Coherent Wave Packets Research Articles

Visualizing conical intersection passages via vibronic coherence maps generated by stimulated ultrafast X-ray Raman signals

The rates and outcomes of virtually all photophysical and photochemical processes are determined by conical intersections. These are regions of degeneracy between electronic states on the nuclear landscape of molecules where electrons and nuclei evolve on comparable timescales and thus become strongly coupled, enabling radiationless relaxation channels upon optical excitation. Due to their ultrafast nature and vast complexity, monitoring conical intersections experimentally is an open challenge. We present a simulation study on the ultrafast photorelaxation of uracil, based on a quantum description of the nuclei. We demonstrate an additional window into conical intersections obtained by recording the transient wavepacket coherence during this passage with an X-ray free-electron laser pulse. Two major findings are reported. First, we find that the vibronic coherence at the conical intersection lives for several hundred femtoseconds and can be measured during this entire time. Second, the time-dependent energy-splitting landscape of the participating vibrational and electronic states is directly extracted from Wigner spectrograms of the signal. These offer a physical picture of the quantum conical intersection pathways through visualizing their transient vibronic coherence distributions. The path of a nuclear wavepacket in the vicinity of the conical intersection is directly mapped by the proposed experiment.

Observation of two-photon coalescence in weak coherent wave packets

We present an experimental investigation of two-photon interference using a continuous-wave laser. Using a Mach–Zehnder interferometer, we observe two-photon coalescence and its complementary effect, Hong–Ou–Mandel interference. We also demonstrate two-photon coalescence using a Michelson interferometer. Our work paves the way for the realization of multi-photon interference in high photon loss scenarios using continuous-wave lasers.

Uncovering the Functional Role of Coherent Phonons during the Photoinduced Phase Transition in a Molecular Crystal

The atomic motions that make up phonons and molecular vibrations in molecular crystals influence their photophysical and electronic properties, including polaron formation, carrier mobility, and phase transitions. Discriminating between spectator and driving motions is a significant challenge hindering optimization. Unlocking this information and developing fine-tuned controls over actively participating phonon modes would not only lead to a stronger understanding of photochemistry but also provide a significant new tool in controlling solid state chemistry. We present a strategy using rationally designed double pulses to unveil the unique function of specific excited state phonon modes. Using ultrafast spectroscopy, we identified 50 and 90 cm-1 phonons involved in modulating the photoinduced spin-Peierls melting of potassium tetracyanoquinodimethane crystals. We show that the 50 cm-1 phonon specifically corresponds to the coherent nuclear wavepacket involved in the charge transfer component of the overall spin-Peierls phase melting process, while the 90 cm-1 phonon facilitates the phase transition component.

Vibrational decoherence induced by ultrafast intramolecular charge separation of an asymmetric bianthryl derivative.

Coherent wavepacket oscillation accompanying the ultrafast photoexcited intramolecular charge separation (CS) of 9,9'-bianthryl (BA) and 10-cyano-9,9'-bianthryl (CBA) in a room temperature ionic liquid, N,N-diethyl-N-methyl-N-(methoxyethyl)ammonium tetrafluoroborate (DemeBF4), was investigated by femtosecond time-resolved transient absorption spectroscopy. The frequency of the coherent oscillation observed for CBA in nonpolar n-hexane solution (Hex) was 377 cm-1, while this oscillation was undetectable in DemeBF4. For BA in DemeBF4, coherent oscillation with a frequency of 394 cm-1 was observed, which is similar to that for CBA in Hex. CS of CBA occurs in the ultrashort time range of ≤100 fs, while that of BA occurs in a few picosecond range [E. Takeuchi et al., J. Phys. Chem. C 120, 14502-14512 (2016)]. Hence, the oscillation of CBA in Hex and that of BA in DemeBF4 are assigned to the molecular vibration in the locally excited state, while this oscillation dephases instantaneously for CBA in DemeBF4 due to the ultrafast CS and no oscillation was generated in the CS state. This result suggests that the CS reaction is not mediated by a specific intramolecular vibration in the CS state but occurs incoherently through higher levels of multiple vibrational modes.

Time-Domain Observation of Surface-Enhanced Coherent Raman Scattering with 105-106 Enhancement.

