Maximum attainable efficiency of a cubic nonlinear response of matter

It is shown that the efficiency of a cubic nonlinear optical response of matter, defined as the ratio of the cubic nonlinear susceptibility to its relaxation time, may reach ~10⁸ cm² J^–1 for electronic mechanisms of this response. This is 10³—10⁴ times higher than the value achieved so far. The conditions for attaining the maximum efficiency are found.