Scientists in Osaka, Japan, have developed a method of measuring light pressure and have used it to show the forces exerted by an evanescent wave. The system works by measuring the 3D displacement of a particle to within a few nanometers and then factoring out other known forces to determine how much of the position change was due to radiation pressure. The technique experimentally confirms theoretical expectations, and may help in the engineering of optical trapping technologies (such as optical tweezers).
Figure 1. In the basic 3D position sensor, focused light from above is used to trap a microparticle, and that scattered from the evanescent wave below is detected. To calculate the result, the four detector outputs are processed by an operation circuit.
The basic apparatus used to measure position (Figure 1) was developed by researchers at PRESTO, the Japan Science and Technology Corp., and the Dept. of Applied Physics at Osaka Univ.1 A beam from an Nd:YAG laser illuminates a microparticle through a microscope, with the resulting 1-µm focus acting as an optical trap. This creates the potential field to be measured.
A second He-Ne laser beam is used for the 3D measurement. The light passes through a glass prism that is optically coupled to a glass plate (with oil) to produce total internal reflection at the top substrate. Through contact with the microparticle, some of the light from the evanescent wave is coupled out and scattered, where it can be imaged by the microscope and detected by the quadrant photodiode. The x- and y- positions of the particle can be determined by looking at the differential outputs of the four detectors, and the intensity detected by the four together show the z-position.
Figure 2. In the new system, the trapping light comes from both above and a second evanescent wave below.
Though this solves the problem of the basic measurement, it does not allow radiation pressure to be detected directly -- primarily because of Brownian motion. Thermal energy causes the particle to be constantly moving. To compensate for this, many measurements are taken over a period of time, and the resulting data is used to calculate a 3D probability function. Other contributions to the particle's position, such as temperature and gravity, can also be pulled out in order to produce curves that show the effect of radiation pressure alone.
To extend this system to measuring the radiation pressure caused by an evanescent wave,2 the Osaka team split the trapping light from the Nd:YAG laser into two parts (Figure 2). The first beam, as before, traps the particle -- which is immersed in water -- from above via the microscope. The second, traveling in the opposite direction to the He-Ne beam used for measurement, is used to create a standing wave at the water/glass interface. To demonstrate that the evanescent wave does indeed make a difference to the particle position, researchers also included a shutter so that it could be selectively turned on and off.
Figure 3. A shutter in the system allows the potential experienced by the particle to be measured with the new evanescent wave both off and on. Shown in (a) and (b) are the fields in both cases in the x- and z-directions, respectively. (The y-direction doesn't change because the evanescent wave does not run in that direction.)
Figure 3 shows the potential well in which the 4.5-µm latex particle was held, both with and without the presence of the evanescent wave. As expected, the x- and z-directions show significant change in the field, whereas the y- direction (not shown) stays the same. As expected, with change in the x-displacement of the trapped particle, the potential due to radiation pressure -- once extracted -- proves to be linear, whereas it is exponential for the z-direction. The team was also able to measure the scattering force caused by the light, demonstrating how it decreased with light intensity.
Researchers hope to use these techniques to further develop micromanipulation systems and to study near-field microscopy.
1. Keiji Sasaki, Mitsuru Tsukima, and Hiroshi Masuhara, Three-dimensional potential analysis of radiation pressure exerted on a single microparticle, App. Phys. Lett. 71 (1), pp. 37-39, 7 July 1997.
2. Ken-ichiro Wada, Keiji Sasaki, and Hiroshi Masuhara, Optical measurement of interaction potentials between a single microparticle and an evanescent field, App. Phys. Lett. 76 (20), pp. 2815-2817, 15 May 2000.