Sub-ns Time Scale Research Articles

Computational modeling of nanosecond time-scale charge carrier dynamics in organic semiconductors

ABSTRACTWe present a study of photoinduced charge carrier dynamics in single crystals and polycrystalline thin films of a functionalized fluorinated anthradithiophene (ADT) derivative, ADT-TES-F, combining measurements of time-resolved photocurrent with computational modeling. Simulations revealed two competing charge generation pathways: ultrafast charge separation and nanosecond (ns) time-scale exciton dissociation. Single crystals exhibited significantly enhanced fast charge photogeneration and charge carrier mobilities, as well as lower charge trap densities and free hole-trapped electron recombination, as compared to thin films. At sub-ns time scales after photoexcitation, the light intensity dependence of the photocurrents obtained in single crystals was determined by the carrier density-dependent recombination. At longer time scales, and at lower intensities, taking into account carrier concentration-dependent mobility improved agreement between numerically simulated and experimentally measured photocurrent data.

Resonant amplification of vortex-core oscillations by coherent magnetic-field pulses

Vortex structures in soft magnetic nanodisks are highly attractive due to their scientific beauty and potential technological applications. Here, we experimentally demonstrated the resonant amplification of vortex oscillations by application of simple coherent field pulses tuned to optimal width and time intervals. In order to investigate vortex excitations on the sub-ns time scale, we employed state-of-the-art time-resolved full-field soft X-ray microscopy of 70 ps temporal and 25 nm lateral resolution. We found that, due to the resonant enhancement of the vortex gyration motion, the signal input power can be significantly reduced to ~ 1 Oe in field strength, while increasing signal gains, by increasing the number of the optimal field pulses. We identified the origin of this behavior as the forced resonant amplification of vortex gyration. This work represents an important milestone towards the potential implementation of vortex oscillations in future magnetic vortex devices.

Acoustically Driven Photon Antibunching in Nanowires

The oscillating piezoelectric field of a surface acoustic wave (SAW) is employed to transport photoexcited carriers, as well as to spatially control exciton recombination in GaAs-based nanowires (NWs) on a subns time scale. The experiments are carried out in core-shell NWs transferred to a SAW delay line on a LiNbO(3) crystal. Carriers generated in the NW by a focused laser spot are acoustically transferred to a second location, leading to the remote emission of subns light pulses synchronized with the SAW phase. The dynamics of the carrier transport, investigated using spatially and time-resolved photoluminescence, is well-reproduced by computer simulations. The high-frequency contactless manipulation of carriers by SAWs opens new perspectives for applications of NWs in opto-electronic devices operating at gigahertz frequencies. The potential of this approach is demonstrated by the realization of a high-frequency source of antibunched photons based on the acoustic transport of electrons and holes in (In,Ga)As NWs.

Time-resolved soft x-ray absorption setup using multi-bunch operation modes at synchrotrons

Here, we report on a novel experimental apparatus for performing time-resolved soft x-ray absorption spectroscopy in the sub-ns time scale using non-hybrid multi-bunch mode synchrotron radiation. The present setup is based on a variable repetition rate Ti:sapphire laser (pump pulse) synchronized with the ~500 MHz x-ray synchrotron radiation bunches and on a detection system that discriminates and singles out the significant x-ray photon pulses by means of a custom made photon counting unit. The whole setup has been validated by measuring the time evolution of the L(3) absorption edge during the melting and the solidification of a Ge single crystal irradiated by an intense ultrafast laser pulse. These results pave the way for performing synchrotron time-resolved experiments in the sub-ns time domain with variable repetition rate exploiting the full flux of the synchrotron radiation.

Photon energy dependence of the spin polarization

Spin polarization in semiconductors was investigated by pump-probe measurements, where the transmissions of the samples were monitored with probe pulses with same and opposite circular polarizations. The spin polarization as a function of the pump-probe delay was estimated, and the polarization was found to decay in a sub-ns timescale. It was also found that the polarization depends strongly on photon energy of the pump beam. The pumping energy dependence of the spin polarization was discussed based on the inter-band transition probabilities.

Spontaneous disordering of nm-grain-sized polycrystals and clusters of silica stishovite

We investigate spontaneous structural disordering of nm-grain-sized polycrystals and clusters of stishovite at ambient conditions using molecular dynamics, and compare to disordering in stishovite nanocrystals and single-crystal α-quartz at high pressures. For nanocrystals and clusters at ambient conditions and sub-ns time scales, the extent and rate of disordering increase with the surface-area-to-volume ratio, and the structure becomes completely amorphous if the size is less than about 2 nm. Within the stability field of stishovite, disordering at the grain boundaries is less pronounced. Low-pressure disordering of stishovite structure involves Si coordination number reduction from six to five and finally to four, a process opposite to the pressure-induced amorphization of quartz.

NMR structure of the conserved hypothetical protein TM0979 from Thermotoga maritima

NMR structure of the conserved hypothetical protein TM0979 from <i>Thermotoga maritima</i>

Picosecond time-resolved X-ray crystallography: probing protein function in real time

A detailed mechanistic understanding of how a protein functions requires knowledge not only of its static structure, but also how its conformation evolves as it executes its function. The recent development of picosecond time-resolved X-ray crystallography has allowed us to visualize in real time and with atomic detail the conformational evolution of a protein. Here, we report the photolysis-induced structural evolution of wild-type and L29F myoglobin over times ranging from 100 ps to 3 μs. The sub-ns structural rearrangements that accompany ligand dissociation in wild-type and the mutant form differ dramatically, and lead to vastly different ligand migration dynamics. The correlated protein displacements provide a structural explanation for the kinetic differences. Our observation of functionally important protein motion on the sub-ns time scale was made possible by the 150-ps time resolution of the measurement, and demonstrates that picosecond dynamics are relevant to protein function. To visualize subtle structural changes without modeling, we developed a novel method for rendering time-resolved electron density that depicts motion as a color gradient across the atom or group of atoms that move. A sequence of these time-resolved images have been stitched together into a movie, which allows one to literally “watch” the protein as it executes its function.

