Ultrafast Regime Research Articles

Fast Molecular Rotor Dynamics Modulated by Guest Inclusion in a Highly Organized Nanoporous Organosilica

Rotor speed regulation in the hybrid walls of porous p-phenylenesilica can be achieved by guest uptake and removal. A graduation from slow to ultrafast motional regimes (k=104–108 s−1) can be experienced by the molecular rotors depending on guest nature and temperature, as detected by solid-state spin-echo 2H NMR spectroscopy (see picture).

Ultrafast terahertz photoconductivity in nanocrystalline mesoporous TiO2 films

Terahertz time-resolved spectroscopy is used to investigate the transport of photoexcited electrons in nanocrystalline mesoporous TiO2 films prepared by the recently proposed “brick and mortar” technology [Szeifert et al.Chem. Mater. 21, 1260 (2009)] with a variable fraction of nanocrystalline titania “bricks” and amorphous titania “mortar.” Both long- and short-range conductivity is significantly enhanced upon calcination. After an ultrafast (subpicosecond) regime where the intrananograin conductivity dominates, the electron conductivity becomes limited by the interaction of electrons with the amorphous mortar. Comparison of the experimental results with Monte Carlo simulations of the electron motion allows us to determine the crystalline grain size after calcination and the yield of mobile photocarriers.

Coherent dynamics and ultrafast excited state relaxation of blue copper protein; plastocyanin

Ultrafast transient absorption measurements in the femtosecond to picosecond time region were carried out for a blue copper protein, plastocyanin (Pc). To compare the dynamical profiles after photoexcitation upon the ligand-to-metal-charge-transfer (LMCT) band and the d-d transition band, the pump wavelength was set at wavelengths of 597 and 895 nm, respectively. The results were nearly identical, indicating that the transition from the LMCT to the lower ligand field (LF) states takes place in an ultrafast time regime of less than 40 fs. Subsequently, relaxation in the LF state occurs with a time constant of 90 fs and the system returns to the ground state with that of 250 fs. The longest time constant of 1.8 ps was attributed to the vibrational cooling in the ground state. Several wavepacket motions were observed, including Franck-Condon type motion at approximately 510 nm and a Herzberg-Teller type motion at 660-720 nm. Critically damped low-frequency oscillation of approximately 30 cm(-1) was also observed with both excitation wavelengths with the strongest amplitude around 600 nm. This oscillation could be due to the motion of the protein that is ballistically stimulated by ultrafast relaxation.

Two electron response to an intense x-ray free electron laser pulse

New x-ray free electron lasers (FELs) promise an ultra-fast ultra-intense regime in which new physical phenomena, such as double core hole formation in at atom, should become directly observable. Ahead of x-ray FEL experiments, an initial key task is to theoretically explore such fundamental laser-atom interactions and processes. To study the response of a two-electron positive ion to an intense x-ray FEL pulse, our theoretical approach is a direct numerical integration, incorporating non-dipole Hamiltonian terms, of the full six-dimensional time-dependent Schroedinger equation. We present probabilities of double K-shell ionization in the two-electron positive ions Ne8+ and Ar16+ exposed to x-ray FEL pulses with frequencies in the range 50 au to 300 au and intensities in the range 1017 to 1022 W/cm2.

What Can We Learn from Three-Dimensional Infrared Spectroscopy?

The low-frequency part of the vibrational spectrum of a liquid is dominated by intermolecular degrees of freedom. Hence, it reports on the motion of solvent molecules with respect to each other rather than on the intramolecular details of individual molecules. In hydrogen-bonded liquids, in particular water, a detailed understanding of the low-frequency spectrum is enormously complicated because of the complex hydrogen-bond network, which constantly rearranges on an ultrafast femtosecond to picosecond time scale. Many of the peculiar properties of water have their origin in these processes. Conventional far-infrared (far-IR) or Raman spectroscopy, as well as two-dimensional IR (2D-IR) spectroscopy, are all linear with respect to the intermolecular (solvent) degrees of freedom. These spectroscopies tell us much about the density of states in the low-frequency range but little about the dynamics of the hydrogen-bond making and breaking. In this Account, we propose three-dimensional IR (3D-IR) spectroscopy as a novel tool that is nonlinear with respect to these low-frequency degrees of freedom; hence, it may provide much more detailed insights into intermolecular dynamics. The first experimental realizations of 3D-IR spectroscopy have been demonstrated in the literature; the information it affords is similar to that of 2D-Raman spectroscopy. Three-dimensional IR spectroscopy will, for the first time, reveal whether the low-frequency part of the vibrational spectrum of liquids has to be considered mostly homogeneously or inhomogeneously broadened. Alternately, we may find that either of these classifications is completely wrong because the normal mode picture fails when thermal energy is of the same order of magnitude as the ruggedness of the intramolecular potential energy surface. We briefly introduce the theoretical background of 3D-IR spectroscopy and discuss two of its most promising applications: (a) the more thorough characterization of non-Gaussian stochastic processes such as the hydrogen-bond dynamics of water and (b) non-Markovian ultrafast exchange processes. In the ultrafast regime, many of the otherwise valid simplifying assumptions of nonequilibrium statistical mechanics (for example, linear response and Markovian dynamics) are likely to fail; 3D-IR spectroscopy will allow us for the first time to experimentally explore their range of validity.

