Multiple optical traps using a mesh diffraction element
Tue Jul 18 08:51:15 BST 2006
physics/0607149
- Title: Multiple optical traps from a single laser beam using a mechanical element
- Authors: J.A. Dharmadhikari, A.K. Dharmadhikari, D. Mathur
- Abstract: The use of a wire mesh facilitates creation of multiple optical traps for manipulation of small micron or sub-micron particles. Such an array of optical traps can be easily controlled. The trap that is formed in this manner is a continuous trap; it obviates the need to time share a laser beam among a set of positions, as is presently done in conventional multiple traps.
- Comments: The aim of the paper is to construct an optical tweezer arrangement having multiple trapping regions. The standard optical tweezer setup produces a single trapping region by focussing a laser through a microscope objective. Multiple trapping regions are created by (a) using multiple beams, (b) time sharing a single beam between multiple locations using an AOM, (c) making use of a diffractive element such as a hologram or spatial light modulator.
In this paper the authors show that a wire mesh acts as an efficient diffractive element to produce multiple traps, with the advantage that the trap spacing can be varied by translating the mesh with respect to the first telescope lens that images the laser beam onto the microscope objective. This is a simple setup than using an SLM, and can be used with higher power lasers. Diffraction efficiency can be increased by using a mesh with thinner wires. The traps are continuous rather than the AOM case where they are pulsed, although the loss of power due to reflection from the mesh offsets this.
- Ideas:
- improve efficiency further by using subwavelength holes in thin metal on insulator materials (ie make use of surface plasmon resonances)?
- variation of diffraction efficiency as a function of radial distance -> different trap strengths. Combine with optical pumping and it may be possible to do some form of spatial Sisyphus cooling. Lower intensity further from centre of beam implies reduced light shift, perhaps better to use annular beam with hole on axis. This would give maximum light shift off-axis, better Doppler cooling capture range. Further out the beam is radially, further the transition is red-shifted => captures hotter atoms.
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