Ulas Ayaz

Tuning of whispering gallery modes of spherical resonators using an external electric field.

In this paper we investigate the electrostriction effect on the whispering gallery modes (WGM) of polymeric microspheres and the feasibility of a WGM-based microsensor for electric field measurement. The electrostriction is the elastic deformation (strain) of a dielectric material under the force exerted by an electrostatic field. The deformation is accompanied by mechanical stress which perturbs the refractive index distribution in the sphere. Both strain and stress induce a shift in the WGM of the microsphere. In the present, we develop analytical expressions for the WGM shift due to electrostriction for solid and thin-walled hollow microspheres. Our analysis indicates that detection of electric fields as small as ~500V/m may be possible using water filled, hollow solid polydimethylsiloxane (PDMS) microspheres. The electric field sensitivities for solid spheres, on the other hand, are significantly smaller. Results of experiments carried out using solid PDMS spheres agree well with the analytical prediction.

High-resolution force sensor based on morphology dependent optical resonances of polymeric spheres

Performance characteristics of a force sensor concept based on the morphology dependent resonance (MDR) shifts of micro-optical resonators have been investigated. Previous experimental studies have indicated that microsphere sensors with diameters ranging between 30 and 950 μm may have force resolutions reaching 10−5 N [T. Ioppolo et al., Appl. Opt. 47, 3009 (2008)]. In the present, we carry out a systematic analysis and experiments to investigate the sensitivity, resolution, and bandwidth limits of MDR-based force sensors. Expressions for MDR shifts due to applied force in the polar direction are obtained for microspheres of various dielectric materials in the diameter range of 300–950 μm. The analyses are compared with experimental results for polymethylmethacrylate and polydimethylsyloxane (PDMS) microsphere sensors. The results show that the strain effect on MDR shifts is dominant over that of mechanical stress. It also indicates that force sensitivities of the order of a 1 pN are feasible using hollo...

Wall shear stress sensor based on the optical resonances of dielectric microspheres

We report an optical wall shear stress sensor based on the whispering gallery mode (WGM) shifts of dielectric micro-resonators. The optical resonators are spheres with a typical diameter of several hundred microns and they serve as the sensing element. The wall shear force acting on a movable plate is transmitted mechanically to the microsphere. As a result of the applied force, the shape of the resonator is perturbed leading to a shift of the optical resonance (WGM). The one-dimensional wall shear stress is measured by monitoring these WGM shifts. Shape perturbations of the order of a nanometer can be detected with this optical method. The measurement resolution and range can be optimized by using dielectric sphere materials of different stiffness covering a wide range of flows. Prototype sensors using PDMS spheres have been built and validated in a laminar Poiseuille flow and in a plane wave acoustic tube.

A photonic wall pressure sensor for fluid mechanics applications.

In this paper, we demonstrate a micro-optical wall pressure sensor concept based on the optical modes of dielectric resonators. The sensing element is a spherical micro-resonator with a diameter of a few hundred micrometers. A latex membrane that is flush mounted on the wall transmits the normal pressure to the sensing element. Changes in the wall pressure perturb the spheres morphology, leading to a shift in the optical modes. The wall pressure is measured by monitoring the shifts in the optical modes. Prototype sensors with polydimethylsiloxane micro-spheres are tested in a steady two-dimensional channel flow and in a plane wave acoustic tube. Results indicate sensor resolutions of ∼20 mPa and bandwidth of up to 2 kHz.

Direct measurement of wall shear stress in a reattaching flow with a photonic sensor

Wall shear stress measurements are carried out in a planar backward-facing step flow using a micro-optical sensor. The sensor is essentially a floating element system and measures the shear stress directly. The transduction method to measure the floating element deflection is based on the whispering gallery optical mode (WGM) shifts of a dielectric microsphere. This method is capable of measuring floating element displacements of the order of a nanometer. The floating element surface is circular with a diameter of ~960 µm, which is part of a beam that is in contact with the dielectric microsphere. The sensor is calibrated for shear stress as well as pressure sensitivity yielding 7.3 pm Pa−1 and 0.0236 pm Pa−1 for shear stress and pressure sensitivity, respectively. Hence, the contribution by the wall pressure is less than two orders of magnitude smaller than that of shear stress. Measurements are made for a Reynolds number range of 2000–5000 extending to 18 step heights from the step face. The results are in good agreement with those of earlier reports. An analysis is also carried out to evaluate the performance of the WGM sensor including measurement sensitivity and bandwidth.

