Optimization of a horizontal slot waveguide biosensor to detect DNA hybridization

Overview of Optical Biosensors for Early Cancer Detection: Fundamentals, Applications and Future Perspectives

Conventional cancer detection and treatment methodologies are based on surgical, chemical and radiational processes, which are expensive, time consuming and painful. Therefore, great interest has been directed toward developing sensitive, inexpensive and rapid techniques for early cancer detection. Optical biosensors have advantages in terms of high sensitivity and being label free with a compact size. In this review paper, the state of the art of optical biosensors for early cancer detection is presented in detail. The basic idea, sensitivity analysis, advantages and limitations of the optical biosensors are discussed. This includes optical biosensors based on plasmonic waveguides, photonic crystal fibers, slot waveguides and metamaterials. Further, the traditional optical methods, such as the colorimetric technique, optical coherence tomography, surface-enhanced Raman spectroscopy and reflectometric interference spectroscopy, are addressed.

Recent Advances in Sensing Materials Targeting Clinical Volatile Organic Compound (VOC) Biomarkers: A Review

In general, volatile organic compounds (VOCs) have a high vapor pressure at room temperature (RT). It has been reported that all humans generate unique VOC profiles in their exhaled breath which can be utilized as biomarkers to diagnose disease conditions. The VOCs available in exhaled human breath are the products of metabolic activity in the body and, therefore, any changes in its control level can be utilized to diagnose specific diseases. More than 1000 VOCs have been identified in exhaled human breath along with the respiratory droplets which provide rich information on overall health conditions. This provides great potential as a biomarker for a disease that can be sampled non-invasively from exhaled breath with breath biopsy. However, it is still a great challenge to develop a quick responsive, highly selective, and sensitive VOC-sensing system. The VOC sensors are usually coated with various sensing materials to achieve target-specific detection and real-time monitoring of the VOC molecules in the exhaled breath. These VOC-sensing materials have been the subject of huge interest and extensive research has been done in developing various sensing tools based on electrochemical, chemoresistive, and optical methods. The target-sensitive material with excellent sensing performance and capturing of the VOC molecules can be achieved by optimizing the materials, methods, and its thickness. This review paper extensively provides a detailed literature survey on various non-biological VOC-sensing materials including metal oxides, polymers, composites, and other novel materials. Furthermore, this review provides the associated limitations of each material and a summary table comparing the performance of various sensing materials to give a better insight to the readers.

Anisotropic slot waveguides with bulk transition metal dichalcogenides for crosstalk reduction and high-efficiency mode conversion

Conventional slot waveguides (CSWs) consisting of an isotropic low-index material sandwiched by two high-index silicon wires have been extensively used in functional photonic devices, including chemical sensing, optical modulating, and all-optical signal processing, due to its significantly enhanced electric field perpendicular to the interfaces in the slot layer. However, there are two drawbacks to be improved if the CSWs are used for signal transmission in photonic integrated circuits, including the crosstalk between waveguides and direct butting mode conversion efficiency (MCE) to a silicon (Si)-strip waveguide. In this study, we propose an anisotropic SW with bulk transition metal dichalcogenide (ASWTMD) to relieve the two shortcomings by replacing the isotropic low-index slot layer with a bulk molybdenum disulfide layer having a high refractive index and giant optical anisotropy. We demonstrated the crosstalk reduction (CR) of the proposed ASWTMD by analyzing the mode profile, power confinement, and coupling strength. We also investigated the MCE by examining the mode overlap ratio and power evolution. The proposed ASWTMD shows significant CR and superior MCE for the transverse electric and transverse magnetic modes compared to those of a CSW with a SiO₂-slot layer. The present design paves the possible extensibility to other transition metal dichalcogenides (TMDs) for designing state-of-the-art TMD-based photonic devices exploiting their extraordinary optical properties.

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.