Combining surface-enhanced Raman scattering (SERS) with the coherent nonlinear Raman technique is a promising route for achieving higher sensitivity and time-resolved SERS measurements, yet such attempts have just been started. Here, we report time-domain Raman measurements of trans-1,2-bis(4-pyridyl)ethylene (BPE) adsorbed on gold nanoparticle assemblies (GNAs), which were carried out with impulsive stimulated Raman spectroscopy using sub-8 fs pulses. We observe coherent nuclear wavepacket motion of BPE on GNAs with drastic enhancement through the surface plasmon resonance, which provides information on the Raman-active vibrations in the time domain. Through Fourier transform of the measured time-domain Raman data, we obtained SERS spectra of BPE on GNAs with enhancement factors as high as 105-106. The present study not only demonstrates applicability of time-domain nonlinear Raman techniques in SERS, i.e., surface-enhanced impulsive stimulated Raman spectroscopy (SE-ISRS), but also provides a technical basis for femtosecond time-resolved SE-ISRS experiments to track ultrafast dynamics of the adsorbates.

Femtosecond Polarization Switching in the Crystal of a [CrCo] Dinuclear Complex.

Capability to control macroscopic molecular properties with external stimuli offers the possibility to exploit molecules as switching devices of various types. However, application of such molecular-level switching has often been limited by its speed and thus efficiency. Herein, we demonstrate ultrafast, photoinduced polarization switching in the crystal of a [CrCo] dinuclear complex by ultrafast pump-probe spectroscopy in the visible and mid-infrared regions. The photoinduced polarization switching was found to have a time constant of 280 fs, which makes the [CrCo] complex crystal the fastest polarization-switching material realized using the metastable state. Moreover, the pump-probe data in the visible region reveal the pronounced appearance of coherent nuclear wavepacket motion with a frequency as low as 22 cm-1 , which we attribute to a lattice vibrational mode. The pronounced non-Condon effect for its resonance Raman enhancement implies that this mode couples the relevant electronic states, thereby facilitating the ultrafast polarization switching.

Femtosecond Polarization Switching in the Crystal of a [CrCo] Dinuclear Complex

AbstractCapability to control macroscopic molecular properties with external stimuli offers the possibility to exploit molecules as switching devices of various types. However, application of such molecular‐level switching has often been limited by its speed and thus efficiency. Herein, we demonstrate ultrafast, photoinduced polarization switching in the crystal of a [CrCo] dinuclear complex by ultrafast pump–probe spectroscopy in the visible and mid‐infrared regions. The photoinduced polarization switching was found to have a time constant of 280 fs, which makes the [CrCo] complex crystal the fastest polarization‐switching material realized using the metastable state. Moreover, the pump–probe data in the visible region reveal the pronounced appearance of coherent nuclear wavepacket motion with a frequency as low as 22 cm−1, which we attribute to a lattice vibrational mode. The pronounced non‐Condon effect for its resonance Raman enhancement implies that this mode couples the relevant electronic states, thereby facilitating the ultrafast polarization switching.

Decoherence of charge density waves in beam splitters for interacting quantum wires

Simple intersections between one-dimensional channels can act as coherent beam splitters for non-interacting electrons. Here we examine how coherent splitting at such intersections is affected by inter-particle interactions, in the special case of an intersection of topological edge states. We derive an effective impurity model which represents the edge-state intersection within Luttinger liquid theory at low energy. For Luttinger K = 1 / 2 , we compute the exact time-dependent expectation values of the charge density as well as the density-density correlation functions. In general a single incoming charge density wave packet will split into four outgoing wave packets with transmission and reflection coefficients depending on the strengths of the tunnelling processes between the wires at the junction. We find that when multiple charge density wave packets from different directions pass through the intersection at the same time, reflection and splitting of the packets depend on the relative phases of the waves. Active use of this phase-dependent splitting of wave packets may make Luttinger interferometry possible. We also find that coherent incident packets generally suffer partial decoherence from the intersection, with some of their initially coherent signal being transferred into correlated quantum noise. In an extreme case four incident coherent wave packets can be transformed entirely into density-density correlations, with the charge density itself having zero expectation value everywhere in the final state.

Complete phase retrieval of photoelectron wavepackets

Coherent, broadband pulses of extreme ultraviolet light provide a new and exciting tool for exploring attosecond electron dynamics. Using photoelectron streaking, interferometric spectrograms can be generated that contain a wealth of information about the phase properties of the photoionization process. If properly retrieved, this phase information reveals attosecond dynamics during photoelectron emission such as multielectron dynamics and resonance processes. However, until now, the full retrieval of the continuous electron wavepacket phase from isolated attosecond pulses has remained challenging. Here, after elucidating key approximations and limitations that hinder one from extracting the coherent electron wavepacket dynamics using available retrieval algorithms, we present a new method called absolute complex dipole transition matrix element reconstruction (ACDC). We apply the ACDC method to experimental spectrograms to resolve the phase and group delay difference between photoelectrons emitted from Ne and Ar. Our results reveal subtle dynamics in this group delay difference of photoelectrons emitted form Ar. These group delay dynamics were not resolvable with prior methods that were only able to extract phase information at discrete energy levels, emphasizing the importance of a complete and continuous phase retrieval technique such as ACDC. Here we also make this new ACDC retrieval algorithm available with appropriate citation in return.

