Name: fabrication of polymer microspheres for optical resonator and laser applications
Uploaded: 2017-06-02T15:00:00
Description: Watch this Scientific Journal Video about Fabrication of Polymer Microspheres for Optical Resonator and Laser Applications at JoVE.com

This work was partly supported by KAKENHI (25708020, 15K13812, 15H00860, 15H00986, 16H02081) from JSPS/MEXT Japan, the Asahi Glass Foundation, and the University of Tsukuba Pre-strategic initiative, "Ensemble of light with matters and life."

1. Fabrication Protocols of Polymer Microspheres
<ol>
	<li>Vapor Diffusion Method
	<ol>
		<li>Dissolve 2 mg of conjugated polymers, such as P1 (poly[(9,9-dioctylfluorene-2,7-diyl)-alt-(5-octylthieno[3,4-c]pyrrole-4,6-dione-1,3-diyl)])28 and P2 (poly[(N-(2-heptylundecyl)carbazole-2,7-diyl)-alt-(4,8-bis[(dodecyl)carbonyl]benzo[1,2-b:4,5-b&#8242;]dithiophene-2,6-diyl)])28, in 2 mL of chloroform (a good solvent) in...

     

<ol>
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C. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Colloidal+Crystal+Lasers+from+Monodisperse+Conjugated+Polymer+Particles+via+Bottom-Up+Coassembly+in+a+Sol-Gel+Matrix.">Colloidal Crystal Lasers from Monodisperse Conjugated Polymer Particles via Bottom-Up Coassembly in a Sol-Gel Matrix.</a> ACS Nano. 10, 10195-10201 (2016).</li><li>Oraevsky, A. N. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Whispering-Gallery+Waves.">Whispering-Gallery Waves.</a> Quant Electron. 32 (5), 377-400 (2002).</li><li>Yamamoto, Y. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Spherical+Resonators+from+&#960;-Conjugated+Polymers.">Spherical Resonators from &#960;-Conjugated Polymers.</a> Polym. J. 48, 1045-1050 (2016).</li><li>Adachi, T., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Spherical+Assemblies+from+&#960;+-Conjugated+Alternating+Copolymers:+Toward+Optoelectronic+Colloidal+Crystals.">Spherical Assemblies from &#960; -Conjugated Alternating Copolymers: Toward Optoelectronic Colloidal Crystals.</a> J. Am. Chem. Soc. 135, 870-876 (2013).</li><li>Tong, L., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Tetramethylbithiophene+in+&#960;-Conjugated+Alternating+Copolymers+as+an+Effective+Structural+Component+for+the+Formation+of+Spherical+Assemblies.">Tetramethylbithiophene in &#960;-Conjugated Alternating Copolymers as an Effective Structural Component for the Formation of Spherical Assemblies.</a> Polym. 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The selection of a good solvent and non-solvent is very important for the self-assembly of well-defined microspheres. If the solubility of a polymer is too high, precipitation will not occur. Also, in general, &#960;-conjugated polymers are hydrophobic, so polar non-solvents, such as MeOH, acetonitrile, and acetone, are often used in the vapor diffusion method to minimize the surface energy required to form a spherical shape. The interface precipitation method is often adopted for the preparation of dye-doped polymer mic...

