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

• URL: http://arxiv.org/abs/2507.20829v1
• Date: Mon, 28 Jul 2025 13:35:01 GMT
• Title: Phase structure of below-threshold harmonics in aligned molecules: a few-level model system
• Authors: Samuel Schöpa, Falk-Erik Wiechmann, Franziska Fennel, Dieter Bauer,
• Abstract summary: We analyze the polarization and phase properties of below-threshold harmonics (BTH) in aligned molecules.<n>For harmonics with photon energies below the transition between the dominant field-dressed states, the phase alternates by $pi$ between successive odd harmonic orders but remains constant above.<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 (BTH) in aligned molecules. In a two-level system (TLS), we find that the phase of emitted harmonics undergoes a distinct change. For harmonics with photon energies below the transition between the dominant field-dressed states, the phase alternates by $\pi$ between successive odd harmonic orders but remains constant above. Exploiting this behavior, we construct a four-level model composed of two uncoupled TLS subsystems 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 BTH regime.

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