Optical sensors based on lossy-mode resonances

Lossy-mode resonance (LMR)–based optical sensing technology has emerged in the last two decades as a nanotechnological platform with very interesting and promising properties. LMR complements the metallic materials typically used in surface plasmon resonance (SPR)–based sensors, with metallic oxides and polymers. In addition, it enables one to tune the position of the resonance in the optical spectrum, to excite the resonance with both transverse electric (TE) and transverse magnetic (TM) polarized light, and to generate multiple resonances. The domains of application are numerous: as sensors for detection of refractive indices voltage, pH, humidity, chemical species, and antigens, as well as biosensors. This review will discuss the bases of this relatively new technology and will show the main contributions that have permitted the optimization of its performance to the point that the question arises as to whether LMR–based optical sensors could become the sensing platform of the near future

Recent Advances in Lossy Mode Resonance-Based Fiber Optic Sensors: A Review

Fiber optic sensors (FOSs) based on the lossy mode resonance (LMR) technique have gained substantial attention from the scientific community. The LMR technique displays several important features over the conventional surface plasmon resonance (SPR) phenomenon, for planning extremely sensitive FOSs. Unlike SPR, which mainly utilizes the thin film of metals, a wide range of materials such as conducting metal oxides and polymers support LMR. The past several years have witnessed a remarkable development in the field of LMR-based fiber optic sensors; through this review, we have tried to summarize the overall development of LMR-based fiber optic sensors. This review article not only provides the fundamental understanding and detailed explanation of LMR generation but also sheds light on the setup/configuration required to excite the lossy modes. Several geometries explored in the literature so far have also been addressed. In addition, this review includes a survey of the different materials capable of supporting lossy modes and explores new possible LMR supporting materials and their potential applications in sensing

High Sensitivity Optical Fiber Interferometric Sensors

Optical fiber interferometers have been widely employed and investigated for monitoring the changes in both physical and chemical parameters, with the advantages of compact size, light weight, immunity to electromagnetic interference, high sensitivity, capability to work in harsh environments and remote operation capabilities. Among the different kinds of fiber sensors based on interferometry, singlemode-multimode-singlemode (SMS) structures has attracted considerable interest due to their inherent advantages of high sensitivity, ease of fabrication and interconnection to other fiber systems and low cost. However, the challenge is that the sensitivity of the traditional SMS based fiber structure is not sufficient in some cases, for example in bio-chemical applications, where detection of a very small variation in a bio-chemicals’ concentration is required. There is thus a need to investigate how to modify or enhance an SMS structure to achieve ultrahigh sensitivity. This thesis presents research and its applications concerning approaches to improve the sensitivity and detection accuracy of a traditional SMS fiber structure based sensor. The key achievements of this thesis include: Traditional SMS fiber structure for breathing state monitoring A bend SMS structure is investigated as a breathing sensor by attaching it to a thin plastic film in an oxygen mask. Breath rate can be monitored using this sensor by detecting power variations due to the macro bending applied to the SMS section during each inhalation and exhalation cycles. Different types of breathing conditions including regular and irregular breath patterns can be distinguished. The proposed sensor is capable of working in a strong electromagnetic field and radioactive environment. Tapered small core singlemode fiber (SCSMF) for the detection of refractive index (RI), ammonia, and volatile organic compounds (VOCs) A modified SMS structure based on a tapered SCSMF is proposed and investigated with significantly improved RI sensitivity. It is found that the sample with a smaller waist diameter gives higher sensitivity. In the experiment, a maximum sensitivity of 19212.5 nm/RIU (RI unit) in the RI range from 1.4304 to 1.4320 has been demonstrated when the waist diameter of the SCSMF is tapered down to 12.5 μm. The best corresponding theoretical resolution of the proposed sensor is 5.025 × 10-7 RIU which is over 10 times higher than that of many previous reported optical fiber based RI sensors. The proposed structure is capable of monitoring relative humidity level change even without coating of the fiber sensor’s surface with a layer of hygroscopic material. A silica sol-gel based coating has been used as a sensitive material to ammonia for the first time, by applying it to the surface of the tapered SCSMF for the detection of ammonia in water. The proposed sensor shows an ultra-high sensitivity of 2.47 nm/ppm with short response and recovery time of less than 2 and 5 minutes respectively. The corresponding theoretical detection limit of ammonia in water is calculated to be 4 ppb, which is 3 orders of magnitude improvement compared to the previous reported interferometry based ammonia sensor. In addition, the sensor has good performance in terms of repeatability of measurement and selectivity for sensing ammonia compared to that of other common ions and organic molecules in water. VOCs sensors are also demonstrated by coating a mixture of sol-gel silica and Nile red on the surface of two different types of tapered fiber sensors (tapered SCSMF) and a microfiber coupler (MFC)). The MFC based sensor shows better sensitivities to ethanol and methanol than that based on a tapered SCSMF due to its smaller waist diameter. The detectable gas concentration changes of the MFC based sensor are calculated to be ~77 ppb and ~281 ppb for ethanol and methanol respectively which are over one order of magnitude improvement than many other reports. The sensors also show fast response times of less than 5 minutes and recovery times varied from 7 to 12 minutes. Simultaneous measurement of ethanol and methanol is achieved by utilizing two different coating recipes. Hollow core fiber (HCF) structure for high temperature and twist sensing. A modified SMS structure with much improved spectral quality factor (Q) is investigated both theoretically and experimentally. The modified structure is based on a HCF. It is found that periodic transmission dips with high spectral extinction ratio and high Q factor are excited because of the multiple beam interferences introduced by the cladding of the HCF. The HCF structure can be used as a high sensitivity (up to 33.4 pm/°C) temperature sensor in a wide working temperature range (from room temperature to 1000 °C). By coating a thin layer of silver (~ 6.7 nm) on one side of the HCF surface, a twist sensor with a maximum sensitivity of 0.717 dB/°has been achieved, which is the highest twist sensitivity reported for intensity modulation based fiber sensors, with excellent measurement repeatability. Further theoretical and experimental investigation attributes this high twist sensitivity to the polarization dependent reflection coefficient at the outer HCF surface associated with the partial silver coating

