**4. Conclusion**

70 Advanced Photonic Sciences

The reconfiguration possibilities of waveguides are somehow limited when compared with that of the cavities. The reconfiguration capabilities of the photonic crystal cavity proposed in Fig. 15 via electro-optic effects are presented next. Recall that the cavity had a resonant frequency at around 1500nm, precisely at 1499.1nm. We alter the refractive index of the photonic crystal material from 0 to 0.1 in steps of 10-5 by exploiting Pockels. The overlap of transmission spectra for different refractive index values proves the resonant wavelength redshift. This diagram resulting from our FDTD simulations is presented in Fig. 18. Subsequently we have focused in index changes achievable for reasonable values of applied electric field in normal applications, i.e. from 0.1 to 10MV/ μm. This reduces the range of achievable index changes to values up to 1.5510-3. In Fig.19 we represent the computed cavity resonant wavelengths as a function of the index change generated by Pockels effect using realistic electric field values. This simulation results are the proof of the concept of fast (switching time bellow 1 ns) and efficient (every |ΔE0|= 64.5 mV/μm implies Δλ= 6pm) reconfigurability.

Fig. 18. Overlapped transmission spectrum for refractive index increments of Δn=(0, 0.001,

Fig. 19. Resonant wavelength versus refractive index change for the photonic crystal cavity

0.01, 0.1)

shown in Fig.15.

Photonic band gap materials are a powerful tool for tailoring light propagation properties. This Chapter has put the emphasis on the control over the signal group velocity given the wide range of applications enabled by slow light. All slow light techniques reviewed rely on slowing down the information or the energy transported by light signals, more than on slowing the photons themselves.

Among the plurality of foreseen slow-light applications we have mainly highlighted two due to its high technological and societal expected impact. The development of optical buffer memories is of utmost importance for the deployment of all-optical networks. Slow light is a promising approach for fast, scalable and low-power consuming on-chip optical buffers. Given the state of the art of the technology, nowadays, such devices cannot replace electronic RAMs in their current functions; however major breakthroughs are still expected in this field. Concerning slow light application to biosensing, not so bandwidth demanding and less affected by losses, near future commercial prospects are encouraging.

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Slow light engineered structures perform better than material-based methods for high bandwidths. On the other hand, their feasibility into devices for practical applications is higher, due to the materials employed and their operation conditions. In particular, photonic crystals are proving to be a suitable technology to take slow light into practice. Small operating power, compact footprint, and the possibility of monolithic integration with electronics and CMOS fabrication are some of their strong points. Along this Chapter the capabilities of fast and efficient switching using electro-optic effects have been justified on the basis of computing and simulations.

So far our work in this field has been focused on high-index contrast photonic crystal structures, exploiting the use of materials as silicon and lithium niobate. Our future research lines will explore the use of *intelligent* materials such as hydrogel polymers and chalcogenide glass (Eggleton et al.,2011), in order to achieve added functionalities to the photonic crystal devices.

In spite of all these promising and almost magic properties of photonic band gap materials, they still remain at research stage. Some difficulties such as high propagation losses in the slow light regime, dispersive effects, coupling inefficiencies and fabrication roughness or inaccuracies are somehow hampering their evolution to commercial devices. Recent advances in dispersion engineering and loss reduction-oriented design approaches, together with the continuous improvement of fabrication processes, are taking slow light in photonic crystal closer to practical applications. In addition to this, the refinement of nanoimprint lithography, as an alternative for accurate mass production, herald a brighter future for real slow-light photonic crystal devices.
