The overall goal of the following experiment is to transport submicron dielectric particles using the optical near field of a linear array of addressable plasmonic resonators, also known as a nano optical conveyor belt. This is achieved by first designing the conveyor belt with software to ensure successful transport along the desired path. As a second step, fabricate the conveyor belt using an electron beam lithography process and a template stripping technique.
Next, trap a sample on the end of the conveyor belt using a laser and rotate a half wave plate in the beam path to induce transport of the particle. The results show linear transport across the width of the laser beam based on rotating the angle of polarization and not by changing the beam intensity profile in any way. The main advantage of this technique over conventional optical tweezers is that transport behavior and resolution depend on structures fabricated using lithography and not on beam steering.
This technique can be applied to on-chip biological analysis and nanoscale manufacturing. We first had the idea for this method when we looked at uses for the optical nearfield produced by a sea aperture Demonstrating the procedure will be Tiffany Wong, a graduate student in our laboratory. To begin, use a CAD program to generate a double linear array of CS shaped polygons along a linear path.
Design the polygon in each pair so that they are consecutively rotated 30 plus or minus 90 degrees about its convex hull. Leave no more than one particle diameter separating consecutive pairs and leave no more than 90%of this distance between polygon centers in a pair. Next, draw up a numerical method geometry, which accommodates the planar pattern dimensions and extends at least 200 nanometers below and 600 nanometers above the pattern.
Plane below the plane include a domain to represent the substrate and above the plane, A domain to represent the fluid chamber, extrude the planer CS shaped pattern 150 nanometers downwards into the substrate, creating 3D domains to represent the interior of the engravings. Then introduce a particle domain with a desired shape. In this particular example, we introduce a sphere with a diameter of 400 nanometers.
Next, add perfectly matched layers at least 500 nanometers in thickness to the open boundaries of the simulation. To absorb outward radiation, set the electromagnetic material properties of the domain above the interface to those of water, the properties of the interior of the C-sharp engravings to those of hydrogen esque ine, and the properties of the remaining material to those of gold. Finally set the material properties of the particle to those of polystyrene.
Next discretized. The simulation volume with an adaptive tetrahedral mesh no larger than 100 nanometers in the bulk. Furthermore, constrain the maximum size of the mesh elements to 30 nanometers on the sphere surface, and 30 nanometers on the engraving surfaces.
To increase accuracy on critical structures for optical excitation, define a background harmonic plane wave with a free space wavelength of 1064 nanometers. Choose the polarization of this wave such that the electric field is aligned with the ridge of a C-sharp engraving. Solve for the scattered electromagnetic fields in a batch of simulations, sweeping the particle position parameter from one end of the path to the other while holding the particle's altitude constant at just a few nanometers off the surface.
Repeat this computation for incident polarization aligned with each of the other CS shape orientations and obtain solutions for the scattered electromagnetic fields at those angles. Verify that the seas aligned with the polarization light up most brightly. Next, calculate the electromagnetic net force on the particle at each position by integrating the Maxwell stress tensor over a surface and closing the particle.
Follow the procedure outlined in the text to ensure that the particle will be both stably trapped and successfully handed off along the conveyor. As the polarization rotates. Begin with a clean polished silicon wafer.
Apply 20 to 25 drops of a 2%polymethyl methacrylate. Resist in anol solution and spin it into a thin film at 5, 000 RPMs for 40 seconds. Post bake the PMMA resist on a hot plate at 200 degrees Celsius for two minutes.
Then change the spin speed to 900 RPMs and the spin duration to one minute. Add 20 to 25 drops of hydrogen squi INE negative tone. Resist and spin the resist into a film 150 nanometers in thickness post bake the HSQ resist on a hot plate at 80 degrees Celsius for two minutes.
Next, prepare for electron beam lithography by mounting the wafer to the wafer holder. Load the wafer holder into the load lock of the electron beam lithography exposure tool and start the vacuum pump Set electron beam parameters to 100 kilovolts accelerating voltage based dose of 800 micro coomes per centimeter squared. And once a vacuum has been established and beam calibrations are complete, begin exposure of the designed array.
Develop the exposed negative tone. Resist by submerging the wafer in a 2.2%tetraethyl ammonium hydroxide developer solution for 90 seconds. During development, gently agitate the solution by jostling the developer dish every 10 seconds after the development time has passed, immediately stop the development by flushing the surface with water for 60 seconds.
Then use magnatron sputtering to add a layer of gold, 200 nanometers thick, followed by a 1000 nanometer thick layer of copper. Next, spread a single drop of UV curable epoxy onto the copper side of the sample in a one centimeter by one centimeter square covering the pattern device area. Then apply a quartz glass backplate to the copper surface, making sure that it completely covers the pattern device area.
Rest the backplate and wafer on a level surface and illuminate the epoxy from above with a UV flood lamp for approximately 30 minutes to cure the sample. Then release the device from the silicon substrate by scoring around the quartz backplate all the way to the silicon wafer and immerse the substrate in an acetone bath. After waiting six to eight hours, remove the wafer from the acetone bath and carefully use a razor to pry the chip away from the silicon substrate.
Clean the surface of the chip with isopropyl alcohol. After calibrating the system as described in the accompanying text protocol, place two to four microliters of diluted fluorescent particles onto a clean cover slip and carefully place the device on top of the solution with the gold surface facing down. Focus on the nano structures by bringing the dark alignment marks into focus.
Then insert a narrow band pass filter in front of the mercury lamp, which blocks all colors other than that corresponding to the fluorescent beads absorption peak. Next, insert a narrow band pass filter in front of the specimen imaging camera, which blocks all colors other than that corresponding to the fluorescent beads emission peak. Bring the fluorescent image of the beads into focus and wait until the beads average drift velocity slows to less than 10 micrometers per second.
Then turn on the focused laser beam and gradually increase the laser output power until a drifting bead can be captured stably at the beam focus. Scanning the microscope stage can assist in trapping a bead, which is off center. Next, adjust the beam contractor built into the beam path until the beam spot in the specimen plane has expanded to nine microns in diameter.
When it is fully in focus, measure this as a straight intensity cross-section through the center of the beam spot in the beam image with a bead trapped by the loosely focused beam. Use the microscope stage to move the substrate pattern so that the end of an array of resonators can be seen directly behind the trapped bead. Once near field trapping has been established, move the microscope translation stage slightly so that the center of the laser spot resides more near to the center of the conveyor.
After displacing the beam, slightly rotate a halfway plate placed in the laser beam path to rotate the angle of linear polarization. This will activate resonators in a sequence down the array and induces controlled linear motion in the fluorescent bead. When a particle reaches the edge of the laser beam, it will jump back to a previous location.
Shown here is a scanning electron microscope image of a simple pair of C-sharp engravings. After resist development. After dissolving away the PMMA and removing the silicon substrate, the entirety of the final pattern is revealed.
The light circle in this image shows the location of a 390 nanometer diameter polystyrene bead as it travels across a nano optical conveyor belt five microns in length. The bead is moved by the rotation of the light polarization causing the nano metallic CS shaped resonators to trap the bead in a stepwise fashion across the length of the conveyor belt Methods to better shape the overhead illumination over the conveyor belt, such as with phase plates or beam steering can improve conveyor belt performance. After watching this video, you should have a good understanding of how to design, fabricate, and operate a nano optical conveyor belt.
Don't forget that working with high powered lasers can be extremely hazardous and laser safety goggles should always be worn while performing this procedure.
