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Optics Express

  • Editor: Michael Duncan
  • Vol. 14, Iss. 9 — May. 1, 2006
  • pp: 4176–4182
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Holographic optical trapping of aerosol droplets

D. R. Burnham and D. McGloin  »View Author Affiliations


Optics Express, Vol. 14, Issue 9, pp. 4176-4182 (2006)
http://dx.doi.org/10.1364/OE.14.004176


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Abstract

We demonstrate the use of holographic optical tweezers for trapping particles in air, specifically aerosol droplets. We show the trapping and manipulation of arrays of liquid aerosols as well as the controlled coagulation of two or more droplets. We discuss the ability of spatial light modulators to manipulate airborne droplets in real time as well as highlight the difficulties associated with loading and trapping particles in such an environment. We conclude with a discussion of some of the applications of such a technique.

© 2006 Optical Society of America

1. Introduction

Optical manipulation techniques have matured considerably in the 35 years since Ashkin first demonstrated the use of radiation pressure to guide particles [1

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

]. The field was firmly established by the demonstration of the optical tweezers technique by Ashkin et al [2

2. 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]

], and these have developed into a tool that is routinely used to probe biological function [3

3. E. A. Abbondanzieri, W. J. Greenleaf, J. W. Shaevitz, R. Landick, and S. M. Block, “Direct observation of base-pair stepping by RNA polymerase,” Nature 438, 460–465 (2005). [CrossRef] [PubMed]

,4

4. S. Dumont, W. Cheng, V. Serebrov, R. K. Beran, I. Tinoco Jr., A. M. Pyle, and C. Bustamante “RNA translocation and unwinding mechanism of HCV NS3 helicase and its coordination by ATP,” Nature 439, 105–108 (2006). [CrossRef] [PubMed]

], colloidal dynamics [5

5. P. T. Korda, G. C. Spalding, and D. G. Grier, “Evolution of a colloidal critical state in an optical pinning potential landscape,” Physical Review B 66, 024504 (2002). [CrossRef]

,6

6. P. M. Hansen, J. K. Dreyer, J. Ferkinghoff-Borg, and L. Oddershede “Novel optical and statistical methods reveal colloid-wall interactions inconsistent with DLVO and Lifshitz theories,”, Journal of Colloid and Interface Science, 287561–571 (2005). [CrossRef]

], properties of light beams [7

7. V. Garcés-Chávez, D. McGloin, M. J. Padgett, W. Dultz, H. Schmitzer, and K. Dholakia, “Observation of the transfer of the local angular momentum density of a multi-ringed light beam to an optically trapped particle,” Phys. Rev. Lett. 91, 093602 (2003). [CrossRef] [PubMed]

,8

8. N. B. Simpson, K. Dholakia, L. Allen, and M. J. Padgett, “The mechanical equivalence of the spin and orbital angular momentum of light: an optical spanner,” Opt. Lett. 22, 52–54 (1997). [CrossRef] [PubMed]

], and to facilitate the stable trapping and manipulation of particles at the micron scale in a wide range of disciplines. Almost the entire body of work on optical manipulation is carried out on particles suspended in a liquid medium. This medium acts to damp out the motion of particles as they are trapped and this fact combined with the buoyancy of particles in such samples makes their trapping relatively straightforward. To trap particles in the absence of such a damping medium (such as air) is more difficult and less relevant for studies to date. Ashkin studied the optical levitation of airborne droplets [9

9. A. Ashkin and J.M. Dziedzic, “Optical levitation of liquid drops by radiation pressure,” Science 1871073–1075 (1975). [CrossRef] [PubMed]

] and others have used these techniques to probe the size and composition of aerosols [10

10. A. Biswas, H. Lati, R. L. Armstrong, and R.G. Pinnick, “Double-resonance stimulated Raman scattering from optically levitated glycerol droplets,” Phys. Rev. A 40, 7413–7416 (1989). [CrossRef] [PubMed]

,11

11. R. Thurn and W. Kiefer, “Raman microsampling technique applying optical levitation by radiation pressure,” Appl. Spectrosc. 38, 78–83 (1984). [CrossRef]

] but it is only relatively recently that their optical tweezing (that is gradient force trapping) has been demonstrated [12

12. R. Omori, T. Kobayashi, and A. Suzuki, “Observation of a single beam gradient-force optical trap for dielectric particles in air,” Opt. Lett. 22, 816–818 (1997). [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]

].

