This PDF file contains the front matter associated with SPIE Proceedings Volume 12572, including the Title Page, Copyright information, Table of Contents and Conference Committee lists.

This video presentation Optical Sensors 2023 was presented at SPIE Optics and Electronics 2023.

Development of novel plasmonic nanopatterns is of great interest for various applications, including chemical and biological analysis. Systems based on gold nanoelements have been designed and tested in several research works for the study and detection of various kinds of biological analytes, giving appreciable results. Plasmonic properties associated to the nanostructure can be tuned by changing the size and the shape of the nanoparticles or the periodicity or, more in general, the geometry of the nanopattern. These features are key to many applications aiming at signal enhancement and low threshold sensing. In this work we present a study of periodic arrangements of novel plasmonic metamolecular unit cells made of triangular nanoelements. Nanostructures analyzed were fabricated using electron beam lithography technique (EBL) that allows to create patterns with high accuracy and repeatability. Morphological analysis was realized by Scanning Electron Microscopy (SEM) and their plasmonic properties were studied and compared using experimental set-up for Surface Plasmon Resonance (SPR) and Surface Enhanced Raman Spectroscopy (SERS) measurements. We tested the sensing performance of our nanostructures by analyzing the SARS-CoV-2 (COVID-19) Spike Antibody (3525) getting its molecular fingerprint. Our results suggest that these plasmonic patterns are promising to develop highly sensitive nanosensors for the detection of biological analytes.

The increasing demand for precise chemical and biological sensing has led to the development of highly efficient plasmonic optical fiber sensors. Therefore, it is essential to optimize and match the operating wavelength region of both the optical fiber configuration and localized surface plasmon resonance of nanoparticles (NPs). This can be achieved by developing NPs that can reach resonance at near-infrared wavelengths, where refractive index sensitivity is enhanced, and silica optical fibers have lower losses. High aspect-ratio bimetallic Au@Ag nanorods and different side-polished fiber structures are tested using numerical simulations. The selected optical fiber configuration was based on a side- polished fiber with a 1 mm polished section. It is compared power losses and power at the NP interface for two configurations: a step-index single-mode fiber (SMF) with core/cladding diameters of 8.2/125 µm and a multimode graded-index fiber (GIF) with 62.5/125 µm at various polishing depths. The results showed that the best performance for both configurations was achieved at similar polishing depths, namely 59.5 and 55.2 µm for the SMF and GIF, respectively. The optical impact of retardation effects due to the proximity with the fiber structure were also observed, which caused a reduction in sensitivity from 1750 nm/RIU to 1500 nm/RIU and a red-shift of

To achieve single-molecule detection based on enzyme-free and isothermal amplification techniques, we have developed a strategy employing coupled catalytic hairpin assembly reactions and investigated several parameters influencing its yield. We employed avidin-biotin assay using biotin-modified trigger DNA and surface plasmon resonance to excite fluorescence-labeled hairpin DNA captured a surface. Our investigation of association and dissociation of fluorescence-labeled hairpin DNA allows to determine the reaction yield and provide guide to design the involved oligonucleotides.

Circulating Tumor Cells (CTCs) have emerged as an eligible biomarker for liquid biopsy. These cells are released in peripheral circulation at a very early stage from the tumor mass via metastatic or non-metastatic cascade. These cells provide a non-invasive method for cancer detection and monitoring. In this work, Gold Nanoparticle (GNP) decorated U-bent optical fiber was used as a sensor platform. For specific detection of cancer cells, an antibody for nucleolin protein which over-expresses on their surface was employed as a receptor. This Localized Surface Plasmon Resonance (LSPR) based biosensor poses salient features like high sensitivity, ease of fabrication, low cost, and handling. For sensor fabrication, optical fiber was decladded, bent in a U-shape, and cleaned. After GNPs, U-bent fiber was coated with cysteamine. The concentration and incubation time of cysteamine was optimized as they played a critical role in the sensitivity and specificity of the sensor. Next, the amine group of cysteamines was treated with glutaraldehyde to which the antibody was attached. Ethanolamine was used as a blocking agent and its incubation time was also optimized. The sensor was introduced to 104 MCF-7 cells and 105 WBCs in PBS buffer and the binding absorbance for 2 hours was monitored. The obtained absorbance for MCF-7 cells and WBCs was around 0.05±0.011 and 0.002±0.0015 O.D. respectively, which indicated a very high specificity of the sensor for cancer cells. The obtained results are promising and pave the way to develop a highly specific and affordable point-of-care device for cancer detection and monitoring.

Radiography is the gold standard imaging technique in dental medicine. While every modern X-ray equipment manufacturer strives to obtain the best possible image characteristics, there are certain limitations for the different types of dental radiographs: panoramic, intraoral, cephalometric, and cone-beam computed tomography (CBCT). Such targeted characteristics include especially resolution, field of view (FOV), and dose radiation, with a clear trade-off between them. The aim of a series of studies we have performed was to address such trade-offs of X-ray imaging using a higher-resolution technique, optical coherence tomography (OCT) [https://doi.org/10.3390/ma13214825, https://doi.org/10.3390/s21134554]. The present work focuses on two of the most common types of dental radiographies (i.e., panoramic and intraoral) obtained with Planmeca X-ray units (Planmeca, Helsinki, Finland). The aim of this work is to present protocol elements and results of their OCT-based optimization. The procedure and working steps are described. Radiographs performed before and after the optimization for patients in a dental clinic in Timisoara, Romania, are presented. Statistical analyses of image characteristics (i.e. contrast, brightness, contrast to noise ratio (CNR) and sharpness) are pointed out, in the trade-off with the radiation dose. The way the ALARA (as-low-as-possible radiation dose) protocol is observed, while improving to the highest possible level image characteristics of considered radiographs is discussed. Limitations that can influence the result of the image characteristics improvement are highlighted. The developed protocol can be applied in every dental clinic or radiology center.

Photoplethysmography (PPG) is a non-invasive optical-based technique used to measure various hemodynamic parameters. State-of-the-art proposed various methods for arrhythmia (premature ventricular contraction (PVC), atrial fibrillation) detection using PPG signals. However, restricted research has been carried out for detecting other arrhythmias that could be life-threatening. In this research work, the detection of atrial flutter (AFl) from Normal, Sinus Tachycardia (ST), and PVC signals have been carried out using PPG signals. The method relies on time-domain and entropy features for characterizing the AFl PPG pulse. A sliding window approach has been applied to extract features, and an artificial neural network has been implemented for feature classification. The ground-truth generation for the PPG signals has been carried out on publically available and prospective data. The comparative analysis of the results obtained from the two datasets is useful in the effective identification of the abnormality.

