Parametric optomechanical excitations have recently been experimentally demonstrated in microfluidic optomechanical resonators by means of optical radiation pressure and stimulated Brillouin scattering. This paper describes the fabrication of these microfluidic resonators along with methodologies for generating and verifying optomechanical oscillations.
Cavity optomechanics experiments that parametrically couple the phonon modes and photon modes have been investigated in various optical systems including microresonators. However, because of the increased acoustic radiative losses during direct liquid immersion of optomechanical devices, almost all published optomechanical experiments have been performed in solid phase. This paper discusses a recently introduced hollow microfluidic optomechanical resonator. Detailed methodology is provided to fabricate these ultra-high-Q microfluidic resonators, perform optomechanical testing, and measure radiation pressure-driven breathing mode and SBS-driven whispering gallery mode parametric vibrations. By confining liquids inside the capillary resonator, high mechanical- and optical- quality factors are simultaneously maintained.
Cavity optomechanics studies the parametric coupling between phonon modes and photon modes in microresonators by means of radiation pressure (RP)1-3 and stimulated Brillouin scattering (SBS)4-6. SBS and RP mechanisms have been demonstrated in many different optical systems, such as fibers7, microspheres4,6,8, toroids1,9, and crystalline resonators5,10. Through this photon-phonon coupling, both cooling11 and excitation6,10 of mechanical modes have been demonstrated. However, almost all reported optomechanics experiments are with solid phases of matter. This is because direct liquid immersion of the optomechanical devices results in greatly increased radiative acoustic loss because of the higher impedance of liquids compared against air. In addition, in some situations dissipative loss mechanisms in liquids may exceed the radiative acoustic losses.
Recently, a new type of hollow optomechanical oscillator with a microcapillary geometry was introduced12-15, and which by design is equipped for microfluidic experiments. The diameter of this capillary is modulated along its length to form multiple ‘bottle resonators’ that simultaneously confine optical whispering-gallery resonances16 as well as mechanical resonant modes17. Multiple families of mechanical resonant modes participate, including breathing modes, wine-glass modes, and whispering-gallery acoustic modes. The wine-glass (standing-wave) and whispering-gallery acoustic (traveling-wave) resonances are formed when a vibration with integer multiple of acoustic wavelengths occurs around the device circumference. Light is evanescently coupled into the optical whispering-gallery modes of these ‘bottles’ by means of a tapered optical fiber18. Confinement of the liquid inside19,20 the capillary resonator, as opposed to outside it, enables high mechanical- and optical- quality factors simultaneously, which allows the optical excitation of mechanical modes by means of both RP and SBS. As has been shown, these mechanical excitations are able to penetrate into the fluid within the device12,13, forming a shared solid-liquid resonant mode, thus enabling an opto-mechanical interface to the fluidic environment within.
In this paper we describe fabrication, RP and SBS actuation, and representative measurement results for this novel optomechanical system. Specific material and tool lists are also provided.
1. Fabrication of Ultra-high-Q Microfluidic Resonators
2. Experimental Setup for Optomechanical Testing
3. Measuring Optomechanical Vibrations
The capillaries produced by this method are thin (between 30 µm and 200 µm), clear, and very flexible, but are sufficiently robust for direct handling. It is important to protect the outer surface of the capillary device against dust and water (moisture) in order to maintain a high optical quality factor (Q). By dipping one end of the capillary in water and blowing air through the capillary by means of a syringe, it can be verified whether the capillary is through or whether was sealed off during fabrication due to overheating.
A tunable laser can be used to probe the optical modes of the fabricated device by means of tapered fiber waveguide coupling. In this test, sharp optical resonances are expected indicating high optical Q-factor. An additional indication for high Q-factor is the thermal broadening of the optical modes25.
