One-, two-, and three-photon absorption induced fluorescence of a novel chromophore in chloroform solution

One-, two-, and three-photon absorption induced fluorescence intensities of a novel nonlinear optical chromophore have been measured by using a tunable femtosecond pulsed laser as the excitation. Four resonance peaks are observed as the excitation wavelength is tuned from 600 to 2000 nm. These peaks correspond to the one-, two- and three-photon fluorescence resonance. Except for intensity difference, the lifetime and the fluorescence spectrum are found to be the same for the one-, two-, or three-photon resonance, hence suggesting that the same excited energy level is involved in emitting the fluorescence intensity. A three-level model is developed to account for the incident excitation laser intensity dependence of the one-photon and multiphoton fluorescence intensity. The model allows the multiphoton absorption cross sections to be extracted; it can also account for the deviation observed in the linear, square, and cubic intensity dependence of the one-, two-, and three-photon fluorescence intensity, respectively. To determine the absorption cross sections, the present method does not require the fluorescence quantum efficiency data, needed in the low intensity technique.     

Rational Design of Cyclometalated Iridium(III) Complexes for Three-Photon Phosphorescence Bioimaging

Compared to 2PE (two-photon excitation) microscopy, 3PE microscopy has superior spatial resolution, deeper tissue penetration, and less defocused interference. The design of suitable agents with a large Stokes shift, good three-photon absorption (3PA), subcellular targeting, and fluorescence lifetime imaging (FLIM) properties, is challenging. Now, two Ir^III complexes (3PAIr1 and 3PAIr2) were developed as efficient three-photon phosphorescence (3PP) agents. Calculations reveal that the introduction of a new group to the molecular scaffold confers a quadruple promotion in three-photon transition probability. Confocal and lifetime imaging of mitochondria using Ir^III complexes as 3PP agents is shown. The complexes exhibit low working concentration (50 nm ), fast uptake (5 min), and low threshold for three-photon excitation power (0.5 mW at 980 nm). The impressive tissue penetration depth (ca. 450 μm) allowed the 3D imaging and reconstruction of brain vasculature from a living specimen.     

Luminescence of alkali halide crystals by multiphoton excitation

Luminescence is observed for CsI and NaBr crystals due to two-photon excitation. Near UV fluorescence of a NaI crystal from three-photon excitation is reported. The mechanism of producing near UV luminescence is discussed.     

Porphyrin bleaching and PDT-induced spectral changes are irradiance dependent in ALA-sensitized normal rat skin in vivo

Photobleaching kinetics of aminolevulinic acid-induced protoporphyrin IX (PpIX) were measured in the normal skin of rats in vivo using a technique in which fluorescence spectra were corrected for the effects of tissue optical properties in the emission spectral window through division by reflectance spectra acquired in the same geometry and wavelength interval and for changes in excitation wavelength optical properties using diffuse reflectance measured at the excitation wavelength. Loss of PpIX fluorescence was monitored during photodynamic therapy (PDT) performed using 514 nm irradiation. Bleaching in response to irradiances of 1, 5 and 100 mW cm-2 was evaluated. The results demonstrate an irradiance dependence to the rate of photobleaching vs irradiation fluence, with the lowest irradiance leading to the most efficient loss of fluorescence. The kinetics for the accumulation of the primary fluorescent photoproduct of PpIX also exhibit an irradiance dependence, with greater peak accumulation at higher irradiance. These findings are consistent with a predominantly oxygen-dependent photobleaching reaction mechanism in vivo, and they provide spectroscopic evidence that PDT delivered at low irradiance deposits greater photodynamic dose for a given irradiation fluence. We also observed an irradiance dependence to the appearance of a fluorescence emission peak near 620 nm, consistent with accumulation of uroporphyrin/coproporphyrin in response to mitochondrial damage.     

