Research Topics


Click the links below to learn more about our research:

Coupled electron-nuclear dynamics at conical intersections

Strong-field modification of atomic and molecular polarization

Attosecond-resolved electron dynamics in strong fields

Non-adiabatic relaxation of superexcited molecules

Ultrafast many-body dynamics in nanomaterials

Generation of high-flux isolated attosecond pulses

Funding:

  • National Science Foundation

  • Army Research Office, Department of Defense

  • Department of Energy


  • Coupled electron-nuclear dynamics at conical intersections

    Many fundamental processes in nature (e.g. photosynthesis, catalysis, metabolism) result from the complex motion of an electron through the potential energy landscape defined by the positions of atoms constituting the polyatomic molecule. However, at the points of electronic degeneracy, called conical intersections, the simplistic Born-Oppenheimer approximation based energy landscape picture breaks down. Near such points the electron is at molecular crossroads and the result of the chemical process depends on the path chosen by the electron. We filmed the complex, coherent evolution of an electron hole near an conical intersection of an molecular ion using extreme ultraviolet, attosecond light bursts as a strobe light. We observe that the character of the electronic states becomes blurred near conical intersections.

    We show that the presence of such an intersection in carbon dioxide molecule causes the electron hole distribution to oscillate back and forth within the molecule, and these quantum beats can persist for hundreds of femtoseconds. These results shed light on the question of how strongly electrons and nuclei influence each other during charge transfer reactions. From a technical viewpoint, this work establishes the attosecond spectroscopy as a powerful took for real-time study and control of electronic motion in complex molecules and materials.

    For more information see:
    "Coherent electron hole dynamics near a conical intersection” Phys. Rev. Lett. 113, 113003 (2014).

    Strong-field modification of atomic and molecular polarization

    Electric dipole response (Lorentz oscillator model) is a basic picture for light-matter interaction that elucidates the origin of absorption, refraction, dispersion, etc. Conventional photoabsorption spectroscopy provides static (time-intergrated) information about the optical response of a medium. We have implemented attosecond transient absorption (ATA) spectroscopy to study ultrafast time-resolved photoabsorption spectra in atoms, molecules and materials.

    Attosecond XUV pulses coherently prepares the atomic or molecular polarizations, and a time-delay femtosecond IR or visible pulse is used to perturb the polarization. This allows us to observe interesting dynamic phenomena such as AC Stark shifts, light-induced virtual states, spectral lineshape manipulation, quantum-path interference, resonant pulse propagation, etc. We have applied ATA to study complex dynamics in optically thick helium target and in oxygen molecule. Experimental investigations are then compared with the TDSE simulations and Multiconfiguration Time-dependent Hartree-Fock (MCTDHF) calculations.

    For more information see:
    "Attosecond transient absorption in dense gases: Exploring the interplay between resonant pulse propagation and laser-induced line-shape control” Phys. Rev. A 93, 033405 (2016).
    "Beyond the Single-Atom Response in Absorption Line Shapes: Probing a Dense, Laser-Dressed Helium Gas with Attosecond Pulse Trains" Phys. Rev. Lett. 114, 143002 93, 033405 (2016).

    Attosecond-resolved electron dynamics in strong fields

    Atomic and molecular structure is highly dynamic in the presence of an intense ultrafast laser pulse. Fundamentally new excitation, relaxation, and ionization pathways materialize within a few femtoseconds due to transient electronic modifications, which cannot be investigated using conventional lasers. We performed attosecond-resolved experiments to investigate the electronic structure of an atom under the action of a strong IR field. Using a two-color scheme, we tuned the XUV photons below the ionization threshold of helium and obtained precision measurements of the ion-yield and electron angular distributions in the presence of IR pulse. Our measurements highlighted the role of quantum interferences between photo-excitation paths. As the intensity ramps on femtosecond time scales, we observe switching between ionization channels mediated by different atomic resonances. The quantum phase difference between interfering paths is extracted for each ionization channel and compared with theTDSE simulations.

    We have introduced new methods to improve the accuracy of attosecond-resolved measurements. Our experiments also shed light on importance of Gouy phase slip in attosecond experiments. We achieve an accurate, in-situ characterization of the Gouy phase slip of an ‘annular’ beam. We have investigated the photoionization dynamics and transient electronic structure of a helium atom in the presence of strong femtosecond laser pulses. We also developed a new method that allows us to extract the absolute timing of XUV attosecond bursts relative to the driving IR pulses.

    For more information see:
    "Attosecond-Resolved Evolution of a Laser-Dressed Helium Atom: Interfering Excitation Paths and Quantum Phases" Phys. Rev. Lett. 108, 193002 (2012).
    "Photoionization dynamics in the presence of attosecond pulse trains and strong fields" Chem. Phys. 414, 139 (2013).
    "Measurement of the absolute timing of attosecond XUV bursts with respect to the driving field" Phys. Rev. A 85, 051802(R) (2012).
    "In situ spatial mapping of Gouy phase slip for high-detail attosecond pump–probe measurements" Optics Letters 35, 3312 (2010).

