Optical Integrated Circuits in a Photonic Crystal

All-optical switch using photonic crystal nanocavity


Since photons interact each other through the dipole moment, to obtain an efficient photon-photon interaction it is important to achieve large coefficient between the material and photons. However, the optical devices are usually made with optically transparent materials, which have only small light-matter interaction coefficient. Recently it is known that by designing the structure of the optical device, it is possible to overcome this fundamental problem. By introducing a new structure that can maximize the photon density, we can effectively enhance the light-matter interaction.

An optical cavity is a good candidate to yield a high photon density, because it can confine the light through a long period of time in one place. The quality factor (Q-factor) gives the extent of the light confinement. To obtain a higher density of photons, one can reduce the size of the cavity. However, it has been regarded as a difficult task to keep the Q while reducing the mode volume V by using conventional light confinement method. Yet it is now possible to obtain large Q/V value by employing light confinement yielding photonic bandgap (PBG).


We fabricated high-Q photonic crystal nanocavity using silicon photonic crystal slabs. We showed that <100 ps switching is possible using this device. Moreover, the operating energy is about 100 fJ. (1 pico = 1 x 10-12, 1 femto =1 x 10-15). This energy value is the smallest yet reported for an all-optical switching device using silicon.


Silicon is one of the most promising material owing to the high fusion with conventional planer semi-conductor processing. This work is significant because the ultra-low energy optical switching is demonstrated using silicon. In addition, since all the light propagate in-the-plane of the chip this study paves the way to the development of the nano-scale all-optical signal processing.

Operation principle

At the moment a control pulse injects the cavity, the refractive index of the silicon changes due to the carriers generated by two-photon absorption. When the signal light is initially at the resonance of the nanocavity, the cavity transmittance of the signal light exhibit ON state. When carriers are generated, the signal transmittance exhibit OFF state because the wavelength of the signal light and the resonance of the nanocavity becomes different. By detuning of the signal light to a shorter wavelength, we can also perform OFF to ON modulation.


  1. T. Tanabe, M. Notomi, A. Shinya, S. Mitsugi, and E. Kuramochi, "All-optical switches on silicon chip realized using photonic crystal nanocavitites," Appl. Phys. Lett. (submitted for publication).
  2. T. Tanabe, M. Notomi, A. Shinya, S. Mitsugi, and E. Kuramochi, "Fast on-chip all-optical switches and memories using silicon photonic crystal with extremely low operating energy," Quantum Electronics and Laser Science Conference (QELS'05), QPDA5, Baltimore, May 22-27, (2005).
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