How it Works – Optofluidics’ Nanotweezer Technology
Optofluidics’ Nanotweezer technology enables chemistry free, reversible, surface immobilization of cells, viruses, nucleic acids, and proteins using light confined within microchip based waveguides. Here we provide a brief description of the physics behind what makes the system work. See our products page to find out how it can work for you!
Waveguiding and evanescent fields
Waveguiding is the act of keeping light confined within a region that has a higher refractive index than its surroundings. This is the same technique used to transmit light through optical fibers.
Light moving through a waveguide exists in two forms. The first part is the propagating part, which is the light that you see. The second part is an invisible “aura” that surrounds the outside of the waveguide called the evanescent field. This field exists as a consequence of quantum mechanics and it’s power decays exponentially as you move away from the waveguide. Usually the evanescent field is very weak and doesn’t have much influence.
As the size of a waveguide gets smaller however, the relative amount of light in the evanescent field increases. Optofluidics’ NanoTweezer technology is based on the use of nanophotonic waveguides that are a couple of hundred nanometers tall. As shown in the figure, for these waveguides the evanescent field extends out of the waveguide as much as a half of a micrometer.
Optical Resonance
To make these evanescent fields stronger, Optofluidics uses optical resonance. We have designed and patented a series of special features that are incorporated into our waveguides that act as special mirrors to trap light. When the right wavelength of light travels down the waveguide and comes in contact with these features, the light bounces back and forth within them, constructively interfering with itself. This causes the intensity of the evanescent field to increase significantly and is what ultimately allows us to trap very small things.
NanoTweezer Trapping
The use of optical forces to manipulate microparticles dates back to the 1970s. By focusing a laser through a microscope objective, one could create a well into which particles and cells could be trapped. Unfortunately the physics of this type of “free-space” trapping limits the size and type of things that can be trapped. The reason for this is that the force that light can apply to a particle is proportional to the intensity of the light and how sharply it is focused. Traditional optics is limited in both these respects by a combination of diffraction and the amount of power that can be safely applied to the trapped object.
Because light in the evanescent field decays exponentially and the resonators make it very bright the amount of force that can be applied is much higher. This combination is what enables the NanoTweezer technology to get around this classic limitation and enables our users to trap a whole new class of previously inaccessible biological and non-biological particles like cells, viruses and molecules.
In operation, when particles flow over the nanotweezer waveguides they interact with the evanescent field and are pulled down and reversibly immobilized on them. The trapped particle remains immobilized until the light is turned off. To see this in action, have a look at our products page.
The NanoTweezer technology is also unique because of its size dependence of the trapping force. This means that objects smaller than the trapped target are unaffected by the trapping force and behave normally. This enables almost any standard immunoassay to be conducted on the trapped particle.
Key publications
The technology behind Optofluidics, inc’s products have been used in a number of different scientific studies. Below we list a few key papers illustrating the technology from Cornell University.
- Chen Y.-F., Serey, X., Sarkar, R., Chen, P., Erickson, D., “Controlled photonic manipulation of proteins and other nanomaterials” Nano Letters 12 (3), 1633-1637 (2012).
- Erickson, D., Serey, X., Chen, Y.-F., Mandal, S., “Review: Nanomanipulation using Near Field Photonics” Lab-on-a-Chip, 11, 995-1009 (2011)
- Mandal, S., Serey, X., Erickson, D., “Nanomanipulation using Silicon Photonic Crystal Resonators” Nano Letters 10, 99-104 (2010).
- Yang, A.H.J., Moore, S.D., Schmidt, B.S, Klug, M., Lipson, M., Erickson, D., “Optical Manipulation of Nanoparticles and Biomolecules in Sub-Wavelength Slot Waveguides” Nature 457, 71-75 (2009).