Esophageal pressure, bile content and pH are important parameters in gastroesophageal diseases. An all-optical technology is described capable to perform simultaneously oesophageal manometry, pH-metry and bilimetry. The three different sensors were integrated in a single optical fibre catheter for the simultaneous measurement of the three parameters. The optoelectronic prototype for the interrogation of the catheter is constituted by two separate optoelectronic modules for the interrogation of the pressure sensor and for the bile and pH sensors, respectively. The prototype and the optical catheter are compliant with European Directives on medical devices in terms of electromagnetic compatibility, electrical safety and biocompatibility.

We designed, fabricated and tested a Vector Diffractive Optical Element (VDOE) to simultaneously determine the Stokes vector of light. It comprises several sectors. Each one is a vector Fresnel zone plate which focuses the light on separate foci and has different polarization properties. The polarization state is calculated from their intensities. From simulations, we could identify the error sources that were analytically removed. The residual uncertainty after applying our corrections was as low as 6x10^(-5). The uncertainty obtained for our fabricated VDOE, 3.33 %, is competitive with the results from state-of-the-art techniques.

Methods for sub 10 nm plasmonic nanopores fabrication are usually complex and require multi-step processes usually suitable for the preparation of single pores. Other processes to fabricate metallic nanopores involve pore shrinking by metal evaporation, which is applied to the whole substrate, increasing its thickness; therefore it is not localized and reduces spatial resolution. For that reason, a process in which metal deposition can be controlled at the nanoscale, is a key advancement in the field. Here, we report on a process for the fabrication of sub 10 nm solid-state plasmonic nanopores, via photocatalytic effect caused by the electromagnetic field enhancement in metallic rings on top of dielectric nanotubes. Under illumination, the areas of maximum field inside these structures trigger sites for metal nucleation and growth. Using this methodology, we fabricated Au-Ag and Au-Au nanopores, with consistent and reproducible shrinkage in pore diameter. Numerical simulations were performed in order to support the findings and to show how the obtained plasmonic structures can be used to confine the electromagnetic field, enhancing the intensity in a volume in the scale of sub 10 nm. The confinement of the field inside the final nanopore can be used for thermoplasmonic effects modifying ionic conductivity inside the pore under different illumination wavelengths.

In this work we introduce the first step toward the development of an Infrared (IR) sensoristic platform aiming to the detection and discrimination of airborne pathogens. SARS-CoV-2 Spike glycoprotein (S) is considered as a biomarker for virus recognition. A comparative spectroscopic analysis is illustrated, studying Spike glycoprotein subunit S1 from three different viruses, MERS-CoV, SARS-CoV and SARS-CoV-2. Moreover, SARS-CoV-2 variants are also investigated. The IR characterization of their S1 secondary conformation was carried out through Attenuated Total Reflection (ATR), analyzing the Amide I absorption band. In addition, pH-dependent conformational changes of SARS-CoV-2 S1 were investigated, too

Mid infrared (MID-IR) band of electromagnetic radiation plays the key role for gas and chemical sensing application because it provides molecular fingerprints for most trace gases and molecules. Photonic crystal cavity coupled waveguide (PC-CWG) based sensors is a remarkable platform for sensing applications due to its capability to confine light modes on small volume displaying high sensitivity and high-quality factors. In this work, we report a novel design of (PC-CWG) based on square lattice of silicon pillars in air with radius of 0.2𝜇m and lattice constant of 1 𝜇m. A waveguide is introduced by removing three columns of silicon pillars, and a microcavity is created by removing a number of silicon pillars forming curved shapes on both sides of the waveguide. The proposed design demonstrates multiple resonances covering a broad spectral range in MID-IR ranging from 2.4 𝜇m to 4.2 𝜇m that represents the bandgap region. Remarkable resonances are observed at operating wavelengths 2.67 𝜇m, 2.88 𝜇m, 3.03 𝜇m, 3.2 𝜇m and 3.5 𝜇m. Moreover, the reported design shows ultra-high sensitivity reaching 2680 nm/RIU with a significant quality factor of Q=6475 giving rise to a figure-of-merit of 5.7 × 10⁶ at the operating wavelength of (𝜆=3.03𝜇m). The suggested photonic crystal design offers simple fabrication and broad applicability for refractive index sensing applications.

The potential of surface enhanced Raman spectroscopy (SERS) in bioanalytics is illustrated by the example for detecting the antibiotic ciprofloxacin in pharmaceutical formulations in its relevant concentrations. Both, Raman and SERS spectroscopy are applied to identify the target analyte dependent on the complexity of the matrices.

In-process optical monitoring has led to impressive enhancements in the quality control of metallic parts made by wire and arc additive manufacturing (WAAM). Identification of material defects during a WAAM process gives an opportunity to reduce post-process inspection, to pause the deposition to address the defect problem or terminate the process to save resources. This is of importance in the aerospace sector, where inferior quality components can have significant cost penalties. Many process factors, including deposition parameters, WAAM equipment, feedstock, the surrounding atmosphere and contaminants can all contribute to create a defect in the component. The contaminants can be introduced as organics (oils / grease) or inorganic elements coming from the deposition atmosphere, the WAAM system itself or the feedstock material. Contamination by tungsten (originating in the plasma torch electrode) is a particular concern as its melting point, 3422C, is much higher than that of titanium alloy (1674C for Ti-6Al-4V)[1]. Within the WAAM melt pool, a solidified drop of tungsten can remain dissolved which could result in a defect in the final part under tensile loading. Here, we demonstrate the development of an optical emission spectroscopy system to identify tungsten as a contaminant. Spectra were obtained from the plasma during the process using a spectrometer with an integration time of 110ms. Data analysis was undertaken to average over longer timescales, and unambiguously identify the emission lines of contaminating tungsten.

The combustible gas sector requires for instrumentation capable to determine the composition and the quality of the gas mixtures present in the transport and distribution networks. The gas parameters need to be monitored in a wide interval, since mixtures are found within an extremely variable range. A compact, fast and highly sensitive instrument based on Raman spectroscopy has been developed with the specific aim to operate directly on-line. This approach is intrinsically non-invasive, since it needs a laser beam passing through the gas, and multi-species sensitive, since the different components of the gas mixture are simultaneously detected. The Raman scattering is stimulated by a laser diode centered at 455 nm with multi-mode emission and 1.5 W optical power. The laser is focused on a gas cell through a window, the Raman emission is collected by a grating spectrometer and finally acquired by a 2D camera. The measured spectra are fitted with the calibration dataset acquired at room temperature to achieve the mixture composition. The system is able to determine the main components of the natural gas: methane, heavier hydrocarbons, nitrogen, carbon dioxide and hydrogen. The Heating Value (HV) is finally calculated using the ISO6976:2016 standard. Several certified gas mixtures have been tested with the instrument operated at different temperatures in the range from -20°C to 50°C, to prove the capability to operate in a wide industrial temperature range. Each measure requires less than 25 seconds, with a sample pressure of 6 bars. The calculated HV value lies in the ±0.5% error range.

