Capturing light

While electronic communication used to rely solely upon miles of copper wire, fiber optics are now vital to telecommunication. By using light to transmit information, fiber optics are able to offer higher bandwidth and are not susceptible to electronic interference. The transistor was arguably the most important development in regards to electronic transmission, as it allowed signals to be easily amplified. Now research led by Ecole Polytechnique Fédérale de Lausanne (EPFL) has succeeded in producing an optical transistor [Weis et al., Science (2010) doi:10.1126/science.1195596].

 

The transistor makes use of a strange effect named optomechanically induced transparency, whereby the device can become transparent to a control laser beam, which effects how a second beam can propagate through the material, due to a change in refractive index. The result is thus analogous to a conventional transistor, where an input signal can be used to modulate a second signal.

 

In order for the system to operate in this manner, the mechanical vibrations of the structure have to directly couple to the optical input. Light incident on a surface results in a form of pressure known as radiation pressure. The presence of two beams (control and probe) with different frequencies means that there is an oscillatory force acting on the structure. When this oscillation is close to the resonant frequency of the device, the cavity is deformed.

 

This is not the first device to couple electromagnetic waves to a mechanical movement, although this is the first in the optical part of the electromagnetic spectrum. According to Prof. Kippenberg, team leader of the EPFL group, “the conversion of radio wave photons into phonons [vibration pseudo-particles] is the basic principal of cell phone filters”. But the potential uses do not stop there, as the device offers the possibility of storing optical pulses by slowing the group velocity of a light wave.

 

In speaking to Materials Today, Kippenberg told us that “the technological challenge is to find a platform that is suitable for applications; the toroid microcavity is the first system to observe the effect, but the relevance of the paper is that this effect can be observed in a wide range of optomechanical systems”. One possibility for the future of such devices are “optomechanical crystals that are fully planar”. It is devices such as these which may then be integrated into systems and provide “complex functionality”.