Optimization of bulk sensitivity for strip, slot, and subwavelength grating-based waveguides for dual-polarization operation.

We propose a dual-polarization optimization method for the bulk sensitivity of silicon-on-insulator (SOI) waveguides by defining a multi-objective function that accounts for the substrate leakage losses. The proposed optimization method was used to design micro-ring resonator bulk sensors with strip, slot, subwavelength grating, and subwavelength grating slot waveguides. The subwavelength grating slot waveguide has a bulk sensitivity of 520 nm/RIU and 325 nm/RIU for the TE and TM modes, respectively, both of which are higher than the bulk sensitivities of strip, slot, and subwavelength grating waveguides. Moreover, our Monte Carlo analysis shows that the subwavelength grating slot waveguide has the highest immunity to fabrication errors.

Planar organization of airway epithelial cell morphology using hydrogel grooves during ciliogenesis fails to induce ciliary alignment.

Topographical cues are known to influence cell organization both in native tissues and in vitro. In the trachea, the matrix beneath the epithelial lining is composed of collagen fibres that run along the long axis of the airway. Previous studies have shown that grooved topography can induce morphological and cytoskeletal alignment in epithelial cell lines. In the present work we assessed the impact of substrate topography on the organization of primary human tracheal epithelial cells (HTECs) and human induced pluripotent stem cell (hiPSC)-derived airway progenitors and the resulting alignment of cilia after maturation of the airway cells under Air-Liquid-Interface (ALI) culture. Grooves with optimized dimensions were imprinted into collagen vitrigel membranes (CVM) to produce gel inserts for ALI culture. Grooved CVM substrates induced cell alignment in HTECs and hiPSC airway progenitors in submerged culture. Further, both cell types were able to terminally differentiate into a multi-ciliated epithelium on both flat and groove CVM substrates. When exposed to ALI conditions, HTECs lost alignment after 14 days. Meanwhile, hiPSC-derived airway progenitors maintained their alignment throughout 31 days of ALI culture. Interestingly, neither initial alignment on the grooves, nor maintained alignment on the grooves induced alignment of cilia basal bodies, an indication of the direction of ciliary beating direction in the airway cells. Planar organization of airway cells during or prior to ciliogenesis therefore does not appear to be a feasible strategy to control cilia organization and subsequent airway epithelial function and additional cues are likely necessary to produce cilia alignment.

Enhancing the Sensitivity of Surface Plasmon Resonance Measurements Utilizing Polymer Film/Au Assemblies.

Surface plasmon resonance (SPR) is used to infer information about a sample that is in contact with an Au-coated glass slide coupled to the SPR prism. Shifts in the angle of the "SPR minimum reflection" can be related to changes in the refractive index (and/or thickness) of the sample that is in contact with the Au film, which can then be used to determine the concentration of an analyte in that sample. Here, we show that by depositing a layer of poly(N-isopropylacrylamide-co-acrylic acid) [p(NIPAm-co-AAc)] microgel on the SPR's Au film, with a subsequent layer of Au deposited on top of the microgels, the sensitivity of SPR to changes in solution properties can be enhanced. We investigated the sensitivity of the SPR to changes in the temperature of water in contact with the SPR's Au film as a function of the microgel immobilization density and the thickness of the Au layer deposited on the microgel layer. The data revealed that the SPR's Au film densely coated with microgels, with 5 nm of Au deposited, exhibited the maximal enhancement. The plasmon coupling effect between the additional Au film on the microgels and the SPR's Au film was further confirmed by 3D finite difference time domain simulations.

Bead-based multiplex detection of dengue biomarkers in a portable imaging device.

Dengue is one of the most rapidly spreading mosquito-borne viral diseases in the world. Differential diagnosis is a crucial step for the management of the disease and its epidemiology. Point-of-care testing of blood-borne dengue biomarkers provides an advantageous approach in many health care settings, and the ability to follow more than one biomarker at once could significantly improve the management of the disease. Bead-based multiplex technologies (suspension array) can measure multiple biomarker targets simultaneously by using recognition molecules immobilized on microsphere beads. The overarching objective of our work is to develop a portable detection device for the simultaneous measurement of multiple biomarkers important in dengue diagnosis, monitoring and treatment. Here, we present a bead-based assay for the detection of one of the four serotypes of dengue virus non-structural protein (DENV-NS1) as well as its cognate human IgG. In this system, the fluorescent microspheres containing the classification fluorophore and detection fluorophore are imaged through a microfluidic chip using an infinity-corrected microscope system. Calibration curves were plotted for median fluorescence intensity against known concentrations of DENV-NS1 protein and anti-NS1 human IgG. The limit of quantitation was 7.8 ng/mL and 15.6 ng/mL, respectively. The results of this study demonstrate the feasibility of the multiplex detection of dengue biomarkers and present its analytical performance parameters. The proposed imaging device holds potential for point-of-care testing of biomarkers on a highly portable system, and it may facilitate the diagnosis and prevention of dengue as well as other infectious diseases.

Design of biomimetic substrates for long-term maintenance of alveolar epithelial cells.

