Light-controlled movement of isotropic droplets in smectic films

This paper reports on optical trapping of micrometre-sized isotropic inclusions in free-standing smectic A* films. Droplet manipulation and trapping potential in such a two-dimensional anisotropic system show that optical trapping has two distinct regimes with unique separation dependence, governed by long-range and short-range trapping forces and enhanced diffusivity at the free surfaces. Molecular ordering in the surface layers of isotropic inclusions, at the liquid crystal–air interface, in addition leads to a new field of light-controlled particle dynamics. For low laser powers, translational motion of a droplet along the laser polarisation is observed. Above the threshold laser power, the transfer of optical angular momentum to the inclusion via linearly polarised light leads to circular-like motion. As the optical torque for a given intensity is counterbalanced by the elastic torque of the smectic film, this motion results in finite angle steps.     

One-dimensional random lasing in a single organic nanofiber

One-dimensional light amplification in individual p-sexiphenyl nanofibers is investigated. The influence of fiber morphology on light propagation properties is studied via optical and atomic force microscopy. Isolated nanofibers are shown to yield low-threshold random laser emission in the deep blue. Model calculations of coherent light propagation in one-dimensional random media qualitatively reproduce the experimental results. Implications for photonic nanosensors are briefly discussed.     

Low threshold polymerised cholesteric liquid crystal film lasers with red,green and blue colour

We demonstrate dye-doped low threshold polymerised cholesteric liquid crystal (PCLC) film lasers with red, green and blue (RGB) colour. The polymer network structure of lasers, which were fabricated by washing-out/refilling method, enhanced the stability of the laser thus avoiding the external interference like temperature and electromagnetic field and prevented the efficiency of laser dye from decreasing during polymerisation. The proposed RGB PCLC film lasers have application potential in such as coherent light source, multi-wavelength biomedical light source, white laser and other photonic applications.     

Towards the development of laser shock test for mechanical characterisation of fibre/matrix interface in eco-composites

This paper deals with the possibility of using the laser shock test for studying the adhesion between fibre and matrix in composite materials. Single hemp yarn in epoxy matrices - a fully synthetic one, Epolam 2020, and a partially bio-based one, Greenpoxy 56 - specimens have been tested. The water sorption effect on interfacial adhesion quality has been studied. Two different types of damage induced by laser shock have been observed: resin cracks appear only for high laser intensity levels, and specific cone-shaped interfacial damage appears for lower intensity values. The reproducibility of the threshold value evaluation has been demonstrated for the two resins. A numerical simulation by finite elements has also been performed to enhance the understanding of laser shock wave propagation in such samples. These preliminary results demonstrate the ability of the laser shock test to study and quantify the mechanical quality of yarn/matrix interface, which is needed to help design of such composites.     

Xenon chloride ultraviolet B laser is more effective in treating psoriasis and in inducing T cell apoptosis than narrow-band ultraviolet B

Earlier we reported that a 308-nm xenon chloride (XeCl) UVB laser is highly effective for treating psoriasis. As ultraviolet B light seems to cause T cell apoptosis, in the present study we studied the ability of the XeCl laser to induce T-cell apoptosis in vitro, and then compared the apoptosis-inducing capacities of narrow-band UVB (NB-UVB) light and the XeCl laser. The role of laser impulse frequency and intensity in the therapeutical and apoptosis-inducing efficacy of XeCl laser was also investigated. Both XeCl laser and NB-UVB induced T cell apoptosis, but quantitative induction was greater with XeCl laser. Changes in the frequency and intensity of impulses of XeCl laser did not influence its therapeutic and T cell apoptosis-inducing efficacy. These results suggest that the more effective induction of T cell apoptosis can be responsible for the greater clinical efficacy of XeCl laser compared to NB-UVB. Additionally, the optical properties of the XeCl laser (a monochromatic, coherent, pulse-mode laser; easier precise dosimetry, there are no 'contaminating' wavelengths) can make this laser light an ideal tool for studies of the mode of action of UVB light.     

