OSA's Digital Library

Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 16, Iss. 19 — Sep. 15, 2008
  • pp: 14550–14560
« Show journal navigation

Fiber based optical trapping of aerosols

D. Rudd, C. López-Mariscal, M. Summers, A. Shahvisi, J. C. Gutiérrez-Vega, and D. McGloin  »View Author Affiliations


Optics Express, Vol. 16, Issue 19, pp. 14550-14560 (2008)
http://dx.doi.org/10.1364/OE.16.014550


View Full Text Article

Acrobat PDF (857 KB)





Browse Journals / Lookup Meetings

Browse by Journal and Year


   


Lookup Conference Papers

Close Browse Journals / Lookup Meetings

Article Tools

Share
Citations

Abstract

We present the use of optical fibers to form a counter-propagating optical trap as a means of manipulating both solid and liquid aerosols. We explore the use of single and multimode fibers to achieve trapping of various particles in air, present the trapping properties of the different fiber types and compare the observed trends to those predicted by theory. Using fibers, we are able to hold suspended particles for extended periods of time and to precisely manipulate them over distances of several hundred microns. We discuss the difficulties and advantages of each fiber configuration and conclude with a demonstration that fiber based trapping offers a good candidate for studying optical binding in air.

© 2008 Optical Society of America

1. Introduction

Since their demonstration by Arthur Ashkin [1

1. A. Ashkin, “Acceleration and trapping of particles by radiation pressure,” Phys. Rev. Lett. 24, 156–159 (1970). [CrossRef]

], the use of optical manipulation techniques has spread into many areas of research [2

2. K. Dholakia, P Reece, and M Gu, “Optical Micromanipulation” Chem. Soc. Rev. 37, 42–55 (2008). [CrossRef] [PubMed]

] as a non invasive tool for the manipulation of microscopic objects. Over the last 35 years, these techniques have been refined and developed [3

3. M. J. Lang and S. M Block, “Resource Letter: LBOT-1: Laser-based optical tweezers,” Am. J. Phys. 71, 201–215 (2003). [CrossRef]

5

5. D. McGloin, “Optical tweezers: 20 years on,” Phil. Trans. R. Soc. 364, 3521–3537 (2006). [CrossRef]

], mainly using particles suspended in a liquid medium. The ability to trap particles in air, however, also offers an additional number of interesting avenues of study. These include examining the Brownian motion of airborne particles in optical traps [6

6. R. D. Leonardo, G. Ruocco, L. Leach, M. J. Padgett, A. J. Wright, J. M. Girkin, D. R. Burnham, and D. McGloin, “Parametric Resonance of Optically Trapped Aerosols,” Phys. Rev. Lett. 99, 010601 (2007). [CrossRef] [PubMed]

] as well a range of applications in atmospheric chemistry [7

7. M. D. King, K. C. Thompson, and A. D. Ward, “Laser Tweezers Raman Study of Optically Trapped Aerosol Droplets of Seawater and Oleic Acid Reacting with Ozone: Implications for Cloud-Droplet Properties,” J. Am. Chem. Soc. 126, 16710–16711 (2004). [CrossRef] [PubMed]

9

9. L. Mitchem and J. P. Reid, “Optical Manipulation and Characterisation of Aerosol Particles,” Chem. Soc. Rev. 37, 756–769 (2008). [CrossRef] [PubMed]

]. Creating robust traps where the particles of interest reside in the air can be difficult, mainly due to the significantly reduced buoyancy and viscosity compared to liquid based traps along with the added difficulty of populating the trapping sites in a controlled fashion.

When wishing to study the properties or dynamics of a single or small number of airborne particles, it is often essential to know their positions. By optically trapping a particle its location is fixed and its properties can be easily investigated. A stable three dimensional optical trap can only be achieved by balancing the scattering and gradient components of the light-matter interaction. This is generally achieved in one of two ways: either a high numerical aperture (NA) objective is used to tightly focus the incident beam [10

10. A. Ashkin, J. M. Dziedzic, J. E. Bjorkhom, and S. Chu, “Observation of a single beam gradient force trap for dielectric particles,” Opt. Lett. 11, 288–290 (1986). [CrossRef] [PubMed]

], in which case, it is referred to as a single beam gradient trap or optical tweezers; alternatively a second equal and opposite beam is added to balance the force of the first, forming a counter-propagating trap [1

1. A. Ashkin, “Acceleration and trapping of particles by radiation pressure,” Phys. Rev. Lett. 24, 156–159 (1970). [CrossRef]

,11

11. G. Roosen and C. Imbert, “Optical levitation by means of 2 horizontal laser beams—Theoretical and experimental study,” Phys. Lett. A. 59, 6–8 (1976). [CrossRef]

]. Both methods have been successfully employed to optically confine aerosol particles [12

12. M. D. Summers, D. R. Burnham, and D. McGloin, “Trapping solid aerosols with optical tweezers: A comparison between gas and liquid phase optical traps,” Opt. Express 16, 7739–7747 (2008). [CrossRef] [PubMed]

,13

13. N. Magome, M. I. Kohira, E. Hayata, S. Mukai, and K. Yoshikawa, “Optical Trapping of a Growing Water Droplet in Air,” J. Phys. Chem. B. 107, 3988–3990 (2003). [CrossRef]

]. Multiple single beam traps have been employed to allow several particles to be simultaneously trapped and independently manipulated using dual beam tweezers [14

14. R. J. Hopkins, L. Mitchem, A. D. Ward, and J. P. Reid, “Control and characterisation of a single aerosol droplet in a single-beam gradient-force optical trap,” Phys. Chem. Chem. Phys. 6, 4924–4927 (2004). [CrossRef]

], a spatial light modulator [15

15. D. R. Burnham and D. McGloin, “Holographic optical trapping of aerosol droplets,” Opt. Express. 14, 4175–4181 (2006). [CrossRef] [PubMed]

] or an acousto-optical deflector [16

16. J. Buajarern, L. Mitchem, A. D. Ward, N. H. Nahler, D. McGloin, and J. P. Reid, “Controlling and Characterising the Coagulation of Liquid Aerosol Droplets,” J. Chem. Phys. 125, 114506 (2006). [CrossRef] [PubMed]

].

