Broad-band Mach-Zehnder interferometers as high performance refractive index sensors: theory and monolithic implementation

Sensing the Future-Frontiers in Biosensors: Exploring Classifications, Principles, and Recent Advances

Biosensors are transforming healthcare by delivering swift, precise, and economical diagnostic solutions. These analytical instruments combine biological indicators with physical transducers to identify and quantify biomarkers, thereby improving illness detection, management, and patient surveillance. Biosensors are widely utilized in healthcare for the diagnosis of chronic and infectious diseases, tailored treatment, and real-time health monitoring. This thorough overview examines several categories of biosensors and their uses in the detection of numerous biomarkers, including glucose, proteins, nucleic acids, and infections. Biosensors are commonly classified based on the type of transducer employed or the specific biorecognition element utilized. This review introduces a novel classification based on substrate morphology, offering a comprehensive perspective on biosensor categorization. Considerable emphasis is placed on the advancement of point-of-care biosensors, facilitating decentralized diagnostics and alleviating the strain on centralized healthcare systems. Recent advancements in nanotechnology have significantly improved the sensitivity, selectivity, and downsizing of biosensors, rendering them more efficient and accessible. The study examines problems such as stability, reproducibility, and regulatory approval that must be addressed to enable the widespread implementation of biosensors in clinical environments. The study examines the amalgamation of biosensors with wearable devices and smartphones, emphasizing the prospects for ongoing health surveillance and individualized medical care. This viewpoint clarifies the distinct types of biosensors and their particular roles, together with recent developments in the "smart biosensor" sector, facilitated by artificial intelligence and the Internet of Medical Things (IoMT). This novel approach seeks to deliver a comprehensive evaluation of the present condition of biosensor technology in healthcare, recent developments, and prospective paths, emphasizing their significance in influencing the future of medical diagnostics and patient care.

Influence of Ion Exchange Process Parameters on Broadband Differential Interference

The paper presents theoretical analyses and experimental investigations of broadband differential interference in planar gradient waveguides made via K⁺-Na⁺ ion exchange in BK-7 glass. This technology, due to its large polarimetric dispersion, is especially useful for applications in differential interferometry. We discuss the influence of technological parameters on the operation characteristics of the structure in terms of sensor applications. The refractive index variation in the measured external surroundings affects the modal properties of TE and TM modes and the spectral distribution at the output of the differential interferometer. The optical system described in this work has been designed specifically for use in biological systems where variations in the index of refraction need to be measured.

Determining the Nutrient Content of Hydroponically-Cultivated Microgreens with Immersible Silicon Photonic Sensors: A Preliminary Feasibility Study

Microgreens have gained attention for their exceptional culinary characteristics and high nutritional value. The present study focused on a novel approach for investigating the easy extraction of plant samples and the utilization of immersible silicon photonic sensors to determine, on the spot, the nutrient content of microgreens and their optimum time of harvest. For the first time, it was examined how these novel sensors can capture time-shifting spectra caused by the molecules' dynamic adhesion onto the sensor surface. The experiment involved four types of microgreens (three types of basil and broccoli) grown in a do-it-yourself hydroponic installation. The sensors successfully distinguished between different plant types, showcasing their discriminative capabilities. To determine the optimum harvest time, this study compared the sensor data with results obtained through standard analytical methods. Specifically, the total phenolic content and antioxidant activity of two basil varieties were juxtaposed with the sensor data, and this study concluded that the ideal harvest time for basil microgreens was 14 days after planting. This finding highlights the potential of the immersible silicon photonic sensors for potentially replacing time-consuming analytical techniques. By concentrating on obtaining plant extracts, capturing time-shifting spectra, and assessing sensor reusability, this research paves the way for future advancements in urban farming.