Ultrafast spin demagnetization by nonthermal electrons of TbFe alloy film

An ultrafast spin demagnetization process of an amorphous Tb35Fe65 alloy film has been investigated by means of an all-optical pump-probe technique. Interestingly, steplike demagnetization on a subpicosecond time scale is observed before a much slower change on a time scale of tens of picoseconds. The steplike demagnetization at the subpicosecond scale is explained by the extended three-temperature model considering the interaction between a nonthermal electron and a spin system. The characteristic of subpicosecond demagnetization of TbFe alloy film is expected to be very useful in the manipulation of the spin state in ultrafast regime.

Naimark-DilatedPT-Symmetric Brachistochrone

The quantum mechanical brachistochrone system with a PT-symmetric Hamiltonian is Naimark-dilated and reinterpreted as a subsystem of a Hermitian system in a higher-dimensional Hilbert space. This opens a way to a direct experimental implementation of the recently hypothesized PT-symmetric ultrafast brachistochrone regime of Bender et al. [Phys. Rev. Lett. 98, 040403 (2007)] in an entangled two-spin system.

S11/4 Fast ligand and electron transfer dynamics in oxidases and cytochrome c

The active site of heme-copper oxidases contains two cofactors, heme a3 and CuB, which can both bind external ligands as the substrate O2 and signalling molecules NO and CO, and which are both involved in electron transfer processes. Over the last few years we have exploited the fact that the heme-ligand bond can be dissociated by a short light pulse to explore the dynamics of CO and NO in the active site and the interaction between the two cofactors using ultrafast spectroscopic techniques. For example, we have time-resolved the CO transfer from heme a3 to CuB and shown that it occurs in a ballistic way in ∼ 500 fs, which presumably reflects rigidity of the active site. Heme a is located close to heme a3 (∼ 7 A edge-to-edge) and acts as electron donor for the active site. Using mixed valence oxidases we have extended the 'reverse electron flow' technique to the ultrafast regime and demonstrated that this electron transfer process occurs in only 1.2 ns. The process is activationless and associated with a very low reorganization energy (< 200 meV), in contrast to common assumptions but in general agreement with the hydrophobic environment of the reactants. Finally, ligand dynamics in native and modified cytochrome c reflects the rigidity required for optimal electron transfer properties.

Terahertz emission from ultrafast optical orientation of spins in semiconductors: Experiment and theory

We discuss the optical injection of magnetization into a nonmagnetic semiconductor by the absorption of circularly polarized light. A microscopic approach, which is based on Fermi's golden rule and $\mathbf{k}\ensuremath{\cdot}\mathbf{p}$ band models, is used to quantify the magnetization-injection rate in GaAs. We find that under conditions typical in optical orientation experiments, the magnetization-injection rate of holes is approximately 20 times larger than it is for electrons, reflecting the large hole magnetic moment. We then turn to the ultrafast excitation regime and explore the possibility that the injected magnetization can radiate a detectable terahertz field. By using a phenomenological approach for the magnetization relaxation dynamics, we predict that the terahertz field from magnetic injection is at the limit of current terahertz detection technology. We provide initial experimental measurements in search of this terahertz radiation.

