We describe an ultrafast time resolved pump-probe spectroscopy setup aimed at studying the switching of nanophotonic structures. Both fs pump and probe pulses can be independently tuned over broad frequency range between 3850 and 21050 cm$^{-1}$. A b
road pump scan range allows a large optical penetration depth, while a broad probe scan range is crucial to study strongly photonic crystals. A new data acquisition method allows for sensitive pump-probe measurements, and corrects for fluctuations in probe intensity and pump stray light. We observe a tenfold improvement of the precision of the setup compared to laser fluctuations, allowing a measurement accuracy of better than $Delta$R= 0.07% in a 1 s measurement time. Demonstrations of the improved technique are presented for a bulk Si wafer, a 3D Si inverse opal photonic bandgap crystal, and z-scan measurements of the two-photon absorption coefficient of Si, GaAs, and the three-photon absorption coefficient of GaP in the infrared wavelength range.
We present ultrafast optical switching experiments on 3D photonic band gap crystals. Switching the Si inverse opal is achieved by optically exciting free carriers by a two-photon process. We probe reflectivity in the frequency range of second order B
ragg diffraction where the photonic band gap is predicted. We find good experimental switching conditions for free-carrier plasma frequencies between 0.3 and 0.7 times the optical frequency: we thus observe a large frequency shift of up to D omega/omega= 1.5% of all spectral features including the peak that corresponds to the photonic band gap. We deduce a corresponding large refractive index change of Dn_Si/n_Si= 2.0% and an induced absorption length that is longer than the sample thickness. We observe a fast decay time of 21 ps, which implies that switching could potentially be repeated at GHz rates. Such a high switching rate is relevant to future switching and modulation applications.
