Fabrication and Testing of Microfluidic Optomechanical Oscillators

1. Fabrication of Ultra-high-Q Microfluidic Resonators

1. Preparation of capillary manufacturing setup
   1. Fabricate the microfluidic optomechanical resonator in the following way – Heat a glass capillary preform with approximately 10 W of CO₂ laser radiation at 10.6 microns wavelength, and draw out the heated capillary linearly using motorized translation stages. Figure 1 shows the arrangement of the linear translation stages, the lasers, and the location of the capillary preform before the pulling process.
   2. Program suitable automation software to simultaneously control the two CO₂ lasers (for heating) and the two linear stages. The two linear stages perform the drawing process for the laser-heated capillary.
   3. One of the linear stages must be fast (e.g. 5 mm/sec) for the linear drawing process. Feed in more material to the heating zone with the second, slower linear stage (e.g. 0.5 mm/sec) since the capillary preform material gets depleted during the pulling process.
   4. Align the sample holders on the linear stages along both vertical and horizontal axes.
   5. Carefully align both CO₂ laser beams such that they target the same spot in space (between the sample holders). A piece of card paper or heat sensitive paper is useful for this process. Do not forget to use eye protection for laser safety. Do not lower eyes to table level. Use suitable beam blocks, fume exhaust, and fire protection.
   6. Select reasonable parameters for the drawing process. For example, the following parameters reliably produce a good capillary size — 10 mm/sec pulling speed, 0.5 mm/sec feed-in speed, 3 sec preheating time, 4.5 W preheating powers for both lasers, and 5 W heating powers for both lasers.
   7. Modulation of the laser power during pulling can be used to control the capillary radius lengthwise during the drawing process to form the ‘bottle’ resonators. An example is shown in Figure 2d. Select the proper modulation parameters: 3 Hz frequency, 6 W and 3 W for laser powers, and 50% duty cycle.
2. Fabrication of microfluidic optomechanical resonators
   1. Cut a sufficiently long segment (about 2-4 cm) of fused silica capillary such that it can reach the two holders attached to the linear translation stages.
   2. Mount the capillary sample on the sample holders such that the laser target zone is roughly in the middle of the capillary. Readjust CO₂ laser alignment if needed.
   3. Pull the capillary using the parameters as stated in 1.1.6. First preheat the capillary for a few seconds (Figure 2a), and then pull it with or without laser modulation (parameters in 1.1.7) as needed.
   4. Remove the drawn capillary (Figure 2b) from the sample holder. Handle the sample with gloves at the two thick ends only, in order not to contaminate the clean resonator surface.
   5. Vary the pulling parameters to fabricate capillaries with different diameters. Typically outer diameter varies from 30 µm to 200 µm depending on pulling conditions.
3. Mounting the fabricated device for testing
   1. Prepare an E shape glass holder (Figure 2c). Cut three 1 cm x 0.5 cm and one 3 cm x 0.5 cm glass pieces from glass slides. Assemble them into an E shape using glass adhesive or superglue.
   2. Cut a length of microfluidic capillary out of the drawn sample. This length should be longer than the distance between two adjacent glass branches on the E shape holder.
   3. Glue the microcapillary device onto the holder using optical adhesive while making sure to keep a portion hanging uncontaminated between two branches of the E shape holder. Cure the optical adhesive with a LED UV curing light source for 10 sec. Figures 2c and 2d show the finished product.
   4. Carefully insert both ends of the mounted resonator into two slightly larger plastic tubes (e.g. 200 µm in inner diameter). Glue and UV cure both ends to the plastic tubes with optical adhesive.
   5. Clamp the E shape structure from the third (free) glass branch to a clamped mounting device for testing. The optical quality factor of the final microfluidic resonator depends on how well the fabrication lasers were aligned and how stable their power levels were.

