Delocalized Two-Exciton States in DNA Scaffolded Cyanine Dimers.

In nondegenerate FWM (NDFWM) experiments employing spectrally narrower pulses, we can limit the FWM processes to specific ones. Thus, NDFWM measurements will give new insights on the excitonic dynamics. In our previous study using spectrally-resolved DFWM techniques on a self-organized quantum-well material, (C/sub 6/H/sub 13/NH/sub 3/)/sub 2/PbI/sub 4/, it has been shown that two exciton states, including biexciton and weakly interacting two-exciton state, play an important role in FWM processes. However, the signals arising from various two-exciton states overlapped spectrally with each other in DFWM experiments. In this study, we have employed NDFWM techniques on (C/sub 6/H/sub 13/NH/sub 3/)/sub 2/PbI/sub 4/ to observe the contributions of two-exciton states separately.

Engineering couplings for exciton transport using synthetic DNA scaffolds

Two-exciton states with lattice relaxation in one-dimensional semiconductors

We examine the effect of electronic coupling on the optical properties of Cy3 dimers attached to DNA duplexes as a function of base pair (bp) separation using steady-state and time-resolved spectroscopy. For close Cy3-Cy3 separations, 0 and 1 bp between dyes, intermediate to strong electronic coupling is revealed by modulation of the absorption and fluorescence properties including spectral band shape, peak wavelength, and excited-state lifetime. Using a vibronic exciton model, we estimate coupling strengths of 150 and 266 cm-1 for the 1 and 0 bp separations, respectively, which are comparable to those found in natural light-harvesting complexes. For the strongest electronic coupling (0 bp separation), we observe that the absorption band shape is strongly affected by the base pairs that surround the dyes, where more strongly hydrogen-bonded G-C pairs produce a red-shifted absorption spectrum consistent with a J-type dimer. This effect is studied theoretically using molecular dynamics simulation, which predicts an in-line dye configuration that is consistent with the experimental J-type spectrum. When the Cy3 dimers are in a standard aqueous buffer, the presence of relatively strong electronic coupling is accompanied by decreased fluorescence lifetime, suggesting that it promotes nonradiative relaxation in cyanine dyes. However, we show that the use of a viscous solvent can suppress this nonradiative recombination and thereby restore the dimer fluorescent emission. Ultrafast transient absorption measurements of Cy3 dimers in both standard aqueous buffer and viscous glycerol buffer suggest that sufficiently strong electronic coupling increases the probability of excited-state relaxation through a dark state that is related to Cy3 torsional motion.

The authors report on the photoluminescence spectroscopy of a single GaSb∕GaAs type II quantum dot (QD) at 8K. A sharp exciton emission with a linewidth of less than 250μeV was observed. Two-exciton emission at the higher energy side of the exciton emission indicates that the two excitons in a type II QD do not form a bound biexciton. The energies of the exciton and two-exciton states were calculated using an atomic pseudopotential model, which provides a quantitative description of the antibound nature of the two-exciton state in type II QDs.

We theoretically analyze the optical response from an ultrathin film built up of oriented molecular aggregates, the operating states of which are represented by Frenkel exciton states. A four-level model, involving transitions between the ground, one-exciton and two-exciton states, exciton-exciton annihilation from the two-exciton state as well as relaxation from the annihilation level back to the one-exciton and ground states, is used for describing the film optical response. It is proved that the exciton-exciton annihilation may act not as a destructive but, on the contrary, as a constructive factor tending towards the occurrence of bistability. In particular, the effect of inhomogeneous broadening of the exciton optical transition, preventing the bistable behavior, may be suppressed considerably due to a fast exciton-exciton annihilation.

