Fabrication and Operation of a Nano-Optical Conveyor Belt

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.....

Protocol

1. Design the C-shaped Engraving (CSE) Array

1. Design the array pattern.

Figure 1. CSE Layout. Depiction of conveyor belt repeating element. Successful transport has been achieved using d_y = 320 nm and d_x = 360 nm. Adjacent pairs of engravings have a 60º relative rotational offset.

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’s position .......

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

List of materials used in this article

Name	Company	Catalog Number	Comments
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