“…The nonlinearity can be quantified, e.g., by the difference of high and low-field permittivity [9,16,21,24]. ii) Application of a high ac field and determination of the higher harmonics of the response, leading to higher-order permittivities [12,13,18,20,21,22,76,77]. iii) Application of a high dc bias voltage and measurement of the permittivity, usually with a smaller ac signal [78,79,80,81,82].…”