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Propagation of optically tunable coherentradiation in a medium of asymmetric molecules

Piotr Gładysz, Karolina Słowik

Published Feb 13, 2020• 1 citation
OpenAlex

Abstract

Coherent, optically dressed media composed of two-level atomic systems without inversion symmetry are considered as all-optically tunable sources of coherent radiation in the microwave to near-infrared domain. Here, a theoretical model and a numerical toolbox are developed to account for buildup and propagation dynamics of such low-frequency impulses in gaseous or bulk media. We discuss the physical mechanisms involved and make a performance study in conditions representative for vapors of heteronuclear diatomic molecules to identify realistic experimental scenarios.

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Propagation of optically tunable coherent radiation in a gas of polar molecules

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Coherent, optically dressed media composed of two-level molecular systems without inversion symmetry are considered as all-optically tunable sources of coherent radiation in the microwave domain. A theoretical model and a numerical toolbox are developed to confirm the main finding: the generation of low-frequency radiation, and the buildup and propagation dynamics of such low-frequency signals in a medium of polar molecules in a gas phase. The physical mechanism of the signal generation relies on the permanent dipole moment characterizing systems without inversion symmetry. The molecules are polarized with a DC electric field yielding a permanent electric dipole moment in the laboratory frame; the direction and magnitude of the moment depend on the molecular state. As the system is resonantly driven, the dipole moment oscillates at the Rabi frequency and, hence, generates microwave radiation. We demonstrate the tuning capability of the output signal frequency with the drive amplitude and detuning. We find that even though decoherence mechanisms such as spontaneous emission may damp the output field, a scenario based on pulsed illumination yields a coherent, pulsed output of tunable temporal width. Finally, we discuss experimental scenarios exploiting rotational levels of gaseous ensembles of heteronuclear diatomic molecules.

arXiv:2002.05469•Scientific Reports•Oct 19, 2020•7 citations

Media of asymmetric atomic systems as low-frequency radiation sources.

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If a two-level atomic system characterized by inversion symmetry in space is coupled to a classical electromagnetic field, it undergoes Rabi oscillations where the population flips between the ground and excited levels. We investigate a scenario, where a system of broken inversion symmetry is used instead. In such case the dynamics is modified for the following reason: Eigenstates of an asymmetric system can be characterized with a permanent electric dipole moment originating from the polarization of charges, which plays a significant role of an additional source of dipole radiation. Its frequency corresponds to the Rabi frequency of population transfer between the eigenstates and therefore is optically tunable with the intensity of the driving field. We construct the numerical solver to simulate the medium of such asymmetric systems driven by the laser beam and by the Bloch-Maxwell equations calculate the generated radiation's propagation.

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Theory of optical near-resonant cone emission in atomic vapor

B. D. Paul, J. Cooper, Alan Gallagher +1 more

A time-dependent theory for conical emission during near-resonant propagation of laser light in an atomic vapor, which includes full propagation for the laser and frequency sidebands in a nonlinear two-level medium is presented. The density-matrix equations for the dipole moment and population are solved in the dressed atomic frame. The polarization source terms are accurate to order \ensuremath{\gamma}/R, where \ensuremath{\gamma} is a damping constant and R is the generalized Rabi frequency. Analytical plane-wave solutions and numerical, cylindrically symmetric propagation simulations including diffraction are presented. It is shown that the calculations with cylindrically symmetric fields and atomic excitation profiles are incapable of accounting for the high levels of optical gain that are responsible for the intense conical emission observed in experiments. This result is at first surprising, since the model accounts rigorously for all of the physical phenomena that have been previously proposed as being responsible for generating large gains, and the calculation matches the symmetry of the observations. The lack of large calculated gain seems to imply the existence of higher-order $(m>0)$ radial modes in the field for the experimental conditions that give rise to cone emission. In the simulations, however, the cylindrically symmetric fields do produce weak red-detuned cones with angular-frequency distributions similar to those seen in experiments.

