Energy transfer in flashlamp pumped organic dye lasers

The pumping characteristics of dye lasers using multiple dyes are discussed. It turns out that varying the dye concentration may change considerably the portion of the pump light spectrum which is effective in creating population inversion. Thus the effect of an energy transfer dye depends strongly on the concentration of the lasing dye. Multiple dye systems are shown to have larger tuning ranges than single dye systems, and should exhibit improved tuned linewidth as well as better dye stability. Experimental results are presented for a common dye combination (Rhodamine 6G and Coumarin 6). Measurements were made both by directly mixing the auxiliary dye with the lasing dye, and also by using the auxiliary dye as a fluorescent spectral filter placed between the pump light source and the laser cell; the results are compared.     

High-resolution imaging of single fluorescent molecules with the optical near-field of a metal tip

We show that a concentration of light at a metal tip allows near-field optical imaging of single fluorescent dye molecules at very high resolution, despite strong quenching effects. Details as small as 10 nm were observed in the fluorescence patterns of single Cy-3 dyes bound to the termini of DNA. Data evaluation by model fitting determines the positions of the dyes to an accuracy even better than 1 nm and also yields their 3D orientation. The metal tip simultaneously provides high-resolution topographic imaging complementing the optical signal for a detailed surface examination.     

Micromixing models for turbulent flows

In transported probability density function and filtered density function methods, micromixing models are required to close the molecular mixing term. The accuracy and computational efficiency of improved versions of the parameterized scalar profile (PSP) model are assessed and compared with commonly used mixing models such as Curl, modified Curl, interaction by exchange with the mean and Euclidean minimum spanning tree. Different generalizations of the PSP mixing model for spatially inhomogeneous flow configurations are presented. The selected test cases focus on molecular mixing and avoid interference with other models. Simulation results for a three-stream problem, involving two inert scalars, and a multi-scalar test case with mean-scalar-gradients are presented.     

Fluorescent polystyrene microspheres with large Stokes shift by fluorescence resonance energy transfer

We prepared fluorescent microspheres with notably large Stokes shift and long-wavelength fluorescence by applying fluorescence resonance energy transfer (FRET) between two common julolidine dyes. Short distance between dye molecules caused by high dye concentration results in efficient FRET in microspheres. However, adequate dye concentration and moderate molar ratio of the donor and acceptor should be chosen to avoid aggregation of dye molecules, which leads to the decrease of fluorescent intensity. Microrspheres with average distance between dye molecules of 1.94 nm and molar ratio of 3.08:1 realize highly efficient FRET with no fluorescence of donor and intense long-wavelength emission of acceptor. In addition, the applied solvent evaporation method for preparing microspheres provided better protection of dyes from ambient medium than traditional surface-labeled method. These results demonstrate the feasibility of applying FRET in microspheres to expand useful fluorescent probes, and reveal their potential application in bioassays field.     

Broadband tunability of dye lasers

A double compartment dye cell to hold two or more dyes has been found to increase the frequency range of dye lasers. Results are compared with those obtained by mixing the same two dyes in a single cell. The dye cells are transversely pumped by a nitrogen laser. Broadband tunability results from dyes having similar absorption ranges but different Stokes shifts.     

Simulation of light generation in cholesteric liquid crystals using kinetic equations: Time-independent solution

We report on the results of calculating the conditions for light generation in cholesteric liquid crystals doped with fluorescent dyes using kinetic equations. Specific features of spectral properties of the chiral cholesteric medium as a photonic structure and spatially distributed type of the feedback in the active medium are taken into account. The expression is derived for the threshold pump radiation intensity as a function of the dye concentration and sample thickness. The importance of taking into account the distributed loss level in the active medium for calculating the optimal parameters of the medium and for matching the calculated values with the results of experiments is demonstrated.     