LIGHT QUENCHING OF FLUORESCENCE: A NEW METHOD TO CONTROL THE EXCITED STATE LIFETIME AND ORIENTATION OF FLUOROPHORES

Abstract Experimental studies have recently demonstrated that fluorescence emission can be quenched by laser light pulses from modem high-repetition rate lasers, a phenomenon we call “light quenching.” In this overview article, we describe the possible effects of light quenching on the steady-state and time-resolved intensity and anisotropy of fluorophores. One can imagine two classes of experiments. Light quenching can occur within the single excitation pulse, or light quenching can be accomplished with a second time-delayed quenching pulse. The extent of light quenching depends on the amplitude of the emission spectrum at the quenching wavelength. Different effects are expected for light quenching by a single laser beam (within a single laser pulse) or for a time-delayed quenching pulse. Depending upon the polarization of the light quenching beam, light quenching can decrease or increase the anisotropy. Remarkably, the light quenching can break the usual z-axis symmetry of the excited state population, and the measured anisotropy (or polarization) depends upon whether the observation axis is parallel or perpendicular to the propagation direction of the light quenching beam. The polarization can increase to unity under selected conditions. Quenching with time-delayed light pulses can result in step changes in the intensity or anisotropy, which is predicted to result in oscillations in the frequency-domain intensity and anisotropy decays. These predicted effects of light quenching, including oscillations in the frequency-domain data, were demonstrated to occur using selected fluorophores. The increasing availability and use of pulsed laser sources requires consideration of the possible effects of light quenching and offers the opportunity for a new class of two-pulse or multiple-pulse time-resolved experiments where the sample is prepared by the excitation pulse and subsequent quenching pulses to modify the excited state population, followed by time- or frequency-domain measurement of the optically prepared excited fluorophores.     

Excitation of Biomolecules by Coherent vs. Incoherent Light: Model Rhodopsin Photoisomerization

Light-induced processes in biological molecules, which occur naturally in continuous incoherent light, are often studied using pulsed coherent light sources. With a focus on timescales, the relationship between excitation due to these two types of light sources is examined through a uniform minimal model of the photoisomerization of retinal in rhodopsin, induced by either coherent laser light or low level incoherent light (e.g. moonlight). Realistic timescales for both processes are obtained and a kinetic scheme involving rates for both coherent and incoherent light excitation is introduced, placing all timescales into a uniform framework. The rate limiting step in the natural light-absorption process is shown to be the low incoherent photon flux.     

The dynamics of self-trapped beams of incoherent white light in a free-radical photopolymerizable medium

Detailed experimental studies of the dynamics of self-trapped beams of white light (400-800 nm) in a photosensitive organosiloxane medium are presented. Self-trapped white light beams with similar spatial profiles formed in the organosiloxane at intensities ranging across an order of magnitude (2.7-22.0 W.cm-2). Beam-profiling measurements showed that these spatially and temporally incoherent wave packets propagate without diffracting (broadening) by initiating free-radical polymerization of methacrylate groups and corresponding refractive index changes in the organosiloxane medium. Analyses of their temporal evolution showed that the intensity-dependent behavior of self-trapped white light is similar to that of self-trapped laser light despite the extreme differences in their phase structure and chromaticity; the self-trapped incoherent beams even show the complementary oscillations of width and intensity that is characteristic of self-trapped coherent light. Furthermore, the dynamics of the self-trapped white light beams was found to be strongly correlated to the kinetics of free-radical polymerization and corresponding rates of refractive index changes in the organosiloxane. These studies provide accessible photochemical routes to self-trapped incoherent wave packets, which are extremely difficult to generate in conventional nonlinear optical media that owe their responses to higher-order dielectric susceptibility tensors. This could enable the experimental verification of theoretical models developed for the nonlinear propagation of white light and stimulate research into more complex self-trapping phenomena such as the interactions of self-trapped incoherent beams and spontaneous pattern formation due to modulation instability in a uniform incoherent optical field. These findings also carry potential for the development of self-induced waveguide, optical solder and interconnect technology for incoherent light emitted by incandescent sources or LEDs.     

反射式有机光导液晶光阀的研究

本文报道用聚乙烯咔唑(PVK)-2,4,7三硝基芴酮(TNF)电荷转移复合物作为光导体与手征向列-向列相转变液晶构成的直流反射式光阀及其性能的研究结果。制得的光阀有明显的光阀效应,可实现非相干光-相干光转换,得到清晰的图象。此外,还详细研究了光阀的阈值电压和上升时间与写入光功率及温度等的依赖关系。     

Fragmentation of leucine enkephalin as a function of laser fluence in a MALDI TOF-TOF

