Characterisation of the electrical properties of materials is fundamental to the development of nanoelectronic technologies. Optical techniques, specifically in the terahertz (THz) range (1-10 THz), have emerged as attractive probes for non-invasive measurement of conductive and dielectric material properties, and could become vital tools for rapid inspection in semiconductor processing facilities [1]. Despite wide success, conventional optical methods have limited resolution with which spatial heterogeneities in non-pristine single-crystal and polycrystalline materials can be determined. This issue is particularly apparent for THz-based methods, where the long-wavelength radiation of the source means that due to diffraction spatial variations in material properties below approximately 100 µm are indistinguishable. Yet the need to resolve smaller and smaller feature sizes is ever present as the electronics industry continues to drive down device sizes to meet demands on speed, capacity and energy efficiency, and for this purpose, THz scanning near-field microscopy (THz-SNOM) can be an ideal tool, especially if such a measurement offers a local signal that is a measure of the local nanoscale conductivity.  

Here, we show unequivocal experimental evidence that under suitable conditions contrast in the scattered THz-SNOM signal is distinguishable in conductive thin-films. We demonstrate this by studying exfoliated graphene transferred onto a SiO₂/Si substrate, where we not only demonstrate clear and quantifiable contrast within a single monolayer, but also contrast between layers in few-layer graphene.

Our results suggest that THz-SNOM can be a powerful probe sensitive to variations in conductive materials and capable of quantifiable measurements down to the nanoscale.