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The manufacture of millimeter-size industrial components requires micrometer-order processing precision. Inspection of these components requires even greater precision, in the form of nanometer precision measurement technology. In the growing field of ultra-micro processing, the Optical Radiation Pressure Microprobe Unit manufactured by Nikon Engineering will prove indispensable. This device utilizes laser trapping technology for “capturing” an object using light, and it can accurately measure the shape of ultra-fine components.
In this interview we asked Professor Yasuhiro Takaya of Osaka University's Graduate School of Engineering about such topics as the principle of laser trapping using optical radiation pressure, the scope of application of the microprobe unit, the nano-CMM coordinate measuring machine—a 3-dimensional coordinate measurement system, and anticipated future developments.
What is laser trapping technology for capturing objects using light?
What is meant by capturing objects using light?
“Capturing” an object refers to the ability to move it by applying a force or to lift it in defiance of gravity. Force is applied to the object in order to capture it, and light is used to apply this force. The lighting that is now illuminating us all is in fact applying an extremely weak force. The effect that light exerts on the surface area of a target object in the micro domain is greater than those exerted by the object's own volume and by gravity. Hence, despite the fact that light only exerts an extremely weak force on the object, it is sufficiently powerful to move it.
This was predicted by Isaac Newton over 300 years ago. James Clerk Maxwell subsequently turned this into theory in 1864 when he published equations that revealed light possesses energy. However, more than 100 years passed before light was actually used to raise an object. Since the energy in light is very weak, moving an object requires that the light be focused into a small area by means of a lens, and this was made possible by the development of the laser. Successfully demonstrated experimentally in 1971, lasers were probably not that widespread at the time. In their experiments*, Arthur Ashkin's team succeeded in raising (levitating) an object in a vacuum by striking it with a laser from beneath.
In our current experiments we raise an object in air by striking it with a laser from above. However, the basic method of focusing the laser light using a lens and striking the object remains essentially unchanged to this day.
- *
A.Ashkin and J.M.Dziedzic:Optical Levitation by Radiation Pressure,
Applied physics letters, Vol.19,No.8(1971)283-285
Raising an object with a light from above is miraculous.
When an object changes its course of direction, it applies a force to the corresponding object from which it rebounds.
When the course of direction of light changes due to reflection or refraction, the light likewise exerts a motive force on each corresponding object. This force is known as “optical radiation pressure.”
Based on the principle of laser trapping, to put it simply, it represents the skillful use of the particle-holding properties of light. For example, when a ball strikes a wall and rebounds, its direction of travel changes, and at the same time it exerts a force on the wall. Light is just the same. For example, if light strikes a transparent glass pane, it will be refracted and enter the glass. In addition, some of the light will be reflected from the surface. It is probably easy to understand that, in the case of reflection, a force is acting on the glass just like a ball. However, since refraction also alters the direction of light, a force is acting in this case too. This force is referred to as “optical radiation pressure.”
If the object is a level surface, it will be pushed down by light from above. However, in these experiments we are using minute particles that are spherical. The path of the light changes twice, with the reflection and refraction that occur at the particle surface, and with reflection and refraction as the light re-emerges from the particle. The vectors of the optical radiation pressure from each change of direction combine to become the force that raises the sphere, and essentially the minute particle moves towards the focus of the laser light. Since gravity is also acting, stability is achieved at the point where the raising force and gravity cancel one another out (slightly below the focal point), and this is the trap point. This position varies according to the power of the laser, the particle's weight, and the numerical aperture (N.A. value) of the lens. However, we are going further and taking into account factors such as the refractive indices of the air and the particle in our calculations, ascertaining the type of conditions that will increase the lifting force (trap force), and verifying them experimentally. The results of these experiments are now also being used in the unit which Nikon has manufactured for us.
- The principle of laser trapping
- When light strikes the sphere, it is mainly refracted, generating optical radiation pressure from the sphere's center to the outside. The compound optical radiation pressure vectors Ft acts to lift the sphere towords the laser's focal point. The sphere stabilizes at the point at which Ft and gravity balance (slightly below the focal point).
What proved troublesome at the basic experimental stage?
Laser trapping in air has rarely been investigated overseas, and there have only been a few cases in Japan. That is to say, it is even more difficult to conduct experiments in air, than in a vacuum or in water. In air, for some reason there is an adsorption force or viscosity acting between the substrate on which the particle is placed and the particle itself. This powerful force strains to tear the particle from the glass substrate. We have been conducting these basic experiments for approximately 10 years now; however, only for the past five or six years have we been able to smoothly achieve levitation in air.
A number of methods exist; initially, however, we tried the technique of striking the adhering surface with a strong pulse laser. With this technique a powerful laser light is first used to alter the properties of the contact area, and the object is then raised. We started using this method in June and were finally successful with it six months later in December. We later realized that the amount of moisture in the air somehow affects the adsorption force. We believe that this is why this method did not work well during the rainy season but was successful in December, when the air was dry.
In order to make the apparatus smaller, we subsequently switched from a method employing a large pulse laser device to a technique for tearing the particle from the glass substrate by using ultrasound to make the stage vibrate. Even with this method, the particle still moved with the glass substrate, so after all we have been going through approximately six months of repetitive trial and error, tinkering with frequency, amplitude, and the like.
At one point we were unsuccessful in placing particles on the stage, in that we could not disperse them evenly and they tended to cluster. Nevertheless, when we went ahead and vibrated the glass substrate, the particles still rose. The particles piled up on the glass substrate, and as the vibration spread belatedly from one individual particle to the next, the uppermost particles became more easily detachable. This method for trapping the particles with a laser exhibited a high level of repeatability and now enjoys a success rate of almost 100 percent. But for this chance failure with the clustering, we would probably have had to expend even more effort.