In this work, we have demonstrated the application of plasmonic quasi crystal (PlQC) as SERS active substrate. SERS, an offshoot of Raman spectroscopy, is a powerful analytical technique that provides chemical information about molecules or molecular assemblies adsorbed or attached to nanostructured metallic surfaces. We have demonstrated the detection of urea (20μL), up to 10-5 M concentration using PlQC as SERS substrate. Urea, a biomolecule, is irradiated by a laser source (λ=785nm), and Raman spectra are collected using the Raman setup (Renishaw). All Raman peaks of urea were distinctly visible for further analysis. The proposed PlQC can be used as a SERS active substrate for on-site narcoanalysis, forensic study, explosive detection, bio-diagnostics, detection of adulterants in food/water etc.

This contribution explains the development of a full-Stokes imaging polarimeter that provides the 2D map of the polarization state of a scene in a single acquisition working in the complete visible band. The nature of polarization makes it impossible to measure it using a single measurement with bare intensity-based detectors. Stokes parameters describe the state of polarization of light and provide the amplitudes of the electric field and the phase difference of the two components orthogonal to propagation using simple experiments that measure the time-averaged intensity of the waves. Our camera can transform the input polarization into intensity by using polarization-sensitive elements to recover the complete Stokes vector at each pixel of the camera. The states used for measuring the input polarization are claimed to be the optimal polarization states allowing for fast acquisition in a single shot while keeping the acquisition immunized to Gaussian and Poisson noise. The acquisition errors for full-Stokes parameters are demonstrated to be lower than 10% showing the capability of the system to perform Stokes imaging both in indoor and outdoor scenes. The camera has great potential in computer vision and deep learning applications due to the complementarity of the information provided when compared to intensity data

This conference presentation was prepared for SPIE Optics + Optoelectronics, 2023.

Optochemical sensor based systems for analysis of cell metabolism are well established and continue to grow providing valuable and detailed information on cell bioenergetics, metabolic signature and ability to withstand stress conditions and drug treatments. The area is currently dominated by the Seahorse/Agilent XF (eXtracellular Flux) Analyzer and Luxcel/Agilent MitoXpress platforms, which provide measurement of Oxygen Consumption (OCR) and Extracellular Acidification (ECA) rates under different physiological conditions (metabolic substrates, drugs, stressors), by means of fluorescent and phosphorescent pH and O₂ sensors or probes. The XF system uses solid-state sensors and specialized microplates, it is highly integrated, sensitive and fast, and user-friendly. However, the outdated sensor chemistry and basic fluorescence intensity readouts limit its performance (unstable sensor calibrations, cross-talk) and make it expensive. The MitoXpress platform, which uses soluble probes, standard microplates and plate readers, provides stable and accurate lifetime based sensing of O₂ and pH with internal referencing and no cross-talk. However, its cost-efficient DIY (do it yourself) approach is less sensitive and less user-friendly than the XF. Several new dual pH/O₂ sensing platforms are emerging, which can potentially improve the above systems and overcome their limitations. Examples include: i) a meso-substituted Pt-porphyrin Schiff base dye, which senses pH and O2 via phosphorescence intensity and lifetime changes; ii) the fully-referenced solid-state dual sensor based on a pH-sensitive fluorescent porphyrin and an O₂-sensitive phosphorescent Pt-porphyrin. iii) a sensor with near-infrared fluorescent pH-indicator and phosphorescence O₂ indicator dyes and phase measurements at multiple frequencies. In this paper, we discuss the merits and limitations of the different sensor systems for cell analysis.

Bacteria are omnipresent. They can be found in food, water, living beings, and in the surrounding environment, being ubiquitous in Nature. Although many bacteria strains are nonpathogenic, several are hazardous to health, causing different health complications. Escherichia coli (E. coli) bacteria, existing in contaminated food and water, have been identified as the main cause of several health outbreaks. In order to prevent disease outbreaks, it is crucial a rapid and real-time detection of E. coli present in contaminated food and water, which is a prerequisite to the food industry. A novel methodology proposed in the present work demonstrates a basic microfluidics system composed of a glass double-chambered cell with an activated surface for E. coli detection. Anti-E. coli antibodies are immobilized on the glass surface through a complex functionalization procedure. During this process, dimethyloctadecyl [3-(trimethoxysilyl) propyl] ammonium chloride (DMOAP) is used to induce an homeotropic alignment of the nematic liquid crystal molecules. These functionalized glass surfaces in contact with analyte samples will allow the formation of immunocomplexes by the binding of E. coli bacteria to the anti-E. coli antibodies, if present. To detect the presence of E. coli bacteria, the immunosensor cell is then filled with a nematic liquid crystal. Using two crossed polarizers, in order to visualize the interaction of the incident light with the liquid crystal, it was possible to observe the results. Different methodologies to interpret the results, such as the quantification of the bacteria, are also discussed. Besides the food industry, these immunosensors can also be used in a large number of point-of-care health applications, even in regions without electrical energy and healthcare means.

A newly developed colorimetric immunosensor based on gold-coated magnetic nanoparticles (Fe3O4@Au) is presented, and its application for the detection of human immunoglobulin G (IgG) in water is demonstrated. By taking advantage of both the localized surface plasmon resonance (LSPR) phenomenon of gold nanoparticles and the magnetic property of the core, the Fe3O4@Au immunosensor provides a fast and effective method for detecting analytes. In a sandwich scheme, Fe₃O₄@Au nanoparticles are used to enhance the response of the nanostructured gold surface made of gold nanoparticles randomly placed onto a glass coverslip. Specifically, the detection of the target analyte (human IgG) occurs when Fe₃O₄@Au nanoparticles bind to the target from the top in the presence of a magnetic field, leading to a change in the absorption spectrum of the nanostructure. Preliminary results have shown that the colorimetric immunosensor can achieve a limit of detection of 1 ng/mL, with a measurement carried out in only 10 minutes. The use of gold-coated magnetic nanoparticles in conjunction with the plasmonic surface offers great potential for the sensitive and specific detection of analytes. This could pave the way for future applications of the immunosensor in rapid testing and mass screening

