The disclosed technology generally relates to characterization of semiconductor structures, and more particularly to optical characterization of high-k dielectric materials. A method includes providing a semiconductor structure comprising a semiconductor and a high-k dielectric layer formed over the semiconductor, wherein the dielectric layer has electron traps formed therein. The method additionally includes at least partially transmitting an incident light having an incident energy through the high-k dielectric layer and at least partially absorbing the incident light in the semiconductor. The method additionally includes measuring a nonlinear optical spectrum resulting from the light having the energy different from the incident energy, the nonlinear optical spectrum having a first region and a second region, wherein the first region changes at a different rate in intensity compared to the second region. The method further includes determining from the nonlinear optical spectrum one or both of a first time constant from the first region and a second time constant from the second region, and determining a trap density in the high-k dielectric layer based on the one or both of the first time constant and the second time constant.