Experimental verification of the intrinsic ultrafast delayed nonlinearity of gold

Surface plasmon polaritons (SPPs) in plasmonic metal waveguides can excite a third-order nonlinear response [1] much akin the well-known self-phase modulation (SPM) and two-photon absorption seen in light propagating in dielectric waveguides. In metals, the nonlinearity mainly arises as a self-acting effect: after absorption of the incident light the nonlinearity arises due to hot-electron contributions from changes of the intrinsic electronic lattice temperature. Using the so-called two-temperature model (TTM), the characteristic thermalization time of free-electrons in noble metals, such as gold, is described by an intrinsic delayed response function acting on a femtosecond-picosecond scale [1], explaining why the measured χ⁽³⁾ values of gold are affected by the laser pulse duration. Here we show experimental data performed using picosecond [2] and femtosecond [3] laser pulses that for the first time allows to quantitatively connect the measured χ⁽³⁾ values with the nonlinear coefficient of a nonlinear Schrödinger equation (NLSE) using the TTM to derive the nonlinearity.