Evanescent Scattering by Ferromagnetic Microparticles for Versatile Optical Sensing


Student thesis: Doctoral Thesis

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Award date4 Sep 2020


Optical waveguide sensing (OWS) technologies have been undergoing rapid development in recent years and emerging as the key enabling technology in environment monitoring, healthcare, manufacturing quality control, transportation, and industrial technologies. In the applications where desire sensor miniaturization, high sensitivity, and remote control, the OWS technologies, in general, exhibit a number of advantages over conventional or competitive solutions. These include compact and lightweight, excellent integrability, immunity to electromagnetic interferences (EMI), adaptability in hazardous environments, excellent multiplexing capabilities, and chemical passivity. Nowadays, many OWS devices such as gyroscopes, vibrometer, temperature probes, biosensors, chemical monitors etc. are commercially available as a result of decades of research and development works.

In most cases, OWS technologies facilitate the optical waveguide as a sensing head relying on the interaction of the evanescent waves with the sensing medium located at the waveguide layers. By adopting proper designs, the evanescent interactions can be implemented by evanescent wave absorption (EWA),evanescent wave coupling (EWC) and evanescent wave scattering (EWS). EWA sensors refer to a pioneer evanescent sensing type for detecting chemical bonds as well as monitoring biomolecular interactions. This type of sensing approach features in simple design, direct measurement and ease of operation. However, it has a major drawback that only specific chemical bonds or biomolecular species are applicable. To circumvent this issue, EWC as an indirect sensing approach could be used. It operates by correlating the analyte concentration with its refractive index change through various optical sensing schemes, such as interferometers, optical gratings and multi-mode interferences (MMI), surface plasmon resonances(SPR) etc. EWC sensors is more favorable over EWA sensors in the sense that it offers design diversities and interrogation flexibilities for various sensing applications. However, EWC sensors do suffer from complex design fabrication as well as high building cost. For instance, surface plasmon resonance (SPR) is one type of versatile EWC sensing technique due to its high response sensitivity and analysis usability. However, SPR sensors are intrinsic polarization and wavelength dependence, which are both not desirable as practical sensing devices. Further, it also demands a deposition of metallic ultra-thin films which requires sophisticated film deposition equipment, resulting in difficult control in production yields.  In view of these limitations, researchers have adopted EWS with moderate optical sensing performances but have evaded restrictions of the abovementioned plasmonic sensors. EWS is an indirect light scattering technique on the evanescent wave residing in the medium of low density, for optical uses. EWS has many advantages over EWC techniques which include broadband functionality, no polarization selectivity, ease of operation, and ease of design implementation with low cost. This phenomenon has been used in various platforms for determining particle size, tracking the motion of biological species, and real-time humidity transducing.

In this thesis, we devote to developing a few simple EWS platforms for versatile optical sensing based on a planar waveguide structure. All these EWS sensing platforms operate on the intensity-interrogation mode, that is, to collect the power transmission at the waveguide output. Since EWS demands nonplanar morphology for necessary scattering efficiency, the low-cost ferromagnetic microparticles were then chosen and manipulated to create the evanescent scattering loss required in our design work. The ferromagnetic microparticles are spherical carbonyl iron powder (CIP) purchased from BASF and consist of highly pure iron made by chemical decomposition of iron pentacarbonyl. Their intrinsic magnetizations lead to various industrial applications in manufacturing magnetic cores, inductive devices in electronics and food additives in pharmaceutics. Beyond that, CIP also exhibit unique extrinsic properties which strongly rely on their morphologies. In particular under an external magnetization, the mass CIPs exert strong particle-to-particle interactions which drive them to realign themselves to form self-assemble chain-like or columnar structures. The induced anisotropic particle-distribution brings the bulky anisotropic mechanical and optical features along or perpendicular to the field directions.

