Graphene Characterization by Hilbert-Huang Transform-Based Terahertz Spectroscopy

We demonstrate a contact-less characterization method of monolayer graphene (MG) with all-fiber terahertz time-domain spectroscopy (THz-TDS) based on the Hilbert-Huang transform (HHT). The MG was grown on a Cu foil by chemical vapor deposition and then transferred to the poly (ethylene terephthalate) substrate by polymethyl methacrylate-assisted wet transfer. In our THz-TDS system, the time-domain pulse signal through MG is decomposed into eight components called the intrinsic mode functions (IMFs). It is found that the decomposed low-order modes are related to the original MG signal oscillations, whereas the other high-order modes correspond to the noise. The first IMF component can be used to extract the apparent graphene absorption peak near 0.7 THz, which could be more obvious with superposition of the second IMF due to the increasing spectral amplitude in the lower frequency range. The proposed HHT-based THz-TDS provides a more efficient way for graphene characterization.