Optically-induced-potential-based image encryption.

We present a technique of nonlinear image encryption by use of virtual optics. The image to be encrypted is superposed on a random intensity image. And this superposed image propagates through a nonlinear medium and a 4-f system with single phase key. The image is encrypted to a stationary white noise. The decryption process is sensitive to the parameters of the encryption system and the phase key in 4-f system. This sensitivity makes attackers hard to access the phase key. In nonlinear medium, optically-induced potentials, which depend on intensity of optical wave, make the superposition principle frustrated. This nonlinearity based on optically induced potentials highly improves the secrecy level of image encryption. Resistance against attacks based on the phase retrieval technique proves that it has the high secrecy level. This nonlinear image encryption based on optically induced potentials is proposed and demonstrated for the first time.

Dynamics and all-optical control of solitons at the interface of optical superlattices with spatially modulated nonlinearity.

We find the existence of two kinds of solitons at the interface of optical superlattices with both spatially modulated nonlinearity and linear refraction index. The first kind of solitons can either drift across the lattice, or deflect to the uniform nonlinear medium. The dynamics of such solitons mainly depends on their powers. The other kind of solitons can stably propagate along the surface, and can be controlled by additional Gaussian beams. In addition, we demonstrate the input-angle-dependent reflection, trapping, and refraction with nearly no losses by launching sech-shaped solitons.

Fraunhofer computer-generated hologram for diffused 3D scene in Fresnel region.

A Fraunhofer computer-generated hologram (CGH) is proved to be valid in display for three-dimensional (3D) objects from the Fresnel to the far-field region without a Fourier lens for reconstruction. To quickly compute large and complicated 3D objects that consist of slanted diffused surfaces in the Fresnel region, a Fraunhofer-based analytical approach using a basic-triangle tiling diffuser is developed. Both theoretical and experimental results reveal that Fraunhofer CGH can perform the same effects as Fresnel CGH but require less calculation time. Impressive 3D solid effects are achieved in the Fresnel region.

Observation of backscattering-immune chiral electromagnetic modes without time reversal breaking.

A strategy is proposed to realize robust transport in a time reversal invariant photonic system. Using numerical simulation and a microwave experiment, we demonstrate that a chiral guided mode in the channel of a three-dimensional dielectric layer-by-layer photonic crystal is immune to the scattering of a square patch of metal or dielectric inserted to block the channel. The chirality based robust transport can be realized in nonmagnetic dielectric materials without any external field.

Wideband slow light and dispersion control in oblique lattice photonic crystal waveguides.

We find that the angle between elementary lattice vectors obviously affects the bandwidth and dispersion of slow light in photonic crystal line-defect waveguides. When the fluctuation of group index is strictly limited in a +/-1% range, the oblique lattice structures with the angle between elementary lattice vectors slightly larger than 60 degrees have broader available bandwidth of flat band slow light than triangular lattice structures. For example, for the angle 66 degrees , there are increases of the available bandwidth from 20% to 68% for several different structures. For the same angle and a +/-10% variation in group velocity, when group indices are nearly constants of 30, 48.5, 80 and 130, their corresponding bandwidths of flat band reach 20 nm, 11.8 nm, 7.3 nm and 3.9 nm around 1550 nm, respectively. The increasing of bandwidth is related to the shift of the anticrossing point towards smaller wave numbers.

High-speed full analytical holographic computations for true-life scenes.

We develop a novel method to generate hologram of three-dimensional (3D) textured triangle-mesh-model that is reconstructed from ordinary digital photos. This method allows analytically encoding the 3D model consisting of triangles. In contrast to other polygon based holographic computations, our full analytical method will free oneself from the numerical error that is in the angular spectrum due to the Whittaker-Shannon sampling. In order to saving the computation time, we employ the GPU platform that is remarkably superior to the CPU's. We have rendered a true-life scene with colored textures as the first demo by our homemade software. The holographic reconstructed scene possesses high performances in many aspects such as depth cues, surface textures, shadings, and occlusions, etc. The GPU's algorithm performs hundreds of times faster than those of CPU.

