Mode-selective Control Research Articles

Innovative mode selective control and parameterization for charging Li-ion batteries in a PV system

Innovative mode selective control and parameterization for charging Li-ion batteries in a PV system

Mode-selective ballistic pathway to a metastable electronic phase.

Exploiting vibrational excitation for the dynamic control of material properties is an attractive goal with wide-ranging technological potential. Most metal-to-insulator transitions are mediated by few structural modes and are, thus, ideal candidates for selective driving toward a desired electronic phase. Such targeted navigation within a generally multi-dimensional potential energy landscape requires microscopic insight into the non-equilibrium pathway. However, the exact role of coherent inertial motion across the transition state has remained elusive. Here, we demonstrate mode-selective control over the metal-to-insulator phase transition of atomic indium wires on the Si(111) surface, monitored by ultrafast low-energy electron diffraction. We use tailored pulse sequences to individually enhance or suppress key phonon modes and thereby steer the collective atomic motion within the potential energy surface underlying the structural transformation. Ab initio molecular dynamics simulations demonstrate the ballistic character of the structural transition along the deformation vectors of the Peierls amplitude modes. Our work illustrates that coherent excitation of collective modes via exciton-phonon interactions evades entropic barriers and enables the dynamic control of materials functionality.

Plasma-enabled mode-selective activation of CH4 for dry reforming: First touch on the kinetic analysis

A mode-selective control of the surface reaction is expected to be a promising approach in the heterogeneous catalysis. Nonthermal plasma is a vital solution for the generation of vibrationally excited molecules, thereby enhancing mode-selective surface chemistry. Especially, plasma-enabled promotion of heterogeneous catalysis for CH4 conversion attracts keen attention because the strong C–H bond breaking is possible via vibrational excitation of CH4 at low temperature. Similarly, vibrational excitation of CO2 possesses unique reactivity in heterogeneous catalysts. Herein, we provide a rigorous determination of kinetic parameters of CH4 dry reforming to elucidate the drastic reaction promotion mechanism enabled by plasma-catalyst interaction. Lanthanum-modified Ni/Al2O3 catalyst was combined with dielectric barrier discharge (DBD) at 5 kPa and 400–700 °C without dilution gas. Reaction order for CH4 and CO2 were determined respectively as 0.68 and −0.17; these values were kept unchanged by DBD, indicating the surface coverage of CH4 and CO2 was not influenced by nonthermal plasma. The Arrhenius plot for forward CH4 rate constant revealed that 12 kHz DBD hybrid reaction is characterized as mixed catalysis where plasma and thermal catalysis are not decoupled. The apparent activation energy was influenced only slightly by the specific energy input (SEI, eV/molecules) and gaseous hourly space velocity (GHSV, h−1), because the electrical properties of streamer swarm are not influenced to a large extent by either SEI or GHSV at fixed frequency. In contrast, 100 kHz DBD yielded significant improvement of CH4 and CO2 conversion via vibrational excitation. Activation energy decreased from 91 kJ/mol to 44.7 kJ/mol which was well correlated with the state-specific gas-surface reactivity of vibrationally excited CH4 on Ni surfaces.

Multimode EDFA With Scalable Mode Selective Gain Control at 1480-nm Pump Wavelength

Erbium-doped fiber amplifiers are a promising solution for the signal amplification in long-haul mode-division multiplexed transmission systems. In order to minimize signal degradations by these amplifiers, low mode dependent gain and noise figures are desirable. We propose a scalable mode dependent gain equalization scheme for the 1480-nm pump wavelength. Contrary to the 980-nm pump wavelength, each signal mode has a maximum overlap with only one pump mode. Due to this, a scalable mode selective gain control is achievable by adjusting the pump modes. In this letter, we demonstrate the scalability of our approach with the number of signal modes by numerical simulation. Furthermore, we propose a two-stage amplifier setup in which the mode dependent gain of both stages is compensated in the second stage, pumped at a wavelength of 1480 nm.

Electromechanically tunable metasurface transmission waveplate at terahertz frequencies

Dynamic polarization control of light is essential for numerous applications ranging from enhanced imaging to materials characterization and identification. We present a reconfigurable terahertz metasurface quarter-waveplate consisting of electromechanically actuated micro-cantilever arrays. Our anisotropic metasurface enables tunable polarization conversion cantilever actuation. Specifically, voltage-based actuation provides mode selective control of the resonance frequency, enabling real-time tuning of the polarization state of the transmitted light. The polarization tunable metasurface has been fabricated using surface micromachining and characterized using terahertz time domain spectroscopy. We observe a ~230 GHz cantilever actuated frequency shift of the resonance mode, sufficient to modulate the transmitted wave from pure circular polarization to linear polarization. Our CMOS-compatible tunable quarter-waveplate enriches the library of terahertz optical components, thereby facilitating practical applications of terahertz technologies.

