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Measurement of the Length of an Optical Trap

This technique aids in the assembly of MEMS devices.

NASA Glenn has been involved in developing optical trapping and optical micromanipulation techniques in order to develop a tool that can be used to probe, characterize, and assemble nano and microscale materials to create microscale sensors for harsh flight environments. In order to be able to assemble a sensor or probe candidate sensor material, it is useful to know how far an optical trap can “reach”; that is, the distance beyond/below the stable trapping point through which an object will be drawn into the optical trap. Typically, to measure the distance over which an optical trap would influence matter in a horizontal (perpendicular to beam propagation) direction, it was common to hold an object in one optical trap, place a second optical trap a known distance away, turn off the first optical trap, and note if the object was moved into the second trap when it was turned on. The disadvantage of this technique is that it only gives information of trap influence distance in horizontal (xy) directions. No information about the distance of the influence of the trap is gained in the direction of propagation of the beam (the z direction).

A method was developed to use a time-of-flight technique to determine the length along the propagation direction of an optical trap beam over which an object may be drawn into the optical trap. Test objects (polystyrene microspheres) were held in an optical trap in a water-filled sample chamber and raised to a pre-determined position near the top of the sample chamber. Next, the test objects were released by blocking the optical trap beam. The test objects were allowed to fall through the water for predetermined periods of time, at the end of which the trapping beam was unblocked. It was noted whether or not the test object returned to the optical trap or continued to fall.

This determination of the length of an optical trap’s influence by this manner assumes that the test object falls through the water in the sample chamber at terminal velocity for the duration of its fall, so that the distance of trap influence can be computed simply by: d = VTt, where d is the trap length (or distance of trap reach), VT is the terminal velocity of the test object, and t is the time interval over which the object is allowed to fall. In order for this methodology to work, it must be established that the test object indeed falls through the water in the sample chamber at terminal velocity. This answers the question of how far below the trap point an object must be to be drawn into an optical trap in order to select and manipulate material for microscale assembly and characterization.

This methodology would make it possible for optical trapping to be incorporated into the assembly of MEMS (micro-electromechanical systems) devices. In particular, adding pieces or connectors to MEMS devices that cannot be positioned via photolithography and vapor or film deposition techniques may be added to a MEMS device via placement by optical traps. In this case, it is imperative to know how far beyond the stable trapping point in the direction of propagation of the beam an object should or must be to be trapped, and also the distance beyond the stable optical trapping point over which the propagating laser beam has no effect.

This work was done by Susan Y. Wrbanek of Glenn Research Center. For more information, download the Technical Support Package (free white paper) at www.techbriefs.com/tsp under the Physical Sciences category.

Inquiries concerning rights for the commercial use of this invention should be addressed to NASA Glenn Research Center, Innovative Partnerships Office, Attn: Steve Fedor, Mail Stop 4–8, 21000 Brookpark Road, Cleveland, Ohio 44135. Refer to LEW-18539-1.

This Brief includes a Technical Support Package (TSP).

Measurement of the Length of an Optical Trap (reference LEW-18539-1) is currently available for download from the TSP library.

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