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Scientific Reports

Population difference gratings created on vibrational transitions by nonoverlapping subcycle THz pulses

Rostislav M. Arkhipov, A. V. Pakhomov, Mikhail V. Arkhipov, Ihar Babushkin, Ayhan Demircan, Uwe Morgner, Nikolay N. Rosanov

Published Jan 21, 2021• 41 citations
Publisher

Abstract

We study theoretically a possibility of creation and ultrafast control (erasing, spatial frequency multiplication) of population density gratings in a multi-level resonant medium having a resonance transition frequency in the THz range. These gratings are produced by subcycle THz pulses coherently interacting with a nonlinear medium, without any need for pulses to overlap, thereby utilizing an indirect pulse interaction via an induced coherent polarization grating. High values of dipole moments of the transitions in the THz range facilitate low field strength of the needed THz excitation. Our results clearly show this possibility in multi-level resonant media. Our theoretical approach is based on an approximate analytical solution of time-dependent Schrödinger equation (TDSE) using perturbation theory. Remarkably, as we show here, quasi-unipolar subcycle pulses allow more efficient excitation of higher quantum levels, leading to gratings with a stronger modulation depth. Numerical simulations, performed for THz resonances of the [Formula: see text] molecule using Bloch equations for density matrix elements, are in agreement with analytical results in the perturbative regime. In the strong-field non-perturbative regime, the spatial shape of the gratings becomes non-harmonic. A possibility of THz radiation control using such gratings is discussed. The predicted phenomena open novel avenues in THz spectroscopy of molecules with unipolar and quasi-unipolar THz light bursts and allow for better control of ultra-short THz pulses.

DOI
10.1038/s41598-021-81275-8

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We study theoretically a possibility of creation and ultrafast control (erasing, spatial frequency multiplication) of population density gratings in a multi-level resonant medium having a resonance transition frequency in the THz range. These gratings are produced by subcycle THz pulses coherently interacting with a nonlinear medium, without any need for pulses to overlap, thereby utilizing an indirect pulse interaction via an induced coherent polarization grating. High values of dipole moments of the transitions in the THz range facilitate low field strength of the needed THz excitation. Our results clearly show this possibility in multi-level resonant media. Our theoretical approach is based on an approximate analytical solution of time-dependent Schrödinger equation (TDSE) using perturbation theory. Remarkably, as we show here, quasi-unipolar subcycle pulses allow more efficient excitation of higher quantum levels, leading to gratings with a stronger modulation depth. Numerical simulations, performed for THz resonances of the [Formula: see text] molecule using Bloch equations for density matrix elements, are in agreement with analytical results in the perturbative regime. In the strong-field non-perturbative regime, the spatial shape of the gratings becomes non-harmonic. A possibility of THz radiation control using such gratings is discussed. The predicted phenomena open novel avenues in THz spectroscopy of molecules with unipolar and quasi-unipolar THz light bursts and allow for better control of ultra-short THz pulses.
Abstract The possibility of selective population of the energy levels of quantum systems using a single unipolar subcycle pulse and a pair of pulses has been studied. The possibility of selective population of quantum levels has been clearly illustrated based on the numerical solution of a system of equations for the density matrix of a three-level medium that interacts with a pair of subcycle attosecond and terahertz pulses. The possibility of creating a population inversion in a three-level medium using a pair of such pulses has been shown. The dynamics of population gratings in a three-level medium exposed to the action of a pair of Gaussian pulses of large amplitude on the system is studied. Whereas the shape of the gratings in a weak field is harmonic, according to analytical calculations performed by the perturbation theory, in the case of a strong field, the spatial profile of the gratings may differ from the sinusoidal profile and may have a complex structure of peaks.
We demonstrate the possibility of inducing, erasing and extra rapidly controlling population difference gratings resulting from coherent interaction of unipolar subcycle pulses with a resonant medium. Gratings can be produced without overlap of pulses in the medium, which is an important distinction of the proposed approach from the traditional one, in which gratings are produced using interference of two or more overlapping quasi-monochromatic light beams. The use of unipolar subcycle pulses ensures faster control over gratings in comparison with bipolar pulses studied by us previously.
Abstract Recently, the possibility of the creation, erasing and ultrafast control of polarization and population inversion gratings by sequences of few-cycle bipolar pulses interacting with a medium in a resonant and coherent way was predicted. In this case, the overlapping of pulses in the medium is not needed for the creation of gratings. In this paper, we study the possibility of the ultrafast creation and control of spatial periodic gratings in a resonant medium when subcycle unipolar pulses (that is ones containing the constant spectral component of an electric field) propagate in the coherent regime.
Abstract We study the possibility of creating gratings of populations in a gas of hydrogen atoms using a pair of UV attosecond pulses that do not overlap in the medium. In this case, the central frequency of the pulses can either coincide with the frequency of the 1–2 resonant transition from the ground state to the first excited state (the main line of the Lyman series), or be detuned from it. The values of numerically calculated quantities agree with those obtained analytically from the solution of an approximate Schrödinger equation in perturbation theory. We show that, upon resonant excitation, the greatest creation efficiency of gratings is achieved with increasing pulse duration. In the case of nonresonant excitation, on the contrary, the system is excited more efficiently by short quasi-unipolar subcycle pulses than by bipolar multicycle once. The obtained results can be applied to coherent excitation of a single atom (thin layer) using a pair of UV pulses. We show that the modulation depth of gratings can be controlled by changing the carrier envelope phase (CEP) of UV attosecond pulses.
Abstract In a fixed spectral range, single- and half-cycle electromagnetic pulses have the shortest duration. Half-cycle pulses are promising tools for ultrafast control of quantum systems. Previously, the possibility of using a sequence of single- and half-cycle attosecond pulses to generate and ultrafast control light-induced population difference gratings has been demonstrated. However, such studies have been carried out using different approximations, such as the sudden perturbation theory and the two-level model for the resonant medium. In this paper, based on the numerical solution of constitutive equations for elements of the density matrix and wave equation it is shown that it is possible to generate and control population gratings in a three-level medium without using the approximation of sudden perturbations used in previous studies. It is shown that taking into account the additional level of the medium does not lead to a violation of the effect of generating such gratings. This extends the applicability of previous results.
Abstract Electromagnetically induced gratings (EIG) are created by standing-wave laser field in resonant media. Such gratings can be also created by few-cycle electromagnetic pulses counter-propagating in the medium via coherent Rabi oscillations of atomic inversion. In this case, instantaneous cross-section of the pulses in the medium is not necessary for grating formation. In this paper, we revise our recent results in study of such grating formation and their control by few-cycle pulses coherently propagating in a resonant medium. We demonstrate the grating formation and their control in three-level medium excited by three subcycle THz pulses.