A Theoretical Study On The Electrodynamics Properties Of Superconducting Ultra-Thin Films And Their Optical Response In The Terahertz Domain
DOI:
https://doi.org/10.63665/k1ve5a38Keywords:
Superconducting ultra-thin films, THz optical conductivity, Mattis–Bardeen theory, London penetration depth, kinetic inductance detectorsAbstract
Superconducting ultra-thin films exhibit distinctive electrodynamics behaviour governed by reduced dimensionality, Cooper-pair condensation, and the formation of an energy gap at the Fermi level. This paper presents a theoretical investigation into the optical conductivity and THz-frequency response of conventional metallic superconducting thin films, with niobium nitride (NbN) and tantalum nitride (TaN) as primary model systems. The Mattis–Bardeen (MB) theoretical framework within BCS formalism is adopted to analyse real (σ₁) and imaginary (σ₂) components of complex conductivity across the 0.1–1.1 THz spectral window. The central hypothesis posits that as film thickness diminishes below the London penetration depth (λL), disorder-induced modifications to the superconducting gap produce measurable deviations from BCS-predicted optical responses. Secondary datasets from terahertz frequency-domain spectroscopy (THz-FDS) experiments are reanalysed to validate theoretical predictions. Results demonstrate that NbN films of 3.9–5.0 nm exhibit Tc suppression, penetration depth enhancement, and spectral weight redistribution consistent with disorder-driven pair-breaking, while TaN films adhere closely to weak-coupling BCS predictions. These findings carry implications for the rational engineering of kinetic inductance detectors, superconducting nanowire single-photon detectors, and quantum sensing platforms.
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