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.     

Manipulated localized surface plasmon resonances in silver nanoshells coated with a spherical anisotropic layer

<正>The influences of the anisotropy of the outer spherically anisotropic(SA) layer on the far-field spectra and nearfield enhancements of the silver nanoshells are investigated by using a modified Mie scattering theory.It is found that with the increase of the anisotropic value of the SA layer,the dipole resonance wavelength of the silver nanoshell first increases and then decreases,while the local field factor(LFF) reduces.With the decrease of SA layer thickness, the dipole wavelength of the silver nanoshell shows a distinct blue-shift.When the SA layer becomes very thin,the modulations of the anisotropy of the SA layer on the plasmon resonance energy and the near-field enhancement are weakened.We further find that the smaller anisotropic value of the SA layer is helpful for obtaining the larger near-field enhancement in the Ag nanoshell.The geometric average of the dielectric components of the SA layer has a stronger effect on the plasmon resonance energy of the silver nanoshell than on the near-field enhancement.     

Control of single emitter radiation by polarization- and position-dependent activation of dark antenna modes

We study the modification of the decay rates of a single dipolar emitter positioned in the vicinity of metallic linear nanoantennas when higher-order plasmonic excitations are induced. We show that it is possible to effectively tune the enhancement or suppression of both the radiative and nonradiative decay processes by controlling the position and orientation of the dipole with respect to the antenna. Transverse polarization of a single emitter, with respect to the antenna axis, located at the center of the antenna activates dark antenna modes that modify dramatically both the intensity and the spectral features of the decay rates.     

Biomolecular imaging based on far-red fluorescent protein with a high two-photon excitation action cross section

Received October 14, 2005; revised January 7, 2006; accepted January 9, 2006; posted January 12, 2006 (Doc. ID 65391) The two-photon excitation action cross section of Hc-Red fluorescent proteins (Hc-RFPs) is measured and found to be of the same order as that of enhanced green fluorescent proteins. With a 618 nm emission wavelength in the far-red region and with an excitation wavelength around 1200 nm, Hc-RPF-based two-photon fluorescence microscopy (2PFM) can offer deep penetration capability inside live samples and is ideal for in vivo gene expression study and biomolecular imaging in live objects. In vivo 2PFM of the developing heart deep inside a transgenic zebrafish embryo tagged by Hc-RFP is also successfully demonstrated.     

Effect of dipole structures on field emission of wide-gap semiconductor emitters

It is shown that dipole structures placed in a thin (less than 1 nm) near-surface layer of a high-resistivity field emitter produce small domains on the emitting surface in which the electric field may exceed 10⁸ V/cm. In these domains, the emitter surface potential is positive, providing effective electron transport from inside the emitter to the emission boundary. Optimal dipole orientations ensuring maximal electric fields at the surface are found. When the surface density of dipoles localized in the near-surface layer is on the order of 10⁶ cm⁻², one can expect an emitter-averaged emission current density of higher than 1 A/cm². The dipole structures in the near-surface layer may persist owing to incorporated impurity molecules having a dipole moment or result from a random combination of positively charged ionized impurities and electrons captured by deep traps. Trap charging/discharging asymmetry accounts for the hysteresis of the emission I–V characteristics.     

Purcell effect in one-dimensional photonic quasicrystals

The change in probability of spontaneous emission for emitter placed in one-dimensional photonic quasicrystal (optical Fibonacci lattice) was examined. When the dipole is placed in Fibonacci lattice two different scenarios can be expected: enhancing (if frequency and direction of the dipole emission correspond to optical eigenmode of structure, and position corresponds to maximum value of modes electric field profile) or suppression (in case of photonic band gap) of spontaneous emission rate. Fact that both effects are expressed in quasicrystals less than in the Bragg reflectors and in the microcavities was demonstrated.     

Preparation of two-photon fluorescent probe and biological imaging application in cells

We report on a novel dibenzothiophene-based two-photon fluorescent probe for selective nuclear bioimaging,which contains bilaterally symmetrical pyridine rings connected by a central conjugated-bridge dibenzothiophene. This probe possesses a large two-photon absorption cross-section of 471 GM, yields a 25-fold enhancement of the fluorescence titration, and a stronger photostability for nuclei labeling than existing probes. The real-time observation period is a minimum of 1800 s under a femtosecond laser excitation, which is significantly longer than that of 40,6-diamidino-2-phenylindole. The above results confirm that this novel molecule is a suitable two-photon fluorescent probe for application to nuclear bioimaging in cells.     