The impact of the pi-electron conjugation length on the three-photon absorption cross section of fluorene derivatives

The three-photon absorption cross sections of three different fluorene derivatives, with extended pi-electron conjugation lengths was experimentally measured and compared with shorter pi-electron conjugation length analogs. The effect of the conjugation length on the three-photon absorption cross section sigma(3) (') of this family of molecules has been elucidated. It is demonstrated that sigma(3) (') of the asymmetric compound D-pi-pi-pi-A is 6.6 times larger than its shorter configuration D-pi-A, while for the symmetric compounds D-pi-pi-pi-D and A-pi-pi-pi-A a two-fold enhancement was found relative to their shorter conjugation length analogs. Measurements of the three-photon excitation of these compounds in THF solution (10(-3)M) were accomplished with a tunable optical pulse generation pumped by a 25 ps Nd-YAG laser.     

Early phase laser induced plasma diagnostics and mass removal during single-pulse laser ablation of silicon

The electron number density and temperature during the early phase (<300 ns) of laser-induced plasmas from silicon using a 266-nm, 3-ns Nd:YAG laser were deduced via spectroscopic methods. These parameters were measured as a function of delay time vs. irradiance in the range of 2–80 GW/cm², and compared with crater volume measurements. A dramatic change in plasma characteristics (electron number density, temperature, and degree of ionization) as well as a sharp increase of mass removal was observed when the irradiance was increased beyond a threshold of 20 GW/cm². Possible mechanisms such as inverse bremsstrahlung and self-regulation were used to describe these data in the low irradiance region. Laser self-focusing and critical temperature are discussed to explain the dramatic changes after the irradiance reaches the threshold.     

臭氧分子在激光场中的多光子激发

采用二次量子化方法和酉变换讨论了O3分子在激光场中的多光子激发.推导出了O3分子的振动Hamiltonian 算子、从基态到各激发态的跃迁几率公式,以及O3分子从激光场中吸收的光子数公式,并分析了计算结果.这包括对O3分子伸缩振动能谱的计算及与实验结果的比较,跃迁几率随外场频率的变化、随时间的变化,以及O3分子在辐射场中的能量吸收情况(取光场强度为5×10-2 W/cm2).建立讨论所有具有C2v对称分子从基态到第四激发态以下各态多光子激发问题的模型.     

Investigation of laser induced structures in multiphoton ionization of magnesium

We examine the effect of the excitation of an autoionizing state on electron spectra resulting from multiphoton-ionization. We study the case of above threshold three-photon ionization of magnesium in the region of the 3p3d ¹ P ⁰ resonance, with conditions similar to a two-colour experiment recently performed. The electron spectrum is compared to the partial spectrum corresponding to the region located just above the threshold (which is populated through two-photon absorption from a probing laser). The calculations confirm the experimental observations, a laser induced continuum structure (LICS) is observed in this partial spectrum. The interference process in examined in detail and a Fano-like parametrization of the two-photon ionization electron profile is proposed.     

Competition between multiphoton fluorescence and multiphoton ionization in NO

A nitrogen laser pumped tunable dye laser has been used to observe the three-photon ionization of NO through a two-photon resonance with the C²II state. Fluorescence is also observed from this state. The wavelength dependence of both signals have been measured. A reaction mechanism is postulated, which includes the initial two-photon excitation of the C²II state as the rate-limiting step. This mechanism predicts the observed second-order intensity dependence of the ionization signal and shows that the simple rate equation treatment is valid in this system.     

Three-Photon Infrared Stimulation of Endogenous Neuroreceptors in Vivo

To interrogate neural circuits and crack their codes, in vivo brain activity imaging must be combined with spatiotemporally precise stimulation in three dimensions using genetic or pharmacological specificity. This challenge requires deep penetration and focusing as provided by infrared light and multiphoton excitation, and has promoted two-photon photopharmacology and optogenetics. However, three-photon brain stimulation in vivo remains to be demonstrated. We report the regulation of neuronal activity in zebrafish larvae by three-photon excitation of a photoswitchable muscarinic agonist at 50 pM, a billion-fold lower concentration than used for uncaging, and with mid-infrared light of 1560 nm, the longest reported photoswitch wavelength. Robust, physiologically relevant photoresponses allow modulating brain activity in wild-type animals with spatiotemporal and pharmacological precision. Computational calculations predict that azobenzene-based ligands have high three-photon absorption cross-section and can be used directly with pulsed infrared light. The expansion of three-photon pharmacology will deeply impact basic neurobiology and neuromodulation phototherapies.     