Single-photon-counting hotspot detector with integrated RSFQ readout electronics

Absorption of an infrared photon in an ultrathin film (such as 10-nm NbN) creates a localized nonequilibrium hotspot on the submicron length scale and sub-ns time scale. If a strip /spl sim/1 /spl mu/m wide is biased in the middle of the superconducting transition, this hotspot will lead to a resistance pulse with amplitude proportional to the energy of the incident photon. This resistance pulse, in turn, can be converted to a current pulse and inductively coupled to a SQUID amplifier with a digitized output, operating at 4 K or above. A preliminary design analysis indicates that this data can be processed on-chip, using ultrafast RSFQ digital circuits, to obtain a sensitive infrared detector for wavelengths up to 10 /spl mu/m and beyond, with bandwidth of 1 GHz, that counts individual photons and measures their energy with 25 meV resolution. This proposed device combines the speed of a hot-electron bolometer with the single-photon-counting ability of a transition-edge microcalorimeter, to obtain an infrared detector with sensitivity, speed, and spectral selectivity that are unmatched by any alternative technology.

Rydberg states in quantum crystals NO in solid H2

Fluorescence, excitation and fluorescence depletion spectra of the lowest Rydberg states of NO trapped in H2 matrices are reported. The absorption bands are shifted by about 0.58 eV to the blue of the gas phase energy. They are strongly broadened and exhibit an asymmetry by a blue wing. The fluorescence bands are significantly narrower, with a red wing, and lie very close to the gas phase energy. The absorption lineshape can be accounted for by the large extension of the ground state wavefunction, due to the strong contribution of the zero point motion in the H2 lattice. The absorption–emission Stokes shift is interpreted in terms of ‘bubble’ formation around the Rydberg excited molecule. A moment analysis of the absorption and emission bands in the harmonic approximation shows that most of the absorption–emission Stokes shift is used up as energy to create the ‘bubble’ around the excited molecule. The fluorescence depletion spectrum yields Rydberg–Rydberg transitions very close to the gas phase energy. This, together with the fluorescence spectra, indicates that the molecule is in a quasi-free, gas-phase-like state in the expanded cage. The excitation spectra and the fluorescence depletion spectra indicate a severe compression of the Rydberg series of NO in H2 matrices, which can be accounted for by a large negative electron affinity of solid H2. Concerning the intramolecular energy relaxation in NO, it is found that the Rydberg↔valence relaxation processes follow much the same pattern as observed in rare gas matrices for the lower valence states. For the higher valence states, a photochemical route is suggested. For the vibrational relaxation by Δv=2 in the A state and for the C–A electronic relaxation, intermolecular energy transfer processes between NO molecules are invoked, which occur in the sub-ns timescale.

Light-Emitting Conjugated Polymers in Optical Microcavities

We studied the luminescence of microcavities consisting of a single-layer of poly(para-phenylenevinylene), PPV, sandwiched between a dielectric mirror coated with a conducting indium-tin oxide layer and a semi-transparent metal electrode. Compared with a device without cavity structure, the emission is spectrally and spatially narrowed, and the forward emission intensity enhanced. The Full Width at Half Maximum of the modes is about 4 nm in photoluminescence and we measure an enhancement of intensity in the forward direction of more than an order of magnitude. The PL efficiencies and lifetimes on a sub-ns time scale show only minor alterations with respect to PPV on glass.

Solution conformation and dynamics of the octadeoxy-nucleotide d(CACTAGTG) 2: a multinuclear n.m.r. relaxation study

The conformation and internal dynamics of the octadeoxynucleotide d(CACTAGTG) 2 have been examined by 1H- and 13C-n.m.r. relaxation. All the non-exchangeable protons, the seven phosphate resonances, and most of the 13C resonances of the proton-bearing carbons have been assigned by conventional two-dimensional n.m.r. methods. The average conformations of each nucleotide have been determined using time-dependent one-dimensional n.O.e.'s and 3 J HH values derived from both NOESY and 2-quantum-filtered COSY experiments. All glycosidic torsion angles are anti, and in the range −95 to 125°, in which the pyrimidines have a significantly larger angle than the purines. All sugars were found mainly (> 80%) in the conformation range C-2′ endo to C-3′exo. The DNA fragment is within the B-family of conformations. The cytosine H-6-H-5 vectors move with an apparent correlation time of 3 ns at 25°. Cross-relaxation rate constants for the H-1′–H-2b vectors and some H-2a–H-2b and H-2a-H-3′ vectors were measured, from which order parameters were determined. The order parameters are all in the range 0.7–0.9, which is consistent with only moderate internal mobility on the sub-ns time scale. The { 1H}- 13C n.O.e. and the spin-lattice relaxation rate constant show that the terminal residues are relatively more mobile than the internal residues, and that the C-2′-H and C-3′%z.sbnd;-H vectors move with order parameters of 0.6–0.75.