基于光学微腔的纳米颗粒尺寸测量研究

Over the past few years, optical whispering gallery mode (WGM) microcavities have become valuable tools in sensing applications due to the significantly enhanced light-matter interaction provided by their ultrahigh-Q factors and small mode volumes. Sensing of nanoscale objects with ultrahigh sensitivity is of crucial importance for many applications including early stage diagnosis of diseases, process control of semiconductor manufacturing, environmental monitoring, and homeland security. During the recent few years, great progress has been achieved with the detection limit down to a single nanoparticle level. However, accurate sizing of a single nanoparticle still remains difficult. A nanoparticle appearing in the near field of the microcavity dramatically affects the mode spectra in the form of resonant frequency shifts, mode splitting and linewidth broadening. The spectrum signals rely on the refractive index, size and location of the nanoparticle. Since the nanoparticle location is hard to be determined, the measurement of the nanoparticle size is generally based on the assumption that the nanoparticle is locked at the position where the field intensity is maximum by optical force. In this proposal, upon one single nanoparticle binding event, rather than just monitoring one mode of the microcavity as usual, we propose to monitor several high Q split polar modes at the same time. Through this way, we try to achieve the accurate location of the nanoparticle in all three dimensions: the polar, azimuthal and radial dimensions. Based on the location, high sensitivity nanoparticle sizing can be fulfilled. Experimentally, we will prepare ultrahigh-Q microtoroids and micro-spheroids. The split polar modes can be excited with the same tapered fiber through near field coupling. By monitoring the spectra of the split polar modes, the accurate location as well as the sizing of a single nanoparticle can be achieved.

回音壁模式光学微腔具有超高品质因子和较小模式体积，可以显著增强光与物质的相互作用，因而在超高灵敏光学传感领域具有重要应用价值。尽管人们已经利用光学微腔实现了纳米尺度颗粒的检测，但往往只能给出颗粒的浓度信息。究其原因，现有的传感方法无法获得纳米颗粒的位置，因而不能测量颗粒的尺寸，严重制约了其在更广泛领域的应用。本项目提出：针对在微腔表面的同一纳米颗粒吸附事件，通过同时监测微腔多个高品质因子模式的移动、劈裂和展宽等信息，可获知纳米颗粒在微腔表面上的三维位置（方位角、极角和径向坐标），进而计算出颗粒的等效尺寸等重要物理参数。实验上，课题组将首先制备超高品质因子微芯圆环腔和微球腔；接着，利用可调谐激光作为激发光源，通过光纤锥同时近场耦合多个微腔模式；最后测量单个纳米颗粒吸附在微腔表面时的模式变化。申请人在微腔传感领域具有多年研究基础，有望取得重要进展，将微腔传感从纳米颗粒检测推进到尺寸测量。

由于具有超高的品质因子(~108)和较小的模式体积，回音壁模式光学微腔能够显著地增强光与物质之间的相互作用。基于回音壁模式光学微腔的高灵敏生物化学传感是十分重要的研究方向。实现纳米尺度的单个颗粒的探测是目前光学传感的重大需求。尽管基于回音壁模式光学微腔的纳米颗粒探测已经取得一系列重要进展，纳米颗粒尺寸的精确测定却很困难。基于回音壁模式光学微腔的纳米颗粒尺寸测量主要受制于一个难题：纳米颗粒吸附位置的精确测定。. 我们对任意形状的瑞利颗粒引起的回音壁模式光学微腔的模式劈裂展开了研究。当纳米颗粒粘附在微腔表面时，微腔的模式劈裂和展宽都依赖于颗粒的极化率。对于实际传感场景中的颗粒，各项异性导致决定模式劈裂和决定模式展宽的两个极化率分量不同。利用微腔的一对逆向传播的模式，当它们具有不同的电场方向且非线性极化时，颗粒的粘附会引起这一对模式都发生改变。通过对两个TE基模劈裂的同时观测，能够获得颗粒的极角坐标。同时监测三个模式的劈裂和展宽，能够测得颗粒的方向以及极化率。. 我们还研究了多个纳米颗粒对类球形微腔的光场散射情况。对于一个稍微偏离球形的微腔，原本模式简并被解除，各阶模式具有不同的频率和电磁场分布。当待测目标纳米颗粒进入微腔倏逝场时，它对这些模式的扰动必定不同。针对在微腔表面的同一纳米颗粒吸附事件，通过同时监测微腔多个高品质因子模式的移动、劈裂和展宽等信息，能够同时获取纳米颗粒的极角、方位角和径向坐标的信息。在精确测定颗粒的吸附位置的前提下，纳米颗粒的尺寸也能得到精确的测量。我们从量子光学原理出发，提出了实现多个纳米颗粒位置及尺寸测定的传感方案。只要微腔的品质因子足够高，我们的方案能够对数百个纳米颗粒实现探测。.该项目将微腔传感推进到基于微腔的物理量测量领域，对微腔传感的实际应用具有重要的意义。