Thermal and Non-Thermal Optical Nonlinearities of 2D Layered Transition Metal Dichalcogenides in Suspension

An accurate characterization of the materials' nonlinear optical (NLO) properties requires an investigation of the physical mechanisms responsible for the effects. In general, third-order optical nonlinearities can be described by an intensity-dependent change in the refractive index given by
$\Delta n=n_{2}I$
, where
$n_{2}$
is the nonlinear refractive index and
$I$
is the optical intensity. This equation holds for third-order nonlinear effects irrespective of their origin. When the response is associated with the optical Kerr effect, for instance,
$n_{2}$
relates to the third-order electronic susceptibility,
$\chi^{(3)}$
. Conversely, thermal effects give rise to a Kerr-type third-order nonlinearity that bears no relationship to the intrinsic electronic properties of the materials, so knowing the physical mechanism responsible for the NLO effect is essential to avoid misleading inferences about the material's properties.