Session N03: Coherent and Quantum Control: Ultrafast and Strong Field Processes

Strong field coherent control of molecular ionization

We use shaped few cycle pulses to control the dynamics during strong field molecular ionization. By varying the optical phase in a pulse sequence we can control the final state of the molecule.The control relies on a subtle interplay between the optical phase and internal conversion dynamics which occur during the ionization process. We compare experiment and theory in order to interpret the laser driven coupled electronic and nuclear dynamics.   

Quantum Coherent Control of H$_{\mathrm{3}}^{\mathrm{+}}$ Formation in Strong Fields

Quantum coherent control (QCC) has been successfully demonstrated experimentally and theoretically for two- and three-photon optical excitation of atoms and molecules. Here we explore QCC using spectral phase functions with a single spectral phase step for controlling the yield of H$_{\mathrm{3}}^{\mathrm{+}}$ from methanol under strong laser field excitation. We observe a significant and systematic enhanced production of H$_{\mathrm{3}}^{\mathrm{+}}$ when a negative \textthreequarters $\pi $ phase step is applied near the low energy region of the laser spectrum and when a positive \textthreequarters $\pi $ phase step is applied near the high energy region of the laser spectrum. In some cases, most notably the HCO$^{\mathrm{+}}$ fragment, we found the enhancement to exceed the yield measured for transform limited pulses. The observation of enhanced yield is surprising and far from the QCC prediction of yield suppression. The observed QCC enhancement implies an underlying strong field process responsible for polyatomic fragmentation controllable by easy to reproduce shaped pulses.   

Non-Adiabatic Control of the Acetylene Dication Using an Infrared Field

Non-adiabatic dynamics are affected by the relative speed of a molecular vibrational wavepacket with respect to the potential energy slope and splitting. We propose that a light field can control the kinetic energy of a molecular wavepacket in order to exert control in non-adiabatic regions. We call this type of control ``kinetic energy control,'' and examine how different photon energies affect the resultant molecular dynamics. To verify our proposal, we performed a control experiment on the acetylene dication deprotonation pathway using both 1300 nm and 800 nm light. We find that a simple parameter relating the speed and potential energy can be used to understand why 1300 nm light is a more effective control field.   

Following population transfer in a pump-dump experiment using time-resolved x-ray scattering

Molecular iodine was photoexcited using a combination of 520 and 800 nm light at variable delay. The 520 nm ‘pump’ pulse photoexcites iodine onto the bound B state, and the 800 nm ‘dump’ pulse can couple the excited population back to the X state when the resonance condition is satisfied. The ‘dump’ population was observed at the LINAC Coherent Light Source (LCLS) using time-resolved x-ray scattering (TRXS) or, rather, the temporal Fourier transform of TRXS, called frequency-resolved x-ray scattering (FRXS). The FRXS identifies a dump population with a beat frequency of $31.8\pm0.9$~THz oscillating about an equilibrium position of $3.2\pm0.1$~\AA, which match the expected values. The dump population varies with a period of $330\pm20$~fs, half of the B state vibrational period.   

Strong-field mechanism of molecular alignment based on transient nonadiabatic charge redistribution.

A new mode of effective interaction of molecular rotational degrees of freedom with an intense, nonresonant, ultrashort laser pulse is explored. This new mode of impulsive-torque interaction replaces the traditional mechanism of molecular alignment based on linear anisotropic polarizability when the strong electric field of the pulse can cause transient nonadiabatic charge redistribution (TNCR) in larger molecules or molecular ions. We explore this new alignment mechanism on a generic model of a tight-binding diatomic molecule. The TNCR mode of effective laser interaction with the rotational degrees of freedom thoroughly changes the composition of the resulting rotational wavepacket and the dynamics of subsequent field-free alignment in the molecular ensemble. The rotational wavepacket emerging from the TNCR interaction is markedly contributed to by the plethora of states with higher rotational quantum numbers, in both perturbative and non-perturbative regime; the after-pulse alignment oscillations are out-of-phase with those resulting from the traditional interaction. This TNCR interaction mode opens a new class of alignment mechanisms associated with considerable nonresonant excitation or ionization of a molecule during the laser pulse.   

