Herstellung und Betrieb eines Nano-Optical Fließband

Capture, interrogation and manipulation of single nanoparticles are of growing importance in nanotechnology. Optical tweezers have become a particularly successful manipulation technique for experiments in molecular biology^1-4, chemistry^5-7 and nano-assembly^7-10, where they have enabled breakthrough experiments such as the measurement of the mechanical properties of single DNA molecules⁴ and the sorting of cells by their optical properties^11,12. Discoveries on these frontiers open up the study of even smaller systems, and they make way for the engineering of new practically beneficial products and techniques. In turn, this trend drives the need for new techniques to manipulate smaller, more rudimentary particles. In addition, there is a push to build 'lab-on-a-chip' devices to perform these functions more cheaply and in a smaller package in order to bring chemical and biological tests out of the lab and into the field for medical and other purposes^13,14.

Unfortunately, conventional optical trapping (COT) cannot meet all of nanotechnology's growing demands. COT operates on the mechanism of using a high numerical aperture (N.A.) objective lens to bring laser light to a tight focus, creating a localized peak in optical intensity and high gradients in the electromagnetic field energy. These energy density gradients exert a net force on light-scattering particles which generally draws them in towards the center of the focus. Trapping smaller particles requires higher optical power or a tighter focus. However, focused beams of light obey the principle of diffraction, which limits the minimal size of the focal spot and places an upper bound on the energy density gradient. This has two immediate consequences: COT cannot trap small objects efficiently, and COT has trouble discriminating between closely spaced particles, a trapping resolution limitation known as the 'fat fingers' problem. In addition, implementing multiple particle trapping with COT requires systems of beam-steering optics or spatial light modulators, components which drastically increase the cost and complexity of an optical trapping system.

One way to circumvent the fundamental limitations of conventional focused beams of light, said to propagate in the far field, is to instead exploit the gradients of optical electromagnetic energy in the near field. The near field decays exponentially away from sources of electromagnetic fields, which means that not only is it highly localized to these sources, but it also exhibits very high gradients in its energy density. The near fields of nano-metallic resonators, such as bowtie apertures, nano pillars, and C-shaped engravings, have been shown to exhibit extraordinary concentrations of electromagnetic energy, further enhanced by the plasmonic action of gold and silver at near-infrared and optical wavelengths. These resonators have been used to trap extremely small particles at high efficiency and resolution^15-22. While this technique has proven effective at trapping small particles, it has also proven to be limited in its ability to transport particles over appreciable range, which is necessary if near-field systems are to interface with far-field systems or microfluidics.

Recently, our group has proposed a solution to this problem. When resonators are placed very close together, a particle can in principle migrate from one near-field optical trap to the next without being released from the surface. The direction of transport can be determined if adjacent traps can be turned on and off separately. A linear array of three or more addressable resonators, in which each resonator is sensitive to a polarization or wavelength of light different from that of its neighbors, works as an optical conveyor belt, transporting nanoparticles over a distance of several microns on a chip.

The so-called 'Nano-Optical Conveyor Belt' (NOCB) is unique among plasmonic resonator trapping schemes, as not only can it hold particles in place, but it can also move them at high speed along patterned tracks, gather or disperse particles, mix and queue them, and even sort them by properties such as their mobility²³. All of these functions are controlled by modulating the polarization or wavelength of illumination, with no need for beam-steering optics. As a near-field optical trap, the NOCB trapping resolution is higher than that of conventional focused-beam optical traps, so it can differentiate between particles in close proximity; because it uses a metal nanostructure to concentrate light into a trapping well, it is power-efficient, and does not require expensive optical components such as a high N.A. objective. Furthermore, many NOCBs may be operated in parallel, at high packing density, on the same substrate, and 1 W of power can drive over 1200 apertures²³.

We have recently demonstrated the first polarization-driven NOCB, smoothly propelling a nanoparticle back and forth along a 4.5 µm track²⁴. In this article we present the steps necessary to design and fabricate the device, optically activate it and reproduce the transport experiment. We hope that making this technique more widely available will help bridge the size gap between microfluidics, far-field optics, and nanoscale devices and experiments.