Light-matter interaction and polarization of single ZnO nanowire lasers

The optical properties of single zinc oxide (ZnO) nanolasers are investigated. ZnO nanowires with different diameters and lengths are prepared by chemical vapor transport. The diameter plays an important role in the stimulated emission process in nanowires. The spectral shift and spacing of Fabry-Pérot-type modes imply a strong light-matter interaction in the lasing nanowires, which is explained by the exciton-polariton model. The polarization of the electric field in the lasing nanowires is perpendicular to the long axis of the nanowire and parallel to the substrate plane. The coexistence of the transverse modes is distinguished by decomposing the peak shape and the degree of polarization. In addition to the transverse mode of the lasing with the polarization parallel to the substrate plane, the lasing mode with the polarization perpendicular to the substrate plane is observed.     

Photogating effects of HgTe nanoparticles on a single ZnO nanowire

In this study, we demonstrate the photogating effects of p-type HgTe nanoparticles (NPs) on an n-type ZnO nanowire (NW). The photogating effects are due to the charge separation of the charge carriers photogenerated in the NPs under illumination and the subsequent accumulation of the photogenerated electrons in the pn junction of the NPs and the NW. The presence of the electrons in the junction reduces the current in the ZnO NW. The photogating effects are proved by the different photocurrent behavior of the ZnO NW to which the HgTe NPs are attached from that of a bare ZnO NW. In addition, the dependence of the photogating effects on the power of the incident light is discussed.     

Ultrafast dynamics of UV-excited imidazole

The ultrafast dynamics of UV-excited imidazole in the gas phase is investigated by theoretical nonadiabatic dynamics simulations and experimental time-resolved photoelectron spectroscopy. The results show that different electronic excited-state relaxation mechanisms occur, depending on the pump wavelength. When imidazole is excited at 239.6 nm, deactivation through the NH-dissociation conical intersection is observed on the sub-50 fs timescale. After 200.8 nm excitation, competition between NH-dissociation and NH-puckering conical intersections is observed. The NH-dissociation to NH-puckering branching ratio is predicted to be 21:4, and the total relaxation time is elongated by a factor of eight. A procedure for simulation of photoelectron spectra based on dynamics results is developed and employed to assign different features in the experimental spectra.     

Ultrafast dynamics in excited ammonia dimers and trimers

Ammonia dimers and trimers are studied in a femtosecond pump probe experiment. The pump laser (6.2 eV) excites the cluster into the à state which is ionized either by 4.6 eV or by 3.1 eV probe photons. Characteristic differences are explained in terms of a kinetic model involving an internal protonated neutral excited state as an intermediate.     

Ultrafast energy transfer and structural dynamics in DNA

Ultrafast structural dynamics concomitant to excitation energy transfer in DNA has been studied using a pair of pyrene-labeled DNA bases. The temporal evolution of the femtosecond pump-probe spectra reveals the existence of two electronic coupling pathways, through-base stack and through-space, which lead to excitation energy transfer and excimer formation even when the labeled DNA bases are separated by one AT base pair. The electronic coupling which mediates through-base stack energy transfer is so strong that a new absorption band arises in the excited-state absorption spectrum within 300 fs. From the analysis of time-dependent spectral shifts due to through-space excimer formation, the local structural dynamics and flexibility of DNA are characterized on the picosecond and nanosecond time scale.     

Ultrafast dynamics of isolated fluorenone

The ultrafast dynamics of isolated 9-fluorenone was studied by femtosecond time-resolved photoionization and photoelectron spectroscopy. The molecule was excited around 264-266 nm into the S(6) state. The experimental results indicate that the excitation is followed by a multistep deactivation. A time constant of 50 fs or less corresponds to a fast redistribution of energy within the initially excited manifold of states, i.e., a motion away from the Franck-Condon region. Internal conversion to the S(1) state then proceeds within 0.4 ps. The S(1) state is long-lived, and only a lower bound of 20 ps can be derived. In addition, we computed excited state energies and oscillator strengths by TD-DFT theory, supporting the interpretation of the experimental data.     

Ultrafast dynamics of water in cationic micelles

The effect of confinement on the dynamical properties of liquid water is investigated for water enclosed in cationic reverse micelles. The authors performed mid-infrared ultrafast pump-probe spectroscopy on the OH-stretch vibration of isotopically diluted HDO in D(2)O in cetyltrimethylammonium bromide (CTAB) reverse micelles of various sizes. The authors observe that the surfactant counterions are inhomogeneously distributed throughout the reverse micelle, and that regions of extreme salinity occur near the interfacial Stern layer. The authors find that the water molecules in the core of the micelles show similar orientational dynamics as bulk water, and that water molecules in the counterion-rich interfacial region are much less mobile. An explicit comparison is made with the dynamics of water confined in anionic sodium bis(2-ethythexyl) sulfosuccinate (AOT) reverse micelles. The authors find that interfacial water in cationic CTAB reverse micelles has a higher orientational mobility than water in anionic AOT reverse micelles.     

