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This paper investigates the factors which can influence the quenching effect from the perspective of energy transfer. The quenched fluorescence of the CdSe semiconductor quantum dots (SQDs) by using plasmonic Au nanocrystals (p-AuNCs) and molecule-like Au nanocrystals (m-AuNCs) in aqueous suspensions and spin-coated films is comparatively investigated. In the aqueous suspensions, the p-AuNCs have larger quenching effect than the m-AuNCs. In the spin-coated films, the p-AuNCs and m-AuNCs have comparable quenching factor. Furthermore, the experiments show that the p-AuNCs simultaneously enhance the radiative and nonradiative rates. But the m-AuNCs only enhance the nonradiative rate of the SQDs, which reveals the difference of quenching process between the p-AuNCs and m-AuNCs. This result of the research has guiding significance for the detection technique based on the fluorescence quenching.

Silver nanoplates have obvious advantages compared with other silver structural nanoparticles due to their wide tunability of the local surface plasmon resonance (LSPR) and significant local field enhancement around the sharp corners. Nonlinear optical absorption (NLA) and nonlinear optical refraction (NLR) are widely employed to characterize the nonlinear optical properties due to their simply testing requirement and comparable valuable results. Z-scan method is widely used in optical characterization of nonlinear properties of different materials. Here, a seed-mediated growth method of the silver nanoplates with well controlled size and local surface plasmon wavelength is presented. The nanoparticle size increases gradually and becomes more uniform distribution as the red-shift of the plasmon resonance wavelength from 650 nm to 950 nm. In particular, we show the dependence of NLA and NLR of the silver nanoplates on the LSPR. The absolute value of effective NLA coefficient increases from 1.38 to 4 cm/GW and that of the NLR index increases as large as 3 times when the LSPR wavelength changes from 728 nm to 898 nm, which is attributed to the strong plasmon absorption and local filed enhancement of larger size silver nanoplates. These findings have great potentials in the explorations of functional non-linear devices.

The controllable synthesis of uniform tungsten diselenide (WSe

We investigate the population dynamics and the photon emission statistics of the coupled semiconductor quantum dots (QDs) with pulse excitation by using quantum regression theorem and optical Bloch equations. The population of the biexciton state vertical bar b> is close to those of two exciton states (vertical bar x> and vertical bar y>) when the effective input pulse area is about pi. The calculation results reveal that both the probabilities of the cross single photon emission and the correlated photon-pair emission decrease as the input pulse area increases in every period. (C) 2007 Elsevier B.V. All rights reserved.

To investigate the influence of surface trapping and dark states on CdSe and CdSe/ZnS quantum dots (QDs), we studied the absorption, fluorescence intensity and lifetime by using one-and two-photon excitation, respectively. Experimental results show that both one- and two-photon fluorescence emission efficiencies of the QDs enhance greatly and the lifetime increase after capping CdSe with ZnS due to the effective surface passivation. The lifetime of one-photon fluorescence of CdSe and CdSe/ZnS QDs increase with increasing emission wavelength in a supralinear way, which is attributed to the energy transfer of dark excitons. On the contrary, the lifetime of two-photon fluorescence of bare and core-shell QDs decrease with increasing emission wavelength, and this indicates that the surface trapping is the dominant decay mechanism in this case.

The relative probability of two- and three-photon absorption in Au nanoparticles was adjusted by tuning the surface-plasmon-resonance (SPR) wavelength and the excitation intensity. Consequently, highly efficient three-photon luminescence from Au nanoparticles with strong SPR enhancement was observed. The decay rate and polarization of two- and three-photon luminescence from Au nanoparticles were comparatively studied. Furthermore, the multiphoton luminescence from Au nanoparticles was used to investigate the resonant energy transfer from CdSe semiconductor quantum dots.

A new class of fluorescent probe produced by conjugating semiconductor quantum dots (QDs) with protein molecule is proposed as an alternative to conventional organic labels. However the fluorescence characteristics of the QD bioconjugates are not clear while they are excitied with one- or two-photon laser pulse. We synthesized specific immunofluorescent probes by linking QDs to alpha fetoprotein (AFP) antibody for specific binding alpha-fetoprotein -an important marker for hepatocellular carcinoma cell lines, and archived specific fluorescence detection with the QDs-Anti-AFP in nude mice. Then, we have analyzed the fluorescence characteristics of QDs-Anti-AFP and original QDs both under one- and two-photon excitations. The results demonstrated that QDs-Anti-AFP's fluorescent spectral and lifetime haven't varied much from that of original QDs. Moreover, QDs-Anti-AFP have exhibited higher fluorescence efficiency than QDs under two-photon examination.

We analyse the population dynamics and the photon emission statistics of a single semiconductor quantum dot with an exciton–biexciton–triexciton multilevel driven by a pulse field, in which the quantum regression theorem and optical Bloch equations are employed. The populations of the three states are comparable under resonant excitation with the input pulse area around 1.5π. Three second-order cross-correlation functions demonstrate the correlated three-photon emissions at a given order from triexciton, biexciton and exciton states driven by single pulses, while the probability of the bunched three-photon cascaded emissions decreases to the minimum as the input pulse area increases to 2π in every period.

