Regensburg 2025 – scientific programme

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DS: Fachverband Dünne Schichten

DS 9: Optical Analysis of Thin Films II

DS 9.7: Talk

Thursday, March 20, 2025, 12:00–12:15, H3

Optical absorption by two-dimensional excitons (with and without screening) — •Stefan Zollner and Carlos Armenta — New Mexico State University, Las Cruces, NM

Two-dimensional excitons form not only in 2D layered materials (such as transition metal dichalcogenides) or in quantum wells. They also dominate the visible absorption by 2D van Hove singularities in 3D semiconductors. As an example, we measured and calculated the optical absorption of germanium near the E₁ and E₁+Δ₁ transitions. The 2D character of these transitions stems from the large optical mass along the (111) direction, where the highest valence bands and the lowest conduction band are nearly parallel. Elliot’s formalism for the Sommerfeld enhancement of optical absorption (as modified by Tanguy for 2D excitons) provides an excellent description of these absorption peaks, if the momentum matrix element from k^→·p^→-theory is used as the optical dipole matrix element. We also discuss how band filling with Fermi-Dirac statistics after intense laser excitation reduces the absorption. We are not aware of a (Banyai-Koch) theory that can be applied to the screening of 2D excitons. This work was supported by the US Air Force (FA-9550-24-1-0061).

Keywords: excitons; screening; ellipsometry; absorption; femtosecond