Imaging of acoustic pressure modes in opto-mechano-fluidic resonators with a single particle probe

Abstract

Opto-mechano-fluidic resonators (OMFRs) are a new platform for high-throughput sensing of the mechanical properties of freely flowing microparticles in arbitrary media. Experimental extraction of OMFR mode shapes, especially the acoustic pressure field within the fluidic core, is essential for determining sensitivity and for extracting the particle parameters. Here we demonstrate a new imaging technique for simultaneously capturing the spatially distributed acoustic pressure fields of multiple vibrational modes in the OMFR system. The mechanism operates using microparticles as perturbative imaging probes, and potentially reveals the inverse path towards multimode inertial detection of the particles themselves.Optical and mechanical resonant modes are the basis for the design and implementation of nearly all sensor technologies. Micro-mechanical resonators are routinely employed for measuring forces 1,2 and inertial motion 3,4 , and for quantifying the properties of fluids 5,6 and particles 7,8 . The typical operating principle for such sensors is to measure the perturbation of resonant modes due to an analyte. For instance, the frequency shift of a mechanical resonator can be used to infer mass of a single microparticle bound to it 8,9 . The sensitivity of such devices depends on the location where the interaction between the analyte and the resonator takes place. For example, mass sensors are insensitive at displacement nodes and most sensitive at anti-nodes 10,11 . It is thus essential to map the resonant mode spatially, in addition to their spectral characteristics, in order to produce calibrated and well-optimized sensor devices. While several methods are available for imaging mode shapes of solid-state microdevices, including laser Doppler vibrometry 12,13 and atomic-force microscopy 14-16 , there are relatively few techniques for mapping modes within fluids. Doppler imaging does permit visualization of acoustic pressure distributions in fluids 17,18 but is limited to resolutions no better than 10's of μm 19 . Characterization of acoustic pressure distributions is thus necessary in resonant sensors in which fluids play a major role, especially for applications in biology and chemistry where fluid-based media are commonly encountered 5,7,20,21 .Previously, we have demonstrated opto-mechanofluidic resonators (OMFRs) as a microfluidic sensor for measurements on bulk fluids 5,20,22 and for determining the properties of individual microparticles 21 at extremely high speeds 23 . An example OMFR vibrational mode is illustrated in Fig. 1(b), showing its hybrid nature in which both the mechanical strain of the shell and the acoustic pressure distribution within the internal fluid are coupled. Specifically, the pressure field in the fluid forms a bridge between the optically measurable mechanical resonance on the shell and the properties of any analytes suspended in the fluid 21 . Thus, knowledge of the acoustic pressure field in the fluid can enable analysis of unknown particle properties such as compressibility, den...

Free-access optomechanical liquid probes using a twin-microbottle resonator

“…The signal-to-noise ratios (i.e., the ratios between the peak top level and the noise floor level) are 3.2 and 1.7, which result in displacement sensitivity (i.e., the minimum detectable displacement) of 4.5 × 10 −18 normalm/Hz in our optomechanical setup and temperature condition [25°C for both the room (air) and water]. This sensitivity is on a level similar to that in microdisks ( 7 , 10 , 12 ) and microcapillaries ( 5 , 6 , 8 ) and will be further improved by constructing a balanced homodyne interferometer ( 35 ). Combining the efficient transduction between two mechanical modes and the strong optomechanical coupling enables us to detect the thermal motion when the TMBR is immersed in liquid.…”

“…Recently, two innovative optomechanical approaches have been demonstrated to maintain the optical sensitivity ( 5 , 8 , 10 , 12 ). The first approach is based on a glass microcapillary.…”

“…The first approach is based on a glass microcapillary. The capillary consists of a hollow-core optomechanical microbottle ( 5 , 6 , 8 ), which forms both the optical whispering gallery modes (WGMs) and mechanical radial breathing modes (RBMs). The liquid flowing inside the core influences the mechanical RBMs and affects the optical WGMs through optomechanical coupling via radiation pressure.…”

Whispering-Gallery Sensors

“…Another indirect technique is based on the optomechanical coupling between optical WGM resonances and long-range phonons, enabling the measurement of particle mass density, mechanical compressibility, and viscoelasticity. 29 , 83 Both of these techniques measure physical quantities other than particle polarizability and have the potential to expand the versatility of WGM sensors further.…”

Coherent Optical Transduction of Suspended Microcapillary Resonators for Multi-Parameter Sensing Applications

“…Optomechanics in liquids [29] is an emerging field that deals with the external damping imposed by the dragging force exerted by a liquid when an immersed object (the nanomechanical resonator surrounded by the liquid) moves. There are two ways to overcome this issue: to use the liquid as a waveguide, including it in the optical cavity [30], or developing a hollow mechanical resonator [31,32] in which the liquid flows inside the structure while the structure vibrates in a vacuum or gaseous atmosphere [33,34]. In this last case, it is possible to perform mass measurements by tracking the resonance frequency of the flexural modes [31].…”