Session S07: Coherent/Quantum Control

Controlling bond rearrangement in ethanol: An interesting failure?

In the past, we have successfully used adaptive control coupled to ultrafast pulse shaping to influence bond rearrangement induced by photofragmentation of a number of small molecules such as acetylene, ethylene, ethane, and methanol.  Our standard control objective is the improvement of a ratio of ionization products, such as D₂H⁺/D₃⁺ from CD₃OH. Typically, the shaped laser pulse optimized by the search algorithm can enhance the desired ratio of product channels by a factor of two or three.  When this technique was extended to ethanol, however, we could not substantially improve either the D₂H⁺/D₃⁺ or D₃⁺/D₂H⁺ ratio from CH₃CD₂OD.  This talk discusses that puzzle.   

Coherent control of coupled field-nuclei-electron dynamics in strong field molecular ionization

Similar to the notion of adiabaticity with regard to the motion of electrons following nuclei, one can consider the adiabaticity of electrons following the variation of an applied electric field. Here we consider a competition between these two types of nonadiabatic electron dynamics in the resonance-enhanced strong-field ionization of a polyatomic molecule. We drive the molecules with a chirped ultrafast laser pulse whose frequency varies with time. The strong field of the laser pulse can drive Stark-shifted multiphoton resonance enhancement of the ionization, with the chirp of the pulse determining exactly when the multiphoton resonance occurs. This creates an opportunity for the molecule to undergo internal conversion before ionization, causing the molecule to reach different states of the molecular cation. We interpret our measurements using strong-field ionization dynamics calculations, which include multiphoton resonance, dynamic Stark shifts, as well as vibrational dynamics and internal conversion during the ionization process. Together, the measurements and calculations demonstrate how the light matter coupling can compete with the coupled motions of electrons and nuclei in strong-field laser-molecule interactions and offers control over the final state of the molecular cation.   

Monitoring nonadiabatic coherence dynamics by time-resolved photoelectron spectroscopy

Time-resolved photoelectron spectroscopy (TRPES) is one of the most widely used techniques to study ultrafast molecular processes and nonadiabatic dynamics at conical intersections (CoIns). The molecular coherences emerging at CoIns contain key information about these nonadiabatic passages. However, the TRPES signal is dominated by background contributions due to state populations, and direct CoIn signatures due to molecular coherences could not be observed to date. Here, we simulate TRPES signals to monitor the relaxation of the RNA base uracil through a CoIn. We show that distinguishable signatures of coherence dynamics appear in the photoelectron signal. These could be accessed using existing ultrashort XUV and x-ray pulses, e.g., from free-electron lasers, providing most valuable information about nonadiabatic CoIn dynamics.   

Control of atomic ionization using interfering polaritonic pathways

Tunable-dressing-field attosecond transient absorption spectroscopy, which employs XUV attosecond pulse trains and strong-field tunable infrared (IR) pulses, is ideal for studying and manipulating autoionization dynamics in atomic systems. The frequency tunability of the dressing field provides control over different IR-driven couplings of the 3s^-14p bright autoionizing state in argon and the neighboring dark states. Near resonance, the degeneracy between different light-induced states and the bright state leads to the formation of autoionizing polaritons (AIPs). We present a comprehensive study on the role of different IR parameters, in particular the field strength, over AIP dynamics in the continuum. Our experimental measurements are compared with theoretical essential-state simulations, showing exceptional agreement. These results provide new avenues for quantum optical control of AIPs in multi-electronic systems.   

Rabi sideband emission from transient excitation gratings in cross-beam filament channels in a dense gas*

Femtosecond laser filamentation in dense gases gives rise to extensive generation of excited atoms or molecules. We consider transient excitation and ionization gratings formed at the crossing of two laser beams and effectively controlled by the temporal shape of the replicated laser pulse. A probe picosecond laser pulse, when incident normally on thus produced transient grating, couples with pairs of states in the excited state manifold, resulting in Rabi oscillations and Rabi sidebands emission at frequencies red- and blue-shifted about the carrier frequency of the probe beam. The emitted radiation is involved in spatial and spectral interference and produces complicated interference patterns available for remote detection. These patterns are sensitive to the excitation grating properties, such as the bean crossing angle, the phase shift between the two grating-producing pulses, the gas pressure, and the orientation of the grating with respect to the probe beam. Addressing the case of high-pressure argon gas, we investigate quantitatively modifications of the Rabi sideband interference patterns controlled by manipulation of these parameters and reflecting the state of the gas excitation in the transient filament-wake gratings.   

Visualizing coherent molecular rotation in a gaseous medium

Inducing and controlling the ultrafast molecular rotational dynamics using shaped laser fields is essential in numerous applications. Several approaches exist that allow following the coherent molecular motion in real-time, including Coulomb explosion-based techniques and recovering molecular orientation from the angular distribution of high harmonics. We theoretically [1] and experimentally [2] consider a non-intrusive optical scheme for visualizing the rotational dynamics in an anisotropic molecular gas. The proposed method allows determining the instantaneous orientation of the principal optical axes of the gas. The method is based on probing the sample using ultrashort circularly polarized laser pulses and recording the transmission image through a vortex wave plate. We consider two example excitations: molecular alignment induced by an intense linearly polarized laser pulse and unidirectional molecular rotation induced by a polarization-shaped pulse. The proposed optical method is promising for visualizing the dynamics of complex symmetric- and asymmetric-top molecules.

[1] I. Tutunnikov, E. Prost, U. Steinitz, P. Béjot, E. Hertz, F. Billard, O. Faucher, I. Sh. Averbukh, Phys. Rev. A 104, 053113 (2021)

[2] J. Bert, E. Prost, I. Tutunnikov, P. Béjot, E. Hertz, F. Billard, B. Lavorel, U. Steinitz, I. Sh. Averbukh, O. Faucher, Laser & Photonics Reviews 14, 5, 1900344 (2020).   

The detuning controlled maximum coherence via C-CARS

Maximizing vibrational coherence in Coherent Anti-Stokes Raman Spectroscopy (CARS) to enhance the signal response from molecules has been studied recently with various applications including remote detection. A chirp control scheme C-CARS for the creation of maximum coherence [1], determined from the conditions of adiabatic passage, is used to analyze the effects of resonance and two-photon detuning on the response from the system. Using the C-CARS, the maximum coherence is achievable for a broad range of Rabi frequency and chirp parameters in the scenario of a large one-photon detuning and the two-photon resonance. It is also found that for positive values of the chirp parameter, a non-zero two-photon detuning gives high coherence for a wide range of the Rabi frequency in the one-photon resonance. We also demonstrate the application of the C-CARS to remote detection of molecules in a framework of a semiclassical model [2].   

Subwavelength propagation-invariant surface plasmon polaritons

Unless deliberately confined by waveguide structures, surface plasmon polaritons (SPP) quickly diverge and disperse.  By correlating spatial and temporal frequencies of a wave packet propagating along a metal dielectric surface we show that it is possible to construct an SPP that propagates invariantly in space and time without any waveguide confinement.  Peak transverse dimensions less than the carrier wavelength as well as natural SPP localization normal to the surface are possible leading to strong localized electric fields.  We also show that the group velocity of space-time SPP wave packets can be readily tuned to subluminal, superluminal, and even negative values by tailoring the spatiotemporal field structure independently of any material properties.