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Journal of the Optical Society of America B

Two tunneling-coupled two-level systems with broken inversion symmetry: tuning the terahertz emission

MirFaez Miri, Farhad Zamani, Hamideh Alipoor

Published Aug 10, 2016• 4 citations
Publisher

Abstract

Exposed to a strong electromagnetic field, a two-level system with broken inversion symmetry radiates at the Rabi frequency ΩR. This can be used to generate terahertz waves [Phys. Rev. Lett.102, 023601 (2009)PRLTAO0031-900710.1103/PhysRevLett.102.023601]. We show that a system composed of two tunneling-coupled asymmetric two-level systems radiates at frequency (ξ1−ξ2)2+ΩR2, where energies ℏξ1 and ℏξ2 characterize the tunneling interaction. This opens the possibility of tuning the terahertz spectrum in a considerable range. We also show that for particular amplitudes of the driving field, the emission at frequency (ξ1−ξ2)2+ΩR2 is suppressed.

DOI
10.1364/josab.33.001873

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Exposed to a strong electromagnetic field, a two-level system with broken inversion symmetry radiates at the Rabi frequency ΩR. This can be used to generate terahertz waves [Phys. Rev. Lett.102, 023601 (2009)PRLTAO0031-900710.1103/PhysRevLett.102.023601]. We show that a system composed of two tunneling-coupled asymmetric two-level systems radiates at frequency (ξ1−ξ2)2+ΩR2, where energies ℏξ1 and ℏξ2 characterize the tunneling interaction. This opens the possibility of tuning the terahertz spectrum in a considerable range. We also show that for particular amplitudes of the driving field, the emission at frequency (ξ1−ξ2)2+ΩR2 is suppressed.
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.
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.
Based on coupled quantum dots, we present an interesting optical effect in a four-level loop coupled system. Both the two upper levels and the two lower levels are designed to be almost degenerate, which induces a considerable dipole moment. The terahertz wave is obtained from the low-frequency component of the photon emission spectrum. The frequency of the terahertz wave can be controlled by tuning the energy levels via designing the nanostructure appropriately or tuning the driving laser field. A terahertz wave with adjustable frequency and considerable intensity (100 times higher than that of the Rayleigh line) can be obtained. It provides an effective scheme for a terahertz source.
This research analyzes the impact of the asymmetry of the quantum system and the range of control possibilities offered by dispersive media and the nanostructures. I investigated the asymmetric behavior of a two level system in the Jaynes-Cummings model (JC) and in the spontaneous emission process discussing respectively the presence of a non trivial emission at the Rabi frequency and the correction at a much more basic level, i.e., in the lifetime of the excited state, and also affect the value of the Lamb shift with broken symmetry in the diagonal terms of dipole moment.
Based on spatial-temporal symmetry breaking mechanism, we propose a novel scheme for terahertz (THz) wave generation from hyper-Raman lines associated with the 0th harmonic (a particular even harmonic) in a two-level quantum system driven by two-color laser fields. With the help of analysis of quasi-energy, the frequency of THz wave can be tuned by changing the field amplitude of the driving laser. By optimizing the parameters of the laser fields, we are able to obtain arbitrary frequency radiation in the THz regime with appreciable strength (as strong as the typical harmonics). Our proposal can be realized in experiment in view of the recent experimental progress of even-harmonics generation by two-color laser fields.
With the purpose to devise a novel lasing scheme, we consider a two level system with both a transversal and longitudinal coupling to the electromagnetic field. If the longitudinal coupling is sufficiently strong, multi-photon transitions become possible. We assume furthermore that the electromagnetic environment has a spectrum with a single sharp resonance, which serves as a lasing cavity. Additionally, the electromagnetic environment should have a very broad resonance around a frequency which differs form the sharp resonance. We use the polaron transformation and derive a rate equation to describe the dynamics of such system. We find that lasing at the frequency of the sharp mode is possible, if the energy difference of the atomic transition is similar to the sum of the frequencies of both peaks in the spectral function. This allows for the creation of lasing over a large frequency range and may in perspective enable THz lasing at room temperature.