快猫短视频

LIGHT can now be used to track movements to within a fraction of an atom鈥檚
width鈥攁nd it鈥檚 all done with mirrors. The precision of the new measurement
technique might lead to the manufacture of faster microchips and even tinier
micro-machines.

For more than a century, scientists have used light to measure tiny
movements, with a method known as interferometry. But the new trick could
improve the precision of this old technique up to 500-fold, report Yuri
Ovchinnikov and Tilman Pfau of the University of Stuttgart.

The simplest type of interferometer splits a single light beam in two and
bounces one beam off a fixed mirror while the other reflects off a mirror
attached to a moving object. The two beams recombine to form a single 鈥渙utput鈥�
beam whose strength depends on the position of the object.

It varies because the two beams travel different distances over the same
time, depending on how far the moving mirror has travelled. So when the light
waves recombine they can line up peak-to-peak, peak-to-trough, or anywhere in
between. When they line up peak-to-peak, they combine to give the maximum
output. When they line up peak-to-trough, the two waves cancel out, and the
output drops to zero. In previous models, the output would fall toward zero and
climb back to its maximum only if the moving mirror had travelled a distance at
least half the wavelength of the light.

Ovchinnikov and Pfau get around that half-wavelength limit by shining a laser
beam at an angle into a narrow gap between two parallel mirrors, one fixed and
one movable. The beam separates into several 鈥渕odes鈥濃�攚aves that bounce off
the mirrors a different number of times as they travel down the long gap, just
like light bouncing off the walls inside an optical fibre.

The modes recombine to produce an output beam whose intensity depends on the
exact distance between the mirrors. As the mirrors are brought together, each
mode bounces more times. The result is that ever smaller shifts of the movable
mirror cause the output intensity to fall toward zero and climb back to its
maximum.

The researchers were able to detect movements as small as one-ninth of a
wavelength, roughly 70 nanometres. And Ovchinnikov says movements as small as a
thousandth of a wavelength鈥攁nd perhaps smaller than an atom鈥檚
width鈥攎ight be detectable if the mirrors could be made sufficiently
flat.

Interferometers typically serve as ultra-precise rulers: the user tracks an
object by counting the peaks in the intensity of the output over a fixed time.
The new method adds a much finer scale to the ruler, says Olivier Pfister, a
physicist at the University of Virginia in Charlottesville. 鈥淚 think this could
open an ocean of technological possibilities,鈥� he says. Examples might be new
generations of micro-machines and chips made with tinier components.