Recent advances in plasmonic sensor-based fiber optic probes for biological applications

Funding: This research was funded by National Natural Science Foundation of China (NSFC), grant number [61675008]. Acknowledgments: KN wishes to thank The Royal Society Kan Tong Po International Fellowship 2018 for the travel fund to visit Hong Kong Polytechnic University and Shenzhen Science and Technology Innovation Commission (Project GJHZ20180411185015272).Peer reviewedPublisher PD

Specially Shaped Optical Fiber Probes: Understanding and Their Applications in Integrated Photonics, Sensing, and Microfluidics

Thanks to their capability of transmitting light with low loss, optical fibers have found a wide range of applications in illumination, imaging, and telecommunication. However, since the light guided in a regular optical fiber is well confined in the core and effectively isolated from the environment, the fiber does not allow the interactions between the light and matters around it, which are critical for many sensing and actuation applications. Specially shaped optical fibers endow the guided light in optical fibers with the capability of interacting with the environment by modifying part of the fiber into a special shape, while still preserving the regular fiber’s benefit of low-loss light delivering. However, the existing specially shaped fibers have the following limitations: 1) limited light coupling efficiency between the regular optical fiber and the specially shaped optical fiber, 2) lack special shape designs that can facilitate the light-matter interactions, 3) inadequate material selections for different applications, 4) the existing fabrication setups for the specially shaped fibers have poor accessibility, repeatability, and controllability. The overall goal of this dissertation is to further the fundamental understanding of specially shaped fibers and to develop novel specially shaped fibers for different applications. In addition, the final part of this dissertation work proposed a microfluidic platform that can potentially improve the light-matter interactions of the specially shaped fibers in fluidic environments. The contributions of this dissertation work are summarized as follows: 1) An enhanced fiber tapering system for highly repeatable adiabatic tapered fiber fabrications. An enhanced fiber tapering system based on a novel heat source and an innovative monitoring method have been developed. The novel heat source is a low-cost ceramic housed electric furnace (CHEF). The innovative monitoring method is based on the frequency-domain optical transmission signal from the fiber that is being tapered. The enhanced fiber tapering system can allow highly repeatable fabrication of adiabatically tapered fibers. 2) A lossy mode resonance (LMR) sensor enabled by SnO2 coating on a novel specially shaped fiber design has been developed. The developed LMR sensor has a D-shape fiber tip with SnO2 coating. It has the capability of relative humidity and moisture sensing. The fiber-tip form factor can allow the sensor to be used like a probe and be inserted into/removed from a tight space. 3) Specially shaped tapered fibers with novel designs have been developed for integrated photonic and microfluidic applications. Two novel specially tapered fibers, the tapered fiber loop and the tapered fiber helix have been developed. The tapered fiber loop developed in this work has two superiority that differentiated itself from previous works: a) the mechanical stability of the tapered fiber loop in this work is significantly better. b) the tapered fiber loops in this work can achieve a diameter as small as 15 ?m while still have a high intrinsic optical quality factor of 32,500. The tapered fiber helix developed in this work has a 3D structure that allows it to efficiently deliver light to locations out of the plane defined by its two regular fiber arms. Applications of the tapered fiber helices in both integrated photonic device characterizations and microparticle manipulations have been demonstrated. 4) Developed an acrylic-tape hybrid microfluidic platform that can allow function reconfiguration and optical fiber integration. A low-cost, versatile microfluidic platform based on reconfigurable acrylic-tape hybrid microfluidic devices has been developed. To the best of the author’s knowledge, this is the first time that the fabrication method of sealing the acrylic channel with a reconfigurable functional tape has been demonstrated. The tape-sealing method is compatible with specially shaped fiber integrations

