Waveguide trapping of hollow glass spheres
Spotlight summary: Hollow microspheres are interesting objects to work with, as they are essentially bubbles of air that are encased in a thin glass shell. The possibility of using them as microcontainers for minute quantities of reagents—and the simple fact that they often float rather than sink—makes them interesting objects to work with and manipulate optically. However, these glass-coated bubbles are trickier to trap than the solid spheres usually used in optical trapping experiments. Their behavior in a laser beam is closer to that of an air bubble than a glass ball, and so they are repelled from a laser focus, making them very hard to manipulate optically. A number of inventive methods, usually based around beams containing some sort of dark spot in which the particle is trapped, have been tried to trap them.
Ahluwalia et al. present a new approach to trap hollow spheres, which is to hold them using their surface. Using the evanescent field leaking out of a waveguide, they show that the light can be allowed to interact only with the glass shell rather than with the air space inside the particle. This means that the optical field encounters a medium with higher refractive index than the water in which it is immersed, and thus the particles trap stably on the surface of the waveguide. As well as being pulled toward the waveguide, the particles are also pushed along its surface in the direction of propagation of the light. These actions create a sort of optically driven conveyor belt, which could be used to construct an optically controlled microreactor on a chip.
The authors explain the criteria for stable trapping very simply: the average refractive index of the particle must be higher than that of the surrounding fluid (in this case, water). For a focused Gaussian beam, this average must be taken over the whole volume of the particle. However, the evanescent field above a waveguide decays rapidly; thus the average should include only the area accessed by the light. As such the latter value is biased toward the refractive index of the shell rather than the air and means that a much larger range of shell particles will trap stably on the waveguide.
Particles trapped by the waveguide sit on its surface, so the technique is necessarily two-dimensional. one. However, by changing the shape of the waveguide or the particle, the team hopes to balance attractive and repulsive forces, perhaps allowing them to levitate particles in the evanescent field. Levitation would liberate the particles from the surface of the waveguide and make the technique much more flexible. The possibility of integrating waveguides into lab-on-a-chip devices makes this work particularly interesting as does its elegantly simple premise: if you can't grab the middle, grab it by the edges!
Technical Division: Optics in Biology and Medicine
ToC Category: Medical Optics and Biotechnology
|OCIS Codes:||(130.0130) Integrated optics : Integrated optics|
|(170.4520) Medical optics and biotechnology : Optical confinement and manipulation|
|(240.6690) Optics at surfaces : Surface waves|
|(350.4855) Other areas of optics : Optical tweezers or optical manipulation|
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