Light-analyte Interaction Research Articles

Graphene-Dielectric metasurface of asymmetric grooved disks for biosensing and surface roughness evaluation

In this paper, we propose a high-performance graphene-dielectric metasurface that is applied as an optical biosensor. The proposed metasurface is based on an array of asymmetric grooved silicon disks grown over a graphene sheet. The asymmetric silicon disks lead to the formation of a high quality factor (Q-factor) Fano resonance. In addition to the sharp resonance, the strong field confinement provided by annular grooves results in an enhanced light-analyte interaction and sensitivity of as high as 640 nm/RIU. According to the presented results, the biosensor exhibits a high linearity R2 value of 0.999 and low resolution of 1.56e-4 RIU. Furthermore, the proposed biosensor is polarization-insensitive with a figure of merit (FOM) as high as 784 RIU−1. To the best of our knowledge, this is the highest FOM ever reported for a polarization-insensitive biosensor. Such characteristics are promising for effective detection of biomolecules like hemoglobin. Finally, the effect of surface roughness on performance of the proposed biosensor is investigated where different resonance characteristics and sensitivities are obtained. It implies that the proposed structure as a multiparameter sensor could be applied for biosensing and also for evaluation of the amount of surface roughness due to fabrication errors.

Sub-ppm Methane Detection with Mid-Infrared Slot Waveguides.

Hybrid integration of photonic chips with electronic and micromechanical circuits is projected to bring about miniature, but still highly accurate and reliable, laser spectroscopic sensors for both climate research and industrial applications. However, the sensitivity of chip-scale devices has been limited by immature and lossy photonic waveguides, weak light-analyte interaction, and etalon effects from chip facets and defects. Addressing these challenges, we present a nanophotonic waveguide for methane detection at 3270.4 nm delivering a limit of detection of 0.3 ppm, over 2 orders of magnitude lower than the state-of-the-art of on-chip spectroscopy. We achieved this result with a Si slot waveguide designed to maximize the light-analyte interaction, while special double-tip fork couplers at waveguide facets suppress spurious etalon fringes. We also study and discuss the effect of adsorbed humidity on the performance of mid-infrared waveguides around 3 μm, which has been repeatedly overlooked in previous reports.

Design of a high-sensitivity polymer double-slot waveguide sensor for point-of-care biomedical applications

Silicon photonics is a rapidly developing field that offers cost-effective biosensors with improved sensitivity and the potential for interconnecting with other electronic devices for instant disease diagnosis. The Mach Zehnder interferometer architecture (MZI) is a key technology for biosensors, as it detects changes in refractive index (RI) caused by the presence of biomolecules. In this study, a silicon-polymer double-slot waveguide-based MZI was designed, with a small mode area and a large evanescent field to enhance light-analyte interaction. The waveguide was optimized by converting a normal slot waveguide into a double-slot waveguide with varying slot widths. In transmission spectrum, the wavelength shift was measured for both normal and disease samples. Additionally, the loss at a specific wavelength was analyzed to understand the impact of the biomolecule on the sensor performance. The results show that this sensor has a high sensitivity of 2.39 X10^5 nm/RIU, making it a promising candidate for biosensing applications.

Balancing detectivity and sensitivity of plasmonic sensors with surface lattice resonance

Abstract Resonators are at the core of optical sensors enhancing light–analyte interaction and leading to higher sensitivities. Maximizing the sensitivity is an obvious objective function for the resonator design. However, high sensitivity does not guarantee sufficient detectivity. When the optical energy budget is limited, as in sensors on mobile platforms, a higher sensitivity usually leads to lower detectivity for nanophotonic sensors. In such scenarios, resonator design requires balancing the trade-off between the sensitivity and detectivity of the resonant sensor. Here, we show the direct dependence of detectivity on the Q-factor and the trade-off between the Q-factor and sensitivity. We study this trade-off in an array of plasmonic resonators. We choose plasmonic resonators because of their high sensitivity arising from large local field enhancements. Then, we show that the detectivity of this sensor may be boosted for limited energy budget applications by making an array of resonators supporting a surface lattice resonance (SLR). We experimentally demonstrate sensing and detection of antimouse IgG protein in a gold nanodisk array–based SLR sensor for various energy budgets.