The Advanced Infrared Detection Assembly Dual Band (AIDA-2B) project, part of the Skyward instrument, is an imager that consists of a main Aluminum alloy metallic frame attached to the Sensor Head Unit (SHU) chassis by means of thirteen screws. The imager features several subassemblies among which the FOV change & focusing mechanism. This mechanism has two separately actioned trolleys that allow the FOV change and focusing movement. Each trolley moves on two linear ball slides and is actuated by a lead screw. In order to achieve challenging optical performance target in the infrared range, the introduction of chalcogenide glass is required. IG4 is the material selected for the lens installed on one of the two trolleys of the focusing mechanism. Such material features excellent thermal properties (such as almost constant refraction index in the whole temperature range), but suffers from an extreme fragility and very weak mechanical properties. In order to employ such material in a challenging mechanical environment such as an airborne IRST instrument a 'floating' design is necessary, with the glass attached to the mechanical mounting by means of adhesive pads, and no metal-glass contact. A description of the design solutions developed, manufactured and qualified for the most critical optical mount inside the Instrument is presented. This paper contains a collection of mechanical results obtained on the optical mount breadboards, including a description of environmental tests performed. Three configurations for the lens mounting have been designed and tested: 1. C-shaped profile; 2. Thin ring; 3. Crown ring. The comparison between these high stability optical mounts based on adhesive joints, as well as the acceptance criteria derived in order to establish the flight worthiness of the manufactured and assembled hardware, are presented in this paper.

We present a study on the application of machine learning to optical fibre distributed sensing, with data recovered using a state-of-the-art, commercial BOTDR distributed sensing system; temperature information was extracted from the power line distribution networks that are part of the Electricity Authority of Cyprus. A machine learning approach was implemented for the prediction task of finding points of abnormal behaviour, mimicking the power cable joints that are prone to failure, along with general monitoring for unusual behaviour and potential cable fault conditions; the task is a binary classification one. Labels “0/1” were assigned to the BOTDR measurements, with “1” corresponding to data points in space and time for which the signal showcased a problematic scenario, such as that recorded by optical fibres that are collocated with power cables where the fibre’s temperature measurement increases to dangerously high values, and conversely “0” for all other scenarios. The algorithm’s base is a variation of the state-of-the-art transformer architecture, which depends solely on attention mechanisms. The field data recovered show the potential of the algorithm to predict spatiotemporally problematic points, using the temperature measurements of the collocated fibre.

Passive athermalization techniques for optical systems combine materials in the mounting structure with different coefficients of thermal expansion (CTE) to minimize thermally driven shifts. However, such an approach requires complex structures to be manufactured with multiple high-precision opto-mechanical components. Our concept utilises a monolithic and additively manufactured mounting structure and a housing made of a second material to generate mechanical stresses caused by temperature fluctuations. The difference in coefficients of thermal expansion induces these mechanical stresses during temperature changes, resulting in elastic deformations of the inner monolithic structure. The magnitude of the local deformation of the monolithic structure is adjusted via the stiffness between the optical elements. This allows to control the displacement for each optical element such that their positions remain unaffected by a thermal load and thus passively athermalizes the optical system.

Recent development in the field of advanced driver assistant system (ADAS) has shown that Light Detection and Ranging (lidar) sensors are essential for 3D object detection. One of the key parameters of automotive lidar is the maximum detection range, which is dependent on background light as well as the lidar signal itself. To make lidar sensors widely accessible for the automotive market, high-volume series production becomes a necessity. Since today's sensors have a maximum detection range far beyond a hundred meters it is neither economically nor logistically viable to build a long-range setup for series production. In this work we will present a table-top setup with a length no longer than one meter, directly measuring the maximum detection range of a lidar sensor - with similar precision (about two meters for one sigma) compared to the long-range distance measurement. This setup can be used to verify the maximum detection range and execute other complementary tests like beam quality and straylight simultaneously. This is particularly important considering the cycle time restriction in series production of the automotive lidar. Our setup mainly consists of a triggerable laser and a background light source. Using the fact that the intensity of the back-scattered light is inversely proportional to the distance squared, it is possible to imitate a far-away object by tuning a laser to a reduced intensity.

In this work, a highly sensitive sensor made of SiN is proposed that can be used in gas or biological sensing, where the choice depends on whether a marker is used or not. The whole sensor is subjected to water cladding. The proposed device is based on a Mach-Zehnder Interferometer (MZI), while the sensing arm is used for sensing the change in the refractive index of the analyte. Both polarizations (TE, TM) are considered in this study, where a higher sensitivity is achieved for the TE-polarized light. The field confinement in the strip waveguide in the sensing region is investigated and verified with a mode solver, whereas the optimum dimensions are obtained using finite difference eigenmode and finite difference time domain solvers. With a sensing arm length of only 180 μm, the proposed sensor achieves a device sensitivity of about 1942 nm/RIU and a figure-of-merit (FOM) as high as 2284 RIU −1 at the wavelength of 1.55 μm. Higher values of FOM can even be attained by employing a longer sensing arm.

Conventional sensing techniques including electrochemical, voltammetric, colorimetry, and non-enzymatic are widely used for the detection of chronic diseases. However, such techniques suffer from poor selectivity, complexity, low sensitivity, monofunctional, and expensive development procedures, which limits its widespread and accessibility across the medical field. Replacing these techniques with localized surface plasmon resonance (LSPR) based optical sensors can be much more beneficial as these are real-time, label-free devices, highly reproducible, cheap, and hold higher sensitivity to changes in the refractive index of samples. The plasmonic nanoparticles like - Ag, Au and Cu are highly sensitive to their local environment and undergoes spectral response due to their strong scattering or absorption. The easy monitoring of these light signals paves the way for its utilization in the sensor market. This work studies the influence of morphology of Au on optical tapered fibers for sensing applications.

The COVID-19 pandemic caused by the SARS-CoV-2 virus has brought global healthcare systems to their knees. The spike protein on the surface of the virus is a critical component for viral entry into human cells and therefore is a prime target for diagnosis and therapeutics. In recent years, the integration of photonic technology with biosensors has emerged as a promising approach due to its high sensitivity, specificity, and real-time detection capabilities. Optical fibres are one of the most versatile platforms for photonic technology-based biosensors, owing to their small size, low cost, and compatibility with various transduction methods. In this work, we present photonic technology based on optical fibres for the detection of the spike protein present in the SARS-CoV-2 virus. The proposed method involves the use of ad hoc synthesized peptides that specifically bind to the spike protein. The synthesized peptides are immobilized on the surface of the external face of an asymmetric Fabry-Perot cavity fabricated at the end face of a standard optical fibre, which acts as a biosensor. The presence of the spike protein in the sample causes a change in the refractive index, which is detected as variations in the visibility of the spectrum generated in Fabry-Perot cavity. The experimental results carried out have detected spike protein on buffered solutions with an LOD of 0.3 ng/ml. The proposed method offers several advantages over existing biosensors, including high sensitivity, real-time detection, and ease of integration into existing diagnostic platforms. We believe that the proposed photonic technology-based approach can significantly contribute to the development of biosensors for the early diagnosis of COVID-19 and other diseases.

