Nonlinear THz Response in Ultrathin Au Films

Gold is one of the most important materials for nano-optics with applications in energy transformation, sensing and imaging. The linear optical response of gold is commonly treated with its frequency-dependent dielectric function, reasoned by Drude free-electron model. The corresponding resonant frequencies are crucial for plasmonic applications and gold-based metamaterials. In current strong-field nano-optics, gold enables the localization of electromagnetic fields on scales of a few nanometers [1], [2]. However, the details of the nonperturbative nonlinear phenomena when approaching high fields are not understood on a microscopic level. Here, we apply intense terahertz (THz) pulses as defined electric field sources to investigate the response of gold to strong fields, where the frequency of the field is slower than the typical momentum relaxation time.