|Title:||Resonant optical tunneling effect for refractive index sensor applications|
|Subject:||Detectors -- Design and construction.|
Hong Kong Polytechnic University -- Dissertations
|Department:||Department of Applied Physics|
|Pages:||xviii, 156 leaves : ill. ; 30 cm.|
|Abstract:||This doctoral study focuses on the theoretical analysis of resonant optical tunneling effect (ROTE) and its potential application for ultrahigh-sensitivity refractive index sensing. More specifically, detailed studies have been conducted on the physical principles from the optics and quantum origins, two sensor device designs using the microfluidic chip and the angled fibers, the analyses of the structural parameters and performance, and the experimental results of the fabricated angled-fiber-based sensor. In the theoretical study of this work, the physical mechanism of the ROTE is examined from two origins nano-optics interpretation and quantum mechanism interpretation. Correspondingly, two theoretical models the transfer matrix model (TMM) and the potential barrier model (PBM) are developed. In this study, the equivalence of the two models is first tested using simple tunneling structures (2 and 3 layers). It is found that they give almost identical results in computing the transmission and reflection properties. For more complicated structures like the 5-layer ROTE structure, the two models share the same period and overlap well after a shift to compensate the phase difference. Through detailed studies of the two models, it is found that TMM is more convenient for simple structures whereas PBM works better for complicated structures.|
In the sensor design, two types of ROTE refractometers have been proposed. One utilizes microfluidic chip and the other makes use of a pair of angled optical fibers. Theoretical study shows that the microfluidic chip design has extremely sharp transmission peak and achieves a detectivity of 85,000 RIU⁻¹ about two orders of magnitude higher than the widely-used FabryPerot (FP) etalons and the surface plasmon resonance (SPR) sensors. In case of the angled fiber-based design, the simulation results show that the intensity-based method could reach a detection limit of 10⁻⁷ RIU and the spectrum-based method promises a sensitivity of 81,000 nm/RIU, which are about 10 times larger than the SPR sensors and over 180 times larger than the FP etalons. As the FP and SPR sensors have already demonstrated a detection limit of 10⁻⁴ 10⁻⁷ RIU, it is reasonable to expect the ROTE sensor offers a detection limit up to 10⁻⁶ 10⁻⁹ RIU. In the experimental studies, the angled fiber-based design is adopted due to its simple structure and easy implementation. For comparison, the refractive index sensor based on SPR effect has also been fabricated and obtained a sensitivity of 7,650 nm/RIU. As to the ROTE sensor, the ROTE sensor exhibits a series of peaks in the transmission spectrum. When the polarization state of the incident light is adjusted between the two polarization states, the intensity and position of the transmission peak can be varied significantly, agreeing well with the theoretical predictions. By slightly changing the fiber separation and then monitoring the peak wavelength shift, the sensor achieves an equivalent sensitivity of 3,500 nm/RIU. This work is probably the first systematic study on the physics mechanism of the ROTE. The investigations based on both the nano-optics and the quantum mechanics interpretation not only bring deep insight into the physical understandings of the ROTE, but also reveal an interesting analogy between two distinct fields－optics and quantum theory. This work is also the first attempt to apply the ROTE for sensing applications. Particularly, the experimental studies, though far from perfect, have well demonstrated the great potential of the ROTE sensors in ultrahigh sensitive detection of refractive index. Equipped with such a new working principle, the ROTE sensors may find broad applications in biochemical analysis, food safety and water quality monitoring.
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