Spontaneous emission in the near field of two-dimensional photonic crystals

We show theoretically that photonic crystal membranes cause large variations in the spontaneous emission rate of dipole emitters, not only inside but also in the near field above the membranes. Our three-dimensional finite-difference time-domain calculations reveal an inhibition of more than five times and an enhancement of more than ten times for the spontaneous emission rate of emitters with select dipole orientations and frequencies. Furthermore, we demonstrate theoretically the potential of a nanoscopic emitter attached to the end of a glass fiber tip as a local probe for mapping the large spatial variations of the photonic crystal local radiative density of states. This arrangement is promising for on-command modification of the coupling between an emitter and the photonic crystal in quantum optical experiments.     

Spontaneous two-photon emission from a single quantum dot

Spontaneous two-photon emission from a solid-state single quantum emitter is observed. We investigated photoluminescence from the neutral biexciton in a single semiconductor quantum dot coupled with a high Q photonic crystal nanocavity. When the cavity is resonant to the half energy of the biexciton, the strong vacuum field in the cavity inspires the biexciton to simultaneously emit two photons into the mode, resulting in clear emission enhancement of the mode. Meanwhile, the suppression of other single photon emission from the biexciton was observed, as the two-photon emission process becomes faster than the others at the resonance.     

Modification of single molecule fluorescence by a scanning probe

We examine the optical near-field interaction between different types of scanning tips and single oriented fluorescent molecules. We demonstrate the influence of a tip on the excitation intensity as well as on the integrated fluorescence signal, the excited state lifetime, and the angular emission of single molecules. By using a standard model describing the radiation of an oscillating dipole close to a nanosphere or a flat interface, we interpret our observations and describe some central criteria for obtaining fluorescence enhancement or quenching. PACS 33.80.-b; 07.79.Fc; 78.90.+t     

Surface plasmon propagation in a pair of metal nanowires coupled to a nanosized optical emitter

The coupling, propagations, and far-field emissions of surface plasmons in a pair of Au nanowires with a dipole emitter have been investigated using the finite-difference time domain method. The surface plasmon wavelength is tunable from 650 to 380 nm by adjusting the distance between the two wires, which leads to an enhancement of coupling constant and density of states of the surface plasmon. The converted energy from the dipole emitter to the propagating surface plasmon as well as the far-field emission intensity of a pair of Au nanowires increase to approximately four times as large as those of a single nanowire.     

Fluorescence characteristics of a molecule in the vicinity of a plasmonic nanomatryoska: Nonlocal optical effects

The fluorescence characteristics of a dipole molecule in the vicinity of a spherical multilayered metallic nanoshell (a plasmonic nanomatryoska) of ultra-small dimensions is studied via electrodynamic modeling, where we have computed the fluorescence decay rates, the shifts in emission frequency, and the overall fluorescence yields for molecular dipoles of both tangential and radial orientations. Our focus is on structures of ultra small dimensions in order to elucidate the possibly novel nonlocal optical effects in such a phenomenon. The results show that at very close distances between the molecule and the nanoshell, the nonlocal effects in general lead to smaller structure-induced effects with broadened and blue-shifted plasmonic resonances. These effects include overall smaller induced decay rates, smaller red-shifts in emission frequency, and somewhat larger fluorescence yields at low emission frequencies. Physical interpretation of our simulation results is provided.     

Thermochromic shifts of the fluorescence spectra of 4-N,N-dimethylaminobenzonitrile in solution

From the temperature effect on the fluorescence bands of 4-N,N-dimethylaminobenzonitrile the dipole moments of the two emitting species are estimated as 5 (normal fluorescence) and 15 (anomalous fluorescence) Debye above the ground-state dipole moment. The dipole moment of the anomalous emitter does not change with solvent polarity and this emitter is therefore probably a molecular species (i.e. twisted charge-transfer state) and not a solvent-solute exciplex.     

“Hot spots” induced near-field enhancements in Au nanoshell and?Au nanoshell dimer

The influence of “hot spots” on the near-field properties of Au nanoshell and Au nanoshell dimers have been investigated by means of the finite element method. It is found with increasing the pinhole radius R that the maximal enhancement of near-field for Au nanoshell with pinhole parallel to the polarization increases from 17.906 at R=0 nm to 36.979 at R=0.8 nm, and then almost shows a negligible radius dependence. Large electric fields also can be observed inside the pinhole perpendicular to the polarization, which increases with increasing the pinhole radius. The near-field of Au nanoshell dimer depends strongly on the polarization and propagation directions of the incident light. Exponential decay behavior is found for the maximal enhancement of the electric field in the dimer junction as a function of the dimer separation. Furthermore, a very strong electric field is found in the junction between two Au nanoshells when the pinholes are located near the gap between the nanoshells.     