Three-dimensional imaging with monocular cues using holographic stereography.

Two quantitative criteria are derived to evaluate monocular cues in holographic stereograms. We find that the reconstruction has correct monocular cues when the whole scene is located in a so-called "monocular cues area" with compatible monocular and binocular cues. In contrast, incorrect monocular cues appear when the scene is in the other two areas, namely, the "visible multi-imaging area" and the "lacking information area." A pupil-function integral imaging algorithm is developed to simulate monocular observation, and a holographic printing system is set up to fabricate full-parallax holographic stereograms. Both simulations and experiments agree with the criteria.

Continuous generation of soliton patterns in two-dimensional dissipative media by razor, dagger, and needle potentials.

We report dynamic regimes supported by a sharp quasi-one-dimensional (1D) ("razor"), pyramid-shaped ("dagger"), and conical ("needle") potentials in the 2D complex Ginzburg-Landau (CGL) equation with cubic-quintic nonlinearity. This is a model of an active optical medium with respective expanding antiwaveguiding structures. If the potentials are strong enough, they give rise to continuous generation of expanding soliton patterns by a 2D soliton initially placed at the center. In the case of the pyramidal potential with M edges, the generated patterns are sets of M jets for M < or = 5, or expanding polygonal chains of solitons for M > or = 6. In the conical geometry, these are concentric waves expanding in the radial direction.

[Room temperature ultraviolet stimulated emission of ZnO nanocrystals].

The characteristics of the exciton in ZnO is essential to the development of ZnO-based optoelectronic devices, therefore it is critical to study the optical properties of exciton and transition process in ZnO quantum dot. In the present work, zinc oxide nanocrystals were prepared by the sol-gel method. X-ray diffraction (XRD) pattern shows that they have a hexagonal wurtzite structure. The pumping power dependent UV emission and their dynamics were studied, the spontaneous emission of the free exciton and the stimulated emission due to exciton-exciton collision and electron hole plasmon were observed, and the pumping power dependent dynamics was reported for the first time. This is helpful for us to understand the near-band-edge excitonic emission, and might be valuable in the realization of semiconductor UV diode.

The transmission characteristics of surface plasmon polaritons in ring resonator.

A two-dimensional nanoscale structure which consists of two metal-insulator-metal (MIM) waveguides coupled to each other by a ring resonator is designed. The transmission characteristics of surface plasmon polaritons are studied in this structure. There are several types of modes in the transmission spectrum. These modes exhibit red shift when the radius of the ring increases. The transmission properties of such structure are simulated by the Finite-Difference Time-Domain (FDTD) method, and the eignwavelengths of the ring resonator are calculated theoretically. Results obtained by the theory of the ring resonator are consistent with those from the FDTD simulations.

Annularly and radially phase-modulated spatiotemporal necklace-ring patterns in the Ginzburg-Landau and Swift-Hohenberg equations.

Annularly and radially phase-modulated spatiotemporal necklace-shaped patterns (SNPs) in the complex Ginzburg-Landau (CGL) and complex Swift-Hohenberg (CSH) equations are theoretically studied. It is shown that the annularly phase-modulated SNPs, with a small initial radius of the necklace and modulation parameters, can evolve into stable fundamental or vortex solitons. To the radially phase-modulated SNPs, the modulated "beads" on the necklace rapidly vanish under strong dissipation in transmission, which may have potential application for optical switching in signal processing. A prediction that the SNPs with large initial radii keep necklace-ring shapes upon propagation is demonstrated by use of balance equations for energy and momentum. Differences between both models for the evolution of solitons are revealed.

[Time-resolved photoluminescence of stimulated emission from ZnO nanoparticles].

