Peiwang Zhu

Computed tomography algorithm based on diffraction-enhanced imaging setup

Recently taking advantage of the novel diffraction-enhanced imaging (DEI) method, one very effective and practical phase contrast imaging technique—a new x-ray computed tomography scheme based on DEI (DEI-CT) showed promising results, really superior to those of conventional CT imaging. In this letter, we show that the application of the conventional CT reconstruction algorithm is not always satisfactory for the new DEI-CT imaging and a new mathematical framework for imaging reconstruction is presented. Experimental data collected at the Beijing synchrotron radiation facility are also discussed using the new algorithm.

Design of novel nonlinear optical chromophores with multiple substitutions

The molecular first hyperpolarizabilities (β) and electronic properties of some tetra-substituted benzenes/pyrazines which can be described as 1,2-didonor-4,5-diacceptor-benzenes/2,3-didonor-5,6-diacceptor-pyrazines have been investigated by employing an AM1/FF and ZINDO/S-CI approach. It is found that these multiple substituted chromophores possess large β values which may be attributed to the positive contribution of the off-diagonal tensorial component βxyy. Two novel chromophores (1 and 2) containing lengthened conjugation and thermally stable imidazole rings are designed under such a multiple substitutions principle. Chromophore 1 shows a very large µβ0 value of approximately 3900×10-48 esu.† The theoretical calculation and preliminary experimental results indicate that a good nonlinearity–transparency–thermal stability trade-off is achieved for chromophore 2, which possesses a µβ0 of 555×10-48 esu, a rather blue-shifted absorption peak at 384 nm in THF and a high decomposition temperature (Td) of 377°C.

Optical limiting properties of phthalocyanine-fullerene derivatives

The solubility of fullerene (C60) is improved by combining it with an alkyl group of substituted copper (II) phthalocyanine (CuPc) through Diels–Alder reaction. Optical limiting properties of this compound were investigated by using a double frequency Nd:YAG laser system at a wavelength of 532 nm with pulse duration of 45 ps. The results show that there are two separated conjugation systems in this kind of molecule. Both of them independently contribute to the reverse saturable absorption. Broadband optical limiting materials might be designed based on the combination of various optical limiting molecules through covalent bonds. Phase separation problems existing in the mixture of various materials can be avoided.

High Tg donor-embedded polyimides for second-order nonlinear optical applications

We present here a class of processible donor-embedded nonlinear optical (NLO) polyimides exhibiting the high glass-transition temperature and exceptional thermal and chemical stability. Due to the rigidity of the donor-embedded structure, the stability of the electric field induced dipole orientation of the NLO chromophores was enhanced. A sizable second-order nonlinear optical coefficient was obtained.

Second order non‐linear optical materials based on poly(p‐chloromethyl styrene)

The second order non-linear optical (NLO) materials were prepared based on poly(p-chloromethyl styrene) (PCMS). Two kinds of NLO chromophores, i. e. azo dye Disperse Red 1 (4-[N-ethyl-N-(2-hydroxy-ethyl)-amino]-4′-nitroazobenzene) and 4-hydroxy-4′-nitrostilbene, were introduced to the poly(p-chloromethyl styrene) by etherification. On the other hand, the acrylates and methacrylates of these two kinds of chromophores were co-polymerzed with methyl methacrylate (MMA) with PCMS as an initiator by atom transfer radical polymerization (ATRP). The obtained polymers were characterized by GPC, DSC and second-harmonic generation (SHG) measurements. The SHG signals began to decline at temperatures above their Tgs, which might result from their branched structures.

Theoretical investigation on the first hyperpolarizability of push–pull polyenes containing non-aromatic cyclic olefines

Abstract Novel push–pull polyenes containing non-aromatic cyclic olefines, such as cyclopentadiene, cyclopropene and cycloheptatriene, have been investigated for application of nonlinear optical (NLO) materials. Their dot products μβ0 of first hyperpolarizability (β0) and dipole moment (μ) are calculated by employing AM1/Finite Field and ZINDO/S approaches. Among them, the largest value is as high as 4.1×10−45 esu. The origin of such high μβ0 was analyzed based on the two-level model. Non-aromatic groups can transform to a stable aromatic anion/cation through gaining/losing an electron in their charge transfer states.

Thermally stable second‐order nonlinear optical addition‐type polyimides functionalized by diamine chromophore

A series of chromophore-functionalized polyimide prepolymers with excellent processibility were prepared by a Michael addition reaction of diamine chromophore 2 with structurally different bismaleimide (BMI) monomers. The effects of the BMI moietys structure and thermal curing condition on glass transition temperature (Tg) and thermal stability of the polyimides were studied by DSC, TGA, and FTIR. Among the five cured polyimides, PI3, bearing a sulfone moiety, exhibited the highest Tg and thermal decomposition temperature (Td). Its corresponding prepolymer, PP3, was selected to evaluate NLO properties in a simultaneously poling and thermal polymerization process. A relatively large poling-order parameter was observed. The second-order nonlinear coefficient, d33, was 25 pm/V at 1064 nm fundamental wavelength. The second harmonic generation signal was almost without decay up to 170°C.

SECOND-ORDER NONLINEAR OPTICAL POLYIMIDES WITH HIGH THERMAL STABILITY

Two high Tg (250°C and 258°C) polyimides embedded with triphenyl imidazole chromophores were synthesized through addition polymerization. Simultaneous poling and polymerization and in situ second h...

Thermally stable nonlinear optical polyimide functionalized by N,N‐diallylamino‐substituted chromophore

Thermally stable nonlinear optical (NLO) polyimide prepolymers with different chromophore-loading levels were prepared through an addition reaction of bismaleimide 1 and N,N-diallylamino-substituted chromophore 2. The prepolymers were used to prepare NLO polyimide in a simultaneously poling/curing process. These prepolymers exhibited excellent film-forming quality and good solubility in low boiling-point solvents. Kinetic parameters (n, In A, Ea) of their solidification reaction were determined by the dynamic method. Thermal and NLO properties of the cured polyimides with various chromophore-loading levels were studied. With increase of the chromophore-loading level, the glass transition temperature (T g ) and the thermal stability of the polyimides decreased, but the second-order nonlinearity of the poled film increased. The largest d 33 coefficient obtained was 76.2 pm/V at 1064 nm. The SHG signal was rather stable up to 120°C.

In-Situ Electrochromic Studies of Corona-Poled Polymer Films

Real-time monitoring of the corona-poling process was accomplished by in-situ electrochromism of doped and functionalized polymer films. The poling properties including poling and depoling processes, dependence of orientation order parameter on poling time etc. of used films were investigated. The referential criteria for deciding the suitable poling conditions were suggested. Based on the one-dimensional rigid orientation gas model, the d33 of poled films was evaluated with the order parameter and bathochromic shift of maximum absorption of films independently. The results are in good coincidence.