以萘为π-中心的双芪类衍生物双光子上转换荧光性能研究

研究了2个新的双光子上转换荧光分子—1,4-双-(9-乙基咔唑基)萘(简称为BECVN)和1,4-双-(4’-N,N-二甲氨基苯乙烯基)萘(简称为BMABN)的单光子、双光子光谱性质。在～375 nm Xe灯光源激发下,两样品的DMF溶液发出很强的蓝、绿色荧光(峰位492～541 nm),视感效果非常明显。BMABN分子的线性吸收/发射光谱的峰位与BECVN分子相比,均发生红移;相对荧光量子产率(Φ_f)比BECVN降低了7.4倍。在飞秒钛宝石激光器泵浦下(760 nm),两样品的DMF溶液发出强的双光子上转换荧光发射,峰位与单光子荧光峰位相比发生红移(500～556 nm)。BMABN的双光子荧光强度和双光子吸收截面分别是BECVN的3倍和30.4倍。     

Photobleaching-Based Quantitative Analysis of Fluorescence Resonance Energy Transfer inside Single Living Cell

The current advances of fluorescence microscopy and new fluorescent probes make fluorescence resonance energy transfer (FRET) a powerful technique for studying protein-protein interactions inside living cells. It is very hard to quantitatively analyze FRET efficiency using intensity-based FRET imaging microscopy due to the presence of autofluorescence and spectral crosstalks. In this study, we for the first time developed a novel photobleaching-based method to quantitatively detect FRET efficiency (Pb-FRET) by selectively photobleaching acceptor. The Pb-FRET method requires two fluorescence detection channels: a donor channel (CH ₁ ) to selectively detect the fluorescence from donor, and a FRET channel (CH ₂ ) which normally includes the fluorescence from both acceptor and donor due to emission spectral crosstalk. We used the Pb-FRET method to quantitatively measure the FRET efficiency of SCAT3, a caspase-3 indicator based on FRET, inside single living cells stably expressing SCAT3 during STS-induced apoptosis. At 0, 6 and 12 h after STS treatment, the FRET efficiency of SCAT3 obtained by Pb-FRET inside living cells was verified by two-photon excitation (TPE) fluorescence lifetime imaging microscopy (FLIM). The temporal resolution of Pb-FRET method is in second time-scale for ROI photobleaching, even in microsecond time-scale for spot photobleaching. Our results demonstrate that the Pb-FRET method is independent of photobleaching degree, and is very useful for quantitatively monitoring protein-protein interactions inside single living cell.     

Analytical Expression for the Relaxation Rate of Emitter-Near-Metal Nanoparticles

We present a simple analytical expression for the full relaxation rate of the excited state of the two-level resonant emitter placed close to a metal nanoparticle with localized plasmon resonance excited by the emitter. We take into account the interaction of the emitter with all multipole polarization modes and the radiation absorption in the nanoparticle. Analytical and numerical estimations of the full relaxation rate are in good agreement. Thus, only two modes are sufficient for describing the electromagnetic interaction of the dipole emitter and the metal nanoparticle, namely, the dipole mode and the mode related to the emitter image under the nanoparticle surface. Such “two-mode” approximation can simplify the analysis of optical properties of nanoplasmonic structures. In particular, the proposed expression for the full relaxation rate is helpful in the modeling of plasmonic nanolasers.     

Identification of molecules through the fluorescence enhancement by a metal tip

A fluorescence enhancement phenomenon, which is realized as a result of a sharp increase in the radiative decay rate of a quantum dipole emitter (QDE) is investigated theoretically in the vicinity of a conical metal tip. The QDE relaxation process is considered as a self-stimulated transition from an excited state into the ground state due to the feedback field formation from the tip. The dynamics of the system shows a stepped relaxation behavior that differs significantly from the conventional exponential decay. This effect can be observed in a small region of the resonance frequency, which is defined by an angle of conical tip. The increase of fluorescence when approaching of molecule to the metal tip on the surface enables one to determine its location.     

Plasmonic effect of nanoshelled nanocavity on encapsulated emitter's spontaneous emission

The plasmonic enhancement of nanoshelled nanocavity (a silica core coated by Ag or Au shell) on the spontaneous emission of an encapsulated emitter (a molecule or quantum dot) is studied systematically by analyzing the excitation rate and the apparent quantum yield together. By averaging all possible locations and orientations of the emitter, the average enhancement factor (AEF) of the emitter randomly located in the core is calculated. Our results show that the AEF is weaker than that of the emitter located at the core center. In addition, Ag nanoshell (NS) is a narrowband enhancer. As the thickness of the shell becomes thinner, the surface plasmon resonance of NS is red-shifted and the peak of AEF increases. The specificity of Ag NS for enhancing a specific spontaneous emission is higher than Au NS. In addition, Ag NS with a smaller core has a larger AEF, while Au NS has an optimal radius of core (30 nm) to obtain the maximum AEF. Moreover, the AEF is reduced, as the Stokes shift increases.