Observations of the Source Region of Whistler Mode Waves in Magnetosheath Mirror Structures

AbstractIn the magnetosheath, intense whistler mode waves, called “Lion roars,” are often detected in troughs of magnetic field intensity in mirror mode structures. Using data obtained by the four Magnetospheric Multiscale (MMS) spacecraft, we show that reversals of gradient of magnetic field intensity along the magnetic field correspond to reversals of the field‐aligned component of Poynting flux of whistler mode waves in the troughs. Such a characteristic is consistent with the idea that the whistler mode waves are effectively generated near the local minima of magnetic field intensity because of the smallest cyclotron resonance velocity and propagate toward regions of larger magnetic field intensity along the magnetic field lines on both sides. We use the reversal of the Poynting flux as an indicator of wave source regions. In these regions, we find that pancake or an outer edge of butterfly electron distributions above ~100 eV are good candidates for wave generation. Unclear correlations of phase difference and amplitude variations of whistler mode waves in cases of ~40 km spacecraft separation indicate that a simple plane wave approximation with a constant amplitude is not valid at this spatial scale that is much smaller than the ion gyroradius. The whistler mode waves consist of small coherent wave packets from multiple sources with spatial scales smaller than tens of electron gyroradii transverse to the background magnetic field in a mirror mode structure.

Generation of nonlinear internal waves by flow over topography: Rotational effects.

We use the forced Ostrovsky equation to investigate the generation of internal waves excited by a constant background current flowing over localized topography in the presence of background rotation. As is now well known in the absence of background rotation, the evolution scenarios fall into three cases, namely subcritical, transcritical, and supercritical. Here an analysis of the linearized response divides the waves into steady and unsteady waves. In all three cases, steady waves occur downstream but no steady waves can occur upstream, while unsteady waves can arise upstream only when there is a negative minimum of the group velocity. The regions occupied by the steady and unsteady waves are determined by their respective group velocities. When the background current is increased, the wave number of the steady waves decreases. In addition, the concavity (canyon or sill), the topographic width, and the relative strength of the rotation play an important role in the generation mechanism. Nonlinear effects modulate the wave amplitude and lead to the emergence of coherent wave packets. All these findings are confirmed by numerical simulations.

Limitations of Generic Chromophore Concept for Femtosecond Vibrational Coherences

The spectrum of vibrational coherences modulating transient triplet–triplet absorption of bis-triisopropylsilylethynylpentacene that propagate in the course of the fission process from the singlet state of this molecule to the triplet manifold has been lately reproduced by ab initio calculations. This has led to the conclusion that, contrary to a recent hypothesis, spontaneous generation of coherent vibrational wave packets is not a constitutive feature of singlet exciton fission. Here we demonstrate by calculations that the corresponding spectrum of unsubstituted pentacene is significantly different, so that the two systems cannot be viewed as different models of the same generic chromophore. This shows that femtosecond pump–probe spectra of vibrational coherences are sensitive to subtle features of molecular structure, which we identify here, thereby rationalizing the registered spectral differences. The crucial factor is the substituent-induced coupling between some pentacene normal modes that in other...

Echo in a single vibrationally excited molecule

Echo is a ubiquitous phenomenon found in many physical systems, ranging from spins in magnetic fields to particle beams in hadron accelerators. It is typically observed in inhomogeneously broadened ensembles of nonlinear objects, and is used to eliminate the effects of environmental-induced dephasing, enabling observation of proper, inherent object properties. Here, we report experimental observation of quantum wave packet echoes in a single isolated molecule. In contrast to conventional echoes, here the entire dephasing-rephasing cycle occurs within a single molecule without any inhomogeneous spread of molecular properties, or any interaction with the environment. In our experiments, we use a short laser pulse to impulsively excite a vibrational wave packet in an anharmonic molecular potential, and observe its oscillations and eventual dispersion with time. A second delayed pulsed excitation is applied, giving rise to an echo: a partial recovery of the initial coherent wavepacket. The vibrational dynamics of single molecules is visualized by time-delayed probe pulse dissociating them one at a time. Two mechanisms for the echo formation are discussed: ac Stark-induced molecular potential shaking and creation of depletion-induced "hole" in the nuclear spatial distribution. Interplay between the optically induced echoes and quantum revivals of the vibrational wave packets is observed and theoretically analyzed. The single molecule wave packet echoes may lead to the development of new tools for probing ultrafast intramolecular processes in various molecules.

