Carrier recombination under one-photon and two-photon excitation in GaN epilayers

Luminescence properties of 100-mum thick GaN epilayers grown by hydride vapor phase epitaxy (HVPE) over three different substrates: high-pressure grown n-type bulk GaN (HP-n-GaN), high-pressure bulk GaN doped with magnesium (HP-GaN:Mg), and free-standing HVPE lifted-off from sapphire (FS-HVPE-GaN), were compared by means of one-photon and two-photon excitations. The contribution of carrier capture to nonradiative traps was estimated by the analysis of luminescence transients with carrier diffusion taken into account. The estimated values of carrier lifetime of about 3ns and diffusion coefficient of 1cm(2)/s indicate the highest quality of GaN epilayers on FS-HVPE-GaN substrates.     

Change of the photon statistics by one-photon absorption and by simultaneous one-photon and two-photon absorption

With the help of the generating function we investigate the change of photon statistics due to one-photon absorption. Photon distributions with antibunching approach by this process the Poisson distribution. In a second part we consider the simultaneous action of one-photon and two-photon absorption. We present the exact solution and the asymptotic state in the limit of great initial photon numbers.     

Dictyostelium Extracellular Vesicles Containing Hoechst 33342 Transfer the Dye into the Nuclei of Living Cells: A Fluorescence Study

Cells of the eukaryotic unicellular microorganism Dictyostelium discoideum are constitutively resistant to vital staining of their nuclei by the DNA-specific dye Hoechst 33342. By studying the mechanisms of this resistance, we evidenced that these cells expel vesicles containing the dye for detoxification (Tatischeff et al., Cell Mol Life Sci, 54: 476-87, 1998). The question to be addressed in the present work is the potential use of these extracellular vesicles as a biological drug delivery tool, using Hoechst 33342 as a model of a DNA-targeting drug. After cell growth with or without the dye, vesicles were prepared from the cell-free growth medium by differential centrifugation, giving rise to two types of vesicles. Negative staining electron microscopy showed their large heterogeneity in size. Using fluorescence techniques, data were obtained on the dye loading and its environment inside the vesicles. By UV video-microscopy, it was demonstrated that the dye-containing vesicles were able to deliver it into the nuclei of naive Dictyostelium cells, thus overcoming their constitutive resistance to the free dye. A vesicle-mediated dye-transfer into the nuclei of living human leukaemia multidrug resistant K562r cells was also observed.     

Fluorescence of horse liver alcohol dehydrogenase using one- and two-photon excitation

We examined the steady-state and time-resolved emission of liver alcohol dehydrogenase resulting from one-photon and two-photon excitation. Previous studies with one-photon excitation revealed that the two nonidentical tryptophan residues display different emission spectra and decay times. The use of two-photon excitation resulted in similar emission spectra, multiexponential intensity decays, time-resolved emission spectra, and anisotropy decays as was observed for one-photon excitation. These results suggest that both nonidentical tryptophan residues are excited to a similar extent for one- and two-photon excitation. However, the limiting anisotropy (r ₀) with two-photon excitation from 585 to 610 nm is below 0.1 and appears distinct from that observed previously forN-acetyl-l-tryptophanamide.Abbreviations LADH liver alcohol dehydrogenase - -NAD⁺ -nicotinamide adenine dinucleotide - OPE one-photon excitation - OPIF one-photon induced fluorescence - TPE two-photon excitation - TCSPC time-correlated single photon counting - TPIF two-photon induced fluorescence     

Fluorescence of Hoechst 33258 Complexes with Serum Albumin and DNA

Journal of Applied Spectroscopy - Fluorescence spectroscopy was used to study the interactions of the bisbenzimidazole compound Hoechst 33258 with bovine serum albumin and calf thymus DNA. The...     

Two photon-induced fluorescence intensity and anisotropy decays of diphenylhexatriene in solvents and lipid bilayers

We measured the fluorescence intensity and anisotropy decays of 1,6-diphenyl-1,3,5-hexatriene (DPH)-labeled membranes resulting from simultaneous two-photon excitation of fluorescence. Comparison of these two-photon data with the more usual one-photon measurements revealed that DPH displayed identical intensity decays, anisotropy decays, and order parameters for one- and two-photon excitation. While the anisotropy data are numerically distinct, they can be compared by use of the factor 10/7, which accounts for the two-photon versus one-photon photoselection. The increased time 0 anisotropy of DPH can result in increased resolution of complex anisotropy decays. Global analysis of the one- and two-photon data reveals consistency with a single apparent angle between the absorption and the emission oscillators. The global anisotropy analysis also suggests that, except for the photoselection factor, the anisotropy decays are the same for one-and two-photon excitation. This ideal behavior of DPH as a two-photon absorber, and its high two-photon cross section, makes DPH a potential probe for confocal two-photon microscopy and other systems where it is advantageous to use long-wavelength (680- to 760-nm) excitation.     

