Photonic crystals have received much attention for their ability to guide light, or even to store light in resonating cavities for brief periods. Now, researchers at Tohoku University in Japan1 have designed photonic crystals that allow the controlled switching of light between waveguide structures.

The guiding of light waves using photonic crystals is a promising means of achieving optical interconnection in optoelectronic microcircuits, where information is transported between optoelectronic components. However, a practical way to achieve active, on-demand control over waveguiding and light storage in photonic crystals has proved elusive — existing systems are either relatively large or too slow.

Fig. 1: Photonic switches. As a resonant cavity (region without holes in the lower half) is moved towards a waveguide structure, light leaks into the cavity, where it is strongly confined.

The research team led by Yoshiaki Kanamori has come up with a system in which the coupling between a photonic crystal waveguide and a cavity is controlled mechanically (Fig. 1). The photonic crystal consists of two halves, separated by an air gap. The structure on one side of the gap hosts the waveguide, with the resonator cavity on the other. The cavity is fabricated on top of a silicon-based actuating structure — commonly used in nanomechanical systems — that brings the waveguide closer to the cavity with the application of an external voltage. When the waveguide approaches the cavity, the interaction between the two photonic crystal structures results in the transfer of light from the waveguide into the resonating structure. The light transfer ceases as the actuator moves the two halves of the device apart.

This approach has several advantages over other schemes. For example, the entire system is based on silicon technology, and therefore promises easy integration with photonic components. Furthermore, explains Kanamori, “the nanomechanical actuator used in our device is ultra-small, with high speed and low power consumption.” While conventional devices operate at frequencies of 132 kHz, says Kanamori, nanomechanical resonant structures with speeds of up to 100 MHz have been reported.

The potential for high-speed operation along with further optimization of the components of the photonic crystals to reduce waveguiding losses make this approach very promising for optoelectronics applications. “Our device has the potential for use as an ultra-small optical switch that cannot be realized in this size by other active control systems and that will be indispensable for light control of nano-photonics devices,” says Kanamori.