Violet-to-red photoluminescence of spiro-TAD organic films doped with different organic dyes

Photoluminescence (PL) and photoluminescence excitation (PLE) spectra of undoped spiro-TAD films and spiro-TAD films doped either by the organic dyes coumarin 7 or DCM as well as by both of these dyes simultaneously were investigated at different dye concentrations. A widened PL spectrum caused by doping was encountered and violet-to-red emission was obtained. It was established that excitation of the dyes is realized most efficiently through spiro-TAD. The overall integral PL intensity of the coumarin 7 doped films increased with dye concentration due to the suppression of nonradiative recombination in the film caused by a transfer of spiro-TAD excitation energy to the dye molecules. Mainly radiative energy transfer from semiconductor to dye molecules occurs in the case of DCM doping. No mutual influence on the luminescence of both dyes in the spiro-TAD film was observed and as a consequence, the PL band intensity of each dye can be adjusted separately.     

Fluorescence confocal polarizing microscopy: Three-dimensional imaging of the director

Much of the modern understanding of orientational order in liquid crystals (LCs) is based on polarizing microscopy (PM). A PM image bears only two-dimensional (2D) information, integrating the 3D pattern of optical birefringence over the path of light. Recently, we proposed a technique to image 3D director patterns by fluorescence confocal polarizing microscopy (FCPM). The technique employs the property of LC to orient the fluorescent dye molecules of anisometric shape, added in small quantities to the LC. In LC, smooth director deformations do not alter mass density of the material. Thus the density of dye is also uniform across the sample, except, perhaps, near the surfaces or at the cores of topological defects. In polarized light, the measured fluorescence signal is determined by the spatial orientation of the molecules rather than by dye concentration (as in regular biological samples stained with tissue-specific dyes). The contrast is enhanced when both excitation and detection of fluorescence light are performed in polarized light. This short review describes the essence of FCPM technique and illustrates some of its applications, including imaging of Frederiks electric-field induced effect in a nematic LC and defects such as dislocations in cholesteric LCs.     

Visualizing Acidophilic Microorganisms in Biofilm Communities Using Acid Stable Fluorescence Dyes

Bacteria in acidophilic biofilm communities, i.e. acid streamers and snottites, obtained from a subsurface mine in Königstein were visualized by fluorescence microscopy using four new fluorescent dyes (DY-601XL, V07-04118, V07-04146, DY-613). The pH of the bulk solution in which these bacteria thrive was pH 2.6 to 2.9. The new fluorescent dyes were all able to clearly stain and microscopically visualize in-situ the bacteria within the biofilm community without changing pH or background ion concentration. The commonly used fluorescent dyes DAPI and SYTO 59 were also applied for comparison. Both dyes, however, were not able to visualize any bacteria in-situ, since they were not stable under the very acid conditions. In addition, dye V07-04118 and dye DY-613 also possess the ability to stain larger cells which were presumably eukaryotic origin and may be attributed to yeast cells or amoeba-like cells. PCR analyses have shown that the dominant bacterial species in these acidophilic biofilm communities was a gram negative bacterium of the species Ferrovum myxofaciens. The presented four new dyes are ideal for in-situ investigations of microorganisms occurring in very acid conditions, e.g. in acidophilic biofilm communities when in parallel information on pH sensitive incorporated fluorescent heavy metals should be acquired.     

Study on Synthesis and Spectrum of Novel Styryl Cyanine Dyes with a Carbazole Bridged Chain

Based on the frequently-used cyanine dye probe thiazole orange (TO), a novel kind of cyanine dye was designed and synthesized. Carbazole was inserted into the methylidyne structure of TO as a bridge to afford a kind of novel styryl cyanine dye with carbazole bridged chain. The dyes were characterized by HNMR and MS. The spectra of the novel dyes were also studied and the results showed that the fluorescent wavelength of novel carbzole dye shifted red for 100 nm, stock shift increased by 70 nm and the fluorescent intensity enhanced by 70 times compared to that of TO. When the novel dye was labeled by BSA, its fluorescent wavelength changed little and the intensity enhanced. It is indicated that it can be used as an excellent fluorescent probe in biological labeling.     

