| Author: | Wang, Zhizheng |
| Title: | High-Q directional-emission whispering-gallery-mode microlaser sensors for label-free biodetection |
| Degree: | M.Phil. |
| Year: | 2024 |
| Subject: | Lasers Biosensors Hong Kong Polytechnic University -- Dissertations |
| Department: | Department of Electrical and Electronic Engineering |
| Pages: | 83 pages : color illustrations |
| Language: | English |
| Abstract: | Optical microcavity is a kind of optical elements that has been widely proposed for high-sensitivity sensing in recent years. With the development of micro/nano fabrication technology and numerical simulation tools, many new sensing mechanisms of high quality-factor (Q) whispering-gallery-mode (WGM) microcavities have been demonstrated for various applications. Conventional optical microcavity sensors use circular microcavities. However, they can usually only be coupled with tapered optical fibers. This scheme requires a very accurate relative position between the tapered fiber and the microcavity, which means that such biosensors are prone to be disturbed by external environmental interferences. To solve this challenge, we demonstrated the deformed microcavities to develop high-Q WGM microlaser sensors with directional emission and far-field coupling capabilities for label-free biodetection. Two different kinds of deformed microcavities are designed for WGM microlaser sensing. The first is the Limacon-shaped microcavity, which has been proved to have a unidirectional emission around 0° direction. Another deformed microcavity we employed is elliptical microcavity. Due to the axial symmetry of its contour, it allows directional emission in four different directions. Numerical simulations using COMSOL have been systematically carried out to calculate the distribution of WGMs and their sensitivity to the refractive index change of external medium as well as their directional emission behaviors. These WGM microlaser sensors with weakly deformed microcavities have been experimentally fabricated and demonstrated for label-free biodetection. An optical 3D microprinting technology using a digital micromirror device (DMD) as optical pattern generator and a UV lamp as a projection light source has been established to rapidly fabricate different designs of WGM microcavities. A negative photoresist SU-8 was used to fabricate the designed WGM microcavities and microlaser sensors. A dynamic exposure scheme is developed to directly print 3D SU-8 microstructures for WGM microcavity and microlaser fabrication. The fabricated WGM microlaser sensors have been tested by using an own-established pumping test setup. A 532-nm pulsed laser was employed to pump these 3D micro-printed WGM microlasers. The spectral peak bandwidths of the fabricated WGM microlasers’ emission spectra were measured to be as narrow as about 20 pm, which corresponds to a very high Q value of around 2.51 x 104. In the experiments, WGM microlasers of both Limacon-shaped and elliptical microcavities showed directional emission clearly. The sensitivities of these WGM microlaser sensors were measured and compared with numerical simulation results. The biodetection ability of such 3D micro-printed WGM microlaser sensors has been demonstrated in experiments. The fabricated WGM microlaser sensors were modified with antibodies to specifically detect human immunoglobulin G (IgG) in the solution. When the antigen-antibody binding reaction occurs on the surface of WGM microlaser sensors, the wavelength of the WGM of microlaser sensor was shifted, allowing for specific detection of human IgG. The experiments revealed that the fabricated WGM microlaser biosensors can detect human IgG at the concentration level of around 15.94 ag/mL. Such WGM microlaser sensors have advantages of small size, high sensitivity, and high level of integration and thus offer new opportunities for the development of future portable disease diagnostic devices. |
| Rights: | All rights reserved |
| Access: | open access |
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