Counter propagating configurations have previously been used to trap both liquid and solid particles [17

17. R. Pastel, A. Struthers, R. Ringle, J. Rodgers, C. Rohde, and P. Geiser, “Laser trapping of microscopic particles for undergraduate experiments,” Am. J. Phys. 68, 993–1001 (2000). [CrossRef]

19

19. M. Guillon, O. Moine, and B. Stout, “Longitudinal Optical Binding of High Optical Contrast Microdroplets in Air,” Phys. Rev. Lett. 96, 143902 (2006). Erratum, Phys. Rev. Lett. 99, 079901 (2007). [CrossRef] [PubMed]

]. Salt and sugar crystals have been trapped from solution using two counter propagating beams in a hollow core fiber using laser powers between 20mW and 1W [17

17. R. Pastel, A. Struthers, R. Ringle, J. Rodgers, C. Rohde, and P. Geiser, “Laser trapping of microscopic particles for undergraduate experiments,” Am. J. Phys. 68, 993–1001 (2000). [CrossRef]

]. Much higher powers of 2.2 W per arm were used when trapping droplets between two free space beams in experiments to study ice formation [18

18. K. Taji, M. Tachikawa, and K. Nagashima, “Laser trapping of ice crystals,” Appl. Phys. Lett. 88, 141111 (2006). [CrossRef]

]. Recently, a trap formed using a dual beam trap making use of an incoming beam and retroreflected beam of 30mW has been used to study optical binding in air [19

19. M. Guillon, O. Moine, and B. Stout, “Longitudinal Optical Binding of High Optical Contrast Microdroplets in Air,” Phys. Rev. Lett. 96, 143902 (2006). Erratum, Phys. Rev. Lett. 99, 079901 (2007). [CrossRef] [PubMed]

]. All these experiments make use of free-space light beams focused through lenses. These lens-based systems are very similar to those used by Ashkin and co-workers in their early work based on radiation pressure traps. However, more recent work using systems of counter propagating beams in liquid media has seen these free beams replaced with light transmitted through optical fibers [20

20. A. Constable, J. Kim, J. Mervis, F. Zarinetchi, and M. Prentiss, “Demonstration of a-fiber-optical light-force trap,” Opt. Lett. 18, 1867–1869 (1993). [CrossRef] [PubMed]

]. The beam properties can then be dictated by the characteristics of the fibers chosen; potentially removing the need for any other optical components. These “fiber traps” [21

21. J. Guck, R. Ananthakrishnan, H. Mahmood, T. J. Moon, C. C. Cunningham, and J. Käs, “The Optical Stretcher: A Novel Laser Tool to Micromanipulate Cells,” Biophys. J. 81, 767–784 (2001). [CrossRef] [PubMed]

24

24. N. K. Metzger, K. Dholakia, and E. M. Wright, “Observation of bistability and hysteresis in optical binding of two dielectric spheres,” Phys. Rev. Lett. 96, 068102 (2006). [CrossRef] [PubMed]

] have several advantages over the single beam trap. They can, by providing large forces, trap a wider range of particle sizes while allowing increased optical access to the trapping area when compared to optical tweezers [25

25. E. Sidick, S. D. Collins, and A. Knoesen, “Trapping forces in a multiple-beam fiber-optic trap,” Appl. Opt. 36, 6423–6433 (1997). [CrossRef]

], making them easy to integrate with simultaneous spectroscopic analysis [22

22. P. R. T. Jess, V. Garcés-Chávez, D. Smith, M. Mazilu, L. Paterson, A. Riches, C. S. Herrington, W. Sibbett, and K. Dholakia, “Dual beam fiber trap for Raman micro-spectroscopy of single cells,” Opt. Express. 14, 5779–5791 (2006). [CrossRef] [PubMed]

] and microfluidic systems [26

26. S. Ebert, K. Travis, B. Lincoln, and J. Guck “Fluorescence ratio thermometry in a microfluidic dual-beam laser trap,” Opt. Express 15, 15493–15499 (2007). [CrossRef] [PubMed]

]. For the purpose of trapping aerosols fiber based traps also have the potential advantage of being more suitable for producing small, integrated devices (analogous to microfluidic systems), suitable for field studies.

In this paper, we demonstrate a dual beam fiber trap capable of trapping airborne particles. We compare the performance of traps using both multimode and single mode fiber and examine the possibility of studying optical binding using such configurations.

2. Theoretical model

To allow us to predict the performance of our dual beam traps, we model the optical forces present by building on existing theoretical work for liquid media [11

11. G. Roosen and C. Imbert, “Optical levitation by means of 2 horizontal laser beams—Theoretical and experimental study,” Phys. Lett. A. 59, 6–8 (1976). [CrossRef]

,25

25. E. Sidick, S. D. Collins, and A. Knoesen, “Trapping forces in a multiple-beam fiber-optic trap,” Appl. Opt. 36, 6423–6433 (1997). [CrossRef]

,27

27. R. Gussgard and T. Lindmo “Calculation of the trapping force in a strongly focused laser beam,” J. Opt. Soc. Am. B. 9, 1922–1930 (1992). [CrossRef]

] and apply it to aerosol trapping.

