In this schematic diagram of the optical cavity, the laser light resonates between the fixed mirror on the left and a single grating on the membrane at right. The gratings are tested individually.

A
team of scientists from PML’s Quantum Measurement Division has designed
and tested a novel device that may lead to substantial progress in the
new and fast-moving field of optomechanics.

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. 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.

Most
researchers use a moving mirror made up of 16 to 40 layers of
dielectric film with different indices of refraction, culminating in a
stack structure a few micrometers thick. That’s not exactly huge. But
“as you add layers,” Lawall says, “the mass per unit area and the
thickness just keep going up. And we need really low mass because the
optical force is so weak.”

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 nm, 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.)

Eventually
81 such gratings, each with slightly different finger widths and
spacings, were arranged on a single membrane. In this way the
researchers could experimentally determine the reflectivity
corresponding to a particular grating design, and choose a grating to
couple optimally to a particular mechanical mode of the membrane.

Top: The gratings are fabricated with approximately 700 nm between ridges. Each has slightly different spacing and thickness, affecting reflectivity and mechanical performance. Bottom: A single grating measuring 50 micrometers on a side.

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.

“In
order to get to the point where the system is down to only a few
motional quanta, as opposed to 10 million or so, what you want to do is
get rid of that thermal excitation,” Lawall says. The brute force way to
do that is just stick it in a cryostat. But in many cases, the
temperatures you need are very low. Instead, you can turn to techniques
that aren’t terribly different from those employed to laser-cool atoms.

“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.”

This
research was partly supported by the National Science Foundation
through the Physics Frontier Center at the Joint Quantum Institute. The
research was performed in part at NIST’s Center for Nanoscale Science
and Technology.

Mechanically compliant grating reflectors for optomechanics

Source: Joint Quantum Institute