z -scan measurement of the nonlinear absorption of a thin gold film

We have used the z-scan technique at a wavelength (532 nm) near the transmission window of bulk gold to measure the nonlinear absorption coefficient of continuous approximately 50-Å-thick gold films, deposited onto surface-modified quartz substrates. For highly absorbing media such as metals, we demonstrate that determination of either the real or imaginary part of the third-order susceptibility requires a measurement of both nonlinear absorption and nonlinear refraction, i.e., both open- and closed-aperture z scans must be performed. Closed-aperture z scans did not yield a sufficient signal for the determination of the nonlinear refraction. However, open-aperture z scans yielded values ranging from β=1.9×10−3 to 5.3×10−3 cm/W in good agreement with predictions which ascribe the nonlinear response to a Fermi smearing mechanism. We note that the sign of the nonlinearity is reversed from that of gold nanoparticle composites, in accordance with the predictions of mean field theories.