High-Q optical resonators allow label-free detection of individual nanoparticles through perturbation of optical signatures but have practical limitations due to reliance on random diffusion to deliver particles to the sensing region. We have recently developed microfluidic optomechanical resonators that allow detection of free-flowing particles in fluid media with near perfect detection efficiency, without requiring labeling, binding, or direct access to the optical mode. Rapid detection of single particles is achieved through a long-range optomechanical interaction that influences the scattered light spectra from the resonator, which can be quantified with postprocessing. Here, we present a hybrid electromechanical-optomechanical technique for substantially increasing the bandwidth of these optomechanofluidic sensors, enabling real-time operation. The presented system demonstrates temporal resolution of better than 20 μs (50,000 events/second) with particle sensing resolution down to 490 nm, operating in the air without any stabilization or environmental control. Our technique significantly enhances the sensing capabilities of high-Q optical resonators into the mechanics domain, and allows extremely high-throughput analysis of large nanoparticle populations.
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...
High-throughput label-free measurements of the optical and mechanical properties of single microparticles play an important role in biological research, drug development, and related large population assays. Mechanical detection techniques that rely on the density contrast of a particle with respect to its environment are blind to neutrally bouyant particles. However, neutrally buoyant particles may still have a high compressibility contrast with respect to their environment, opening a window to detection. Here we present a label-free high-throughput approach for measuring the compressibility (bulk modulus) of freely flowing microparticles by means of resonant measurements in an opto-mechano-fluidic resonator.
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