Ultrafast dynamics of a spiropyran in water

The reversible switching of a water-soluble spiropyran compound is recorded over 1 ns by means of femtosecond vis-pump/vis- and IR-probe spectroscopy under aqueous conditions. Our investigations reveal that the photochemical conversion from the closed spiropyran to the open merocyanine takes 1.6 ps whereas the reversed photoreaction is accomplished within 25 ps. The combination of time-resolved and steady-state observations allows us to reveal central parts of the reaction pathway leading to either form. The enhanced water solubility, its fast and efficient switching behavior, and its stability against hydrolysis over a time range of several weeks make this compound an attractive and versatile tool for biological applications.     

Ultrafast dynamics in aqueous polyacrylamide solutions.

We have investigated the ultrafast dynamics of aqueous polyacrylamide ([-CH(2)CH(CONH(2))-](n), or PAAm) solutions using femtosecond optical heterodyne-detected Raman-induced Kerr effect spectroscopy (OHD-RIKES). The observed aqueous PAAm dynamics are nearly identical for both M(w) = 1500 and 10 000. Aqueous propionamide (CH(3)CH(2)CONH(2), or PrAm) solutions were also studied, because PrAm is an exact model for the PAAm constitutional repeat unit (CRU). The longest time scale dynamics observed for both aqueous PAAm and PrAm solutions occur in the 4-10 ps range. Over the range of concentrations from 0 to 40 wt %, the picosecond reorientation time constants for the aqueous PAAm and PrAm solutions scale linearly with the solution concentration, despite the fact that the solution shear viscosities vary exponentially from 1 to 264 cP. For a given value of solution concentration in weight percent, constant ratios of measured reorientation time constants for PAAm to PrAm are obtained. This ratio of PAAm to PrAm reorientation time constants is equal to the ratio of the volume for the PAAm constitutional repeat unit (-CH(2)CHCONH(2)-) to the molecular volume of PrAm. For these reasons, we assign the polymer reorientation dynamics to motions of the entire constitutional repeat unit, not only side group motions. Simple molecular dynamics simulations of H[-CH(2)CH(CONH(2))-](7)H in a periodic box with 180 water molecules support this assignment. Amide-amide and amide-water hydrogen-bonding interactions lead to strongly oscillatory femtosecond dynamics in the Kerr transients, peaking at 80, 410, and 750 fs.     

Ultrafast stimulated emission and structural dynamics in nickel porphyrins

The excited-state structural dynamics of nickel(II)tetrakis(2,4,6-trimethylphenyl)porphyrin (NiTMP) and nickel(II)tetrakis(tridec-7-yl)porphyrin (NiSWTP) in a toluene solution were investigated via ultrafast transient optical absorption spectroscopy. An ultrashort stimulated emission between 620 and 670 nm from the S1 state was observed in both nickel porphyrins only when this state was directly generated via Q-band excitation, whereas such a stimulated emission was absent under B (Soret)-band excitation. Because the stimulated emission in the spectral region occurs only from the S1 state, this photoexcitation-wavelength-dependent behavior of Ni(II) porphyrins is attributed to a faster intersystem crossing from the S2 state than the internal conversion S2 --> S1. The dynamics of the excited-state pathways involving the (pi, pi*) and (d, d) states varies with the meso-substituted peripheral groups, which is attributed to the nickel porphyrin macrocycle distortion from a planar configuration. Evidence for intramolecular vibrational relaxation within 2 ps and vibrational cooling in 6-20 ps of a (d, d) excited state has been established for NiTMP and NiSWTP. Finally, the lifetimes of the vibrationally relaxed (d, d) state also depend on the nature of the peripheral groups, decreasing from 200 ps for NiTMP to 100 ps for the bulkier NiSWTP.     

Ultrafast charge-transfer dynamics of donor-substituted truxenones

The photophysics of two donor-substituted truxenone derivatives has been studied by femtosecond time-resolved transient absorption spectroscopy. The systems consist of a central truxenone acceptor with three triarylamine (TARA) branches which act as electron donors. Upon excitation in the visible regime an electron is transferred from the donor to the acceptor, generating a charge-separated state. This state can be probed via the characteristic absorption of the TARA radical cation around 700 nm. A second absorption band around 420 nm exhibits the same kinetics and is assigned to an absorption of the radical anion of the truxenone moiety. The back electron transfer and the recovery of the ground state can be interpreted within the framework of Marcus theory. To study the dependence of the back electron transfer on the electronic coupling, the distance between the donor and the acceptor was adjusted. Two solvents were employed, dimethylsulfoxide and dichloroethane. A biexponential decay of the bands assigned to the charge-separated state was observed, with time constants in the picosecond range. Surprisingly, the rates for electron back transfer do not follow the simple picture of the donor-acceptor distance being the determining factor. The observations are explained within a model that additionally takes steric interactions between the donor and the acceptor into account.     

