Linear response in the strong-field domain: ultrafast wave packet interferometry in the continuum

Abstract

We demonstrate that there are cases in which the response of a molecular system to the effects of a strong laser field is linear. By numerically solving the quantum dynamics of a wave packet of photo-dissociated H, we show that the initial state depletion and the photodissociation yields to various final channels may at times be an essentially linear function of the laser peak intensity. We have also investigated the case when the excitation is performed with delayed pulses, where the linear response regime is accompanied by oscillations in the population of vibrational and rotational states as a function of the delay time. In keeping with the observation of linear response beyond the weak field perturbative limit, coherent control over branching ratios, which can only exist in the strong field regime, is also shown to be extensive.

Considerations regarding one-photon phase control

“…In doing so, a well-defined, physically motivated and experimentally testable definition of one-photon phase control is obtained. Importantly, we propose, as did others, [33][34][35] that weak multiphoton control is the source of the weak, long-lived phase control previously measured in the linear regime in a number of experimental and numerical investigations. [27][28][29][30][31]36 Significantly, the novel, general bounds on one-photon phase control provided herein strongly support a multiphoton, rather than one-photon, phase control mechanism.…”

“…We hypothesize that multiphoton effects are responsible for reported long-lived phase control results, as was previously proposed. [32][33][34][35] Phase-dependent multiphoton effects can appear in linear regime experiments and numerical simulations. For example, phase-dependent higher order corrections (e.g., as shown in Fig.…”

“…2 can be important even in the low-intensity regime. [33][34][35]49 Significantly, the linear regime may not be the same for all pulse shapes and for all observables. For example, consider the control of isomer populations in rhodopsin.…”

“…32 Steady-state onephoton phase control has been shown to be impossible for certain classes of open quantum systems, 9 as it is for closed systems. 7 In addition, it has been demonstrated that multiphoton effects can create large contribution even in the linear absorption regime, 14,[33][34][35] which might lead to multiphoton phase control in seemingly one-photon experiments.…”

An efficient spectral method for numerical time-dependent perturbation theory

“…For example, higher order processes can appear in "linear regime" experiments and simulations thereof. 6,[28][29][30] In fact, non-perturbative microscopic simulations (i.e. where only the wavefunction of a single molecule is evolved) suffer in particular from uncontrolled contributions of unwanted optical signals as they are not subject to the phase matching conditions arising from the macroscopic Maxwell equations.…”