The ability to manipulate nanoscopic matter precisely is critical for the development of active nanosystems. Optical tweezers1,2,3,4 are excellent tools for transporting particles ranging in size from several micrometres to a few hundred nanometres. Manipulation of dielectric objects with much smaller diameters, however, requires stronger optical confinement and higher intensities than can be provided by these diffraction-limited5 systems. Here we present an approach to optofluidic transport that overcomes these limitations, using sub-wavelength liquid-core slot waveguides6. The technique simultaneously makes use of near-field optical forces to confine matter inside the waveguide and scattering/adsorption forces to transport it. The ability of the slot waveguide to condense the accessible electromagnetic energy to scales as small as 60 nm allows us also to overcome the fundamental diffraction problem. We apply the approach here to the trapping and transport of 75-nm dielectric nanoparticles and λ-DNA molecules. Because trapping occurs along a line, rather than at a point as with traditional point traps7,8, the method provides the ability to handle extended biomolecules directly. We also carry out a detailed numerical analysis that relates the near-field optical forces to release kinetics. We believe that the architecture demonstrated here will help to bridge the gap between optical manipulation and nanofluidics.
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We thank A. Stroock for discussions, and A. Nitkowski and S. Manipatruni for technical support. This work was done in part at the Cornell NanoScale Facility, a member of the National Nanotechnology Infrastructure Network, which is supported by the US National Science Foundation. This work was funded by the US National Science Foundation NIRT: Active Nanophotofluidic Systems for Single Molecule/Particle Analysis (award number 0708599).
Author Contributions A.H.J.Y. and S.D.M. were responsible for running the bulk of the trapping experiments and analysing data. A.H.J.Y. carried out the simulation calculations. B.S.S was responsible for running initial experiments and fabrication of the slot waveguide chips. M.K. developed the DNA imaging methods. A.H.J.Y., M.L. and D.E. were responsible for writing the paper. All authors discussed the results and commented on the manuscript.
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Stable nanoparticle trapping and controlled release by the slot waveguide. This movie demonstrates the high stability, long term trapping of many 75nm polystyrene particles by and optically excited 100nm slot waveguide. In this case, the slot waveguide is excited and particles flowing over it are allowed to accumulate for some time. In the first half of the movie the particles are released by removing the excitation power and in the second half they are released by switching polarity. This capture and release is also shown in Figure 2. (MOV 2065 kb)
Nanoparticle capture by the slot waveguide. This movie demonstrates the capture and trapping of two individual 100nm polystyrene particles by an optically excited 120nm slot waveguide. In this movie, the particles are initially flowing from left to right by a microfluidic flow. They are trapped near the stability point and the particle is released when the random thermal energy exceeds the work required to break the trap. (MOV 2740 kb)
Optical force transport of a single nanoparticle inside the slot waveguide. In this movie, we demonstrate the optical force transport of a single 100nm particle inside a 120nm slot waveguide. This transport is also shown in Figure 2. (MOV 3979 kb)
Biomolecular trapping inside the slot waveguide. In this movie, we demonstrate optical trapping of Yo-Yo tagged lambda-DNA by the slot waveguide. The DNA is initially flowing from left to right in the microchannel and becomes trapped on a 60nm slot waveguide. The DNA is released at the end of the movie when the optical power is removed. Note that when no optical power is applied, no DNA is trapped indicating that the effect is entirely optical, not hydrodynamic. This trapping is also shown in Figure 3. (MOV 10734 kb)
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Yang, A., Moore, S., Schmidt, B. et al. Optical manipulation of nanoparticles and biomolecules in sub-wavelength slot waveguides. Nature 457, 71–75 (2009). https://doi.org/10.1038/nature07593
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