Designer surfaces rewrite laws of refraction and reflection
CAMBRIDGE, Mass. – A new technique
called phase discontinuity helps light
break the age-old laws of reflection and
refraction – and could lead to the development of flat lenses that can focus images
with no aberrations.
For centuries, it has been recognized
that light travels at different speeds
through different media. Reflection and refraction occur whenever light encounters a
material at an angle, because one side of
the beam is able to race ahead of the other.
As a result, the wavefront changes
Conventional laws in physics predict
the angles of reflection and refraction
based only upon the incident (incoming)
angle and the properties of the two media.
However, researchers at Harvard School
of Engineering and Applied Sciences
(SEAS) have discovered that the boundary
between the two media, if specially patterned, can itself behave as though it were
a third medium.
Using designer surfaces, the researchers
created a fun-house mirror effect on a flat
plane, according to Federico Capasso, professor and senior researcher. The key component is an array of gold nanoantennae
etched into the surface of the silicon used
in the SEAS lab. The array is structured
on a scale much thinner than the wavelength of the light hitting it. This means
that, unlike in a conventional optical system, the engineered boundary between the
air and the silicon imparts an abrupt phase
shift – or phase discontinuity – to the
crests of the light wave crossing it.
Each antenna in the array acts as a tiny
resonator that can trap the light, holding
its energy for a given amount of time before releasing it. A gradient of different
types of nanoscale resonators across the
surface of the silicon can effectively bend
the light before it even begins to propagate
through the new medium.
An array of nanoscale resonators that are much
thinner than a wavelength creates a constant
gradient across the surface of the silicon. In this
visualization, the light ray hits the surface from
below at a perpendicular angle. The resonators
on the left hold the energy slightly longer than those
on the right, so the wavefront (red line) propagates
at an angle. Without the array, it would be parallel
to the surface. Courtesy of Nanfang Yu.
(Above) Clockwise from left: Patrice Genevet, Nanfang Yu, Federico Capasso, Zeno Gaburro and
Mikhail A. Kats. (Below) A simulation of the image that would appear in a large mirror patterned with
the team’s new phase mirror technology. Courtesy of Nanfang Yu and Eliza Grinnell, Harvard SEAS.
Harvard researchers have created strange optical
effects, including corkscrewlike vortex beams, by
reflecting light off a flat, nanostructured surface.
Courtesy of Nanfang Yu.