New Device Would Enable Optical and Mechanical Waves Vibrating at Tens of Gigahertz to Interact

A silicon photonic device capable of enabling the interaction of mechanical and optical waves vibrating at tens of gigahertz (GHz) has been theoretically developed by researchers at the University of Campinas's Gleb Wataghin Physics Institute (IFGW-UNICAMP) in São Paulo State, Brazil.

The proposed device was the outcome of the "Optomechanics in photonic and phononic crystals" and "Nanophotonics in Group IV and III-V semiconductors", both supported by FAPESP. A description of this device was presented in an article featured in Scientific Reports, an online journal published by Springer Nature.

Through computer simulations, we proposed a device that could exploit a mechanism for the scattering of light by mechanical vibrations, called Brillouin scattering, and could be transposed to photonic microchips.

Gustavo Silva Wiederhecker, Professor, IFGW-UNICAMP

This mechanism, initially described in 1922 by French physicist León Nicolas Brillouin (1889-1969), has been the focus of study by Wiederhecker and his group at IFGW-UNICAMP in the recent years. In Brillouin scattering, light, which is made up of photons, interacts with elastic vibrations comprising of phonons at extremely high frequencies (tens of GHz) in a transparent medium.

It was not possible to exploit this effect in an efficient manner until the 1960s, when the laser was incented by US physicist Theodore Harold Maiman (1927-2007).

During this time, it was discovered that the electromagnetic field of an intense beam of light transmitted along an optical fiber with the help of a laser source induces acoustic waves that circulate along the material and then scatter the light at a totally different frequency from that of the laser's.

This light scattering mechanism is easy to observe in optical fibers, which can be hundreds of kilometers long, because it's cumulative. It's harder to observe and exploit in an optomechanical device at the micrometer scale because of the tiny space in which the light circulates.

Gustavo Silva Wiederhecker, Professor, IFGW-UNICAMP

optomechanical devices simultaneously confine mechanical waves and light waves to allow interaction between them.

Wiederhecker and his group succeeded in overcoming this size limitation with reference to light propagation by developing silicon disks with a diameter of approximately 10 μm, corresponding to one tenth of the thickness of a strand of human hair. The disks perform as microcavities.

The researchers used an optical fiber with a diameter of approximately 2 μm to couple light to this system. Before dissipating, the light is reflected from the material’s edge and then spins around the disk cavity thousands of times over a few nanoseconds.

As an outcome, the light continues to be in the cavity for a longer time and thus interacts more with the material, and the optomechanical effects are then augmented. "It's as if the light is propagated over a much larger distance," Wiederhecker explained.

The problem here is that such a microcavity fails to allow light at any arbitrary frequency to be resonant, that is, to propagate via the cavity, even though it does enable the light initially emitted by the laser to propagate. "So you can't exploit the Brillouin scattering effect in these microcavities," he said.

The researchers used computer simulations to theoretically construct a system made up of two silicon microdisks with one cavity each instead of a microdisk with one cavity. The disks are laterally coupled, and the distance existing between their cavities is small, of the order of hundreds of nanometers (a nanometer refers to one billionth of a meter). A frequency separation effect is developed by this system.

The frequency of the light scattered by the acoustic waves is slightly separated by this effect from the frequency of the light emitted by the laser, which is 11-25 GHz -- precisely the same as that of the mechanical waves – and guarantees that the thousands of phonons (referring to elementary excitations of acoustic waves) generated each second in this system (at rates ranging from 50 kHz to 90 kHz) are capable of propagating in the cavities.

According to Wiederhecker, it is thus possible to observe and exploit Brillouin scattering in this micrometric system.

We show that with a laser power of about 1 milliwatt - equivalent to the power of a laser pointer for use in a slide presentation, for example - it would be possible to observe the Brillouin scattering effect in a double-disk cavity system.

Gustavo Silva Wiederhecker, Professor, IFGW-UNICAMP