General theory for the interference of two-photon and one-photon processes in semiconductor heterostructures

We develop a general theory for the dynamics of multi-photon processes in semiconductor heterostructures. The resulting effective multi-band Bloch equations describe the dynamics of electrons in the reduced set of bands between which the optical pulses induce quasi-resonant transitions. The model is specialized to the case of interfering one- and two-photon transitions across the band gap. The withdrawn bands are included as intermediate states for an effective interaction that is quadratic in the electromagnetic fields. The benefit of this perturbative approach is to lead to equations of motion for slowly varying quantities only, in the spirit of the rotating wave approximation. Coulomb interaction and relaxation can also easily be included. Finally, a general expression for the time dependent polarization current that is consistent with the approximations involved by the effective multi-band Bloch equations is derived.     

Fluorescence intensity and anisotropy decays of the intrinsic tryptophan emission of hemoglobin measured with a 10-GHz fluorometer using front-face geometry on a free liquid surface

We measured the intensity and anisotropy decays of the intrinsic tryptophan emission from hemoglobin solutions obtained using a 10-GHz frequency-domain fluorometer and a specially designed cuvette which allows front-face excitation on a free liquid surface. The cuvette eliminates reflections and stray emissions, which become significant for low-intensity fluorescence such as in hemoglobin. Three lifetimes are detectable in the subnanosecond range. The average lifetime of hemoglobin emission is ligand dependent. The measured values of average lifetimes are 91, 174, and 184 ps for deoxy-, oxy-, and carboxyhemoglobin, respectively. Fluorescence anisotropy decays of oxy-, deoxy-, and carbonmonoxyhemoglobin can be fitted with up to three correlation times. When three components are used, the floating initial anisotropyr _o is, in each case, higher than the steady-state anisotropy of tryptophan in vitrified solution. For deoxy hemoglobin it is close to 0.4. The data are consistent with an initial loss of anisotropy from 0.4 to about 0.3 occurring in the first 2 ps.     

Nonappearance of some two-photon and one-photon absorption lines between the stark sublevels of rare-earth ions

Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 54, No. 2, pp. 198–201, February, 1991.     

CdTe量子点与罗丹明B水溶液体系下的双光子激发荧光共振能量转移

采用时间分辨荧光光谱技术研究了在双光子激发下不同尺寸的量子点与罗丹明B 之间的荧光共振能量转移. 研究结果表明, 在800 nm的双光子激发条件下, 体系间能量转移效率随着供体吸收光谱与受体荧光光谱的光谱重叠程度增加而增加; 理论分析表明, 供体和受体间的Förster半径增加是导致其双光子能量转移效率增大的物理原因. 同时, 研究了罗丹明B浓度对荧光共振能量转移效率的影响. 研究结果表明, 量子点的荧光寿命随着罗丹明B浓度的增加而减小; 量子点与罗丹明B之间的荧光共振能量转移效率随着罗丹明B浓度的增加而增加; 当罗丹明B浓度为3.0×10^-5 mol·L^-1时, 双光子荧光共振能量转移效率为40.1%.     

On the interpretation of fluorescence anisotropy decays from probe molecules in lipid vesicle systems

Measurements of fluorescence depolarization decays are widely used to obtain information about the molecular order and rotational dynamics of fluorescent probe molecules in membrane systems. This information is obtained by least-squares fits of the experimental data to the predictions of physical models for motion. Here we present a critical review of the ways and means of the data analysis and address the question how and why totally different models such as Brownian rotational diffusion and wobble-in-cone provide such convincing fits to the fluorescence anistropy decay curves. We show that while these models are useful for investigating the general trends in the behavior of the probe molecules, they fail to describe the underlying motional processes. We propose to remedy this situation with a model in which the probe molecules undergo fast, though restricted local motions within a slowly rotating cage in the lipid bilayer structure. The cage may be envisaged as a free volume cavity between the lipid molecules, so that its position and orientation change with the internal conformational motions of the lipid chains. This approach may be considered to be a synthesis of the wobble-in-cone and Brownian rotational diffusion models. Importantly, this compound motion model appears to provide a consistent picture of fluorescent probe behavior in both oriented lipid bilayers and lipid vesicle systems.     

