Ioppolo Tindaro, Ötügen Volkan, Ayaz Ulas
Mechanical Engineering Department, Southern Methodist University.
J Vis Exp. 2013 Jan 29(71):e50199. doi: 10.3791/50199.
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.
近年来，介电微腔的光学模式因其在广泛应用中的潜力而备受关注。这些光学模式常被称为“回音壁模式”（WGM）或“形态依赖共振”（MDR），并具有很高的光学品质因数。微腔光学谐振器的一些应用提议包括光谱学、微腔激光技术、光通信以及传感器技术。基于WGM的传感器应用包括生物领域、痕量气体检测以及液体中的杂质检测。基于微球谐振器的机械传感器也已被提出，包括用于力、压力、加速度和壁面剪应力的传感器。目前，我们展示了一种基于WGM的电场传感器，它建立在我们之前的研究基础之上。这种传感器的一个潜在应用是检测神经元动作电位。该电场传感器基于聚合物多层介电微球。外部电场在球体上产生表面力和体力（电致伸缩效应），导致弹性变形。球体形态的这种变化会导致WGM发生偏移。通过用激光激发球体的光学模式来检测电场引起的WGM偏移。来自分布反馈（DFB）激光器（标称波长约为1.3μm）的光通过单模光纤的锥形部分侧面耦合到微球中。球体的基础材料是聚二甲基硅氧烷（PDMS）。使用了三种微球几何形状：（1）基础材料与固化剂混合物体积比为60:1的PDMS球体；（2）具有60:1 PDMS核心的多层球体，为了提高球体的介电常数，中间层是与不同量（体积比2%至10%）钛酸钡混合的60:1 PDMS，外层是60:1 PDMS；（3）涂有一层未固化PDMS基础材料薄层的实心二氧化硅球体。在每种类型的传感器中，来自锥形光纤的激光耦合到提供高光学品质因数WGM（Q约为10(6)）的最外层。微球在约1 MV/m的电场下极化数小时，以提高其对电场的灵敏度。
