High Broadband Optical Absorption and Bandstop Filter Characteristics of Pb/Nb2O5 Interfaces

In this study, semitransparent lead films serve as substrates for depositing niobium pentoxide thin films, forming versatile electro-optical devices. Using vacuum evaporation and ion sputtering techniques at ≈10⁻⁵ mbar, stacked layers of crystalline Pb and amorphous Nb₂O₅ are created. This process reduces free carrier absorption in Nb₂O₅ and forms Urbach tail states with a width of 0.91 eV. Pb/Nb₂O₅ thin films exhibit remarkable broadband absorption, exceeding 440% in the visible and 98% in the infrared. Moreover, Pb substrates induce a redshift in Nb₂O₅’s energy bandgap. Electrical analysis using impedance spectroscopy on Pb/Nb₂O₅/Ag structures reveals their series/parallel resonance and bandstop filter properties. Notably, the bandstop filters exhibit reflection coefficient minima at a notch frequency of 1.66 GHz, with a bandwidth of 280 MHz, return loss of 26 dB, and voltage standing wave ratio of 1.13. These findings underscore the device's potential for wide-ranging electro-optical applications across the electromagnetic spectrum.