Non-Born-Oppenheimer Molecular Dynamics Observed by Coherent Nuclear Wave Packets.

The reaction dynamics of a photochemical reaction is typically described by reaction coordinates based on the Born-Oppenheimer (BO) approximation. A strong interaction between electrons and nuclei, conventionally occurring at conical intersections, however, breaks the BO approximation and has major consequences for the efficiency of a photochemical reaction. Despite its importance, related studies into the non-BO dynamics are scarce. Here, we investigate the non-BO dynamics of excited-state intramolecular proton transfer (ESIPT) occurring in 10-hydroxybenzo[h]quinoline (HBQ). Two coherent vibrational modes at 237 and 794 cm-1 representing molecular dynamics on a diabatic surface in HBQ are identified by a wave packet analysis based on a transient absorption measurement with a time resolution of 11 fs and with a density functional theory-based model calculation. It is also revealed that the strong Coulomb field effect in HBQ leads to the completion of ESIPT within about two cycles of the OH stretching mode. The work paves the way for time-domain studies of molecular dynamics beyond the BO approximation in other photochemical reactions.

Unraveling Phononic, Optoacoustic, and Mechanical Properties of Metals with Light-Driven Hypersound

A thorough understanding of the electron-phonon, thermo-optic, and acoustic properties of materials is of paramount importance for many applications in materials science and advanced applications adopting laser-induced sound waves. Even though metals are usually opaque to light, optical methods for materials characterization can still be developed, especially in the high-frequency regime, where phonon dynamics governs the thermal, acoustic, and even optical properties of metals. Ultrafast laser pulses incident on metals can lead to the generation of coherent phonon wave packets with frequencies in the gigahertz to terahertz range, providing a means to study the material properties in this otherwise inaccessible frequency range. While this principle has been known, the complex interplay of light and matter in both the generation and detection of such ultrafast hypersound pulses has limited its use mainly to geometrical effects. Here, we demonstrate the quantitative characterization of a range of different material properties using laser-driven hypersound. We use all-optical generation and detection of hypersound pulses to sensitively probe the bulk properties of various metals. We introduce an advanced two-dimensional numerical model that captures the generation, propagation, and detection of these hypersound waves in full detail. The combination of experiment and simulation allows us to unravel and elucidate various physical effects that appear over a wide range of different time scales. Through least-squares fitting of the data to the simulation results, we extract quantitative information about the electron-phonon, thermo-optic, and acoustic properties of metal films, establishing the ability to use light as a sensitive probe for the study of opaque materials.

Basic Quantum Field Model of the Self-Organization of Microtubules in Eukaryotic Cells

The background of this contribution is the ongoing extended debate over quantum effects in the biology since decades. Typical examples of the quantum biology are photosynthesis, enzymatic activities, bird navigation and especially the coherent movement of microtubules. The reason for all these effects is dominantly the quantum coherence of waves. The objective of this contribution is the quantum description of the instability dynamics of microtubules at their assembly and disassembly phases during the interphase. The corresponding theoretical investigations of this article confirm the existence of quantum coherence of microtubules. Experimental results assert such vibrations of microtubules by the observation of γ-waves in the human brain generated by bundles of microtubules. Tubulin subunits and the accessory proteins are of nano size; therefore, they are modeled as field quanta in the framework of non-relativistic quantum field theory. This approach describes the dynamics of these quantum particles and their interactions, by accentuation of their different performances as coherent or incoherent waves. The achieved results strongly depend from the preconditions: whether the fluctuating forces are turned on or turned off. With the inset of fluctuations, the quantum coherence is destroyed, and only incoherent particle solutions are obeyed. Without the impact of fluctuations wave solutions dominate. Another type of wave solution are coherent wave packets which are counter-running, where their superposition can extinct or enhance them. This kind of interfering coherent solutions is applied on the polymerization of protofilaments. The calculations of this contribution demonstrate that the quantum coherence can be only observed when fluctuations are excluded. The conclusion is that that dedicated biological processes must be able to suppress the destroying influences of the local environment. In contrary to technical-based experiments, where coherence is only obtained when the fluctuations are deliberately excluded (e.g. quantum computer). Therefore, the answer why the processes of quantum biology can generate quantum coherence at least in case of microtubules is actually not answered. Kinesins in combination with microtubules are fundamental for cellular functions and morphogenesis. Recent genetic experiments uncovered their role for tumor suppression and developmental patterning. However, these findings which open exciting new areas of kinesin research are not included in this contribution, because the description of the kinesin-microtubule system is to comprehensive for one article.

