Enhanced mid-infrared light-matter coupling in heavily-doped p-germanium due to interplay between plasmonic and inter-subband contributions

Subramanian, Aravind N. and Bin Anooz, Saud and Pavlov, Sergey G. and Richter, Carsten and Szántó, Géza Levente and Sidorova, Mariia and Petrik, Péter and Sumathi, R. Radhakrishnan (2026) Enhanced mid-infrared light-matter coupling in heavily-doped p-germanium due to interplay between plasmonic and inter-subband contributions. COMMUNICATIONS MATERIALS, 7 (1). No. 161. ISSN 2662-4443

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Abstract

The development of germanium-based mid-infrared (MIR) optoelectronic technologies is central to advancing compact, high-performance on-chip systems. While plasmonic field enhancement in heavily-doped semiconductors is widely exploited to intensify light-matter interactions, yet p- type germanium has been discarded due to its complex valence band structure. Here, we show that heavily-doped p- type germanium exhibits a dual light-matter interaction, where an ε-near-zero band delineates a plasmonic-like regime below the plasma frequency and a broadband absorption (dominated by inter-subband transitions) above it. This duality extends the spectral range of strong interaction beyond conventional Drude plasmonics. Using the Czochralski method, we have grown high-quality heavily-doped germanium single crystals with gallium concentrations up to ~2×10 20 cm −3 , exhibiting a high, near 100% acceptor activation. Mid-infrared spectroscopic ellipsometry and reflectance measurements reveal a continuous blue shift of the plasma wavelength with increasing dopant concentration, up to 4.5 μm, the shortest wavelength reported in Ge to date. Our findings manifest the potential of heavily-doped p- type bulk germanium crystals for applications that benefit from strong mid-infrared absorption and high crystal lattice quality.

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