Core-shell Ag/TiO 2 nanocomposites with different TiO 2 concentrations were synthesized by the photoreduction method, in the presence of ethanol. The diameter of the nanoparticles, with various TiO 2 and Ag + content, was determined to be 20–60 nm using a transmission electron microscope (TEM), and the crystal lattice fringes of silver particles were observed. The prepared nanocomposites were characterized by ultraviolet-visible (UV-Vis) spectroscopy and X-ray Diffraction (XRD). The intensity and width of the characteristic peak near 420 nm of the Ag nanoparticles increased as the TiO 2 or silver concentration was increased. The nonlinear optical properties of thin films of nanocomposites with different TiO 2 concentrations were analyzed using Z-scan technique at the wavelength of 790 nm with a pulse width of 130 fs. Two different nonlinear regimes in nonlinear absorption were found by changing the content of TiO 2 : an optical limiting effect at a low concentration, and a sign change at a higher concentration.

The nanosized (Au,Ag)–Ti three-component granular composite films on glass substrates were prepared using an rf co-sputtering method, and the deposited films were annealed in air. The optical absorption and the ultrafast third-order nonlinearity of the (Au,Ag)–TiO2 films annealed at different temperatures were investigated. The composite film annealed at 600°C exhibits strong resonant absorption at wavelengths of 400 and 670 nm. The ultrafast third-order nonlinear optical susceptibility at 820 nm of the phase separated (Au,Ag)–TiO2 composite film is measured to be 7.4 × 10−9 esu. This is about 70% larger than that of the Au–TiO2 composite film.

The porous nanosized poly-crystal LiNbO3 film on the fused SiO2 substrate was prepared by using r.f. sputtering method. The microstructure and the third-order optical nonlinearity of the film were investigated. The size of the micro-crystal grains was in the range of 110–160 nm, and the average pore size was 80 nm. The magnitude of ultrafast response χ (3) of the sputtered thin LiNbO3 film at 830 nm wavelength was estimated to be as large as 1.9 × 10 −8 esu, which is 4 orders of magnitude larger than that of LiNbO3 single crystal.

Recently, we have successfully developed a two-photon excitation fluorescence resonance energy transfer (TPE-FRET)-based homogeneous immunoassay using two-photon excitable small organic molecule as the energy donor. In the present work, the newly emerging TPE-FRET technique was extended to the determination of oligonucleotide. A new TPE molecule with favorable two-photon action cross section was synthesized [2-(2,5-bis(4-(dimethylamino)styryl)-1H-pyrrol-1-yl)acetic acid, abbreviated as TP-COOH], with the tagged reactive carboxyl group allowing facile conjugation with streptavidin (SA). Employing the TP-COOH molecule as energy donor and black hole quencher 1 (BHQ-1) as acceptor, a TPE-FRET-based homogeneous competitive hybridization model was constructed via a biotin-streptavidin bridge. Through the hybridization between a biotinylated single-stranded DNA (ssDNA) and a BHQ-1-linked ssDNA, and the subsequent capture of the as-formed hybrid by TP-COOH labeled SA, the donor fluorescence was quenched due to the FRET between TP-COOH and BHQ-1. Upon the competition between a target ssDNA and the quencher-linked ssDNA toward the biotinylated oligonucleotide, the donor fluorescence was recovered in a target-dependent manner. Good linearity was obtained with the target oligonucleotide ranging from 0.08 to 1.52 microM. The method was applied to spiked serum and urine samples with satisfying recoveries obtained. The results of this work verified the applicability of TPE-FRET technique in hybridization assay and confirmed the advantages of TPE-FRET in complicated matrix.

A fluorescence resonance energy transfer (FRET) model using two-photon excitable small organic molecule DMAHAS as energy donor has been constructed and tried in an assay for avidin. In the FRET model, biotin was conjugated to the FRET donor, and avidin was labeled with a dark quencher DABS-Cl. Binding of DABS-Cl labeled avidin to biotinylated DMAHAS resulted in the quenching of fluorescence emission of the donor, based on which a competitive assay for free avidin was established. With using such donors that are excited in IR region, it is capable of overcoming some primary shortcomings of conventional one-photon FRET methods, especially in bioassays, such as the interference from background fluorescence or scattering light, the coexcitation of the energy acceptor with the donor. And such small molecules also show advantages over inorganic up-converting particles that also give anti-Stokes photoluminescence and have been applied as FRET donor recently. The results of this work suggest that two-photon excitable small molecules could be a promising energy donor for FRET-based bioassays.

We demonstrate highly efficient avalanche multiphoton luminescence (MPL) from ordered-arrayed gold nanowires (NWs) with low time-average excitation intensity, Iexc (5.0-9.1 kW/cm2). The intensity of avalanche MPL, IMPL, is about 10(4) times larger than that of three-photon luminescence, the slope partial differential log IMPL/ partial differential log Iexc of avalanche MPL reaches as high as 18.3, and the corresponding polarization dependence of IMPL has a form of cos50 phip. The emission dynamics of avalanche MPL and three-photon luminescence are also studied comparatively. These observations indicate that the highly efficient avalanche MPL is attributed to the giant enhancement and coupling of longitudinal surface plasmon resonance of ordered-arrayed gold NWs.

We report the spatial intensity distributions of the laser-induced charge and spin polarizations in GaAs∕AlGaAs two-dimensional electron gas (2DEG) detected by using second harmonic generation (SHG) imaging technique. The spin polarized electrons in the 2DEG are pumped by a single linear polarized laser beam and probed by another beam which produces reflective SHG. By comparing the images of SHG probed with left- and right-circular polarized laser beams, the spatial distributions of the effective charge and spin polarizations in the 2DEG sample are mapped out.