In this paper we outline the first use of holographic optical tweezers [16

16. M. Reicherter, T. Haist, E. U. Wagemann, and H. J. Tiziani “Optical particle trapping with computer generated holograms written on a liquid crystal display,” Opt. Lett. 24, 608–610 (1999). [CrossRef]

,17

17. J. A. Curtis, B. A. Koss, and D. G. Grier, “Dynamic Holographic Optical Trapping,” Opt. Commun. 207, 169–175 (2002). [CrossRef]

] for the manipulation of airborne particles, in this case aerosol droplets. Making use of such devices allows for arrays of particles to be trapped simultaneously, as well as allowing their controlled xyz translation. We show that controlled coagulation can be achieved and discuss the performance of the spatial light modulator in this context as well as outlining future applications of such a technique.

2. Experimental setup

Holographic tweezers are now a well established technique for the simultaneous manipulation of many particles. Recent work using these tools has explored applications in microfluidics [18

18. J. Enger, M. Goksor, K. Ramser, P. Hagberg, and D. Hanstorp, “Optical tweezers applied to a microfluidic system,” Lab on a Chip 4, 196–200 (2004). [CrossRef] [PubMed]

,19

19. K. Ramser, J. Enger, M. Goksör, D. Hanstorp, K. Logg, and M. Käll, “A microfluidic system enabling Raman measurements of the oxygenation cycle in single optically trapped red blood cells,” Lab on a Chip 5, 431–436 (2005). [CrossRef] [PubMed]

], Raman imaging [20

20. C. Creely, G. Volpe, M. Soler, and D. Petrov, “Raman imaging of floating cells,” Opt. Express 13, 6105–6110 (2005). [CrossRef] [PubMed]

] and the creation of complex colloidal structures [21

21. Y. Roichman and D. G. Grier, “Holographic assembly of quasicrystalline photonic heterostructures,” Opt. Express 13, 5434–5439 (2005). [CrossRef] [PubMed]

]. Our system makes use of a Holoeye LC-R 2500 spatial light modulator (SLM) to modulate the phase of 532nm cw light from a Laser Quantum Finesse laser (maximum output 4W). A schematic of our system is shown in Fig. 1. We first rotate the laser’s plane of polarisation to optimize the efficiency of the phase modulation, then having expanded the beam via a telescope (L1 and L2) it is incident on, and covers, the SLM. Two 4f (L3 and L4, and L5 and L6) imaging systems are placed directly after the SLM to reduce the beam size to slightly overfill the back aperture of the microscope objective (Nikon CFI E Plan Achromat 100X oil, N.A. 1.25). The 4f imaging systems also make the SLM and back aperture planes conjugate. Having focused the light into the trapping plane the same objective is used for imaging.

Fig. 1. Experimental setup. W is a half-wave plate. Lenses L1 and L2, with focal lengths 75mm and 750mm respectively act to expand the laser source. Lenses L3, L4, L5, and L6 form the two 4f imaging systems with focal lengths 400mm, 250mm, 200mm and 100mm respectively. Finally lens L7, with a focal length of 200mm, is the tube lens used in conjuction with the microscope objective, OBJ, which focuses the desired trapping pattern into the trapping chamber containing a water soaked tissue. Mirrors M1 and M2 are broadband dielectric mirrors. Mirror DM is a green dichroic mirror and MM is a metallic mirror for imaging into the camera CCD. A beam block, BB, is used to remove the SLM’s zeroth diffraction order. The aerosols are produced using an ultrasonic nebuliser, NEB.

We generate our holograms making use of a custom LabVIEW program implementing an adaptive-additive algorithm [22

22. V. Soifer, V. Kotlyar, and L. Doskolovich, Iterative Methods for Diffractive Optical Elements Computation (Taylor and Francis, London, 1997).

] which treats the SLM as a secondary microdisplay.

The aerosol droplets are created using an ultrasonic nebuliser (Omron U22(NE-U22-E)), which produces aerosols with a mass median aerodynamic diameter (MMAD) between approximately 3 and 5 microns [23

23. 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.