2. Experimental Setup for Optomechanical Testing

1. Fabrication of tapered optical fiber
   1. Prepare a single mode telecom-band optical fiber of desired length (e.g. a few feet). Fiber segment should be long enough to be both mounted in the tapering area and connected to the setup (Figure 4). The tapering method explained here is similar to what is suggested and demonstrated in ²².
   2. Connect the prepared fiber segment to the rest of the experimental setup using any convenient fiber-splicing method.
   3. Mount the spliced fiber segment onto two linear pullers that face each other.
   4. Strip off the fiber jacket in the center of the mounted fiber fragment to expose cladding area. This is where taper will be fabricated. Clean the stripped area with methanol.
   5. Turn on the tunable laser to see real-time transmission on an oscilloscope. Make sure to set attenuators so that photodetectors are not damaged.
   6. Place a narrow nozzle hydrogen gas burner immediately underneath the unjacketed portion of the fiber. Follow all recommended safety procedures when working with pressurized flammable gases such as hydrogen. Other “clean burning” sources of flame or ceramic heaters could also be used.
   7. Before lighting up the gas, check the flow rate so that flame will not be too large (a 1-2 cm tall flame is adequate). Note that the flame is mostly invisible but may be seen as a faint orange glow in a dark room. The hydrogen flow rate should be set to a point where lighted flame will adequately soften the glass fiber.
   8. Light up the flame. As soon as flame is on, start pulling the fiber using motorized stages. Appropriate pulling speed depends on flow rate of hydrogen gas and vicinity of the flame. NOTE: Transmission through the fiber will begin to show temporal oscillation behavior as pulling continues. This indicates multimode operation.
   9. When oscillatory behavior stops and shows an unchanging signal over time, stop pulling and turn-off the flame immediately. This is when single-mode taper is obtained. Check the transmission. If transmission is too low, repeat the procedure from 2.1.1. with modified gas flow rate, flame size, and flame location. On occasion, low transmission could be due to bad alignment at step 2.1.3. or due to contamination of the exposed cladding.
   10. If resultant transmission through the taper is satisfactory, wait a few minutes to cool down the taper.
   11. Inspect the taper under a microscope. For 1,550 nm operational wavelength, typical diameter of the single-mode taper is in the order of 1-2 µm.
2. Taper-coupling to WGR and searching for electronic signals indicating vibration
   1. Set up the experiment in the configuration shown in Figure 3. Mechanical vibrations can be generated through both SBS and RP by the same experimental configuration. In order to clearly detect back-scattered signals as in the case of backward-SBS^4,21, use a circulator between taper and tunable laser.
   2. Before turning on the tunable IR laser, make sure to set attenuators in place so that photodetectors are not damaged.
   3. Turn on and stabilize the tunable IR laser. A function generator is used to sweep the frequency of the input IR laser.
   4. Mount the resonator holder on a nanopositioning stage. Carefully bring the resonator close to the tapered fiber in order to obtain evanescent coupling. As the laser frequency is swept, optical resonances will appear as dips in transmission in the oscilloscope, as in Figure 2b of ²².
   5. Connect the photodetector output to an electrical spectrum analyzer (ESA), where the temporal interference (i.e. beat note) between the input laser light and the scattered light can be observed. This temporal interference occurs at the mechanical oscillation frequency. The “peak hold” function on the spectrum analyzer is often useful in the initial search for mechanical vibrations.
   6. Use higher input power while performing the initial search for mechanical vibration, especially when liquids are present inside the device. NOTE: Typically, input power in the order of 100 µW to the device is sufficient to excite mechanical vibration.
   7. If mechanical oscillation is observed, attempt to lock to the relevant optical mode by turning off the laser frequency scan and controlling the laser wavelength in CW mode. Here, both oscilloscope and spectrum analyzer are useful in tandem. Periodic signals appear on the oscilloscope when a mechanical mode is present, as seen in Figure 5 and ^1,6.

3. Measuring Optomechanical Vibrations

1. Optical and electronic signature of radiation pressure (RP) modes
   1. As described in 2.2, mechanical oscillations will be observed when the taper and device are correctly coupled, the device optical and mechanical modes have sufficient Q-factors, and sufficient input optical power is provided. If oscillations in the range of 10 MHz – 1 GHz are not observed, attempt to change polarization to investigate different resonances, or increase the input power from tunable laser in order to overcome the minimum threshold for oscillation. When increasing the input power, always be careful not to saturate the photodetectors. Also, as described in ⁸, coupling distance is a key factor for exciting different RP modes.
   2. If mechanical modes are still not observed, try measuring optical quality factor. For microfluidic optomechanical resonators, results show that optical quality factor of 10⁶ is sufficient to excite parametric oscillations¹³.
      NOTE: Usually, RP modes will manifest as electronic oscillations on the spectrum analyzer accompanied by their harmonics, as seen in Figure 5. Representative results will be discussed in section 4.
   3. Use a scanning Fabry-Perot interferometer or high-resolution optical spectrum analyzer to detect the optical side bands that are generated due to amplitude and phase modulation, which is in turn induced by the periodic cavity deformation. An example measurement may be seen in Figure 3h of ¹.
2. Optical and electronic signature of whispering-gallery acoustic modes
   1. The acoustic frequency of backward-SBS for silica glass is approximately 11 GHz when a 1.5 micron pump laser is used^4,23. Use a circulator that monitors the back-scattered light and some small amount of Rayleigh-scattered pump, to observe electronic signals for these vibrational modes. Use a high-resolution optical spectrum analyzer to resolve the scattered light. An example measurement is shown in Figure 2 of ⁴.
   2. Use the beat note between forward scattered light and the pump laser to observe lower frequency (sub-1 GHz) whispering-gallery acoustic modes.
   3. Due to the lower mechanical rigidity in the breathing direction, the signal from SBS is sometimes weaker than the signal from the RP modes. Again, sweep the laser at slow speed, and use “peak hold” on the spectrum analyzer to help in finding the SBS signal.
   4. Note that unlike RP-excited breathing modes, SBS-excited whispering-gallery acoustic modes do not exhibit harmonics in the optical and electronic spectra (unless cascaded excitation takes place^4,24). Instead only one Stokes sideband appears for SBS modes.