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Journal of the Optical Society of America B

Three-pulse scattering for femtosecond dephasing studies: theory and experiment

Andrew M. Weiner, S. De Silvestri, Erich P. Ippen

Published Apr 1, 1985• 315 citations
Publisher

Abstract

A novel transient three-pulse scattering technique for measuring ultrafast dephasing times in condensed matter is analyzed using a perturbative solution of the density matrix equation. The advantages of this technique include subpulsewidth resolution, a clear distinction between homogeneous and inhomogeneous broadening, and sensitivity to spectral cross-relaxation. Its application to the case of a multilevel resonance is also considered. We report results of femtosecond dephasing experiments with dye molecules in liquids and in a polymer host. The dephasing time is determined to be less than 20 fsec for dyes in solution at room temperature. At low temperatures in polymers, a transition from homogeneous to inhomogeneous broadening has been observed and studied as a function of temperature.

DOI
10.1364/josab.2.000654

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The nonlinear absorption of intense laser light is used to obtain information about the times required for spectral cross relaxation (${T}_{3}$) and collisional dephasing (${T}_{2}$) in organic-dye solutions. The steady-state transmission experiment of Huff and DeShazer indicates that ${T}_{2}$ is in the range 0.1-1 psec for cryptocyanine-methanol, while ${T}_{3}$ is in the range 1-10 psec. For DDI-glycerin (1, 1\ensuremath{'}-diethyl-2, 2\ensuremath{'}-dicarbocyanine-iodide-glycerin), measurements of ultrashort-pulse transmission as a function of incident energy and optical polarization suggest a value ${T}_{2}=0.4\ifmmode\pm\else\textpm\fi{}0.2$ psec for that system. These results are based on a new set of rate equations for absorption by inhomogeneously broadened organic-dye solutions. The equations display the effects of spectral cross relaxation and are not limited to light intensities less than those at which coherent interaction effects become important. For the case of ultrashort incident pulses, the equations are extended to account for excited-state absorption, the dependence of absorption on optical polarization, and the existence of a multiplicity of interacting excited states.

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Photon Echo and Single Molecule Fluorescence Measurements of Organic Dyes in Thin Polymer Films

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The extraordinarily short optical dephasing time of organic dye molecules in polymer hosts is known to be under 20 fs between 100 and 200 K. Short dephasing times suggest enormously broadened homogeneous linewidths on the order of 10 nm. Direct observation of single dye molecule fluorescence spectra shows a similarly broadened line, giving comparable dephasing times for room temperature measurements. While this single molecule linewidth agrees with that of the bulk sample, it is inaccurate to attribute this broadening to homogeneous mechanisms. Repeated single molecule fluorescence measurements permit determination of the homogeneous nature of this linewidth, as distinct from the inhomogeneous nature of dispersion in bulk samples. Spectral diffusion in single molecule spectra is evidenced by multiple peaks, visible despite broad spectra lines at room temperature. In the particular experiment discussed in this thesis, fluorescence from single organic dye molecules in thin polymethyl methacrylate films was studied with scanning confocal microscopy. Photon antibunching exhibited by the photon-pair correlation function g(2) (τ) at τ = 0 confirms that single molecule emission spectra were obtained.

•Jan 1, 2003•3 citations
A novel transient three-pulse scattering technique for measuring ultrafast dephasing times in condensed matter is analyzed using a perturbative solution of the density matrix equation. The advantages of this technique include subpulsewidth resolution, a clear distinction between homogeneous and inhomogeneous broadening, and sensitivity to spectral cross-relaxation. Its application to the case of a multilevel resonance is also considered. We report results of femtosecond dephasing experiments with dye molecules in liquids and in a polymer host. The dephasing time is determined to be less than 20 fsec for dyes in solution at room temperature. At low temperatures in polymers, a transition from homogeneous to inhomogeneous broadening has been observed and studied as a function of temperature.
Summary form only given. Coherent optical transients such as photon echo are a versatile method far measurement of the homogeneous absorption spectrum of molecular systems in the presence of strong inhomogeneous broadening. Such experiments require measurement of the decay of the optical coherence, typically accomplished by gradually varying the time delay between the excitation pulses. Time-domain information is then Fourier transformed to frequency domain by a computer. In this paper we demonstrate a new method that yields the homogeneous spectrum in a single shot of femtosecond laser pulse illumination by performing Fourier transformation of coherent optical transient signal with a lens. We apply this method to measure the homogeneous spectrum of dye molecules in a polymer film in a broad range of temperatures.
Electronic dephasing of solutions of all-trans-bis-(dimethylamino)-heptamethinium chloride, its photoisomer, and of 1,1∣IH{1}^{|\mathrm{I}\mathrm{H}}1∣IH,3,3,3∣IH{3}^{|\mathrm{I}\mathrm{H}}3∣IH,3∣IH{3}^{|\mathrm{I}\mathrm{H}}3∣IH-hexamethylindotricarbocyanine iodide (HITCI) is studied using forced light scattering by broad-bandwidth incoherent pump lasers in the wavelength region from 440 to 770 nm. The dephasing slows down as the pump laser wavelength is tuned over the 0-0 transition. Photoisomer and parent heptamethine molecule show comparable behavior in dephasing experiments. Absorption line-shape analysis including resonance Raman experiments indicates a considerable spectral cross relaxation in the absorption band of the parent molecule. The results for HITCI are close to those reported for two-pulse femtosecond photon echoes.
The nonlinear absorption of intense laser light is used to obtain information about the times required for spectral cross relaxation (T3{T}_{3}T3​) and collisional dephasing (T2{T}_{2}T2​) in organic-dye solutions. The steady-state transmission experiment of Huff and DeShazer indicates that T2{T}_{2}T2​ is in the range 0.1-1 psec for cryptocyanine-methanol, while T3{T}_{3}T3​ is in the range 1-10 psec. For DDI-glycerin (1, 1\ensuremath{'}-diethyl-2, 2\ensuremath{'}-dicarbocyanine-iodide-glycerin), measurements of ultrashort-pulse transmission as a function of incident energy and optical polarization suggest a value ${T}_{2}=0.4\ifmmode\pm\else\textpm\fi{}0.2$ psec for that system. These results are based on a new set of rate equations for absorption by inhomogeneously broadened organic-dye solutions. The equations display the effects of spectral cross relaxation and are not limited to light intensities less than those at which coherent interaction effects become important. For the case of ultrashort incident pulses, the equations are extended to account for excited-state absorption, the dependence of absorption on optical polarization, and the existence of a multiplicity of interacting excited states.
The extraordinarily short optical dephasing time of organic dye molecules in polymer hosts is known to be under 20 fs between 100 and 200 K. Short dephasing times suggest enormously broadened homogeneous linewidths on the order of 10 nm. Direct observation of single dye molecule fluorescence spectra shows a similarly broadened line, giving comparable dephasing times for room temperature measurements. While this single molecule linewidth agrees with that of the bulk sample, it is inaccurate to attribute this broadening to homogeneous mechanisms. Repeated single molecule fluorescence measurements permit determination of the homogeneous nature of this linewidth, as distinct from the inhomogeneous nature of dispersion in bulk samples. Spectral diffusion in single molecule spectra is evidenced by multiple peaks, visible despite broad spectra lines at room temperature. In the particular experiment discussed in this thesis, fluorescence from single organic dye molecules in thin polymethyl methacrylate films was studied with scanning confocal microscopy. Photon antibunching exhibited by the photon-pair correlation function g(2) (τ) at τ = 0 confirms that single molecule emission spectra were obtained.