Robin Seibel

University of Akron, Akron, Ohio

Manipulation of Micro Scale Particles in an

Optical Trap Using Interferometry

NASA/CR—2002-211975

October 2002
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Robin Seibel

University of Akron, Akron, Ohio

Manipulation of Micro Scale Particles in an

Optical Trap Using Interferometry

NASA/CR—2002-211975

October 2002

National Aeronautics and

Space Administration

Glenn Research Center

Prepared under Cooperative Agreement NCC3–904
Acknowledgments

I would like to thank Rex Ramsier for alerting me to this opportunity at NASA Glenn and supporting me

throughout my physics career to date; Art Decker, NASA GRC, for bringing me to NASA and giving me free reign;

Ken Weiland who always knew where everything was or how to make what I needed; and Carolyn Mercer, Optical

Instrumentation Technology Branch Chief, for giving me the opportunity to continue this project into the future.

Available from

NASA Center for Aerospace Information

7121 Standard Drive

Hanover, MD 21076

National Technical Information Service

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Springfield, VA 22100

Trade names or manufacturers’ names are used in this report for

identification only. This usage does not constitute an official

endorsement, either expressed or implied, by the National

Aeronautics and Space Administration.

This report contains preliminary

findings, subject to revision as

analysis proceeds.

The Aerospace Propulsion and Power Program at

NASA Glenn Research Center sponsored this work.

Available electronically at http://gltrs.grc.nasa.gov
NASA/CR—2002-211975 1

MANIPULATION OF MICRO SCALE PARTICLES IN AN OPTICAL TRAP

USING INTERFEROMETRY

Robin Seibel

University of Akron

Akron, Ohio 44325

Abstract

This research shows that micro particles can be manipulated, via interferometric

patterns superimposed on an optical tweezers beam. Interferometry allows the

manipulation of intensity distributions, and thus, force distributions on a trapped particle.

To demonstrate the feasibility of such manipulation, 458 nm light, from an argon-ion

laser, was injected into a Mach Zender interferometer. One mirror in the interferometer

was oscillated with a piezoelectric phase modulator. The light from the interferometer

was then injected into a microscope to trap a 9.75 μm polystyrene sphere. By varying the

phase modulation, the sphere was made to oscillate in a controlled fashion.

Introduction

With the rapid growth of interest in nano technology has come the possibility of

manually constructing or interrogating nano structures held in optical tweezers. However,

using optical tweezers for direct, high resolution control of nano scale particles is

untenable given the diffraction limits of the system. The minimum diameter of a focused

spot in such a system is given by the following equation:

. . 1.22

min N A d = λ

where dmin is the minimum diameter of the focused spot, and N.A. is the numerical

aperture of the objective. For N.A. = 1.25, λ = 458 nm, dmin ≈ λ . Such a relatively

large focus precludes the high-resolution manipulation and measurement of anything with

size on the order of the wavelength of light used or smaller (1).

Complicating direct manipulation of nano particles is the relative complexity of

current optical tweezers configurations. Manipulation of particles in one plane currently

requires either coupling microscope stage movement with vertical translation of the trap

or the use of at least two trapping beams: one to hold the particle, the other to manipulate

it. Multiple beam trapping requires coordinating active beam steering modalities—piezo

driven mirrors, acoustic beam steerers, et al.—for each beam. The size and complexity of

both methods will limit their ability to manipulate nano particles.

The objective of this research, performed at NASA Glenn Research Center, was

to qualitatively prove viable a technique for manipulating a micron scale tool, via

interferometry. Such a technique would simplify an apparatus by requiring only a single
NASA/CR—2002-211975 2

beam to manipulate a tool. An appropriate diffraction solution would allow the

translation of a tool across the objective’s field of view, without having to move the

microscope stage. Moreover, such a solution would reduce the complexity of the optical

train, thus reducing apparatus size and complexity.

Experimental Method

The first step in this project was to assemble and optimize an optical tweezers

apparatus (Figure 1), and the second step was to perfect the trapping technique.

Figure 1. 1. Laser; 2. Beam Expander; 3. Microscope; 4. Dichroic Filter;

5. CCD Camera

To this end, 190 mW of 458 nm light from a 5 W Argon ion laser (2) was collimated and

injected into a microscope (3), via a yellow, subtractive dichroic filter (4). The dichroic

filter was inserted into the light path such that it reflected the laser beam into the

microscope but prevented laser light from returning to the digital camera. A 100X, oilimmersion objective, N.A. 1.25 (5) was used to focus the laser on the 9.75 μm

polystyrene spheres in suspension (6). The spheres were contained in a well, constructed

by sandwiching two, 100 μm thick, parafilm strips between a microscope slide and a

0.17 mm coverslip (Figure 2).

Figure 2. Trapping Well
NASA/CR—2002-211975 3

The sphere suspension was entrained into the well via capillary action, and the well was

sealed with grease. A CCD video camera (7) was used in place of the microscope head to

image the trapped particles.