]. Custom glassware is attached to the nebuliser allowing the droplets to be fed accurately into a sealed glass cube, with an inlet and outlet hole in either side, which acts as our trapping chamber. The bottom of the cube is formed from a glass coverslip having been soaked in a 2% weight by volume solution of sodium dodecyl sulphate, for reasons explained in the discussion. To help control the humidity inside the cell we place a small water soaked paper tissue in the cube. In the experiments making use of one type of aerosol we block the second hole to avoid unnecessary air currents.

For the work outlined here we make use of water aerosols. However, water droplets generally only exist in oversaturated atmospheres (that is with a relative humidity greater than 100%) so we use aqueous solutions of NaCl to decrease the vapour pressure of the droplets which allows them to form in relative humidity <100% [24

24. J. H. Seinfeld and S. N. Pandis, Atmospheric Chemistry and Physics: Air Pollution to Climate Change (John Wiley and Sons Inc., 1997).

]. The majority of the work has been carried out with 0.34817 Molar concentrations [25

25. Laura Mitchem, Particle Dynamics Group, School of Chemistry, University of Bristol, Bristol, BS8 1TS (personal communication, 2006).

] but others have been tested, for example 40mM and seawater, which provide similar results, although they do alter the droplet dynamics. The ambient laboratory temperature is 22.5 ± 1°C.

3. Experimental results

We are able to trap arrays of aerosol droplets as shown in Fig. 2, which also outlines one of the difficulties of trapping in air. In a liquid medium it can be hard to reliably load a large array of trap sites with colloidal particles. One has to actively seek out the particles, but this can result in particles falling out of the trap or in multiple particles being trapped at a single site. Alternatively the trap can be static with particles flown in to try and fill the sites and this can lead to jamming, or particles knocking each other out. In air we have no choice but to wait for particles to fall into the traps. In the experimental geometry trap loading is far from optimized, and the images shown illustrate that although we have a relatively simple trap pattern, which would be easy to fill in a liquid, we may be unable to fill the sites at all if no aerosol flies into the trapping region of that site.

Fig. 2. Top left image shows the backscattered light from the bottom of the microscope slide revealing the holographic trapping pattern. The remaining images show the resulting trapped water droplets after multiple uses of the nebuliser in attempts to fill all trap sites. As indicated in the text, although we have a relatively simple trapping pattern it is hard to fill all the sites.

The current system allows xy translation of droplets in single jumps of up to 3.40 ± 0.02μm, which combined with a high refresh rate of ~25 frames per second allows xy translation at speeds up to 85μms-1. The precision to which we can translate droplets in x and y is 227 ± 1nm with control of droplets over the entire field of view; 45 by 60μm. The apparatus also allows z control of the droplets as shown in Fig. 3.

Fig. 3. A series of microscope images demonstrating the z control of water droplets from -10 to +10 microns. The white bar indicates a scale of 5 microns.

We are able to trap a range of droplet sizes from ~2.5 to 12μm in diameter and determine that the minimum power required to trap an aerosol is 0.38 ± 0.02mW and an axial Q value of 0.22 ± 0.01, which are all comparable to results in conventional tweezers [15

15. 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]

]. The droplet size is dependent on power with larger droplets requiring greater powers, as shown in Fig. 4.

Fig. 4. Graph showing the variation of droplet size as a function of power. The vertical error has standard error bars increasing in size with power indicating that not only can higher powers trap, on average, larger droplets but also a greater distribution of sizes. The horizontal error bars mostly arise from the non perfect intensity uniformity of the trapping sites. However, the graph cannot convey the difficulty of achieving initial trapping at low powers.

This stable trapping and precise manipulation of water droplets allows not only their individual control but also the ability to coagulate multiple droplets both of which are demonstrated in Fig. 5.

Fig. 5. Movie showing control and coagulation of multiple water droplets. [Media 1]

One may imagine the Brownian motion of aerosol particles is too high, and the SLM refresh rate too slow to enable such precise control of water droplets. However, by calculating the displacement due to Brownian diffusivity using Eq. (1):

x2=2kTCct3πμDp,
(1)

where k is Boltzmann’s constant, T is the temperature in Kelvin, Cc is the slip correction factor, μ is the viscosity of the surrounding fluid (air), Dp is the droplet diameter, and t is the time. Plotting the results, Fig 6, it is clear that for the droplet sizes studied here the displacement due to Brownian motion after the SLM response time of 16ms is significantly below a single droplet radius [24

24. J. H. Seinfeld and S. N. Pandis, Atmospheric Chemistry and Physics: Air Pollution to Climate Change (John Wiley and Sons Inc., 1997).