Terahertz photonic crystal fiber for sensing the creatinine level in the blood

In this work, THz photonic crystal fiber (PCF) is used to detect the creatinine level in the blood with high sensitivity. The sensing technique depends on increasing the light interaction with the analyte infiltrated into the air holes in the fiber core region. In this regard, most of the light power should be confined through the analyte region. This will increase the relative sensitivity coefficient that is proportional to the analyte power fraction. The operation of the suggested sensor is based on studying the light–analyte interaction at different creatinine concentrations by detecting the change of the analyte power fraction against the change of the creatinine level in the blood at the THz range 0.5–1.5 THz. The effective mode index (neff), effective material loss (EML), effective modal area (Aeff), and relative sensor sensitivity are calculated using the full vectorial finite element method. Additionally, the different geometrical parameters are studied to maximize the sensor’s sensitivity. The proposed THz-PCF has a 93% and 95% relative sensitivity for x- and y-polarized modes, respectively. Therefore, the suggested THz-PCF biosensor gives a promising usage in measuring the creatinine level in the blood.

Rapid detection of SARS-CoV-2 genetic targets using nanoporous waveguide based competitive displacement assay

Rapid detection of unlabeled SARS-CoV-2 genetic target was demonstrated using a competitive displacement hybridization assay made by a nanostructured anodized alumina oxide (AAO) membrane. The assay applied the toehold-mediated strand displacement reaction. The nanoporous surface of the membrane was functionalized with a complementary pair consisting of Cy3-labeled probe and quencher-labeled nucleic acids through a chemical immobilization process. In the presence of the unlabeled SARS-CoV-2 target, the quencher-tagged strand of the immobilized probe-quencher duplex was separated from the Cy3-modifed strand. A stable probe-target duplex formed and regained a strong fluorescence signal, thus enabling real-time and label-free SARS-CoV-2 detection. Assay designs with different numbers of base pair (bp) matches were synthesized to compare their affinities. Because of the large surface of a free-standing nanoporous membrane, two orders enhancement of the fluorescence was observed, where the detection limit of the unlabeled concentration can be improved to 1 nM. The assay was miniaturized by integrating a nanoporous AAO layer onto an optical waveguide device. The detection mechanism and the sensitivity improvement of the AAO-waveguide device were illustrated from the finite difference method (FDM) simulation and the experimental results. Light-analyte interaction was further improved due to the presence of the AAO layer, which created an intermediate refractive index and enhanced the waveguide's evanescent field. Our competitive hybridization sensor is an accurate and label-free testing platform applicable to the deployment of compact and sensitive virus detection strategies.

Effective optical biosensing using a graphene-based metasurface of asymmetric split silicon bars

In this paper, we propose an effective optical biosensor based on a graphene-dielectric metasurface. The metasurface is constructed by an array of asymmetric silicon bars deposited on a sheet of graphene. A high quality factor (Q-factor) Fano resonance is supported by the presented structure. In addition, there is a gap within each silicon bar where the optical field is highly confined and a strong light-analyte interaction takes place. Various geometrical parameters of the metasurface are optimized. As a result, a high sensitivity of 392 nm/RIU and figure of merit (FOM) of as high as 587 RIU−1 are obtained. According to the presented results, the resolution of the proposed biosensor is as low as 2.55e-4 and the linearity R2 value is 0.999. The underlying graphene sheet of the metasurface could potentially adsorb and immobilize biomolecules. Such characteristics make the proposed biosensor promising for detection of various biomolecules like hemoglobin.

Plasmonic Ag/ZnO Nanoscale Villi in Microstructure Fibers for Sensitive and Reusable Surface-Enhanced Raman Scattering Sensing

Microstructure fibers, integrating microfluidic channels and light guidance in one fiber, enable three-dimensional surface-enhanced Raman scattering (SERS) detection for large signal accumulation. However, the available fiber SERS probes are complicated to prepare and they are not reusable. In addition, light interacts with analytes in a form of an evanescent field, which is very weak. In this paper, we developed a SERS platform based on suspended-core photonic crystal fibers decorated with Ag/ZnO nanocomposites on the inner surface for direct, ultrasensitive, and reusable analyte detection. The unique configuration not only transfers a core-localized field to the liquid interface to greatly enhance the light–analyte interaction but also facilitates charge transfer to further improve the SERS detection sensitivity and degradation efficiency. The detection limit of crystal violet solution is 10–13 M, and the enhancement factor reaches 1011. The relative standard deviation is as low as 5.4%, ensuring the reproducibility of SERS detection. The probe has good photocatalytic performance and can degrade molecules under ultraviolet-light illumination within 20 min. This ultrasensitive and reusable SERS probe shows great application potential in rapid and in situ liquid detection.

