Light resonators used to move nano-sized objects
Scientists at Cornell University report they can now use a light beam carrying a single milliwatt of power to move objects and even change the optical properties of silicon from opaque to transparent at the nanometric scale. Such an advancement could prove very useful for the future of micro-electromechanical (MEMS) and micro-optomechanical (MOMS) systems.
As with any other electromagnetic wave, light can be characterized as the union of an electric and a magnetic field oscillating in perpendicular directions that form small but periodic peaks and valleys in potential energy. These oscillations aren't enough to influence massive objects; on a small enough scale, though, particles that are hit by the wave tend to slide towards the "valleys" and distribute evenly on a surface. This is the principle exploited by optical and, more recently, sound tweezers to pattern tiny droplets in a predefined way.
However, it is thing is to move a nanoscale object, but another to hit it with a beam strong enough to change its geometry and optical properties, which requires much higher energy levels. To address the issue, the Cornell researchers created two "ring resonators," circular waveguides whose circumference are a multiple of the wavelength of the light used, and exploited the relationship between beams of light traveling through the rings to make exerting high forces with small energy levels possible.
The two waveguides are three microns wide, one micron apart, and just 190 nanometers thick. When light at an appropriate frequency enters the rings, the waveguides either strongly attract or repel each other depending on whether the beams traveling through them are in phase (meaning a peak in one beam corresponds to a peak in the other) or out of phase.
The repulsion phenomenon might be useful in MEMS, micro-electromechanical systems with moving parts, where a yet unresolved problem is posed by the tendency of silicon components to stick too close together. The repulsion force generated by the resonators could, in other words, separate these components and keep them at the desired, optimal distance increasing the system's efficiency. MOMS, or micro-optomechanical systems, could also benefit from the team's research to create tunable filters for a specific optical wavelength.
The research is due to appear in a forthcoming edition of the journal Nature. The work is supported by the National Science Foundation and the Cornell Center for Nanoscale Systems.