Using a microscopic optical
sensor that can be batch-fabricated on a silicon chip at low cost, researchers
from the NIST Center for Nanoscale Science and Technology have measured the
mechanical motion between two nanofabricated structures with a precision close
to the fundamental limit imposed by quantum mechanics.
Combining a microelectromechanical system (MEMS) with a
sensitive optical resonator that can be accessed using conventional optical
fibers, the device provides a model for dramatically improving MEMS-based
sensors such as accelerometers, gyroscopes, and cantilevers for atomic force
microscopy. Traditional MEMS sensors depend on integrated electrostatic
transducers with slow response times and low signal-to-noise ratios, and most
scientific instruments that detect motion use bulky optics that require costly
instrumentation, careful alignment, and mechanical isolation.
To overcome these difficulties, the researchers created a
highly sensitive position detector that relies on a silicon microdisk optical
cavity that is only ten micrometers in diameter and has a similarly-sized
silicon nitride ring suspended a few hundred nanometers above it. The proximity
allows the evanescent light at the surface of the disk to interact with the
ring, and changes in the strength of this interaction can be used to measure
changes in the distance between them. The cavity has a high optical quality
factor, meaning that light from an optical fiber can make several thousand
round-trips in the cavity before leaking out, accumulating information about
the ring’s position with each round-trip. The cavity’s very sharp optical
resonance, combined with the high sensitivity of the optical mode to the
disk-ring distance, enables precision displacement measurements close to the
limit imposed by the quantum mechanical uncertainty principle. By using the
signal from the optical cavity as an input to an electronic feedback circuit
controlling a MEMS actuator that moves the ring, the researchers are able to
reduce the Brownian motion of the actuator (the displacement caused by random
molecular motion) by a factor of 1,000.
This feedback system effectively increases the mechanical
force-sensing bandwidth by more than a factor of 2,000, reducing the system
response time to ten microseconds. The overall device achieves a combination of
speed and precision that is completely unreachable with conventional MEMS
sensors. It combines extraordinary sensing performance, low power dissipation,
and wide tunability with the high stability and practicality of a fully
integrated silicon microsystem.
The researchers are now working to integrate these sensors
and actuators into highly sensitive, stable and compact cantilever probes for
atomic force microscopy.