Optimization of light-analyte interaction in Si3N4/polymer hybrid waveguide for sensitive sensing of pyridine vapor with ppb-level detection limit

High-sensitivity and miniature volatile organic compound (VOC) sensor is considerably significant for environment monitoring. Herein we performed the theoretical and experimental optimization of waveguide sensor to realize the sensitive detection of volatile toxic pyridine vapor with ppb-level detection limit. The miniature sensor is based on the CMOS-compatible silicon nitride (Si3N4) Mach-Zehnder interferometer (MZI) waveguide with dipolar polycarbonate as sensitive cladding material. Theoretical optimization by varying the waveguide width and the thickness of sensitive cladding is carried out to tune the group refractive index and evanescent field. Experimentally, two MZIs with distinct waveguide widths and sensitive cladding thickness are fabricated to compare their performance in pyridine vapor sensing. The sensing results show that the thickness of sensitive layer plays a key role in sensing sensitivity and response time. The final obtained pyridine sensing sensitivity can reach 63 pm/ppm and the detection limit can be as low as 476 ppb. Our demonstration provide a simple and efficient strategy for sensitive sensing of hazardous chemical vapors, which may find application in biochemical sensing.

Controlled fiber core mode and surface mode interaction for enhanced SERS performance.

Three-dimensional surface-enhanced Raman scattering (SERS) platform based on microstructure fibers has many advantages for rapid liquid detection due to its microfluidic channels and light guidance. The fiber mode field distribution determines the light-analyte interaction strength but has rarely been studied in SERS applications. In this paper, we numerically and experimentally investigate the mode field distribution in suspended-core fibers decorated with gold nanoparticles. The interaction between the core mode and surface mode is controlled by changing the density of gold nanoparticles on the inner surface. The avoided crossing wavelength shifts linearly to red with the decrease of the nanoparticle spacing. With an optimized nanoparticle spacing of 20 nm, the avoided crossing occurs near the laser wavelength of 633 nm, which greatly increases the power ratio in the liquid channels and hence improves the SERS performance. The detection limit for crystal violet was 10-9 M, and the enhancement factor was 108. The avoided crossing mechanism can be applied to all fiber SERS probes for sensitivity improvement.

SNROW-based highly sensitive label-free surface biosensor for hepatitis B detection.

Despite the availability of effective hepatitis B vaccinations, the hepatitis B virus remains a serious global health concern. It is expected that early detection could aid in initiating therapy before the infection progresses to liver damage. A silicon nanowire rectangular optical waveguide has been demonstrated theoretically to detect the surface antigen of hepatitis B "HBsAg" based on label-free surface sensing using finite-element method-based COMSOL Multiphysics. Different procedural segments of the biomarker detection have been mimicked on the surface of a waveguide as adlayers to investigate the device theoretically. Initially, the parameters of the waveguide have been optimized to provide a large interaction of light and bio-analyte, i.e., to provide high sensitivity. The analyses are first performed at the waveguide level based on the light-analyte interaction. Furthermore, performances of the sensor have been obtained by incorporating this waveguide structure in the sensing arm of the Mach-Zehnder interferometer. The device structure shows ultra-high surface sensitivities such as phase surface sensitivity of 7.03×2πrad/nm and MZI surface sensitivity of 3421.89 µW/nm with an excellent detection limit of 2.92×10-3pg/mm2 for HBsAg detection. The proposed device can measure the HBsAg concentration as low as 0.00973 ng/mL, which is significantly low to detect the infection in an early stage.

Label-Free Imaging of Single Proteins and Binding Kinetics Using Total Internal Reflection-Based Evanescent Scattering Microscopy.