We consider two horizontally aligned optical fibers centered along the z-axis, separated by a distance s. At the midpoint between these fibers we place a lossless dielectric particle of radius r 0, which is offset from the optical axis by a distance d along the x-axis. This particle is illuminated by a Gaussian beam emerging from each fiber with a minimum beam waist of ω 0. Note that as we are only considering Gaussian beams this model is only valid for single mode fiber and not multimode fiber. By considering a ray incident on the particle at point P, at an angle θ to the optical axis, and following a geometrical force calculation the two main results of Sidick’s work can be obtained [28

28. J. H. Dennis, C. A. Pieron, and K. Asai, “Aerosol Output and Size from Omron NE-U22 nebulizer,” in Proceedings of the 14th International Congress International Society for Aerosols in Medicines, Baltimore June 14–18 2003. Journal of Aerosol Medicine 16:2213, (2003)

, Eqs. 9(a) and 9(b)] which give the trapping efficiencies in the x and z directions due to a single beam as:

Qx=2r02π0πφ0θmaxθsin2θexp(2r2ω2)ω2Rc
×{qs(r0sinθcosφd)+qgtanγ[r0sinθcosφd(1Rcacosγ)]}
(1)
Qz=2r02π0πφ0θmaxθsin2θexp(2r2ω2)ω2Rc
+{qsRz+qgtanγ[RzRc(Rz+r0cosθ)acosγ]}
(2)

where qs, qg, are the fractions of momentum transferred to the parallel and perpendicular directions of the incident ray respectively. Both of these functions are dependent on the transmittance and reflectance of the particle and hence the trapping efficiencies Qx, Qz are dependent on the ratio of the refractive index of the particle and medium, N. Rc is the radius of curvature of the ray, which when projected onto the z-axis gives Rz. r and ω are the beam radius and beam waist at a given position along the z-axis, γ is the half apex angle of the beam and a is the separation between the rays’ source and particle centre. The portion of the particle illuminated by the incident beam is defined by the boundary θmax at which the angle of incidence=π/2. All of these variables are defined in greater detail in [25

25. E. Sidick, S. D. Collins, and A. Knoesen, “Trapping forces in a multiple-beam fiber-optic trap,” Appl. Opt. 36, 6423–6433 (1997). [CrossRef]

]. Note that for the case of an on-axis beam (d=0) the Qz component can be simplified significantly and will become independent of φ. It is also important to note that due to symmetry, an on-axis particle will experience no force in the x-direction. Equally a particle positioned at the centre of the two fibers will not experience a force along z, assuming equal power in each arm. The trapping efficiency relates to the force from a fiber via the constant (n1P)/c where n1 is the refractive index of the medium, P is the laser power coming out of the fiber and c is the speed of light giving:

Fx,z=n1PcQx,z
(3)

Equations 1 and 2 define the trapping efficiencies due to a single beam. To find the total force experienced by a particle trapped using a counter-propagating setup the force from each fiber, given by equation 3, must be summed. Along the x-axis, the forces act to reinforce one another, making the restoring force across the x-axis greater than that along the z. We concentrate on forces due to displacements along the x-axis for the remainder of this section. In our experimental configuration we are limited in the number of variables we can control: primarily we cannot control the size of the particles falling into the trap (unlike most liquid based traps where the particle size is well known). The size of the particle trapped has a direct effect on the magnitude of the optical force it experiences in the trap as can be seen in Fig. 1.

Fig.1 Theoretical plot of the restoring force perpendicular to the optical axis as a function of the axial offset. The water droplets have a radii 2.5, 5, 7.5 and 10µm and are trapped in air. These forces were calculated using a power of 100mW in each fiber. A fiber separation of 170µm and a refracting index ratio of 1.344. The minimum beam waist is 3.5µm and a wavelength of 532nm. These values remain unchanged unless otherwise stated in the figure. caption.

While we may not be able to accurately control the particle sizes, two things that can be controlled are the type of substance trapped and the fiber separation. In the former case the larger the index ratio between particle and medium the stronger the trapping will be, making trapping in air more effective than trapping in water. This can be seen in Fig. 2(a) where the theoretical force and axial offsets are shown for all the materials experimentally trapped in air, and for reference for a silica particle trapped in water (N=1.08).

In our experimental work we looked at trapping achieved with fiber separation of 170 and 240µm. The theoretical force curves for these separations can be seen in Fig. 2(b). Together Figsures 1 and 2 demonstrate the effect that any combination of changes to the fiber system such as a larger, higher refractive index or a particle trapped at a decreased fiber separation would have on the restoring force that balances gravity.

Fig. 2. (a). Theoretical force curves for the different materials used experimentally at a separation of 170µm. N=1.08 corresponds to a silica sphere in water, N=1.344 a 50g/L saltwater droplet in air, N=1.357 a 20% glycerol-water solution trapped in air and N=1.445 a silica particle in air. (b). Theoretical force curves showing the effect of changing fiber separation a 50g/L salt doped water particle trapped in air (N=1.344). In both A and B the power in each fiber was set to 100mw, the particle radius is 10µm and a minimum beam waist of 3.5µm emerging from the fiber was used. In both case the particle is also considered to be at the midpoint of the fibers.

Using the theoretical model it is possible to consider the maximum sized particle that could be stably trapped at a given power. This is done by finding particles whose weight is equal to the maximum restoring optical force. Doing this for even the lowest experimental powers produces values far larger than any trapped droplet observed. While this would seem a contradiction, this process assumes that the particles have no initial kinetic energy and ignores any effects due to air currents. As the particles are actively introduced into the trapping cell during the experiment, the model allows us to safely state that the particles we trap are well within the theoretical limits of the system.

