Trapping of Micro Particles in Nanoplasmonic Optical Lattice

Summary

We describe a procedure to optically trap micro-particles in nanoplasmonic optical lattice.

Abstract

The plasmonic optical tweezer has been developed to overcome the diffraction limits of the conventional far field optical tweezer. Plasmonic optical lattice consists of an array of nanostructures, which exhibit a variety of trapping and transport behaviors. We report the experimental procedures to trap micro-particles in a simple square nanoplasmonic optical lattice. We also describe the optical setup and the nanofabrication of a nanoplasmonic array. The optical potential is created by illuminating an array of gold nanodiscs with a Gaussian beam of 980 nm wavelength, and exciting plasmon resonance. The motion of particles is monitored by fluorescence imaging. A scheme to suppress photothermal convection is also described to increase usable optical power for optimal trapping. Suppression of convection is achieved by cooling the sample to a low temperature, and utilizing the near-zero thermal expansion coefficient of a water medium. Both single particle transport and multiple particle trapping are reported here.

Introduction

The optical trapping of micro-scale particles was originally developed by Arthur Askin in the early 1970s. Ever since its invention, the technique has been developed as a versatile tool for micro- and nanomanipulation^1,2. Conventional optical trapping based on the far-field focusing principle is inherently limited by the diffraction in its spatial confinement, wherein the trapping force decreases dramatically (following an ~a³ law for a particle of radius a)³. To overcome such diffraction limits, researchers have developed near-field optical trapping techniq....

Protocol

1. Optical Setup

Note: The principle of the optical setup is illustrated in Figure 1.

1. Set up the optical tweezer kit (see the Table of Materials) and the fluorescence module (see Table of Materials) as per their manuals. Connect a 470 nm blue light emitting diode (LED) light source to the fluorescent module.
2. Replace the high numerical aperture (NA) (NA= 1.25, magnification 100x) oil immersion objective by a long working distance (WD) microscope objective (focal length 3.6 mm, WD=10.6 mm, NA=0.5).
3. Remove the lens in the beam expansion section of t....

Results

Single particle trajectories were recorded by a CCD camera in our experiment and the images were then processed with a custom program to extract each particle's trajectory¹⁶. Representative results are displayed in Figure 3 and Video 1 for micro-spheres with diameters of 2 µm. Multiple particle trappings inside the optical lattice were observed. Successive images extracted from a representative motion video of the part.......

Discussion

The procedure described here enables the reader to reliably reproduce trapping on a daily basis. A general empirical guideline to design a usable optical lattice is to use a comparable size for plasmonic nanoarray, interdisc distance, and trapped particle size. Compared to a single, isolated plasmonic nanostructure, the optical lattice design in conjunction with the high optical power afforded by cooling the sample to ~4 °C used here greatly enhances the trapping probability. If well separated, plasmonic nanostructu.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Thermoelectric cooling element	Thorlabs	TEC 1.4-6	TEC element for sample cooling
RTD thermometer	Omega Engineering	RTD Thermometer 969C
Forward looking infrared camera	FLIR	FLIR One	IR camera for temperature monitoring
light emitting diode light source	Touchbright		Light source for illumination for fluorescent imaging
Long working distance objective	Olympus	LMPLFLN	For illuminating the sample and imaging
Optical trap kit	Thorlabs	OTKB/M
Cover slip			thickness 0.17 mm
Scanning electron microscope	Hitachi	SEM-Hitachi S3400N
Electron beam blanker	DEBEN	PCD beam blanker	the blanker is added to the scanning electron microscope
Thermal evaporator	SYSKEY Technology
Mask aligner	Karl Suss	MJB 3	For marker fabrication
Electron beam resist	Sigma Alrich	PMMA 120K	For e-beam lithography
Electron beam resist	Sigma Alrich	PMMA 960K	For e-beam lithography
Fluoresent labeled polystyrene microspheres	Polyscience		2 um diameter
Bipolar transistor	Mouser	2N3904	quantity 2 for TEC driver circuit
Bipolar transistor	Mouser	2N3906	quantity 2 for TEC driver circuit
MOSFET power transistor	Mouser	IRF5305	quantity 2 for TEC driver circuit
MOSFET power transistor	Mouser	IRF131ON	quantity 2 for TEC driver circuit
10 kOhm resistor	Mouser		quantity 6 for TEC driver circuit
910 Ohm resistor	Mouser		quantity 2 for TEC driver circuit
Photoresist	Microchemicals	AZ4620	For marker fabrication
Acetone	Sigma Alrich		For marker fabrication
Fluorescence Module for the OTKB/M, Metric Threads	Thorlabs	OTKB-FL/M
Fluorescent filter set	Thorlabs	MDF-FITC	For Fluorescein Isothiocyanate (FITC)
Ultrasonic cleaner	Delta	DC150H	For the lift off step