Optical parameters and upconversion fluorescence in Tm3+/Yb3+-doped alkali-barium-bismuth-tellurite glasses

Tm(3+)/Yb(3+)-doped alkali-barium-bismuth-tellurite (LKBBT) glasses have been fabricated and characterized. Density, refractive index, optical absorption, absorption and emission cross-sections of Yb(3+), Judd-Ofelt parameters and spontaneous transition probabilities of Tm(3+) have been measured and calculated, respectively. Intense blue three-photon upconversion fluorescence and near-infrared two-photon upconversion fluorescence were investigated under the excitation of a 980 nm diode laser at room temperature. Wide infrared transmission window, high refractive index and strong blue three-photon upconversion emission of Tm(3+) indicate that Tm(3+)/Yb(3+) co-doped LKBBT glasses are promising upconversion optical and laser materials.     

Excitation volumetric effects (EVE) in metal-enhanced fluorescence

Metal-Enhanced Fluorescence (MEF) effects from different density silver island films (SiFs) and the effects of far-field excitation irradiance on the observed enhancement of fluorescence were studied. It is shown that MEF non-linearly depends on silver nanoparticle (NP) size/density, reaching a maximum value for SiFs made at a deposition time (DT) of ～5 minutes, i.e. just before SiFs become continuous. Numerical simulations of the silver-islands growing on glass revealed that the near-field magnitude depends non-linearly on size and interparticle distance exhibiting dramatic enhancement at ～10 nm distance between the NPs. In addition, a remarkable effect of modulation in MEF efficiency by far-field excitation irradiance has been observed, which can be correlated well with numerical simulations that show an excitation power volume dependence. The near-field volume changes non-linearly with far-field power. This unique observation has profound implications in MEF, which has rapidly emerged as a powerful tool in the biosciences and ultimately allows for tunable fluorescence enhancement factors.     

Resonance-enhanced,multiphoton-ionization spectroscopy of NH3

The multiphoton-ionization spectroscopy of NH₃ has been explored using three-photon ionization through a two-photon resonant intermediate state. This method provides greatly increased sensitivity when compared to three-photon excitation to the intermediate state.     

A DIURNAL RHYTHM IN THE RATE OF LIGHT-INDUCED ELECTRON FLOW IN MAIZE BUNDLE SHEATH STRIPS

Abstract— A technique for the rapid mechanical isolation of bundle sheath strips from Zea mays was used to study the rate of light-induced electron flow as a function of time of day. Seedlings were raised for 10 days in light/dark cycles and then exposed to various intensities of light. The rate of light-induced electron flow was measured five times during the 24-h cycle using a whole cell, H₂O to methyl viologen assay in the presence of gramicidin. The uncoupled rate of electron flow reached a maximum at midday and then decreased for the remainder of the cycle.
If at the end of the 14-h dark period plants were either maintained in darkness, or exposed to very low irradiance white light. then no rhythmicity was observed. If continuous light at higher irradiance was used, then only one additional peak of activity was observed. The minimal light irradiance necessary to allow the continued expression of the rhythm in light/dark cycles is in the range between 11 to 54 W m_-2. A 15-min bright-light (54 W m_-2) pulse administered at dawn is not sufficient to stimulate the expression of the rhythm indicating that the light during the light/dark cycle is doing more than just acting as a "light-on" synchronizing signal. An imposed dark period is also necessary if the rhythm is to be expressed for more than one cycle.     

