Multifunctional and tunable trigate graphene metamaterial with "Lakes of Wada" topology.

Many plasmon-induced transparency (PIT) metamaterials previously reported had limited functions. Their tunabilities were realized by complex discrete structures, which greatly increased the difficulty and cost of device fabrication and adversely affected their resonance characteristics. It is an open question to adjust the Fermi levels of many graphene patterns with only a few in-plane electrodes. We propose and numerically study a novel electrically tunable and multifunctional trigate graphene metamaterial (TGGM) based on the concept of "Lakes of Wada". Benefiting from the trigate regulation, our proposed TGGM turns out to exhibit excellent characteristics, that can not only be used for terahertz band-stop filter, terahertz refractive index sensor, near-field optical switch, slow-light device, but also for double PIT window metamaterial with broad transparency windows and large tunable frequency range.

Laser direct writing of graphene nanostructures beyond the diffraction limit by graphene oxidation.

The fabrication ability of graphene nanostructures is the cornerstone of graphene-based devices, which are of particular interest because of their broad optical response and gate-tunable properties. Here, via laser-induced redox reaction of graphene and silica, we fabricate nano-scale graphene structures by femtosecond laser direct writing. The resolution of destructed graphene lines is far beyond the diffraction limit up to 100 nm with a precision as small as ± 7 nm. Consequently, graphene nanostructures are fabricated precisely and excellent plasmon responses are detected. This novel fabrication method of graphene nanostructures has the advantages of low costs, high efficiency, maskless and especially high precision, which would pave the way for practical application of graphene-based optical and electronic devices.

Time-resolved photoluminescence of silicon microstructures fabricated by femtosecond laser in air.

Green photoluminescence (PL) from silicon microstructures fabricated by femtosecond laser in air was studied at different temperature by time-resolved spectroscopy. The PL decay profiles are well fitted by a stretched exponential function: I(t)=I(0)*exp[-(t/τ)ß]. The dependence of the decay time constant τ and of the stretching index ß on PL photon energy and on the temperature is investigated. A model of transport and recombination of the carriers is introduced as a possible explanation of the stretched exponential decay. The nonradiative recombination rate of the localized carriers, which is dependent on the carrier density and influenced by the trapping site density and the temperature, is deduced to be responsible for this kind of decay.

Optical refocusing three-dimensional wide-field fluorescence lifetime imaging microscopy.

Three-dimensional fluorescence lifetime microscopy is achieved by combining wide-field fluorescence lifetime imaging with a remote optical refocusing method. As required for some applications in dynamic research for physics, chemistry, or biology, it is thereby not necessary to move the sample, i.e., the specimen is not disturbed during measurement. Using a fluorescent microsphere the performance of the system has been tested successfully with respect to three-dimensional fluorescence lifetime microscopy as well as time-resolved fluorescence spectroscopy.

Transfer matrix method for light propagation in variable complex chiral media.

By taking a cholesteric liquid crystal (CLC) as an example and treating it as a multilayer stack of birefringent plates, we use a transfer matrix method to analyze light propagation in a common chiral medium in consideration of interlayer reflection and transmission. Based on the transfer matrix, the electric field distribution can be expressed in the form of linearly as well as circularly polarized components, so as to discuss the change of the polarization state of light in the transmission process. The transfer matrix of the same medium with different chirality can be converted by only changing the rotation matrix in the calculation process. Electric field distributions, band structure, transmission, and reflection spectra are calculated when circularly polarized light is incident normally on CLCs or on composite periodic structures of left- and right-handed CLCs. The results obtained by using this transfer matrix method are in good agreement with those obtained by the method based on solving the eigenvalues of Maxwell's equations. Finally, the transfer matrix method is used to calculate the dynamic transmission properties of CLCs under external magnetic field, which is of great significance for the research of noncontact controllable optical devices. The presented computational method saves computing time and can be used for constructing new photonic microstructures with different chiral media.

Ultraviolet irradiation induces autofluorescence enhancement via production of reactive oxygen species and photodecomposition in erythrocytes.