Physical Review A•Dec 31, 2002•8 citations
Coherent, optically dressed media composed of two-level atomic systems without inversion symmetry are considered as all-optically tunable sources of coherent radiation in the microwave to near-infrared domain. Here, a theoretical model and a numerical toolbox are developed to account for buildup and propagation dynamics of such low-frequency impulses in gaseous or bulk media. We discuss the physical mechanisms involved and make a performance study in conditions representative for vapors of heteronuclear diatomic molecules to identify realistic experimental scenarios.
Coherent, optically dressed media composed of two-level molecular systems without inversion symmetry are considered as all-optically tunable sources of coherent radiation in the microwave domain. A theoretical model and a numerical toolbox are developed to confirm the main finding: the generation of low-frequency radiation, and the buildup and propagation dynamics of such low-frequency signals in a medium of polar molecules in a gas phase. The physical mechanism of the signal generation relies on the permanent dipole moment characterizing systems without inversion symmetry. The molecules are polarized with a DC electric field yielding a permanent electric dipole moment in the laboratory frame; the direction and magnitude of the moment depend on the molecular state. As the system is resonantly driven, the dipole moment oscillates at the Rabi frequency and, hence, generates microwave radiation. We demonstrate the tuning capability of the output signal frequency with the drive amplitude and detuning. We find that even though decoherence mechanisms such as spontaneous emission may damp the output field, a scenario based on pulsed illumination yields a coherent, pulsed output of tunable temporal width. Finally, we discuss experimental scenarios exploiting rotational levels of gaseous ensembles of heteronuclear diatomic molecules.
If a two-level atomic system characterized by inversion symmetry in space is coupled to a classical electromagnetic field, it undergoes Rabi oscillations where the population flips between the ground and excited levels. We investigate a scenario, where a system of broken inversion symmetry is used instead. In such case the dynamics is modified for the following reason: Eigenstates of an asymmetric system can be characterized with a permanent electric dipole moment originating from the polarization of charges, which plays a significant role of an additional source of dipole radiation. Its frequency corresponds to the Rabi frequency of population transfer between the eigenstates and therefore is optically tunable with the intensity of the driving field. We construct the numerical solver to simulate the medium of such asymmetric systems driven by the laser beam and by the Bloch-Maxwell equations calculate the generated radiation's propagation.
The first part of this work describes theoretical and experimental studies of Stark-induced three-wave mixing in gaseous NH2D. Application of a dc electric field to a gaseous system destroys the basic inversion symmetry and allows two-photon mixing processes to occur. A theoretical derivation of this effect under conditions of resonantly enhanced non-linearities is given for a three-level system. Calculations are presented for mixing of a CO2 laser with a 4 GHz microwaves in the molecule NH2D, producing single lower sideband radiation. Experimental observation of resonantly enhanced, dc-induced, three-wave mixing in gaseous NH2D is presented. The dependence of this effect on gas pressure, microwave frequency, applied dc field, and microwave power are presented and compared with theoretical predictions. The experiment was done at Hughes Research Laboratories by Abrams and his coworkers. The second part of this work describes the propagation of electromagnetic waves in periodic layered media. The propagation of electromagnetic radiation in periodically stratified media is considered. Media of finite, semi-finite and infinite extent are treated. A diagonalization of the unit cell translation operator is used to obtain exact solutions for the Bloch waves, the dispersion relations, and the band structure of the medium. The theory of electromagnetic Bloch waves in periodic stratified media is then applied to the problems of birefringence, and group velocity in these media. The relevance of periodic media to phase matching in nonlinear mixing experiments-and to laser action in the x-ray region is discussed. New types of guided waves such as Bragg guided waves and optical surface waves are theoretically predicted and experimentally observed.
Strong and short coherent pulses of electromagnetic radiation propagate through extended absorbing media with anomalously low absorption, they are delayed and reshaped. This article presents experimental results and calculations for propagation of 10.6-\ensuremath{\mu} wavelength pulses through gaseous SF6{\mathrm{F}}_{6}F6​. The degeneracy of overlapping levels of SF6{\mathrm{F}}_{6}F6​ in that spectral region is important, and enables us to use a simple model to describe observed phenomena. We assume a continuous distribution of dipole moments. Results are particularly interesting in two regimes of excitation. One corresponds to very strong excitation and \ensuremath{\sim}20 absorption lengths in the medium: Pulses sharpen at their leading edge, while their front part gets absorbed. The other pertains to much lower excitation and only 2 absorption lengths: Reradiation can be distinguished and long delays result which qualitatively resemble the behavior of a two-level system in self-induced transparency. Fairly conclusive agreement is obtained between a previous theoretical treatment of a homogeneously broadened multilevel medium and experiment. The agreement is substantiated by computer calculations. In three Appendixes we present the observation of focusing effects, the theory of an experiment related to transient nutation, and some remarks on photon-echo polarization experiments in optically thick media.
Coherent preparation by laser light of quantum states of atoms and molecules can lead to quantum interference in the amplitudes of optical transitions. In this way the optical properties of a medium can be dramatically modified, leading to electromagnetically induced transparency and related effects, which have placed gas-phase systems at the center of recent advances in the development of media with radically new optical properties. This article reviews these advances and the new possibilities they offer for nonlinear optics and quantum information science. As a basis for the theory of electromagnetically induced transparency the authors consider the atomic dynamics and the optical response of the medium to a continuous-wave laser. They then discuss pulse propagation and the adiabatic evolution of field-coupled states and show how coherently prepared media can be used to improve frequency conversion in nonlinear optical mixing experiments. The extension of these concepts to very weak optical fields in the few-photon limit is then examined. The review concludes with a discussion of future prospects and potential new applications.
A time-dependent theory for conical emission during near-resonant propagation of laser light in an atomic vapor, which includes full propagation for the laser and frequency sidebands in a nonlinear two-level medium is presented. The density-matrix equations for the dipole moment and population are solved in the dressed atomic frame. The polarization source terms are accurate to order \ensuremath{\gamma}/R, where \ensuremath{\gamma} is a damping constant and R is the generalized Rabi frequency. Analytical plane-wave solutions and numerical, cylindrically symmetric propagation simulations including diffraction are presented. It is shown that the calculations with cylindrically symmetric fields and atomic excitation profiles are incapable of accounting for the high levels of optical gain that are responsible for the intense conical emission observed in experiments. This result is at first surprising, since the model accounts rigorously for all of the physical phenomena that have been previously proposed as being responsible for generating large gains, and the calculation matches the symmetry of the observations. The lack of large calculated gain seems to imply the existence of higher-order (m>0)(m>0)(m>0) radial modes in the field for the experimental conditions that give rise to cone emission. In the simulations, however, the cylindrically symmetric fields do produce weak red-detuned cones with angular-frequency distributions similar to those seen in experiments.