Band structure anisotropy effects on the ultrafast electron transport in 4H-SiC

We study band structure anisotropy effects on the electron transport transient in 4H-SiC subjected to electric fields parallel and perpendicular to the c -axis direction. Coupled Boltzmann-like energy–momentum balance transport equations are solved numerically within a single equivalent isotropic valley picture in the momentum and energy relaxation time approximation. The electron drift velocity is shown to be higher in the direction parallel to the c -axis than that perpendicular to it, due to the electron effective mass being larger in the former direction. The ultrafast transport regime develops on a subpicosecond scale (≲0.2 ps) in both directions, during which an overshoot in the electron drift velocity is observed at 300 K for sufficiently high enough electric fields (> 60 kV/cm).

High lattice temperature effects on the ultrafast electron transport in 4H-SiC

High lattice temperature effects on the electron transport transient in the 4H-SiC c-parallel direction are studied within a single equivalent isotropic valley picture in the momentum and energy relaxation time approximation. The ultrafast transport regime occurs in a subpicosecond scale (&amp;lt;0.2ps), during which an overshoot in the electron drift velocity starts to be evident for high electric fields (≳60kV∕cm), depending on the lattice temperature. An increase of the electric field strength shifts the overshoot peak of the electron drift velocity to an earlier time. For a strong enough electric field, a higher lattice temperature cannot eliminate the electron drift overshoot effect, but can reduce it considerably due to a stronger electron-phonon scattering.

Normal and hyperspherical mode analysis of NO-doped Kr crystals upon Rydberg excitation of the impurity

Molecular dynamics simulations and both normal mode and hyperspherical mode analyses of NO-doped Kr solid are carried out in order to get insights into the structural relaxation of the medium upon electronic excitation of the NO molecule. A combined study is reported on the time evolution of the cage radius and on the density of vibrational states, according to the hyperspherical and normal mode analyses. For the hyperspherical modes, hyper-radial and grand angular contributions are considered. For the normal modes, radial and tangential contributions are examined. Results show that the first shell radius dynamics is driven by modes with frequencies at approximately 47 and approximately 15 cm-1. The first one is related to the ultrafast regime where a large part of the energy is transmitted to the lattice and the second one to relaxation and slow redistribution of the energy. The density of vibrational states gamma(omega) is characterized by a broad distribution of bands peaking around the frequencies of approximately 13, approximately 19, approximately 25, approximately 31, approximately 37, approximately 47, and approximately 103 cm-1 (very small band). The dominant modes in the relaxation process were at 14.89, 23.49, and 53.78 cm-1; they present the largest amplitudes and the greatest energy contributions. The mode at 14.89 cm-1 is present in both the fit of the first shell radius and in the hyper-radial kinetic energy spectrum and resulted the one with the largest amplitude, although could not be revealed by the total kinetic energy power spectrum.

Nonadiabatic response of molecules to strong fields of picosecond, femtosecond, and subfemtosecond duration: An experimental study of the methane dication

The double ionization of methane has been accomplished using strong optical fields that are generated using moderately intense lasers, and by strong fields that are induced by fast-moving, highly charged ions. In the former case laser intensities in the range 10(14) W cm(-2) generate fields whose durations are of 35 ps and 36 fs while in the latter case equivalent fields last for only 200-300 as. The dynamics of the field-ionized electrons are different in the two temporal regimes, fast (picoseconds), and ultrafast (few tens of femtoseconds and subfemtoseconds). Our experiments show that nonadiabatic effects come into play in the ultrafast regime; we directly monitor such effects by measuring the kinetic energy that is released when a specific bond in the doubly charged methane molecular ion breaks.

Effect of Varying Dwell Time During Non-melt Laser Annealing of Boron Implanted Silicon

AbstractAs the fabrication for the CMOS technology trends to decreasing the size of transistors for the benefit of increased performance and device density, the implant and annealing process technologies are being forced to meet increasingly challenging demands for ultra-shallow junctions. As the current annealing preferences shift from rapid thermal annealing toward sub-second RTP technologies such as flash and laser annealing, the dynamics of micro-structural evolution on the order of milliseconds and microseconds become of increasing importance. To investigate the kinetics in this ultra-fast annealing regime, this study investigated the effect of varying the temperature and time of a scanning laser anneal in the millisecond and microsecond timeframe. Amorphous layers were created in silicon substrates using a 30 keV silicon implant at a dose of 1x1015 ions/cm3. These wafers were then processed using a scanning continuous wave near infra-red laser at varying temperatures and scan rates in order to vary the dwell time of each condition. A combination of sheet resistance, optical interferometry, and secondary ion mass spectrometry characterization techniques were used in order to investigate the activation and diffusion of these laser annealed samples. The regrowth temperature across all samples showed a positive correlation to the sheet resistance; trending toward lower values as temperatures increased. Dwell time, however, showed an inverse effect at lower temperatures. The microsecond anneal at 1200 °C showed a lower Rs than the millisecond anneal, while at higher temperatures the trend reversed. This trending shows an obvious competition between activation and diffusion mechanisms and their dependence on regrowth temperature.

