Jiangying Zhang

Nonlinearly optical–optical isomerization cycle in azobenzene liquid crystal polymers

A nonlinearly optical method to induce the birefringence in azo liquid crystal polymers by the optical–optical isomerization cycle (O–O cycle) process is proposed in theory and implemented by experiments. In the O–O cycle, the two isomerization steps of trans-cis and cis-trans are both induced by light. In the nonlinearly O–O cycle, at least one step involves the nonlinearly optical absorption. It has been demonstrated that the permanent birefringence was induced in azo polymer P–CN films with a visible light source at 532 nm wavelength by the nonlinearly O–O cycle. The nonlinear nature of the photoisomerization process in P–CN was verified and a high nonlinear absorption coefficient of 40 cm/W was measured by use of the Z-scan technique.

Nonlinear optical and optical-limiting properties of Azobenzene liquid crystal polymer☆

The nonlinear absorption coefficient β of Azo (Azobenzene) liquid crystal polymer (LCP) was measured by using open aperture Z-scan technique with 10 ns, 532 nm laser pulses. The value of β is 9.02 cm/GW. The optical limiting (OL) response of Azo LCP in tetrahydrofuran (THF) was measured also by use of the Q-switched (10 ns) second-harmonic generation Nd:YAG pulse. We attribute the nonlinear absorption and OL property of Azo LCP to TPA effect at 532 nm.

Increase the storage density of solid immersion lens system by high-pass angular spectrum filter method☆

In this paper the optical distribution in the near-field region of solid immersion lens (SIL) is investigated from the viewpoint of angular spectrum. Because of the big numeric aperture (NA) of objective in SIL storage system, the polarization of incident laser is also considered. Based on this investigation, a kind of high-pass angular spectrum filter method is proposed to decrease the optical spot size. The effect of filter in SIL system is analyzed in detail. Our theoretical analysis shows that this kind of filter can decrease the spot size by nearly 40% and light intensity loss by around 11%.

First-principles study on the spin polarization of benzene adsorbed on Fe(100) surface

In this paper, we investigate the spin-resolved electronic states and spin polarization of the benzene adsorbed Fe(100) surface using density functional theory. The existence of the adsorbate-induced π* state and backdonation of the electrons have been demonstrated clearly by the differential charge densities. The calculated plane-averaged density of states of benzene∕Fe(100) in the vacuum side is found to be consistent with the experimental result of spin asymmetry in the spin-polarized metastable-atom deexcitation spectra for pentacene consisting of five benzene rings adsorbed Fe(100) surface.

Independent control of upper and lower cutoff frequencies in two-dimensional photonic crystal waveguides

The independent control of the upper and lower cutoff frequencies of the guided modes in coupled-cavity wavguides (CCWs) is investigated numerically. The CCWs are formed in a two-dimensional photonic crystal (2D PhC) consisting of a square array of dielectric rods in the air. The dielectric constant and radius of the rods in the perfect PhC is 11.56 and 0.18α, respectively, in which α is lattice constant. By using the plane wave expansion method (PWEM), the impact of two influencing factors on cutoff frequencies in CCWs is calculated systematically. Efficient methods have been demonstrated for tuning one cutoff frequency while keeping the other unchanged. The indepent control ranges of the upper and lower cutoff can be up to 67.85% and 68.57% of photonic band gap (PBG), respectively. The results can be applied to the design of PhC-based optical devices such as band filters and optical switches.

Holographic lens in azobenzene liquid crystal polymer films

In this paper, an off-axis holographic lens with the focal length of about 70mm is recorded in the azobenzene liquid crystal polymer film (azo-LCP) by 532nm YbVO4 double-frequency lasers and is reconstructed by a 633nm He-Ne laser. The particularity and main advantage of the work consists in writing and reading out the hologram at the same place, without moving, post-exposure adjusting or chemical processing. The converging and imaging property of the holographic lens is measured and analyzed.

Two-photon bit data storage in azobenzene polymer liquid crystal using low-power cw laser

In this paper, two-photon bit data storage in azobenzene polymer liquid crystal (PLC) film using low-power continuous wave (CW) laser beam at 532 nm is proposed and demonstrated for the first time, the exposure threshold of the material is below 4 X 106 mw/cm2, and a bit data storage pattern is recorded by two-photon absorption (TPA). The data is easily erasable and rewritable by the light of the same wavelength as the recording light.

Photo-induced polarized emission of the Eu3+-doped Azo-polymer waveguide

The polarized emission behaviors of the Eu3+ doped azo-polymer waveguide were reported in this paper. Affected by the azobenzene groups in the photoinduced orientation process by the 532nm linearly polarized laser, the ligands were realigned orderly perpendicular to the direction of the orientation direction. This leads to the polarized absorption and emission of the waveguide in the orientation direction. By an m-line apparatus based on the prism coupling technique, two guided modes propagation were observed in the waveguide at 650nm in TE polarizations.

Effect of heat on photoinduced birefringence in azobenzene-side-chain copolymer

Azobenzene polymers show large optical anisotropy, and one can control the optical anisotropy by changing the alignment of the molecules with light. Azo groups fall perpendicularly to the direction of the electric field vector of the linear polarized light by photoisomerization. This reorientation induces anisotropy and the difference of refractive index parallel and perpendicular to the irradiating laser polarization direction. In this paper, the photoinduced birefringence induced with 532 nm light in films of azobenzene polymer has been studied as a function of temperature, light intensity. From the evolution of the birefringence induced by laser, the temperature effects on photochemical trans-cis-trans isomerization cycles have been probed and analyzed. The effect of laser-induced heating has been introduced to the buildup of photoinduced birefringence in azobenzene-side-chain copolymer. The curves for the buildup of birefringence were fitted with a modified function, namely, biexponential curves and Gaussian curves. The relationship of each parameter has been also presented. With the modified fitting function, we obtained better fitting result.

Near-field optical research on the azobenzene polymer films

We report the near-field investigation on azobenzene contained polymer films using scanning near-field optical microscopy. Nanometer scale dots and lines were inscribed on these films, and the topographies and transmitting images of these patterns could be obtained at the same time. The transmitting images were in agreements with the topographies, and it proved that trans-cis isomerization of the azobenzene moieties in the illuminated area induced both surface relief and anisotropy. The experimental results helped to discover the mechanism of the photo induced surface relief and anisotropy, and the agreement between the transmitting images and the topographies could be applied in pure optical writing/reading in high-density data storage.