Strong optical coupling between 3D confined resonant modes in microtube cavities

Coupled whispering-gallery-mode (WGM) optical microcavities have been extensively explored to tune the resonant eigenfrequencies and spatial distributions of the optical modes, finding many unique photonic applications in a variety of fields, such as nonlinear optics, laser physics, and non-Hermitian photonics. As one type of WGM microcavities, microtube cavities with axial potential wells support 3D confined resonances by circulating light along the microtube cross-section and axis simultaneously, which offers a promising possibility to explore multidimensional and multichannel optical coupling. In this thesis, the optical coupling of 3D confined resonant modes is investigated in coupled microtube cavities fabricated by self-rolling of prestrained nanomembranes. In the first coupling system, multiple sets of 3D optical modes are generated in a single microtube cavity owing to nanogap induced resonant trajectory splitting. The large overlap of optical fields in the split resonant trajectories triggers strong optical coupling of the 3D confined resonant modes. The spectra anticrossing feature and changing-over of one group of coupled fundamental modes are demonstrated as direct evidence of strong coupling. The spatial optical field distribution of 3D coupling modes was experimentally mapped upon the strong coupling regime, which allows direct observation of the energy transfer process between two hybrid states. Numerical calculations based on a quasi-potential model and the mode detuning process are in excellent agreement with the experimental results. On this basis, monolithically integrated twin microtube cavities are proposed to achieve the collective coupling of two sets of 3D optical modes. Owing to the aligned twin geometries, two sets of 3D optical modes in twin microtubes are spectrally and spatially matched, by which both the fundamental and higher-order axial modes are respectively coupled with each other. Multiple groups of the coupling modes provide multiple effective channels for energy exchange between coupled microcavities, which are illustrated by the measured spatial optical field distributions. The spectral anticrossing and changing-over features of each group of coupled modes are revealed in experiments and calculations, indicating the occurrence of strong coupling. In addition, the simulated 3D mode profiles of twin microcavities confirm the collective strong coupling behavior, which is in good agreement with the experimental results. Our work provides a compact and robust scheme for realizing 3D optical coupling, which is of high interest for promising applications such as 3D non-Hermitian systems and multi-channel optical signal processing