Ultraviolet (UV) light has a significant influence on human health. In this study, human erythrocytes were exposed to UV light to investigate the effects of UV irradiation (UVI) on autofluorescence. Our results showed that high-dose continuous UVI enhanced erythrocyte autofluorescence, whereas low-dose pulsed UVI alone did not have this effect. Further, we found that H(2)O(2), one type of reactive oxygen species (ROS), accelerated autofluorescence enhancement under both continuous and pulsed UVI. In contrast, continuous and pulsed visible light did not result in erythrocyte autofluorescence enhancement in the presence or absence of H(2)O(2). Moreover, NAD(P)H had little effect on UVI-induced autofluorescence enhancement. From these studies, we conclude that UVI-induced erythrocyte autofluorescence enhancement via both UVI-dependent ROS production and photodecomposition. Finally, we present a theoretical study of this autofluorescence enhancement using a rate equation model. Notably, the results of this theoretical simulation agree well with the experimental data further supporting our conclusion that UVI plays two roles in the autofluorescence enhancement process.

Nonlinear spectrum broadening of femtosecond laser pulses in photorefractive waveguide arrays.

In photorefractive waveguide arrays, the process and extent of spectral broadening of femtosecond laser pulse caused by self-phase modulation are studied theoretically and experimentally. The threshold of self-phase modulation is more than two times larger than the common threshold in a bulk sample, which affects the extent of spectral broadening dramatically. The coupling length and the ratio between the common threshold and the input peak intensity of the femtosecond laser pulse are the two key parameters dominating these phenomena. The experimental results confirm the theoretical expectation.

Light-controllable coherent backscattering from water suspension of lithium niobate microcrystalline particles.

We experimentally study coherent backscattering of light for a water suspension of lithium niobate microcrystalline particles. Light-controllable weak localization of photons in a suspension is demonstrated for the first time to our knowledge. The effect is attributed to the reorientation of microcrystalline particles in the field of a linearly polarized pump beam. Thus the isotropic suspension becomes partially anisotropic.

Optical trapping and manipulation of metallic micro/nanoparticles via photorefractive crystals.

A simple method to trap and manipulate metallic micro/nano-particles on the surface of photorefractive crystals is proposed. After inducing inhomogeneous charge density and space-charge fields in photorefractive crystals by non-uniform illumination, both uncharged and charged metallic particles can be trapped on the illuminated surface due to dielectrophoretic force and electrophoretic force, respectively. A transition from dielectrophoresis to electrophoresis is observed when manipulating nano-silver particles with high surface space-charge field. Our results show that this method is simple and effective to form surface microstructures of metallic particles.

Coupling of Defect Modes in Cholesteric Liquid Crystals Separated by Isotropic Polymeric Layers.

Cholesteric liquid crystal structures with multiple isotropic defect layers exhibit localized optical modes (defect modes). Coupling effects between these modes were simulated using the finite difference time domain method. Analogous to the well-known result of the tight-binding approximation in solid state physics, splitting of the defect modes takes place, as soon as the structure contains more than one defect layer. The dispersion relation of the mini-bands forming within the photonic band gap of the structure is calculated numerically. The structures might have promising applications for multiwavelength filters and low-threshold lasers.

Protection of the biconcave profile of human erythrocytes against osmotic damage by ultraviolet-A irradiation through membrane-cytoskeleton enhancement.

To perform various physiological functions, erythrocytes possess a unique biconcave shape provided by a special architecture of the membrane-skeleton system. In the present work, we use a simple irradiation method to treat human erythrocytes with 365 nm ultraviolet-A (UVA) light at the single-cell level in vitro. Depending on the irradiation dose, UVA show protection of the biconcave profile against the detrimental action of distilled water. This protective effect can also be confirmed for saponin that damages the membrane-skeleton by vesiculation and pore formation. Interestingly, at two irradiation doses of UVA pretreatment, erythrocytes still seem to exhibit cell viability as tested by trypan blue assay even if distilled water or saponin is added. The oxidants hydrogen peroxide and cumene hydroperoxide partly simulate the protective effects. Taken together, these results demonstrate that 365 nm UVA irradiation can protect the biconcave profile of human erythrocytes through membrane-skeleton enhancement associated with a production of oxidants.

Structuring by multi-beam interference using symmetric pyramids.