Polymer nano/micro-sized particles are widely used for a variety of applications, including as catalyst support, column chromatography fillers, drug delivery agents, fluorescent probes for cell tracking, optical media, and so forth1,2,3,4,5,6,7,8,9. In particular, &#960;-conjugated polymers have inherent luminescent and charge conducting properties that are beneficial to optical, electronic, and optoelectronic applications using polymer spheres10,11,12,13,14, especially laser applications using soft organic materials15,16,17. For example, the three-dimensional integration of spheres with several hundred nanometer diameters forms colloidal crystals, which show photonic band gaps at a certain wavelength18,19. When light is confined in the intersphere periodic structure, lasing action appears at the middle of the stop band. On the other hand, when the size of the spheres increases to the several-micrometer scale, light is confined inside a single microsphere via total internal reflection at the polymer/air interface20. Propagation of the light wave at the maximum circumference results in interference, leading to the appearance of a resonant mode with sharp and periodic emission lines. These optical modes are so-called &#34;whispering gallery modes&#34; (WGMs). The term &#34;whispering gallery&#34; originated from St. Paul&#39;s Cathedral in London, where sound waves propagate along the circumference of the wall, allowing whispers to be heard by a person on the other side of the gallery. Because the wavelength of light is on the sub-micrometer scale, which is far smaller than sound waves, such a large dome is not necessary for the WGM of light: tiny, micrometer-scale, well-defined vessels, such as microspheres, microdiscs, and microcrystals, fulfill the WGM conditions.
Equation 1 is a simple form of the WGM resonating condition21:
n&#960;d = l&#955;&#160;&#160;&#160;&#160;&#160; (1)
where n is the refractive index of the resonator, d is the diameter, l is the integer number, and &#955; is the wavelength of the light. The left part of (1) is the optical path length through one circle propagation. When the optical path coincides with the integer multiple of the wavelength, resonance occurs, while at the other wavelength, the light wave is diminished upon rounding.
This paper introduces several experimental methods to prepare microspheres for WGM resonators from conjugated polymers in solution: vapor diffusion22,23,24,25,26,27,28,29,30, mini-emulsion31, and interface precipitation32. Each method has unique characteristics; for example, the vapor diffusion method affords well-defined microspheres with very high sphericity and smooth surfaces, but only low-crystallinity polymers can form these microspheres. On the other hand, for the mini-emulsion method, various kinds of conjugated polymers, including high-crystalline polymers, can form spheres, but the surface morphology is inferior to that obtained from the vapor diffusion method. The interface precipitation method is preferable for creating microspheres from dye-doped, non-conjugated polymers. In all cases, the selection of the solvent and the non-solvent plays an important role in the formation of spherical morphology.
In the second half of this paper, &#181;-PL and micro-manipulation techniques are presented. For the &#181;-PL technique, microspheres are dispersed on a substrate, and a focused laser beam, through a microscope lens, is used to irradiate a single isolated microsphere24. The generated PL from a sphere is detected by a spectrometer through the microscope lens. Moving the sample stage can vary the position of the excitation spot. The detection point is also variable by tilting the collimator optics of the excitation laser beam with respect to the optical axis of the detection path28,32. To investigate intersphere light propagation and wavelength conversion, the micro-manipulation technique can be used32. To connect several microspheres with different optical properties, it is possible to pick up one sphere using a micro-needle and put it on another sphere. In conjunction with the micromanipulation techniques and the &#181;-PL method, various optical measurements can be carried out using conjugated polymer spheres, which are prepared by a simple self-assembly method. This video paper will be useful to readers who wish to use soft polymer materials for optical applications.

