EO modulator using randomness in photonic crystal line defect waveguides

Silicon photonics is a research field that uses silicon as a material to propagate light and handle optical signals. Compared to glass used for optical fibers, silicon has a higher refractive index, which allows light to be confined to a smaller space, making it possible to miniaturize devices and drive them with low energy. Photonic crystals are one of the elements belonging to silicon photonics, and have a special periodic structure that enables even stronger light confinement than silicon materials inherently provide. Various devices have been realized using photonic crystals, including waveguides, switches, detectors, and oscillators.

While photonic crystals enable miniaturization, they are susceptible to fabrication errors. Since the absolute amount of fabrication error is process-specific, the smaller the device to be fabricated, the greater the effect of the error. In addition, the recent trend in the fabrication process of photonic crystals is to use photolithography, which is capable of mass production but has lower fabrication accuracy, instead of the conventional EB lithography.

In this study, we have demonstrated that the effects of fabrication errors in photonic crystals can be structurally controlled and used with high yields. EO modulators are shown as a possible application, and operation at GHz has been confirmed.

We chose the structure shown in Fig. 1(a), which is called a photonic crystal line-defect waveguide. Light propagates through the center of the waveguide where there is no hole. Figure 1(b) and (c) show the cutoff frequency of this structure without and with fabrication error. When light is incident on the structure at the frequencies indicated by the red dashed lines and arrows, light confinement occurs only when there is a certain amount of fabrication error [Fig. 1(c)]. It is also important to emphasize that light confinement occurs only at W0.98 because the waveguide width is varied. Although this phenomenon originates from the randomness of the fabrication error, the structure allows us to control the area where the randomness appears.