3. Experimental setup

The experimental setup is shown in Fig. 3. We found that using connectorised fibers as opposed to fibers with bare ends within the trapping cell made the system more mechanically robust. Alignment was achieved and maintained by fixing one arm in place and mounting the other on an adjustable linear motion stage. 1.27cm diameter lens tubes were used to hold the fibers in place and to make room for a long working distance objective (Mitutoyo OBJ PLAN NIR 50X) through which the system was observed. A custom piece of glassware was used to create a trapping cell into which the aerosol could be sprayed while minimizing the effect of air currents. The aerosols were produced using an ultrasonic nebuliser (Omron U22(NE-U22-E)), which when used with pure water produces aerosols with a mass median aerodynamic diameter of between 3 and 5 microns [28

28. J. H. Dennis, C. A. Pieron, and K. Asai, “Aerosol Output and Size from Omron NE-U22 nebulizer,” in Proceedings of the 14th International Congress International Society for Aerosols in Medicines, Baltimore June 14–18 2003. Journal of Aerosol Medicine 16:2213, (2003)

]. It should be noted that as water droplets are not able to exist in a non saturated environment, salt (NaCl) was added to the water before being nebulised. Salt reduces the vapour pressure and allows stable droplets to be formed. The concentration of salt also affects the size of droplets [29

29. J. R. ButlerL. MitchemK. L. Hanford L. TreuelJ. P. Reid “In situ comparative measurements of the properties of aerosol droplets of different chemical composition,” Faraday Discuss 137, 351–366 (2008). [CrossRef] [PubMed]

] produced from the nebuliser, with higher concentrations producing larger drops.

A 532nm laser (Quantum Finesse 4W) was used for all of the work. The same setup was used with both multimode fiber (MMF) and single mode fiber (SMF) by simply replacing the fibers with those of the desired properties. The MMF used was 0.22-NA 50µm Core Multimode Vis-IR Fiber (Thorlabs part# AFS50/125Y), while the single mode fiber (Thorlabs part# 460HP) had a mode field diameter of 3.5±0.5µm@515nm and both were used with ST connectors. When using MMF with equal power down each arm and with the variable neutral desnity (ND) filters set to ND=0, 27% of the total laser output could be transmitted down each arm. Due to the small core size of the SMF fibers, mechanical drift in the launching stages produced significant and random power fluctuations in the transmitted power. Approximately 5–8% total laser power could be transferred using SMF. To account for this, the power down each arm was measured directly using a power meter, before and after each trapping sequence. By keeping the trapping sequences to around 5 minutes no change in transmitted power was observed. As the power remained constant over this period any effects that might result from salt or other particles building up on the exposed fiber ends can also be neglected. However, to minimise any effect this might have, the fiber ends were cleaned thoroughly before the start of each sequence. The trap was positioned horizontally so that the gradient force is balancing gravity.

Fig. 3. Diagram of the experimental setup. From the laser the beam is split into two arms using a half waveplate (1/2 WP) and polarising beam cube. This waveplate can be adjusted to control the relative power down each arm while variable neutral density (ND) filers in each arm provide independent power adjustment. The beams are launched into the fibers using stage mounted 5X microscope objectives. The fibers feed directly into the trapping cell where trapping was observed and recorded using a long working distance objective and CCD camera.

4. Multimode fiber trapping

Due to their larger core size, multimode fibers (MMF) are much easier to couple into providing greater power with which to trap, and are simpler to align. This makes the multimode system relatively easy to set up. However, being multimode they produce a complex intensity pattern as can be seen in Fig. 4. While Fig. 1 showed the optical force to be dependent on particle size, using MMF makes this behaviour more complex. Sufficiently large droplets will straddle the peaks and troughs of the optical field and experience an average field emitted from each fiber. Smaller droplets, however, experience a local force dependent on their exact position in the overall overlapping mode patterns.

Fig. 4. Beam profile produced from a MMF of the type used. The intensity pattern is that of a super Gaussian with added peaks and troughs.

Due to the complex nature of this overlapping field any number of stable local trapping sites can be created, each with a different power, up to and including the total output of the fiber. This makes trapping very unpredictable. We were able to simultaneously trap anywhere up to 10 water droplets. The local nature of the trapping field is again emphasised when the power balance between each fiber is adjusted. The trapped particles then move independently of one another according to the change in their particular local field. Hence while some particles will move a large distance, others may not move at all, as can be seen in the Fig. 5.

Fig. 5. Video of a salt-water (20g/L) droplet trapped in air using multimode fibers at a power of 135mW in each arm. The power in the left hand fiber is increased as that in the right is decreased moving the trapped particles to the right. Due to the multimode nature the particles moved different amounts and at different rates. The field of view of the image and video is 218µm by 164 µm. The droplets are all approximately 8µm in diameter. (Media 1)

For our work with multimode fibers we concentrated on trapping water. As previously mentioned the amount of salt added to the water used in the nebuliser will affect the size of the droplets produced, with higher concentrations producing larger drops. Figure 6 shows how this affects trapping behaviour, with an increase in average trapped droplet size at a given power if the salt concentration is higher. Maximum droplet sizes were determined visually using a custom LabView program.

Fig. 6. Fig. 6. Plot of the maximum droplet sizes trapped using MMF at increasing power in each arm. Each point represents the mean size obtained from a large sample of at least 20 particles trapped at each power. The error bars represent the standard deviation of these droplets. While there is a slight increase in the size and range of the droplets that can be trapped, due to the inconsistency between global and local fields the lines are essentially flat.

