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Terahertz Emitters, Receivers, and Applications

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

Piotr Gładysz, Karolina Słowik

Published Aug 20, 2020• 0 citations
Publisher

Abstract

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.

DOI
10.1117/12.2567963

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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.
The fluorescent radiative properties of a two-level quantum polar system with broken spatial inversion symmetry being realized as a model of a single-electron two-level “atom” with permanent atomic electric dipole moment, so that its electric dipole operator diagonal matrix elements are not equal to each other, were examined. In practice, such a system can be realized as an asymmetric quantum dot. A particular case when this system is driven by a trichromatic external field, with two high-frequency bichromatic field components of equal Rabi frequencies and carrier frequencies being placed symmetrically around the atomic transition frequency, and one more low-frequency component being in resonance with the Rabi frequency of the high-frequency components, was analyzed. It was shown that the high-frequency spectral properties of the fluorescent radiation can be altered with equal efficiency either by controlling the intensity of the low-frequency component of the trichromatic driving field or by changing the degree of the quantum dot asymmetry. Possible options to apply this potentially use
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.
Abstract On the basis of numerical solution to the Maxwell–Bloch equations within the one-dimensional two-level model of a superradiant laser with a symmetric cavity in which the photon lifetime is less than the incoherent relaxation time of the optical-dipole oscillations of active centers, the phenomenon of the spontaneous asymmetric generation of counter-propagating waves under continuous uniform pumping of the active medium is revealed. It is shown that the observed symmetry breaking of the spatial profiles of counter-propagating waves of the electromagnetic field and, the polarization and inversion of the population of active-medium levels in the case of a small inhomogeneous broadening of the spectral line of its working transition occurs due to the asymmetric half-wave nonlinear grating of inversion of the energy-level population produced by these waves.
On the basis of numerical solution to the Maxwell-Bloch equations within an one-dimensional two-level model of a superradiant laser with a symmetric cavity where a photon lifetime is less than an incoherent relaxation time of the optical dipole oscillations of active centers, we find that a spontaneous asymmetric generation of the counter-propagating waves is possible under a continuous homogeneous pumping of an active medium. We show that such a phenomenon of a symmetry breaking of the spatial profiles of the counter-propagating waves of an electromagnetic field as well as the polarization and population inversion of an active medium in the considered case of a weak inhomogeneous broadening of an operating transition is caused by an asymmetric half-wavelength nonlinear grating of the population inversion of the transition’s energy levels which is produced by these waves.
We study spectral properties of fluorescent radiation from a two-level quantum system with broken inversion spatial symmetry, which can be implemented as a model of a one-electron two-level atom whose electric dipole moment operator has permanent unequal diagonal matrix elements. We consider the case of the excitation of this system by a bichromatic laser field consisting of a high-frequency resonance component with the frequency coinciding with the atomic transition frequency and a low-frequency component whose frequency coincides with the Rabi frequency of the high-frequency component. We show that by changing the intensity of the low-frequency component, we can efficiently control spectral properties of the fluorescent radiation of the system in the high-frequency range. We discuss possible methods for the experimental detection and practical use of the effects under study.
We demonstrate theoretically the parametric oscillator behavior of a two-level quantum system with broken inversion symmetry exposed to a strong electromagnetic field. A multitude of resonance frequencies and additional harmonics in the scattered light spectrum as well as an altered Rabi frequency are predicted to be inherent to such systems. In particular, dipole radiation at the Rabi frequency appears to be possible. Since the Rabi frequency is controlled by the strength of the coupling electromagnetic field, the effect can serve for the frequency-tuned parametric amplification and generation of electromagnetic waves. Manifestation of the effect is discussed for III-nitride quantum dots with strong built-in electric field breaking the inversion symmetry. Terahertz emission from arrays of such quantum dots is shown to be experimentally observable.