DID YOU
KNOW?
A test photo,
placed at differing distances
from the lens,
shows the focusing ability of the
diffractive liquid
crystal lens when
no voltage is
applied (a) and
when focal length
is 40 cm (b), 20
cm (c), 13. 3 cm
(d), 10 cm (e) and
6. 6 cm (f).
The world is get- ting smaller all
the time—or, at least,
maps of the world
are. Silicon photonics researchers at the
Photonics Research
Group of Ghent University and associated
with the Interuniversity
Microelectronics Center (both in Belgium)
used standard CMOS
tools to fabricate a
map of the world a
trillion times smaller
than the real thing.
The 40,000-km
circumference of the
globe at the equator
spans only 40 µm on
the map. The smallest features resolved
are about 100 nm
(corresponding to
100 km). The map was
made using a 30-step
fabrication process on
a silicon-on-insulator
wafer. It was created
in one corner of a chip
for testing silicon-based waveguides
with low propagation
loss and structures to
provide more efficient
coupling on and off
the chip.
Flat Lens Zooms Fast
An adaptive liquid crystal (LC) diffractive lens developed at the University of Arizona (U.S.A.) focuses with high efficiency and
zooms with millisecond-fast switching times.
Doctoral student Pouria Valley and colleagues
at the University of Arizona (U.S.A.) reported
a lens based on a 3-µm-thick layer of nematic
liquid crystals (P. Valley et al. “Tunable-focus
flat liquid crystal diffractive lens,” Opt. Lett.
35, 336).
Potential applications include zoom lenses
with no moving parts. His team can change
the optical power in a relatively large range
( 40 diopters or more) with a very low power
source (± 2. 4 V ac) and very fast (20-150 ms).
The moving parts in regular zoom lenses
are bulky, expensive and fragile, and the
process of physically moving lenses is slow
compared to the speed of electronic switching.
This explains the lack of zoom capacity in
camera phones, for example.
Previous LC lenses have shown lower
efficiency, and some have required notably
higher voltages. Compared to the Arizona
group’s previous lenses developed for ophthalmic applications, “the new lenses have
higher optical powers and higher diffraction
efficiency so we can tune them in a wider
range,” explains Valley.
The LC lens consists of a flat diffractive
optical element and a layer of LC sandwiched
between two glass substrates coated with transparent and conductive indium tin oxide. One
of the substrates is patterned like a diffractive
optical element. The effective refractive index
of the LC varies with applied voltage. One type
of diffractive element is a binary Fresnel zone
plate, with each zone divided into several subzones to digitize the phase profile.
With 12 phase levels, the diffraction efficiency (the fraction of light intensity passing
through the lens that is focused into a specific
diffraction order) reached 95 percent. High
efficiencies are easier to achieve with small-aperture systems, although lens apertures of
10 mm in diameter or more are possible.
The test images were made using 540-nm
light. Later work will be done with broadband
white light. “The chromatic aberration can be
reduced either by a proper hybrid design of a
diffractive and refractive lens or by adoption
of multi-order or harmonic diffraction,” Valley
says. “We plan to show this in future articles.”
The images were captured by placing the
test photo at different distances from the lens
and bringing it into focus. Although the effi-
ciency drops as the focal length shortens, the
image quality remains good.
—Yvonne Carts-Powell
8 | OPN Optics & Photonics News
www.osa-opn.org
