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The Optical Radiation Pressure Microprobe Unit—Interview with Yasuhiro Takaya, Dr. Eng., Professor, Department of Mechanical Engineering, Graduate School of Engineering, Osaka University

The reality of laser trapping

How exactly is measurement performed?

An 8 µm diameter particle (a glass sphere) is captured by laser trapping for use as a probe and brought up close to the object to be measured. The object's surface position is ascertained by reading the changes in the reflection of light from the particle as the probe makes contact with the object. This also entails various problems, however. Since the probe is extremely small, it is affected by the thermal motion (or Brownian motion) of air molecules. Merely bringing it up close to the measurement object causes significant movement of the probe, such that the position at which contact with the object is made cannot be ascertained. In addition, it has been known for the probe to adsorb to the measurement object. Initially, we did not know what to do about these problems; however, we solved them by vibrating the probe and reading the resulting changes in vibration frequency and amplitude.

It is probably easy to understand this by looking at the actual form of the probe. Shot using a microscope, this movie shows how the probe is actually floating. The object being measured is a glass sphere 160 µm in diameter.


Image 1

This image was taken in air during our basic experiments. It would be impossible to shoot such a clear image in water, due to the relative refractive indices. Taken several years ago, this photograph in fact required a considerable amount of time to shoot. Here, the distance between the objective lens and the particle was approximately 2 mm. On the current optical radiation pressure microprobe unit, this distance is now approximately 0.3 mm. The greater this distance, the better; however, this entails numerous as yet unresolved problems.

What does vibrating the particle (or probe) entail?


Image 2

The image illustrates the particle's lateral movement. For demonstration purposes, the particle is shown as slowly making large movements. In reality, however, it would probably be impossible to tell that it was moving, given the low amplitude of no more than 100 nanometers and the high vibration frequency of 1 kHz.

It was initially thought by some that if the particle (or probe sphere) were made to make such pronounced movements, it would fly off. However, the viscous resistance of the air prevents this from happening. It was only later that we realized how important a role the air's viscous resistance plays. Vibrating the particle and bringing it close to the object creates a thin layer of air between the particle and the object, and increases the resistance between them. It is thus now possible to measure an object without making any physical contact with it.


Image 3

The pyramid-shaped object shows the difficulty of measuring a three dimensional ridge line using conventional image measurement. This is a picture used in demonstrations of the optical radiation pressure microprobe unit. Since the image was taken with a digital camera, it is close to what the naked eye would see. This is how it appears. Although the 8 µm particle (or probe) cannot be seen, bright points seem to be visible and the floating structure is readily apparent. Only the scattered light rays of the normal illumination from the microscope can be seen.

The particle is brought closer to the object in this way and the changing position of the reflected light is read with a resolution of 1 nm, giving the X, Y, and Z coordinates. Although control would be extremely difficult, it is probably possible to move the particle so as to trace over the object to be measured. Implementing this would likely enable high-speed measurement.

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