Exciton Trapping Dynamics in DNA Multimers

Using as a model the single adenine strand (dA)₂₀, we study the ultrafast evolution of electronic excitations in DNA with a time resolution of 30 fs. Our transient absorption spectra in the UV and visible spectral domains show that internal conversion among photogenerated exciton states occurs within 100 fs. Subsequently, the ππ* states acquire progressively charge-transfer character before being completely trapped, within 3 ps, by fully developed charge-transfer states corresponding to transfer of an electron from one adenine moiety to another (A⁺A^–)

Low Sensitivity of Acoustic Breathing Mode Frequency in Co Nanocrystals upon Change in Nanocrystallinity

Cobalt nanocrystals (NCs) with narrow size distribution and polycrystalline structure in their native form are synthesized in reverse micelles. After annealing at 350 °C, these NCs are transformed into single crystalline phase with hexagonal close-packed structure. The vibrational dynamics of NCs differing by their nanocrystallinity is studied by femtosecond pump–probe spectroscopy. By recording the differential reflectivity signal in the native and annealed Co NCs, the frequency of their fundamental breathing acoustic mode can be measured in the time domain. A small decrease of the breathing mode frequency is observed in single crystalline Co NCs compared to that measured in polycrystals, indicating low sensitivity of their fundamental radial mode upon change in crystallinity. This result is in agreement with predictions from calculations using the resonant ultrasound approach

Widefield phototransient imaging for visualizing 3D motion of resonant particles in scattering environments

Identifying, visualising and ultimately tracking dynamically moving non-fluorescent nanoparticles in the presence of non-specific scattering is a long-standing challenge across the nano- and life-sciences. In this work we demonstrate that our recently developed ultrafast holographic transient (UHT) microscope is ideally suited for meeting this challenge. We show that UHT microscopy allows reliably distinguishing off-resonant, dielectric, from resonant, metallic, nanoparticles, based on the phototransient signal: a pre-requisite for single-particle tracking in scattering environments. We then demonstrate the capability of UHT microscopy to holographically localize in 3D single particles over large volumes of view. Ultimately, we combine the two concepts to simultaneously track several tens of freely diffusing gold nanoparticles, within a 110x110x110 $\mu$m volume of view at an integration time of 10 ms per frame, while simultaneously recording their phototransient signals. The combined experimental concepts outlined and validated in this work lay the foundation for background-free 3D single-particle tracking applications or spectroscopy in scattering environments and are immediately applicable to systems as diverse as live cells and tissues or supported heterogeneous catalysts

Electron Transfer from Organic Aminophenyl Acid Sensitizers to Titanium Dioxide Nanoparticle Films

Electron transfer from three conjugated amino-phenyl acid dyes to titanium and aluminum oxide nanocrystalline films was studied by using transient absorption spectroscopy with sub 20 fs time-resolution over the visible spectral region. All the dyes attached to TiO2 showed long-lived ground state bleach signals indicative of formation of new species. Global analysis of the transient kinetics of the dyes on TiO2 revealed stimulated emission decays of about 40 fs and less than 300 fs assigned to electron injection. The same dyes on Al2O3 substrates displayed long stimulated emission decays (ns) suggesting that electron transfer is blocked in this high band gap semiconductor. For two of the dyes (NK1 and NK2) with amino methyl terminal groups cation formation was seen at 670 nm probe wavelength, in the onset region of cation absorption predicted by time-dependent density functional theory calculations, with dynamics matching that of stimulated emission decay. Early excited-state dynamics observed in the NK7 dye bound to both TiO2 and Al2O3, with pulse limited rise and 30−40 fs decay times, was assigned to rearrangement of charge in the amino phenyl moiety and/or possibly isomerization, which competes with electron injection in NK7 sensitized TiO2 films. This additional relaxation channel could be the reason for the low efficiency reported for the NK7 sensitized solar cell

Transition Metal Doping in CdS Quantum Dots: Diffusion, Magnetism, and Ultrafast Charge Carrier Dynamics

