Herstellung und Betrieb eines Nano-Optical Fließband

Summary

The scalability and resolution of conventional optical manipulation techniques are limited by diffraction. We circumvent the diffraction limit and describe a method of optically transporting nanoparticles across a chip using a gold surface patterned with a path of closely spaced C-shaped plasmonic resonators.

Abstract

The technique of using focused laser beams to trap and exert forces on small particles has enabled many pivotal discoveries in the nanoscale biological and physical sciences over the past few decades. The progress made in this field invites further study of even smaller systems and at a larger scale, with tools that could be distributed more easily and made more widely available. Unfortunately, the fundamental laws of diffraction limit the minimum size of the focal spot of a laser beam, which makes particles smaller than a half-wavelength in diameter hard to trap and generally prevents an operator from discriminating between particles which are closer together than one half-wavelength. This precludes the optical manipulation of many closely-spaced nanoparticles and limits the resolution of optical-mechanical systems. Furthermore, manipulation using focused beams requires beam-forming or steering optics, which can be very bulky and expensive. To address these limitations in the system scalability of conventional optical trapping our lab has devised an alternative technique which utilizes near-field optics to move particles across a chip. Instead of focusing laser beams in the far-field, the optical near field of plasmonic resonators produces the necessary local optical intensity enhancement to overcome the restrictions of diffraction and manipulate particles at higher resolution. Closely-spaced resonators produce strong optical traps which can be addressed to mediate the hand-off of particles from one to the next in a conveyor-belt-like fashion. Here, we describe how to design and produce a conveyor belt using a gold surface patterned with plasmonic C-shaped resonators and how to operate it with polarized laser light to achieve super-resolution nanoparticle manipulation and transport. The nano-optical conveyor belt chip can be produced using lithography techniques and easily packaged and distributed.

Introduction

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.

Protocol

1. Design the C-shaped Engraving (CSE) Array Design the array pattern. Figure 1. CSE Layout. Depiction of conveyor belt repeating element. Successful transport has been achieved using dy = 320 nm and dx = 360 nm. Adjacent pairs of engravings have a 60º relative rotational offset. <a href="https://www.jove.com/files/ftp_up…

Representative Results

Figure 7 is a picture of the final device. At the center of the 1 cm x 1 cm gold surface is the matrix of CSE and conveyor patterns, which can be barely seen from an angled view. Figure 6 is a scanning electron microscopy image of an example CSE pattern on the final device. The particle motion of a 390 nm polystyrene bead traveling across a nano-optical conveyor belt 5 µm in length is shown in Figure 9. The curve shows the particle’…

Discussion

The NOCB combines the strong trapping forces and small trap size of plasmonic approaches with the capability to transport particles, long available only for conventional focused-beam techniques. Unique to the NOCB, the trapping and transport properties of the system are a result of surface patterning and not of shaping the illumination beam. Provided the illumination is bright enough and its polarization or wavelength can be modulated, particles can be held or moved in complicated protocols on the surface. We have demons…

Materials

HSQ e-beam resist	Dow Corning	XR-1541-006
PMMA	MicroChem	950A2 M230002
Fast curing optical adhesive	Norland Optical Adhesive	NOA 81
Fluorescent carboxyl microspheres	Bangs Laboratories	FC02F, FC03F
Fluorescent carboxylate-modified microspheres	Molecular Probes	F-8888
Quartz slide	SPI Supplies	1020-AB
Inverted fluorescent microscope	Nikon	ECLIPSE TE2000-U
Nd:YAG laser	Lightwave Electronics	221-HD-V04
sCMOS camera	PCO	EDGE55
CCD camera	Watec	WAT-120N
Zero-order half-wave plate	Thorlabs	WPH05M-1064
Triton X-100	Sigma-Aldrich	T8787
Distilled water	Invitrogen	10977-023
Si Wafer	Silicon Quest International	708069
Optical lenses	Thorlabs