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

Abstract

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 spectroscopy, micro-cavity laser technology, optical communications as well as sensor technology. The WGM-based sensor applications include those in biology, trace gas detection, and impurity detection in liquids. Mechanical sensors based on microsphere resonators have also been proposed, including those for force, pressure, acceleration and wall shear stress. In the present, we demonstrate a WGM-based electric field sensor, which builds on our previous studies. 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 ~ 10(6)). The microspheres are poled for several hours at electric fields of ~ 1 MV/m to increase their sensitivity to electric field.

Applications of Optical Microcavity Resonators in Analytical Chemistry

“…Various organic polymer materials have been used for optical microcavity fabrication (54, 55, 67, 74–85). The principal advantage of these materials is low-cost, simple manufacturing, and many of the resulting devices, such as the polymer microgoblets, maintain remarkable optical qualities with Q-factors as high as 10 6 (54).…”

Effect of Dangling Bonds on De-Poling Time for Polymeric Electric Field Optical Sensors

“…Tangentially coupling the light to the cavity is one of the easiest methods to let the light start to interrogate through the total internal reflection of the cavity [12,[16][17][18], as shown in Figure 1a. Recently, MDR micro-cavities have been used in applications including spectroscopy [19,20], micro-cavity laser technology [21], optical communications (switching [22], filtering [23], and multiplexing [24]), and sensor technologies [3,4,8,12,15,17,[25][26][27][28][29][30][31][32][33]. In this paper, the electric field imposes a force on the dielectric microsphere (commonly referred to as the electrostriction effect) leading to the deformation of the sphere.…”

“…Recently, an MDR-based photonic electric field sensor was demonstrated [9][10][11][12][13]. The theory of operation is based on changing the radius of the cavity due to the electrostriction effect.…”

Mathematical Model for Electric Field Sensor Based on Whispering Gallery Modes Using Navier’s Equation for Linear Elasticity

“…Then, a sphere of PDMS will form at the end of the fiber due to the surface tension and the gravitational forces. The fiber and sphere assembly is heated to cure of the polymeric material as reported in [19].…”