Studies of multiphoton dissociation (MPD) over the past few years have led to a good semi-quantitative understanding of the process for SF6 and similar molecules. Several reviews have appeared recently.1 Applications have progressed to the point that a pilot plant is operating for the production of 13C by MPD of CF3I.2 The model developed for understanding MPD divides the vibrational energy levels of a molecule into three regions.3 Region I includes the lowest, “discrete” levels of the vibrational modes that are nearly resonant with the laser frequency. Vibrational anharmonicity prevents exact resonances for the sequential absorption of several photons. However, the combined effects of rotational energy compensation, Coriolis shifts, and other perturbations, along with power broadening, permit the absorption of several photons. In region II, strong coupling among vibrational modes drains energy from the pumped mode and broadens vibration-rotation levels so that absorption occurs over a broad frequency range. Sequential absorption of photons becomes possible. In region III, where levels are above the dissociation threshold, this absorption continues, but with the competing process of dissociation occurring simultaneously. The boundary between regions I and II is poorly defined. From an operational point of view, region II begins when absorption becomes non-frequency selective for any molecule at any laser frequency of interest to the experimenter. In region II and especially in region III there is good evidence that energy is transferred rapidly among all vibrations of the excited molecule and that the rate of dissociation as a function of energy may be estimated using the RRKM theory.4 The main features of the dynamics of molecules in regions II and III appear to apply rather generally to all types of molecules,5 with the possible exception of very large ones.6

The effect of the excited two-exciton state on the transition from the ground state to the third molecular state is studied for a three-level molecular aggregate. Based on a Green function technique, the analytical expression is given for the line shape of pump–probe differential spectrum. A redshift peak of the transition from the ground state to the third state has been found because of introducing the coupling of the excited two-exciton states to the third state. Further, the dependence of the spectra on the aggregate length shows that the delocalization length of the exciton is decreased with an increase in the coupling strength. This result indicates that the coupling induces the exciton localization, leading to the reduction of the effective molecular number in the molecular aggregates.

Optical sectioning is performed by collecting the fluorescent emission of two-exciton states in colloidal quantum dots. The two-exciton state is created by two consecutive resonant absorption events, thus requiring unprecedented low excitation energy and peak powers as low as 10(5) W/cm(2). The depth resolution is shown to be equivalent to that of standard multiphoton microscopy, and it was found to deteriorate only slowly as saturation of the two-exciton state is approached, owing to signal contribution from higher excitonic states.

Excitonic nonlinearities in the weak-excitation limit condition are examined with time-integrated and time-resolved four-wave mixing (FWM). Below an excitation of 3\ifmmode\times\else\texttimes\fi{}${10}^{9}$ excitons/${\mathrm{cm}}^{2}$, coherent third-order processes prevail. Echo-type signals are observed in parallel and perpendicular polarization configurations. The coherent process induced by the two-photon coherence of the two-exciton state is attributable to signals in the perpendicular configuration. The polarization dependences of a temporal profile and the intensity of FWM signals are well explained with third-order perturbational analysis that takes into account the two-exciton states and inhomogeneity.

A four-level model, involving the ground, one-exciton and two-exciton states as well as a high-lying molecular term through which the two-exciton state annihilates, is used for analyzing the bistable optical response of a thin film built up of oriented molecular aggregates. We focus on the effects of inhomogeneous broadening of the exciton optical transitions and exciton-exciton annihilation on bistability of the thin film response. It turns out that the inhomogeneous broadening, preventing generally the occurrence of bistability, may be suppressed considerably due to a fast exciton-exciton annihilation.