Limits on the asymmetry parameters for two- and three-photon photofragmentation

The known constraint, -1 相似文献   

Quantum chemical studies of three-photon absorption of some stilbenoid chromophores

Three-photon absorption of a series of donor-acceptor trans-stilbene derivatives is studied by means of density functional theory applied to the third-order response function and its residues. The results obtained by using different functionals are compared with experimental data for similar systems obtained from the literature. With a Coulomb attenuated, asymptotically corrected functional, the excitation energy to the first resonance state is much improved. Comparison with experiment indicates that this is the case for the three-photon cross section as well. In particular, the overestimation of the cross sections and underestimation of excitation energies offered by the density functional theory using common density functionals is corrected for. It is argued that a reliable theory for three-photon absorption in charge transfer and other chromophore systems thereby has been obtained. Further elaboration of the theory and its experimental comparison call for explicit inclusion of solvent polarization and pulse propagation effects.     

RAPID COMMUNICATION

Abstract— We show that the calcium fiuorophore Indo-1 can be excited by simultaneous absorption of three-photons at 885 nra, a wavelength readily available from Ti:sapphire lasers. Three-photon excitation was demonstrated by the emission intensity of Indo-1 which depended on the cube of the laser power, and by a higher anisotropy than was observed for two-photon excitation. Excitation of Indo-1 becomes a two-photon process when the wavelength is decreased to 820 nm. Three-photon excitation was accomplished at a low 17μ concentration of Indo-1. Examination of the spatial profile of the excited Indo-1 showed a smaller volume for three- versus two-photon excitation. These results suggest that three-photon excitation may be useful in fluorescence microscopy using the long wavelength output of Tksapphire lasers, and may provide higher spatial resolution than available using two-photon excitation.     

Emission of Au nanoparticles with and without rhodamine 6G dye

We have observed Stokes and anti-Stokes emission of Au nanoparticles suspended in methanol and rhodamine 6G dye solution. Photoluminescence of Au nanoparticles is a three-step process involving single-photon or three-photon excitation of electron-hole pairs, relaxation of excited electrons and holes, and emission from electron-hole recombination, possibly enhanced by surface plasmons. In the presence of dye, the excitation of anti-Stokes emission of gold involves two-photon absorption in rhodamine 6G molecules followed by the energy transfer to Au nanoparticles with simultaneous absorption of one pumping photon by Au. This mechanism significantly enhances anti-Stokes emission of gold nanoparticles in the presence of dye.     

High-quality manganese-doped zinc sulfide quantum rods with tunable dual-color and multiphoton emissions

We report a simple, fast and green phosphine-free colloidal chemistry to synthesize high-quality wurtzite-type Mn-doped ZnS quantum rods (QRs) with tunable diameters (1.6-5.6 nm), high aspect ratios (up to 50), variable Mn doping levels (0.18-1.60%), and high quantum yields (up to 45%). The electron paramagnetic resonance spectra with modeling reveal the successful doping of paramagnetic Mn(2+) ions in the host ZnS QRs. The Mn-doped ZnS QRs demonstrate tunable dual-color (orange and blue) emissions by tuning the doping levels and UV excitation wavelengths. The orange emission with long decay lifetime (3.3 ms) originates from the doped Mn(2+) states, while the blue emission with fast decay lifetime (0.31 ns) is attributed to the QR surface states. The bright two- and three-photon excitation upconversion luminescence from the Mn-doped ZnS QRs have been observed using tunable near-infrared femtosecond laser. Our strategy provides a versatile route to programmably control the optical properties of anisotropic semiconductor nanomaterials, which may create new opportunities for photonic devices and bioimaging applications.     

Spectroscopic measurements of biphenylene singlet states

We report the observation of fluorescence from higher singlet levels of biphenylene. The molecule was excited by either two- or three-photon absorption and the resulting fluorescence to the ground state was observed in a broad band centered near 265 nm. We have measured the decay time of the lowest excited singlet state to be 240 ± 20 ps in hexane at 300 K.