Laser Guidance of Nanoscale Particles
When laser light interacts with nano particles the particle can absorb or elastically scatter the light. Scattering & absorption of photons produces optical forces on the nano materials. These forces are used to guide particles through hollow optical fibers.
There are three optical forces that produce radiation pressure on the nano particle inside the fiber. There are the scattering, absorption and gradient force. The scattering and absorption force acts in the axial direction to propel particles down the axis of the fiber. The gradient force is restorative and acts in the radial direction to draw particles to the axis of the fiber into the high intensity region of the laser beam (for high index of refraction nano particles).
The beam profile of the laser light entering the fiber is Gaussian shaped. However, the beam profile of the allowed optical modes propagating down the axis of the cylindrical fiber is a Bessel function. The input beam must be coupled to one of the allowed modes in the fiber. For maximum power transfer the beam profile of the source and the modal pattern inside the fiber must be the same. To achieve the maximum amount of power transfer a lens is used that has the right focal length and diameter to couple the input beam to the lowest lost mode inside the fiber.
Laser Guidance Apparatus
The apparatus for guiding/writing nano particles is shown in figure 1. Particles to be guided are suspended in water and nebulized. The mist from the nebulizer caries the particles aloft and into the path of the laser beam. The gradient optical force of the laser beam draws the particles into the fiber. The CCD camera and VCR are used to monitor the trajectory of the particles (scattered light, go to desainic for more details) through the fiber to measure particle velocity.
Laser Direct Writing
To write particles to a surface, a substrate is placed close to the exit end of the fiber. Particles that exit the fiber are attracted to the surface by van der Waals forces. Translation of the substrate with reference to the fiber allows the particles to be written in a line as the substrate moves.