Ultrafast vibrational dynamics of interfacial water

We report investigations of the vibrational dynamics of water molecules at the water–air and at the water–lipid interface. Following vibrational excitation with an intense femtosecond infrared pulse resonant with the O–H stretch vibration of water, we follow the subsequent relaxation processes using the surface-specific spectroscopic technique of sum frequency generation. This allows us to selectively follow the vibrational relaxation of the approximately one monolayer of water molecules at the interface. Although the surface vibrational spectra of water at the interface with air and lipids are very similar, we find dramatic variations in both the rates and mechanisms of vibrational relaxation. For water at the water–air interface, very rapid exchange of vibrational energy occurs with water molecules in the bulk, and this intermolecular energy transfer process dominates the response. For membrane-bound water at the lipid interface, intermolecular energy transfer is suppressed, and intramolecular relaxation dominates. The difference in relaxation mechanism can be understood from differences in the local environments experienced by the interfacial water molecules in the two different systems.     

Large-scale Ni-doped ZnO nanowire arrays and electrical and optical properties

Large-scale Ni-doped ZnO nanowire (NW) arrays are grown. The electrical conductivity of a single Ni-doped ZnO NW has been increased for 30 times. The photoluminescence (PL) spectrum of the doped ZnO NWs has a red shift, suggesting possible doping induced band edge bending. The doped NW arrays could be the basis for building integrated nanoscale transistors, sensors, and photodetectors.     

Ultrafast dynamics of exciton fission in polycrystalline pentacene

We use ultrafast transient absorption spectroscopy with sub-20 fs time resolution and broad spectral coverage to directly probe the process of exciton fission in polycrystalline thin films of pentacene. We observe that the overwhelming majority of initially photogenerated singlet excitons evolve into triplet excitons on an ～80 fs time scale independent of the excitation wavelength. This implies that exciton fission occurs at a rate comparable to phonon-mediated exciton localization processes and may proceed directly from the initial, delocalized, state. The singlet population is identified due to the brief presence of stimulated emission, which is emitted at wavelengths which vary with the photon energy of the excitation pulse, a violation of Kasha's Rule that confirms that the lowest-lying singlet state is extremely short-lived. This direct demonstration that triplet generation is both rapid and efficient establishes multiple exciton generation by exciton fission as an attractive route to increased efficiency in organic solar cells.     

Ultrafast relaxation dynamics of perchlorinated cycloheptatriene in solution

The photochemistry of perchlorinated cycloheptatriene (CHTCl(8)) has been studied by means of ultrafast pump-probe, transient anisotropy and continuous UV-irradiation experiments in various solvents as well as by DFT calculations. After UV-excitation to the 1A' '-state, two competing reactions occur--a [1,7]-sigmatropic chlorine migration via two ultrafast internal conversions and a [4,5]-electrocyclization forming octachlorobicylo[3.2.0]hepta-[2,6]-diene. The first reaction has been studied by excitation with a 263 nm femtosecond-laser pulse. Pump-probe experiments reveal a first, solvent-independent time constant, tau1(CHTCl(8)) = 140 fs, that can be associated with the electronic relaxation of the 2A'-1A' ' transition, while a second one, tau2(CHTCl(8)), ranges from 0.9 to 1.8 ps depending on the polarity of the solvent. This finding is consistent with a [1,7]-chlorine migration during the 1A'-2A' transition where the migrating chlorine atom is partly negatively charged. The charge separation has also been confirmed by DFT calculations. Transient anisotropy measurements result in a time zero value of r(0) = 0.35 after deconvolution and a decay constant of tau1(a) = 120 fs, which can be explained by vibrational motions of CHTCl(8) in the electronically excited states, 1A' ' and 2A'. After continuous UV-irradiation of CHTCl(8), octachlorobicylo[3.2.0]hepta-[2,6]-diene is primarily formed with a solvent-dependent yield. From these investigations, we suggest a relaxation mechanism for CHTCl(8) after photoexcitation that is comparable to cycloheptatriene.     

