Sub-100 fs Optical Nonlinearities in 2D Metallic Photonic Crystals

We used the pump-probe spectroscopy technique in the fs time domain to study the ultrafast optical nonlinearities in various 2D metallic photonic crystals (MPC), which include perforated films of cobalt, silver and aluminum. The pump beam was extracted from a fs Ti:sapphire amplifier laser system with pulses of 100 fs duration that were frequency doubled at 400 nm, with 10 micro-joule of energy/pulse at 1KHz repetition rate; whereas the probe beam was a white light super-continuum from 900 to 450 nm. In contrast to regular metals we found that the optical nonlinearity in the 2D MPC is in the sub-100 fs time domain in the form of an oscillatory time response. The fs transient spectrum contains oscillations in both non-linear transmission and reflection. The results are discussed in terms of resonant excitation of surface plasmon polaritons on the metal surfaces.