Ultrafast Optical Switching of 2-D Photonic Crystals

The optical properties of photonic crystals can be altered most rapidly if a refractive index is altered using ultrashort optical pulses. I review several experiments in which we have used the Kerr effect or the Drude effect to switch optical properties of 2-D silicon photonic or GaAs photonic crystals on a picosecond or subpicosecond time scale. Tuning of the optical properties of a photonic crystal (PhC) has been the subject of intense research efforts related to the potential development of active photonic crystal components. With femto- or picosecond pulse excitation of PhCs, tuning can behaved in the fastest possible way via the free-carrier induced Drude effect or the Kerr effect. In this paper we summarize some of our recent work on the tuning of both 2-D silicon and 2-D GaAs photonic crystals using short optical pulses. In the case of the silicon PhCs, earlier we showed how tuning of a Bragg gap edge by as much as 30 nm could be achieved via the Drude effect. However, our results were partly limited by the absorption depth of an 800 nm pump pulse. As others have recognized as well the use of two photon absorption pumping might provide more uniform pumping. We have therefore used the real (Kerr) and imaginary (two photon absorption) parts of a third order optical nonlinearity to tune the long (1.6 mum) and short wavelength (1.3 mum) band edges of a stop-gap. From pump-probe reflectivity experiments using 130 fs pulses, we observe that a 2 mum pulse induces optical tuning of the 1.3 mum edge via the Kerr effect whereas a 1.76 mm pulse induces tuning of the 1.6 mum band edge via both Kerr and Drude effects with the latter related to two-photon induced generation of free carriers with a lifetime of 900 ps. In the course of this tuning research we noticed that little attention has been paid in the literature to how the spatial characteristics of the excited eigenmodes influence the temporal or spectral dependence of the optical response. We have therefore undertaken experiments in which 1.5 mum, 130 fs pulses excite carriers in the 2-D PhC by two photon absorption (2PA) via PhC eigenmodes with different spatial characteristics. We demonstrated a time- and pump mode-dependent modification of the silicon refractive index via the Drude effect and monitored this using time-resolved reflectivity in the vicinity of a PhC mode at 1.9 mum. The 2PA absorption depth is sufficiently large that a mode description of the initial carrier description is possible. A simple perturbation technique was adapted to determine how PhC eigenfrequencies in general are influenced by small, refractive index changes in the PhC. The degree of tuning is controlled by the spatial overlap of the induced refractive index change (governed by the pump mode) with the probe mode energy density. Although the generated carriers are initially distributed according to the pump mode (actually the fourth power of the mode's field distribution for 2PA), this distribution can be influenced by disorder within the PhC but not appreciably affected by surface termination and weak beam focusing. Subsequently, the dynamics and spatial distribution of the carriers are governed by diffusion as well as surface and bulk recombination. Owing to the fact that bulk recombination in defect-free Si is slow and carriers are generated within 100's of nm of a surface recombination was found to dominate bulk recombination. Femtosecond pump-probe reflectivity and second harmonic spectroscopy experiments have also been used to investigate tuning of leaky modes in a two-dimensional GaAs photonic crystal waveguide. For above band-gap excitation with 270 fs, 800 nm pulses with a pump fluence of 100 mucm^-2, a blue shift of 16 nm is measured for a 1900 nm leaky mode as observed from the shift of the resonantly enhanced second harmonic pulse. Theoretical calculations of carrier-induced changes in the refractive index from band filling, band gap shrinkage and Drude contributions are in good agreement with the results. Recovery occurs within 2~ps and is attributed to recombination via deep-centered defects. For below band-gap excitation with 166 fs, 1900 nm pulses with a fluence of 800 muJcm^-2, red shifts of 1.5 nm of the ~1360 nm leaky mode are induced via the optical Kerr effect during the early pan of the pump pulse, but thereafter the mode is blue shifted, likely due to free carriers injected into or from deep-centered defects