Directly immersible silicon photonic probes: Application to rapid SARS-CoV-2 serological testing

Silicon photonic probes based on broad-band Mach-Zehnder interferometry are explored for the first time as directly immersible immunosensors alleviating the need for microfluidics and pumps. Each probe includes two U-shaped waveguides allowing light in- and out-coupling from the same chip side through a bifurcated fiber and a mechanical coupler. At the opposite chip side, two Mach-Zehnder interferometers (MZI) are located enabling real-time monitoring of binding reactions by immersion of this chip side into a sample. The sensing arm windows of the two MZIs have different length resulting in two distinct peaks in the Fourier domain, the phase shift of which can be monitored independently through Fast Fourier Transform of the output spectrum. The photonic probes analytical potential was demonstrated through detection of antibodies against SARS-CoV-2 in human serum samples. For this, one MZI was functionalized with the Receptor Binding Domain (RBD) of SARS-CoV-2 Spike 1 protein, and the other with bovine serum albumin to serve as reference. The biofunctionalized probes were immersed for 10 min in human serum sample and then for 5 min in goat anti-human IgG Fc specific antibody solution. Using a humanized rat antibody against SARS-CoV-2 RBD, a detection limit of 20 ng/mL was determined. Analysis of human serum samples indicated that the proposed sensor discriminated completely non-infected/non-vaccinated from vaccinated individuals, and the antibodies levels determined correlated well with those determined in the same samples by ELISA. These results demonstrated the potential of the proposed sensor to serve as an efficient tool for expeditious point-of-care testing.

Metasurface Micro/Nano-Optical Sensors: Principles and Applications

Metasurfaces are 2D artificial materials consisting of arrays of metamolecules, which are exquisitely designed to manipulate light in terms of amplitude, phase, and polarization state with spatial resolutions at the subwavelength scale. Traditional micro/nano-optical sensors (MNOSs) pursue high sensitivity through strongly localized optical fields based on diffractive and refractive optics, microcavities, and interferometers. Although detections of ultra-low concentrations of analytes have already been demonstrated, the label-free sensing and recognition of complex and unknown samples remain challenging, requiring multiple readouts from sensors, e.g., refractive index, absorption/emission spectrum, chirality, etc. Additionally, the reliability of detecting large, inhomogeneous biosamples may be compromised by the limited near-field sensing area from the localization of light. Here, we review recent advances in metasurface-based MNOSs and compare them with counterparts using micro-optics from aspects of physics, working principles, and applications. By virtue of underlying the physics and design flexibilities of metasurfaces, MNOSs have now been endowed with superb performances and advanced functionalities, leading toward highly integrated smart sensing platforms.

Simultaneous Detection of Salmonella typhimurium and Escherichia coli O157:H7 in Drinking Water and Milk with Mach–Zehnder Interferometers Monolithically Integrated on Silicon Chips

The consumption of water and milk contaminated with bacteria can lead to foodborne disease outbreaks. For this reason, the development of rapid and sensitive analytical methods for bacteria detection is of primary importance for public health protection. Here, a miniaturized immunosensor based on broadband Mach–Zehnder Interferometry for the simultaneous determination of S. typhimurium and E. coli O157:H7 in drinking water and milk is presented. For the assay, mixtures of bacteria solutions with anti-bacteria-specific antibodies were run over the chip, followed by solutions of biotinylated anti-species-specific antibody and streptavidin. The assay was fast (10 min for water, 15 min for milk), accurate, sensitive (LOD: 40 cfu/mL for S. typhimurium; 110 cfu/mL for E. coli) and reproducible. The analytical characteristics achieved combined with the small chip size make the proposed biosensor suitable for on-site bacteria determination in drinking water and milk samples.

Simultaneous Detection of Salmonella typhimurium and Escherichia coli O157:H7 in Drinking Water and Milk with Mach-Zehnder Interferometers Monolithically Integrated on Silicon Chips

The consumption of water and milk contaminated with bacteria can lead to foodborne disease outbreaks. For this reason, the development of rapid and sensitive analytical methods for bacteria detection is of primary importance for public health protection. Here, a miniaturized immunosensor based on broadband Mach-Zehnder Interferometry for the simultaneous determination of S. typhimurium and E. coli O157:H7 in drinking water and milk is presented. For the assay, mixtures of bacteria solutions with anti-bacteria-specific antibodies were run over the chip, followed by solutions of biotinylated anti-species-specific antibody and streptavidin. The assay was fast (10 min for water, 15 min for milk), accurate, sensitive (LOD: 40 cfu/mL for S. typhimurium; 110 cfu/mL for E. coli) and reproducible. The analytical characteristics achieved combined with the small chip size make the proposed biosensor suitable for on-site bacteria determination in drinking water and milk samples.