Role of coherent nuclear motion in the ultrafast intersystem crossing of ruthenium complexes.

Ultrafast intersystem crossing (ISC) in transition metal complexes leads to a long-lived active state with a high yield, which leads to efficient light energy conversion. The detailed mechanism of ISC may lead to a rational molecular design of superior transition metal complexes. Coherent nuclear wave packets observed in femtosecond time-resolved spectroscopies provide important information on the excited-state dynamics. In particular, analyzing the nuclear wave packets in both the reactant and the product may unveil the molecular dynamics of an ultrafast reaction. In this study, experimental evidence proving the reaction coordinates of the ultrafast ISC of ruthenium(ii) complexes is presented using coherent vibrational spectroscopy with a quantum chemical simulation of coherent vibrational motion. We observed vibrational modes strongly coupled to the ISC, whose vibrational coherences undergo remarkable attenuation after the ISC. The coupled modes contain metal-ligand stretching or symmetry breaking components, and the faster ISC rates of lower-symmetry ruthenium(ii) complexes support the significance of the latter.

Controlling the Orbital Alignment in Atoms Using Cross-Circularly Polarized Extreme Ultraviolet Wave Packets.

We report on the use of pairs of 10-cycle extreme ultraviolet wave packets with attosecond-controlled spacing emitted by individual relativistic electrons within an electron bunch passing through a tandem undulator. Based on the temporal coherent control technique with circular polarization, we succeeded in controlling the excited state alignment in the photoexcitation of helium atoms, which we verified through the observation of oscillation in fluorescence yield depending on the attosecond-controlled delay time. Our work demonstrates the potential of undulator radiation for the generation of longitudinally coherent wave packets suitable for attosecond coherent control, an application which has hitherto been hidden in the incoherent nature of the radiation pulse emitted by a bunch of electrons.

Cosmology as a CFT1

We show that the simplest FLRW cosmological system consisting in the homo- geneous and isotropic massless Einstein-Scalar system enjoys a hidden conformal symmetry under the 1D conformal group SL(2, ℝ) acting as Mobius transformations in proper time. This invariance is made explicit through the mapping of FLRW cosmology onto conformal mechanics. On the one hand, we identify the corresponding conformal Noether charges, as combinations of the Hamiltonian scalar constraint, the extrinsic curvature and the 3D volume, which form a closed \U0001d530\U0001d529 (2, ℝ) Lie algebra. On the other hand, this approach allows to write FLRW cosmology in terms of a AdS2 phase space and a Schwarzian action. Preserving this conformal structure at the quantum level fixes the ordering ambiguities in the Wheeler-de Witt quantization and allows to formulate FLRW quantum cosmology as a CFT1. We show that the CFT two-points correlator is realized as the overlap of the evolution in proper time of cosmological coherent wave-packets. In particular, the two-points function is built from a vacuum state which, although not conformally invariant, coincides with the cosmological vacuum annihilated by the scalar constraint. These results suggest new perspectives in classical and quantum cosmology, among which the possibility to apply the conformal bootstrap program to quantize cosmological backgrounds.

Rotating Stall Control in an Axial Fan with Pulsed-Direct-Current Plasma Actuation

The use of dielectric barrier discharge plasma actuators to suppress traveling stall cells and thereby improve stall margin is examined. This involved the design and construction of a nonconductive high-solidity fan facility. The experiments were meant to support the application of similar systems in transonic axial compressors. Eight 63.5-mm-long plasma actuator segments were installed in the casing over the blade row at three axial locations that were indicated in the literature to be optimum for stall margin improvement. Independent control of fan revolutions per minute (RPM) and mass flow was used to set conditions that produced traveling stall cells. These were documented using an azimuthal array of microphones. The microphone pressure time series were spatially correlated to reveal the coherent wave packets. Different azimuthal combinations of the plasma actuators were investigated from a single actuator to all eight. A single actuator reduced the local spatial correlation metric (SCM), a measure of stall cell activity, by 40% and the azimuthally averaged SCM by 30%. A “memory effect” on the stall cell suppression was revealed, and it persisted for approximately 180° of the casing azimuth. The effect of the plasma actuator axial location was investigated at the design RPM and at two off-design RPMs. At the design RPM, the two most optimum locations were slightly upstream of the blade row and at the midchord of the blade row. This changed for the off-design conditions. For the range of the fan RPMs and actuator locations investigated, the stall margin improvements ranged from 1.63 to 5.45%. Finally, a plasma actuator stall extension model is presented for relating the required plasma actuator body force to the achieved stall margin improvement.