5. Single mode fiber trapping

The output from a single mode fiber (SMF) generally results in only one stable equilibrium position [25

25. E. Sidick, S. D. Collins, and A. Knoesen, “Trapping forces in a multiple-beam fiber-optic trap,” Appl. Opt. 36, 6423–6433 (1997). [CrossRef]

,27

27. R. Gussgard and T. Lindmo “Calculation of the trapping force in a strongly focused laser beam,” J. Opt. Soc. Am. B. 9, 1922–1930 (1992). [CrossRef]

]. While being harder to align, the trapping position and behaviour of these fibers is much more predictable. By adjusting the power down each arm the particle position can be fully controlled as is shown in Fig. 7

Fig. 7. Video of a trapped 8µm salt-water drop (50g/L) being moved 120 micron between the fibers by adjusting the power transmitted by each arm. The droplet is moved at speeds of up to 75µm/s without them falling out of the trap. The fiber separation is also adjusted to demonstrate the effect it has on the equilibrium position. (Media 2)

The optical force experienced by an aerosol is dependent, amongst other things, on the fiber separation and power, as shown in equations 1–3. To establish the trapping ability of the fibers, shown in Fig. 8, droplet sizes were measured at a range of powers at fiber separations of 170 and 240µm. At separations greater than these trapping became less reliable while closer separations impeded the flow of particles. The aerosol used was water doped with NaCl at 50g/L.

Fig. 8. Plot of the maximum and minimum droplets sizes trapped at increasing power at 170 and 240µm. NaCl concentration is 50g/L. Solid and dashed lines are approximate guides to the maximum and minimum size of droplet that can be trapped at a power respectively. As can be seen the minimum size of droplet observed remains approximately constant regardless of power, but as power is increased larger droplets can be held. For larger separations the trapping efficiencies are smaller requiring more power to hold a particle of equal size.

We also trapped a number of other substances in air, including ethanol, glycerol (20% in water), and 3.01µm solid silica particles using nebulization. The ethanol evaporated too quickly to get any useful sizing data. Fig. 9 shows data for glycerol trapped at a fiber separation of 170µm. We were able to trap solid particles, loading them into the trap by placing them into solution with ethanol, which quickly evaporated leaving behind the silica particle [12

12. M. D. Summers, D. R. Burnham, and D. McGloin, “Trapping solid aerosols with optical tweezers: A comparison between gas and liquid phase optical traps,” Opt. Express 16, 7739–7747 (2008). [CrossRef] [PubMed]

]. The solid particle was stably trapped for in excess of half an hour.

Fig. 9. Plot of the maximum and minimum size of airborne glycerol (20% in water) droplets trapped versus power. The maximum line is roughly equivalent to that of the salt doped water show in Fig. 8.

6. Optical binding

Optical trapping occurs as a result of a particle being held in an equilibrium position due to a balance in both the gradient and scattering components of the matter-light interaction. The presence of this particle, will also distort the optical field slightly, for example by refocusing light from each fiber to a point on the opposite side of the particle. Given the correct conditions, this refocusing can result in another stable trapping position being formed. Were a particle to be trapped at this new equilibrium point, it would have a similar distorting effect on the light field experienced by the first particle. As a result, the position of each individual particle is dependent on the position of all of the others, creating a self-sustaining system where optical forces create arrays of trapped particles, an effect referred to as optical binding.

While these systems have been extensively investigated using liquid media [24

24. N. K. Metzger, K. Dholakia, and E. M. Wright, “Observation of bistability and hysteresis in optical binding of two dielectric spheres,” Phys. Rev. Lett. 96, 068102 (2006). [CrossRef] [PubMed]

,30

30. M. M. Burns, J.-M. Fournier, and J. A. Golovchenko, “Optical Matter: Crystallization and Binding in Intense Optical Fields,” Science 249, 749–754 (1990).

,31

31. S. A. Tatarkova, A. E. Carruthers, and K. Dholakia, “One-Dimensional Optically Bound Arrays of Microscopic Particles,” Phys. Rev. Lett. 89, 283901 (2002). [CrossRef]

], there may be advantages in carrying out experiments in air, including the ability to increase the relative refractive index of the particles and media and the possibility of seeing dynamics such as breathing modes.

During the course of our investigation into trapping single particles, we recorded many examples of optical binding using the single mode fiber trap. A separate, detailed study of optically-bound systems is required, but this falls outside the scope of the present work. However, examples of optical binding of water and, separately, glycerol can be seen in Fig. 10. It is interesting to note that the bound arrays always seem to have a large particle in the presence of much smaller particles.

Fig. 10. Example images of optical binding observed with SMF. Images A, B and C show binding with glycerol droplets while D is an example of the binding of salt doped water droplets(20g/L). Image A shows a 16.4 and 5.7µm particle separated by 30 µm. The Field of view is the same in all the images.

7. Discussion and summary

As can be seen in Figs. 5 and 7, fiber based trapping of aerosol particles can be achieved using both single and multimode systems. Using MMF allows for easy coupling, alignment and for multiple particles to be trapped simultaneously, however, due to the complex nature of the light field a quantitative study of the trap is difficult and the trap behaviour is unpredictable. Using SMF the coupling and alignment becomes more critical but the trap behaviour is more predictable and lower laser powers are required to trap. Using both types of fibers we were able to manipulate particles over distances of several hundred microns. With SMF the range of motion extends fully from one side of the trap to the other, which is often not the case using MMF due to the more complex nature of the optical field.