].

Fig. 6. Theoretical curve showing particle displacement after 16ms (SLM response time) due to Brownian diffusion.

4. Discussion and conclusion

Clearly from Figs. 3, 4, and 6 a major hurdle in quantitative analysis is the inability to accurately size droplets from video images alone, due not only to their dynamic nature but also to the poor definition of their edges on the video output. However, precise droplet size measurement techniques have already been demonstrated [15

15. 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]

] allowing for future accuracy improvement and more precise quantitative results.

The relationship between laser power and droplet size combined with the ability of holographic optical tweezers to control each individual trap site’s intensity will eventually allow some degree of size selective trapping.

One problem encountered was the formation of water ‘puddles’ from parts of the aerosol cloud settling onto the microscope slide. Measuring tens of microns across they create sufficient aberrations in the beam to cause the loss of droplets from traps unless they are sited directly over the puddle’s centre where curvature is at a minimum. As a solution we applied a detergent surfactant to the slide thus increasing the wetting and reducing the formation of puddles.

The quantitative results shown above are specific to one concentration of sodium chloride solution, with any alteration in the aerosol composition significantly altering the droplet’s properties. This obviously increases the complexity over simple monodispersed or even polydispersed microspheres as they have fixed dimensions and properties where as droplets can condense, evaporate, absorb, and constantly interact with their surroundings.

The experiments described above show for the first time the optical trapping and control of arrays of aerosol particles. Further they demonstrate the applicability of using the slow (in comparison to acousto-optical deflection techniques) phase only SLMs to manipulate airborne particles and to coagulate them. However issues remain with the techniques as described for some applications. Better kinoform generation algorithms need to be implemented [26

26. M. Polin, K. Ladavac, S. -H. Lee, Y. Roichman, and D. Grier, “Optimized holographic optical traps,” Opt. Express 13, 5831–5845 (2005). [CrossRef] [PubMed]

] in order to reduce the effects of ghost traps and increase the intensity uniformity of the pattern. As we are using a flow of aerosols from a nebuliser source we mimic how atmospheric aerosols might behave, and for real sampling strategies we must make robust devices that can cope with a range of particle sizes and velocities. To some extent this is what we have demonstrated. For other applications, such as airborne digital microfluidics, where we wish to have less random choices about which particles we trap and carry out droplet reactions with we must come up with better loading strategies for the traps. We envisage using single droplet maker devices (such as ink jet printer heads) to position the drops ready for accurate loading. Combining this sort of technique with the random process would also allow better implementation of digital microfluidics for real time sampling.

Acknowledgments

We would like to thank both Jonathan Reid and Laura Mitchem from Bristol University for helpful discussions and Kishan Dholakia (St. Andrews) for the use of the SLM. DRB would like to thank EPSRC for support, while DM is a Royal Society University Research Fellow. The work was partly funded by NERC and EPSRC.

References and Links

1.

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

2.

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]

3.

E. A. Abbondanzieri, W. J. Greenleaf, J. W. Shaevitz, R. Landick, and S. M. Block, “Direct observation of base-pair stepping by RNA polymerase,” Nature 438, 460–465 (2005). [CrossRef] [PubMed]

4.

S. Dumont, W. Cheng, V. Serebrov, R. K. Beran, I. Tinoco Jr., A. M. Pyle, and C. Bustamante “RNA translocation and unwinding mechanism of HCV NS3 helicase and its coordination by ATP,” Nature 439, 105–108 (2006). [CrossRef] [PubMed]

5.

P. T. Korda, G. C. Spalding, and D. G. Grier, “Evolution of a colloidal critical state in an optical pinning potential landscape,” Physical Review B 66, 024504 (2002). [CrossRef]

6.

P. M. Hansen, J. K. Dreyer, J. Ferkinghoff-Borg, and L. Oddershede “Novel optical and statistical methods reveal colloid-wall interactions inconsistent with DLVO and Lifshitz theories,”, Journal of Colloid and Interface Science, 287561–571 (2005). [CrossRef]

7.