The effects of laser fluence on ion formation in MALDI were studied using a tandem TOF mass spectrometer with a Nd-YAG laser and alpha-cyano hydrocinnamic acid matrix. Leucine enkephalin ionization and fragmentation were followed as a function of laser fluence ranging from the threshold of ion formation to the maximum available, that is, about 280-930 mJ/mm2. The most notable finding was the appearance of immonium ions at fluence values close to threshold, increasing rapidly and then tapering in intensity with the appearance of typical backbone fragment ions. The data suggest the presence of two distinct environments for ion formation. One is associated with molecular desorption at low values of laser fluence that leads to extensive immonium ion formation. The second becomes dominant at higher fluences, is associated initially with backbone type fragments, but, at the highest values of fluence, progresses to immonium fragments. This second environment is suggestive of ion desorption from large pieces of material ablated from the surface. Arrhenius rate law considerations were used to estimate temperatures associated with the onset of these two processes.     

Applications of ultrashort shaped pulses in microscopy and for controlling chemical reactions

This article presents a new perspective on laser control based on insights into the effect of spectral phase on nonlinear optical processes. Gaining this understanding requires the systematic evaluation of the molecular response as a function of a series of pre-defined accurately shaped laser pulses. The effort required is rewarded with robust, highly reproducible, results. This approach is illustrated by results on selective two-photon excitation microscopy of biological samples, where higher signal and less photobleaching damage are achieved by accurate phase measurement and elimination of high-order phase distortions from the ultrashort laser pulses. A similar systematic approach applied to laser control of gas phase chemical reactions reveals surprising general trends. Molecular fragmentation pattern is found to be dependent on phase shaping. Differently shaped pulses with similar pulse duration have been found to produce similar fragmentation patterns. This implies that any single parameter that is proportional to the pulse duration, such as second harmonic generation intensity, allows us to predict the molecular fragmentation pattern within the experimental noise. This finding, is illustrated here for a series of isomers. Bond selectivity, coherent photochemistry and their applications are discussed in light of results from these systematic studies.     

3D mapping of the output laser beam from a PDLC sample

Self-transparency due to thermal non-linearities is presented as a basic switching effect in a thick polymer dispersed liquid crystal sample. For the first time a detailed 3D mapping of the output laser beam as a function of the x-y coordinates is presented: changes of the transmitted beam profile are recorded vs. both incident power and time. It is discussed how light intensity and temperature can be used as control parameters for the non-linear part of the refractive index. The experimental results confirm the existence of a threshold value of the incident light intensity at which the device switches from the scattering state to the transmissive state.     

3D mapping of the output laser beam from a PDLC sample

Self-transparency due to thermal non-linearities is presented as a basic switching effect in a thick polymer dispersed liquid crystal sample. For the first time a detailed 3D mapping of the output laser beam as a function of the x-y coordinates is presented: changes of the transmitted beam profile are recorded vs. both incident power and time. It is discussed how light intensity and temperature can be used as control parameters for the non-linear part of the refractive index. The experimental results confirm the existence of a threshold value of the incident light intensity at which the device switches from the scattering state to the transmissive state.     

Upconverting‐Nanoparticle‐Assisted Photochemistry Induced by Low‐Intensity Near‐Infrared Light: How Low Can We Go?

Upconverting nanoparticles (UCNPs) convert near‐infrared (NIR) light into UV or visible light that can trigger photoreactions of photosensitive compounds. In this paper, we demonstrate how to reduce the intensity of NIR light for UCNP‐assisted photochemistry. We synthesized two types of UCNPs with different emission bands and five photosensitive compounds with different absorption bands. A λ=974 nm laser was used to induce photoreactions in all of the investigated photosensitive compounds in the presence of the UCNPs. The excitation thresholds of the photoreactions induced by λ=974 nm light were measured. The lowest threshold was 0.5 W cm^?2, which is lower than the maximum permissible exposure of skin (0.726 W cm^?2). We demonstrate that low‐intensity NIR light can induce photoreactions after passing through a piece of tissue without damaging the tissue. Our results indicate that the threshold for UCNP‐ assisted photochemistry can be reduced by using highly photosensitive compounds that absorb upconverted visible light. Low excitation intensity in UCNP‐assisted photochemistry is important for biomedical applications because it minimizes the overheating problems of NIR light and causes less photodamage to biomaterials.     

