Pump-Selective Spectral Shaping of the Ultrafast Response in Plasmonic Nanostars

Plasmonic nanostructures are, to date, well-known to offer unique possibilities for the tailoring of light–matter interactions at the nanoscale. Most recently, a new route to ultrafast all-optical modulation has been disclosed by combining the resonant features of plasmonic nanostructures with the giant third-order optical nonlinearity of noble metals regulated by highly energetic (hot) carriers. In this framework, a variety of nanostructures have been designed, with special attention to shapes featuring tips, where extreme and highly sensitive field enhancements (hot spots) can be attained. Here, we report on a broadband pump–probe spectroscopy analysis of an ensemble of spiky star-shaped nanoparticles, exploring both the perturbative and nonperturbative regimes of photoexcitation. The experiments are corroborated by semiclassical numerical simulations of the ultrafast optical response of the sample. We found that the peculiar hot spots supported by the star tips allow one to easily control the spectral shape of the transient optical signal, upon tuning of the pump wavelength. Our results elucidate the ultrafast response of hot electrons in star-shaped nanostructures and contribute to the understanding of the tip-mediated enhanced nonlinearities. This work paves the way to the development of ultrafast all-optical plasmonic modulators for pump-selective spectral shaping

Exploring Solvent and Substituent Effects on the Excited State Dynamics and Symmetry Breaking of Quadrupolar Triarylamine End-Capped Benzothiazole Chromophores by Femtosecond Spectroscopy

We investigate herein the excited state dynamics and symmetry breaking processes in three benzothiazole-derived two-photon absorbing chromophores by femtosecond fluorescence and transient absorption (fs-TA) spectroscopies in solvents of various polarity. The chromophores feature a quasi-quadrupolar D-π-A-π-D architecture comprised of an electron-withdrawing benzothiazole core and lateral triphenylamine donors (Qbtz-H), while the acceptor strength of the central unit is enforced by attached cyano groups (Qbtz-CN) and the electron-donating strength of the arylamine moieties by introduction of peripheral methoxy groups (Qbtz′-CN). Steady state spectroscopy reveals positive solvatochromism, which is mostly pronounced for Qbtz′-CN. Femtosecond spectroscopy of Qbtz-H reveals the coexistence of the Franck–Condon (FC) state and states populated after symmetry breaking (SB) in low-polarity solvents such as toluene and tetrahydrofuran, while the SB state becomes favorable in polar acetonitrile. For the other two molecules possessing a stronger electron-accepting unit and thus more polar excited state, SB takes place even in low-polarity solvents, as shown by fs-TA spectroscopy. Global fitting of the fs-TA spectra together with investigation of the evolution associated spectra (EAS) reveals the existence of an initial FC state in Qbtz-H, in all studied solvents, which relaxes toward Intermediate Charge Transfer (I-CT) and SB states. On the other hand, for Qbtz-CN and Qbtz′-CN in more polar solvents, the FC state undergoes ultrafast relaxation toward symmetry-broken charge transfer (SB-CT) states which in turn show very fast recombination to the ground state. Our measurements confirm that the extent of symmetry breaking is larger for D-π-A-π-D systems with the stronger acceptor core and increases further by increasing electron-donating strength of triarylamine moieties, giving rise to symmetry breaking in these nonionic quadrupolar molecules with ethynylene (triple bond) π-spacers also in less polar solvents

Effect of the DNA Polarity on the Relaxation of Its Electronic Excited States

The DNA polarity, i.e., the order in which nucleobases are connected together via the phosphodiester backbone, is crucial for several biological processes. But, so far, there has not been experimental evidence regarding its effect on the relaxation of DNA electronic excited states. Here we examine this aspect for two dinucleotides containing adenine and guanine: 5′-dApdG-3′ and 5′-dGpdA-3′ in water. We used two different femtosecond transient absorption setups: one providing high temporal resolution and broad spectral coverage (330–650 nm) between 30 fs and 50 ps, and the other recording decays at selected wavelengths until 1.2 ns. The transient absorption spectra corresponding to the minima in the potential energy surface of the first excited state were computed by quantum chemistry methods. Our results show that the excited charge transfer state in 5′-dGpdA-3′ is formed with a ∼75% higher quantum yield and exhibits slower decay (170 ± 10 ps vs 112 ± 12 ps) compared to 5′-dApdG-3′

All-optical Reconfiguration of Ultrafast Dichroism in Gold Metasurfaces

Optical metasurfaces have come into the spotlight as a promising platform for light manipulation at the nanoscale, including ultrafast all-optical control via excitation with femtosecond laser pulses. Recently, dichroic metasurfaces have been exploited to modulate the polarization state of light with unprecedented speed. Here, we theoretically predict and experimentally demonstrate by pump-probe spectroscopy the capability to reconfigure the ultrafast dichroic signal of a gold metasurface by simply acting on the polarization of the pump pulse, which is shown to reshape the spatio-temporal distribution of the optical perturbation. The photoinduced anisotropic response, driven by out-of-equilibrium carriers and extinguished in a sub-picosecond temporal window, is readily controlled in intensity by tuning the polarization direction of the excitation up to a full sign reversal. This work proves that nonlinear metasurfaces offer the flexibility to tailor their ultrafast optical response in a fully all-optically reconfigurable platform

