A light-driven motor operated by a single source of light is demonstrated for the first time. The device utilizes a NiTi shape memory alloy (SMA) wire initially trained straight shape. The SMA wire was formed in a loop and put around two wheels. In the experiment a steady and continuous rotation was achieved with a bundled loop of 0.3-mm diameter wire. The rotational speed was proportional to the incident light power and 60 rpm was obtained at 3.6 W of light power. The experimental result was consistent with the theory.
The purpose of this paper is to present a possibility of improving the efficiency of light driven actuators by using pulsed
laser and to investigate theoretical limit of its efficiency, thereby providing an insight for the design of an efficient light
driven actuator in the future. Many light driven actuators based on the heat deposit can be regarded as heat engines, and
consequently their efficiencies are subjected to the limitation from Carnot's theory, which dictates that the maximum
efficiency is larger for larger temperature difference between the high- and the low-temperature reservoirs. If one uses
focused laser pulses, an extremely high temperature difference can be created for a short fraction of time, therefore, there is
a possibility of achieving a higher efficiency. In the theoretical treatment, a cycle in which heat is given instantaneously in
an adiabatic condition was considered. The formula for the maximum efficiency was derived, which gives about the half
that of Carnot cycle efficiency at an ambient temperature of 300K. In an ideal case, for the instantaneous temperature
increase of 200K the maximum efficiency can be about 20 %.
The purpose of this paper is to demonstrate a light-driven actuator with high energy conversion efficiency. Ordinarily,
light-driven actuators have low conversion efficiency of about 10-5%. This is partly due to an inefficient coupling of the
material itself and the actuating mechanism. Our idea is to use a primarily tensioned thin wire as the material since wire can
shrink and stretch in a specific direction. Therefore, we created a light-driven actuator with a wire of shape memory alloy.
Using Argon-ion laser to irradiate the shape memory alloy, we checked the energy conversion efficiency of this actuator.
As a result, we obtained an energy conversion efficiency of about 3.9% that is far greater than the ordinary value for
light-driven actuators. If the reflectivity of the shape memory alloy is considered, the conversion efficiency is doubled to
7.8%. When viewed as a heat engine, 7.8% efficiency corresponds to approximately 27% of the theoretical limit from
Carnot's theory.
The concept of a light driven speaker was proposed as a new category of light driven actuator, and its feasibility was
experimentally demonstrated. If a diaphragm of a speaker is made up of a light-driven actuator, sound would be produced
upon irradiation of a modulated light. In this scheme, the light doubles as the signal and the energy source. With a
conversion efficiency of 1 %, an audible sound is supposed to be generated from a 100 mW light source, therefore,
energy-wise light actuated speaker should be possible. The limitation on the energy conversion efficiency of this scheme is
discussed taking into account the energy loss due to the presence of a carrier wave that accompanies transmission.
Unfortunately, a material that satisfies the requirements for this scheme is not found at present. Yet, since the purpose of
this work is to show the feasibility of this concept and evaluate its possibility, it is an option to employ a combination of
materials. In this preliminary study, we chose to combine a piezoelectric actuator and a solar cell with an appropriate
interface for matching impedance. In the experiment, an 808 nm diode laser beam was modulated at audio frequency, and
shined a solar cell. The audio signal was reproduced from the connected piezoelectric speaker as an audible sound. The
maximum sound output of 3.1 mW was obtained. In a different scheme the maximum energy conversion efficiency of 4.4
% was obtained. These results show that the light actuated speaker is indeed feasible.
KEYWORDS: Solar cells, Energy efficiency, Energy conversion efficiency, Visualization, Light sources, Acoustics, Power supplies, Actuators, Light emitting diodes, Resistance
As one category of light-driven actuators, we propose a light-actuated device which outputs sound. To verify the
feasibility of the concept of the light-actuated speaker, we performed experiments using a scheme where the light energy
is first converted into an electrical current which then drives the connected speaker. The method seems rather indirect,
but given the efficiency of the solar cell, which can reach around 25% with the use of an appropriate wavelength, and
with the adoption of impedance matching between the solar cell and the speaker, a high overall conversion efficiency is
expected. With its realization, it would be possible to create a sound system that is wireless and can be controlled
remotely without the need of a power supply which will provide advantages over the conventional electronic
transmission such as wireless operation with no external power source, and precise control of listening area compared to
radio transmission. Possible applications include a sound guidance system for the visually impaired.
Pulsed Laser beam irradiation induced nm peak to peak resonant vibrations in solids were generated, detected and
analyzed. For the evaluation of the induced vibrations, transducers and optical methods were used.
The enhancement effect of vibrational amplitude by resonance and other methods was confirmed.
The existence of vibrations of picometer amplitude induced by mechanical means in solids are visualized by use of
synchronous illumination and optical manipulation.
Light driven actuators that have already been proposed are intended for applications on a rather small scale, however,
commercially available laser oscillators have sufficient energy to drive much larger objects. Is it possible to realize
light-driven actuators that can replace electrical motors? In this paper, a discussion regarding this goal is presented basic of
the conversion efficiencies from light energy into mechanical energy. Several methods of actuation, including the one that
is based on radiation pressure, were compared from this perspective. The energy conversion efficiencies for converting the
motion of the actuator element into a useful form of motion are separately considered. It was concluded that
light-absorption type actuators with a continuous operation scheme are the most promising for achieving a high efficiency.
Based on these findings, a new scheme, called the laser motor, is proposed. In the proposed scheme, a pulsed laser shines
on an elastic material and induces a specific form of vibrations in it. By using two lasers of different wavelengths, a
traveling wave is formed. Another object is pressed against the vibrating surface and a relative movement between the two
objects is then created.
We investigated the role of the photovoltaic effect on the photorefractive grating formation under illumination of weak ultrashort light pulses whose duration is much shorter than the time constants of the carrier and compared it with the cw case. In our experiments using mode-locked picosecond and cw Nd:YAG lasers, the bulk photovoltaic current in an iron- doped lithium niobate crystal were confirmed to be in the same order of magnitude for both light sources. The evolution of photorefractive gratings was also essentially the same. The Glass expression for the bulk photovoltaic effect is not compatible with the scheme of the band transport model for a very fast response. Therefore we developed a model in which the bulk photovoltaic current was implemented as a spatial shift in the initial carrier distribution. The resultant expression agreed with that of the quasi-cw approximation in the limit of a small spatial shift.
A novel method to measure a short light pulse using the photorefractive effect is proposed. Since photorefractive crystals are volume holograms, if we assign one axis to time, we can use them as materials which record the change of spatially two dimensional information. We recorded 3.5 ps pulses of a mode-locked Nd:YAG laser in a Fe:LiNbO3 crystal and read out the information by cw He-Ne laser by measuring the amplitude and phase of the diffracted wave.
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