That discipline studies and exploits the ways in which the feeble force of light interacts with very small mechanical objects. Or, as the team leader John Lawall says, "it involves coupling optical to mechanical systems in which the coupling is provided by radiation pressure."
To be sure, the forces and motions involved are extremely small. But understanding and measuring them is important to projects ranging in size from the lasers and mirrors at the giant Laser Interferometer Gravitational Wave Observatory to the minuscule cantilever probes used in atomic force microscopy.
A typical lab setup for this kind of work involves an optical cavity with two mirrors: a fixed mirror on one end, and a movable mirror on the other. (See schematic below.) Laser light injected into the cavity reflects back and forth, pushing on the mirrors as it goes. The resulting displacement in the movable mirror changes the distance between the mirrors, which in turn affects the resonance frequency of the cavity.
So Lawall, Utku Kemiktarak, Michael Metcalfe, and Mathieu Durand of the Quantum Metrology and Processes Group decided to take a completely different approach. Metcalfe, a former postdoc in Lawall's group, pointed out that with careful design, gratings can work as mirrors as long as the spacing between the ridges is below the wavelength of light, which in the team's case is about 1560 nm.
With colleagues at NIST's Center for Nanoscale Science and Technology, they produced a grating with a spacing of about 700 nanometers, beginning with a membrane of silicon nitride – a material known to have exceedingly low mechanical losses – and then etching it with reactive ions. (The original membrane has a reflectivity of about 27%; the final product has a reflectivity of 99.6% and has a smaller mass.)
The result is a micromechanical reflector that is more than an order of magnitude less massive than conventional stack reflectors, has a mechanical quality factor (Q = 7.8 X 105) two orders of magnitude higher, and reflects 99.6 % of the incident light. "We are not the first people to use gratings as reflectors," says Kemiktarak, "but we think we are the first to study them closely for their mechanical properties as well as their optical properties."
"These devices will likely be of great importance in MEMS devices and optomechanical systems employing radiation pressure," Lawall says.
They may also be of considerable use in pursuing some of the most intriguing phenomena that occur when optomechanical systems approach quantum limits. For example, scientists are interested in preparing fabricated mechanical systems in a state of motion that is equivalent to the ground state (lowest energy level) of a single atom. That will require, among other things, elimination of virtually all the thermal noise in the system – difficult to achieve in mesoscopic conditions.
"Basically, you can just take the excess thermal energy corresponding to mechanical motion and transfer it to the optical field via scattered light and therefore reduce the amount of thermal energy in the device itself."
 "Mechanically compliant grating reflectors for optomechanics," Utku Kemiktarak, Michael Metcalfe, Mathieu Durand, and John Lawall, App. Phys. Lett. 100, 061124 (2012).