One obvious advantage of using a fiber trap is the ability to trap larger particles than with a single beam gradient trap, an indication of the larger optical forces present when using a counter propagating configuration. The combination of the large trapping forces, the large trapping area and the possible compactness of a fibre trap system, when compared to a normal optical tweezers make the system an attractive option for developing robust field based devices. In addition, the fiber trap concept opens up analogues to microfluidic systems for aerosols. One can imagine integrated light sources combined with covered microchannels in which aerosols can flow and be manipulated and analysed.

Acknowledgments

This work was funded by a Royal Society Joint Project award and the UK EPSRC. CL-M and JCG-V acknowledge financial support from CONACyT México and from Tecnológico de Monterrey grant CAT141. DM is a Royal Society University Research Fellow.

References and links

1.

A. Ashkin, “Acceleration and trapping of particles by radiation pressure,” Phys. Rev. Lett. 24, 156–159 (1970). [CrossRef]

2.

K. Dholakia, P Reece, and M Gu, “Optical Micromanipulation” Chem. Soc. Rev. 37, 42–55 (2008). [CrossRef] [PubMed]

3.

M. J. Lang and S. M Block, “Resource Letter: LBOT-1: Laser-based optical tweezers,” Am. J. Phys. 71, 201–215 (2003). [CrossRef]

4.

K. C. Neuman and S. M. Block, “Optical trapping,” Rev. Sci. Instrum. 75, 2787–2809 (2004). [CrossRef]

5.

D. McGloin, “Optical tweezers: 20 years on,” Phil. Trans. R. Soc. 364, 3521–3537 (2006). [CrossRef]

6.

R. D. Leonardo, G. Ruocco, L. Leach, M. J. Padgett, A. J. Wright, J. M. Girkin, D. R. Burnham, and D. McGloin, “Parametric Resonance of Optically Trapped Aerosols,” Phys. Rev. Lett. 99, 010601 (2007). [CrossRef] [PubMed]

7.

M. D. King, K. C. Thompson, and A. D. Ward, “Laser Tweezers Raman Study of Optically Trapped Aerosol Droplets of Seawater and Oleic Acid Reacting with Ozone: Implications for Cloud-Droplet Properties,” J. Am. Chem. Soc. 126, 16710–16711 (2004). [CrossRef] [PubMed]

8.

J. Buajarern, L. Mitchem, and J. P. Reid, “Characterising the formation of organic layers on the surface of inorganic/aqueous aerosols by Raman spectroscopy,” J. Phys. Chem. A. 11111852–11859 (2007). [CrossRef] [PubMed]

9.

L. Mitchem and J. P. Reid, “Optical Manipulation and Characterisation of Aerosol Particles,” Chem. Soc. Rev. 37, 756–769 (2008). [CrossRef] [PubMed]

10.

A. Ashkin, J. M. Dziedzic, J. E. Bjorkhom, and S. Chu, “Observation of a single beam gradient force trap for dielectric particles,” Opt. Lett. 11, 288–290 (1986). [CrossRef] [PubMed]

11.

G. Roosen and C. Imbert, “Optical levitation by means of 2 horizontal laser beams—Theoretical and experimental study,” Phys. Lett. A. 59, 6–8 (1976). [CrossRef]

12.

M. D. Summers, D. R. Burnham, and D. McGloin, “Trapping solid aerosols with optical tweezers: A comparison between gas and liquid phase optical traps,” Opt. Express 16, 7739–7747 (2008). [CrossRef] [PubMed]

13.

N. Magome, M. I. Kohira, E. Hayata, S. Mukai, and K. Yoshikawa, “Optical Trapping of a Growing Water Droplet in Air,” J. Phys. Chem. B. 107, 3988–3990 (2003). [CrossRef]

14.

R. J. Hopkins, L. Mitchem, A. D. Ward, and J. P. Reid, “Control and characterisation of a single aerosol droplet in a single-beam gradient-force optical trap,” Phys. Chem. Chem. Phys. 6, 4924–4927 (2004). [CrossRef]

15.

D. R. Burnham and D. McGloin, “Holographic optical trapping of aerosol droplets,” Opt. Express. 14, 4175–4181 (2006). [CrossRef] [PubMed]

16.

J. Buajarern, L. Mitchem, A. D. Ward, N. H. Nahler, D. McGloin, and J. P. Reid, “Controlling and Characterising the Coagulation of Liquid Aerosol Droplets,” J. Chem. Phys. 125, 114506 (2006). [CrossRef] [PubMed]

17.

R. Pastel, A. Struthers, R. Ringle, J. Rodgers, C. Rohde, and P. Geiser, “Laser trapping of microscopic particles for undergraduate experiments,” Am. J. Phys. 68, 993–1001 (2000). [CrossRef]

18.

K. Taji, M. Tachikawa, and K. Nagashima, “Laser trapping of ice crystals,” Appl. Phys. Lett. 88, 141111 (2006). [CrossRef]

19.

M. Guillon, O. Moine, and B. Stout, “Longitudinal Optical Binding of High Optical Contrast Microdroplets in Air,” Phys. Rev. Lett. 96, 143902 (2006). Erratum, Phys. Rev. Lett. 99, 079901 (2007). [CrossRef] [PubMed]

20.

A. Constable, J. Kim, J. Mervis, F. Zarinetchi, and M. Prentiss, “Demonstration of a-fiber-optical light-force trap,” Opt. Lett. 18, 1867–1869 (1993). [CrossRef] [PubMed]

21.

J. Guck, R. Ananthakrishnan, H. Mahmood, T. J. Moon, C. C. Cunningham, and J. Käs, “The Optical Stretcher: A Novel Laser Tool to Micromanipulate Cells,” Biophys. J. 81, 767–784 (2001). [CrossRef] [PubMed]

22.

