Research Of Terahertz Plasmon In A Grating-Gate Coupled Graphene With A Resonant Cavity

Owing to its unique properties which differ from other wavebands such as infrared, microwave and X-rays, terahertz(THz) electromagnetic wave have a wide range of potential applications. In contrast to the conventional two-dimensional electron gas(2DEG), graphene has many novel electronic properties, including high temperature stability, high mobility, nonlinear and ambipolar characteristic of charge transport. Two-dimensional graphene plasmons are collective charge oscillations whose frequency can be tuned into the THz range. The density of 2DEG in graphene is usually in the range from 1010 to 1013 cm-2 and can be tuned by a gate voltage by using the field effect. However, applications of graphene plasmon devices are hindered by the weak interaction between the graphene and the electromagnetic wave. Here, we introduce a device model which combines a grating coupler with a THz Fabry-Pérot cavity. The integration enhances the interaction between graphene(matter) and THz cavity modes(light). Devices including graphene field-effect transistors and grating-cavity coupled graphene plasmon are designed, fabricated and tested. The main results are:1. Plasmon modes in graphene can be tuned into resonance with an incident THz electromagnetic wave in a range of 1-4 THz by setting a proper gate voltage. By using finite-difference-time domain(FDTD) method, we simulate a graphene plasmon device comprising a single-layer graphene, a metallic grating and a THz cavity. Simulations suggest that THz electric field can be enhanced by several times due to the grating-cavity configuration. The direct outcome of this near-field enhancement is that the maximal absorption of the incident THz wave reaches up to about 45%.2. We have successfully fabricated graphene dual-gated field-effect transistors. Two different gate processes are developed. Tune of the graphene channel by both the top-gate and the back-gate are demonstrated. The mobility and the charge density are extracted from the DC characterization. A mobility value of 2200 cm2 /Vs at room temperature is obtained.3. By using the terahertz time-domain spectroscopy(THz-TDS) technique, terahertz resonant absorption by graphene plasmon is observed in graphene plasmon devices. The frequency of the graphene plasmon is controlled in the terahertz range, and the maximal relative absorption can reach up to about 95%. However, the tunability of graphene plasmon frequency by the gate voltage is not achieved due to the large current density through the gate dielectrics. Possible improvement on the gate dielectrics to revive the plasmon tunability is suggested and discussed.