Signal intensity enhancement of μ-LIF by using ultra-thin laser sheet illumination and aqueous mixture with ethanol/methanol for micro-channel applications

Measurement of concentration distributions of suspended particles in a micro-channel is one of the most crucial necessities in the area of Lab-on-a-chip to be used for various bio-chemical applications. One most feasible way to measure the concentration field in the micro-channel is using μ-LIF (micro-scale laser-induced fluorescence) method. However, an accurate concentration field at a given cross-plane in a micro-channel has not been successfully achieved so far due to various limitations in the light illumination and fluorescence signal detection. The present study demonstrates a novel method to provide an ultra-thin laser sheet beam having 5 μm thickness using a micro-focus laser line generator. The laser sheet beam illuminates an exact plane of concentration measurement field to increase the signal-to-noise ratio and considerably reduce the depth uncertainty.Nile Blue A was used as fluorescent dye for the present LIF measurement. The enhancement of the fluorescent intensity signals was performed by a solvent mixture of water (95%) and ethanol (EtOH)/methanol (MeOH) (5%) mixture. To reduce the rms errors resulting from the CCD electronic noise and other sources, an expansion of grid size was attempted from 1×1 to 3×3 or 5×5 pixel data windows and the pertinent signal-to-noise level has been noticeably increased accordingly.     

Tri- and Pentamethine Cyanine Dyes for Fluorescent Detection of α-Synuclein Oligomeric Aggregates

The pathogenesis of Parkinson’s disease that is the second most common neurodegenerative disease is associated with formation of different aggregates of α-synuclein (ASN), namely oligomers and amyloid fibrils. Current research is aimed on the design of fluorescent dyes for the detection of oligomeric aggregates, which are considered to be toxic and morbific spices. Fluorescent properties of series of benzothiazole trimethine and pentamethine cyanines were characterized in free state and in presence of monomeric, oligomeric and fibrilar ASN. The dyes with wide aromatic systems and bulky phenyl and alkyl substituents that are potentially able to interact with hydrophobic regions of oligomeric aggregates were selected for the studies. For majority of studied dyes noticeable changes in fluorescence characteristics were shown in the presence of fibrillar or oligomeric ASN, while the dyes slightly responded on the presence of monomeric protein. For pentamethine cyanine SL-631 and trimethine cyanine SH-299 certain specificity to oligomeric aggregates over fibrils was observed. Using these dyes at 10^?6 M concentration permits the detection of oligomeric ASN in the concentrations range of at least 0.2–2 microM. Pentamethine cyanine SL-631 is proposed as dye for fluorescent detection of oligomeric aggregates of ASN, while trimethine cyanine SH-299 is shown to be a sensitive probe both on oligomeric and fibrillar ASN. It is proposed that wide aromatic system of SL-631 pentamethine dye molecule could better fix on the less dense and structured oligomeric formation, while less bulky and more “crescent-shape” molecule of trimethine dye SH-299 could easier enter into the groove of beta-pleated structure.     

Modification of glass transition and liquid—liquid transition in PMMA doped with fluorescent dyes

PMMA doped with fluorescent dyes presents important modifications of thermally stimulated current spectra at high temperature. Glass and liquid-liquid transitions are no longer resolved and only one intense peak appears at 127°C. This peak cannot be resolved by the thermal sampling method. It has an Arrhenius behavior, and the plot of τ(1/T) shows the existence of a compensation phenomenon with a characteristic temperature which does not depend on the dye concentration. The modification is induced even if the dye concentration is as weak as some 10s of ppm. Thus it cannot be considered as a plasticization effect.     

Digital Holography for Instantaneous Spray Diagnostics on a Plane

We present a measurement technique that is capable of simultaneously determining sizes and positions of multiple transparent droplets in a plane from scattered light features. The technique is largely independent of particle intensity and mutual obscuration. Reflected and refracted light from the droplets in a pulsed laser sheet is recorded holographically to yield the smallest possible probe volume and the largest possible number density. Larger droplets are best analyzed at the image plane; in this case, the droplets appear as two spots (glare points), whose separation is proportional to the droplet diameter. Smaller droplets are easier to analyze at an out‐of‐focus plane, where their images appear as fringe patterns whose spatial frequency is related to droplet size. Photographic techniques allow only one of these planes to be chosen and are therefore not suitable for multidisperse sprays. Optical holography allows to analyze arbitrary depths, but often suffers from low sensitivity and long analysis times. With digital holography, the spray images are captured digitally by a CCD camera and reconstructed numerically; as in optical holography, the particle reconstruction plane can be freely chosen a posteriori to optimize the measurement. We discuss the issues raised by the transition from holographic film to a CCD sensor as the recording medium, and demonstrate the capabilities of the digital technique.     

