Recent advances in photobleaching and blinking prevention have aided the advancement of single-molecule and super-resolution fluorescence microscopy. However, a common mechanism of the action of ...antifading agents such as Trolox is still missing. In this communication we present evidence that Trolox acts in accordance with a mechanism that involves triplet quenching through electron transfer and subsequent recovery of the resulting radical ion by the complementary redox reaction. The required oxidant for this unifying mechanism based on a reducing and oxidizing system (ROXS) is formed via (photo-) reaction with molecular oxygen. We present evidence that this oxidized form is a quinone derivative of Trolox with strong oxidizing properties. These findings shed light on many contradicting results regarding the action of antifading agents and might lead to a common mechanistic understanding of photobleaching and its prevention. Finally, a recipe on the proper use of Trolox as an antifading agent is provided.
The aggregation of conjugated polymers and electronic coupling of chromophores play a central role in the fundamental understanding of light and charge generation processes. Here we report that the ...predominant coupling in isolated aggregates of conjugated polymers can be switched reversibly between H-type and J-type coupling by partially swelling and drying the aggregates. Aggregation is identified by shifts in photoluminescence energy, changes in vibronic peak ratio, and photoluminescence lifetime. This experiment unravels the internal electronic structure of the aggregate and highlights the importance of the drying process in the final spectroscopic properties. The electronic coupling after drying is tuned between H-type and J-type by changing the side chains of the conjugated polymer, but can also be entirely suppressed. The types of electronic coupling correlate with chain morphology, which is quantified by excitation polarization spectroscopy and the efficiency of interchromophoric energy transfer that is revealed by the degree of single-photon emission.
Sharp metallic nanotapers irradiated with few-cycle laser pulses are emerging as a source of highly confined coherent electron wave packets with attosecond duration and strong directivity. The ...possibility to steer, control or switch such electron wave packets with light is expected to pave the way towards direct visualization of nanoplasmonic field dynamics and real-time probing of electron motion in solid-state nanostructures. Such pulses can be generated by strong-field-induced tunnelling and acceleration of electrons in the near-field of sharp gold tapers within one half-cycle of the driving laser field. Here, we show the effect of the carrier-envelope phase of the laser field on the generation and motion of strong-field-emitted electrons from such tips. We observe clear variations in the width of plateau-like photoelectron spectra characteristic of the subcycle regime. This is a step towards controlling the coherent electron motion in and around metallic nanostructures over ultrashort lengths and timescales.
The chemistry of life is founded on light, so is it appropriate to think of light as a chemical substance? Planck's quantization offers a metric analogous to Avogadro's number to relate the number of ...particles to an effective reaction of single molecules and photons to form a new compound. A rhodamine dye molecule serves as a dehalogenating photocatalyst in a consecutive photoelectron transfer (conPET) process which adds the energy of two photons, with the first photon inducing radical formation and the second photon triggering PET to the substrate molecule. Rather than probing catalytic heterogeneity and dynamics on the single-molecule level, single-photon synthesis is demonstrated: the light quantum constitutes a reactant for the single substrate molecule in a dye-driven reaction. The approach illustrates that molecular diffusion and excited-state internal conversion are not limiting factors in conPET reaction kinetics because of catalyst-substrate preassociation. The effect could be common to photoredox catalysis, removing the conventional requirement of long excited-state lifetimes.
We report photoelectron emission from the apex of a sharp gold nanotaper illuminated via grating coupling at a distance of 50 μm from the emission site with few-cycle near-infrared laser pulses. We ...find a fifty-fold increase in electron yield over that for direct apex illumination. Spatial localization of the electron emission to a nanometer-sized region is demonstrated by point-projection microscopic imaging of a silver nanowire. Our results reveal negligible plasmon-induced electron emission from the taper shaft and thus efficient nanofocusing of few-cycle plasmon wavepackets. This novel, remotely driven emission scheme offers a particularly compact source of ultrashort electron pulses of immediate interest for miniaturized electron microscopy and diffraction schemes with ultrahigh time resolution.
Spatiotemporal coupling of attosecond pulses Wikmark, Hampus; Guo, Chen; Vogelsang, Jan ...
Proceedings of the National Academy of Sciences - PNAS,
03/2019, Letnik:
116, Številka:
11
Journal Article
Recenzirano
Odprti dostop
The shortest light pulses produced to date are of the order of a few tens of attoseconds, with central frequencies in the extreme UV range and bandwidths exceeding tens of electronvolts. They are ...often produced as a train of pulses separated by half the driving laser period, leading in the frequency domain to a spectrum of high, odd-order harmonics. As light pulses become shorter and more spectrally wide, the widely used approximation consisting of writing the optical waveform as a product of temporal and spatial amplitudes does not apply anymore. Here, we investigate the interplay of temporal and spatial properties of attosecond pulses. We show that the divergence and focus position of the generated harmonics often strongly depend on their frequency, leading to strong chromatic aberrations of the broadband attosecond pulses. Our argument uses a simple analytical model based on Gaussian optics, numerical propagation calculations, and experimental harmonic divergence measurements. This effect needs to be considered for future applications requiring high-quality focusing while retaining the broadband/ultrashort characteristics of the radiation.
