Enhancement and quenching of single-molecule fluorescence

We present an experimental and theoretical study of the fluorescence rate of a single molecule as a function of its distance to a laser-irradiated gold nanoparticle. The local field enhancement leads to an increased excitation rate whereas nonradiative energy transfer to the particle leads to a decrease of the quantum yield (quenching). Because of these competing effects, previous experiments showed either fluorescence enhancement or fluorescence quenching. By varying the distance between molecule and particle we show the first experimental measurement demonstrating the continuous transition from fluorescence enhancement to fluorescence quenching. This transition cannot be explained by treating the particle as a polarizable sphere in the dipole approximation.     

Excitation and emission enhancement of single molecule fluorescence through multiple surface-plasmon resonances on metal trimer nanoantennas

We study theoretically the light scattering from trimers of metal nanowires, with emphasis on the occurrence of multiple surface-plasmon resonances, showing that such resonances can be exploited to achieve twofold-enhanced fluorescence from a single molecule placed in the nanotrimer gaps, even if excitation and emission frequencies are separated. Pump enhancement stems from the local field enhancement coinciding with one of the resonances, whereas a strong enhancement of the radiative decay rate (and quantum yield) is revealed at a different resonance, leading to a large overall signal emission.     

胶体金纳米颗粒的荧光光谱特性

研究了胶体金纳米颗粒的荧光激发光谱和荧光发射光谱特性.增大激发光的波长,发射峰相应红移并在629nm达到最强,对应的灵敏激发波长为473nm.增加金纳米颗粒粒径,观察到发射谱的峰值先增大然后减小.发射峰产生的原因在于电子与带间空穴的复合导致非线性共振光散射.发射峰强度和金颗粒粒径的关系可以用表面等离子体震荡引起的局域场增强来解释,数值计算结果和实验结果能较好的吻合.     

碘分子单频及双频多光子光谱

采用多光子离化谱和荧光激发谱技术，对碘分子单频和双频五光子离化谱和三光子荧光谱进行了研究和标识．对OODR五光子离化谱中一组间隔为72cm^-1的振动序列进行了归属，并确定了其激发、发射和离化通道 关键词：     

Indistinguishable photons from a single molecule

We report the results of coincidence counting experiments at the output, of a Michelson interferometer using the zero-phonon-line emission of a single molecule at 1.4 K. Under continuous wave excitation, we observe the absence of coincidence counts as an indication of two-photon interference. This corresponds to the observation of Hong-Ou-Mandel correlations and proves the suitability of the zero-phonon-line emission of single molecules for applications in linear optics quantum computation.     

Laser-excited spectra of Lu2SiO5:Ce scintillator

The emission spectra of Lu₂SiO₅:Ce single crystal under the excitation of 266 nm laser were investigated. The emission spectra of LSO single crystal show no temperature quenching from 20 to 300 K, under the excitation of 266 nm laser with 2 mJ pulse energy. With rising temperature, the Ce1 emission is slightly decreased, while the Ce2 emission is slightly increased. These results show the emissions of Ce1 and Ce2 is not only dependent on the concentration ratio but also influenced by the possible energy transfer processes, including Ce1 to Ce2, intrinsic STHs to Ce2 and the phonon-assisted transfer processes. The spectral thermal broadening and the spectral overlap become evident at high temperature, leading to the enhancement of energy transfer. When the excitation power lowers, the ratio of Ce1 and Ce2 emission increases, and is close to the Xe lamp ultraviolet (UV) excitation, suggesting that the energy transfer from Ce1 center to Ce2 center may be also dependent on the excitation power.     

Effects of the reaction time and size on the up conversion luminescence of NaYF4:Yb(20%),Er(1%) microcrystals

NaYF₄:Yb(20%),Er(1%) microcrystals were synthesized using a hydrothermal method, and the effects of the reaction time and the particle size on the up conversion (UC) emission properties were systematically studied. The UC emission intensities of the samples are directly related to the particle size and phase structure. The increase in the reaction time resulted in the phase transformation from cubic nanoparticles to hexagonal microprisms, leading to the enhancement of the UC emission. Moreover, the emission intensities and the intensity ratios of the green and red emissions (F_g/r) of the microprisms, in which a normal two-photon UC process was displayed under excitation with a 980 nm laser, remarkably increased with the increase of grain sizes, resulted from the increase of Yb³⁺ concentrations in crystals.     