When RP-actuated parametric oscillation takes place, harmonics of the mechanical mode will be seen in the optical spectrum obtained on the output port of the tapered waveguide. This occurs due to the large modulation depth of the amplitude and phase modulation of the light, caused by the mechanical vibrations. Examples of the typically observed electrical spectrum are seen in Figure 5a and also in 1. An oscilloscope trace of the signal exhibits periodic behavior (Figure 5b). Finite element analysis can be invoked to model the mechanical modes of the system, to confirm that the observed optical modulation corresponds to an eigenmechanical frequency. SBS driven mechanical modes are easily identified by the absence of harmonics of the fundamental mechanical signal, since only a single Stokes sideband is generated6. These modes typically occur at higher frequencies than the RP modes, although low frequencies are possible as well.
Figure 1. Schematic of the capillary pulling setup. The microfluidic optomechanical resonators are drawn from a larger capillary preform attached to two linear stages while the glass is heated by the CO2 laser. Both laser beams are carefully aligned to the same spot of the capillary. Moving direction and relative speeds of the linear stages are indicated by the arrows. Please click here to view a larger version of this figure.
Figure 2. Optomechanical bottle resonator fabrication. (a) The capillary preform is pulled at a constant speed while being heated by means of CO2 laser radiation. Note the glowing region is the laser target spot (where the beams heat the silica). When the required length and diameter are reached, (b) stop the linear stage motion and turn the lasers off. The pulled capillary is thin, clear, and very flexible. (c) Employ an E shape glass structure to mount the microcapillary resonator device as described in section 1.3. The optomechanical bottle resonator is now ready to be taken to the experimental setup and connected to tubing that will provide analytes. (d) Scanning electron micrograph of the fabricated optomechanical bottle resonator. Resonator radius and wall thickness can be varied as needed. Please click here to view a larger version of this figure.
Figure 3. Schematic of the testing setup. Light is evanescently coupled into the resonator through a tapered fiber. A tunable IR laser (1,520-1,570 nm) is used as the light source and is fine-tuned to match a chosen optical mode of the resonator. Mechanical vibrations actuated by light in the resonator cause modulation of the input light at the mechanical vibration frequency. The electric fields of the optical pump and vibration-scattered light in the forward direction interfere temporally on the photodetector (PD) at the end of the tapered fiber. A beat note between the two optical signals is thus generated through the optical-power-to-current transduction taking place in the photodetector. This beating can be observed on an electric spectrum analyzer (ESA). A scanning Fabry-Perot cavity (FPC) and optical spectrum analyzer (OSA) can also be used to directly observe the optical side bands that are generated due to the modulation. Please click here to view a larger version of this figure.
Figure 4. Coupling light from a fiber to micro resonator. The E shape structure is mounted just above the tapered fiber so that light can be evanescently coupled into the resonators. Please click here to view a larger version of this figure.
Figure 5. Representative results. (a) A breathing mechanical mode at 24.94 MHz in the microcapillary is excited by centrifugal radiation pressure by light circulating in an optical mode. Modulation of the input light by this mechanical vibration is observable on an electrical spectrum analyzer through beat-note generation on a photodetector placed in the forward-scattering direction (See Figure 3). (b) An oscilloscope trace of the photodetector output signal (i.e. transmitted power) shows the periodic temporal interference of the input light and scattered light. (c) Finite element simulation for the corresponding breathing mode confirms that the observed optical modulation corresponds to an eigenmechanical frequency. Colors represent deformation and the simulation is sliced at the capillary mid-point for presentation. Please click here to view a larger version of this figure.
Figure 6. The mechanical frequency is presented as a function of the fluid density. The same mechanical mode is measured on the same device with different concentrations of sucrose solutions present inside. Please click here to view a larger version of this figure.
