SPIE Newsroom
10.1117/2.1200701.0589
A laser motor directly
transforms light energy into
mechanical energy
Hideki Okamura
A novel light-driven actuator can induce relative movement between
two objects.
After showing that the radiation pressure from focused laser
light was capable of manipulating small neutral particles,1
Ashkin invented the optical tweezer, now widely used in the
biological sciences to manipulate objects.2 The most basic type
of optical tweezer uses a high-precision microscope to focus a
laser beam to a spot within a sample, creating an optical trap
capable of holding a small particle in its center. The technique
is particularly useful because one can drive and control the motion of objects in a non-contact mode. Its application, however, is
limited to small objects, typically 100µm or smaller, because the
technique can only apply forces in the pN range. It is estimated
that only about 10−10 % of the light energy is used for driving
the object.3 Hence, if the efficiency of converting light into mechanical energy could be improved, a new range of applications
could emerge. The light energy is actually high enough to manipulate much larger objects. For instance, a laser output of 10W
corresponds to power capable of accelerating a 1kg-object from
rest to a speed of 5m/s in one second, or moving it upward at a
constant speed of 1m/s.
Various types of light-driven actuators have been proposed.3–5
In most designs, laser irradiation induces heat, electricity, or
phase transition and the subsequent volume change produces
mechanical motion. Their energy conversion efficiency is generally higher than that of the optical tweezer itself (10−5 −
10−1 %). However, walk-like cycles consisting of several strokes
are required to convert the motion of the actuator element (usually bending) into a useful form of translational or rotary motion. This not only degrades the overall conversion efficiency
but, worse, also slows down the operation speed, leading to a
significant reduction in actuator power.
Figure 1. Two laser motor configurations: rotary (top) and linear
(bottom).
Based on the above considerations, I designed the light-driven
actuators shown in Figure 1, called laser motors3 by analogy
with conventional motors. The schemes are based on light pulses
from two pulsed lasers that generate a traveling elastic wave
in a solid. This can be achieved using various methods including heat deposition, ablation, or the photovoltaic effect.
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Typically, a Rayleigh wave is generated, although a bulk wave
will also work.6 A Rayleigh wave is a surface wave such that
each portion exhibits circular motion along the surface of the object (see Figure 2). Hence, if another object is pressed against the
first, relative motion is induced as the second object is driven
in the direction opposite to that of wave propagation. Rotary or
translational motion can thus be generated without any cycle
requirement.
To demonstrate elastic wave generation, we performed an
experiment using a Q-switched Nd:YAG laser (wavelength =
1064nm) and a copper ring as target. A clearly-defined elastic
wave of a single frequency was observed by tuning the repetition rate to the resonant frequency of the target (see Figure
3).7 The amplitude of the vibration was of the order of 10nm,
which could be increased by using a higher Q-factor material, or
by resorting to excitation methods more efficient than heat deposition.
One of the advantages of light-driven actuators is that the
driving light doubles as the control signal, which eliminates the
need for other control channels.
The laser motor can also easily be reversed, simply by changing the irradiation timing. If two elastic waves, A1 ( z, t) =
sin(ωt) cos(kz) and A2 ( z, t) = sin(ωt + π /2) cos(kz + π /2),
are excited by two sets of laser pulse trains, their superposition,
Figure 2. Generation of relative motion between two objects induced
by a Rayleigh wave.
Figure 4. Irradiation timing of two laser pulse trains, B1 and B2. The
top and bottom schemes rotate the motor in opposite directions.
A1 ( z, t) + A2 ( z, t) = sin(ωt − kz), yields a traveling wave in the
+ z direction. If we shift the irradiation timing of A2 ( z, t) and use
A02 ( z, t) = sin(ωt − π /2) cos(kz + π /2) instead, the superposition becomes A1 ( z, t) + A02 ( z, t) = sin(ωt + kz), i.e. a traveling
wave in the − z direction, thus effectively reversing the motor
direction (see Figure 4).
Light-driven actuators have attractive properties. First, energy
is delivered in a non-contact mode and an object can be remotely
driven. Moreover, these devices can be controlled by light, so
other control methods are unnecessary. Since there is no need
for batteries or other energy sources, they can also be very small
and light. Another advantage is their high tolerance for electromagnetic noise. These properties make them promising candidates for new applications seeking to replace conventional motors, while allowing them to be used in environments in which
conventional actuators cannot be used.
In summary, I have designed a potentially-efficient light-tomechanical-energy converter, a new laser motor that works by
irradiation from two sets of pulsed lasers, controlled by their irradiation timing. It operates continuously and is potentially fast
and efficient. Its unique properties may enable the development
of various novel applications such as micro-robots, ultra-light
climbers, and remotely operated screws.
Author Information
Figure 3. Induction of an elastic wave of single frequency: oscilloscope
trace of the voltage signal from the transducer attached on the surface
of the copper ring target (green trace). The bottom trace is the photodetector signal showing the irradiation timing. The horizontal scale is
200µs/division.
Hideki Okamura
Physics Department
International Christian University
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Mitaka, Tokyo, Japan
http://subsite.icu.ac.jp/people/okamura/
http://www.icu.ac.jp
Hideki Okamura is currently assistant professor of physics at International Christian University. He received his PhD from the
University of Tokyo in 1994. His research interests include lightdriven actuators, trapping of neutral molecules by laser, laser
isotope separation, high-resolution spectroscopy of atoms and
molecules, laser-assisted time-of-flight spectroscopy, and nonlinear optics.
References
1. A. Ashkin, “Acceleration and trapping of particles by radiation pressure,” Phys.
Rev. Lett. 24(4), pp. 156–159, 1970.
2. A. Ashkin, “History of optical trapping and manipulation of small-neutral particle, atoms, and molecules,” IEEE J. Sel. Top. Quant. El. 6(6), pp. 841–856, 2000.
3. Hideki Okamura, “Laser motor,” 6374, p. 637401, 2006. Proc. SPIE.
4. S. S. Sarkisov, M. J. Curley, L. Huey, A. Fields, S. S. Sarkisov II, and G.
Adamovsky, “Light-driven actuators based on polymer films,” Opt. Eng. 45,
p. 034302, 2006.
5. Y. Yu, M. Nakano, and T. Ikeda, “Directed bending of a polymer film by light,”
Nature 425, p. 145, 2003.
6. Z. Shen, S. Zhang, and J. Cheng, “Theoretical study on surface acoustic wave
generated by a laser pulse in solids,” Anal. Sci. 17, pp. S204–S207, 2001.
7. Bodo Richert and Hideki Okamura, “Laser irradiation induced vibrations in
solids,” 6374, p. 63740M, 2006. Proc. SPIE.
c 2007 SPIE—The International Society for Optical Engineering