Figure 2: Z-scan nonlinear measurement.

(a) Schematic z-scan setup for third-order nonlinearity characterization. A femtosecond laser beam (Ti-sapphire laser, Mai Tai HP from Spectra-Physics, linearly polarized, 80 fs width and 80 MHz repetition rate) is focused onto the MQW sample through a lens (f=3 cm). The raw laser beam diameter is about 3.5 mm, resulting in a Gaussian beam waist about 9 μm at the focus. The MQW is moved along the optical axis near the focus, two power metres are used to record the open and closed aperture z-scan signal through one collimation lens and a beam splitter. The closed aperture (b) and open aperture (d) z-scan curves for the 3 nm (red-circle) and 15 nm (black-cross) Au films using 900 nm incident wavelength with power of 111 and 690 mW. The closed aperture data are normalized to that of the open aperture. Z-scan signal of 3 nm sample is fitted by the standard z-scan theory (blue-dashed), from which the Kerr coefficient is extracted. (c) The measured nonlinear phase shift for different incident power. The dashed line is the mathematically linear fitting for the measurement data and nonlinear phase change is zero at zero intensity.