Mode-selective control of thermal Brownian vibration of micro-resonator (Generation of a thermal no-equilibrium state by mechanical feedback control)

Recently, there have been various attempts to dampen the vibration amplitude of the Brownian motion of a microresonator below the thermal vibration amplitude, with the goal of reaching the quantum ground vibration level. To further develop the approach of reaching the quantum ground state, it is essential to clarify whether or not coupling exists between the different vibration modes of the resonator. In this paper, the mode-selective control of thermal Brownian vibration is shown. The first and the second vibration modes of a micro-cantilever moved by a random Brownian motion are cooled selectively and independently below the thermal vibration amplitude, as determined by the statistical thermodynamic theory, using a mechanical feedback control method. This experimental result shows that the thermal no-equilibrium condition was generated by mechanical feedback control.

Mode-selective control of the crystal lattice.

CONSPECTUS: Driving phase changes by selective optical excitation of specific vibrational modes in molecular and condensed phase systems has long been a grand goal for laser science. However, phase control has to date primarily been achieved by using coherent light fields generated by femtosecond pulsed lasers at near-infrared or visible wavelengths. This field is now being advanced by progress in generating intense femtosecond pulses in the mid-infrared, which can be tuned into resonance with infrared-active crystal lattice modes of a solid. Selective vibrational excitation is particularly interesting in complex oxides with strong electronic correlations, where even subtle modulations of the crystallographic structure can lead to colossal changes of the electronic and magnetic properties. In this Account, we summarize recent efforts to control the collective phase state in solids through mode-selective lattice excitation. The key aspect of the underlying physics is the nonlinear coupling of the resonantly driven phonon to other (Raman-active) modes due to lattice anharmonicities, theoretically discussed as ionic Raman scattering in the 1970s. Such nonlinear phononic excitation leads to rectification of a directly excited infrared-active mode and to a net displacement of the crystal along the coordinate of all anharmonically coupled modes. We present the theoretical basis and the experimental demonstration of this phenomenon, using femtosecond optical spectroscopy and ultrafast X-ray diffraction at a free electron laser. The observed nonlinear lattice dynamics is shown to drive electronic and magnetic phase transitions in many complex oxides, including insulator-metal transitions, charge/orbital order melting and magnetic switching in manganites. Furthermore, we show that the selective vibrational excitation can drive high-TC cuprates into a transient structure with enhanced superconductivity. The combination of nonlinear phononics with ultrafast crystallography at X-ray free electron lasers may provide new design rules for the development of materials that exhibit these exotic behaviors also at equilibrium.

Molecular alignment using coherent resonant excitation: A new proposal for stereodynamic control of chemical reactions

For the mode-selective control of chemical reaction, we present a new approach of molecular alignment using coherent resonant interaction with low intensity midinfrared optical pulses. Under coherent excitation, the alignment of vibrationally excited molecules becomes a function of the optical pulse area. Depending on the type of transition, with certain values of the pulse areas, a narrow group of magnetic substates are selectively excited, which results in aligning the rotational axis of the molecular ensemble. It is shown that for a P-type transition, significant alignment in the excited vibrational state can be realized using a resonant midinfrared pulse of area approximately 2pi. Under the steady state excitation (pulse duration longer than the vibrational relaxation time), the molecular alignment is destroyed due to saturation. We design a polarization spectroscopy experiment to coherently excite and probe the molecular alignment in real time.

Mode selective control of flow turbulence

We reported the control of flow turbulence governed by the two-dimensional Navier-Stokes equations. Control is achieved by using mode selective control technique which sporadically filters some modes with small wave numbers out of control signal. The numerical results show that this control strategy can significantly improve control efficiency when control parameters are suitably chosen. The physical mechanism of the control scheme is heuristically analyzed, based on mode-mode interactions.

Mode-selective excitation of hydrogen atoms on a Si surface: Non-Markovian and Markovian treatment of infrared laser driven dissipative quantum dynamics

Mode-selective excitation of adsorbates by shaped infrared laser pulses is investigated here theoretically, for the example of a H atom on a hydrogen-covered $\mathrm{Si}(100)\text{\ensuremath{-}}2\ifmmode\times\else\texttimes\fi{}1$ surface. The mode-selective excitation is perturbed by the intermode coupling within the system (bending and stretching modes) and by system-bath coupling to substrate phonons. Using a force-field based model, vibration-phonon coupling was found and predicted to lead to vibrational relaxation of the H-Si stretching mode on a ns timescale, and of the Si-Si-H bending mode on a ps timescale [I. Andrianov and P. Saalfrank, J. Chem. Phys. 124, 034710 (2006)]. To address the question as to whether in such a dissipative situation mode-selective control of adsorbate vibrational dynamics is still possible, a system-bath ansatz is used to derive an open-system density matrix theory in which the H vibrations are driven either by ${\mathrm{sin}}^{2}$, or by freely optimized infrared ps laser pulses. Both for the Si-H stretching and Si-Si-H bending vibrations mode-selective excitation is predicted to be possible. It is also found that the Markov approximation works well in most of the applications, and that simple ${\mathrm{sin}}^{2}$ are nearly as effective as pulses which were freely optimized by optimal control theory.