香豆素、罗丹明衍生物和DCM激光染料分子间的激发能传递

通过考察香豆素4-DCM、香豆素481-DCM、否豆素540A-罗丹明6G、香豆素540A-罗丹明640在乙醇、乙二醇等极性溶剂中的激发光谱和荧光发射光谱,研究了它们之间的激发能传递过程。     

Single molecule spectroscopy on photosynthetic pigment-protein complexes

Single molecule spectroscopy was applied to unravel the energy transfer pathway in photosynthetic pigment-protein complexes. Detailed analysis of excitation and fluorescence emission spectra has been made for peripheral plant antenna LHC II and Photosystem I from cyanobacterium Synechococcus elongatus. Optical transitions of individual pigments were resolved under nonselective excitation of antenna chlorophylls. High-resolution fluorescence spectroscopy of individual plant antenna LHC II indicates that at low temperatures, the excitation energy is localized on the red-most Chl a pool absorbing at 680 nm. More than one pigment molecule is responsible for the fluorescence emission of the LHC II trimer. The spectral lines of single Chl a molecules absorbing at 675 nm are broadened because of the Foerster energy transfer towards the red-most pigments. Low-temperature spectroscopy on single PS I trimers indicates that two subgroups of pigments, which are present in the red antenna pool, differ by the strength of the coupling with the protein environment and by the efficiency of the triplet state quenching. Based on single molecule and hole burning data, we envisage the dimeric nature of the strongly phonon coupled red-most Chl state.     

Plasmonic Enhancement of Single-Molecule Fluorescence Near a Silver Nanoparticle

In this short paper, we reported the enhanced fluorescence from a single fluorophore bound to a 50nm silver nanoparticle. We found that on average the Cy5 molecules bound to metal nanoparticles are approximately 15-fold brighter than that of free dyes, and that single molecule lifetimes are shorter as compared to free fluorophores. The increased emission rate is primarily the result of local plasmon enhancement. These results demonstrate that the use of fluorophore-metal interactions can increase the brightness and photostability of fluorophores for single molecule detection.     

纳米银粒子的发光特性研究

研究了不同粒径和表面修饰的纳米银粒子的发光特性。研究结果表明,在不同波长光激发下,纳米银粒子在362 nm附近出现较强的发射峰,592和725 nm附近出现较弱的发射峰。随着激发光波长增加,发射峰强度下降,362 nm附近的发射峰红移。纳米银颗粒对210 nm的激发光最为敏感。发射峰波长与纳米银粒子表面修饰状态和颗粒尺寸关系不大,只是随着颗粒尺寸的减小,发射峰强度下降。随着狭缝宽度的减小,发射峰强度下降。随着纳米银胶浓度减少,发射峰逐渐聚拢合并为426 nm的单峰,且发射峰的强度先增强后逐渐下降。通过纳米银粒子表面光电子的吸收-再发射和表面能级杂化探讨了纳米银粒子的发光机理。     

Fluorescence enhancement of radix angelica dahurica by binding to single silver sphere

We present a theoretical study of the influence of a single silver sphere on the fluorescence of radix angelica dahurica,which is a kind of traditional Chinese medicine. The enhancement factors of the excitation and the relaxation processes are deduced. The excitation can be enhanced more than 100 times at 315 nm. The enhancement factor of the emission can reach up to 9 at a center wavelength of 400 nm.     

Enhancement or quenching effect of metallic nanodimer on spontaneous emission

The plasmonic effects of a metallic (Au or Ag) nanodimer on the excitation and emission of a single emitter placed within the gap of the nanodimer are studied to identify its overall performance (enhancement or quenching) for the spontaneous emission. The process of a spontaneous emission is divided into two stages for analysis: the excitation and the subsequent emission stages. For the excitation stage, the amplification of the local electric field around the gap region is studied to show the converging-lens effect of the nanodimer for focusing an incident light. For the emission stage, the apparent quantum yield of an electric dipole (the excited emitter) in the presence of the nanodimer is studied in terms of its radiative and nonradiative decay rates. Both models are simulated by the multiple multi-pole methods for solving Maxwell's equations. The results indicate that the overall enhancement factor of a metallic nanodimer on the spontaneous emission depends not only on its dimension (radius and gap) but also on the absorption and emission spectra of the emitter. Moreover, there is an optimal dimension (radius and gap) of a nanodimer for obtaining the maximum enhancement to a specific spontaneous emission. In addition, the observed emission spectrum of the emitter can be modified by the nearby nanodimer (a low-pass filter), and its lifetime can be reduced by two or three orders of magnitude due to the energy transfer between them.     