Active Opto-Magnetic Biosensing with Silicon Microring Resonators

Integrated optical biosensors are gaining increasing attention for their exploitation in lab-on-chip platforms. The standard detection method is based on the measurement of the shift of some optical quantity induced by the immobilization of target molecules at the surface of an integrated optical element upon biomolecular recognition. However, this requires the acquisition of said quantity over the whole hybridization process, which can take hours, during which any external perturbation (e.g., temperature and mechanical instability) can seriously affect the measurement and contribute to a sizeable percentage of invalid tests. Here, we present a different assay concept, named Opto-Magnetic biosensing, allowing us to optically measure off-line (i.e., post hybridization) tiny variations of the effective refractive index seen by microring resonators upon immobilization of magnetic nanoparticles labelling target molecules. Bound magnetic nanoparticles are driven in oscillation by an external AC magnetic field and the corresponding modulation of the microring transfer function, due to the effective refractive index dependence on the position of the particles above the ring, is recorded using a lock-in technique. For a model system of DNA biomolecular recognition we reached a lowest detected concentration on the order of 10 pm, and data analysis shows an expected effective refractive index variation limit of detection of 7.5×10-9 RIU, in a measurement time of just a few seconds.

Optical biosensors: an exhaustive and comprehensive review

Optical biosensors have exhibited worthwhile performance in detecting biological systems and promoting significant advances in clinical diagnostics, drug discovery, food process control, and environmental monitoring. Without complexity in their pretreatment and probable influence on the nature of target molecules, these biosensors have additional advantages such as high sensitivity, robustness, reliability, and potential to be integrated on a single chip. In this review, the state of the art optical biosensor technologies, including those based on surface plasmon resonance (SPR), optical waveguides, optical resonators, photonic crystals, and optical fibers, are presented. The principles for each type of biosensor are concisely introduced and particular emphasis has been placed on recent achievements. The strengths and weaknesses of each type of biosensor have been outlined as well. Concluding remarks regarding the perspectives of future developments are discussed.

Rapid detection of mozzarella and feta cheese adulteration with cow milk through a silicon photonic immunosensor

Mozzarella di Bufala Campana and Feta are two cheeses with Protected Designation of Origin the fraudulent adulteration of which with bovine milk must be routinely checked to ensure that consumers actually buy these high-end products and avoid health issues related to bovine milk allergy. Here, we employed, for the first time, a silicon-based photonic immunosensor for the detection of mozzarella and feta adulteration with bovine milk. The photonic immunosensor used relies on Mach-Zehnder interferometers monolithically integrated along with their respective light sources on a silicon chip. A rabbit polyclonal antiserum raised against bovine κ-casein was used for the development of a competitive immunoassay realized in three steps, including a reaction with the antiserum, a biotinylated anti-rabbit IgG antibody, and streptavidin. The implementation of this assay configuration significantly reduced the non-specific signal due to the cheese matrix, and allowed completion of the assay in ∼9 min. After optimization of all assay conditions, bovine cheese could be quantified in mozzarella or feta at concentrations as low as 0.5 and 0.25% (w/w), respectively; both quantification limits were below the maximum allowable content of bovine milk in mozzarella and feta (1% w/w) according to the EU regulations. Equally important, the assays were reproducible with intra- and inter-assay coefficients of variation <10%, and exhibited a wide linear dynamic range that extended up to 50 and 25% (w/w) for mozzarella and feta, respectively. Taking into account its performance, the proposed immunosensor may be transformed to a new tool against fraudulent activities in the dairy industry.