P. R. T. Jess, V. Garcés-Chávez, D. Smith, M. Mazilu, L. Paterson, A. Riches, C. S. Herrington, W. Sibbett, and K. Dholakia, “Dual beam fiber trap for Raman micro-spectroscopy of single cells,” Opt. Express. 14, 5779–5791 (2006). [CrossRef] [PubMed]

23.

W. Singer, M. Frick, S. Bernet, and M. Ritsch-Marte, “Self-organized array of regularly spaced microbeads in a fiber-optical trap,” J. Opt. Soc. Am. B 20, 1568–1574 (2001). [CrossRef]

24.

N. K. Metzger, K. Dholakia, and E. M. Wright, “Observation of bistability and hysteresis in optical binding of two dielectric spheres,” Phys. Rev. Lett. 96, 068102 (2006). [CrossRef] [PubMed]

25.

E. Sidick, S. D. Collins, and A. Knoesen, “Trapping forces in a multiple-beam fiber-optic trap,” Appl. Opt. 36, 6423–6433 (1997). [CrossRef]

26.

S. Ebert, K. Travis, B. Lincoln, and J. Guck “Fluorescence ratio thermometry in a microfluidic dual-beam laser trap,” Opt. Express 15, 15493–15499 (2007). [CrossRef] [PubMed]

27.

R. Gussgard and T. Lindmo “Calculation of the trapping force in a strongly focused laser beam,” J. Opt. Soc. Am. B. 9, 1922–1930 (1992). [CrossRef]

28.

J. H. Dennis, C. A. Pieron, and K. Asai, “Aerosol Output and Size from Omron NE-U22 nebulizer,” in Proceedings of the 14th International Congress International Society for Aerosols in Medicines, Baltimore June 14–18 2003. Journal of Aerosol Medicine 16:2213, (2003)

29.

J. R. ButlerL. MitchemK. L. Hanford L. TreuelJ. P. Reid “In situ comparative measurements of the properties of aerosol droplets of different chemical composition,” Faraday Discuss 137, 351–366 (2008). [CrossRef] [PubMed]

30.

M. M. Burns, J.-M. Fournier, and J. A. Golovchenko, “Optical Matter: Crystallization and Binding in Intense Optical Fields,” Science 249, 749–754 (1990).

31.

S. A. Tatarkova, A. E. Carruthers, and K. Dholakia, “One-Dimensional Optically Bound Arrays of Microscopic Particles,” Phys. Rev. Lett. 89, 283901 (2002). [CrossRef]

OCIS Codes
(010.1110) Atmospheric and oceanic optics : Aerosols
(140.7010) Lasers and laser optics : Laser trapping
(350.4855) Other areas of optics : Optical tweezers or optical manipulation

ToC Category:
Optical Trapping and Manipulation

History
Original Manuscript: July 3, 2008
Revised Manuscript: August 12, 2008
Manuscript Accepted: August 19, 2008
Published: September 2, 2008

Virtual Issues
Vol. 3, Iss. 11 Virtual Journal for Biomedical Optics

Citation
D. Rudd, C. Lopez-Mariscal, M. Summers, A. Shahvisi, J. C. Gutiérrez-Vega, and D. McGloin, "Fiber based optical trapping of aerosols," Opt. Express 16, 14550-14560 (2008)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-16-19-14550