Single-molecule detection can push beyond ensemble averages and reveal the statistical distributions of molecular properties. Measuring the binding kinetics of single proteins also represents one of the critical and challenging tasks in protein analysis. Here, we report total internal reflection-based evanescent scattering microscopy with label-free single-protein detection capability. Total internal reflection is employed to excite the evanescent field to enhance light-analyte interaction and reduce environmental noise. As a result, the system provides wide-field imaging capability and allows excitation and observation using one objective. In addition, this system quantifies protein binding kinetics by simultaneously counting the binding of individual molecules and recording their binding sites with nanometer precision, providing a digital method to measure binding kinetics with high spatiotemporal resolution. This approach does not employ specially designed microspheres or nanomaterials and may pave a way for label-free single-protein analysis in conventional microscopy.

Evanescent scattering imaging of single protein binding kinetics and DNA conformation changes

Evanescent illumination has been widely used to detect single biological macromolecules because it can notably enhance light-analyte interaction. However, the current evanescent single-molecule detection system usually requires specially designed microspheres or nanomaterials. Here we show that single protein detection and imaging can be realized on a plain glass surface by imaging the interference between the evanescent lights scattered by the single proteins and by the natural roughness of the cover glass. This allows us to quantify the sizes of single proteins, characterize the protein–antibody interactions at the single-molecule level, and analyze the heterogeneity of single protein binding behaviors. In addition, owing to the exponential distribution of evanescent field intensity, the evanescent imaging system can track the analyte axial movement with high resolution, which can be used to analyze the DNA conformation changes, providing one solution for detecting small molecules, such as microRNA. This work demonstrates a label-free single protein imaging method with ordinary consumables and may pave a road for detecting small biological molecules.

Localized photonic nanojet based sensing platform for highly efficient signal amplification and quantitative biosensing

Light-analyte interaction systems are key elements of novel near-field optics based sensing techniques used for highly-sensitive detection of various kinds of targets. However, it is still a great challenge to achieve quantitative analysis of the targets using these sensing techniques, since critical difficulties exist on how to efficiently and precisely introduce the analytes into the desired location of the near-field light focusing, and quantitatively measure the enhanced optical signal reliably. In this work, we present for the first time a localized photonic nanojet (L-PNJ) based sensing platform which provides a strategy to achieve quantitative biosensing via utilizing a unique light-analyte interaction system. We demonstrate that individual fluorescent microsphere of different sizes can be readily introduced to the light-analyte interaction system with loading efficiency more than 70%, and generates reproducible enhanced fluorescence signals with standard deviation less than 7.5%. We employ this sensing platform for fluorescent-bead-based biotin concentration analysis, achieving the improvement on the detection sensitivity and limit of detection, opening the door for highly sensitive and quantitative biosensing. This L-PNJ based sensing platform is promising for development of next-generation on-chip signal amplification and quantitative detection systems.

Single Protein Detection and Imaging with Evanescent Scattering Microscopy.

Single-molecule measurements provide statistical distributions of molecular properties, in addition to the ensemble averages. Evanescent detection approaches have been widely used for single-molecule detection because the evanescent field can significantly enhance the light-analyte interaction and reduce the background noise. However, current evanescent single-molecule detection systems mostly require specially designed sensing components. Here, we show that single proteins can be imaged on a plain cover glass surface by detecting the evanescent waves scattered by the target molecules. This allows us to quantify the protein-antibody interactions at the single-molecule level. This protocol describes a label-free single-molecule imaging approach with conventional consumables and may pave the road for detecting single molecules with commercial optical microscopy.

Optimization of Light-Analyte Interaction in Si3n4/Polymer Hybrid Waveguide for Sensitive Sensing of Pyridine Vapor with Ppb-Level Detection Limit

Optimization of Light-Analyte Interaction in Si3n4/Polymer Hybrid Waveguide for Sensitive Sensing of Pyridine Vapor with Ppb-Level Detection Limit

Suspended silicon waveguide platform with subwavelength grating metamaterial cladding for long-wave infrared sensing applications

