Two-photon Excitation Conditions Research Articles

Using a Streak Camera to Resolve the Motion of Molecular Excited States with Picosecond Time Resolution and 150 nm Spatial Resolution

A one-dimensional streak camera imaging (1D-SCI) experiment to visualize the spatial motion of luminescent excited-state species in condensed phase systems is presented. A femtosecond pulse is focused through an objective lens to create a localized excited-state population, using either one-photon or two-photon excitation conditions. The time-dependent emission from this spot is then imaged onto the horizontal entrance slit of a streak camera. The resulting one-dimensional projection of the emission spot is detected with picosecond time resolution. Using this setup, the ability to resolve broadening of the excitation spot due to diffusive motion on the order of 150 nm is demonstrated. The 1D-SCI technique is successfully employed to image the translational diffusion of excited-state platinum octaethylporphyrin molecules in a toluene solution at various concentrations. In addition, the absence of long-range (>1 μm) electronic energy transfer in the polymer polyvinylcarbazole is confirmed. In both of these ...

Femtosecond Spectroscopic Studies of the One- and Two-Photon Excited-State Dynamics of 2,2,17,17-Tetramethyloctadeca-5,9,13-trien-3,7,11,15-tetrayne: A Trimeric Oligodiacetylene

The excited-state dynamics of an oligomer of polydiacetylene, 2,2,17,17-tetramethyloctadeca-5,9,13-trien-3,7,11,15-tetrayne, dissolved in n-hexane have been studied by femtosecond fluorescence upconversion and polarized transient absorption experiments under one- and two-photon excitation conditions. Spectroscopically monitoring the population relaxation in the excited states in real time results in a distinct time separation of the dynamics. It has been concluded that the observed dynamics can be fully accounted for on the basis of the two lower excited states of the target molecule. The S1 (2(1)Ag) state, which cannot be excited from the ground state with one-photon absorption, is verified to be populated via internal conversion in 200+/-40 fs from the strong dipole-allowed S2 (1(1)Bu) state. The population in the "hot" S1 state subsequently cools with a time constant of 6+/-1 ps and decays back to the ground state with a lifetime of 790+/-12 ps.

An Iridium(III) Complex that Exhibits Dual Mechanism Nonlinear Absorption

The photophysical properties of the complex (L)Ir(ppy)(2)(+), where ppy = 2-phenylpyridine and L = 4,4'-(2,2'-bipyridine-5,5'-diylbis(ethyne-2,1-diyl))bis(N,N-dihexylaniline), have been investigated under one- and two-photon excitation conditions. In THF solution, the complex exhibits broad ground-state absorption with lambda(max) approximately 500 nm and weak photoluminescence with lambda(max) approximately 730 nm. Excitation of (L)Ir(ppy)(2)(+) at 355 nm produces a long-lived excited state (tau approximately 1 mus) that features a strong excited-state absorption in the near-infrared (lambda(max) approximately 875 nm, Deltaepsilon approximately 6.1 x 10(4) M(-1) cm(-1)). Photoluminescence and transient absorption studies of (L)Ir(ppy)(2)(+) carried out using 5 ns, 1064 nm pulsed excitation demonstrate that the same long-lived and strongly absorbing excited state can be efficiently produced by two-photon absorption. Solutions of the complex in THF display nonlinear absorption of 5 ns, 1064 nm pulses in a process that is believed to involve a combination of two-photon absorption and reverse saturable absorption.

