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

Robin Smith

Published Jan 1, 2003• 3 citations
OpenAlex

Abstract

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.

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The photon echo (PE) spectroscopy and single-molecule spectroscopy (SMS) may be combined to give a very powerful tool for comprehensive study of low-temperature dynamics in dye-doped disordered solids (polymers, glasses). At the same time, this type of studies are likely to reveal discrepancies when comparing characteristic times of optical dephasing T2 and single-molecule zero-phonon spectral lines (ZPL) broadening obtained from PE and SMS, correspondingly, for tetra-tert-butylterrylene in polyisobutylene in the temperature range of a few–dozen of Kelvins [see Phys. Status Solidi B 241, 3480 and 3487 (2004)]. Inexplicably, PE-experiments demonstrated T2-times to be much shorter than it is sufficient to cause the corresponding ZPL broadening. Here we experimentally solve this problem and show that at T = 4.5–15 K the incoherent PE gives T2-times which correspond to the narrowest SM ZPL. On the SM-level there is a pronounced additional ZPL-broadening due to spectral diffusion processes which are strongly dependent on the characteristics time of the measurement (tens of nanoseconds for PE and seconds for SMS). There is also a broad distribution of ZPL spectral widths for different SMs due to different local environments, that contribute differently to both the optical dephasing and the spectral diffusion processes, but always in addition to the value of inverse optical dephasing times measured using a PE technique.

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Photon antibunching in the fluorescence of a single dye molecule embedded in a thin polymer film

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The concept of the homogeneous spectral linewidth of an impurity center is more complicated in disordered media such as organic glasses and polymers than it is in crystals. In amorphous media the local environments for impurity centers are continuously changing with time. As a result transition frequencies of individual centers diffuse within the inhomogeneous broadened spectral contour, which is called spectral diffusion. When the timescale for spectral diffusion processes is much longer than the dephasing time, it is possible to distinguish homogeneous dephasing from spectral diffusion. On the other hand, when the timescales of spectral diffusion and dephasing processes are comparable these phenomena become indistinguishable.

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Single organic molecules and light transport in thin films

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Two important processes are at the base of light-matter interaction: absorption and scattering. The first part of this work focuses on the interaction of light with single absorbers/emitters embedded in thin films. In the second part, diffusion of light through thin films of scattering materials is numerically investigated. \n \nQuantum emitters based on organic fluorescent molecules in thin films are investigated in the first part of this thesis. The focus of this work is on the experimental characterization of a specific system consisting of single DBT molecules embedded in a thin crystalline matrix of anthracene. The system under investigation exhibits some unique optical properties that enable its use in many applications, especially as a single-photon source and as a sensitive nanoprobe. In particular, single DBT molecules are very bright and stable within the anthracene matrix. At cryogenic temperatures, dephasing of the molecular dipole due to interactions with the phonons of the matrix vanishes, and as a result the purely electronic transition or 00-ZPL becomes extremely narrow, approaching the limit set by its natural linewidth. Under pulsed excitation, the system can be operated as a source of indistinguishable, lifetime-limited single photons. Furthermore, the spectral shifts of the narrow ZPL can be exploited as a sensitive probing tool for local effects and fields. \n \nIn this work we perform a complete optical characterization of the DBT in anthracene system. Using a home-built scanning epifluorescence microscope, we study its optical properties at room temperature: fluorescence saturation intensity, dipole orientation and emission pattern, fluorescence and triplet lifetime are investigated. At temperatures down to 3K, we observe a lifetime-limited absorption line. Also, we demonstrate photon antibunching from this system. We then show that single DBT molecules can be effectively used for sensing applications. Indeed, at the nanometre scale, i.e. on a scale of the order of their physical size, the optical properties of a single molecule are affected by the surrounding environment. In particular, we here demonstrate energy transfer between single DBT molecules and a graphene sheet, a process that can be exploited to measure the distance d between a single molecule and the graphene layer. Based on the universality of the energy transfer process and its sole dependence on d, we provide a proof of principle for a nanoscopic ruler. \n \nIn the second part of this thesis we look at the interaction of light with matter from a different perspective. By means of numerical simulations, we address the problem of light transport in turbid media, with a particular focus on optically thin systems. The problem is usually modelled by the Radiative Transfer Equation and its simple Diffusive Approximation which holds for the case of a single, thick slab of turbid material but fails dramatically for thin systems. Alternatively, the problem of light transport can be modelled as a random walk process and therefore it can be numerically investigated by means of Monte Carlo algorithms. \n \nIn this work we develop a Monte Carlo software library for light transport in multilayered scattering samples, introducing several advancements over existing Monte Carlo solutions. We use the software to build a lookup table which allows us to solve the so-called inverse problem of light transport in a thin slab, i.e. the determination of the microscopic properties at the base of light propagation (such as the scattering mean free path ls and the scattering anisotropy g) starting from macroscopic ensemble observables. We then study diffusion of light in thin slabs, with a particular attention on transverse transport. Indeed, even if a diffusive behaviour is usually associated with thick, opaque media, as far as in-plane propagation is concerned, transport is unbounded and will eventually become diffusive provided that sufficiently long times are considered. By means of Monte Carlo simulations, we characterise this almost two-dimensional asymptotic diffusive regime that sets in even for optically thin slabs (OT=1). We show that geometric and boundary conditions, such as the refractive index contrast, play an active role in redefining the very asymptotic value of the diffusion coefficient by directly modifying the statistical distributions underlying light transport in a scattering medium.