This investigation proposes and experimentally validates a method to increase the sensitivity of a bending fiber optic sensor based on anti-resonant reflecting optical waveguide (ARROW) guidance. The sensing device is fabricated by splicing a small piece of capillary hollow-core fiber (CHCF) between two single-mode fibers (SMF), then, this structure is placed on a steel sheet to measure different curvature values. The sensor by itself shows a low curvature sensitivity in a curvature small range. However, if half of the CHCF length is covered with polydimethylsiloxane (PDMS), the curvature sensitivity increases, even in a bigger curvature range. Furthermore, the coated device reveals a really small temperature sensitivity, proving that temperature variations do not have an influence on the bending fiber optic sensor operation. The ARROW sensor created with this method is cost-effective and can be used for real sensing applications like structural health monitoring.

Femtosecond laser pulses are more and more spread for the micro/nano-machining of various materials. They were successfully used for the manufacturing of Bragg gratings in optical fibres through the implementation of the so-called point-by-point, line-by-line and plane-by-plane processes. In this work, we report the use of such laser for Bragg grating manufacturing in pure fused silica planar substrates. In particular, we rely on the commercial system called Femtoprint. This machine has efficiently produced Bragg gratings from bulk silica following several steps. First of all, a waveguide was imprinted in the glass substrate by tight control of the laser pulses and path. Then, an access point was created at one edge of the substrate so that a standard optical fibre can be easily connected with the engraved waveguide for light injection and collection. This was again done with femtosecond laser pulses and a subsequent etching with KOH was performed to create the required open spaces in the substrate. Finally, a Bragg grating was imprinted within the waveguide thanks to a third femtosecond laser process. The reflected amplitude spectrum of the grating was characterized using a dedicated interrogator and the obtained experimental results will be presented in this paper.

A freeform-curved sensor is presented here to demonstrate its highlights in off-axis optical system design. First, we take the extremely demanding TMA telescope as an example, the introduction of the freeform sensor makes the imaging performance reach the diffraction limit, and the PV sag departure of the mirror surface is reduced by 71% compared with the traditional design using flat sensor. Next, we performed finite element analysis on the silicon die with freeform shape to ensure that the stress distribution of the curved sensor is within a safe range when bending. Finally, the prototype of freeform-curved sensor will be manufactured, and its surface shape will be tested in the laboratory.

Photoacoustic microscopy (PAM) plays a vital role in label-free microscopic imaging of the optical absorption contrast in tissues. It usually combined single ultrasound transducer to receive the acoustic waves converted from absorbed optical energy by transient thermoelastic expansion. The opaqueness of conventional ultrasound transducers makes the system to misalignment, complicated and bulky. However, recent developed transparent transducer has lower bandwidth as lack of appropriate matching and backing, which will cause lower axial resolution for imaging. Hence, developing 30-MHz transparent transducer with a -6 dB bandwidth higher than 50% will bring more feasibilities to achieve photoacoustic imaging with higher resolution. And this study indicated the potential of developing new transparent ultrasound transducer for photoacoustic imaging, which will bring more possibilities to develop a fast, compact, and, hand-held PAM imaging device.

The real-time monitoring of physical and chemical parameters in running fluids is of importance for biomedical, biochemical, and environmental applications, such as the presence of biomarkers or chemomarkers, or the departure from some preset values of critical parameters. In this contribution we present a new generation of Permeable Diffractive Optical Elements (PDOE) based on photon sieves. In brief, the PDOE is made of passing holes properly placed on specific locations on a rigid surface. This arrangement makes PDOEs ideal to work with running fluids. Our PDOE is optimized maximizing the irradiance at is focal plane, maintaining an appropriate permeability ratio. The starting point is the classical Fresnel zone distribution. We have used two different optimization strategies to design a working PDOE: i) Particle Swarm Optimization has been applied to modify the distribution of holes on the PDOE simultaneously considering all of them; ii) an iterative minimization algorithm adding one hole at the time until filling the PDOE aperture. Both optimization algorithms generate focal spots that are compared to choose the design better suited for the proposed application. Once the PDOE is optimized and fabricated, the surface of the remaining rigid structure is nanostructured (for example using Laser Induced Periodic Surface Structures), or functionalized, to provide specific sensing capabilities. In addition, the PDOE is integrated within a pipe where the fluid under analysis circulates through. A proposal for the optoelectronic assembly of the device-including auxiliary optical elements, light sources, and detectors - is also presented in this contribution.

In every field of science, new techniques for target molecule detection are increasingly required. This work presents itself as a method for developing new sensors that can detect target molecules even in samples with low concentrations. The aim of the current research project is to create an optical sensor substrate that is precise and specific for fluorescent or fluorescently marked targets. We produced nanostructured surfaces on bulk silver substrates using a femtosecond pulsed laser (Laser Libra Ti:Saphira from Coherent). These surfaces have a larger superficial area, which increases the fluorescence of molecules nearby by due to the surface plasmon resonance, in the effect of metal-enhanced fluorescence (MEF). The COVID virus spike antibody (SARS-CoV-2(2019-nCoV)) and a secondary fluorescently marked antibody (Alexa Fluor™ 633 goat anti-human IgG (H+L)) was used to functionalize the nanostructures. Initial results indicated that the functionalization process was successful in achieving our initial goals, presenting the proposal's methods as an effective route for the development of biosensors. The primary antibody was initially detected in a very low concentration (0.0525 ng/uL), and the fluorescent signal was enhanced on the nanostructured portion of the surface by 6.3 times more than it was on the surface without modification.

The interest in LiDAR imaging systems has recently increased in outdoor ground-based applications related to computer vision, in fields like autonomous vehicles. However, for the complete settling of the technology, there are still obstacles related to outdoor performance, being its use in adverse weather conditions one of the most challenging. When working in bad weather, data shown in point clouds is unreliable and its temporal behavior is unknown. We have designed, constructed, and tested a scanning-pulsed LiDAR imaging system with outstanding characteristics related to optoelectronic modifications, in particular including digitization capabilities of each of the pulses. The system performance was tested in a macro-scale fog chamber and, using the collected data, two relevant phenomena were identified: the backscattering signal of light that first interacts with the media and false-positive points that appear due to the scattering properties of the media. Digitization of the complete signal can be used to develop algorithms to identify and get rid of them. Our contribution is related to the digitization, analysis, and characterization of the acquired signal when steering to a target under foggy conditions, as well as the proposal of different strategies to improve point clouds generated in these conditions.