Two-Photon Excitation of Substituted Enediynes

Electronic spectroscopy of nine benzannelated enediynes and a related fulvene was studied under one-photon and two-photon excitation conditions. We utilize measured absorbance and emission spectra and time-resolved fluorescence decays of these molecules to calculate their radiative lifetimes and fluorescence quantum yields. The fluorescence quantum yields for the other compounds were referenced to the fluorescence quantum yield of compound 3 and used to determine relative two-photon absorption cross-sections. Further insight into experimental studies has been achieved using time-dependent density functional (TD-DFT) computations. The probability of two-photon absorption (TPA) increases noticeably for excitation to the higher excited states. The photophysical properties of benzannelated enediynes are sensitive to substitutions at both the core and the periphery of the enediyne chromophore. Considerably enhanced two-photon absorption is observed in an enediyne with donor substitution in the middle and acceptor substitution at the termini. Excited states with B symmetry are not active in TPA spectra. From a practical point of view, this study extends the range of wavelengths applicable for activation of the enediyne moiety from 350 to 600 nm and provides a rational basis for future studies in this field. Our theoretical computations confirmed that lowest energy TPA in benzannelated enediynes involves different orbitals than lowest energy one-photon absorbance and provided further support to the notion that introduction of donor and acceptor substituents at different ends of a molecule increases TPA.

Singlet oxygen generation via two-photon excited FRET.

A new approach to two-photon excited photodynamic therapy has been developed. A dendritic array of eight donor chromophores capable of two-photon absorption (TPA) was covalently attached to a central porphyrin acceptor. Steady-state fluorescence measurements demonstrated that the donor chromophores transfer excited-state energy to the porphyrin with 97% efficiency. Two-photon excitation of the donor chromophores at 780 nm resulted in a dramatic increase in porphyrin fluorescence relative to a porphyrin model compound. Enhanced singlet oxygen luminescence was observed from oxygen-saturated solutions of the target compound under two-photon excitation conditions.

Assessment of One- and Two-Photon Excited Luminescence for Directly Measuring O2, pH, Na+, Mg2+, or Ca2+ in Optically Dense and Biologically Relevant Samples

We compare the emission spectra and analytical response profiles of several semiselective luminescent probes that are commonly used to quantify nonfluorescent analytes (ruthenium(II) tris(2,2′-bipyridyl) dication, tris(4,7-diphenyl-1,10-phenanthroline)ruthenium(II) dication, SNARF®-1, fluorescein, Rhodol Green™, and Sodium™, Magnesium™, and Calcium Green™) when they are excited under one- and two-photon excitation conditions in water, aqueous solutions of dyes or fluorophores, undiluted mouse or human serum, human urine, and mouse whole blood. The results from this work can be summarized as follows. First, in cases where the probes possess ground states that are composed of multiple species in equilibrium (e.g., acid and base forms of a luminophore), the analyte-dependent emission spectra can be significantly different under one- and two-photon excitation conditions due to differences in the relative one- and two-photon cross-sections associated with the individual ground-state species. Second, for those probes that exhibit one- and two-photon dependent emission spectra, an assessment of the analytical response vs. target analyte concentration profiles show that the response dynamic range can be improved by up to 100 fold, but the response sensitivity can decrease by up to 25% for two- vs. one-photon excitation. Third, two-photon excited luminescence provides a means to use these semiselective luminescent probes to quantify nonfluorescent analytes directly within optically dense mixtures, including solutions of dyes, mouse or human serum, human urine, and mouse whole blood. Finally, two-photon excited luminescence allows one to quantify nonfluorescent analytes in a spatially defined manner within complex samples.

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.

A parallel multiharmonic frequency-domain fluorometer for measuring excited-state decay kinetics following one-, two-, or three-photon excitation.

We report on the performance of a new, multiharmonic frequency-domain instrument that uses the high harmonic content of a passively mode-locked, pulse-picked femto-second Ti-sapphire laser as the excitation source for the determination of one-, two-, or three-photon excited time-resolved fluorescence anisotropy and intensity decay kinetics. In operation, the new instrument can provide a complete frequency-domain data set at 100 modulation frequencies in less than 1 min. The new instrument exhibits 5-10-ps measurement precision and it can rapidly and accurately recover complex excited-state fluorescence anisotropy and intensity decay kinetics under one-, two-, or three-photon excitation for dilute or optically dense samples that exhibit single or multiexponential decay kinetics. This latter aspect of the instrument is demonstrated by successfully determining the excited-state intensity decay kinetics for a dilute aqueous solution of rhodamine 6G dissolved in a high concentration of bromocresol green. This approach is extended by determining the excited-state fluorescence intensity decay kinetics of dilute fluorescein directly in undiluted, whole blood as a function of pH under two-photon excitation conditions. The high-speed capabilities of the new instrument are exploited by performing two-photon excited fluorescence anisotropy decay experiments on the fly for site-selectively labeled bovine serum albumin as it undergoes enzymatic digestion by trypsin.