AFM1 Detection in Milk by Fab' Functionalized Si3N4 Asymmetric Mach-Zehnder Interferometric Biosensors

Aflatoxins (AF) are naturally occurring mycotoxins, produced by many species of Aspergillus. Among aflatoxins, Aflatoxin M1 (AFM1) is one of the most frequent and dangerous for human health. The acceptable maximum level of AFM1 in milk according to EU regulation is 50 ppt, equivalent to 152 pM, and 25 ppt, equivalent to 76 pM, for adults and infants, respectively. Here, we study a photonic biosensor based on Si 3 N 4 asymmetric Mach-Zehnder Interferometers (aMZI) functionalized with Fab' for AFM1 detection in milk samples (eluates). The minimum concentration of AFM1 detected by our aMZI sensors is 48 pM (16.8 pg/mL) in purified and concentrated milk samples. Moreover, the real-time detection of the ligand-analyte binding enables the study of the kinetics of the reaction. We measured the kinetic rate constants of the Fab'-AFM1 interaction.

Study of a Broadband Difference Interferometer Based on Low-Cost Polymer Slab Waveguides

A model and the waveguide parameters of a broadband, polymer-based slab waveguide difference interferometer is presented in this paper. The parameters were determined based on knowledge of the dispersion in the structure materials used to fabricate the waveguide. The impact of the waveguide layer thickness, propagation path length, and change in the waveguide cover refractive index on the output signal from the system was determined. It has been shown that the direction of the maximum shifting is determined by the thickness of the waveguide layer. A relationship describing the shift in the signal extrema for a change in the waveguide cover refractive index was derived. The results show that the use of a propagation constant simplifies the description of the interferometer. Polymer waveguides, although they have a small contrast in refractive indices, allow for large shifts in the maxima of the signal. The determined shifts in the output signal extrema for polymer waveguides are comparable, and these shifts are larger for some waveguide thicknesses compared to waveguides based on Si₃N₄.

Fundamental limit of detection of photonic biosensors with coherent phase read-out

Photonic biosensors offer label-free detection of biomolecules for applications ranging from clinical diagnosis to food quality monitoring. Both sensors based on Mach-Zehnder interferometers and ring resonators are widely used, but are usually read-out using different schemes, making a direct comparison of their fundamental limit of detection challenging. A coherent detection scheme, adapted from optical communication systems, has been recently shown to achieve excellent detection limits, using a simple fixed-wavelength source. Here we present, for the first time, a theoretical model to determine the fundamental limit of detection of such a coherent read-out system, for both interferometric and resonant sensors. Based on this analysis, we provide guidelines for sensor optimization in the presence of optical losses and show that interferometric sensors are preferable over resonant structures when the sensor size is not limited by the available sample volume.

Study on the limit of detection in MZI-based biosensor systems

Mach-Zehnder interferometers are integrated photonic sensors that have yielded excellent detection limits down to 10⁻⁷ RIU. They are of particular interest due to their large design freedom, allowing for example application in promising point-of-care compatible read-out schemes. The attainable detection limit of such sensors can interact with the sensor design in different ways, depending on the dominant origin of noise which can either be influencing a single sensor arm, both sensor arms or can be unrelated to the sensor itself. In this work, the interaction of these three noise regimes with the sensor design is examined. The regimes are combined into a framework that predicts the limit of detection as a function of sensor design. A set of experimental results confirms the validity of this obtained theoretical framework. This analysis provides a blueprint for optimization of MZI photonic sensors under any combination of read-out method and measurement circumstances.

Luminescence from Si-Implanted SiO₂-Si₃N₄ Nano Bi-Layers for Electrophotonic Integrated Si Light Sources

In this paper, we present structural and luminescence studies of silicon-rich silicon oxide (SRO) and SRO-Si 3 N 4 bi-layers for integration in emitter-waveguide pairs that can be used for photonic lab-on-a-chip sensing applications. The results from bi and mono layers are also compared. Two clearly separated emission bands are respectively attributed to a combination of defect and quantum confinement⁻related emission in the SRO, as well as to defects found in an oxynitride transition zone that forms between the oxide and the nitride films, while ruling out quantum-confinement phenomena in the silicon nitride.