Mode-Selective Control of Surface Reactions

Researchers have achieved the goal of controlling chemical reactions by selectively exciting a single vibrational mode. A free-electron laser selectively desorbs Hfrom a silicon surface coated with hydrogen and deuterium.

Mode-selective mapping and control of vectorial nonlinear-optical processes in multimode photonic-crystal fibers

We demonstrate an experimental technique that allows a mapping of vectorial nonlinear-optical processes in multimode photonic-crystal fibers (PCFs). Spatial and polarization modes of PCFs are selectively excited in this technique by varying the tilt angle of the input beam and rotating the polarization of the input field. Intensity spectra of the PCF output plotted as a function of the input field power and polarization then yield mode-resolved maps of nonlinear-optical interactions in multimode PCFs, facilitating the analysis and control of nonlinear-optical transformations of ultrashort laser pulses in such fibers.

Theoretical state-of-the art in adsorbate motions and reactions induced by inelastic tunneling current with STM

Current understanding of the physical mechanisms of motion and reaction of single adsorbates induced by inelastic tunneling current with scanning tunneling microscope (STM) is described in relation to various novel experimental results. It is a central issue to elucidate how the energy stored in the specific vibrational mode excited by tunneling electrons activates the configurational change of an adsorbate, in competing with the vibrational relaxation. There are two reaction paths, i.e., direct or indirect excitation of the reaction coordinate (RC) mode to overcome the relevant potential barrier. This includes vibrational ladder climbing in the potential well following a direct excitation of the RC mode, and indirect excitation of the RC mode through its anharmonic coupling to the vibrational mode excited by tunneling electrons. A unified picture is proposed for a qualitative scenario of mode selective control of different motions of single adsorbates by a combination of bias voltage and tunneling current.

Motions and Reactions of Single Adsorbed Molecules Induced by Vibrational Excitation with STM

Recent progress in the study of motions and reactions of single adsorbed molecules on metal surfaces induced by inelastic tunneling electrons with a scanning tunneling microscope (STM) is given an overview, with the focus on our current theoretical understanding of the elementary processes behind these phenomena. The selected topics include rotation and dissociation of O 2 on Pt (111), rotation of a C 2 H(D) 2 on Cu (100), lateral hopping of CO on Pd (110), lateral translation and desorption of NH 3 on Cu (100), and controlled manipulation of chemical transformation as well as bimolecular reaction of coadsorbed species on metal surfaces. Brief descriptions are presented of how an adsorbate to overcome the potential barrier for motion and reaction by incoherent stepwise and coherent single multistep climbing of the vibrational ladders in the potential well along the reaction coordinate, and indirect excitation of the reaction coordinate mode via anharmonic coupling to the vibrational mode excited by tunneling current. Elementary processes of the mode-selective control of different motions are also discussed in conjunction with a recent experimental result of lateral hopping and desorption of a single NH 3 molecule on Cu (100). Although still at a premature stage, these novel phenomena open a new world of "nano-surface-science," in which the manipulation and reaction of single adsorbates, and synthesis of a new molecular system are realized by a selective excitation of the relevant vibrational mode by tunneling electrons with an STM.

Mode selective control of drift wave turbulence.

Experiments on spatiotemporal open-loop synchronization of drift wave turbulence in a magnetized cylindrical plasma are reported. The synchronization effect is modeled by a rotating current profile with prescribed mode structure. Numerical simulations of an extended Hasegawa-Wakatani model show good agreement with experimental results.