    Non-adiabatic relaxation of superexcited molecules

    A pervasive theme in ultrafast science is the characterization and control of energy distribution in elementary processes. Ultrashort light pulses are used to excite and probe electronic and nuclear dynamics, whose evolution is the essence of many physical and chemical phenomena. Specifically, the ultrashort XUV pulses can form highly-excited states, where the correlation effects play an important role during attosecond excitation and femtosecond fragmentation phases. Using the attosecond XUV pulses, synchronized with probe IR pulses; we aim to time-resolve the dynamics that occur within attoseconds to few femtoseconds of photon-molecule interaction. To image the photo-fragments we use two-dimensional detection scheme called velocity map imaging (VMI).

    We performed a direct measurement of the relaxation dynamics of neutral superexcited states of O2. We investigated the competing predissociation and autoionization mechanisms for superexcited molecules and found that autoionization is dominant for the low n Rydberg states. We measured an autoionization lifetime of 92 fs and 180 fs for (5s;4d)Sigma_g and(6s;5d)Sigma_g Rydberg state groups respectively. We determine that the disputed neutral dissociation lifetime for the v = 0 vibrational level of the Rydberg series is 1100 fs.

    For more information see:
    "Ultrafast dynamics of neutral superexcited Oxygen: A direct measurement of the competition between autoionization and predissociation" Phys. Rev. Lett. 109, 173001 (2012).
    "Femtosecond and Attosecond Spectroscopy in the XUV Regime " IEEE Journal of Selected Topics in Quantum Electronics 18, 351 (2012).

    Ultrafast many-body dynamics in nanomaterials

    Ultrafast dynamics of photoexcited nanomaterials is a interdisciplinary area, which offers unique opportunities for development of new optoelectronic devices. We have expertise in application of femtosecond techniques to study non-equilibrium condensed phase processes. The attosecond XUV pulses open up new avenues by allowing creation of multiple excitations within short temporal duration, thus providing a unique starting point for observation of many-particle correlation effects.

    Our first experiments focus on response of graphene to femtosecond laser irradiation. We establish that graphene has a high single-shot damage threshold. Above this threshold, a single laser pulse cleanly ablates graphene, leaving microscopically defined edges. Below this threshold, we observe laser-induced defect formation leading to degradation of the lattice over multiple exposures. We identify the lattice modification processes through in-situ Raman microscopy. Our latest work measures the electron-phonon coupling induced ultrafast band structure modification of graphene near the saddle point. Using time-resolved different transmission technique, we have measured the acoustic deformation potential which characterizes the strength of electron-phonon coupling. We have also conducted non-linear optical absorption measurements in graphene nanofragments (polycyclic aromatic hyrdocarbons) spanning the phonon energy from near-IR to UV region. We observe that the location of lowest two-photon absorption line relative to the one-photon absorption is a good indication of the strength of electronic correlation effects.

    For more information see:
    “Optical characterization of electron-phonon interactions at the saddle point in graphene”, Phys. Rev. Lett. 112, 187401 (2014).
    “Subgap Two-Photon States in Polycyclic Aromatic Hydrocarbons: Evidence for Strong Electron Correlations”, J. Phys. Chem. C 118, 3331 (2014).
    “Optical thickness determination of hexagonal boron nitride flakes”, Appl. Phys. Lett. 102, 161906 (2013).
    “Gate dependent Raman spectroscopy of graphene on hexagonal boron nitride”, J. Phys.: Condens. Matter 25, 505304 (2013).
    "Response of graphene to femtosecond high-intensity laser irradiation" Appl. Phys. Lett. 99, 051912 (2011).

    Generation of high-flux isolated attosecond pulses

    The extremely non-linear interaction of femtosecond laser pulses with noble gas atoms leads to odd high-harmonic generation in the XUV regime, which corresponds to the emission of a train of attosecond pulses in the time-domain.However, probing attosecond to few-femtosecond correlated electron dynamics requires the use of ‘isolated’ attosecond pulses. We have employed Double Optical Gating (DOG) approach to create an effective half-laser-cycle gate for ‘isolated’ attosecond pulse production. The few cycle (<8fs) NIR driver pulses were obtained by filamenting an intense 35fs laser pulse in a long hollow core Neon filled fiber, followed by the chirped mirror based pulse compression.


    The successful application of the DOG scheme results in the transformation of odd-harmonic frequency comb to a continuum of XUV emission. The XUV continuum observed in our lab corresponds to an isolated burst of 250 attosecond duration. In contrast to the gas jets or semi-infinite gas cells used in other labs, we are the first to use the hollow gas-filled waveguide geometry for isolated XUV pulse generation. The use of waveguide affords energy tunability as well as higher flux through a fine control over the phase matching conditions. We applying these isolated attosecond pulses for attosecond transient absorption for study of correlated electron dynamics in molecules and nanomaterials.

    For more information see:
    Optimization of few-cycle pulse generation: Spatial size, mode quality and focal volume effects in filamentation based pulse compression” Optics Express 17, 23894 (2009).