Picosecond-continuum measurements of ultrafast dynamic refractive nonlinearities in a ZnSe interference filter

A powerful technique for studying the excitation dynamics of thin-film condensed matter is demonstrated. The field-enhancement within a Fabry-Perot etalon, and the sensitivity of such a structure to the optical coefficients of the spacer layer material, is combined with the time and spectral resolution afforded by use of time-delayed picosecond-continuum monitoring. Applied to ZnSe interference-filter material the optoelectronic refractive nonlinearity is identified, however rapid lattice heating is observed leading to dominance of the optothermal nonlinearity (of opposite sign) within 3 ps after photoexcitation.