Quantum optimal control of photoelectron dynamics by interfering resonantly enhanced multiphoton ionization pathways

We present a model [Phys. Rev. Lett. 122, 013204 (2019)] based on many-body electron dynamics, variational scattering theory and optimal control theory to efficiently describe photoelectron continuum states and control photoionization dynamics in randomly oriented molecules. We show that a finite manifold of indistinguishable even-parity $(1+1^\prime)$ REMPI pathways interfering at a common photoelectron energy but probing different intermediate molecular states results in a significant enhancement of the photoelectron circular dichroism (PECD) in chiral molecules, outperforming widely used schemes including interference between opposite-parity photoionization pathways driven by bichromatic ($\omega,2\omega$) fields and sequential pump-probe ionization. Based on the Reconstruction of Attosecond Beating by Interference of Two-photon Transition (RABITT) technique, we also demonstrate coherent control over the PECD at a given RABITT sideband by manipulating the quantum interference arising from different continuum-continuum photon pathways. Finally, we consider a combination of different photon polarization directions in the driven field and analyze the parity of the interfering photoionization pathways to achieve $100\%$ of anisotropy in the photoelectron emission probability.   

Quantum Control Operations in a Non-Hermitian Atomic System

Driven dynamics within a two level system that is described by a non-Hermitian Hamiltonian can be surprisingly complex. In the presence of exponential loss and/or gain, they exhibit complex eigenenergy surfaces whose real and imaginary parts are chiral functions of coupling strength, $\gamma$, and the energy separation, $\delta$ between the bare states. There has been substantial interest in exploiting the chirality of these systems to induce robust quantum state control by transporting the system around closed loops in the $\gamma$-$\delta$ parameter space. Previous studies have stressed the importance of encircling an exceptional point (EP) of degeneracy in the real part of the energy surface as the critical transformation characteristic. However, in cases where the figure of merit is the relative amplitude or population in the two eigenstates, we find that the variation in the relative gain and decay rates of the two states also plays a significant role such that encircling the EP does not necessarily determine the result of the applied controls. We propose an equivalent three-level atomic system that could enable experimental verification of these previously unexplored effects.   

Decoherence analysis in a super-effective two level CARS scheme

The use of Coherent anti-Stokes Raman Spectroscopy (CARS) for remote detection is practical under the condition that the coherence between vibrational states in the target molecules is maximized. To this end, a new adiabatic control method consisting of reducing the four level CARS scheme into a super-effective two level scheme was developed in previous works [1]. In this work, we have applied the theory of decoherence to the Liouville-von Neumann equation for the time evolution of the density matrix. The theory was developed on the basis of collisional decay in two coupled lambda systems. The analysis and numerical results guided further models on how the fields will propagate through the target molecules.\\ \\References: Neil Pandya, Gengyuan Lui, Elliot Pachniak, Jabir Chathanathil, Svetlana Malinovskaya, Maximum coherence control technique in a super-effective two-level CARS system (paper in progress).   

Minimizing the Duration of Isolated Attosecond Pulses

We have employed an analytic method of calculating high-order harmonic generation (HHG) by few-cycle pulses to explore how the duration of an isolated attosecond pulse can be minimized by carefully selecting and coherently combining frequencies in the HHG spectra produced by ultrashort driving pulses. One advantage of the analytic method is that it allows very fast calculations even for very long driving wavelengths. In the analytic description, all the short and long trajectory contributions from one laser cycle are encoded into an Airy function. The computation time using this analytic method is negligibly short comparing to solving the time-dependent Schr\"{o}dinger equation (TDSE). HHG spectra obtained from the analytic method are accurate near the cutoff and compare well with TDSE results over the entire plateau region. Moreover, this analytic method provides clear physical interpretations that we have previously utilized to explore how HHG can be enhanced by time delays[1] or chirps[2] in a two-pulse setup. Using this analytic method, we present attosecond-pulse results obtained by selecting various frequency portions of the HHG spectrum produced by a short driving pulse. [1] D. Peng et al., Phys. Rev. A 95, 033413 (2017). [2] D. Peng et al., Phys. Rev. A 97, 053414 (2018).   

Control of Harmonic Generation by the Time Delay Between Two-Color, Bicircular Few-Cycle Mid-IR Laser Pulses

We study control of high-order harmonic generation (HHG) driven by time-delayed, few-cycle $\omega$ and $2\omega$ counter-rotating mid-IR pulses~[1]. Our numerical and analytical study shows that the time delay between the two-color pulses allows control of the harmonic positions, both those allowed by angular momentum conservation and those seemingly forbidden by it. Moreover, the helicity of any particular harmonic is tunable from left- to right-circular without changing the driving pulse helicity. The highest HHG yield occurs for a time delay comparable to the fundamental period $T=2\pi/\omega$. \\ \noindent[1]~M.V. Frolov \textit{et al.}, Phys.Rev.Lett. \textbf{120}, 263203 (2018).