TriPleX™ waveguide-based fluorescence biosensor for multichannel environmental contaminants detection

In order to realize the multi-analyte assays for environmental contaminants, an optical biosensor utilizing laser-induced fluorescence-based detection via the binding of biomolecules to the surface of an integrated TriPleX™ waveguide chip on a glass substrate (fused silica, FS) is described. As far as we know, this is the first demonstration of using the TriPleX™ technology to fabricate the waveguide chip on a FS substrate. The sensor consists of 32 individually addressable sensor patches, which were formed on the chip surface by exploiting 3 Y-junction splitters, creating four equal rows of eight evanescently excited windows in parallel. The basic low-loss SiO₂/Si₃N₄ TriPleX™ waveguide configuration in combination with on-chip spotsize convertors allows for both high fiber-to-chip coupling efficiency and enables at the same time individually optimized high chip surface intensity and low patch-to-patch deviation. Moreover, the complementary metal-oxide-semiconductor compatible fabrication of waveguide chip allows for its mass production at low cost. By taking MC-LR, 2,4-D, atrazine and BPA as the model analytes, the as-proposed waveguide based biosensor was proven sensitive with the detection limits of 0.22 μg/L for MC-LR, 1.18 μg/L for 2, 4-D, 0.2 μg/L for atrazine and 0.06 μg/L for BPA. Recoveries of the biosensor towards simultaneous detection of MC-LR, 2, 4-D, atrazine and BPA in spiked real water samples varied from 84% to 120%, indicating the satisfactory accuracy of the established technology.

Broad-band difference interferometer as a refractive index sensor

Integrated Optical Broad-Band Difference Interferometer (IO BB DI) is introduced as an alternative and economical measurement method to integrated optical label-free affinity sensors. A detailed theoretical analysis of the method is presented and the effects of the waveguide layer on the operation of the system are shown. A very short operating distance of less than 0.5 mm allows miniaturization of the interferometer. The analysis was performed for Si₃N₄/SiO₂ layers that can be obtained in standard microelectronics technological processes.

Interferometer based on strongly coupled multi-core optical fiber for accurate vibration sensing

We report on the use of a simple interferometer built with strongly-coupled core optical fiber for accurate vibration sensing. Our multi-core fiber (MCF) is designed to mode match a standard single-mode optical fiber (SMF). The interferometer consists of a low insertion loss SMF-MCF-SMF structure where only two super-modes interfere. The polymer coating of the MCF was structured and the interferometer was sandwiched between a flat piece and a V-groove. In this manner our device is highly sensitive to force with sensitivity reaching -4225 pm/N. To make the MCF interferometer sensitive to vibrations the flat piece was allowed to move, thus, its periodic movements exert cyclic localized pressure on the MCF which makes the interference pattern to shift periodically. Our sensors can be used to monitor vibrations in a broad frequency range with the advantage that the measurements are unaffected by temperature changes.

Accurate strain sensing based on super-mode interference in strongly coupled multi-core optical fibres

We report on the use of a multi-core fibre (MCF) comprising strongly-coupled cores for accurate strain sensing. Our MCF is designed to mode match a standard single mode optical fibre. This allows us to fabricate simple MCF interferometers whose interrogation is carried out with light sources, detectors and fibre components readily available from the optical communications tool box. Our MCF interferometers were used for sensing strain. The sensor calibration was carried out in a high-fidelity aerospace test laboratory. In addition, a packaged MCF interferometer was transferred into field trials to validate its performance under deployment conditions, specifically the sensors were installed in a historical iron bridge. Our results suggest that the MCF strain sensors here proposed are likely to reach the readiness level to compete with other mature sensor technologies, hence to find commercial application. An important advantage of our MCF interferometers is their capability to operate at very high temperatures.

Detection of ochratoxin A in beer samples with a label-free monolithically integrated optoelectronic biosensor

An optical biosensor for label-free detection of ochratoxin A (OTA) in beer samples is presented. The biosensor consists of an array of ten Mach-Zehnder interferometers (MZIs) monolithically integrated along with their respective broad-band silicon light sources on the same Si chip (37mm²). The chip was transformed to biosensor by functionalizing the MZIs sensing arms with an OTA-ovalbumin conjugate. OTA determination was performed by pumping over the chip mixtures of calibrators or samples with anti-OTA antibody following a competitive immunoassay format. An external miniaturized spectrometer was employed to continuously record the transmission spectra of each interferometer. Spectral shifts obtained due to immunoreaction were transformed to phase shifts through Discrete Fourier Transform. The assay had a detection limit of 2.0ng/ml and a dynamic range 4.0-100ng/ml in beer samples, recoveries ranging from 90.6 to 116%, and intra- and inter-assay coefficients of variation of 9% and 14%, respectively. The results obtained with the sensor using OTA-spiked beer samples spiked were in good agreement with those obtained by an ELISA developed using the same antibody. The good analytical performance of the biosensor and the small size of the proposed chip provide for the development of a portable instrument for point-of-need determinations.