We have fabricated and tested a new device that bridges between cavity optomechanics and microfluidics by employing high-Q optical resonances to excite (and interrogate) mechanical vibration. It is surprising that multiple excitation mechanisms are available in the very same device, which generate a variety of mechanical vibrational modes at rates spanning 2 MHz to 11,300 MHz. Centrifugal radiation pressure supports both wineglass modes and breathing modes in the 2-200 MHz span, Forward stimulated Brillouin scattering allows mechanical whispering gallery modes in the 50-1,500 MHz range, and lastly, backward stimulated Brillouin scattering excites mechanical whispering gallery modes near 11,000 MHz.
The methods that are described in the current work enable the fabrication of these microfluidic resonators with ultra-high optical quality factors of about 108. Simultaneously, since liquids are now confined within the device, acoustic losses are brought under control and the device is able to maintain a high mechanical quality factor as well. With this platform, we have demonstrated that the density changes of a fluid contained within the device can be measured (Figure 6). In order to fully understand the opto-mechano-fluidic coupling that enables this, future work will involve multiphysical modeling of the device.
There are a few practical challenges associated with this fabrication method. For instance, the capillary material must be a good absorber for the 10.6 micron CO2 laser radiation so that it can heat up sufficiently for the pulling process to take place. In this regard, the materials that have been tested for capillary fabrication are silica and quartz. Furthermore, the circular symmetry of the capillary is dictated by the relative power balance between the two lasers that are employed during the pulling step, and by the location of the capillary in the laser target zone. Since the circular symmetry of the device is a key parameter for maintaining high optical and mechanical quality factor, misalignment of the capillary preform in the CO2 laser target zone before pulling or during pulling can be a concern and care must be taken to keep this under control.
On the other hand, this fabrication method provides a lot of flexibility in the fabrication of silica-based optomechanical capillary resonators. By modulating the CO2 laser power, the capillary diameter can be varied quite easily to suit the application. On demand spacing between adjacent bottle resonators is possible thanks to the high degree of computer control. Finally, control of the rate of pulling and the rate of “feed in” of the capillary preform provides an easy knob for controlling the capillary diameter.
In conclusion, the silica-based microcapillary platform as described is a low-cost, high-performance optical and optomechanical system that can be applied to a variety of studies with non-solid phase materials, including superfluids, and bio-analytes such as living cells. These devices can additionally leverage the very large body of literature on surface acoustic wave sensing of gases and liquids. As a result, this is an enabling technology for optical sensing applications.
The authors have nothing to disclose.
This work was funded by Startup funding from the University of Illinois at Urbana-Champaign, DARPA ORCHID program through a grant from AFOSR, the National Science Foundation through grant CMMI-1265164, and the National Science Foundation Graduate Research Fellowship program. We acknowledge enlightening discussions with Prof. Jack Harris, Prof. Pierre Meystre, Dr. Matt Eichenfield, Prof. Taher Saif, and Prof. Rashid Bashir.
Tunable IR laser | Newfocus | TLB-6328 | |
Photodetectors | Newfocus | 1811-FC (Low speed 125MHz) / 1611-FC-AC (High speed 1GHz) | |
Optical fiber | Corning | SMF28 | |
Silica capillary | PolyMicro | TSP700850 | |
10.6 um wavelength CO2 laser | Synrad | 48-1KWM and 48-2KWM | |
UV-curing optical adhesive | Thorlabs | NOA81 | |
Tubing | Tygon | EW-06418-01 | |
Syringes | B-D | YO-07940-12 | |
Needles | Weller | KDS201P | |
Electrical spectrum analyzer | Agilent Technologies | N9010A (EXA Signal Analyzer) | |
Tektronix | 6114A (RSA, Real-time spectrum analyzer) | ||
Optical spectrum analyzer | Advantest | Q8384 | |
Oscilloscope | Tektronix | DPO 4104B-L | |
Gold mirrors | II-VI Infrared | 836627 | |
Linear stage (slow) | DryLin | H1W1150 | |
Linear stage (fast) | PBC Linear | MTB055D-0902-14F12 | |
Fabry Perot optical spectrum analyser | Thorlabs | SA 200-14A (FSR: 1.5 GHz) |