| Author: | Jiang, Xiaoping |
| Title: | Novel fabry-pérot resonators for sensing applications |
| Advisors: | Chen, Wen (EEE) Lun, P. K. Daniel (EEE) Somekh, Michael (EEE) |
| Degree: | Ph.D. |
| Year: | 2023 |
| Subject: | Optical detectors Optical transducers Detectors Hong Kong Polytechnic University -- Dissertations |
| Department: | Department of Electrical and Electronic Engineering |
| Pages: | xxi, 181 pages : color illustrations |
| Language: | English |
| Abstract: | Optical thin-film sensors and transducers have been developing rapidly for decades with applications in various fields, e.g., chemical detection, biological sensing and medical imaging. Optical thin-film sensors utilize the interaction of light with materials or objects for information regarding their properties and the environment. This thesis is focused on two types of sensing applications: (i) refractive index sensing and (ii) photoacoustic sensing and imaging. For refractive index sensing, one well-established type of sensor is the surface plasmon resonator (SPR), where a refractive index change causes the resonance to drift thus resulting in amplitude or phase shifts. For photoacoustic sensing and imaging, optical transducers detect either local pressure changes, e.g., SPR, or resonance light path changes, e.g., Fabry-Pérot (FP) thin-film resonators. An FP thin-film resonator consists of a cavity sandwiched between two reflecting mirrors. When in resonance, incident light wave interferes with the light wave circulating inside the cavity, resulting in a decrease in reflectivity. For refractive index sensing, sensors can be evaluated by their figure of merit (FOM) and sensitivity, which refers to the noise-equivalent refractive index unit. In this thesis, at 633 nm wavelength, a novel open cavity FP structure based on a high-aspect-ratio-grating (HARG-FP) was investigated for refractive index sensing applications. A lateral comparison with SPR was drawn. The results indicated 10- and 44-fold improvement in FOM with TM and TE polarization compared to the conventional SPR, respectively. TE samples exhibited a single-shot sensitivity of 1.87 × 10−8 RIU. With 794 frames averaging (~1-second acquisition), the sensitivity can be further improved to 1.61 × 10−9 RIU, demonstrating ultra-sensitivity. For the field of photoacoustic sensing, optical transducers offer exceptional potential of high responsivity, broad bandwidth and significantly reduced acoustic impedance mismatch compared with conventional piezoelectric transducers. Besides sensitivity, bandwidth is another crucial factor characterizing an acoustic transducer. A total-internal-reflection-based Fabry-Pérot resonator (TIR-FP) was evaluated for ultra-sensitive wideband ultrasound and photoacoustic applications at around 1550-nm wavelength. The proposed transducer consisted of a 12-nm-thick gold layer and a dielectric cavity. 1.9-μm-thick-PMMA- and 3.4-μm-thick-PDMS-based resonators showed responsivities of 3.6- and 30-fold improvements compared with the conventional SPR, respectively. Examined with photoacoustic signals generated by focused nanosecond laser pulses, bandwidths for the PMMA- and PDMS-based devices were 110 MHz and 75 MHz, respectively. Single-shot sensitivity of 160 Pa was obtained for the PDMS-based TIR-FP. The results indicated that, with the proposed resonator in imaging applications, sensitivity and the signal-to-noise ratio can be improved significantly without compromising the bandwidth. Research has been proposed for a grating-based Fabry-Pérot (GFP) resonator as a highly sensitive detector. The structure features two diffraction gratings as the mirrors forming the FP. The period of the grating is engineered so that only the zero-th and first orders exist inside the cavity and only the zero-th order exists outside the cavity. with a 50-μm cavity, double gratings enable rich interaction between the zero-th and the first order, which gives resonance linewidth ranging from 1 pm to 1 nm with Q ranging from 1.5 × 103 to 1.5 × 106. The freedom in Q and dynamic range offered within the same structure promises great potential for photoacoustic sensing. |
| Rights: | All rights reserved |
| Access: | open access |
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