Coupling Sommerfeld-Zenneck surface waves for enhancing absorption in infrared detectors with deeply subwavelength absorber layers

Sommerfeld-Zenneck wave is a long-wavelength asymptotic form of a surface wave at a metal-dielectric boundary. Away from the SPP resonance, the Sommerfeld-Zenneck mode is weakly confined and does not feature decreased phase or group velocities. Still coupling a Sommerfeld-Zenneck wave in the vicinity of absorbing materials contributes to enhanced absorption. This paper focuses on the interaction between Sommerfeld-Zenneck and guided modes in a type-II superlattice high operating temperature long-wavelength infrared photodetector. Owing to enhanced absorption in the active layer, it is possible to retain the original level of absorptance and to reduce the absorber thickness down to a few hundred nanometers. Coupling surface and guided modes at similar resonance frequencies is possible thanks to a two-dimensional sub-wavelength hole array in a gold film integrated with the detector. The coupling mechanism may be approximately explained based on a semi-analytical model dependent on the dispersion relations of the main detector components partly obtained from ellipsometric measurements. Yet 3D vectorial optical modelling going down to deeply sub-wavelength dimensions is needed in a refined approach. Excitation of hybrid modes provides a mechanism for enhancing the detector’s quantum efficiency and tailoring the detector’s response at specific resonance frequencies.