The possibility of using light to manipulate small objects was first exploited by Ashkin for moving and trapping inorganic micron-scale beads.1,2
The subsequent use of optical tweezers to trap bacteria and viruses created an entire field for studying biological objects with the help of light-induced forces.3
Indeed, optical tweezers are widely applied in cell biology, immunology, genetics and other areas due to their numerous desirable properties. These include exquisite spatial resolution, noninvasive nature, and independence of the charge state of the particle or the surrounding medium.4
Microfluidic approaches towards miniaturization of chemical and biological analysis exploit minute sample volumes, short reaction times, and versatile designs of chemical and physical microenvironments. Recently, microfluidic devices have started to incorporate integrated optical methods. These optofluidic approaches have already resulted in the demonstration of on-chip light sources, detectors, liquid-core waveguides, and biological Raman and fluorescence sensing down to the single particle level.5-7
What has been missing from this integrated optofluidic toolbox is the equivalent of bulk optical tweezers, i.e.
a universal means for optical particle manipulation in integrated form. Several steps in this direction have recently been taken, including particle pushing in hollowcore photonic crystal fibers8
and planar waveguides,9
and particle trapping in fluidic channels either on top of10
an optical waveguide. Since conventional optical tweezers cannot easily be implemented in a waveguide due to the requirement for beam foci with high numerical aperture, on-chip traps have relied on gradient forces in evanescent fields and conventional dual beam traps created by diverging beams exiting opposing waveguides. These approaches are limited by the inefficient use of power in evanescent trapping10
or by being restricted to a few points along the liquid channel.11
Dielectrophoresis, a related technique using alternating quasi-static electical fields, has been used successfully to trap and manipulate sub-micron bio-particles.12,13
However, this technique requires specific electrical properties of buffer and particles, is restricted to the vicinity of electrodes and is hard to reconcile with optofluidic detection schemes.
We introduce a new method for on-chip optical particle control that does not suffer from these restrictions and serves as a genuine manipulator for trapping and actuating particles within and along a liquid waveguide channel. This technique takes advantage of using waveguide loss to form a stable dual beam trap and provides maximum flexibility for controlling the particle location on an optofluidic chip. In addition to characterizing this trapping method, we demonstrate the key attributes for complete on-chip particle control and analysis: (1) control of particle location with micron-scale precision over large distances of several millimeters, (2) definition of multiple, independent traps inside the same fluidic channel, and (3) simultaneous trapping and fluorescence detection of a single particle in fully planar beam geometry.