This review summarizes progress in coherent control as well as relevant recent achievements, highlighting, among several different schemes of coherent control, wave-packet interferometry (WPI). WPI is a fundamental and versatile scenario used to control a variety of quantum systems with a sequence of short laser pulses whose relative phase is finely adjusted to control the interference of electronic or nuclear wave packets (WPs). It is also useful in retrieving quantum information such as the amplitudes and phases of eigenfunctions superposed to generate a WP. Experimental and theoretical efforts to retrieve both the amplitude and phase information are recounted. This review also discusses information processing based on the eigenfunctions of atoms and molecules as one of the modern and future applications of coherent control. The ultrafast coherent control of ultracold atoms and molecules and the coherent control of complex systems are briefly discussed as future perspectives.

Wave-Packet and Coherent Control Dynamics

http://dipc.ehu.es/etxenike/admin/documentos/archivos/publicaciones/419SSR2013.pdf

Vibrational spectroscopy and theory of alkali metal adsorption and co-adsorption on single-crystal surfaces

In this essay we discuss the light-matter interactions at molecule-covered metal surfaces that initiate surface photochemistry. The hot-electron mechanism for surface photochemistry, whereby the absorption of light by a metal surface creates an electron-hole pair, and the hot electron scatters through an unoccupied resonance of adsorbate to initiate nuclear dynamics leading to photochemistry, has become widely accepted. Yet, ultrafast spectroscopic measurements of molecule-surface electronic structure and photoexcitation dynamics provide scant support for the hot electron mechanism. Instead, in most cases the adsorbate resonances are excited through photoinduced substrate-to-adsorbate charge transfer. Based on recent studies of the role of coherence in adsorbate photoexcitation, as measured by the optical phase and momentum resolved two-photon photoemission measurements, we examine critically the hot electron mechanism, and propose an alternative description based on direct charge transfer of electrons from the substrate to adsorbate. The advantage of this more quantum mechanically rigorous description is that it informs how material properties of the substrate and adsorbate, as well as their interaction, influence the frequency dependent probability of photoexcitation and ultimately how light can be used to probe and control surface femtochemistry.

Photoexcitation of adsorbates on metal surfaces: one-step or three-step.

Vibrational dynamics of adsorbates - Quo vadis?

This review outlines advances that have been made in recent years (2005–2007) in the study of ultrafast dynamics occurring in physical, chemical and biological research fields by using pump-probe techniques. Special emphasis is placed on coherent phenomena in materials and experimental achievements of coherent control of wave packets dynamics. After a brief theoretical description of pump-probe methods, an overview of nuclear and electronic coherences appearing in pump-probe spectroscopy is presented. Some of the highlights include molecular alignments, wave packet interferometry, hydrogen migration, and attosecond electron dynamics.

Applications of pump-probe spectroscopy

Electron-phonon coupling at an atomically defined interface : Na quantum well on Cu(111)

We have investigated coherently excited surface phonons at K-covered Pt(111) surfaces by using femtosecond time-resolved second harmonic generation spectroscopy. The frequency of the K-Pt stretching phonon mode depends on the superstructure of K: 5.0--5.3 and $4.5--4.8\phantom{\rule{0.3em}{0ex}}\mathrm{THz}$ for $(2\ifmmode\times\else\texttimes\fi{}2)$ and $(\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3})R30\ifmmode^\circ\else\textdegree\fi{}$ superstructures, respectively. In addition to the stretching mode, three surface phonon modes are simultaneously observed when the $(\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3})R30\ifmmode^\circ\else\textdegree\fi{}$ superstructure is formed. The decay time of the K-Pt stretching mode becomes shorter and its frequency redshifts as the absorbed fluence of the pump pulse increases. This is in stark contrast to the Pt surface phonon modes whose frequencies are independent of fluence. The fluence dependence of the K-Pt stretching mode is interpreted to be due to anharmonic coupling between the K-Pt stretching and lateral modes.

Coherent surface phonon dynamics at K-covered Pt(111) surfaces investigated by time-resolved second harmonic generation

Electron and coherent phonon dynamics at potassium-covered Cu(111) surfaces have been studied by using femtosecond time-resolved second-harmonic generation (TRSHG). At the coverages from 0.22 to 0.35 monolayer (ML), TRSHG traces show the oscillatory component with a frequency of $3.05\ifmmode\pm\else\textpm\fi{}0.05\text{ }\text{THz}$. The amplitude of this component decreases as coverage increases higher than $\ensuremath{\sim}0.35$ ML, whereas another oscillating component with a frequency of $1.26\ifmmode\pm\else\textpm\fi{}0.03\text{ }\text{THz}$ grows. Both components are ascribed to K-Cu stretching motion. The spectral changes with coverage suggest that the overlayer structure varies with lateral compression. The fast transient peak in TRSHG traces at around zero delay changes its sign from negative to positive when coverage exceeds $\ensuremath{\sim}0.22$ ML. Since the quantum-well state (QWS) is partly filled at around this coverage, electrons in the QWS are principally responsible for the transition of the electronic response. Furthermore, the excitation photon energy dependence of TRSHG traces indicates that the excitation of substrate $d$-band electrons, giving rise to rapid charge fluctuations in the QWS, generates the coherent K-Cu stretching vibration. Consequently, the QWS plays a major role in the electronic and nuclear dynamics induced by pump pulses at $h\ensuremath{\nu}=2.2\text{ }\text{eV}$.

Masanori Fuyuki

Papers

Electron-phonon coupling at an atomically defined interface : Na quantum well on Cu(111)

Coherent surface phonon dynamics at K-covered Pt(111) surfaces investigated by time-resolved second harmonic generation

Ultrafast electron and lattice dynamics at potassium-covered Cu(111) surfaces

Observation of Adsorbate Vibrational Dynamics Studied by Ultrafast Nonlinear Spectroscopy