Transition metal (TM) doping in pristine II–VI semiconductor quantum dots (QDs) is known to add several otherwise unavailable properties by introducing midgap states in the host material. Albeit being extensively investigated, the periodicity of the observed properties with respect to the electronic structure has not been attempted so far. In this work, we investigate CdS QDs doped with several different TM ions (Mn, Fe, Co, Ni, and Cu) using extended X-ray absorption fine structure spectroscopy to study dopant-induced structural perturbations and femtosecond transient absorption (TA) spectroscopy to study the ultrafast charge carrier dynamics. This provides solid evidence for the origin of magnetization in doped QDs that has been lacking despite extensive studies. Further, we demonstrate that the ionic radius and the dopant oxidation state play crucial roles in determining the dopant–anion bond lengths. Based on the investigation of the relaxation pathways of excited charge carriers using ultrafast TA spectroscopy, we hypothesize that there exists photoinduced switching between multiple oxidation states in some dopants

Field-Induced Stimulated Emission in a Polymer–Liquid Crystal Mixture

We present a spectroscopic study on a novel blend of a light-emitting polymer (F8BT) and a liquid crystal (5CB). We investigate the possibility to control the optical behavior of such blend with an external stimulus. By means of ultrafast pump–probe spectroscopy we observe a modulation of the stimulated emission of the polymer driven by an external applied voltage. We attribute the rise of the stimulated emission to a rearrangement of the polymer that modifies its packing as a consequence of the alignment of the liquid crystal. Such field-induced stimulated emission modulation can find applications in information and communication technology, lasing and optical sensing

Ultra-broadband Optical Diffraction Tomography

Optical diffraction tomography (ODT) is a powerful non-invasive 3D imaging technique, but its combination with broadband light sources is difficult. In this study, we introduce ultrabroadband ODT, covering over 150 nm of visible spectral bandwidth with a lateral spatial resolution of 150 nm. Our work addresses a critical experimental gap by enabling the measurement of broadband refractive index changes in 3D samples, a crucial information that is difficult to assess with existing methodologies. We present spectrally resolved ODT images of HeLa cells, obtained via pulse-shaping based Fourier transform spectroscopy. We, further, conduct comparative experiments using gold-nanoparticle incubated as well as non-incubated cells. The spectral observations enabled by ultrabroadband ODT, combined with material-dependent refractive index responses, allow for precise three-dimensional identification of the nano-particles within cellular structures. Our work represents a crucial step towards spectrally and time-resolved tomography of complex 3D structures with implications for life and materials science applications

In Operando Photoelectrochemical Femtosecond Transient Absorption Spectroscopy of WO₃/BiVO₄ Heterojunctions

The WO3/BiVO4 heterojunction is a promising photoanode architecture for water splitting applications. Here, the photoinduced charge carrier dynamics occurring in this system in operando photoelectrochemical conditions, i.e., under an applied anodic potential, are studied through femtosecond transient absorption spectroscopy to unveil the effects of an applied bias on the early charge carrier dynamics following WO3/BiVO4 excitation. Electrochromic measurements on BiVO4 suggest the presence of intraband gap (IBG) states in this oxide, which play an important role in the charge carrier dynamics in the heterojunction. The differences observed in WO3/BiVO4 with respect to individual BiVO4 electrodes are rationalized in terms of the electron equilibration between the two oxides at the heterojunction, dominated by the WO3/BiVO4 interfacial electric field arising from their band energy offset, and the bias-dependent alteration of the IBG states, both determining the rate of hole transfer and accumulation at the BiVO4 surface

Transient Absorption Imaging of P3HT:PCBM Photovoltaic Blend: Evidence For Interfacial Charge Transfer State

Solution-processed bulk heterojunction (BHJ) based on electron-donor (D) polymer and acceptor (A) fullerene is a promising technology for organic photovoltaics. Geminate charge recombination is regarded as one of the main loss mechanisms limiting device performances. This stems from the dynamics of the initial charge transfer state (CTS), which depend on the blend morphology, the molecular conformation, and the energetics of the D:A interface. Here we study the photophysics of a crystalline phase-separated blend of regioregular poly(3-hexylthiophene) (P3HT) with [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM) with a coarsened morphology, by mapping the transient absorption signal with submicrometer space and subpicosecond time resolution. At the P3HT:PCBM interface, we detect a long-lived photoinduced dynamic that we assign to a peculiar coherent CTS forming in ∼10 ps, not affected by geminate recombination and characterized by a different polarization with respect to the one in the usual polydispersed blend. Quantum chemical calculations on supramolecular P3HT:PCBM complexes confirm the presence of low-lying and highly polarized CTS, validating the experimental findings

Visualization 1: Broadband stimulated Raman scattering with Fourier-transform detection

sample response at various Raman frequencies Originally published in Optics Express on 21 September 2015 (oe-23-19-25235