Ultrafast non-linear optical signal from a single quantum dot: exciton and biexcitoneffects

We present results on both the intensity and phase dynamics of the transient non-linearoptical response of a single quantum dot (SQD). The time evolution of the four wavemixing (FWM) signal on a subpicosecond timescale is dominated by biexcitoneffects. In particular, for the cross-polarized excitation case a biexciton boundstate is found. In this latter case, mean-field results are shown to give a poordescription of the non-linear optical signal at small times. By properly treatingexciton–exciton effects in an SQD, coherent oscillations in the FWM signal are clearlydemonstrated. These oscillations, with a period corresponding to the inverse of thebiexciton binding energy, are correlated with the phase dynamics of the system’spolarization, giving clear signatures of non-Markovian effects in the ultrafast regime.

Coherence properties of a radiating electron–hole condensate

We analyze both the spatial as well as the temporal coherence of an electron–hole condensate and the radiation emitted from it. These coherences evolve from being full for the low density Bose–Einstein condensate to a chaotic behavior for a high density Bardeen–Cooper–Schrieffer-like state. Time coherence is transferred, to the emitted radiation in the ultrafast regime, in a damped oscillatory way.

Magneto-optics in the ultrafast regime: thermalization of spin populations in ferromagnetic films.

We have characterized by pump-probe polarimetry the time-dependent dielectric tensor in a CoPt3 ferromagnetic film excited by 20 fs laser pulses. It is shown that, after the thermalization time of the electrons (approximately 50 fs), the dynamics of the real and the imaginary parts of the Voigt vector are identical. In addition, their relative variation is 10 times larger than that of the diagonal elements of the tensor, which allows one to infer that the spins dominate the magneto-optical response. During the thermalization process, the temporal behavior of the tensor elements opens new questions concerning the dynamics of the spins associated to a nonthermal electronic population in a ferromagnet.

Restrictions on the coherence of the ultrafast optical emission from an electron-hole-pair condensate.

We report on the transfer of coherence from a quantum-well electron-hole condensate to the light it emits. As a function of density, the coherence of the electron-hole-pair system evolves from being full for the low-density Bose-Einstein condensate to a chaotic behavior for a high-density BCS-like state. This degree of coherence is transferred to the light emitted in a damped oscillatory way in the ultrafast regime. Additionally, the photon field exhibits squeezing properties during the transfer time. Our results suggest a new type of ultrafast experiments for detecting electron-hole-pair condensation.

Femtosecond Z-scan Measurement of the Third Order Nonlinear Optical Coefficient of CuPc Thin Film

The third order nonlinear refractive index and absorption coefficient of copper-phthalocyanine (CuPc) thin film were measured by Z-scan method in ultrafast time regime. By changing pulse separation, we found that the nonlinear absorption behavior is different for the different pulse repetition rate. At 4MHz, the film exhibits RSA (reverse saturable absorption), while at 800kHz and 400kHz it shows SA (saturable absorption). Also the nonlinear refractive index is found to be different at 4MHz and other repetition rates.

Pulse-length dependence of high-order harmonic generation in dissociating cyclic organic molecules

We compare high-order harmonic generation in gas jets of benzene, cyclohexane, and xenon using \ensuremath{\sim}800-nm laser pulses of 70 fs and 240 fs duration. The harmonic intensities of the 7th to 15th orders are studied as a function of laser intensity in the range $5\ifmmode\times\else\texttimes\fi{}{10}^{13}$ to $5\ifmmode\times\else\texttimes\fi{}{10}^{15}{\mathrm{W}\mathrm{}\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}.$ Using 240-fs pulses, the dependence of the harmonic intensity on the laser intensity of the organic molecules exhibits significant deviations from the behavior of atoms, and shows strong evidence for harmonic generation from molecular fragments as well as the parent molecule. When the pulse duration is reduced to 70 fs, the intensity dependences of the harmonics are similar to those of xenon. This is attributed to the significant reduction in dissociation occurring during the shorter pulse and suggests that high-order harmonic generation in the ultrafast regime will be important in the understanding of molecular electron dynamics in strong fields.