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Physical Review A

Magnus expansion for laser-matter interaction: Application to generic few-cycle laser pulses

Michael Klaiber, Darko Dimitrovski, J S Briggs

Published Apr 1, 2009• 17 citations
Publisher

Abstract

We treat the interaction of an atom with a short intense few-cycle laser pulse by the use of the Magnus expansion of the time-evolution operator. Terms of the Magnus expansion up to the third order in the pulse duration are evaluated explicitly, and expressions for the transition probability between atomic eigenstates are given. The transition operators up to this order are particularly simple, involving only momentum and position shifts arising from the laser field alone. The connection to other existing approaches is established. For a variety of generic short-pulse forms the results of the Magnus approximation are in excellent agreement with time-dependent transition probabilities obtained from accurate ab initio numerical calculations. However, the limitation of the Magnus expansion for pulses having both vanishing momentum and position shifts is demonstrated also.

DOI
10.1103/physreva.79.043402

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We treat the interaction of an atom with a short intense few-cycle laser pulse by the use of the Magnus expansion of the time-evolution operator. Terms of the Magnus expansion up to the third order in the pulse duration are evaluated explicitly, and expressions for the transition probability between atomic eigenstates are given. The transition operators up to this order are particularly simple, involving only momentum and position shifts arising from the laser field alone. The connection to other existing approaches is established. For a variety of generic short-pulse forms the results of the Magnus approximation are in excellent agreement with time-dependent transition probabilities obtained from accurate ab initio numerical calculations. However, the limitation of the Magnus expansion for pulses having both vanishing momentum and position shifts is demonstrated also.
Currently, the Magnus expansion method is widely used in studying Rydberg atoms in intense laser fields. In these studies, pulse lengths are usually set as an integer multiple of wavelength, at the same time, the initial phase of the laser pulse are usually set as zero, i.e. , without considering the effect of the initial phase in the calculation. However, this will result in some items multiplied by zero. Then, contributions from these items will be lost. Herein, we investigated the effects of the non-integer-cycle pulse and laser initial phase in the calculation of the transition probability of Rydberg atoms in intense laser fields. Our calculation result shows that these effects are trivial when the nondipole approximation is adopted. In dipole approximation i.e. , when the wavelength of the laser field exceeds the characteristic size of atomic or molecular orbitals, these effects are trivial only in the case of a long pulse duration and a large principal quantum number, however in the case of ultrashort laser pulses, these effects are obvious.
Using the Magnus expansion to the fourth order, we obtain analytic expressions for the atomic state of a two-level system driven by a laser pulse of arbitrary shape with small pulse area. We also determine the limitation of our obtained formulas due to limited range of convergence of the Magnus series. We compare our method to the recently developed method of Rostovtsev et al. (PRA 2009, 79, 063833) for several detunings. Our analysis shows that our technique based on the Magnus expansion can be used as a complementary method to the one in PRA 2009.
Explicit analytic expressions, involving elementary functions, are given for occupation probabilities of atomic states after one or a few cycles of a strong short laser pulse. These formulae are strictly valid in the asymptotic limit of large momentum transfer from the field to the atom during a half cycle of the pulse. The cases of a hydrogenic atom, of a negative ion and of multielectron atoms in the independent-electron approximation are considered. The asymptotic expressions, derived in the limit of infinite momentum transfer from the field, have also the correct limit for vanishingly small momentum transfers, so that fortuitously they are valid for arbitrary momentum transfer.
We consider the interaction of Rydberg atoms with strong infrared laser pulses using an approach based on the Magnus expansion of the time evolution operator. First-order corrections beyond the electric dipole approximation are also included in the theory. We illustrate the dynamics of the interaction at the parameters of the experiment [Eichmann et al., Phys. Rev. Lett. 110, 203002 (2013)]. It emerges that the depletion of Rydberg atoms in this regime comes predominantly from the nondipole effects.
Using a method based on the Magnus expansion, the survival of Rydberg atoms in intense laser fields is studied. Usually, for atoms in intense laser fields, effects beyond the electric dipole approximation need to be considered. In the past few years, researchers have primarily paid attention to the nondipole effects induced by the electric quadrupole term of the optical field. The quadrupole term plays a more important role than the electric dipole term in the calculation of the survival probability of Rydberg atoms in intense laser fields even though the magnitude of the former is smaller than that of the latter in terms of the multipole expansion of the optical vector potential. Meanwhile, the effects of higher-order corrections beyond the quadrupole term remain ambiguous. In this paper, we confirm that corrections containing higher-order terms, except for the quadrupole term, can be neglected in the calculation of the transitions of Rydberg atoms in external fields. We then use the second-order time evolution operator containing the quadrupole term to study the scaling law of the transition probability of Rydberg He in laser fields with different parameters.
Magnus expansion (ME) provides a general way to expand the real-time propagator of a time-dependent Hamiltonian within the exponential such that the unitarity is satisfied at any order. We use this property and explicit integration of Lagrange interpolation formulas for the time-dependent Hamiltonian within each time interval and derive approximations that preserve unitarity for the differential time evolution operators of general time-dependent Hamiltonians. The resulting second-order approximation is the same as using the average of Hamiltonians for two end points of time. We identify three fourth-order approximations involving commutators of Hamiltonians at different times and also derive a sixth-order expression. A test of these approximations along with other available expressions for a two-state time-dependent Hamiltonian with sinusoidal time dependences provides information on the relative performance of these approximations and suggests that the derived expressions can serve as useful numerical tools for time evolution in time-resolved spectroscopy, quantum control, quantum sensing, real-time ab initio quantum dynamics, and open system quantum dynamics.