Phase structure of below-threshold harmonics in aligned molecules: a few-level model system

• URL: http://arxiv.org/abs/2507.20829v2
• Date: Wed, 29 Oct 2025 15:20:19 GMT
• Title: Phase structure of below-threshold harmonics in aligned molecules: a few-level model system
• Abstract summary: We analyze the polarization and phase properties of below-threshold harmonics generated from aligned molecules.<n>For harmonics with photon energies below the transition energy between the dominant field-dressed states, the phase alternates by $pi$ between successive odd harmonic orders.<n>We demonstrate that with selected transition frequencies lower-order harmonics follow the polarization of the linearly polarized driving field while higher-order harmonics exhibit a mirrored polarization.
• Score: 0.0
• License: http://creativecommons.org/licenses/by/4.0/
• Abstract: We utilize few-level model systems to analyze the polarization and phase properties of below-threshold harmonics generated from aligned molecules. In a two-level system, we find that the phase of emitted harmonics undergoes a distinct change. For harmonics with photon energies below the transition energy between the dominant field-dressed states, the phase alternates by $\pi$ between successive odd harmonic orders. In contrast, the phase remains constant for harmonics above the transition energy. Exploiting this behavior, we construct a four-level model composed of two uncoupled two-level systems aligned along orthogonal directions. We demonstrate that with selected transition frequencies lower-order harmonics follow the polarization of the linearly polarized driving field while higher-order harmonics exhibit a mirrored polarization. The model predicts that aligned systems with orthogonal transition dipoles may show analogous phase and polarization features in the below-threshold regime.

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