Optical Nonlinearity of Transparent Conducting Oxides: More Metallic than Realized

Transparent conducting oxides (TCOs) have recently been shown to have a remarkably strong nonlinear optical response. We show that the popular ascription of their nonlinearity to the temperature-dependence of the plasma frequency is only a partial description of their response to intense illumination. Specifically, we show that an increase in the electron collision rate upon illumination and consequent heating contributes to the permittivity in a manner that can be quantitatively comparable to the contribution of the temperature-dependent plasma frequency. This behavior makes the optical nonlinearity of TCOs more similar to that of noble metals than realized so far, and as far as the imaginary part of the permittivity is concerned, this behavior is qualitatively opposite compared to that assumed so far.