Study On Micro - Cavity Coupled Echo Wall Mode Micro - Cavity Laser Sensor

Whispering-gallery-mode (WGM) microcavitys can confine light in the dimension of mircometers by total internal reflection. Usually they possess high quality factor and small mode volume which are conducive to enlarging the interaction area and increasing the interaction time between light and analyte, thus they can realize high sensitivity and low detection limit for label-free optical bio-sensing, especially for single particle detection. Currently, the research that using single WGM microcavity as sensing component is limited by the structure of microcavity and signal resolution, and their best performance is less than1000nm/RIU for bulk refractive index detection and12.5nm in radius polystyrene sphere for particle detection.In order to improve the sensing performance of WGM microcavity, many enhancing mechanics have been introduced, including active microcavity, plasmon, cascaded or coupled microcavity and optical tweezer. This thesis will focus on the design, fabrication and measurement of ultrahigh sensitive optical bio-sensor based on the coupled optofluidic microcavity structure. The main results include:1. Theoretically, the WGMs in single optofluidic ring resonator(OFRR) and their sensitivity have been analyzed. Based on the experimental structure, we analyzed the amplification of the mode sensitivity when different order of modes were coupled with each other according to the Vernier effect. The results showed that in our experiment, the amplified sensitivity by the Vernier effect could reach103～105nm/RIU for bulk refractive index detection, which is several orders of magnitude greater than the utmost sensitivity of single OFRR. The sensitivity limitation of single OFRR could be broken through by the coupled OFRR structure.2.We investigated the coupling area of on-chip coupled microdisks by focused beam heating method experimentally. And the results showed that the special properties of coupling area didn’t influence the Vernier effect of coupled WGM microcavity laser. In this experiment, a532nm CW laser beam was focused as small as40μm in diameter to heat different areas of RhB doped coupled SU-8microdisks, resulting in the refractive index change of localized area. By increasing the laser power, we can change the refractive index and the emitted spectrum shifted as a result. The spectral shift conforms with the Vernier effect.3.We designed and fabricated two kinds of ultrahigh sensitive coupled optofluidic ring laser sensor:coupled optofluidic ring laser (COFRL) and coupled double optofluidic rings laser (CDOFRL). Both have realized ultrahigh sensitivity for aqueous sample detection. In the COFRL design, a thin-wall capillary (optofluidic ring) was coupled with a solidstate ring laser. In this structure the sensitivity of optofluidic ring was amplified by the Vernier Effect, and the amplified factor reached355. The sensitivity of COFRL for bulk refractive index detection (BRID) was2510nm/RIU, which corresponded to a noise equivalent detection limit (NEDL) of1.6×10-5RIU for BRID and6.9pg/mm2for protein molecules such as Bovine serum albumin (BSA) detection. In order to improve the detection limit, we used optofluidic ring laser instead of the solidstate ring laser in the second design to decrease the noise. In this structure sensitivity of2227nm/RIU experimentally has been realized, which corresponded to a NEDL of4.5×10-6RIU for BRID and1.9pg/mm2for BSA detection, respectively. This coupled optofluidic ring laser sensor provided a novel way for developing high sensitive and low detection limit bio-sensor.