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Development of Whispering Gallery Mode Polymeric Micro-optical Electric Field Sensors

Published: January 29th, 2013



1Mechanical Engineering Department, Southern Methodist University

A high-sensitivity photonic micro sensor was developed for electric field detection. The sensor exploits the optical modes of a dielectric sphere. Changes in the external electric field perturb the sphere morphology leading to shifts in its optical modes. The electric field strength is measured by monitoring these optical shifts.

Optical modes of dielectric micro-cavities have received significant attention in recent years for their potential in a broad range of applications. The optical modes are frequently referred to as "whispering gallery modes" (WGM) or "morphology dependent resonances" (MDR) and exhibit high optical quality factors. Some proposed applications of micro-cavity optical resonators are in spectroscopy1, micro-cavity laser technology2, optical communications3-6 as well as sensor technology. The WGM-based sensor applications include those in biology7, trace gas detection8, and impurity detection in liquids9. Mechanical sensors based on microsphere resonators have also been proposed, including those for force10,11, pressure12, acceleration13 and wall shear stress14. In the present, we demonstrate a WGM-based electric field sensor, which builds on our previous studies15,16. A candidate application of this sensor is in the detection of neuronal action potential.

The electric field sensor is based on polymeric multi-layered dielectric microspheres. The external electric field induces surface and body forces on the spheres (electrostriction effect) leading to elastic deformation. This change in the morphology of the spheres, leads to shifts in the WGM. The electric field-induced WGM shifts are interrogated by exciting the optical modes of the spheres by laser light. Light from a distributed feedback (DFB) laser (nominal wavelength of ~ 1.3 μm) is side-coupled into the microspheres using a tapered section of a single mode optical fiber. The base material of the spheres is polydimethylsiloxane (PDMS). Three microsphere geometries are used: (1) PDMS sphere with a 60:1 volumetric ratio of base-to-curing agent mixture, (2) multi layer sphere with 60:1 PDMS core, in order to increase the dielectric constant of the sphere, a middle layer of 60:1 PDMS that is mixed with varying amounts (2% to 10% by volume) of barium titanate and an outer layer of 60:1 PDMS and (3) solid silica sphere coated with a thin layer of uncured PDMS base. In each type of sensor, laser light from the tapered fiber is coupled into the outermost layer 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.

1. PDMS Microsphere Preparation (Sphere I)

  1. Polydimethylsiloxane (PDMS) base and the curing agent are mixed with a volume ratio of 60:1.
  2. A strand of silica optical fiber, about 2 cm long, is first stripped of its plastic cladding using an optical stripper.
  3. One end of the fiber is heated and stretched to provide a stem end that is ~25-50 μm in diameter at the tip.
  4. The stretched end of the fiber is submerged into the PDMS mixture by a length of approximately 2-4 mm and then is pu.......

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An optical mode (WGM) of the sphere is excited by the laser light when optical path length traveled by the light is a multiple integer of the laser wavelength. For the arrangement shown in Figure 3, the optical path length is 2πrn, where n and r are the refractive index and radius of the sphere, respectively. Using geometric optics approximation, a WGM condition is satisfied when 2πrn = lλ wherel is an integer and λ is the laser's .......

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The spheres are initially poled by connecting the electrodes to a DC high voltage supply. At the end of the poling duration, the electrode leads are disconnected from the DC voltage supply and connected to a function generator as indicated in Figure 4. The results presented in Figures 5 through 8 show that positive and negative electric fields (relative to direction of poling) lead to sphere elongation and compression, respectively. Sphere I, which is a single layer 60:1.......

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This research is sponsored by the US Defense Advanced Research Projects Agency under Centers in Integrated Photonics Engineering Research (CIPhER) program with Dr. J. Scott Rodgers as project manager. The information provided in this report does not necessarily reflect the position or the policy of the US Government and no official endorsement should be inferred.


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Name Company Catalog Number Comments
Company Catalogue number Comments (optional)
PDMS Dow Corning Sylgard 184
Silica fiber Fiber Instrument Sales E-37AP15-FIS
Barium Titanate (BaTiO3) nanoparticles Sigma Aldrich 467634-100G
Laser Controller ILX Lightwave LDC-3724B
DFB Laser Agere Agere 2300 1.310 μm central wavelength
Photodiode Thorlabs PDA10CS
A/D Card National Instruments PXI 6115

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