Raman scattering and fluorescence emission in a single nanoaperture: Optimizing the local intensity enhancement

We investigate the potential of a single subwavelength aperture milled in an aluminium film to enhance the local electromagnetic field. We compare the Raman scattering of unadsorbed chlorobenzene molecules and the fluorescence emission of Cyanine-5 dyes, having the same excitation and collection setup for both experiments. For the optimal nanoaperture diameter, we report a clear enhancement factor of about 5 of the Raman scattering intensity per unit volume. Since Raman scattering probes the molecular vibrational levels and avoids the resonant pumping of a real excited state, the observed Raman enhancement is disconnected from the effects of the molecular energy levels alteration previously reported for fluorescent dyes. The observations are similar for both Raman and fluorescence experiments, and stand in good agreement with numerical electromagnetic computations of the excitation intensity inside the nanoaperture.     

单分子-光子制冷泵的热力学行为(Ⅰ)

建立了一般意义上的单分子－光子泵模型,并用计算机模拟了在不同能量的光子激发下单个分子作为制冷泵时的量子跃迁过程,研究了单分子－光子泵处于基态和激发态时的热激发过程以及在发生荧光辐射时的声子参与情况,得出了基态具有较强的制冷能力的结论。基态和激发态在光吸收和发射过程中热力学效应是不同的,因此在选择反斯托克斯芝光制冷材料时,发光中心的基态能级劈裂更具重要性。     

Coherent electronic coupling versus localization in individual molecular dimers

We have investigated electronic excitation transfer in individual molecular dimers by time and spectrally resolved confocal fluorescence microscopy. The single molecule measurements allow for directly probing the distribution of the electronic coupling strengths due to static disorder in the polymer host. We find dimers where the excitation is delocalized (superradiant emission) while for others emission originates from a localized state. Transitions between delocalized and localized states as observed for a given dimer are attributed to structural fluctuations of the guest-host system.     

金纳米颗粒的紫外、蓝紫光波段光致荧光特性

采用电化学方法制备出粒径在20～30 nm的悬浮胶体金纳米颗粒。研究了金纳米颗粒的荧光发射光谱特性。在377和459 nm观察到两个荧光发射峰,分别位于紫外和蓝紫光波段,对应的敏感激发波长为220 nm。减小激发光强度或降低金纳米颗粒的粒子数密度都会使377 nm处的荧光发射峰强度减弱。位于459 nm处的荧光发射峰对激发光强度和颗粒数密度变化更为敏感,并且在激发光强度降低到一定阈值或粒子数密度高于一定阈值后消失。随着激发光强度的增加,位于377和459 nm处的两发射峰强度逐渐靠近,其比值逼近于1。实验结果与随机分布介质的自组织散射式谐振腔理论符合得较好。这表明胶体金纳米颗粒中存在循环多重散射,并由此引发了蓝光及紫外波段的荧光增强,这在光存储和全色显示等方面具有潜在的应用前景。     

Effects of Isospin on Pre-scission Particle Multiplicity of Heavy Systems and Its Excitation Energy Dependence

Isospin effects on particle emission of fissioning isobaric sources ²⁰²Fr, ²⁰²Po, ²⁰²Tl and isotopic sources ^189,202,212Po, and its dependence on the excitation energy are studied via Smoluchowski equations. It is shown that with increasing the isospin of fissioning systems, charged-particle emission is not sensitive to the strength of nuclear dissipation. In addition, we have found that increasing the excitation energy not only increases the influence of nuclear dissipation on particle emission but also greatly enhances the sensitivity of the emission of pre-scission neutrons or charged particles to the isospin of the system. Therefore, in order to extract dissipation strength more accurately by taking light particle multiplicities it is important to choose both a highly excited compound nucleus and a proper kind of particles for systems with different isospins.     

Controlled coupling of a single nitrogen-vacancy center to a silver nanowire

We report on the controlled coupling of a single nitrogen-vacancy (NV) center to a surface plasmon mode propagating along a chemically grown silver nanowire (NW). We locate and optically characterize a single NV center in a uniform dielectric environment before we controllably position this emitter in the close proximity of the NW. We are thus able to control the coupling of this particular emitter to the NW and directly compare the photon emission properties before and after the coupling. The excitation of single plasmonic modes is witnessed and a total rate enhancement by a factor of up to 4.6 is demonstrated.