The overall goal of this experiment is to demonstrate how to trap microparticles with plasmonic optical lattices with a technique that suppresses photothermal convection. The main feature of this technique is that we use the array of plasmonic nanostructures to enhance the trapping efficiency. Also we use a unique brewing property of water, 0%thermal expansion coefficient at low temperature to suppress the photothermal convection.
Demonstrating this procedure will be Dinesh Bhalothia, a grad student from my laboratory. The setup for the experiment builds on a optical tweezer kit. This modified optical tweezer kit with a florescence module is ready on a breadboard.
An LED and a diode laser provide light for florescence and manipulation. Mirrors direct light through an objective lens. The objective focuses the light on a sample stage which also serves as a heat sink.
A CCD camera captures images from the sample. These elements are more evident in this schematic. A 470 nanometer blue LED is the light source for the florescence module.
A 980 nanometer laser diode provides laser light in loose focus for manipulation. The lens is a long working distance microscope objective. Activity on the sample stage is recorded by a CCD camera.
Turn on the power supply and current for the 980 nanometer laser diode. Use the CCD camera to check the laser beam alignment. If the beam is well aligned, the camera image will be a Gaussian spot.
Turn off the laser for the next steps. An important aspect of the setup is the cooling system. The sample stage is a heat sink designed to accommodate a thermoelectric cooler.
Work with the system electronics to prepare to add the thermoelectric cooler. There is a custom driver circuit for this experiment. Make the connections between the driver circuit and the electronic control board.
Next, get a thermoelectric cooling element that will fit in the sample stage and with a hole in it to allow the laser beam through. Connect the output of the driver circuit to the thermoelectric cooling element. Move the cooling element to the sample stage.
Before continuing, connect the driver circuit to a five volt power supply. To monitor the temperature, use a forward-looking infrared camera, and check that the system is cooling properly before proceeding. For this, use a resistance thermometer unit and its sensor.
Begin with the sensor on a glass cover slip. Transfer the sensor and cover slip to the sample stage. Apply a small amount of thermal paste to ensure thermal contact.
There, put the the assembly in contact with the stage. At the controls, adjust the power to the thermoelectric cooling element. After three minutes, read the temperature using the resistance temperature detector thermometer.
In addition, record the temperature with the forward-looking infrared camera. Repeat these two measurements at various output power settings to obtain a temperature calibration curve similar to this one. Calibration is essential.
Before proceeding, shut off the power to the thermoelectric cooler. The final element of the setup is the nanoplasmonic array. The slide cover has a custom fabricated array and is ready to mount in the experiment.
This scanning electron microscope image of the array provides more detail. It is an array of about 16 micrometers square of 22 by 22 gold nanodiscs. Each nanodisc has a thickness of 40 nanometers and a diameter of 550 nanometers.
The center to center distance between discs is 750 nanometers. Put the slide cover with the nanoplasmonic array on the sample stage. It should be in contact with the thermoelectric cooler.
Next, set up the light source. Turn on the florescent light source, and set the power to five milliwatts for bright field imaging. Monitor the CCD image while manipulating the slide.
Use the marker on the slide to locate and align the array. Make sure the array is in the center of the region of interest on the computer screen. Now, turn to the sample for the experiment.
It is a mixture of two micrometer diameter polystyrene particles in deionized water. Use a micropipette to dispense 10 microliters onto the nanoarray slide. Move to the current supply of the 980 nanometer laser diode.
Turn it on to excite the plasmonic resonance of the array. Then work with the power supply for the cooling system to achieve a temperature of four degrees Celsius. Finally, start recording video of the microparticles with the CCD camera.
This is an example of the video recorded during an experiment with two micrometer polystyrene spheres. The optical power of the 980 nanometer wavelength laser used for plasmonic resonance excitation is five milliwatts. Note the particles cluster in a hexagonal, close-packed structure.
These still images are of the accumulated trapped microparticles over time. Again, the hexagonal close-pack structure is clear. Images such as these can provide data to produce a plot of the number of trapped microparticles as a function of time.
These five colored curves are examples of microparticle trajectories that can be extracted from the recorded video using image processing techniques, and the centroid algothrithm. The scale bar represents two micrometers. The procedure reported here enables a researcher to reproduce trapping on a daily basis.
While attempting this procedure, it's important to remember to use the infrared camera to monitor the sample temperature to avoid the sample breakage. After its development, this technique pave the way for researchers in the field of optical trapping to study a larger class of transport phenomena in plasmonic optical lattice. After watching this video, you should have a good understanding of how to do the optical trapping with the plasmonic optical lattice,