An Organic Microlaser Array Based on a Lateral Microcavity of a Single J‐aggregation Microbelt

A laser array on the nano‐ and microscale is a key component for integration in photonic devices, but remains a challenge when using semiconductor nanowire lasers. Here we report a low‐threshold lateral‐cavity microlaser, formed between two lateral‐faces of a single‐crystalline organic microbelt (OMB) of 1,4‐dimethoxy‐2,5‐di[4′‐(cyano)styryl]benzene (COPV). By cutting a single OMB into six pieces by a top‐down two‐photon processing technique, we successfully fabricated a compact and uniform 1×6 microlaser array along the length direction of the OMB. The microlasers had excellent reproducibility and addressable high precision, thus making them attractive candidates as miniaturized coherent light sources for future nanophotonics.     

Spontaneous pattern formation due to modulation instability of incoherent white light in a photopolymerizable medium

Spontaneous pattern formation due to modulation instability was observed in a broad uniform beam of incoherent white light propagating in an optically isotropic, photopolymerizable organosiloxane. Pattern formation originates from intensity-dependent refractive index changes due to polymerization, which cause competition between the natural diffraction (broadening) and self-induced refraction of the beam. Under these nonlinear conditions, weak intensity modulations in the beam, noise, that would be negligible under linear conditions are amplified. The amplified patterns become unstable over time and spontaneously divide into individual self-trapped filaments of white light of essentially identical diameter (76 +/- 3 microm), which propagate through the medium without diffracting. In the case of noise with a weak 1-D periodic modulation, for example, the uniform beam transformed into a 1-D periodic array of self-trapped lamellae, which in turn formed a 2-D array of self-trapped cylindrical filaments. Although the rate of pattern formation varied inversely with optical power (measured from 8.4 to 59.8 mW), the uniform beam always split into discrete filaments, demonstrating that they are the most stable form of light propagation under the nonlinear conditions created by polymerization. Each filament of light retained the spectral composition and incoherence of white light, which showed that the entire polychromatic, incoherent and unpolarized wavepacket collectively participated in pattern formation. These findings are consistent with recent theoretical models of nonlinear white light propagation and with experimental observations of pattern formation in coherent and partially coherent light. Because refractive index changes due to polymerization are permanent, pattern formation imparts microstructure to the organosiloxane. Optical micrographs revealed that, after pattern formation, the initially homogeneous medium consisted entirely of a closely packed array of narrow channel waveguides induced by self-trapped filaments.     

二(乙酰丙酮根)合铜(Ⅱ)热分解动力学研究

金属有机化合物气相化学沉积(OMCTD)形成铜膜常用的母体化合物是铜(Ⅱ)的β-二酮类配合.本文首次采用CW二氧化碳激光研究二(乙酰丙酮根)合铜(Ⅱ)[Cu相似文献   

Efficient and robust strong-field control of population transfer in sensitizer dyes with designed femtosecond laser pulses

We demonstrate control of electronic population transfer in molecules with the help of appropriately shaped femtosecond laser pulses. To this end we investigate two photosensitizer dyes in solution being prepared in the triplet ground state. Excitation within the triplet system is followed by intersystem crossing and the corresponding singlet fluorescence is monitored as a measure of population transfer in the triplet system. We record control landscapes with respect to the fluorescence intensity on both dyes by a systematic variation of laser pulse shapes combining second order and third order dispersion. In the strong-field regime we find highly structured topologies with large areas of maximum or minimum population transfer being insensitive over a certain range of applied laser intensities thus demonstrating robustness. We then compare our experimental results with simulations on generic molecular potentials by solving the time-dependent Schr?dinger equation for excitation with shaped pulses. Control landscapes with respect to population transfer confirm the general trends from experiments. An analysis of regions with maximum or minimum population transfer indicates that coherent processes are responsible for the outcome of our excitation process. The physical mechanisms of joint motion of ground and excited state wave packets or population of a vibrational eigenstate in the excited state permit us to discuss the molecular dynamics in an atom-like picture.     

Coherent and diffusional motion of a chemisorbed atom

Experiments indicate that there are two extreme types of motion of an atom on a solid surface. One is characterized by an average velocity and has a mean square displacement proportion to the square of the time (we call this coherent). The other (called purely diffusional) is characterized by a diffusion coefficient and has a mean square displacement proportional to time. We present a simple stochastic model to explain the microscopic origin of these two extreme types of motion. In the case in which both types of motion coexist, the motion becomes diffusional for times longer than an intrinsic time depending on the intensity of the thermal fluctuations of the atom—lattice coupling.