Sort:  Author  |  Year  |  Journal  |  Reset  

References

  1. A. Ashkin, "Acceleration and trapping of particles by radiation pressure," Phys. Rev. Lett. 24, 156-159 (1970). [CrossRef]
  2. K. Dholakia, P Reece, and M Gu, "Optical Micromanipulation" Chem. Soc. Rev. 37, 42 - 55 (2008). [CrossRef] [PubMed]
  3. M. J. Lang and S. M Block, "Resource Letter: LBOT-1: Laser-based optical tweezers," Am. J. Phys. 71, 201-215 (2003). [CrossRef]
  4. K. C. Neuman and S. M. Block, "Optical trapping," Rev. Sci. Instrum. 75, 2787-2809 (2004). [CrossRef]
  5. D. McGloin, "Optical tweezers: 20 years on," Phil. Trans. R. Soc. 364, 3521-3537 (2006). [CrossRef]
  6. R. D. Leonardo, G. Ruocco, L. Leach, M. J. Padgett. A. J. Wright, J. M. Girkin, D. R. Burnham, and D. McGloin, "Parametric Resonance of Optically Trapped Aerosols," Phys. Rev. Lett. 99, 010601 (2007). [CrossRef] [PubMed]
  7. M. D. King, K. C. Thompson, and A. D. Ward, "Laser Tweezers Raman Study of Optically Trapped Aerosol Droplets of Seawater and Oleic Acid Reacting with Ozone: Implications for Cloud-Droplet Properties," J. Am. Chem. Soc. 126, 16710-16711 (2004). [CrossRef] [PubMed]
  8. J. Buajarern, L. Mitchem, and J. P. Reid, "Characterising the formation of organic layers on the surface of inorganic/aqueous aerosols by Raman spectroscopy," J. Phys. Chem. A. 111, 11852-11859 (2007). [CrossRef] [PubMed]
  9. L. Mitchem and J. P. Reid, "Optical Manipulation and Characterisation of Aerosol Particles," Chem. Soc. Rev. 37, 756-769 (2008). [CrossRef] [PubMed]
  10. A. Ashkin, J. M. Dziedzic, J. E. Bjorkhom, and S. Chu, "Observation of a single beam gradient force trap for dielectric particles," Opt. Lett. 11, 288-290 (1986). [CrossRef] [PubMed]
  11. G. Roosen and C. Imbert, "Optical levitation by means of 2 horizontal laser beams???Theoretical and experimental study," Phys. Lett. A. 59, 6-8 (1976). [CrossRef]
  12. M. D. Summers, D. R. Burnham, and D. McGloin, "Trapping solid aerosols with optical tweezers: A comparison between gas and liquid phase optical traps," Opt. Express 16, 7739-7747 (2008). [CrossRef] [PubMed]
  13. N. Magome, M. I. Kohira, E. Hayata, S. Mukai, and K. Yoshikawa, "Optical Trapping of a Growing Water Droplet in Air," J. Phys. Chem. B. 107, 3988-3990 (2003). [CrossRef]
  14. R. J. Hopkins, L. Mitchem, A. D. Ward, and J. P. Reid, "Control and characterisation of a single aerosol droplet in a single-beam gradient-force optical trap," Phys. Chem. Chem. Phys. 6, 4924-4927 (2004). [CrossRef]
  15. D. R. Burnham and D. McGloin, "Holographic optical trapping of aerosol droplets," Opt. Express. 14, 4175-4181 (2006). [CrossRef] [PubMed]
  16. J. Buajarern, L. Mitchem, A. D. Ward, N. H. Nahler, D. McGloin, and J. P. Reid, "Controlling and Characterising the Coagulation of Liquid Aerosol Droplets," J. Chem. Phys. 125, 114506 (2006). [CrossRef] [PubMed]
  17. R. Pastel, A. Struthers, R. Ringle, J. Rodgers, C. Rohde, and P. Geiser, "Laser trapping of microscopic particles for undergraduate experiments," Am. J. Phys. 68, 993-1001 (2000). [CrossRef]
  18. K. Taji, M. Tachikawa, and K. Nagashima, "Laser trapping of ice crystals," Appl. Phys. Lett. 88, 141111 (2006). [CrossRef]
  19. M. Guillon, O. Moine, and B. Stout, "Longitudinal Optical Binding of High Optical Contrast Microdroplets in Air," Phys. Rev. Lett. 96, 143902 (2006). Erratum, Phys. Rev. Lett. 99, 079901 (2007). [CrossRef] [PubMed]
  20. A. Constable, J. Kim, J. Mervis, F. Zarinetchi, and M. Prentiss, "Demonstration of a-fiber-optical light-force trap," Opt. Lett. 18, 1867-1869 (1993). [CrossRef] [PubMed]
  21. J. Guck, R. Ananthakrishnan, H. Mahmood, T. J. Moon, C. C. Cunningham, and J. Käs, "The Optical Stretcher: A Novel Laser Tool to Micromanipulate Cells," Biophys. J. 81, 767-784 (2001). [CrossRef] [PubMed]
  22. P. R. T. Jess, V. Garcés-Chávez, D. Smith, M. Mazilu, L. Paterson, A. Riches, C. S. Herrington, W. Sibbett, and K. Dholakia, "Dual beam fiber trap for Raman micro-spectroscopy of single cells," Opt. Express. 14, 5779-5791 (2006). [CrossRef] [PubMed]
  23. W. Singer, M. Frick, S. Bernet, and M. Ritsch-Marte, "Self-organized array of regularly spaced microbeads in a fiber-optical trap," J. Opt. Soc. Am. B  20, 1568-1574 (2001). [CrossRef]
  24. N. K. Metzger, K. Dholakia, and E. M. Wright, "Observation of bistability and hysteresis in optical binding of two dielectric spheres," Phys. Rev. Lett. 96, 068102 (2006). [CrossRef] [PubMed]
  25. E. Sidick, S. D. Collins, and A. Knoesen, "Trapping forces in a multiple-beam fiber-optic trap," Appl. Opt. 36, 6423-6433 (1997). [CrossRef]
  26. S. Ebert, K. Travis, B. Lincoln, and J. Guck "Fluorescence ratio thermometry in a microfluidic dual-beam laser trap," Opt. Express 15, 15493-15499 (2007). [CrossRef] [PubMed]
  27. R. Gussgard and T. Lindmo "Calculation of the trapping force in a strongly focused laser beam," J. Opt. Soc. Am. B. 9, 1922-1930 (1992). [CrossRef]
  28. J. H. Dennis, C. A. Pieron, and K. Asai, "Aerosol Output and Size from Omron NE-U22 nebulizer," in Proceedings of the 14th International Congress International Society for Aerosols in Medicines, Baltimore June 14-18 2003. Journal of Aerosol Medicine 16, 2 213, (2003)
  29. J. R. Butler, L. Mitchem, K. L. Hanford, L. Treuel, and J. P. Reid "In situ comparative measurements of the properties of aerosol droplets of different chemical composition," Faraday Discuss 137, 351-366 (2008). [CrossRef] [PubMed]
  30. M. M. Burns, J.-M. Fournier, and J. A. Golovchenko, "Optical Matter: Crystallization and Binding in Intense Optical Fields," Science 249, 749-754 (1990).
  31. S. A. Tatarkova, A. E. Carruthers, and K. Dholakia, "One-Dimensional Optically Bound Arrays of Microscopic Particles," Phys. Rev. Lett. 89, 283901 (2002). [CrossRef]

Cited By

Alert me when this paper is cited

OSA is able to provide readers links to articles that cite this paper by participating in CrossRef's Cited-By Linking service. CrossRef includes content from more than 3000 publishers and societies. In addition to listing OSA journal articles that cite this paper, citing articles from other participating publishers will also be listed.

Supplementary Material


» Media 1: MOV (1337 KB)     
» Media 2: MOV (3711 KB)     

« Previous Article  |  Next Article »

OSA is a member of CrossRef.

CrossCheck Deposited