Investigation of an In-Line Slot Waveguide Sensor Built in a Tapered D-Shaped Silicon-Cored Fiber

An in-line slot waveguide sensor built in a polished flat platform of a D-shaped silicon cored fiber with a taper coupled region is proposed and investigated thoroughly. Simulation results show that the single-mode light field sustained in the silicon cored fiber can be efficiently transferred to the slot waveguides through the tapered region. The geometry parameters of the slot waveguide sensors are optimized to have the corresponding highest power confinement factors and the resultant sensor sensitivities. The three-slot waveguide sensor is found to have the best performance among one-, two- and three-slot waveguides at the mid-IR wavelength.

Releasing the light field in subwavelength grating slot microring resonators for athermal and sensing applications

Silicon-on-insulator (SOI) platforms have attracted increasing interest for photonic integrated devices with an ultra-small footprint. The distinct feature is the strong light confinement in the silicon region due to a high refractive-index-contrast. In contrast, releasing the light field out of the silicon region is also of great significance for providing a useful supplement to existing light guiding mechanisms and for facilitating versatile applications. Here, subwavelength grating slot (SWGS) microring resonators, which can effectively release light out of the silicon region for athermal and sensing applications, are proposed and demonstrated. The mechanism of releasing light relies on the combination of a surface enhanced supermode in a slot waveguide and a Bloch mode in a subwavelength grating waveguide. Four types of racetrack microring resonators (strip, slot, strip-SWGS, and slot-SWGS) were fabricated for comparison. The slot-SWGS microring resonator shows the best performance for athermal and sensing applications. The demonstrations may be useful for new releasing-light-enabled devices and applications.

Multi-band MIM refractive index biosensor based on Ag-air grating with equivalent circuit and T-matrix methods in near-infrared region

In this paper, a multi-band metal-insulator-metal (MIM) perfect absorber with refractive index sensing capability has been investigated in near-infrared region. The proposed structure has been studied for biomedical applications such as detection of solution of glucose in water, diagnosis of different stages of malaria infection, bacillus bacteria and cancer cells. The MIM configuration improves the sensing parameters of the biosensor due to the good interaction with the analyte. The high sensitivity and figure of merit of 2000 nm/RIU and 100 RIU⁻¹ have been achieved, respectively. Also, the Ag-air grating in the suggested plasmonic sensor helps the localized surface plasmons excitation and makes the structure sensitive to the incident lightwave polarization. Therefore, the presented biosensor behaves like a polarization switch with the high extinction ratio and fast response time of 25.15 dB and 100 fs, respectively. The methods of equivalent circuit model and transmission matrix have been utilized to verify the simulation results, as a new challenge in near-infrared region. The new idea of multi-application plasmonic devices, the feasibility of fabrication for the presented structure and utilizing mentioned analytical methods in near-infrared region could pave the way for the future of plasmonic structures.

Electric field sensing with liquid-crystal-filled slot waveguide microring resonators

We consider the operation principles of the sensor of an external electric field on the bases of microring resonators that consist of bent-strip waveguides with vertical or horizontal slots filled with nematic liquid crystal. The mode-field distribution and dispersion parameters of the bent-slot waveguides are calculated by using the algorithm based on the method of lines. The influence of the waveguide and microresonator structure (slot width, position and orientation, microresonator radius, etc.) on the sensor sensitivity is analyzed.

Integrated refractive index sensor using silicon slot waveguides

We propose an integrated refractive index (RI) sensor based on evanescent field absorption (EFA) within a silicon slot waveguide, where the RI variation is translated into a varied attenuation coefficient and eventually the output power at the end of the waveguide. To demonstrate the operating principle of such a RI-EFA sensor, a specific structure is designed and discussed with numerical simulations. The calculated results indicate that the detection limit of our proposed RI-EFA sensor could be as good as ∼10^-8  RIU for homogeneous sensing and ∼10^-7  RIU for surface sensing with optimized structural parameters at a wavelength of 1064 nm. Since only a straight slot waveguide and optical power detection are required for our proposed sensor, we believe that it is promising to achieve an integrated and portable sensor on a single chip.