Silicon on-chip 1D photonic crystal nanobeam bandstop filters for the parallel multiplexing of ultra-compact integrated sensor array

We propose a novel multiplexed ultra-compact high-sensitivity one-dimensional (1D) photonic crystal (PC) nanobeam cavity sensor array on a monolithic silicon chip, referred to as Parallel Integrated 1D PC Nanobeam Cavity Sensor Array (PI-1DPC-NCSA). The performance of the device is investigated numerically with three-dimensional finite-difference time-domain (3D-FDTD) technique. The PI-1DPC-NCSA consists of multiple parallel-connected channels of integrated 1D PC nanobeam cavities/waveguides with gap separations. On each channel, by connecting two additional 1D PC nanobeam bandstop filters (1DPC-NBFs) to a 1D PC nanobeam cavity sensor (1DPC-NCS) in series, a transmission spectrum with a single targeted resonance is achieved for the purpose of multiplexed sensing applications. While the other spurious resonances are filtered out by the stop-band of 1DPC-NBF, multiple 1DPC-NCSs at different resonances can be connected in parallel without spectrum overlap. Furthermore, in order for all 1DPC-NCSs to be integrated into microarrays and to be interrogated simultaneously with a single input/output port, all channels are then connected in parallel by using a 1 × n taper-type equal power splitter and a n × 1 S-type power combiner in the input port and output port, respectively (n is the channel number). The concept model of PI-1DPC-NCSA is displayed with a 3-parallel-channel 1DPC-NCSs array containing series-connected 1DPC-NBFs. The bulk refractive index sensitivities as high as 112.6nm/RIU, 121.7nm/RIU, and 148.5nm/RIU are obtained (RIU = Refractive Index Unit). In particular, the footprint of the 3-parallel-channel PI-1DPC-NCSA is 4.5μm × 50μm (width × length), decreased by more than three orders of magnitude compared to 2D PC integrated sensor arrays. Thus, this is a promising platform for realizing ultra-compact lab-on-a-chip applications with high integration density and high parallel-multiplexing capabilities.

New set of design rules for resonant refractive index sensors enabled by FFT based processing of the measurement data

It is still a common belief that ultra-high quality-factors (Q-factors) are a prerequisite in optical resonant cavities for high refractive index resolution and low detection limit in biosensing applications. In combination with the ultra-short steps that are necessary when the measurement of the resonance shift relies on the wavelength scanning of a laser source and conventional methods for data processing, the high Q-factor requirement makes these biosensors extremely impractical. In this work we analyze an alternative processing method based on the fast-Fourier transform, and show through Monte-Carlo simulations that improvement by 2-3 orders of magnitude can be achieved in the resolution and the detection limit of the system in the presence of amplitude and spectral noise. More significantly, this improvement is maximum for low Q-factors around 10⁴ and is present also for high intra-cavity losses and large scanning steps making the designs compatible with the low-cost aspect of lab-on-a-chip technology. Using a micro-ring resonator as model cavity and a system design with low Q-factor (10⁴), low amplitude transmission (0.85) and relatively large scanning step (0.25 pm), we show that resolution close to 0.01 pm and detection limit close to 10^-7 RIU can be achieved improving the sensing performance by more than 2 orders of magnitude compared to the performance of systems relying on a simple peak search processing method. The improvement in the limit of detection is present even when the simple method is combined with ultra-high Q-factors and ultra-short scanning steps due to the trade-off between the system resolution and sensitivity. Early experimental results are in agreement with the trends of the numerical studies.

Last Advances in Silicon-Based Optical Biosensors

We review the most important achievements published in the last five years in the field of silicon-based optical biosensors. We focus specially on label-free optical biosensors and their implementation into lab-on-a-chip platforms, with an emphasis on developments demonstrating the capability of the devices for real bioanalytical applications. We report on novel transducers and materials, improvements of existing transducers, new and improved biofunctionalization procedures as well as the prospects for near future commercialization of these technologies.

