Absolute Two-photon Absorption Cross Sections Research Articles

Polymer Monoliths Containing Two-Photon Absorbing Phenylenevinylene Platinum(II) Acetylide Chromophores for Optical Power Limiting.

A series of platinum(II) acetylide complexes containing p-phenylenevinylene and moieties end-capped with triphenylamine groups have been incorporated into poly(methyl methacrylate) (PMMA) monoliths for optical power limiting applications. The one- and two-photon photophysical properties were investigated and compared to the photophysical properties in THF. The absolute two-photon absorption cross-section values for the monolith samples were measured and are comparable to the values obtained in solution. In the PMMA monoliths, the complexes retained the important two-photon absorption and reverse saturable absorption properties necessary for optical power limiting via dual mode mechanism, and their strong nonlinear absorption property was demonstrated by the open-aperture Z-scan method. Photostability studies of the p-phenylenevinylene platinum(II) acetylide complexes showed two photodegradation processes: a trans-to-cis isomerization and a singlet-oxygen sensitized self-oxidative cleavage. The photostability of the least photostable complex TPV0 was increased upon incorporation into a PMMA matrix.

One- and Two-Photon Absorption in CdS Nanodots and Wires: The Role of Dimensionality in the One- and Two-Photon Luminescence Excitation Spectrum

We investigate the spectral dependence of the linear and two-photon absorption of wurtzite CdS nanoparticles (dots and rods) by means of quantitative one- and two-photon photoluminescence excitation spectroscopy and effective mass theory modeling. Absolute two-photon absorption cross sections free from spectrally varying beam related uncertainties are obtained by means of a new reference dye-based method. The two-photon spectrum features of rods strongly differ from those of dots, due to the distinct energy structure of quasi-one-dimensional systems. The transversal confinement is found to dominate the energy of the absorption maxima while the longitudinal one dominates their absorption intensity. This suggests two-photon transition energy and intensity can be controlled independently in nanorods. For both geometries we observe a sizable spectral shift between the first one- and two-photon absorption maxima, which we conclude is inherent to the small rates of near-bandgap two-photon transitions rather than to the particular geometry of the absorber. The provided understanding of the spectral dependence of the two-photon absorption of CdS dots and rods is of strong interest for the design of nanocrystals with optimized two-photon absorption properties for bioimaging and phototherapy applications.

Two-Photon Photosensitized Production of Singlet Oxygen: Optical and Optoacoustic Characterization of Absolute Two-Photon Absorption Cross Sections for Standard Sensitizers in Different Solvents

Singlet molecular oxygen, O2(a1Deltag), can be produced upon resonant two-photon excitation of a photosensitizer. In the present study, two molecules that have received recent attention in studies of nonlinear organic materials were characterized for use as standard two-photon sensitizers: 2,5-dicyano-1,4-bis(2-(4-diphenylaminophenyl)vinyl)-benzene, CNPhVB, and 2,5-dibromo-1,4-bis(2-(4-diphenylaminophenyl)vinyl)-benzene, BrPhVB. Absolute two-photon absorption cross sections, delta, were independently determined for these molecules using two techniques that have heretofore not been applied to this problem: an optical technique (time-resolved detection of O2(a1Deltag) phosphorescence) and a nonoptical technique (a time-resolved laser-induced optoacoustic experiment). For experiments performed in toluene, a solvent commonly used for such nonlinear optical studies, appreciable absorption by the solvent itself complicates the measurements. In cyclohexane, however, delta values could be obtained without the interfering effects of solvent absorption. On the basis of these results, we discuss key aspects of the respective techniques used to quantify values of delta. The information reported herein provides some explanation for the lack of consensus that is routinely observed in published values of delta, certainly for experiments performed in aromatic solvents such as toluene and benzene.

A novel fluorophore for two-photon-excited single-molecule fluorescence

We have studied the two-photon excited fluorescence properties of a new fluorophore, DCDHF-6, at single-molecule and bulk concentrations. This molecule features several beneficial properties including suppressed blinking, high fluorescence quantum yield, and viscosity-dependent fluorescence. Here, analysis of the two-photon excited fluorescence (TPF) behavior of single DCDHF-6 molecules demonstrates their applicability to experiments combining single-molecule and two-photon microscopy. A method is developed for measuring the absolute two-photon absorption cross-section δ of a single dye copy, and DCDHF-6 is shown to have a δ of 44 GM at an excitation wavelength of 940 nm.

