I. Ultrafast dynamic imaging
Current focus of this part of the program is the development of novel approaches which:
- push the limits of temporal resolution to one-femtosecond and sub-femtosecond (attosecond) scale
- combine sub-femtosecond temporal and Angstrom-scale spatial resolution.
The approaches are complementary to those pursued at large-scale facilities such as XFEL. A route to combining high temporal and Å-scale spatial resolution without resorting to X-rays or ultra-relativistic electron beams is to use nonlinear response to intense femtosecond laser pulses (F. Krausz and M. Ivanov, Rev. Mod. Phys. 81, 163 (2009)). The physics behind this response is ionization in an intense IR laser pulse, which first liberates an electron and then brings the electron back and re-scatters it on the parent ion. The Å-scale de-Broglie wavelength of the laser-driven electron and the attosecond duration of the rescattering event yield high spatio-temporal resolution. The projects below require strong interaction of theory and experiment.
I.1. Spectroscopy of ultrafast charge/hole migration across molecules
Ultrafast sub-fsec to few-fsec charge migration is predicted to be a universal primary event that follows sudden electronic excitation (J.Breidbach and L.S. Cederbaum, Phys. Rev. Lett. 94, 033901 (2005)). Given the key role of charge & energy flow in various systems, understanding this process may have profound impact on various areas of technology and science. The objectives are:
a. Apply high harmonic spectroscopy to track charge migration (short- to mid-term).
b. Understand the coupling of charge migration to femtosecond chemical dynamics (long-term)
c. Investigate the possible role of ultrafast electron dynamics in electron transport in bio-molecules and photo-voltaic systems (long-term).
I.2. Time-resolved holography with laser-driven electrons
When an electron wavepacket, produced by strong-field ionization, is re-scattered on the parent ion, the scattering yields a hologram: the interference of scattered and transmitted waves. The IR driving laser field imposes <100 asec correlation between the electron energy and the instant of scattering. Thus, the hologram has a potential to record both amplitude and phase information about scattering, with sub-fsec temporal resolution of the scattering target. The short-term to mid-term goal is to use simple examples such as spin-orbit hole dynamics in Xe+ to check this idea, and to see if and how hole dynamics triggered by ionization is encoded in the hologram. The long-term goal is to look at polyatomic molecules.
I.3. Entanglement-enhanced measurements of correlated dynamics
This project brings together ideas from quantum optics and attosecond physics. From the quantum-optics perspective, the goal is to measure both amplitude and phase of an entangled two-electron wavefunction. With both electrons in the continuum, we are dealing with entanglement of continuous variables. From atto-physics perspective, the goal is time-resolving electron correlations and core rearrangement at ~10-asec time-scale, without ever using 10-asec pulses.
Examples include one-photon and two-photon two-electron ionization, starting with Helium. For one-photon ionization, the aim is to time-resolve two competing processes: the knock-out and the shake-off, which should have manifestly different time-scales. The required time-resolution is ~ 10 asec. For the two-photon ionization, the focus is on the interplay of direct versus sequential ionization.
Achieving sub-10 asec temporal resolution should be possible by using correlated measurement of both electrons, in the presence of a weak IR field phase-locked to ionizing XUV pulse. Absorption of IR photons creates ‘replicas'' of the original two-electron wavepacket. The phase of the two-electron wavefunction is retrieved from amplitude modulation produced by the interference of the original wavepacket with its ‘replicas''. Reconstruction of complete two-electron wavefunction from such measurements gives full information in time and energy domain. There is no fundamental limit to time-resolution - only the one set by finite count statistics, finite spectrometer resolution, and IR-XUV phase-jitter.
II. Control of quantum dynamics
Imaging and control are the two sides of the same coin. My interests in weak-field and strong-field control include:
- control of non-adiabatic transitions, charge transfer & charge localization;
- coherent /quantum control in open systems;
- quantum information (QI) -based approaches to control, and the applications of quantum control in QI.
II.1. Laser control of charge and energy flow in quantum systems
Coupled electron-nuclear dynamics at the crossings of potential energy surfaces (conical intersections) controls charge and energy flow in molecules. It plays key role in biological functions, from light harvesting to the resistance of DNA to UV damage. The long-term goals of this project include systematic study of:
- the role of electronic coherence prepared in a controlled way prior to conical intersection;
- the effects of tailored modification of intersections by shaped laser pulses;
- the control of wavepackets in many coupled degrees of freedom and implications for open systems.
Amplitudes and phases of states that make up the electronic wavepacket will matter in how it passes through the intersection of the potential energy surfaces. Topology of the intersection can also be controlled in time-dependent manner by shaped mid-IR pulses locked to the UV pump. The long-term goal is to look at exciton migration in artificial quantum systems.
II.2 Quantum control & quantum information.
The two general directions in this project are:
- Application of quantum information ideas to designing new control schemes,
- Control of wavepackets and ‘system-bath'' coupling for quantum information storage.
Precise manipulation of population amplitudes in the individual quantum states is unnecessary when only incomplete information about the system is desired or collected. This observation has far-reaching consequences. It leads to new formulation of the controllability problem, new ‘coarse-grained'' control schemes for the wavepacket dynamics, the possibility to introduce ‘qubits'' and analogues of one- and two-qubit operations in a wavepacket. Design of ‘decoherence-free subspaces'' without using the degeneracy of quantum states becomes possible. In the mid-term, we will work on (i) using nonlinear resonance to suppress decoherence in a wavepacket and (ii) interaction of nonlinear resonances and information transfer between them.
One of the longer-term plans is to work on multiplexed photon storage schemes, and the generation of arbitrarily shaped photon wavepackets. One proposal is the development of multiplexed schemes, attempting to combine EIT-type and CARS-type ideas (K. F. Reim, et al, Nature Photonics 4, 218(2010)) for storing ultra-short photon wavepackets, and using femtosecond combs for this purpose. Common wisdom suggests that EIT schemes require narrow-band pulses. The same was thought to be the case for STIRAP, until its multiplexed analogues which utilize trains of phase-stabilized few-cycle pulses where proposed (E.A. Shapiro, et al, Phys. Rev. Lett. 99, 033002 (2007), Phys. Rev. Lett. 101, 023601 (2008), Phys. Rev. A 79, 023422 (2009)). The scheme transfers wavepackets between different multi-level manifolds in the three-manifold (rather than the three-level) system.
III. Connections and Applications
The ideas driving the research directions I and II find connections and applications in areas as diverse as laser machining of dielectric materials, the development of new light sources, converting IR light into THZ and XUV/soft Xray (especially using molecules in the vicinity of nano-structured surfaces), and mass-spectrometry and nonlinear optical spectroscopy.
IV. The Marie Curie Training Network
A key component of my research and training activities until at least 2015, and possibly beyond, will be the coordination of the Marie Curie Training Network Correlated Multielectron Dynamics in Intense Light Fields (CORINF). It includes 11 nodes in the EU.
The research focus of this theory-driven network is on developing new generation of theoretical tools for intense laser-matter interaction and for imaging structures and dynamics at sub-fsec and Angstrom scales, in systems ranging from atoms to clusters to macro-molecules. Urgent demand for these methods is created by advances in ultrahsort XUV/Xray sources, from FEL-based to attosecond high harmonic generation-based sources. With the EU investing billions into high-power laser systems (ELI, HiPER), the network''s activities are perfectly timed.
CORINF will promote synergy of different areas in atomic and molecular physics, quantum chemistry, molecular spectroscopy and dynamics, and software development. The network will create unique multidisciplinary environment for training young researchers in areas developed by different communities, in the disciplines taught at different departments, and in the science in technology that will be in high demand.