<table border="1" fo:keep-together.within-page="1" fo:keep-with-next.within-page="always">
	<tbody>
		<tr>
			<td>polystyrene</td>
			<td>Aldrich</td>
			<td>132427-25G</td>
			<td></td>
		</tr>
		<tr>
			<td>sodium dodecylsulfate</td>
			<td>Kanto Kagaku</td>
			<td>372035-31</td>
			<td></td>
		</tr>
		<tr>
			<td>tetrahydrofuran</td>
			<td>Wako</td>
			<td>206-08744</td>
			<td></td>
		</tr>
		<tr>
			<td>chloroform</td>
			<td>Wako</td>
			<td>038-18495</td>
			<td></td>
		</tr>
		<tr>
			<td>methanol</td>
			<td>Wako</td>
			<td>139-13995</td>
			<td></td>
		</tr>
		<tr>
			<td>Poly(9,9-di-n-octylfluorenyl-2,7-diyl)</td>
			<td>Aldrich</td>
			<td>571652-500MG</td>
			<td></td>
		</tr>
		<tr>
			<td>Poly[2-methoxy-5-(3&#8242;,7&#8242;-dimethyloctyloxy)-1,4-phenylenevinylene] (MDMOPPV)</td>
			<td>Aldrich</td>
			<td>546461-1G</td>
			<td></td>
		</tr>
		<tr>
			<td>poly[(9,9-dioctylfluorene-2,7-diyl)-alt-(5-octylthieno[3,4-c]pyrrole-4,6-dione-1,3-diyl)] (P1)</td>
			<td>synthesized</td>
			<td>-</td>
			<td>reference 28</td>
		</tr>
		<tr>
			<td>poly[(N-(2-heptylundecyl)carbazole-2,7-diyl)-alt-(4,8-bis[(dodecyl)carbonyl]benzo[1,2-b:4,5-b&#8242;]dithiophene-2,6-diyl)] (P2)</td>
			<td>synthesized</td>
			<td>-</td>
			<td>reference 28</td>
		</tr>
		<tr>
			<td>fluorescent dye (boron dipyrrin; BODIPY)</td>
			<td>synthesized</td>
			<td>-</td>
			<td>reference 32</td>
		</tr>
		<tr>
			<td>Optical Microscope</td>
			<td>Nicon</td>
			<td>Eclipse LV-N</td>
			<td></td>
		</tr>
		<tr>
			<td>laser_405 nm</td>
			<td>Hutech</td>
			<td>DH405-10-5</td>
			<td></td>
		</tr>
		<tr>
			<td>laser_355 nm</td>
			<td>CNI</td>
			<td>MPL-F-355-10&#65357;W</td>
			<td></td>
		</tr>
		<tr>
			<td>Spectrometer</td>
			<td>Lambda Vision</td>
			<td>LV-MC3/T</td>
			<td></td>
		</tr>
		<tr>
			<td>Homogenizer</td>
			<td>Microtech Nichion</td>
			<td>Physcotron NS-360D</td>
			<td></td>
		</tr>
		<tr>
			<td>micromanipulation</td>
			<td>Microsupport</td>
			<td>Quick Pro QP-3RH</td>
			<td></td>
		</tr>
	</tbody>
</table>

fabrication of polymer microspheres for optical resonator and laser applications

This paper describes three methods of preparing fluorescent microspheres comprising &#960;-conjugated or non-conjugated polymers: vapor diffusion, interface precipitation, and mini-emulsion. In all methods, well-defined, micrometer-sized spheres are obtained from a self-assembling process in solution. The vapor diffusion method can result in spheres with the highest sphericity and surface smoothness, yet the types of the polymers able to form these spheres are limited. On the other hand, in the mini-emulsion method, microspheres can be made from various types of polymers, even from highly crystalline polymers with coplanar, &#960;-conjugated backbones. The photoluminescent (PL) properties from single isolated microspheres are unusual: the PL is confined inside the spheres, propagates at the circumference of the spheres via the total internal reflection at the polymer/air interface, and self-interferes to show sharp and periodic resonant PL lines. These resonating modes are so-called "whispering gallery modes" (WGMs). This work demonstrates how to measure WGM PL from single isolated spheres using the micro-photoluminescence (&#181;-PL) technique. In this technique, a focused laser beam irradiates a single microsphere, and the luminescence is detected by a spectrometer. A micromanipulation technique is then used to connect the microspheres one by one and to demonstrate the intersphere PL propagation and color conversion from coupled microspheres upon excitation at the perimeter of one sphere and detection of PL from the other microsphere. These techniques, &#181;-PL and micromanipulation, are useful for experiments on micro-optic application using polymer materials.

Figure 1 shows schematic representations of the vapor diffusion method (a), mini-emulsion method (b), and interface precipitation method (c). For the vapor diffusion method (Figure 1a), a 5 mL vial containing a CHCl3 solution of polymers (0.5 mg mL-1, 2 mL) was placed in a 50 mL vial containing 5 mL of a non-solvent, such as MeOH. The outside vial was capped and then allowed to stand for 3 days at 25 &#176;C...

Watch this Scientific Journal Video about Fabrication of Polymer Microspheres for Optical Resonator and Laser Applications at JoVE.com

Fabrication of Polymer Microspheres for Optical Resonator and Laser Applications

Protocols for the synthesis of microspheres from polymers, the manipulation of microspheres, and micro-photoluminescence measurements are presented.

This paper describes three methods of preparing fluorescent microspheres comprising &#960;-conjugated or non-conjugated polymers: vapor diffusion, ...

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.

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