Abstract Long-wave infrared (LWIR, 6–14 µm) processes enormous potential for chem/biosensing as it covers abundant molecular absorption fingerprints. Waveguides provide an attractive chip-scale miniaturization solution for optical sensors. However, the exploration of waveguide sensors in this wavelength range is limited. Here, an LWIR photonic platform for fast and sensitive on-chip gas sensing is developed using suspended silicon (Si) waveguide supported by subwavelength grating (SWG) metamaterial claddings. This platform provides a viable approach to fully exploit the transparency window of Si. The SWG structure provides a promising solution to engineer the mode profile for strong light–analyte interaction. Propagation loss and bending loss are studied in the broad wavelength range of 6.4–6.8 µm. Functional devices including grating couplers, Y-junctions, and directional couplers are also demonstrated with high performance. Sensing demonstration based on our platform is presented using toluene vapor detection as an example. The corresponding limit of detection reaches 75 ppm. The response and recovery time to 75 ppm toluene are about 0.8 and 3.4 s, respectively. This good performance makes our platform a promising candidate for on-site medical and environmental applications.

Sol-Gel Thin Film Processing for Integrated Waveguide Sensors

Integrated opto-chemical sensors present great advantages in comparison to the current lab equipment. They bring portability, reduced costs, facilitate in-situ measurements, as well as largely reduced sample volumes. In this quest, standard processing protocols over established materials, such as silicon nitride, silicon, silicon dioxide, titanium oxide, and even a wide variety of polymers have so far been the key toward on-chip devices. However, if very specific materials in terms of composition and tailored properties are required, the deposition via a solution represents a viable alternative. In this review, we highlight the role of sol-gel chemistry and top-down processing of sol-gel thin film layers in the design of waveguide-based optical sensors. In particular, we stress the advantages of porous sol-gel based materials as a new approach to increase sensitivity and selectivity, first when used as claddings, and, more recently, as waveguides with enhanced light–analyte interaction. We finally discuss the future perspectives of such devices to increase specificity in complex matrices, which is of utmost importance for bio-sensing.

Detection of SARS-CoV-2 DNA Targets Using Femtoliter Optofluidic Waveguides.

Chip-scale SARS-CoV-2 testing was demonstrated using silicon nitride (Si3N4) nanoslot fluidic waveguides to detect a tagged oligonucleotide with a coronavirus DNA sequence. The slot waveguides were fabricated using complementary metal–oxide–semiconductor (CMOS) fabrication processes, including multiscale lithography and selective reactive ion etching (RIE), forming femtoliter fluidic channels. Finite difference method (FDM) simulation was used to calculate the optical field distribution of the waveguide mode when the waveguide sensor was excited by transverse electric (TE) and transverse magnetic (TM) polarized light. For the TE polarization, a strong optical field was created in the slot region and its field intensity was 14× stronger than the evanescent sensing field from the TM polarization. The nanoscale confinement of the optical sensing field significantly enhanced the light–analyte interaction and improved the optical sensitivity. The sensitivity enhancement was experimentally demonstrated by measuring the polarization-dependent fluorescence emission from the tagged oligonucleotide. The photonic chips consisting of femtoliter Si3N4 waveguides provide a low-cost and high throughput platform for real-time virus identification, which is critical for point-of-care (PoC) diagnostic applications.

Thermally Stable Optical Filtering Using Silicon-Based Comb-Like Asymmetric Grating for Sensing Applications

A silicon-based compact comb-like asymmetric grating is proposed as a thermally stable optical filter for label-free sensing applications. The device is designed and fabricated with a cavity section introduced between the two grating regions which are partially etched in the lateral direction to ensure the nonzero coupling of the fundamental and first-order modes in the propagation and counter-propagation direction. To demonstrate refractive index sensing, resonance shift in the device is measured with sodium chloride (NaCl) dissolved in DI water. In contrast to conventional Bragg grating where stopband lies in the transmission spectrum, the proposed device allows a single narrow passband transmission peak with a large Free Spectral Range (FSR) attributed to the engineered photonic bandgap of two modes present in the waveguide region. The device is deliberately designed such that slightly wide resonance peak is obtained that makes device operation thermally stable for a large temperature variation of ±15 K. A higher-order leaky mode with strong light-analyte interaction (due to long corrugation width) in the gratings governs high sensitivity of ≈352 nm/RIU for different concentrations of NaCl from 0% to 10% in the Deionized (DI) water with a small footprint area of $18~{\mu } {m}^{\textsf {2}}$ only. Proposed filter characteristics are well suited for multifunctional applications in integrated photonic devices.