Nonlinear interaction of an ultrashort light pulse with a thin semiconductor film under conditions of two-photon excitation of biexcitons

Analytic solutions are used to describe a transient interaction of resonant laser radiation with a thin semiconductor film under the conditions of two-photon excitation of biexcitons from the ground state of a crystal. Several nonlinear effects are predicted for the transmission of ultrashort pulses by a thin film: intensity discrimination, compression and transformation of the profile of a pulse incident on a film, and the possibility of 'superradiative' amplification of a short pulse traversing a previously excited crystal.

Bulk photoluminescence of molecular crystals under two-photon excitation conditions

A study is reported of the spectral characteristics of two-photon-excited luminescence of polycrystalline samples of anthracene, diphenyl, and fluorene. The luminescence spectra recorded under two-photon, one-photon, and x-ray excitation conditions were compared. Considerable differences between these spectra were found. The main factors influencing the nature of the two-photon-excited luminescence spectra are reabsorption, surface energy states, and transition from spontaneous luminescence to superluminescence.

Laser subnanosecond fluorescence spectrometer with single photon counting

A description is given of a new automated laser fluorescence spectrometer. The fluorescence is excited using picosecond (YAG:Nd and dye) lasers at a high pulse repetition frequency (5 kHz). A recording system based on counting single photons is characterized by subnanosecond time resolution. Examples are reported of the use of this spectrometer in the study of the luminescence of biomolecules under one- and two-photon excitation conditions.

Spectral and energy characteristics of four-photon parametric scattering in sodium vapor

An analysis was made of four-photon interaction processes under conditions of two-photon resonance excitation of the 3D sodium level by two-frequency radiation from a single-pulse picosecond YAG:Nd laser with frequency doubling and a KDP optical parametric oscillator. The spatial and frequency spectra of the four-photon parametric scattering (FPPS) were recorded and studied experimentally at various sodium vapor densities (1015–1017 cm −3) and also using collinear and noncollinear excitation. It was found that the observed conical structure of the FPPS radiation may be interpreted by an analysis of whether the frequency and spatial phase-matching conditions were satisfied. Dependences of the intensity of the FPPS radiation on the intensity of the exciting radiation, the sodium vapor density, and the detuning of the exciting radiation frequency from the two-photon resonance were obtained.

Competition between amplified spontaneous emission and the four-wave-mixing process.

Competition between amplified spontaneous emission (ASE) and the four-wave-mixing (FWM) process has been observed under conditions of two-photon resonant excitation of the sodium 3d level. The nature of the competition is that the FWM process is able to prevent the occurrence of ASE, even though the gain of the ASE process calculated in the absence of competition effects is much larger than that of FWM. The ASE is suppressed because the fields generated by the FWM process create a new excitation pathway connecting the ground and 3d levels, and under quite general conditions this pathway interferes destructively with that due solely to the applied laser field. These effects are modeled theoretically by solving perturbatively the density-matrix equations of the atomic system, thereby determining the population in the upper level and the nonlinear polarization of the medium. The coupling between the various optical fields due to the nonlinear polarization is described by coupled amplitude equations. The solution to these equations predicts that when the wave-vector mismatch is not too large the fields evolve spatially to reach steady-state values, and that the population excited to the 3d level by the total steady-state optical field is much smaller than that due to the incident laser field alone. We have observed experimentally the suppression of ASE by FWM and have observed that this suppression does not occur when the medium is excited with counterpropagating beams that cannot efficiently excite the FWM process. In addition, we have conducted a series of experiments that shows that the degree of suppression of ASE depends on the intensity and focusing characteristics of the incident laser as expected on the basis of our theoretical model.