The wave packet motion and intramolecular vibrational redistribution in CHX3 molecules under infrared multiphoton excitation

We report results from quantum dynamical simulations of ultrafast vibrational redistribution processes in the CH chromophore of CHX3 molecules (CHD3, CHF3) during and after infrared-multiphoton excitation. The vibrational Hamiltonian is based on results from high resolution spectroscopy and ab initio calculations of the potential hypersurfaces for these molecules. The quantum dynamical calculations involve accurate solutions of the time dependent quantum equations of motion by means of both Floquet and quasiresonant approximations. We find mode selective redistribution between the CH stretching and bending modes on a time scale of 50 to 100 fs. Other modes participate only on much longer time scales (>1 ps), as was shown previously by analysis of the spectra. For the real, strongly anharmonic systems (k′sbb≂30 to 100 cm−1 ), the redistribution is nonclassical with fast spreading to a quasimicrocanonical distribution, which is particularly pronounced if a narrow range of energies (for example, the N=6 polyad) is initially excited. The effect can be interpreted as an intrinsic quantum statistical behavior induced by anharmonicity. In comparison, a weakly anharmonic hypothetical model system (ksbb≤2 cm−1) leads to quasiclassical motion of the wave packet with quasiperiodic exchange between stretching and bending motions. We present an approximate analytical investigation of the Fermi modes underlying the dynamics which provides a semiquantitative understanding of the Fermi-resonance spectra. On the basis of these results, we discuss possibilities of mode selective reaction control in unimolecular processes with laser excitation and some aspects of intramolecular ‘‘chaos.’’

A new quantum isotope effect: Extreme local mode selectivity in unimolecular dissociations imposed by antagonism between dynamic propensities of educts and zero point energies of products

We predict a new quantum isotope effect for unimolecular dissociations of molecules with two equivalent but isotopically substituted bonds l (light isotope) and h (heavy isotope), e.g., HOT where l=HO and h=OT. Consider two near-degenerate local vibrational excitations of bonds l or h, with energies between the gap of product zero point energies. Dynamically, these excitations should induce preferential fissions of bonds l or h, but energetically, these decay channels are open and closed, respectively. Therefore, local excitation of bond h must be followed by extremely slow internal vibrational energy redistribution to bond l before dissociation, whereas local excitation of bond l induces direct, rapid decay. The resulting decay rates differ by many orders of magnitudes. The effect is demonstrated by fast Fourier transform propagation of representative wavepackets for a model system, HOT→H+OT. Extended applications to more excited educts HOT also confirm an effect discovered previously for HOD, i.e., local mode selective control of competing bond fissions H+OT←HOT→HO+T.

One photon mode selective control of reactions by rapid or shaped laser pulses: An emperor without clothes?

Mode selective approaches to controlling reactions which are based upon one photon absorption of a fast and/or shaped laser pulses creating localized excitation are formally examined and found deficient. Specifically, we show that if such control is possible with laser pulses, then the same result may be obtained with incoherent sources operating over essentially cw time scales. Since the time dependence imparted by the preparation is irrelevant, related concepts, such as the idea that rapid excitation is necessary to “beat out intramolecular vibrational relaxation” are misguided.

Mode selective control of unimolecular dissociations: Survey, and model simulations for HDO → H+DO, D+HO

A survey of mode selective bond fissions, decay rates and vibrational structures in unimolecular dissociations is followed by model simulations of dissociative HDO resonance decays HDO∗( m a n b)→ H+DO, D+HO. Accordingly, the system yields extreme variations of dissociative lifetimes from ps to > ns andfissions of specific bonds, either (a) OH or (b) OD, depending on selective investments of similar energies for dominant excitations of m a plus n b quanta in the stretches of bonds a and b, in the electronic ground state. Both effects result from the decay mechanism of vibrational Feshbach resonances: diabatic transfers of preferably few vibrational quanta from the “spectator” bond that survives as product hydroxyl towards the “promoting” bond to be broken. For example, preferable transfers of single quanta from spectator bonds b and a of near-degenerate local modes resonancesHDO* (17 a1 b) and HDO* (1 a22 b) induce exclusive fissions of “promoting” bonds a (OH) and b (OD), respectively, on the picosecond-time scale. In contrast, dissociation of the near-degenerate hyperspherical-type resonance HDO*(9#a#7#b#) would require rather improbable transfers of several (>/7) vibrational quanta to be accumulated in a single bond, implying much longer (>ns) decay times. Mode selective dissociation dynamics correlate with vibrational structures: local and hypersphericalotype modes have wavefunctions which extend along the potential valleys of dissociative bonds and along ellipsoidal arcs, respectively. The nodal patterns correlate well with the quantum numbers m a n b of HDO* ( m a n b). These as well as complementary, e.g., toroidal structures are also observed for excited bound states HDO( m a n b). In general, mode selectivity in HDO is more extreme than in the symmetric reference H 2O, allowing even better control of reaction rates, and novel control of exclusive product branching ratios induced by selective bond fissions. The results are derived for the simple Thiele-Wilson model of HDO, neglecting effects of bending, rotations, and potential couplings. The simulations employ fast Fourier-transform propagations of wavepackets prepared bysuperpositions of products of Morse oscillator wavefunctions adapted to bonds of asymmetric dihydrides.