•Jan 1, 2016•0 citations

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Каримуллин Камиль Равкатович, А. В. Наумов

Low temperature dynamics (tunneling and vibrational relaxation) in doped polyisobutylene film has been reinvestigated using 2-pulse incoherent photon echo (2IPE) and compared with single-molecule spectroscopy (SMS) data. It has been shown that in a very wide range of low temperatures the 2IPE gives optical dephasing times which correspond to the narrowest zero-phonon lines of single dye molecules.

EPJ Web of Conferences•Jan 1, 2018•0 citations

Contributions of The Different Line Broadening Mechanisms in Photon Echoes and Single Molecule Spectra in Amorphous Solids

Yu. G. Vainer, Roman I. Personov, Stephan J. Zilker +1 more

Experimental data on Photon Echoes (PE) in the system terrylene/poly(isobutylene) in the temperature range 0.4–8K are presented and compared with Single Molecule Spectroscopy (SMS) results. From this comparison we conclude that the linewidth distribution in SMS is caused mainly by a large T 2-dispersion (but not slow spectral diffusion). The temperature line broadening is discussed.

Molecular crystals and liquid crystals science technology. Section A, Molecular crystals and liquid crystals/Molecular crystals and liquid crystals science and technology. Section A, Molecular crystals and liquid crystals•Dec 1, 1996•8 citations
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.
The photon echo (PE) spectroscopy and single-molecule spectroscopy (SMS) may be combined to give a very powerful tool for comprehensive study of low-temperature dynamics in dye-doped disordered solids (polymers, glasses). At the same time, this type of studies are likely to reveal discrepancies when comparing characteristic times of optical dephasing T2 and single-molecule zero-phonon spectral lines (ZPL) broadening obtained from PE and SMS, correspondingly, for tetra-tert-butylterrylene in polyisobutylene in the temperature range of a few–dozen of Kelvins [see Phys. Status Solidi B 241, 3480 and 3487 (2004)]. Inexplicably, PE-experiments demonstrated T2-times to be much shorter than it is sufficient to cause the corresponding ZPL broadening. Here we experimentally solve this problem and show that at T = 4.5–15 K the incoherent PE gives T2-times which correspond to the narrowest SM ZPL. On the SM-level there is a pronounced additional ZPL-broadening due to spectral diffusion processes which are strongly dependent on the characteristics time of the measurement (tens of nanoseconds for PE and seconds for SMS). There is also a broad distribution of ZPL spectral widths for different SMs due to different local environments, that contribute differently to both the optical dephasing and the spectral diffusion processes, but always in addition to the value of inverse optical dephasing times measured using a PE technique.
We used scanning confocal microscopy to study the fluorescence from a single terrylene molecule embedded in a thin polymer film of polymethyl methacrylate, at room temperature, with a high signal-to-background ratio. The photon-pair correlation function g((2))(tau) exhibits perfect photon antibunching at tau = 0 and a limit of 1.3, compatible with bunching associated with the molecular triplet state. Application of this molecular system to a triggered single-photon source based on single-molecule fluorescence is investigated.
The concept of the homogeneous spectral linewidth of an impurity center is more complicated in disordered media such as organic glasses and polymers than it is in crystals. In amorphous media the local environments for impurity centers are continuously changing with time. As a result transition frequencies of individual centers diffuse within the inhomogeneous broadened spectral contour, which is called spectral diffusion. When the timescale for spectral diffusion processes is much longer than the dephasing time, it is possible to distinguish homogeneous dephasing from spectral diffusion. On the other hand, when the timescales of spectral diffusion and dephasing processes are comparable these phenomena become indistinguishable.
