Создан световой нано-двигатель повышенной мощности
Переведено 05.07.10 с оригинала: newscenter.lbl.gov.
|
Nano-sized light mill drives micro-sized disk While
those wonderful light sabers in the Star Wars films remain the figment of
George Lucas’ fertile imagination, light mills – rotary motors driven by
light – that can power objects thousands of times greater in size are now
fact. Researchers with the U.S. Department of Energy (DOE)’s Lawrence
Berkeley National Laboratory and the University of California (UC) Berkeley
have created the first nano-sized light mill motor whose rotational speed and
direction can be controlled by tuning the frequency of the incident light
waves. It may not help conquer the Dark Side, but this new light mill does
open the door to a broad range of valuable applications, including a new
generation of nanoelectromechanical systems (NEMS), nanoscale solar light
harvesters, and bots that can perform in vivo manipulations of DNA and other biological
molecules. “We
have demonstrated a plasmonic motor only 100 nanometers in size that when
illuminated with linearly polarized light can generate a torque sufficient to
drive a micrometre-sized silica disk 4,000 times larger in volume,” says
Xiang Zhang, a principal investigator with Berkeley Lab’s Materials Sciences
Division and director of UC Berkeley’s Nano-scale Science and Engineering
Center (SINAM), who led this research. “In addition to easily being able to
control the rotational speed and direction of this motor, we can create
coherent arrays of such motors, which results in greater torque and faster
rotation of the microdisk.”
This STM image shows a gammadion gold light
mill nanomotor embedded in a silica microdisk. Inset is a magnified top view
of the light mill. The
success of this new light mill stems from the fact that the force exerted on
matter by light can be enhanced in a metallic nanostructure when the
frequencies of the incident light waves are resonant with the metal’s
plasmons – surface waves that roll through a metal’s conduction electrons.
Zhang and his colleagues fashioned a gammadion-shaped light mill type of
nanomotor out of gold that was structurally designed to maximize the
interactions between light and matter. The metamaterial-style structure also
induced orbital angular momentum on the light that in turn imposed a torque
on the nanomotor. “The
planar gammadion gold structures can be viewed as a combination of four small
LC-circuits for which the resonant frequencies are determined by the geometry
and dielectric properties of the metal,” says Zhang. “The imposed torque
results solely from the gammadion structure’s symmetry and interaction with
all incident light, including light which doesn’t carry angular momentum.
Essentially we use design to encode angular momentum in the structure itself.
Since the angular momentum of the light need not be pre-determined, the
illuminating source can be a simple linearly polarized plane-wave or Gaussian
beam.”
Ming Liu (foreground), Xiang Zhang and Thomas
Zentgraf have created the first nano-sized light mill motor whose rotational
speed and direction is controlled by the frequency of incident light waves.
This should open the door to a broad range of valuable applications in energy
and biology as well as in nanoelectromechanical systems. (Photo by Roy
Kaltschmidt, The
results of this research are reported in the journal Nature Nanotechnology in a paper titled, “ Light-driven
nanoscale plasmonic motors.” Co-authoring the paper with Zhang were Ming Liu,
Thomas Zentgraf, Yongmin Liu and Guy Bartal. It
has long been known that the photons in a beam of light carry both linear and
angular momentum that can be transferred to a material object. Optical
tweezers and traps, for example, are based on the direct transfer of linear
momentum. In 1936, “The
typical motors had to be at least micrometres or even millimeters in size in
order to generate a sufficient amount of torque,” says lead author Ming Liu,
a PhD student in Zhang’s group. “We’ve shown that in a nanostructure like our
gammadion gold light mill, torque is greatly enhanced by the coupling of the
incident light to plasmonic waves. The power density of our motors is very
high. As a bonus, the rotational direction is controllable, a
counterintuitive fact based on what we learn from wind mills.” The
directional change, Liu explains, is made possible by the support of the
four-armed gammadion structure for two major resonance modes – a wavelength
of 810 nanometers, and a wavelength of 1,700 nanometers. When illuminated
with a linearly polarized Gaussian beam of laser light at the shorter
wavelength, the plasmonic motor rotated counterclockwise at a rate of 0.3
Hertz. When illuminated with a similar laser beam but at the larger
wavelength, the nanomotor rotated at the same rate of speed but in a
clockwise direction.
Optical forces induced on a light mill motor
by an illumination wavelength of 810 nanometers (left) and 1,700 nanometers.
At 810 nm wavelength, light impacts the outer side of the arms, inducing a
counterclockwise torque on the motor. At 1,700 nm, light passes through the
gaps and impacts the elbow of the motor, providing a clockwise torque. (Image
courtesy of Zhang group) “When
multiple motors are integrated into one silica microdisk, the torques applied
on the disk from the individual motors accumulate and the overall torque is
increased,” Liu says. “For example, a silica disk embedded with four
plasmonic nanomotors attains the same rotation speed with only half of the
laser power applied as a disk embedded with a single motor.” The
nanoscale size of this new light mill makes it ideal for powering NEMS, where
the premium is on size rather than efficiency. Generating relatively powerful
torque in a nanosized light mill also has numerous potential biological applications,
including the controlled unwinding and rewinding of the DNA double helix.
When these light mill motors are structurally optimized for efficiency, they
could be useful for harvesting solar energy in nanoscopic systems. “By
designing multiple motors to work at different resonance frequencies and in a
single direction, we could acquire torque from the broad range of wavelengths
available in sunlight,” Liu says. This
research was supported by DOE’s Office of Science. Berkeley
Lab is a U.S. Department of Energy national laboratory located in |
Создан световой
нано-двигатель повышенной мощности Исследователям из Калифорнийского университета в Беркли и Национальной лаборатории Лоуренса удалось построить наноразмерный световой двигатель, обладающий повышенным крутящим моментом. Как рассказал Xiang Zhang, руководитель исследования, созданный двигатель имеет размер всего 100 нанометров. Двигатель использует принцип облучения плазмонов поляризованным светом и генерирует момент силы, способный вращать кварцевый диск размером в 4000 раз больше самого двигателя.
Образцы двигателей, основанные на данном принципе, создавались и ранее, однако они обладали гораздо худшими характеристиками: размеры их были на несколько порядков больше, а крутящий момент – меньше. Успех эксперимента обусловлен особой формой наноструктур, из которых состоит двигатель. Форма, напоминающая свастику, позволяет взаимодействовать практически со всем падающим светом, почти вне зависимости от угла падения. «Свастика» сконструирована из четырех маленьких колебательных контуров. Поэтому резонансные частоты определяются только геометрией «мельницы» и диэлектрическими свойствами золота, из которого она состоит. И, следовательно, скорость и направление вращения диска можно регулировать, изменяя частоту света, приводящего двигатель в действие, что и подтвердил эксперимент. «Кроме того, что мы теперь можем достаточно просто управлять направлением и скоростью вращения мотора, мы можем построить также когерентную решетку, состоящую из таких моторов и получить в результате больший момент и большую скорость вращения микродиска» - утверждает Xiang Zhang.
Создание этой световой мельницы
открывает новые перспективы в таких областях, как создание нового поколения
наноэлектромеханических систем (NEMS), наноразмерных элементов солнечных
батарей и микророботов, способных к непосредственным манипуляциям с ДНК и
другими биологическими молекулами. |
