Fabrication and Testing of Microfluidic Optomechanical Oscillators

The overall goal of this procedure is to fabricate and demonstrate optomechanical oscillators that operate with fluid phase media. This is accomplished by first fabricating microfluidic optomechanical resonators through heating and drawing of fused silica glass capillary preforms under CO2 laser illumination. The second step is to mount the fabricated devices for testing.

Next, a diode laser is used to pump optical whispering gallery modes of the fabricated device by means of a tapered optical fiber. Ultimately, electronic spectrum analysis of the output optical signals is used to show optomechanical interaction and sensing through mechanical modes in fluids. These devices provide a previously unavailable capability of performing opt mechanics experiments with fluid phase materials.

Generally, individuals new to this method will struggle cause fabrication, process, need, practice, and fabrication parameters must be optimized. Demonstration of this method is critical as the actuation and the identification of optomechanical mode is difficult to master. The microfluidic resonators will be fabricated by heating and drawing glass capillary preforms to accomplish this first align.

Two software controllable linear translation stages. So they move along the same line. They should be separated by a few centimeters so that the capillary can be extended across the gap.

Sample holders on the stages should be aligned to hold a capillary on axis. Along the line of motion of the stages, the heating will be done by two CO2 lasers before proceeding. Take appropriate safety precautions for working with high power lasers.

Then direct the output of the lasers such that they target the same spot in the gap between the linear stages. This spot should be at a point where a capillary will be suspended. Make use of beam blocks to stop the beams after the target spot.

Use software for the simultaneous control of the two translation stages and the power levels of the two CO2 lasers. For this setup. Reliable capillaries are produced when one stage moves fast, about 10 millimeters per second, and the other slow, about 0.5 millimeters per second in the same direction to feed in more material to the heating zone.

Both lasers are set at 4.5 watts for a three second preheating period. Then five watts or higher during the drawing process. To fabricate the resonator use fused silica, capillary preform, cut a length that can reach between the two sample holders on the linear translation stage about two to four centimeters.

Next, attach the capillary to the sample holders. So the laser target zone is roughly in the middle of the capillary length. Proceed by starting the control software.

The laser preheats the capillary for three seconds. Then the capillary is pulled for about 10 seconds. When the process is over, remove the drawn capillary by only handling the thick un unfolded end to prevent contamination.

Hang the capillary so the clean resonator's surface is suspended in air. Create capillaries with different diameters by varying the pulling parameters and repeating the fabrication process to ensure the capillaries are open. Work with a shallow pan of water and a syringe.

Carefully pick up a finished capillary and dip one end into the water. Use the syringe to blow air through the other end. Air bubbles indicate the capillary is open.

Before testing the capillaries, a holder must be built to mount them. To do this, cut three one centimeter by 0.5 centimeter glass pieces from glass slides. In addition, cut one three centimeter by half centimeter piece also from a glass slide.

Then use glass bonding adhesive or super glue to assemble these pieces into an eha. Next cut from a drawn capillary, a length that can span the distance between two adjacent branches on the U-shaped holder and extend beyond them. Use optical adhesive to glue the capillary so its ends extend beyond the two branches of the holder and its center is suspended between them.

Then cure the optical adhesive with an LED UV curing light for as long as needed. About 10 seconds proceed by obtaining plastic tubing with an inner diameter slightly larger than the capillary. Carefully insert a tube over each end of the mounted capillary.

Again, use optical adhesive to seal the connection between the capillary and the and cure with UV light. After curing, the assembly is ready for mounting. Using the third free glass branch to clamp the structure in place.

This experiment makes use of a wavelength tunable fiber coupled infrared laser connected to a function generator. Used to periodically sweep the laser wavelengths. The laser output is first passed through a polarization controller.

From there, it is coupled to a single mode telecom band tapered fiber wave guide that is close to a nano positioning stage. After the tapered fiber, the light passes through attenuators to protect the downstream analyzers. For this experiment following the attenuator, the light is directed to a photo detector attached to an electrical spectrum analyzer and an oscilloscope.

Begin the experiment by stabilizing the laser and beginning sweeps of the laser beam wavelength. Next, retrieve the mounted capillary resonator mount the capillary resonator holder on the nano positioning stage near the tapered fiber. Then carefully translate the stage to bring the resonator about one micron from the tapered fiber to obtain evanescent coupling.

As the wavelengths are swept dips in the laser transmission as a function of time viewed on the oscilloscope, indicate the presence of evanescent coupling and resonances. Use the spectrum analyzer to search for temporal interference between the input laser light and the scattered light. This beat note occurs at the mechanical oscillation frequency.

In this example, a mechanical mode at 24.94 megahertz is excited in the capillary by radiation pressure attempt to lock to the relevant optical mode for a mechanical oscillation by turning off the laser frequency scan and controlling the laser wavelength. In continuous wave mode, periodic signals will appear on an oscilloscope when a mechanical mode is present. Once mastered, the device fabrication and the testing can be completed in about 45 minutes if it's done properly.

While attempting this procedure, it is important to remember that the resonator and tapered fiber must never be handled manually, even with the tweezers. This is to maintain the cleanliness of the surfaces. Since its development, this technique has paved the way for researchers in the field of cavity optic mechanics to explore novel interactions with fluids and bio analytes.