Abstract
The graphene sheet, which exhibits outstanding properties, such as high transparency in both visible and terahertz (THz) regions and superb electrical conductivity, are measured by terahertz time-domain spectroscopy (THz-TDS) based on photoconductive (PC) antenna and laser-induced air-plasma, respectively. The optical and electrical properties of monolayer and bilayer graphene films in the THz frequency range of 0.3~ 1.4 THz are investigated. Complex conductivities of the graphene samples can be extracted from the TDS measurements and fitted with the Drude free-electron model. Electrical properties of the samples, such as plasma frequency (ωp), scattering time (τ), dc mobility (μ), and dc conductivity (σ0), are obtained. Comparing to the indium-tin-oxide nanowhiskers (ITO NWhs), another promising candidate as transparent conductor for THz optoelectronic devices, graphene has lower transmittance but much higher conductivity and mobility. Furthermore, we have constructed and characterized THz phase shifters based on liquid crystals (LCs) with graphene grown by chemical vapor deposition (CVD) and indium-tin-oxide nanowhiskers (ITO NWhs) obliquely evaporated by electron-beam glancing-angle deposition (GLAD) as transparent conducting electrodes. A graphene-based phase shifter can achieve a phase shift of π/2 at 1.0 THz with the operating voltage of ~2.2 V (rms) versus ~ 5.6 V (rms) for ITO-NWhs-based phase shifter in previous work. On the other hand, 2π phase shift at 1.0 THz was achieved in an ITO-NWhs-based phase shifter with a multi-sandwiched structure by applying ~2.6 V (rms). The low operation voltage of both two kinds of phase shifters imply the compatibility with thin-film transistor (TFT) and complementary metal-oxide-semiconductor (CMOS) technologies. The experimental results of phase shifters are in good agreement with the theoretical predictions.