Optical biosensors

Continuous glucose monitoring facilitates the stringent control of blood glucose concentration in diabetic and intensive care patients. Low-cost, robust, and reusable continuous glucose monitoring systems that can provide quantitative measurements at point-of-care settings is an unmet medical need. Phenylboronic acids (PBAs) have emerged as synthetic receptors that can reversibly bind to cis-diols such as glucose molecules. The incorporation of phenylboronic acids in hydrogels offer exclusive attributes as the binding process with glucose induces Donnan potential that leads to osmotic pressure, resulting in volumetric changes in the hydrogel matrix. Optical glucose sensors based on PBA-functionalized hydrogels have emerged as strong candidates for commercialization; however, their complex and time-consuming fabrication process, and their bulky and expensive readouts methods made them undesirable for quantitative analyses. In this dissertation, optical hydrogel sensors have been developed and attached to contact lenses for continuous glucose detection in physiological conditions. A simple fabrication method was utilized, and smartphone technology was employed for recording the output signals. A 1D photonic structure was replicated on a PBA-functionalized hydrogel to function as a transducer and to improve the sensor performance. Upon binding glucose with boronate anions immobilized in the hydrogel matrix, a positive volumetric shift occurred modifying the periodic constant of the photonic structure, consequently its diffraction properties altered. A correlation has been established between the sensor’s periodic constant and glucose concentration in the range of 0-50 mM. The hydrogel sensor was attached to a soft commercial contact lens (ACUVUE) and was interrogated for glucose detection in artificial tears. The ambient light sensor of a smartphone captured the intensity of the laser diffracted signals and was correlated with glucose concentrations. The smart contact lens showed very short response time (3 s), and a saturation time of near 4 minutes in continuous monitoring conditions. However, a laser beam was necessary to interrogate the contact lens which is uncomfortable, and the frequent exposure might be harmful to the eye cornea. Alternatively, a novel transducer has been introduced to enable interrogating the smart contact lens by using a white light beam. Light diffusing microstructures (LDMs) have been introduced for the first time for the sensing applications. The LDMs can be considered as densely-packed microparticles of different shapes and dimensions which have the capacity to diffuse the polychromatic and monochromatic light in the forward and backward directions. The LDMs were imprinted on the glucose -responsive hydrogel to monitor the volumetric shift due to glucose complexation. The volumetric modification of the hydrogel upon glucose complexation induced a change in the dimensions and refractive index of the LDMs, resulting in a variation in the diffusion efficiency. The glucose sensor was attached to a commercial contact lens and a smartphone measured the optical output signals. The alternative transducer enabled interrogating the smart contact lens by a white light beam and retained on the simplicity of the fabrication and the readout methodology; however, the response time of the senor increased significantly. The proposed smart contact lenses can be considered as a non-invasive way for continuous glucose monitoring, and can detect many other biomarkers that are beneficial for medical diagnostic applications. For implantable and remote monitoring of glucose concentration, fiber optic probes have been developed. Fiber optics have inherent advantages such as immunity to electromagnetic interference, miniaturization, and small volumes of samples. These merits candidate them for biosensing applications; however, complexity of the manufacturing process, poor mechanical properties, unpracticality of the readout methodology have hindered their practical applications. We have developed fiber optic probes for glucose detection that overcome the limitations mentioned above. Capability of the LDMs to scatter/diffuse the incident light beam in the forward and backward directions was exploited. The glucose responsive hydrogel imprinted with the LDMs was attached to the tip of a multimode silica fiber. Swelling of the attached hydrogel led to a decrease in the refractive index of the LDMs, inducing a decrease in the light scattering angles. Whereas the numerical aperture of the optical fiber indicates the range of the angles of the incident rays those satisfy the total internal reflection condition. Accordingly, swelling the hydrogel attached to the fiber result in more incident rays fall within the accepted range of angles to be guided in the optical fiber. Hence, the optical power guided in the fiber increased with glucose concentration. The fabricated fiber probe was interrogated for glucose detection in transmission configuration and the smartphone was utilized to pick up the fiber’s signals. Also, the probe was tested in reflection configuration which is a more practical mode for implantable biosensing applications. The probe overcame some limitations of the existing probes such as interferometric, SPR, and fluorescent probes in terms of ease of the fabrication and the interrogation processes. Additionally, the probe showed high sensitivity, rapid response, and selectivity for glucose over lactate. The lactate interference was found to be ~ 0.1% in the physiological condition. Furthermore, biocompatible hydrogel fibers were introduced to prevent or reduce the immune reaction in the implantation sites. The probes were tested for glucose detection and showed similar response to that of silica fiber probes; however, they presented lower sensitivity which might be the result of a higher light loss in the hydrogel fiber. In order to emphasize the variety of applications of these novel fiber optic probes that we developed, two more probes were fabricated for alcohol detection and pH measurements. The alcohol probe showed real-time sensing of ethanol, propanol, and dimethyl sulfoxide with a response time in seconds and a saturation time around 60 s. Also, the pH probe showed high sensitivity and rapid response in the acidic region with a sensitivity near 20% pH-1. For medical applications, the pH sensor was attached to a biocompatible fiber optic and was tested for pH sensing in reflection configuration. The probe can be recommended for gastric pH detection. The fabricated optical fiber sensors may also have applications in wearable and implantable point-of-care and intensive-care continuous monitoring systems

Energy Technology Division research summary 2004.

The Energy Technology (ET) Division provides materials and engineering technology support to a wide range of programs important to the US Department of Energy (DOE). The Division's capabilities are generally applied to technical issues associated with energy systems, biomedical engineering, transportation, and homeland security. Research related to the operational safety of commercial light water nuclear reactors (LWRs) for the US Nuclear Regulatory Commission (NRC) remains another significant area of interest for the Division. The pie chart below summarizes the ET sources of funding for FY 2004

High Sensitivity Refractometer Based on TiO2-Coated Adiabatic Tapered Optical Fiber via ALD Technology

Atomic layer deposition (ALD) technology is introduced to fabricate a high sensitivity refractometer based on an adiabatic tapered optical fiber. Different thicknesses of titanium dioxide (TiO2) nanofilm were coated around the tapered fiber precisely and uniformly under different deposition cycles. Attributed to the higher refractive index of the TiO2 nanofilm compared to that of silica, an asymmetric Fabry–Perot (F-P) resonator could be constructed along the fiber taper. The central wavelength of the F-P resonator could be controlled by adjusting the thickness of the TiO2 nanofilm. Such a F-P resonator is sensitive to changes in the surrounding refractive index (SRI), which is utilized to realize a high sensitivity refractometer. The refractometer developed by depositing 50.9-nm-thickness TiO2 on the tapered fiber shows SRI sensitivity as high as 7096 nm/RIU in the SRI range of 1.3373–1.3500. Due to TiO2’s advantages of high refractive index, lack of toxicity, and good biocompatibility, this refractometer is expected to have wide applications in the biochemical sensing field