field peaks located in the HfO2 layers, once they have the similar nodular defects in them.

Guided-mode resonance in tunable orthogonal grating

In this paper, we present a new tunable orthogonal grating consisting of two weakly modulated subwavelength gratings perpendicularly to one another. Using the rigorous coupled-wave analysis (RCWA), we calculate and analyze the characteristics of guided-mode resonance on incident angle for TE/TM polarization. We demonstrate that a wide range tunability of such structure accomplished with only a nanoscale variation of the spacing between the gratings. For a normally incident wave of both TE and TM polarization, the structure turns out the same spectrum and resonance peaks. A gap of reflection peaks is found, and electric field distributions manifest that this gap is due to the interaction between the leaky modes of TE0-TE0, TM0-TM0 or TE0-TM0. We investigate the reflection efficiency of TE/TM polarization as a function of the wavelength with the angle of incidence as a parameter. As incident angle increases, the separation of the resonances increases also. At the same time, some specific wavelengths are maintained.

Nanometer gap detection method using GMR grating

In this paper the displacement of the reflection resonant peak resulting from the change of the gap between the guided-mode resonance (GMR) grating and substrate is used to measure the nanometer gap. The paper calculates double layer model and metal substrate model using rigorous coupled-wave analysis (RCWA). It is revealed that nano-gap detection using GMR grating is feasible for both dielectric and metal substrate. The detection range of resonant wavelength and gap is tunable. The detect sensitivity is investigated by varying the parameters of grating (thickness, period and refractive index), the thickness of films, and polarization. Tolerance of grating implies an advantage for manufacture. An optimized result presents an 18nm resonant shift for 100nm gap with the max sensitivity achieving 0.85.

Optimized light trapping in thin film silicon solar cells by metal nanoparticle

Detailed design for nanoparticle plasmons-enhanced solar cells is presented in this article. Optimal structure for the max enhancement from the nanoparticle arrays is investigated by varying the size, shape and period of the nanoparticle, and thickness of Si absorber. Tolerance of the nanoparticle arrays provides guidance for fabrication in practice. Contribution of front and back structure as well as the nanoparticle arrays to the photocurrent is analyzed respectively. After optimization short circuit current density attains 20mA/cm2 with a 185% enhancement compared to that of the 700nm thick Si.

Preparation and characterization of free-standing Zr, PI and Zr/PI filter

A thin PI film equal to or less than 200nm was fabricated on a Zr film to improve the mechanical characteristics of the latter. The PI film was prepared by two-step process. Througth fully reaction between Pyromellitic Dianhydride (PMDA) and Oxydianiline (ODA) in Dimethylacetamide (DMAC), polyamic acid (PAA) was produced. After the deposition of Zr film on floating glass using direct-current magnetron sputtering, PAA was prepared on the Zr film through dip-coating and then thermally imidized to form the PI film. The transmission spectrum obtained by using synchrotron radiation fits with calculation result fairly well. Although the combination of the PI film with Zr film results in the decline of the transmission, the mechanical strength of the composite film is improved, and the transmittances of the Zr(300nm)/PI(200nm) and Zr(400nm)/PI(200nm) films reach 14.9% and 7.5% respectively at 13.9 nm, still satisfying the actual requirement.

An Automatic Method for Optimization of Optical Parameters and Electric Filed Distributions in Thin-Film Polarizers

An efficient method based on the needle optimization of a new merit function is proposed to design thin-film polarizers. Improved electric field characteristics are observed in comparison with those obtained by the traditional merit function.

Automated method for optimization of electric field distributions and optical parameters in thin-film polarizers

An efficient method based on the modified needle optimization technique is proposed to design high-power laser thin-film polarizers. In order to minimize the influence of the standing-wave electric field on the laser-induced damage threshold of the polarizers, a crucial optimization parameter, the maximum electric field intensity in the high-refractive-index layers, is included in the proposed merit function. The electric field distribution and optical performance obtained by the proposed method are studied. Improved electric field and identical optical characteristics are observed in comparison with those of the designs obtained by optimizing the traditional merit function without a standing-wave electric field term and by the analytical synthesis method.