The detailed room-temperature stimulated emission including its optical characteristics from nanosized ZnO particles prepared by homogenous precipitation method was investigated by time-resolved spectroscopy both from the frequency domain and time domain. As the excitation power was increased, sharp lasing peaks with the full width at half maximum less than 0.5 nm, similar to the FP lasing mode resonator mode, emerged rapidly from the emission spectra. Additionally, a narrow emission line intensity increased rapidly with increasing the excitation intensity, which was due to an exciton-exciton collision recombination, and the threshold excitation intensity was 7.2 GW x cm(-2). Upon higher excitation power, the lasing mechanism switched to electron-hole plasma (EHP). The EHP emission in the case of a higher excitation intensity appeared at a lower energy side of the E-E emission and replaced completely the E-E emission at the higher intensity. The EHP emission was red-shifted when further increasing the excitation intensity, compared to the E-E emissions. It was demonstrated that the red-shift of the EHP peak was attributed to the relevant band gap renormalization effect in the electron-hole plasma regime. At the same time, the emission lifetime was drastically reduced. Time-resolved spectrum of P band suggested a Gaussian-like decay time with only a few tens of picoseconds, compared to 2 ps, which is the limit of streak camera time resolution. The dynamic processes of lasing behavior and characteristics of the lasing emission in ZnO nanoparticles could be valuable and provide the information on crystal quality, exciton and lasing action in ZnO.

[Surface emission characteristics of ZnO nanoparticles].

The size-dependence of photoluminescence (PL) of ZnO nanoparticles with diameter from 17 to 300 nm was demonstrated by time resolved laser spectroscopy. Broad PL spectra were obtained and found to consist of three Gaussian components. The authors found that the PL peak position and relative amplitude depend on the size of nanoparticles for the Gaussian PL band inside the energy band-gap. The Gaussian band Xc3 is believed to be associated with the transitions between surface states. It is well known that as the dimensions of a semiconductor are reduced to the nanometer scale, one of the key features is the large surface-to-volume ratio. The larger surface-to-volume ratio in nanostructures means the larger bulk density of dangling bonds. The existence of dangling bond in a crystal surface is likely to change a state localized by splitting the state out of the border of the energy gap. The Gaussian peak Xc3 lying inside the band gap of ZnO indicates the existence of such initial states in the forbidden bulk band gap. Furthermore, we found that with the decrease in particle size, the Gaussian curve Xc3 shifts to the lower energy. With the particle size increaseing, the comparative amplitude of PL band Xc3 descends rapidly and so does the corresponding relative PL intensity. The sensitive correlation between the particle size and the optical properties of the below gap broadband shows a key role of the surface states recombination in PL of nanosized particles. Furthermore, our exploration indicates that the surface states recombination plays a dominant role in PL of nanostructures with particle sizes down to a certain degree.

[Time resolved photoluminescence of ZnO nanoparticles under low photon energy excitation].

Time-resolved photoluminescence (TRPL) and time-integrated photoluminescence (TIPL)measurements were performed at ZnO nanoparticles with grain size from 17 nm to 110 nm prepared by homogenous precipitation method (HPM). With a quasi-continuous picoseconds laser excitation at a low photon energy of 2. 33 eV (wavelength 532 nm)from a Vanguard 2000-HM532 laser, ultrafast emission with the wavelength ranging from 550 to 1000 nm was detected. It was observed that the wavelength position of all the peaks of the samples regularly redshifted with the increase in the particle sizes. The PL spectra were fitted with Gaussian curves, in which a good fitting consisting of four Gaussian peaks was obtained. The Gaussian analysis indicates that the redshift in PL is due to the increased relative magnitude of the Gaussian combination in the low energy region. The analysis of the experiment suggests the existence of surface states in the bad gap near Fermi level of ZnO. At the same time, a strong dependence of the PL lifetime on grain size was found. Upon the increase in particle size, the decay of the emission shortened much, in which the PL intensity reveals bi-exponential decay for samples with small diameters while no identified decay of emission was observed within 2 nanoseconds for large particles. The dependence of the decay time on the grain size of nanoparticles can be suitably attributed to the surface related mechanism. Combined analysis of the lifetime dependence on particle size validates the model of surface states recombination in nanostructures due to large surface-to-volume ratio.

