Laser Guided Direct Writing
- PDF / 2,177,280 Bytes
- 8 Pages / 396 x 630 pts Page_size
- 33 Downloads / 229 Views
INTRODUCTION Laser-induced optical forces, arising from the scattering of light by microscopic particles, are widely used for the non-contact manipulation of biological particles. Arthur Ashkin, a pioneer in optical force-based manipulation, first applied optical forces to levitate aerosol droplets and dielectric spheres [1], and later demonstrated the optical manipulation of a variety of biological materials in aqueous suspension [2-5]. Optical forces are now commonly used for noncontact manipulation of cells, subcellular components, and biomolecule-coated particles in a configuration known as optical tweezers" [6-
10]. Despite the ability to control particle positioning to submicron accuracy, optical tweezers have not been applied extensively to microfabrication. The main drawback stems from the fact that optical tweezer-based surface pattering is tedious, requiring repeated cycles of particle capture in the fluid phase, transport through the fluid, deposition on a solid surface, and release. Furthermore, the small trapping volume of conventional optical traps severely limits the number of particles that can be manipulated at one time. Given these limitations it would appear that optical forces are not well-suited to provide both the micrometer-scale positioning accuracy and the high throughput deposition rates that direct-write microfabrication demands. However, by simply changing the laser beam focus, we have found that optical forces can be used to manipulate thousands of particles simultaneously and deposit them in a continuous stream onto surfaces with micrometer accuracy [11-13]. In addition, hollow-core optical fibers can assist laser guidance [11,12,14,15], and allow particles to be guided more accurately and over much longer distances than with free space beams.
107
Mat. Res. Soc. Symp. Proc. Vol. 624 © 2000 Materials Research Society
Physical Basis of Laser Guidance The model for particle-light interaction depicted in Fig. 1 was first proposed by Ashkin and is the working model for the ray optics regime where the particle is larger than the wavelength of light [ 1,16]. The key physical property defining the interaction between the light and the particle is the refractive index of the particle relative to that of the surrounding fluid. Larger refractive indices lead to stronger interactions.
/
Light
Z
a
-~
Light path
Force due to reflectio •.. •
b
-•
Force due to refractio Sum of forces
r Figure 1. Optical forces on a dielectric sphere. Laser light is reflected and refracted at each interface, resulting in a redirection of the light. Since photons have momentum, their redirection by interaction with the particle results in a corresponding momentum transfer to the particle as indicated by the dashed arrows. The net result of the interactions from ray A is to push the particle along the beam axis and pull the particle radially inward. By symmetry, ray B pushes the particle axially and pushes the particle radially outward. However, ray A is stronger than ray B so it overcomes the radial forc
Data Loading...
