Fabrication of Polymer Microspheres for Optical Resonator and Laser Applications

The overall goal of this procedure is to produce optical microresonators for laser devices, optical circuits, and chemical and biological optical sensors. This method can help answer key questions in the functional polymer material sphere such as polymer optics, optoelectronics, and electronics. The main advantage of this simple and low energy consumption technique is that it uses functional polymers to fabricate optical resonators which have both optical and electrical properties.

Demonstrating the procedure will be Mr.Okada, Mr.Zakarias, and Mr.Oki, graduate students from my laboratory. To begin the vapor diffusion method for this procedure, add five milliliters of methanol to a 50 milliliter vial. Next, dissolve two milligrams of conjugated polymers in two milliliters of chloroform in a five milliliter vial.

Carefully place the five milliliter vial inside the 50 milliliter vial. After this, cap the 50 milliliter vial. Incubate at 25 degrees Celsius for three days to precipitate the polymer microspheres.

To begin the mini-emulsion method, dissolve five milligrams of conjugated polymers in one milliliter of chloroform. Then, dissolve 30 milligrams of SDS in two milliliters of deionized water. Add 100 microliters of the polymer mixture to the SDS solution.

Using an ultra high speed homogenizer, stir for two minutes at 30, 000 RPM. Allow the emulsion to rest for one day without the cap to evaporate the chloroform. The next day, centrifuge the dispersion in a 1.5 milliliter centrifuge tube for five minutes at 2, 200 times g.

After removing the supernatant, add one milliliter of deionized water and shake vigorously. Repeat the centrifugation, supernatant removal, and water wash three times to completely remove any residual SDS. To begin the interface precipitation method, dissolve 20 milligrams of polystyrene in one milligram of fluorescent dye in 20 milliliters of tetrahydrofuran.

Add one milliliter of deionized water to an empty 1.5 milliliter microtube. Then, gently pour 0.2 milliliters of the THF solution onto the water layer, being careful to keep the layers from mixing together. Allow the vial to rest for six hours without the cap to precipitate the polymer microspheres.

To begin, dilute a suspension of prepared microspheres by 10 times with a poor solvent such methanol or deionized water. Using a spin coater, spin cast one drop of the diluted suspension onto a quartz substrate. Air dry the resulting casted film until the solvents have evaporated completely.

Then, transfer the quartz substrate to the sample stage of an optical microscope. Identify an isolated and well-defined microsphere for micro-photoluminescence measurement. Set the laser conditions as outlined in the text protocol and use a focused laser beam to irradiate the microsphere.

Using a spectrometer, record the photoluminescence spectrum. Next, take a fluorescent image of the irradiated microsphere. To begin, place the quartz substrate onto the sample stage of an optical microscope.

Identify a single well-define microsphere. Then, set a tungsten microneedle on a micromanipulation apparatus. Using a touch panel controller, move the microneedle and pick up the microsphere.

Connect the collected microsphere to another single well-defined microsphere. After this, irradiate the microsphere with a focused laser beam. Use a spectrometer to record the photoluminescence spectrum.

Then, take a fluorescent image of the irradiated microspheres. In this study, well-defined polymer microspheres are fabricated using three different methods. The vapor diffusion method, the mini-emulsion method, and the interface precipitation method.

Scanning electron microscopy shows that the vapor diffusion and interface precipitation methods produce microspheres with very smooth surface morphology. However, those obtained from the mini-emulsion method are rough because the surfactant covers the whole surface. The whispering gallery mode’s photoluminescence is then recorded for a single microsphere of polymer one, polymer two, and their blend.

The Q factor was seen to reach as 2, 200 for polymer one. Microspheres composed of polymer two however only reached as high as 300, possibly because of their rough surface morphology. Microspheres composed of the polymer blend maintained a high Q factor of 1, 500 likely because of its smooth morphology.

Polymorphic polaron dye-doped polystyrene microspheres with photoluminescent colors of green, yellow, orange, and red were connected forming tetraspheres to investigate the intersphere energy transfer cascade. The light energy transfer from green to yellow and from yellow to orange is seen to be efficient while the energy transfer from orange to red did not occur because of the small overlap between the photoluminescent band of the energy donor and the adsorption band of the energy acceptor. Therefore, upconverted energy transfers such as from red to orange, yellow, and green hardly occur.

While attempting this procedure, it is important to remember to avoid shaking the vials which may cause a rapid precipitation leading to ill-defined aggregates. Following this procedure, other method like emulsion polymerization and dispersion polymerization methods can be performed in order to answer additional questions like how to prepare monodisperse polymer spheres for application to colloidal photonic crystals. After its development, this technique paved the way for researchers in the field of polymer science to explore optical and laser applications using functional polymers.

After watching this video, you should have a good understanding of how to prepare microoptical resonators by bottom up self-assembly process. Don’t forget that working with laser devices have to take special care to protect from laser beams that harms your eyes.