The strong influence of exciton/exciton interaction on the nonlinear optical response of the 2D exciton in GaAs quantum wells has been demonstrated in several recent experimental studies. The theoretical model for the microscopic coupling mechanism is still the subject of controversy, however. In ref. [1], we have presented a model including a density dependent coupling between opposite spin excitons which assumes that the interaction results in a renormalization of the matrix elements, dephasing rates and energies of the transition from the single-exciton state to the two-exciton state with respect to the corresponding quantities for the single exciton transitions. In contrast, Wang et al. claim that all experimental observations are explained by a density induced dephasing rate and that biexciton states play no role [2]. Here we present a new 3-pulse degenerate-four-wave-mixing (DFWM) configuration which is able to differentiate between pure local field effects and biexcitonic contributions to the time-integrated signal. Experiments were performed on an almost homogeneously broadened GaAs/Al0.3Ga0.7As single QW (well width 20 nm, photoluminescence line 0.3 meV, homogenous linewidth 0.15 meV) in the backward reflection geometry. The sample was cooled to 10 K and excited by a sequence of three pulses (1.1 ps duration) with equal intensity, wave vectors k→1,k→2,k→3 and delays τ12 and τ13 between the second and first and the third and first pulse, respectively. The signal was monitored in the direction k→ s =k→1+k→2−k→3 which provides no signal for the chosen pulse length if local field and renormalization effects are negligible. The peak intensity of the time-integrated DFWM signal has been calculated by solving the optical Bloch for a system of two non-interacting two-level systems (2LS) with opposite circular polarization selection rules (local field effect) and for the renormalized four-level system (4LS) depicted in Fig. 1 which assumes the formation of biexcitons between excitons with opposite spins. The signal strength is proportional to the strength of the local field in the case of the 2LS and to the renormalization for the 4LS. The peak signal intensities expected for pure local field and pure biexction contributions are summarized in the second and third column of Table 1 for eight experimental configurations applying different linear and circularly polarized pulses. The values are normalized to the signal strength predicted for three parallel linearly polarized pulses. Close inspection of the data reveals remarkable differences of the peak amplitude with polarization geometry for the two coupling models.

Electron-transfer spectroscopy: donor–acceptor electronic coupling, reorganizational energies, reaction pathways and dynamics

Strong inter-nanocrystal electronic coupling is a prerequisite for delocalization of exciton wave functions and high conductivity. We report 170 meV electronic coupling energy of short chain poly(ethylene glycol) thiolate-coated ultrasmall (<2.5 nm in diameter) CdSe semiconductor nanocrystals (SNCs) in solution. Cryo-transmission electron microscopy analysis showed the formation of a pearl-necklace assembly of nanocrystals in solution with regular inter-nanocrystal spacing. The electronic coupling was studied as a function of CdSe nanocrystal size where the smallest nanocrystals exhibited the largest coupling energy. The electronic coupling in spin-cast thin-film (<200 nm in thickness) of poly(ethylene glycol) thiolate-coated CdSe SNCs was studied as a function of annealing temperature, where an unprecedentedly large, ∼400 meV coupling energy was observed for 1.6 nm diameter SNCs, which were coated with a thin layer of poly(ethylene glycol) thiolates. Small-angle X-ray scattering measurements showed that CdSe SNCs maintained an order array inside the films. The strong electronic coupling of SNCs in a self-organized film could facilitate the large-scale production of highly efficient electronic materials for advanced optoelectronic device application.

A defect engineering of inorganic solids garners great deal of research activities because of its high efficacy to optimize diverse energy‐related functionalities of nanostructured materials. In this study, a novel in situ defect engineering route to maximize electrocatalytic redox activity of inorganic nanosheet is developed by using holey nanostructured substrate with strong interfacial electronic coupling. Density functional theory calculations and in situ spectroscopic analyses confirm that efficient interfacial charge transfer takes place between holey TiN and Ni−Fe‐layered double hydroxide (LDH), leading to the feedback formation of nitrogen vacancies and a maximization of cation redox activity. The holey TiN−LDH nanohybrid is found to exhibit a superior functionality as an oxygen electrocatalyst and electrode for Li−O2 batteries compared to its non‐holey homologues. The great impact of hybridization‐driven vacancy introduction on the electrochemical performance originates from an efficient electrochemical activation of both Fe and Ni ions during electrocatalytic process, a reinforcement of interfacial electronic coupling, an increase in electrochemical active sites, and an improvement in electrocatalysis/charge‐transfer kinetics.