In this paper, an external cavity diode laser (ECDL) in Littrow configuration with narrowband emission is presented. The laser system is based on a commercially available GaN Fabry-Perot laser diode. Longitudinal mode selection is performed using a reflective holographic grating. Tuning range over 3.4 nm is achieved with a short linewidth of 0.02 nm at the rated current. The ECDL system is integrated into an optical sensor for remote detection of Nitrogen Dioxide (NO2) gas.

Absorption fingerprints of substances such as glucose, acetone and CO2 fall within the short-wave infrared range (SWIR), in the wavelength range 1.7 μm – 2.4 μm; improved detection of these substances will be impactful to health, wellbeing and the environment. A design of detector based on the emerging material system InGaAsSb with cut-off wavelengths in this range, 2.25 μm, is presented. By controlling the composition of the InGaAsSb, the cut-off wavelength can be extended beyond GaSb (1.7 μm) to a particular target with minimal leakage increase. Unbiased operation has been obtained using a p-B-n structure design and a quasi-planar device design with good optical power resolution (40 pW). At the current state of optimisation, D* is 9.4×10¹⁰ Jones at 0 V bias and 2.0 μm which is approaching the much more mature extended InGaAs technology, grown mismatched on InP, with the same cut-off wavelength. InGaAsSb is grown on GaSb substrates which are increasingly popular for IR optoelectronics and being lattice matched will offer a higher yield in production compared to mismatched growth. With InGaAsSb, the GaSb-matched material system can support lattice matched epilayers exhibiting cut-off wavelengths from the near to the longwave infrared. This work looks towards future applications through evaluations and measurements of low concentration glucose solutions. These detectors show great promise for future commercial applications.

We demonstrate an integrated on-chip whispering gallery mode (WGM) ring resonator tested for humidity sensing. When developed, SU-8 is chemically and mechanically stable, as well as optically transparent above 400 nm wavelength and it has a high refractive index. Therefore, it is a suitable material for optical WGM resonators. When light is coupled in the resonator, it circulates along the surface for prolonged periods of time interacting with the surrounding environment. Resonance wavelength depends on the refractive index and/or the radius of the resonator. Polymers, including SU-8, are sensitive to gas and temperature changes in the environment. We tested the ring resonators in changing relative humidity (RH). Due to changes in RH, the refractive index of SU-8 changes, and we observed a shift in the resonance wavelength. While the sensitivity was average compared to similar studies using other materials or geometries, the ring geometry showed excellent response and recovery times. This property is important in many areas, such as industrial production and environmental monitoring.

Due to their superior properties in single-molecule detection, plasmonic and nanopore-based sensors have attracted research interest. In recent times, they have been combined in a single device, resulting in plasmonic nanopores-based sensors. These solid-state devices featured unprecedented enhancements in single-molecule and nanoparticle detection, optical spectroscopies and trapping, control of local temperature. In this context, we have investigated two kinds of nanostructures: plasmonic nanopores and plasmonic nanoantennas, both of which were fabricated on free-standing Si3N4 membranes. As regards the nanopores, we were able to prove that their plasmonic coating enhanced their conductance when illuminated at 631 nm. On the other side, antenna-shaped nanopores (i.e., nanoantennas) were fabricated via plasmonic photochemical deposition. At this regard, we demonstrated that it was possible to fabricate nanoantennas with different internal diameters by different time of plasmon-induced photochemical deposition of metal precursors at the free tip of the nanoantenna. In conclusion, we proved that it was possible to use each nanoantenna (i.e., each decreasing internal diameter) to detect the translocation of nanoparticles with correspondingly decreasing diameters or of DNA.

In the contribution, we present the design of a probe responding to the presence of a magnetic field, consisting of a capillary fiber filled with magnetic fluid and connected to polarization-maintaining optical fibers on both sides. Magnetic fluid in the capillary responds to a transverse magnetic field by changing its birefringence. As a result, there is a change in the polarization state of the optical wave passing through the fluid in the capillary, and this change is registered using an optical fiber plane polarizer placed between the fiber coming out of the capillary and the optical spectrum analyzer. A relationship between the magnitude of the magnetic field flux density and the spectral change of the signal detected by the optical spectrum analyzer is observed. The result indicates a strong potential of a magnetic fluid in the field of the optical fiber sensors of magnetic fields

Nanoimprint lithography is the simple method using stamp and UV or thermal curable resins for nano-structures/patterns with low cost, high-throughput, and high resolution. Residual-layer free NIL provides good performance of micro/nano-scale structures functional arrangements with 2/3D layouts. We demonstrated nanohole patterns of 200 nm pore size using residual-layer-free NIL without further process for removing residual layers for reflectance biosensor. The reflectance peaks of gold substrate are enhanced to 8 times using the hexagonal hole patterns of diameter 200 nm, and pitch 400 nm. So, this substrate can be applied for immune reflectance biosensor with magnetic nanoparticles for pre-treatment.

Non-Hermitian systems with varying loss-gain profiles are receiving significant attention due to their exotic behavior at a certain point called the exceptional point (EP). EPs are singularities of non-Hermitian systems where the eigenfrequencies as well as the associated eigenstates coalesce. These EP singularities are ultrasensitive to small perturbations. A conventional system follows a linear relation with perturbation whereas these singularities follow a square root dependence for small perturbations.

The article evaluates a surface plasmon resonance (SPR) sensor that uses a graphene microribbon (GMR) array on a silicon waveguide to detect benzoic acid in terahertz (THz) frequencies. The graphene micro-strips are periodically spaced to efficiently excite SPR in THz. The application of planar waveguide technologies enables the development of miniature and compact multisensor devices that can connect to instrumentation using optical fibers, providing the ability for remote operation. Here, the impact of modifying variables as ribbon width (r_w) and the number of ribbons (num_rib) are examined for a specific structure of GMR, tuned to E_F = 0.45 eV and Γ = 3.7 meV with periodicity λ = 4 µm, deposited on a silicon waveguide of h = 15 µm and SiO_2 substrate. The results show that the manipulation of these variables enhances the plasmon formation, but also highly affect the plasmonic modal distribution along the array. The article concludes that the balance between these features can lead to the sensor's performance optimization. Therefore, changing the analyte refractive index with the acid concentration, a very high sensitivity sensor of 8658 nm/RIU is presented for r_w = 3 µm and num_rib = 200.