Ultrafast dynamics of halogens in rare gas solids

We perform time resolved pump-probe spectroscopy on small halogen molecules ClF, Cl2, Br2, and I2 embedded in rare gas solids (RGS). We find that dissociation, angular depolarization, and the decoherence of the molecule is strongly influenced by the cage structure. The well ordered crystalline environment facilitates the modelling of the experimental angular distribution of the molecular axis after the collision with the rare gas cage. The observation of many subsequent vibrational wave packet oscillations allows the construction of anharmonic potentials and indicate a long vibrational coherence time. We control the vibrational wave packet revivals, thereby gaining information about the vibrational decoherence. The coherence times are remarkable larger when compared to the liquid or high pressure gas phase. This fact is attributed to the highly symmetric molecular environment of the RGS. The decoherence and energy relaxation data agree well with a perturbative model for moderate vibrational excitation and follow a classical model in the strong excitation limit. Furthermore, a wave packet interferometry scheme is applied to deduce electronic coherence times. The positions of those cage atoms, excited by the molecular electronic transitions are modulated by long living coherent phonons of the RGS, which we can probe via the molecular charge transfer states.     

Ultrafast dynamics of flavins in five redox states

We report here our systematic studies of excited-state dynamics of two common flavin molecules, FMN and FAD, in five redox states--oxidized form, neutral and anionic semiquinones, and neutral and anionic fully reduced hydroquinones--in solution and in inert protein environments with femtosecond resolution. Using protein environments, we were able to stabilize two semiquinone radicals and thus observed their weak emission spectra. Significantly, we observed a strong correlation between their excited-state dynamics and the planarity of their flavin isoalloxazine ring. For a bent ring structure, we observed ultrafast dynamics from a few to hundreds of picoseconds and strong excitation-wavelength dependence of emission spectra, indicating deactivation during relaxation. A butterfly bending motion is invoked to get access to conical intersection(s) to facilitate deactivation. These states include the anionic semiquinone radical and fully reduced neutral and anionic hydroquinones in solution. In a planar configuration, flavins have a long lifetime of nanoseconds, except for the stacked conformation of FAD, where intramolecular electron transfer between the ring and the adenine moiety in 5-9 ps as well as subsequent charge recombination in 30-40 ps were observed. These observed distinct dynamics, controlled by the flavin ring flexibility, are fundamental to flavoenzyme's functions, as observed in photolyase with a planar structure to lengthen the lifetime to maximize DNA repair efficiency and in insect type 1 cryptochrome with a flexible structure to vary the excited-state deactivation to modulate the functional channel.     

Ultrafast bimolecular electron transfer dynamics in micellar media

Ultrafast photoinduced bimolecular electron transfer (ET) dynamics between 7-aminocoumarin derivatives and N,N-dimethylaniline (DMAN) has been studied in neutral (TX100), cationic (DTAB) and anionic (SDS) micellar media. A very fast decay time constant (tau(fast)) shorter than approximately 10 ps has been observed for the coumarins in the presence of DMAN in all of the three micellar media. In this time scale, reactants in the micellar phase undergo ET interactions without involving diffusion or reorientation of the reactants and thus can be envisaged as equivalent to nondiffusive bimolecular ET reaction. The fastest ET rates estimated as the inverse of the shortest lifetime components of the fluorescence decay (k(et) congruent with tau(fast)(-1)) nicely follow the predicted Marcus inversion behavior with reaction exergonicity (-DeltaG degrees), irrespective of the nature of micelles considered. Onset of inversion in ET rates occur at approximately 0.61 eV lower exergonicity in SDS and TX100 micelles compared with that in DTAB micelle and are rationalized following two-dimensional ET (2DET) theory. These differences suggest the possibility of tuning Marcus inversion by proper selection of micelles. Interestingly, ET rates (k'(et)) obtained from the conventional Stern-Volmer analysis of the relatively longer time constants of the fluorescence decays also exhibit similar Marcus correlation with DeltaG degrees, showing clear inversion behavior. Fitting of Marcus correlation curves for k(et) and k'(et) indicate two largely different values for the electronic coupling parameters. In micellar media, as the interacting donor-acceptor molecules are on an average expected to be separated by an intervening surfactant chain and the reorientation rate of the reactants is quite slow, it is predicted that the ultrafast ET (k(et)) component arises because of the surfactant separated donor-acceptor pairs that are orientated perfectly to give the maximum electronic coupling. The slower ET (k'(et)) is predicted to arise because of those pairs where the donor-acceptor orientations are not very suitable but good enough to give a sizable electronic coupling.     

Ultrafast dynamics of resonance energy transfer in cryptochrome

In this communication, we report the ultrafast dynamics of resonance energy transfer in a blue-light photoreceptor, Vibrio cholerae cryptochrome. The transfer was observed to occur in 60 ps. We also studied the local rigidity and solvation around the binding site of the photoantenna molecule. The results for the first time show energy transfer in cryptochrome suggesting some mechanistic similarities between photolyase that repairs damaged DNA and cryptochrome that mediates blue-light signaling.