Frequency-domain gratings by simultaneous absorption of two photons

We study frequency-domain coherence gratings produced by interference in process of simultaneous absorption of two photons in an inhomogeneously broadened medium. In our experiment, we create a grating at a visible wavelength by illuminating an organic dye-doped polymer at low temperature with phase-locked pairs of 100-fs laser pulses at a near-infrared wavelength equal to two times the transition wavelength. We show that two-photon excited coherence can be detected either by measuring the spectrum of fluorescence or, after prolonged illumination, by observing spectrally modulated persistent spectral hole. We analyze the phase and contrast of the grating at different temperatures and obtain, for the first time to our knowledge, temperature dependence of Debye–Waller factor and phonon spectrum of two-photon transition of organic molecule in presence of large inhomogeneous broadening. We also study wavelength dependence of absolute two-photon absorption cross-section in a series of porphyrins and show that at some wavelengths in porphyrin B-band region the cross-section is resonantly enhanced by nearby one-photon allowed Q-transition. These experiments indicate that large organic molecules such as porphyrins are promising candidates for further investigation of multi-photon excitation of coherence.

Sensitive measurement of absolute two-photon absorption cross sections

We develop a method for measuring absolute two-photon absorption cross sections (σ2) and employ it to determine the σ2 of Rhodamine-6G in methanol (16.2±2.4 GM at 806 nm). Our measurement calibrates the relative excitation spectrum previously reported for this chromophore. The method is based on our derivation of an analytical expression describing the transmission of Gaussian laser pulses through a two-photon absorbing medium. The expression is valid for arbitrary absorber thickness, at all distances from the focus. This generalizes the prevalent “z-scan” (translation of the sample along the beam direction) technique for measuring two-photon absorbance, removing the requirements of a “thin” (thickness ≪ Rayleigh range of the focused laser beam) sample and of placing the sample at the focus. This leads to an improvement of the sensitivity of the technique by over two orders of magnitude, enabling measurement of the two-photon absorption cross sections of even weakly absorbing specimens at moderate intensities. The results are significant for applications such as nonlinear microscopy, optical data storage and optical power limiting.

Two-photon excitation of atomic oxygen at 200.6, 192.5, and 194.2 nm: Absolute cross sections and collisional ionization rate constants.

Absolute two-photon absorption cross sections have been measured for the np $^{3}P_{2}$,1,0\ensuremath{\leftarrow}2p $^{3}\mathrm{P}_{2}$ transitions in atomic oxygen (n=4 and 5) using the technique of two-photon--excited fluorescence. For the n=4 transition at 200.6 nm the integrated cross section is ${\mathcal{J}}_{J\mathcal{'}}$${\ensuremath{\sigma}}_{0}^{(2)}$(J'\ensuremath{\leftarrow}2) =(4.8\ifmmode\pm\else\textpm\fi{}2.4)\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}36}$ ${\mathrm{cm}}^{4}$. The corresponding number for the n=5 transition at 192.5 nm is (0.${7}_{\ensuremath{-}0.5}^{+0.7}$)\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}36}$ ${\mathrm{cm}}^{4}$. These numbers are in moderately good agreement with ab initio calculations presented previously [Phys. Rev. A 34, 199 (1986)] for n=4 and in this paper for n=5. Calculations are also reported for the 4f $^{3}F_{4}$,3,2\ensuremath{\leftarrow}2p $^{3}\mathrm{P}_{\mathrm{J}\mathcal{'}\mathcal{'}}$ transition at 194.2 nm. Atoms in the np $^{3}P$ excited states produce positive ions and electrons upon collisions with ${\mathrm{O}}_{2}$. In each case, Penning ionization and associative ionization are possible, and cannot be distinguished.Total rate constants for collisional ionization by ${\mathrm{O}}_{2}$ at room temperature are (2\ifmmode\pm\else\textpm\fi{}1)\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}10}$ ${\mathrm{cm}}^{3}$ ${\mathrm{sec}}^{\mathrm{\ensuremath{-}}1}$for n=4 and (4\ifmmode\pm\else\textpm\fi{}3)\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}10}$ ${\mathrm{cm}}^{3}$${\mathrm{sec}}^{\mathrm{\ensuremath{-}}1}$ for n=5. Einstein A coefficients are calculated for all allowed fluorescence transitions originating from the 4p $^{3}P$ and 5p $^{3}P$ electronic states. For each state, the sum of the calculated Einstein A coefficients agrees well with the experimental zero-pressure decay rate. Rate constants for collisional removal of population from these states by the background gas (which is mostly ${\mathrm{O}}_{2}$) are 1.5\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}9}$ ${\mathrm{cm}}^{3}$${\mathrm{sec}}^{\mathrm{\ensuremath{-}}1}$ for n=4 and 7.2\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}10}$ ${\mathrm{cm}}^{3}$ ${\mathrm{sec}}^{\mathrm{\ensuremath{-}}1}$ for n=5. The photoionization cross section for atoms in the 4p $^{3}P$electronic state is (3\ifmmode\pm\else\textpm\fi{}2)\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}18}$ ${\mathrm{cm}}^{2}$. For atoms in the 5p $^{3}P$ state the photoionization cross section is less than 1\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}17}$ ${\mathrm{cm}}^{2}$.

Luminescence of liquids and solids by two-photon excitation

Absolute two-photon absorption cross sections of various neat liquids and alkali halide crystals were measured. Fluorence excitation spectra are also used to study the excitation and decay mechanisms of liquids and crystals.