Fluorescent molecular switches have widespread potential for use as sensors, material applications in electro-optical data storages and displays, and superresolution fluorescence microscopy. We ...demonstrate that adjustment of fluorophore properties and environmental conditions allows the use of ordinary fluorescent dyes as efficient single-molecule switches that report sensitively on their local redox condition. Adding or removing reductant or oxidant, switches the fluorescence of oxazine dyes between stable fluorescent and nonfluorescent states. At low oxygen concentrations, the off-state that we ascribe to a radical anion is thermally stable with a lifetime in the minutes range. The molecular switches show a remarkable reliability with intriguing fatigue resistance at the single-molecule level: Depending on the switching rate, between 400 and 3,000 switching cycles are observed before irreversible photodestruction occurs. A detailed picture of the underlying photoinduced and redox reactions is elaborated. In the presence of both reductant and oxidant, continuous switching is manifested by "blinking" with independently controllable on- and off-state lifetimes in both deoxygenated and oxygenated environments. This "continuous switching mode" is advantageously used for imaging actin filament and actin filament bundles in fixed cells with subdiffraction-limited resolution.
Triplet excitons have been the focus of considerable attention with regards to the functioning of polymer solar cells because these species are long-lived and quench subsequently generated singlet ...excitons in their vicinity. The role of triplets in poly(3-hexylthiophene) (P3HT) has been investigated extensively with contrary conclusions regarding their importance. We probe the various roles triplets can play in P3HT by analyzing the photoluminescence (PL) from isolated single-chain aggregates and multichain mesoscopic aggregates. Solvent vapor annealing allows deterministic growth of P3HT aggregates consisting of ∼20 chains, which exhibit red-shifted and broadened PL compared to single-chain aggregates. The multichain aggregates exhibit a decrease of photon antibunching contrast compared to single-chain aggregates, implying rather weak interchain excitonic coupling and energy transfer. Nevertheless, the influence of triplet-quenching oxygen on PL and a photon correlation analysis of aggregate PL reveal that triplets are quenched by intermolecular interactions in the bulk state.
Controlling the morphology of π‐conjugated polymers for organic optoelectronic devices has long been a goal in the field of materials science. Since the morphology of a polymer chain is closely ...intertwined with its photophysical properties, it is desirable to be able to change the arrangement of the polymers at will. We investigate the π‐conjugated polymer poly(9,9‐dioctylfluorene) (PFO), which can exist in three distinctly different structural phases: the α‐, β‐, and γ‐phase. Every phase has a different chain structure and a unique photoluminescence (PL) spectrum. Due to its unique properties and the pronounced spectral structure‐property relations, PFO can be used as a model system to study the morphology of π‐conjugated polymers. To avoid ensemble averaging, we examine the PL spectrum of single PFO chains embedded in a non‐fluorescent matrix. With single‐molecule spectroscopy the structural phase of every single chain can be determined, and changes can be monitored very easily. To manipulate the morphology, solvent vapor annealing (SVA) was applied, which leads to a diffusion of the polymer chains in the matrix. The β‐ and γ‐phases appear during the self‐assembly of single α‐phase PFO chains into mesoscopic aggregates. The extent of β‐ and γ‐phase formation is directed by the solvent‐swelling protocol used for aggregation. Aggregation unequivocally promotes formation of the more planar β‐ and γ‐phases. Once these lower‐energy more ordered structural phases are formed, SVA cannot return the polymer chain to the less ordered phase by aggregate swelling.
The π‐conjugated polymer poly(9,9‐dioctylfluorene) (PFO) exists in three different phases: the α‐, β‐, and γ‐phase, which are associated with different chain morphologies. The β‐ and γ‐phases appear during the self‐assembly of single α‐phase PFO chains into mesoscopic aggregates. The extent of β‐ and γ‐phase formation is directed by the solvent‐swelling protocol used for aggregation.
We present a compact, few-cycle, short-wave infrared light source delivering 13 µJ, carrier-envelope phase (CEP) stable pulses around 2 µm, operating at 200 kHz repetition rate. Starting from an ...ytterbium fiber amplifier, the seed is produced via white-light generation followed by difference frequency generation, and later amplified in two BiBO nonlinear crystals. A pulse duration of 15.8 fs is measured with the dispersion scan technique, while the CEP stability is assessed via a monolithic spectral interferometry scheme. We demonstrate the potential of the system to drive strong-field experiments by performing high-order harmonic generation in argon gas.
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