In the third step of this project, a Mach Zender interferometer was inserted into

the optical path, between the collimator and the microscope (Figures 3, 4).

Figure 3. 1. Collimator; 2.CCD Video; 3. Ramp Generator; 4. Interferometer;

5. Piezo Mirror; 6. Particle Well, Microscope Stage, Objective

The interferometer was adjusted to provide a finite fringe pattern. One mirror in the

interferometer was replaced with a mirror driven on three axes by piezo electric actuators.

Figure 4. 1. Laser; 2.Beam Expander; 3. Mach Zender Interferometer; 4. Piezo Mirror;

5. Ramp Generator; 6. Microscope and CCD Camera
NASA/CR—2002-211975 4

This driven mirror provided phase modulation of the interference pattern and was

controlled by a ramp generator (8), along only one axis. The behavior of trapped

particles was observed as a function of ramp duration, where the duration was varied

from 100 ms to 1000 ms on the ramp generator. The objective was to find a ramp

duration that allowed the easiest viewing of particle movement.

Results and Discussion

The first successful trap was established at a laser power of 190 mW (60 mW at

the particle well coverslip). The trap plane differed significantly from the focal plane of

the

Figure 5. Untrapped Sphere

camera, being 10 μm – 15 μm above the camera focal plane, as measured by the scale on

the microscope’s vertical translation knob. It was felt that this was a result of significant

spherical aberration in the microscope objective. The microscope objective used was not

apochromatic and was only a semi-plan objective. Further, it is known that spherical

aberration can be exacerbated as an image is projected back through a non-infinity

corrected objective to the back aperture image plane. This effect is worsened if

components are introduced into the path between the objective and the back aperture

image plane. Due to the distance between the viewing plane and the trapping plane,

trapped particles were not directly visualized, but instead were seen as an Airy pattern

projected onto the bottom of the well (Figure 6).

Figure 6. Trapped Sphere

Subsequent traps were achieved at a laser output of 86 mW (approximately 25 mW at the

coverslip.).

With the Mach Zender interferometer in place, it was noted that with a laser

output of 80 mW, the power at the coverslip was 78 percent less than when there was no

interferometry loop. Nevertheless, a trap could still be formed, and it performed nearly

identically to the traps formed without the interferometer. With the ramp generator

driving the mirror, motion could be seen in the Airy pattern created by the sphere. The

movement was best visualized at ramp durations between 200 ms and 500 ms. At these

durations, the sphere’s movement did not coincide with the interference pattern

movement. It appeared that the sphere’s motion had a sharp rise, matching the ramp
NASA/CR—2002-211975 5

generator input, which was followed by a slow relaxation period. This seemed to indicate

that actual induced movement was being witnessed since the ramp generator did not

exhibit a similar “relaxation” period.

Concluding Remarks

This project has demonstrated the viability of using interferometry to manipulate

a micro scale particle in an optical trap. The results further indicate that trap strength is

sensitive to spherical and chromatic aberrations in the optical train. Finally, it is apparent

that before progressing to the next phase, coated optics, tuned to the specified wavelength

need to be used to improve light transmission throughout the optical train, and an infinity

corrected microscope objective is necessary to bring the viewing and trapping planes into

phase.

References

1. M. Born and E. Wolf, Principles of Optics: Electromagnetic Theory of

Propagation, Interference and Diffraction of Light, 7th ed. (Cambridge University

Press, New York, 1999).

2. Coherent, Inc., Innova 90–5

3. Accuscope, 3001 Plan Optic Trinocular

4. Edmund Optics, model #NT52–543

5. Edmund Optics, Semi-plan, model #38–344

6. Duke Scientific, 7000 series

7. Sony, ExWave HAD

8. Burleigh Instruments, PZ–91 Series Aligner/Translator
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Manipulation of Micro Scale Particles in an Optical Trap Using Interferometry

Robin Seibel

Optical tweezers; Interferometry; Nanotechnology

Unclassified -Unlimited

Subject Category: 35 Distribution: Nonstandard

University of Akron

302 Buchtel Mall

Akron, Ohio 44325–0002

Project Manager, Arthur J. Decker, Instrumentation and Controls Division, NASA Glenn Research Center, organization

code 5520, 216–433–3639.

This research shows that micro particles can be manipulated via interferometric patterns superimposed on an optical

tweezers beam. Interferometry allows the manipulation of intensity distributions, and thus, force distributions on a trapped

particle. To demonstrate the feasibility of such manipulation, 458 nm light, from an argon-ion laser, was injected into a

Mach Zender interferometer. One mirror in the interferometer was oscillated with a piezoelectric phase modulator. The

light from the interferometer was then injected into a microscope to trap a 9.75 mm polystyrene sphere. By varying the

phase modulation, the sphere was made to oscillate in a controlled fashion.