Two important processes are at the base of light-matter interaction: absorption and scattering. The first part of this work focuses on the interaction of light with single absorbers/emitters embedded in thin films. In the second part, diffusion of light through thin films of scattering materials is numerically investigated. \n \nQuantum emitters based on organic fluorescent molecules in thin films are investigated in the first part of this thesis. The focus of this work is on the experimental characterization of a specific system consisting of single DBT molecules embedded in a thin crystalline matrix of anthracene. The system under investigation exhibits some unique optical properties that enable its use in many applications, especially as a single-photon source and as a sensitive nanoprobe. In particular, single DBT molecules are very bright and stable within the anthracene matrix. At cryogenic temperatures, dephasing of the molecular dipole due to interactions with the phonons of the matrix vanishes, and as a result the purely electronic transition or 00-ZPL becomes extremely narrow, approaching the limit set by its natural linewidth. Under pulsed excitation, the system can be operated as a source of indistinguishable, lifetime-limited single photons. Furthermore, the spectral shifts of the narrow ZPL can be exploited as a sensitive probing tool for local effects and fields. \n \nIn this work we perform a complete optical characterization of the DBT in anthracene system. Using a home-built scanning epifluorescence microscope, we study its optical properties at room temperature: fluorescence saturation intensity, dipole orientation and emission pattern, fluorescence and triplet lifetime are investigated. At temperatures down to 3K, we observe a lifetime-limited absorption line. Also, we demonstrate photon antibunching from this system. We then show that single DBT molecules can be effectively used for sensing applications. Indeed, at the nanometre scale, i.e. on a scale of the order of their physical size, the optical properties of a single molecule are affected by the surrounding environment. In particular, we here demonstrate energy transfer between single DBT molecules and a graphene sheet, a process that can be exploited to measure the distance d between a single molecule and the graphene layer. Based on the universality of the energy transfer process and its sole dependence on d, we provide a proof of principle for a nanoscopic ruler. \n \nIn the second part of this thesis we look at the interaction of light with matter from a different perspective. By means of numerical simulations, we address the problem of light transport in turbid media, with a particular focus on optically thin systems. The problem is usually modelled by the Radiative Transfer Equation and its simple Diffusive Approximation which holds for the case of a single, thick slab of turbid material but fails dramatically for thin systems. Alternatively, the problem of light transport can be modelled as a random walk process and therefore it can be numerically investigated by means of Monte Carlo algorithms. \n \nIn this work we develop a Monte Carlo software library for light transport in multilayered scattering samples, introducing several advancements over existing Monte Carlo solutions. We use the software to build a lookup table which allows us to solve the so-called inverse problem of light transport in a thin slab, i.e. the determination of the microscopic properties at the base of light propagation (such as the scattering mean free path ls and the scattering anisotropy g) starting from macroscopic ensemble observables. We then study diffusion of light in thin slabs, with a particular attention on transverse transport. Indeed, even if a diffusive behaviour is usually associated with thick, opaque media, as far as in-plane propagation is concerned, transport is unbounded and will eventually become diffusive provided that sufficiently long times are considered. By means of Monte Carlo simulations, we characterise this almost two-dimensional asymptotic diffusive regime that sets in even for optically thin slabs (OT=1). We show that geometric and boundary conditions, such as the refractive index contrast, play an active role in redefining the very asymptotic value of the diffusion coefficient by directly modifying the statistical distributions underlying light transport in a scattering medium.