  We report a large third order nonlinear optical response of palladium
diselenide (PdSe2) films, a two dimensional (2D) noble metal dichalcogenide
material. Both open aperture (OA) and closed aperture (CA) Z scan measurements
are performed with a femtosecond pulsed laser at 800 nm to investigate the
nonlinear absorption and nonlinear refraction, respectively. In the OA
experiment, we observe optical limiting behaviour originating from large two
photo absorption (TPA) in the PdSe2 film of beta = 3.26 x 10-8 m/W. In the CA
experiment, we measure a peak-valley response corresponding to a large and
negative Kerr nonlinearity of n2 = -1.33 x 10-15 m2/W, two orders of magnitude
larger than bulk silicon. We also characterize the variation of n2 as a
function of laser intensity, observing that n2 decreases in magnitude with
incident laser intensity, becoming saturated at n2 = -9.96 x 10-16 m2/W at high
intensities. These results verify the large third order nonlinear optical
response of 2D PdSe2 as well as its strong potential for high performance
nonlinear photonic devices.