V. Garcés-Chávez, D. McGloin, M. J. Padgett, W. Dultz, H. Schmitzer, and K. Dholakia, “Observation of the transfer of the local angular momentum density of a multi-ringed light beam to an optically trapped particle,” Phys. Rev. Lett. 91, 093602 (2003). [CrossRef] [PubMed]

8.

N. B. Simpson, K. Dholakia, L. Allen, and M. J. Padgett, “The mechanical equivalence of the spin and orbital angular momentum of light: an optical spanner,” Opt. Lett. 22, 52–54 (1997). [CrossRef] [PubMed]

9.

A. Ashkin and J.M. Dziedzic, “Optical levitation of liquid drops by radiation pressure,” Science 1871073–1075 (1975). [CrossRef] [PubMed]

10.

A. Biswas, H. Lati, R. L. Armstrong, and R.G. Pinnick, “Double-resonance stimulated Raman scattering from optically levitated glycerol droplets,” Phys. Rev. A 40, 7413–7416 (1989). [CrossRef] [PubMed]

11.

R. Thurn and W. Kiefer, “Raman microsampling technique applying optical levitation by radiation pressure,” Appl. Spectrosc. 38, 78–83 (1984). [CrossRef]

12.

R. Omori, T. Kobayashi, and A. Suzuki, “Observation of a single beam gradient-force optical trap for dielectric particles in air,” Opt. Lett. 22, 816–818 (1997). [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.

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]

15.

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]

16.

M. Reicherter, T. Haist, E. U. Wagemann, and H. J. Tiziani “Optical particle trapping with computer generated holograms written on a liquid crystal display,” Opt. Lett. 24, 608–610 (1999). [CrossRef]

17.

J. A. Curtis, B. A. Koss, and D. G. Grier, “Dynamic Holographic Optical Trapping,” Opt. Commun. 207, 169–175 (2002). [CrossRef]

18.

J. Enger, M. Goksor, K. Ramser, P. Hagberg, and D. Hanstorp, “Optical tweezers applied to a microfluidic system,” Lab on a Chip 4, 196–200 (2004). [CrossRef] [PubMed]

19.

K. Ramser, J. Enger, M. Goksör, D. Hanstorp, K. Logg, and M. Käll, “A microfluidic system enabling Raman measurements of the oxygenation cycle in single optically trapped red blood cells,” Lab on a Chip 5, 431–436 (2005). [CrossRef] [PubMed]

20.

C. Creely, G. Volpe, M. Soler, and D. Petrov, “Raman imaging of floating cells,” Opt. Express 13, 6105–6110 (2005). [CrossRef] [PubMed]

21.

Y. Roichman and D. G. Grier, “Holographic assembly of quasicrystalline photonic heterostructures,” Opt. Express 13, 5434–5439 (2005). [CrossRef] [PubMed]

22.

V. Soifer, V. Kotlyar, and L. Doskolovich, Iterative Methods for Diffractive Optical Elements Computation (Taylor and Francis, London, 1997).

23.

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.

24.

J. H. Seinfeld and S. N. Pandis, Atmospheric Chemistry and Physics: Air Pollution to Climate Change (John Wiley and Sons Inc., 1997).

25.

Laura Mitchem, Particle Dynamics Group, School of Chemistry, University of Bristol, Bristol, BS8 1TS (personal communication, 2006).

26.

M. Polin, K. Ladavac, S. -H. Lee, Y. Roichman, and D. Grier, “Optimized holographic optical traps,” Opt. Express 13, 5831–5845 (2005). [CrossRef] [PubMed]

27.

P. Kaye, W. R. Stanley, E. Hirst, E. V. Foot, K. L. Baxter, and S. J. Barrington, “Single particle multichannel bio-aerosol fluorescence sensor,” Opt. Express 13, 3583–3593 (2005). [CrossRef] [PubMed]

28.

K. Davitt, Y. -K. Song, W. Patterson III, A. Nurmikko, M. Gherasimova, J. Han, Y. -L. Pan, and R. Chang, “290 and 340 nm UV LED arrays for fluorescence detection from single airborne particles,” Opt. Express 13, 9548–9555 (2005). [CrossRef] [PubMed]

29.