Performance of a Micro-Optical Wall Shear Stress Sensor Based on Whispering Gallery Mode Resonators

In this paper, we discuss the dynamic performance of a novel wall shear stress sensor concept based on whispering gallery mode (WGM) shifts of a dielectric microsphere resonator. In the shear stress sensor model, the wall shear stress acting on a sensing element, typically 125 m in diameter, is transmitted mechanically to the microsphere and the transmitted force leads to shifts in the WGMs of the microsphere. By monitoring these WGM shifts, the magnitude as well as the direction of the wall shear stress can be measured. The measurement principle was demonstrated in a previous paper which presented static shear stress results. The sensor used in the present study is a dielectric microsphere made of Polydimethylsyloxane (PDMS), and is tested for loading for dynamic measurements, An electronic circuit is developed to track the fast moving optical resonances (WGM) under dynamic loading.

Composite micro-sphere optical resonators for electric field measurement

Polymer-based, multi-layered dielectric microspheres are investigated for high-resolution electric field sensing. The external electric field induces changes in the morphology of the spheres, leading to shifts in the whispering gallery modes (WGMs). Light from a distributed feedback (DFB) laser is sidecoupled into the microspheres using a tapered section of a single mode optical fiber to interrogate the optical modes. The base material of these multi-layered spheres is polydimethylsiloxane (PDMS). Three microsphere geometries are investigated: (1) cores comprised of a 60:1 volumetric ratio of PDMS-to-curing agent mixture that are mixed with varying amounts of barium titanate (BaTiO3) nano particles, (2) cores comprised of 60:1 PDMS that are coated with a thin layer of 60:1 PDMS that is mixed with varying amounts of barium titanate and (3) a composite Carbon Black-BaTiO3 prototype. The outermost layer for all sphere geometries is a thin coat of 60:1 PDMS which serves as the shell waveguide. Light from the tapered laser is coupled into this outermost shell that provides high optical quality factor WGM (Q ~ 106). The microspheres are poled for several hours at electric fields of ~ 1 MV/m to increase their sensitivity to electric field. Preliminary results show that electric fields of the order of 100 V/m can be detected using these composite micro-resonators.

Tuning of whispering gallery modes of polymeric micro-spheres and shells using external electric field

The electrostriction effect on the whispering gallery modes (WGM) of polymeric micro-spheres is investigated analytically and experimentally. Electrostriction is the elastic deformation (mechanical strain) of a dielectric material due the force exerted by an electric field. The elastic deformation also leads to mechanical stress which perturbs the refractive index of the sphere. Both if these effects (strain and stress) induce a shift in the WGM of the dielectric microsphere. We develop analytical expressions for the WGM shift due to electrostriction for solid and thin-walled micro-shells. The analytical results show that detection of electric fields < 1000 V/m is possible using water filled PDMS microshells. The electric field sensitivities for solid spheres, on the other hand, are significantly smaller. Results of experiments carried out using solid polydimethylsiloxane (PDMS) spheres agree well with the analytical prediction. These results are encouraging for future development of WGM-based optical switches/filters as well as electric field sensors.

Whispering Gallery Mode Based-Micro-Optical Sensor for Electromagnetic Field Detection

In this paper we investigate the electrostriction effect on the whispering gallery modes (WGM) of polymeric microspheres and the feasibility of an WGM-based microsensor for electric field measurements. The electrostriction is the elastic deformation (strain) of a dielectric material under the force exerted by an electrostatic field. The deformation is accompanied by mechanical stress which perturbs the refractive index distribution in the sphere. Both the strain and the stress induce a shift in the WGM of the microsphere. In the present, we develop analytical expressions for the WGM shift due to electrostriction for solid and thin-walled hollow microspheres. Our analysis indicates that measurements of electric fields as small as ~500V/m may be possible using water filled, hollow PDMS micro-spheres. The electric field sensitivities for solid spheres, on the other hand, are significantly smaller. The effect of dielectric constant perturbations in the ambient medium on sphere WGM has also been investigated. A preliminary analysis indicates that changes of the order of ~10 -3 in dielectric constant of the medium surrounding the microsphere can be observed by using a water-filled hollow PDMS sphere.

High-Resolution Whispering Gallery Mode Force Micro- Sensor Based on Polymeric Spheres

Previous experimental studies of whispering gallery mode (WGM) sensors have indicated that microspheres with diameters ranging between 300-950 μm may have force resolutions reaching 10N [1]. In the present, we expand on the previous investigatons. Here, we carry out a systematic analysis and experiments to investigate the sensitivity, resolution and bandwidth limits of WGM-based force sensors. Expressions for WGM shifts due to applied force in the polar direction are obtained for microspheres of various dielectric materials, in the diameter range of 300-950 μm. The analyses are compared with experimental results for Polymethylmethacrylate (PMMA) Polydimethylsyloxane (PDMS) microsphere sensors. The present analysis shows that the strain effect on WGM shifts dominate over that of mechanical stress. It also indicates that force sensitivities of the order of a 1pN are possible using hollow PDMS spheres. The sensor bandwidths (based on the mechanical properties of the sensor material alone) range between 1 kHz and 1 MHz. These results have significance also from the point of view wall shear stress since the same force sensing concept can be used for the development of high-sensitivity skin friction sensors.