A method for producing optical structures using rotationally symmetric pyramids is proposed. Two-dimensional structures can be achieved using acute prisms. They form by multi-beam interference of plane waves that impinge from directions distributed symmetrically around the axis of rotational symmetry. Flat-topped pyramids provide an additional beam along the axis thus generating three-dimensional structures. Experimental results are consistent with the results of numerical simulations. The advantages of the method are simplicity of operation, low cost, ease of integration, good stability, and high transmittance. Possible applications are the fabrication of photonic micro-structures such as photonic crystals or array waveguides as well as multi-beam optical tweezers.

Light-induced phase and amplitude gratings in centrosymmetric Gadolinium Gallium garnet doped with calcium.

The photosensitive properties of a centrosymmetric gadolinium gallium garnet crystal doped with calcium are investigated at room temperature. Elementary holograms can be recorded over a wide range of wavelengths in the visible spectral range. The photosensitive properties are studied experimentally using beam coupling and angular response experiments. Mixed absorption and refractive-index gratings are observed and their amplitudes and relative phases determined. Moreover, the candidate centers that are responsible for the photorefractive effect are discussed.

Ultraviolet irradiation-dependent fluorescence enhancement of hemoglobin catalyzed by reactive oxygen species.

Ultraviolet (UV) light has a potent effect on biological organisms. Hemoglobin, an oxygen-transport protein, plays an irreplaceable role in sustaining life of all vertebrates. In this study we scrutinize the effects of ultraviolet irradiation (UVI) as well as visible irradiation on the fluorescence characteristics of bovine hemoglobin (BHb) in vitro. Data show that UVI results in fluorescence enhancement of BHb in a dose-dependent manner. Furthermore, UVI-induced fluorescence enhancement is significantly increased when BHb is pretreated with hydrogen peroxide (H(2)O(2)), a type of reactive oxygen species (ROS). Meanwhile, The water-soluble antioxidant vitamin C suppresses this UVI-induced fluorescence enhancement. In contrast, green light irradiation does not lead to fluorescence enhancement of BHb no matter whether H(2)O(2) is acting on the BHb solution or not. Taken together, these results indicate that catalysis of ROS and UVI-dependent irradiation play two key roles in the process of UVI-induced fluorescence enhancement of BHb.

Real-time imaging of autofluorescence NAD(P)H in single human neutrophils.

Real-time detection of NAD(P)H is particularly important for understanding physiological activities of neutrophils. We scrutinize the performance of weak light detection systems with electron multiplying CCDs (EMCCDs) with regard to the feasibility of valid investigations by autofluorescence NAD(P)H in single human neutrophils. The low-noise amplification facility of EMCCDs is indeed just adequate to permit detection at an irradiation level where neither quenching nor phototoxic effects occur. For demonstration, a neutrophil respiratory burst was triggered and observed in real time. Our low-intensity detection system fulfills all requirements for real-time investigations at high spatiotemporal resolution in the field of neutrophil physiology and pathology.

Effect of reconstruction beam polarization on the kinetics of anisotropic gratings in bacteriorhodopsin.

Anisotropic gratings are recorded on bacteriorhodopsin films by two parallelly polarized beams, and the effect of the polarization orientation of the reconstructing beam on the diffraction efficiency kinetics is studied. A theoretical model for the diffraction efficiency kinetics of the anisotropic grating is developed by combining Jones-matrix and photochromic two-state theory. It is found that the polarization azimuth of the reconstructing beam produces a cosine modulation on the kinetics of the diffraction efficiency, being positive at the peak efficiency and negative for steady state. By adding auxiliary violet light during grating formation, the saturation of the grating can be restrained. As a result, the negative cosine modulation for the steady-state diffraction efficiency changes to a positive one. In addition, the steady-state diffraction efficiency is increased appreciably for all reconstructing polarization orientations.

Specific recording kinetics as a general property of unconventional photorefractive media.

We introduce a model for the kinetics of grating formation during holographic recording in optically excitable two-state systems. An unexpected characteristic time dependence of the diffraction efficiency is found. We show that it originates from a nonlinear transformation of the light interference pattern into a refractive-index profile. Our findings strongly resemble and explain by nature the experimental data of two-state systems in general, here represented by two examples: sodium nitroprusside and terbium gallium garnet.