Microfluidic devices have the capability to assist in the development of basic miniaturized devices. Optical measurements are powerful in the application of sensors. Optofluidic, or the integration of optics with microfluidics, provides an appealing framework for the implementation of optical devices with a wide range of characteristics and functions. We provide a manufacturing approach for optofluidic devices by integrating a microfluidic channel made of PDMS with silicon nitride substrate to enable optical measurements. The SiN thin film was prepared by means of plasma-enhanced chemical vapour deposition (PECVD), such that SiN was deposited on a Si wafer using only SiH4 and N2 precursors to reduce the hydrogen content. The SiN thin-film thickness is in the range of 300 nm to 350 nm. A microfluidic channel was prepared by casting PDMS on a fabricated mold patterned on a commercial FR4 board. The device was composed by integrating the SiN substrate with the microfluidic channel and tested for optical measurements.

We report on linewidth narrowing and stabilization of semiconductor DFB laser implemented through its self-injection locking to an external fiber ring cavity in conjunction with an active optoelectronic feedback circuit controlled by a simple low-cost USB-DAQ card. The system enables narrowing of the DFB laser linewidth below ~0.5 kHz and drastically reduced the laser phase noise. Specifically, the laser configuration is fully spliced from the polarization maintaining (PM) single-mode optical fiber that exhibits significantly improved stability against the environment noise. Drastic narrowing of the DFB laser linewidth down to ~310 Hz and a phase noise less than –100 dBc/Hz (<30 kHz) are achieved with the PM fiber ring cavity built from a single fiber coupler. The reported PM laser configuration is of great interest for many laser applications where a narrow sub-kHz linewidth, simple design and low cost are important.

A multimode optical fiber supports excitation and propagation of a pure single optical mode, i.e., the field pattern that satisfies the boundary conditions and does not change along the fiber. When two counterpropagating pure optical modes are excited, they could interact through the stimulated Brillouin scattering (SBS) process. Here, we present a simple theoretical formalism describing SBS interaction between two individual optical modes selectively excited in an acoustically isotropic multimode optical fiber. Employing a weakly guiding step-index fiber approach, we have built an analytical expression for the spatial distribution of the sound field amplitude in the fiber core and explored the features of SBS gain spectra, describing the interaction between modes of different orders. In this way, we give a clear insight into the sound propagation effects accompanying SBS in multimode optical fibers, and demonstrate their specific contributions to the SBS gain spectrum.

We examine the use of state-of-the-art distributed sensing systems to extract temperature information from the optical fibre infrastructure already of the Electricity Authority of Cyprus power distribution network (~25-year old installation); as a means of optical fibre distributed sensing in the underground cables. The optical fibres are collocated with existing power distribution cables, for the purpose of power line monitoring cable joints that are prone to failure, along with general monitoring for unusual behaviour and potential cable fault conditions. Detection is achieved using DTS: Distributed Temperature Sensors (Silixa Ltd) that use RAMAN-based measurements in combination with BOTDR (Brillouin Optical Time-domain Reflectometry) for high-precision temperature detection. We examine the correlation between the temperature of the power cable with the power consumption provided by the EAC and the weather conditions. Furthermore, our data will give an indication of how important is uniform spacing between power and optical cables. The real-time and continuous monitoring of the temperature of the optical cables through the distributed sensing systems may help identifying abnormal cable behavior (hot spots) and possible future network failures in the power network.

Metallic nanostructures allow for strong enhancement of field intensity by plasmonic effects and offer efficient means for the amplification of weak optical spectroscopy signals. Typically, the metallic nanostructures are made static. A possible route to expand the spectrum of applications and performance of plasmon-enhanced spectroscopy tools is pursued, based on responsive hydrogel materials that act as artificial muscles and provide on-demand, reversible reconfiguration of plasmonic hotspots. Hydrogels are three-dimensional polymer networks with the ability to intake large amounts of water. Some classes of responsive hydrogels can be reversibly toggled between two states – swollen and collapsed – by modulating their temperature T. In this work, we use poly(N-isopropylacrylamide)-based responsive terpolymers (pNIPAAm) and we disperse polystyrene (PS) nanoparticles in the hydrogels, allowing precise control on the temperature-induced changes of the swelling ratio and allowing for a mechanically more rigid structure . This controlled actuation mechanism finds various applications in plasmonic nanomaterials. Here we present the concept of a microscopic responsive hydrogel structure that allows the modulation of the distance between metallic nanoparticles and a flat metal surface, for reversible near-field coupling and formation of a gap mode. The plasmonic coupling can be exploited for probing of molecules, by plasmonically-enhanced optical spectroscopy.

Monitoring pH and extracellular acidification in biological samples containing live mammalian cells can provide valuable information on the glycolytic activity and bioenergetic status of cells. Compared to pH electrodes, optochemical pH sensors look more advantageous, since they allow rapid, non-invasive parallel analysis of multiple samples with stable readout of pH. We have developed new fluorescent pH sensors based on hydrophobic protonable metal-free porphyrins (OEP and OEPK) embedded in a proton-permeable polymeric matrix together with a proton transfer agent. These pH sensors provide internally-referenced calibration-free operation, both in ratiometric intensity and lifetime based detection modes. Sensor development included optimization of the indicator dye and its photophysical characteristics, screening of different proton transfer agents to minimize sensor toxicity, tuning of protonation range and pKa, long-term storage stability and response time studies. Optimised pH sensor coatings were then deposited on plastic substrates (96-well microplates) and used for real-time monitoring of Extracellular Acidification Rate (ECAR) for cultured cancer cells and 3D spheroid structures on standard laboratory equipment (multi-label plate reader and confocal FLIM microscope). The advanced pH sensors tailored for use with biological samples have high potential for cell analysis and related applications.

The technology we are developing consists of the use of coordination compounds with metals to carry out this detection of Acetic acid. This compound normally reacts with acetic acid changing its colour, making it a suitable compound for use as a detector. The proposed method allows detecting acetic acid in any medium, whether in solution, in the gas phase, in the solid phase, or in any combination of these. Upon contact with the acid, a colour change occurs that can be detected visually or through optical means. After its use, the active medium can be regenerated by a simple procedure and be available again for new use. This allows the creation of simple and intuitive detection devices, usable by non-experts and that can be regenerated and reused. The main advantage of this sensor is to allow the specific detection of acetic acid and quantification of its concentration, using coordination compounds with metals that are present in the yellow dye.