To facilitate the EWS, the ferromagnetic particles were controlled to form proper morphologies which are well positioned on top of a liquid-cladded optical waveguide. The gap distance between particles and waveguide surface will be manipulated dynamically within submicron range. As a result, this gap distance is sufficiently small to allow evanescent scattering between the particles and the propagating light within the waveguide. Due to nonplanar morphology and metallic dissipation of iron particles, a portion of evanescent waves will be scattered during the propagation. As a result, the scattering of evanescent field can be regulated by an external actuation, namely, the measurand (electronic current, magnetic field or mechanical vibration), to modulate the movement of ferromagnetic particles, leading to a change in the waveguide output intensity for optical sensing. This is the fundamental principle of EWS for optical sensing. This thesis presents studies on several EWS platforms based on ferromagnetic particles for versatile optical sensing, which include electronic current sensor, submicron vibration sensor, and magnetic field sensor. The proposed sensing applications will find potentials of further packaging for industrial and commercial products. In addition, the proposed EWS platforms show great potentials to be extended for other sensing uses by proper designs.

Firstly, we propose an EWS platform for optical sensing based on precise manipulation of self-assembled ferromagnetic columns. The degree of evanescent scattering was realized by controlling the gap distance between the column tips and waveguide interface which is achieved by the realignment of ferromagnetic columns actuated by external magnetic field. Consequently, the scattering interactions cause changes in the waveguide output intensity for optical sensing. The optical characteristics, including polarization dependence, static and dynamic responses, tunable sensitivity, signal reversibility, stability, and robustness, were investigated experimentally based on a liquid-cladded planar waveguide. The platform shows excellent signal reversibility and stability. A current sensor was proposed and realized to illustrate its potential in sensing application. Its current sensitivity to the TM polarization can be varied dynamically between 9 and 20 dB/A by varying the DC-biased current between 0.15 and 0.3 A. Moreover, a higher sensitivity is possible by further design optimization. The platform has the advantages of real-time response, short interaction length, broadband operation, and easy implementation at potentially low cost.

Secondly, we investigate the dynamic performance of the EWS platform reported in our 1st work and apply it for low distortions submicron vibration sensing. The platform consists of self-assembled ferromagnetic cantilevers located above a liquid-cladded optical waveguide. Theoretical analyses show that both enhancement of sensitivity and dynamic sensing range are achieved by reducing the waveguide core-cladding index difference. Moreover, a careful tradeoff between sensitivity and linearity is required, which is determined by the bias position of the cantilever tip. Experimental results confirm that our platform can offer low total-harmonic-distortions (THD) of< 3.00% with a submicron displacement of 0.40 µm over the frequency range from 80 Hz to 750 Hz. The measured THD value is very close to our theoretical prediction. Thus, our platform can be employed in submicron vibration sensing with high-precision requirements.

In 3rd work, we present a simple concept to implement a high sensitivity magnetic sensor that uses evanescent scattering by a suspended magnetorheological (MR) film above an optical waveguide. The soft MR film, which was pre-biased into the evanescent field, generates magnetic torques under an external magnetization, thereby driving the embedded particles to scatter the evanescent field.  As a result, the waveguide output power changes in response to the magnetic intensity. Two sensor prototypes of different film thickness were designed and tested showing a tradeoff between the sensitivity and dynamic sensing range. A maximum sensitivity of ~2.62 dB/mT was obtained while the minimum limit of detection(LOD) value was down to ~0.016 mT. The sensor shows high sensitivity and low detection limit with moderate response time of ~750 ms. Moreover, the design is extremely simple to implement at low cost and has high potential to be integrated into portable devices.

To summarize, we report a few EWS platforms for optical sensing based on ferromagnetic microparticles, which operates in form of either ferromagnetic columns or MR thin film. Our proposed platforms can achieve high sensitivities with good linearity through proper designs. Three examples, namely, an AC current sensor, a submicron vibration sensor and a magnetic field sensor were successfully designed, experimentally verified and compared with other sensing schemes. In addition, our proposed sensors illustrate good potentials to be deployed for other uses, such as sensing ultrasonic waves, variable optical attenuations (VOA), incorporated into a pair of directional couplers for optical switching and magnet-steerable fiber coupling array systems etc. In biomedical uses, the platforms can be further packaged for blood pressure monitoring or breath inspection. All these potentials would bring opportunities and boost the development to optical sensing and communication applications.