Axial and conical parametric emissions from potassium atoms under two-photon fs excitation

We report on the emissions near the 5P_3/2,1/2–4S_1/2 and 4P_3/2,1/2–4S_1/2 transitions of potassium atoms which are excited by a fs laser beam. The field at the transition 5P_3/2,1/2–4S_1/2 is mainly the result of a parametric process with an axial profile when the excitation frequency is tuned above resonance and a conical one below resonance. Similar but not identical far-field patterns were also observed for the 4P_3/2,1/2–4S_1/2 emission. No amplified spontaneous emission was observed for the fs case, in contrast to the ns excitation for the 4P_3/2,1/2–4S_1/2 transition.     

双光子技术在三维成像和三维存储技术中的应用

对双光子过程的空间分辨能力进行了理论分析。阐明了双光子技术在三维成像和三维存储中的独特优势。着重介绍了双光子技术与扫描共焦显微术、近场显微术相结合进行三维成像 ,以及一种多焦点多光子显微术和用连续光源激发的双光子三维成像技术的研究和进展情况。对建立在共焦显微镜基础之上的双光子三维光存储和微细加工方面的研究也作了回顾与展望     

Single- and two-photon excitation of a GaAs random laser

We have demonstrated an anti-Stokes random laser emission at lambda approximately 885 nm in micrometer-size powder of GaAs. The maximal pumping wavelength, lambda=1,300 nm, exceeded the lasing wavelength by nearly 50%. We conclude that both one- and two-photon pumping contribute to optical excitation of the material, with the two-photon pumping being a key process in creating the population inversion and gain.     

Comparison of the Raman detection limit for IRS- and CARS-spectroscopy at excitation near molecular one-photon resonance

The nonlinear Raman methods IRS and CARS are compared according to the signal detectability at excitation of molecules under the condition of one-photon resonance. At one photon resonance it is the background contribution resulting from the scattering molecules themselves that determines the maximum attainable signal to noise ratio. These contributions acting for IRS and CARS respectively are compared. The essential difference between IRS and CARS results from a 3rd order saturation contribution to IRS, which may mask the IRS Raman signal near exact resonance while it does not contribute to CARS. This gives to CARS the preference before IRS at resonance excitation. The situation for IRS with respect to background is similar to that of spontaneous Raman scattering, where the resonance fluorescence — corresponding to the saturation contribution at IRS — masks the resonance Raman signal.     

Light Reflection from the end of a semi-infinite semiconductor under conditions of two-photon excitation of biexcitons

It is shown that reflectivity of the end of a semi-infinite semiconductor under conditions of twopulse two-photon excitation of biexcitons from the ground state of the crystal is characterized by a multistable behavior as a function of amplitudes of the incident pulse fields.     

The search for a molecule to measure an autocorrelation trace of the second/third harmonic emission of a Ti:sapphire laser based on two-photon resonant excitation and subsequent one-photon ionization

The temporal profile of the second and third harmonic emissions of a Ti:sapphire laser was measured using an autocorrelator consisting of a mass spectrometer as a two-photon-response detector. A number of organic compounds that are potentially applicable for two-photon excitation and subsequent one-photon ionization were investigated using density functional theory calculations. N,N′-dimethylaniline and acetonitrile were used for the measurement of the pulse width for the second and third harmonic emissions of the Ti:sapphire laser. This approach has the potential for use in measuring pulse widths as short as 1–3 fs in the ultraviolet region.     

Solvent effect on the steady-state fluorescence anisotropy of two-photon absorbing fluorene derivatives

The effect of solvent polarity on the fluorescence properties of several two-photon absorbing fluorene derivatives was investigated. A strong solvatochromic effect was observed for symmetrical fluorene compound, which can be explained by large changes in the quadrupolar moment under excitation. Limiting values of excitation anisotropy of the investigated fluorenes exhibited a dependence on solvent polarity, and the angles between the absorption and emission transition dipole moments decrease in polar solvents by more than a factor of two.