N. J. Beeching, D. A. B. Dance, A. R. O. Miller, and R. C. Spencer, “Biological warfare and bioterrorism,” BMJ 324, 336–339 (2002). [CrossRef] [PubMed]

30.

E. F. Mikhailov, S. S. Vlasenko, Lutz Krämer, and Reinhard Niessner, “Interaction of soot aerosol particles with water droplets: influence of surface hydrophilicity,” J. Aero. Sci. 32, 697–711 (2001). [CrossRef]

31.

P.F. Smith, “Direct detection of weakly interacting massive particles using non-cryogenic techniques,” Phil. Trans. R. Soc. A 361, 2591–2606 (2003). [CrossRef] [PubMed]

OCIS Codes
(010.1110) Atmospheric and oceanic optics : Aerosols
(010.7340) Atmospheric and oceanic optics : Water
(090.2890) Holography : Holographic optical elements
(120.5060) Instrumentation, measurement, and metrology : Phase modulation
(140.7010) Lasers and laser optics : Laser trapping
(230.6120) Optical devices : Spatial light modulators

History
Original Manuscript: March 31, 2006
Manuscript Accepted: April 21, 2006
Published: May 1, 2006

Citation
D. R. Burnham and D. McGloin, "Holographic optical trapping of aerosol droplets," Opt. Express 14, 4176-4182 (2006)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-14-9-4176


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References

  1. A. Ashkin, "Acceleration and trapping of particles by radiation pressure," Phys. Rev. Lett. 24, 156 (1970). [CrossRef]
  2. 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]
  3. E. A. Abbondanzieri, W. J. Greenleaf, J. W. Shaevitz, R. Landick and S. M. Block, "Direct observation of base-pair stepping by RNA polymerase," Nature 438, 460-465 (2005). [CrossRef] [PubMed]
  4. S. Dumont, W. Cheng, V. Serebrov, R. K. Beran, I. TinocoJr., A. M. Pyle and C. Bustamante "RNA translocation and unwinding mechanism of HCV NS3 helicase and its coordination by ATP," Nature 439, 105-108 (2006). [CrossRef] [PubMed]
  5. P. T. Korda, G. C. Spalding and D. G. Grier, "Evolution of a colloidal critical state in an optical pinning potential landscape," Physical Review B 66, 024504 (2002). [CrossRef]
  6. P. M. Hansen, J. K. Dreyer, J. Ferkinghoff-Borg, and L. Oddershede "Novel optical and statistical methods reveal colloid-wall interactions inconsistent with DLVO and Lifshitz theories,", Journal of Colloid and Interface Science,  287561-571 (2005). [CrossRef]
  7. V. Garcés-Chávez, D. McGloin, M. J. Padgett, W. Dultz, H. Schmitzer and K. Dholakia, "Observation of the transfer of the local angular momentum density of a multi-ringed light beam to an optically trapped particle," Phys. Rev. Lett. 91, 093602 (2003). [CrossRef] [PubMed]
  8. N. B. Simpson, K. Dholakia, L. Allen and M. J. Padgett, "The mechanical equivalence of the spin and orbital angular momentum of light: an optical spanner," Opt. Lett. 22, 52-54 (1997). [CrossRef] [PubMed]
  9. A. Ashkin and J.M. Dziedzic, "Optical levitation of liquid drops by radiation pressure," Science 1871073-1075 (1975). [CrossRef] [PubMed]
  10. A. Biswas, H. Lati, R. L. Armstrong and R.G. Pinnick, "Double-resonance stimulated Raman scattering from optically levitated glycerol droplets," Phys. Rev. A 40, 7413-7416 (1989). [CrossRef] [PubMed]
  11. R. Thurn and W. Kiefer, "Raman microsampling technique applying optical levitation by radiation pressure," Appl. Spectrosc. 38, 78-83 (1984). [CrossRef]
  12. R. Omori, T. Kobayashi and A. Suzuki, "Observation of a single beam gradient-force optical trap for dielectric particles in air," Opt. Lett. 22, 816-818 (1997). [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. 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]
  15. 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]
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  17. J. A. Curtis, B. A. Koss and D. G. Grier, "Dynamic Holographic Optical Trapping," Opt. Commun. 207, 169-175 (2002). [CrossRef]
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