Super-precision characterization of ITO nanostructured material by waveguide coupled Terahertz time-domain spectroscopy

In this work, we used a silicon-based waveguide technique that can be easily employed with common THz time-domain spectroscopy setups and enable the characterization of the ITO nanostructured samples with great precision. We were measuring the so-called M-line spectrum of the device when guide modes are excited in the silicon waveguide thanks to a grating coupler engraved at its top surface. When a 418.8 nm-thick ITO sample is deposited on the waveguide, we observed a 9 GHz-frequency shift of the 5 GHz-FWHM M-lines and a 27-times transmission ratio at 0.644 THz. Over 0.1-1 THz, we carefully extracted the complex refractive index and permittivity by fitting the recorded curves with differential methods. By defining a two-dimensional error function, we gave the frequency-dependent experimental error bar with great precision. The conductivity is then calculated and fitted with the Drude-Smith model. The so-determined conductivity shows a nice dielectric property, which is due to the large carrier concentration ($N=1.21\times {10}^{19}$ cm$^{-3}$) in such ITO nanostructured material. The carrier lifetime (41 fs) is linked to the morphology of the structure, which offers an ultrafast dielectric response. Our work indicates that waveguide-coupled terahertz spectroscopy at guided frequencies provides a more precise characterization of the optoelectronic parameters of ITO nanostructured materials, which will serve as a crucial supplement in the development of ITO nanostructures for advanced functional devices.