Single-crystal-to-single-crystal transformation involving release of bridging water molecules and conversion of chain helicity in a chiral three-dimensional metal-organic framework.

An unprecedented release of the bridging aqua molecules and conversion of chain helicity within a 3-D chiral cadmium(ii) dicarboxylate coordination polymer was observed in a unique and drastic single-crystal-to-single-crystal transformation of dehydration.

[Spectral properties of carbazole molecules and three-photon induced lasing].

Spectral properties of a series of new heterocyclic molecules were studied in the present work. The spectral experimental results show that the two-photon absorption cross-sections of these new molecules are dependent on their charge transfer dipole moments, which relate to the donor acceptor end groups. At high density pumping, the spectral and temporal properties of the high directional emission of a new molecule with two-photon absorption cross-section of 617.3 GM demonstrate that three-photon induced lasing occurs.

[Photoluminescence of one dimension ZnO single crystal columns array excited by different space variation direction].

Highly-oriented single crystal ZnO columns array was prepared by a simple two-step evaporation oxidation method. And the photoluminescence properties of different direction excitation were investigated. Upon different direction excitation, the emission spectra were greatly different. Besides the relative intensity of different emission peak of PL changed; some new emission peaks werealso detected at high excitation power. This showed that the energy absorption, transmission and emission in one dimension ZnO column array were directional. The up-conversion polarized photoluminescence spectra were also found, and the intensities of 400 nm emission peak were affected by the laser polarization orientation. These results showed that the intensity and shape of PL were related with the direction and polarization of the excitation laser. The results also give some important references for subminiature one dimension line array optoelectric device.

Studies on the Time-Resolved Polarized Fluorescence of R-Phycoerythrin Crystal from Red Alga Porphyra yezoensis.

The time-resolved polarized fluorescence of oriented R-phycoerythrin single crystal from red alga Porphyra yezoensis was studied in picosecond time scale. The fluorescence decay of the crystal exhibited two exponential processes, one of which was related to the energy transfer and excitation equilibrium between chromophores (tau(1)

[Two-photon absorption spectrum].

Two-photon absorption (TPA) spectrum of a new organic compound has been directly measured by an approach of using tunable laser from an optical parameter amplifier as an excitation source. The output intensity of optical parameter amplifier can cover a broad spectra range of 400-2,000 nm, and identical intensity in the whole spectral range can be easily adjusted. Therefore, the measurement based on this method is directly and reliable. The experimental results show that this compound exhibit large TPA cross section in a broad spectral range from 750 to 1,100 nm. The profiles of TPA spectrum is quite similar to that of the linear absorption spectra. The peak wavelength of TPA band has a little blue-shift compared with the twice of the peak wavelength of the linear absorption due to the different selection rules for two-photon transition and one-photon transition.

[Shoot type morphology and growth characteristics of winter wheat sown at different dates].

Aiming at the delayed sowing of winter wheat induced by the drought and water logging often occurred in Huanghuai Plains of China, six sowing dates (15 October, normal sowing; 30 October, moderate delay; 15 November, delay; 30 November, seriously delay; 15 February, early spring sowing; and 1 March, spring sowing) were designed to investigate the effects of different sowing dates on the shoot type morphology and growth characteristics of winter wheat. With the delay of sowing date, the winter wheat grew and developed faster, and the growing period of the wheat sown in early spring and spring was 115-130 days shorter than that with normal sowing. As compared with those of the wheat with normal sowing, the shoot height, spike number per unit area, and productive spikelets per unit ear of the wheat sown delayed had a decrease, leaf position and canopy moved down, and leaf area reduced. When the sowing was delayed from the date 15 October (normal sowing) to 1 March (spring sowing), the harvest index increased from 0.46 to 0.53. Delaying sowing date also resulted in the significant reduction of grain yield, with the maximum decrement as high as 43. 6%. The spring-sown winter wheat not going through vernalization could still form yield.