A lens-less optical fiber designed for enhanced-fluorescence biosensor applications is presented. In order to obtain the enhanced sensor performances, two elements are essential: a planar antenna that redirects fluorescence emission into a narrow cone and an automated fiber-based optical system for multi-spot analysis. In particular, the potential early diagnosis of sepsis via C-reactive protein (CRP) detection is here demonstrated, reaching a limit of detection of 1.5 ng/mL), which is in the clinical range of interest for such biomarker. Upon the combination with other sepsis biomarkers, the presented sensor can become relevant for the early diagnosis of sepsis. These results validate the developed prototype as a simple, affordable, easy-to-operate, plug&play device with fast turnaround times, compatible with standardized micro-well arrays, and potentially suitable for POC applications with respect to the diagnosis of sepsis. It is also suitable for implementation with other biomarkers and liquid environments.

Intracellular Optical Oxygen sensing is a convenient approach for monitoring and imaging molecular oxygen (O2) in biological samples, however existing intracellular O2-sensing probes still have some limitations. This study describes one new phosphorescent hetero-substituted derivative of Pt(II)-tetrakis(pentafluorophenyl)porphyrin (PtPFPP) obtained via two-step thiol click-chemistry. Particularly, thio-glucose (Glc) and thio-methyl-polyethylene-glycol (mPEG) moieties were covalently attached to the phosphorescent dye, producing the trans-di-glucosylated-di-PEGylated derivative PtGlc2PEG2 (trans). In a previous publication, we demonstrated the ability of these short PEG oligomers to drastically reduce the ability of the tetra- or tri-PEGylated conjugates to translocate across the cellular membrane. However, in this study, we show the capability of the trans-di-PEGylated-di-glucosylated conformation to allow intracellular staining and O2 sensing (IcO2) in murine embryonic fibroblasts (MEFs) mammalian cells.

The article describes a fiber-optic sensor based on a Bragg grating, which is implemented in oxygen glasses, which are commonly used in real medical practice. The realized experimental fiber-optic sensor can be used to monitor the respiratory activity of the human body over time. The article primarily describes the design improvement of the Bragg grating implementation itself in used conventional oxygen glasses. The practical part also describes the experimental verification of the functionality and the subsequent evaluation of the measured data. The experimental measurement was carried out on a group of 5 volunteers in laboratory conditions.

This paper discusses a proposed fiber-optic sensor that was based on the combination of the material called polyethylene terephthalate glycol (PETG) and a fiber Bragg grating (FBG). The practical experiments were conducted under laboratory conditions with a group of 6 volunteers. The data were compared with a conventional electrocardiogram (ECG) and processed using a Bland-Altman method. The results described in this paper show that the 3D printing technique can be used for encapsulating the FBG and for the purpose of biomedical engineering.

Water salinity analysis is critical for water quality monitoring and evaluation in order to guarantee water safety. Unregulated salinity may be harmful to human health, crops, industry, and the ecosystem. The salinity levels in water sources are constantly changing as a result of natural and anthropogenic ecological change. Exceeding specific salt levels might endanger human health, particularly through drinkable water. In this work, we present a simulation of an optofluidic sensor to measure the water’s salinity. We built a design to measure the change in the refractive index in a microfluidic channel based on the dielectric grating. The generated design was modelled to alleviate the manufacturing process of the microfluidic sensor of the saline water. The design was optimized for the range of measurements of the refractive index of saline water by the selection of the material of the sensor.

Plasmonic nanostructures are widely studied for the construction of affinity-based biosensors. In these biosensors, the plasmonic resonances in visible (VIS) and near-infrared (NIR) regions are used to probe the vicinity of the nanostructure, where the molecules of interest (an analyte) are bound to the surface. Although biosensing is limited to the VIS/NIR regions in conventional plasmonic sensors, sensing in the ultraviolet (UV) region provides the capability of detecting proteins or biological compounds which are fluorescent or have absorption bands in the UV region. In this work, we report on the novel approach that employs plasmonic nanostructures with multiple modes supported in UV and VIS regions to provide high-performance biosensing. The UV mode is spectrally tuned to the targeted biomolecules, and the VIS mode provides high refractive-index sensitivity. Using numerical electromagnetic simulations, we analyzed two bimetallic (aluminum and gold) nanostructures. We demonstrated that optimizing the geometrical parameters of these nanostructures allows us to tune the short-wavelength resonance to a region suitable for UV fluorescence/absorption while providing sufficient electromagnetic field overlap with the long-wavelength resonance in the VIS region for high-performance biosensing. The designed plasmonic nanostructures thus can be employed to identify and quantify the biomolecules simultaneously.

Surface plasmon polariton (SPP)-based methods enable the investigation of biological objects, such as biomolecules and cells, with high sensitivity and spatial resolution. These methods employ various types of SPPs to match the size of the investigated biological object with the penetration depth of an SPP. Large penetration depths are provided by long-range SPPs that are supported by dielectric-metal-dielectric structures with dielectric materials of similar refractive indices (RIs). However, the development of such structures for biosensing is difficult due to the limited availability of dielectrics with a RI close to that of water and desired properties, such as stability and compatibility with relevant fabrication techniques. Here, we describe the development of diffractive structures supporting long-range SPPs. The described structure consists of a low RI dielectric grating with a sine profile with a thin layer of gold. The geometric properties of the structure are optimized using rigorous coupled wave analysis to achieve high sensitivity of the SPPs to bulk RI change and to allow for the excitation of SPPs from both sides of the structure under the normal incidence of light. The laboratory prototypes of the structure were fabricated by creating a grating using soft lithography in a low RI polymer (CYTOP) layer on a glass slide and then coating it with a layer of gold using vacuum deposition. The fabricated structures were characterized experimentally and sensitivity to bulk RI changes was determined and compared with the theoretical predictions. The potential of such structures for SPP biosensing and imaging is also discussed.

A Brillouin Optical Time-Domain Analysis (BOTDA) Lorentzian data fitting method to estimate the Brillouin Frequency Shift (BFS) is proposed. Data is obtained from an experimental setup used to conduct the temperature and strain measurements. Before Lorentzian fitting the noisy data is averaged and filtered. The proposed method attempts to lower computational complexity in determining the Brillouin frequency. The resulting parameters of a completed BGS curve fitting are used as initial set of parameters for the next location point BGS fitting. Completion of the Lorentzian fitting using the Levenberg-Marquardt nonlinear curve fitting algorithm is achieved in a small number of iterations which improves the performance in obtaining the Brillouin frequency shift.

In this study, the performance of the experimental laser speckle imaging system and image processing algorithm for early evaluation of microbial activity in a noisy environment was tested. The proposed sub-pixel correlation algorithm was applied to recognize useful signals attributed to the microbial colony forming units in solid media