Analysis of pulse parameters in synchronously pumped infrared dye lasers

Equations describing steady-state pulses circulating in synchronously pumped dye lasers with dyes of arbitrary fluorescence lifetime are derived and solved by numerical methods. Special attention is devoted to infrared emitting dyes characterized by short fluorescence lifetimes in the order of 10 ps. The results are compared with the case of slow dyes typically used in the visible spectral region.     

染料掺杂液晶可调谐光纤荧光光源的研究

本文提出了染料掺杂液晶填充空心光纤构造荧光可调谐光源.基于染料分子能级结构理论分析B4400荧光光谱依赖温度的变化特性,采用脉宽8 ns,波长为532 nm YAG倍频脉冲激光器抽运,向列相液晶作基体,实验分析染料B4400掺杂液晶填充空心光纤荧光光谱选择性荧光放大规律及温度调谐特性.结果表明:通过控制染料浓度可控制荧光输出功率水平;当温度升高时,中心波长发生红移,中心波长调谐范围为590—605 nm;荧光谱宽呈单调展宽,调制范围为228—236 nm;染料掺杂液晶填充空心光纤荧光光源可实现一定范围内的温度调谐.     

Quantum lithography with classical light

We show how to achieve subwavelength diffraction and imaging with classical light, previously thought to require quantum fields. By correlating wave vector and frequency in a narrow band, multiphoton detection process that uses Doppleron-type resonances, we show how to achieve arbitrary focal and image plane patterning with classical laser light at submultiples of the Rayleigh limit, with high efficiency, visibility, and spatial coherence. A frequency-selective measurement process thus allows one to simulate, semiclassically, the path-number correlations that distinguish a quantum entangled field.     

Transverse pseudo-nonlinear effects measured in solid-state laser materials using a sensitive time-resolved technique

The fluorescence of different types of planthopper (Hemiptera) and moth (Lepidoptera), which constitute important Chinese agricultural pests, was investigated both in situ in a laboratory setting and remotely using a fluorescence light detection and ranging (lidar) system operating at a range of about 50 m. The natural autofluorescence of different species, as well as the fluorescence from insects that had been dusted with fluorescent dye powder for identification were studied. Autofluorescence spectra of both moths and planthoppers show a maximum intensity peak around 450 nm. Bleaching upon long-time laser illumination was modest and did not affect the shape of the spectrum. A single dyed rice planthopper, a few mm in size, could be detected at 50 m distance by using the fluorescence lidar system. By employing various marking dyes, different types of agricultural pest could be determined. We suggest that lidar may be used in studies of migration and movement of pest insects, including studies of their behavior in the vicinity of pheromone traps and in pheromone-treated fields.     

Spectral properties of naphthalimide dyes mixed with 4-heptyl-4′-cyanobiphenyl (7CB) in Langmuir-Blodgett films

Langmuir-Blodgett (LB) films formed of some fluorescent dyes, namely derivatives of 4-aminonaphthalimide, and their binary mixtures with the liquid crystal 4-heptyl-4′-cyanobiphenyl (7CB) have been studied. The electronic absorption and fluorescence spectra were recorded. On the basis of these spectra, the spectral properties of the dyes and the intermolecular interactions in ultrathin ordered films were determined. The conclusions about the tendency to the creation of aggregates by dye molecules in LB films have been drawn. The measurements of absorption by using linearly polarised light have allowed us to determine the arrangement of the dye and liquid crystal molecules on the air-solid substrate interface.     

Fluorescence Reduction by a Triplet-State Population

The range of applications for fluorescent dyes in medicine and biology is increasing greatly. At the same time, the demands on the dyes are getting bigger. The dye molecules are often expected to emit maximum-intensity fluorescence. The primary requirement for this is naturally a high fluorescence quantum yield. Beyond this, it must be considered that fluorescent molecules located in a meta-stable triplet state are not able to emit fluorescent radiation. This represents a reduction of the intensity of the fluorescence, which is generally underestimated in its importance. The size of this perturbation can be grasped by knowledge of the quantum yield for occupation of the triplet state and its lifetime. Unfortunately these parameters are completely unknown, even for frequently used dyes. On investigating some medically and biologically relevant dyes, it is shown that the attainable fluorescence intensity could be reduced strongly by occupation of the triplet state.