Vibrational Dephasing along the Reaction Coordinate of an Electron Transfer Reaction

The role of molecular vibration in photoinduced electron transfer (ET) reactions has been extensively debated in recent years. In this study, we investigated vibrational wavepacket dynamics in a model ET system consisting of an organic dye molecule as an electron acceptor dissolved in various electron donating solvents. By using broad band pump–probe (BBPP) spectroscopy with visible laser pulses of sub-10 fs duration, coherent vibrational wavepackets of naphthacene dye with frequencies spanning 170–1600 cm–1 were observed in the time domain. The coherence properties of 11 vibrational modes were analyzed by an inverse Fourier filtering procedure, and we discovered that the dephasing times of some vibrational coherences are reduced with increasing ET rates. Density functional theory calculations indicated that the corresponding vibrational modes have a large Huang–Rhys factor between the reactant and the product states, supporting the hypothesis that the loss of phase coherence along certain vibrational modes elucidates that those vibrations are coupled to the reaction coordinate of an ET reaction

Activated Singlet Exciton Fission in a Semiconducting Polymer

Singlet exciton fission is a spin-allowed process to generate two triplet excitons from a single absorbed photon. This phenomenon offers great potential in organic photovoltaics, but the mechanism remains poorly understood. Most reports to date have addressed intermolecular fission within small-molecular crystals. However, through appropriate chemical design chromophores capable of intramolecular fission can also be produced. Here we directly observe sub-100 fs activated singlet fission in a semiconducting poly­(thienylenevinylene). We demonstrate that fission proceeds directly from the initial 1B_u exciton, contrary to current models that involve the lower-lying 2A_g exciton. In solution, the generated triplet pairs rapidly recombine and decay through the 2A_g state. In films, exciton diffusion breaks this symmetry and we observe long-lived triplets which form charge-transfer states in photovoltaic blends

Chemically-Controlled Ultrafast Photothermal Response in Plasmonic Nanostructured Assemblies

Plasmonic nanoparticles are renowned as efficient heaters due to their capability to resonantly absorb and concentrate electromagnetic radiation, trigger excitation of highly energetic (hot) carriers, and locally convert their excess energy into heat via ultrafast nonradiative relaxation processes. Furthermore, in assembly configurations (i.e., suprastructures), collective effects can even enhance the heating performance. Here, we report on the dynamics of photothermal conversion and the related nonlinear optical response from water-soluble nanoeggs consisting of a Au nanocrystal assembly trapped in a water-soluble shell of ferrite nanocrystals (also called colloidosome) of ∼250–300 nm in size. This nanoegg configuration of the plasmonic assembly enables control of the size of the gold suprastructure core by changing the Au concentration in the chemical synthesis. Different metal concentrations are analyzed by means of ultrafast pump–probe spectroscopy and semiclassical modeling of photothermal dynamics from the onset of hot-carrier photogeneration (few picosecond time scale) to the heating of the matrix ligands in the suprastructure core (hundreds of nanoseconds). Results show the possibility to design and tailor the photothermal properties of the nanoeggs by acting on the core size and indicate superior performances (both in terms of peak temperatures and thermalization speed) compared to conventional (unstructured) nanoheaters of comparable size and chemical composition

Sub-50 fs Formation of Charge Transfer States Rules the Fate of Photoexcitations in Eumelanin-Like Materials

Eumelanins play a crucial role as photoprotective agents for living organisms, yet the nature of the stationary and transient species involved in the light absorption and deactivation processes remains controversial. Moreover, the critical sub-100 fs time scale, which is key to the characterization of the primary excited species, has remained unexplored. Here, we study the eumelanin analogue polydopamine (PDA) and employ a combination of steady-state and transient optical spectroscopies to reveal the presence of spectrally broad coupled electronic transitions with, at least partial, charge-transfer (CT) character. We monitor the CT state dynamics using tunable sub-20 fs pulses. We find that high photon energy excitation results in accelerated (sub-20 fs) CT formation times while activating pathways, which lead to long-lived (≫1 ns), possibly reactive CT species. On the other hand, visible light excitation results in a slower (≈45 fs) formation of bound CT states, which, however, recombine on the ultrafast sub-2 ps time scale

Sub-50 fs Formation of Charge Transfer States Rules the Fate of Photoexcitations in Eumelanin-Like Materials

Eumelanins play a crucial role as photoprotective agents for living organisms, yet the nature of the stationary and transient species involved in the light absorption and deactivation processes remains controversial. Moreover, the critical sub-100 fs time scale, which is key to the characterization of the primary excited species, has remained unexplored. Here, we study the eumelanin analogue polydopamine (PDA) and employ a combination of steady-state and transient optical spectroscopies to reveal the presence of spectrally broad coupled electronic transitions with, at least partial, charge-transfer (CT) character. We monitor the CT state dynamics using tunable sub-20 fs pulses. We find that high photon energy excitation results in accelerated (sub-20 fs) CT formation times while activating pathways, which lead to long-lived (≫1 ns), possibly reactive CT species. On the other hand, visible light excitation results in a slower (≈45 fs) formation of bound CT states, which, however, recombine on the ultrafast sub-2 ps time scale