Kyu Hyun Kim

Brillouin cavity optomechanics with microfluidic devices

Cavity optomechanics allows the parametric coupling of phonon- and photon-modes in microresonators and is presently investigated in a broad variety of solid-state systems. Optomechanics with superfluids has been proposed as a path towards ultra-low optical- and mechanical-dissipation. However, there have been no optomechanics experiments reported with non-solid phases of matter. Direct liquid immersion of optomechanics experiments is challenging, as the acoustic energy simply leaks out to the higher-impedance liquid surrounding the device. Here we confine liquids within hollow resonators to circumvent this issue and to enable optical excitation of mechanical whispering-gallery modes at frequencies ranging from 2 to 11,000 MHz. Our device enables optomechanical investigation with liquids, while light is conventionally coupled from the outer dry side of the capillary, and liquids are provided by means of a standard microfluidic inlet.

Cavity optomechanics on a microfluidic resonator with water and viscous liquids

Scientists in the USA have successfully used optomechanics to probe a microfluidic system. Kyu Hyun Kim and co-workers at the University of Michigan and the University of Illinois at Urbana-Champaign fabricated a bulbous hollow-glass capillary and filled it with sugar-water solution. They then used a tapered optical fibre to couple light evanescently in the perimeter of the capillary and thus excite optical whispering gallery modes. This optical mode excites a vibrational mode of the liquid-filled glass structure via the centrifugal radiation pressure that light applies while circumferentially circulating, which in turn modulated the light at a particular frequency. The frequency of the induced vibrations varied with the sugar concentration of the liquid, thus indicating the potential of the structure to function as a sensor. The researchers found that input optical powers as small as 1 mW were sufficient to induce vibrations.

Fabrication and testing of microfluidic optomechanical oscillators.

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.

Air-coupled ultrasound detection using capillary-based optical ring resonators

We experimentally demonstrate and theoretically analyze high Q-factor (~107) capillary-based optical ring resonators for non-contact detection of air-coupled ultrasound. Noise equivalent pressures in air as low as 215 mPa/√Hz and 41 mPa/√Hz at 50 kHz and 800 kHz in air, respectively, are achieved. Furthermore, non-contact detection of air-coupled photoacoustic pulses optically generated from a 200 nm thick Chromium film is demonstrated. The interaction of an acoustic pulse and the mechanical mode of the ring resonator is also studied. Significant improvement in detection bandwidth is demonstrated by encapsulating the ring resonator in a damping medium. Our work will enable compact and sensitive ultrasound detection in many applications, such as air-coupled non-destructive ultrasound testing, photoacoustic imaging, and remote sensing. It will also provide a model system for fundamental study of the mechanical modes in the ring resonator.

Regular oscillations and random motion of glass microspheres levitated by a single optical beam in air.

We experimentally report on optical binding of many glass particles in air that levitate in a single optical beam. A diversity of particle sizes and shapes interact at long range in a single Gaussian beam. Our system dynamics span from oscillatory to random and dimensionality ranges from 1 to 3D. The low loss for the center of mass motion of the beads could allow this system to serve as a standard many body testbed, similar to what is done today with atoms, but at the mesoscopic scale.

Bridging Two Worlds: Microfluidic Optomechanics

Optomechanical systems that enable strong phonon-photon coupling have been with us for a while but have never been demonstrated with non-solid phases of matter.

Metabolic Alterations Associated with Atorvastatin/Fenofibric Acid Combination in Patients with Atherogenic Dyslipidaemia: A Randomized Trial for Comparison with Escalated-Dose Atorvastatin

In the current study, the metabolic effects of atorvastatin dose escalation versus atorvastatin/fenofibric acid combination were compared using metabolomics analyses. Men and women with combined hyperlipidaemia were initially prescribed atorvastatin (10 mg, ≥4 weeks). Patients who reached low-density lipoprotein-cholesterol targets, but had triglyceride and high-density lipoprotein-cholesterol levels ≥150 mg/dL and <50 mg/dL, respectively, were randomized to receive atorvastatin 20 mg or atorvastatin 10 mg/fenofibric acid 135 mg for 12 weeks. Metabolite profiling of serum was performed and changes in metabolites after drug treatment in the two groups were compared. Analysis was performed using patients’ samples obtained before and after treatment. Of 89 screened patients, 37 who met the inclusion criteria were randomized, and 34 completed the study. Unlike that in the dose-escalation group, distinct clustering of both lipid and aqueous metabolites was observed in the combination group after treatment. Most lipid metabolites of acylglycerols and many of ceramides decreased, while many of sphingomyelins increased in the combination group. Atorvastatin dose escalation modestly decreased lysophosphatidylcholines; however, the effect of combination therapy was variable. Most aqueous metabolites decreased, while l-carnitine remarkably increased in the combination group. In conclusion, the atorvastatin/fenofibric acid combination induced distinct metabolite clustering. Our results provide comprehensive information regarding metabolic changes beyond conventional lipid profiles for this combination therapy.

Contactless ultrasound detection using an optical ring resonator

We develop an air-couple ultrasound detector based on an optical fluidic ring resonator (OFRR) suspended on a Ushaped holder. The OFRR is a glass capillary with an outer diameter of approximately 130 μm and a wall thickness in the order of 1~10 μm. The circular cross section of the OFRR supports the high-Q whispering gallery mode (WGM) that circulates along the circumference. Incoming ultrasound pressure results in a small refractive index change in the glass wall and geometrical change in the OFRR shape, both of which in turn lead to a spectral shift in the WGM that can be sensitively detected owing to WGM with high optical Q-factors (>107). Due to the suspension nature of the OFRR, the ultrasound detection can be carried out in air, which is advantageous in comparison with other ultrasound detections that require acoustic coupling media such water, gel or solid. The sensitivity can be tuned and optimized by changing the diameter and wall thickness. Besides the optical detection, we also demonstrate optomechanical ultrasound mixing, in which optomechanical vibration is first excited within the OFRR that subsequently modulates the ultrasound wave. Our work will lead to the development of a new type of air-coupled ultrasound detector that can be used for photo-acoustic imaging, non-invasive ultrasound detection of external objects, and ultrasound detection/characterization of internal objects (such as particles and liquids) flowing inside the capillary.

Regular oscillations and random motion of glass microspheres levitated by a single optical beam in air: publisher's note.

This publishers note amends a recent publication [Opt. Express24(3), 2850-2857 (2016)] to include Acknowledgments.

Surface sensitive microfluidic optomechanical ring resonator sensors

Sensitivity of the optomechanical resonator to the surface mass change is demonstrated to be 1.2 Hz per pg/mm² by gradually removing SiO₂ molecules from the resonator surface. A detection limit of 83 pg/mm² is achieved.