| Abstract [eng] |
Domantas Vizbaras, NONLINEAR RESPONSE OF A GRAPHENE FIELD-EFFECT TRANSISTOR TERAHERTZ DETECTOR TO FEMTOSECOND TERAHERTZ PULSES Today, terahertz (THz) electronics receive significant attention in the fields of 6G telecommunications, high-frequency electronics, non-ionizing medical imaging, and security. Over the past decade, graphene has been demonstrated to be a highly nonlinear material in the terahertz frequency range (300 GHz – 10 THz). Observing this nonlinearity in graphene field-effect transistors (GFETs) could lead to a new generation of high-frequency mixers and amplifiers. Additionally, graphene could be utilized for applications such as pump-probe spectroscopy. This thesis aimed to analyze the nonlinear response of a graphene field-effect transistor terahertz detector to the autocorrelation of femtosecond terahertz pulses. First, the GFET THz detector antennas and the transistor were modeled. Then, the devices were electronically packaged to protect them from static discharges and external electromagnetic noise. Subsequently, the static volt-ampere and responsivity characteristics of the GFET THz detector were measured. Using a Michelson interferometer, femtosecond pulse autocorrelation interferograms were obtained. GFET modeling yielded typical values of charge carrier mobility (2000-3000 cm2/(Vs), 1-3% standard error) for GFETs with graphene on SiO2. It also enabled the evaluation of the carrier density and effective mass dependence on transistor gate voltage. During the nonlinear response experiment, the GFET THz detector exhibited a modulation depth of 3,7 and nonlinear disturbances at interference minima, indicating that graphene devices demonstrate not only saturation characteristics but also higher than second-order moment response to incident THz power. This phenomenon has the potential to be used during PhD studies to develop an opto-electronic high-frequency mixer. |