Point-of-Need bioanalytics based on planar optical interferometry

This review brings about a comprehensive presentation of the research on interferometric transducers, which have emerged as extremely promising candidates for viable, truly-marketable solutions for PoN applications due to the attested performance that has reached down to 10(-8) in term of effective refractive index changes. The review explores the operation of the various interferometric architectures along with their design, fabrication, and analytical performance aspects. The issues of biosensor functionalization and immobilization of receptors are also addressed. As a conclusion, the comparison among them is attempted in order to delve into and acknowledge their current limitations, and define the future trends.

Monolithically integrated broad-band Mach-Zehnder interferometers for highly sensitive label-free detection of biomolecules through dual polarization optics

Protein detection and characterization based on Broad-band Mach-Zehnder Interferometry is analytically outlined and demonstrated through a monolithic silicon microphotonic transducer. Arrays of silicon light emitting diodes and monomodal silicon nitride waveguides forming Mach-Zehnder interferometers were integrated on a silicon chip. Broad-band light enters the interferometers and exits sinusoidally modulated with two distinct spectral frequencies characteristic of the two polarizations. Deconvolution in the Fourier transform domain makes possible the separation of the two polarizations and the simultaneous monitoring of the TE and the TM signals. The dual polarization analysis over a broad spectral band makes possible the refractive index calculation of the binding adlayers as well as the distinction of effective medium changes into cover medium or adlayer ones. At the same time, multi-analyte detection at concentrations in the pM range is demonstrated.

Assessment of goat milk adulteration with a label-free monolithically integrated optoelectronic biosensor

The label-free detection of bovine milk in goat milk through a miniaturized optical biosensor is presented. The biosensor consists of ten planar silicon nitride waveguide Broad-Band Mach-Zehnder interferometers (BB-MZIs) monolithically integrated and self-aligned with their respective silicon LEDs on the same Si chip. The BB-MZIs were transformed to biosensing transducers by functionalizing their sensing arm with bovine k-casein. Measurements were performed by continuously recording the transmission spectra of each interferometer through an external spectrometer. The amount of bovine milk in goat milk was determined through a competitive immunoassay by passing over the sensor mixtures of anti-k-casein antibodies with the calibrators or the samples. The output spectra of each BB-MZI recorded during the reaction were subjected to Discrete Fourier Transform in order to convert the observed spectral shifts to phase shifts in the wavenumber domain. The method had a detection limit of 0.04 % (v/v) bovine milk in goat milk, dynamic range 0.1-1.0 % (v/v), recoveries 93-110 %, and intra- and inter-assay coefficients of variation less than 12 and 15 %, respectively. The proposed biosensor compared well in terms of analytical performance with a competitive ELISA developed using the same monoclonal antibodies. Nevertheless, the duration of the biosensor assay was 10 min whereas the ELISA required 2 h. Thus, the fast and sensitive determinations along with the small size of the sensor make it ideal for incorporation into portable devices for assessment of goat or ewe's milk adulteration with bovine milk at the point-of-need.

All-silicon monolithic Mach-Zehnder interferometer as a refractive index and bio-chemical sensor

A complete Mach-Zehnder interferometer monolithically integrated on silicon is presented and employed as a refractive index and bio-chemical sensor. The device consists of broad-band light sources optically coupled to photodetectors through monomodal waveguides forming arrays of Mach-Zehnder interferometers, all components being monolithically integrated on silicon through mainstream silicon technology. The interferometer is photonically engineered in a way that the phase difference of light travelling through the sensing and reference arms is approximately wavelength independent. Consequently, upon effective medium changes, it becomes feasible even with a broad-band source to induce sinusoidal-type of detector photocurrents similar to the classical monochromatic counterparts. The device is completed with its fluidic and interconnect components so that on chip interferometric measurements can be performed. Examples of refractive index and protein sensing are presented to establish the potential of the proposed device for real-time in situ monitoring applications. This is the only silicon device that has achieved complete on-chip interferometry.