There is a need to establish in vitro lung alveolar epithelial culture models to better understand the fundamental biological mechanisms that drive lung diseases. While primary alveolar epithelial cells (AEC) are a useful option to study mature lung biology, they have limited utility in vitro. Cells that survive demonstrate limited proliferative capacity and loss of phenotype over the first 3-5 days in traditional culture conditions. To address this limitation, we generated a novel physiologically relevant cell culture system for enhanced viability and maintenance of phenotype. Here we describe a method utilizing e-beam lithography, reactive ion etching, and replica molding to generate poly-dimethylsiloxane (PDMS) substrates containing hemispherical cavities that mimic the architecture and size of mouse and human alveoli. Primary AECs grown on these cavity-containing substrates form a monolayer that conforms to the substrate enabling precise control over cell sheet architecture. AECs grown in cavity culture conditions remain viable and maintain their phenotype over one week. Specifically, cells grown on substrates consisting of 50 µm diameter cavities remained 96 ± 4% viable and maintained expression of surfactant protein C (SPC), a marker of type 2 AEC over 7 days. While this report focuses on primary lung alveolar epithelial cells, our culture platform is potentially relevant and useful for growing primary cells from other tissues with similar cavity-like architecture and could be further adapted to other biomimetic shapes or contours.

Benefit of birefringence for the direct generation of polarization-entangled photon pairs.

Generating polarization-entangled photon pairs on chip is generally complicated by the birefringence of waveguides. In this work, we propose a technique that uses waveguide birefringence and lends itself to simple device designs. The technique relies on two orthogonal spontaneous four-wave mixing processes. We employ the full quantum optics theory and dispersion analysis, and show that the technique can produce highly entangled states, with concurrence as high as 0.976 and covering the entire C-band.

Polarization-entangled photon pair sources based on spontaneous four wave mixing assisted by polarization mode dispersion.

Photonic-based qubits and integrated photonic circuits have enabled demonstrations of quantum information processing (QIP) that promises to transform the way in which we compute and communicate. To that end, sources of polarization-entangled photon pair states are an important enabling technology. However, such states are difficult to prepare in an integrated photonic circuit. Scalable semiconductor sources typically rely on nonlinear optical effects where polarization mode dispersion (PMD) degrades entanglement. Here, we directly generate polarization-entangled states in an AlGaAs waveguide, aided by the PMD and without any compensation steps. We perform quantum state tomography and report a raw concurrence as high as 0.91 ± 0.01 observed in a 1,100-nm-wide waveguide. The scheme allows direct Bell state generation with an observed maximum fidelity of 0.90 ± 0.01 from another (800-nm-wide) waveguide. Our demonstration paves the way for sources that allow for the implementation of polarization-encoded protocols in large-scale quantum photonic circuits.

Realization of an ultra-compact polarization beam splitter using asymmetric MMI based on silicon nitride / silicon-on-insulator platform.

We have experimentally demonstrated a compact polarization beam splitter (PBS) based on the silicon nitride/silicon-on-insulator platform using the recently proposed augmented-low-index-guiding (ALIG) waveguide structure. The two orthogonal polarizations are split in an asymmetric multimode interference (MMI) section, which was 1.6 µm wide and 4.8 µm long. The device works well over the entire C-band wavelength range and has a measured low insertion loss of less than 1 dB. The polarization extinction ratio at the Bar Port is approximately 17 dB and at the Cross Port is approximately 25 dB. The design of the device is robust and has a good fabrication tolerance.

Kinetic analysis of nanoparticle-protein interactions using a plasmon waveguide resonance.

A plasmon waveguide resonance (PWR) sensor is proposed for studying the interaction between gold nanoparticles and proteins. The ability of the PWR sensor to operate in both TM and TE Polarizations, i.e. its polarization diversity, facilitates the simultaneous spectroscopy of the nanoparticles surface reactions using both polarizations. The response of each polarization to streptavidin-biotin binding at the surface of gold nanoparticles is investigated in real time. Finally, using the principles of multimode spectroscopy, the nanoparticle's surface reactions are decoupled from the bulk solution refractive index variations. Schematic diagram of the NP-modified PWR sensor.

Tuneable four-wave mixing in AlGaAs nanowires.

We have experimentally demonstrated broadband tuneable four-wave mixing in AlGaAs nanowires with the widths ranging between 400 and 650 nm and lengths from 0 to 2 mm. We performed a detailed experimental study of the parameters influencing the FWM performance in these devices (experimental conditions and nanowire dimensions). The maximum signal-to-idler conversion range was 100 nm, limited by the tuning range of the pump source. The maximum conversion efficiency, defined as the ratio of the output idler power to the output signal power, was -38 dB. In support of our explanation of the experimentally observed trends, we present modal analysis and group velocity dispersion numerical analysis. This study is what we believe to be a step forward towards realization of all-optical signal processing devices.

Fiber optic stress-independent helical torsion sensor.

Femtosecond laser-fabricated waveguides have been formed into helical paths throughout the cladding of single-mode optical fibers to demonstrate a strain-independent fiber torsion sensor. A comparison between a Bragg grating sensor and a Mach-Zehnder based on helical waveguides (HWs) showed a much weaker twist sensitivity of 1.5 pm/(rad/m) for the grating in contrast with a value of 261 pm/(rad/m) for the interferometer. The HW geometry provided an unambiguous determination of the rotational direction of the twist while facilitating a convenient and efficient means for optical coupling into the single-mode core of the fiber. The flexible three-dimensional writing by the femtosecond laser fabrication method enabled the direct inscription of compact and robust optical cladding devices without the need for combining or splicing multiple-fiber segments.

Self-referenced spectroscopy using plasmon waveguide resonance biosensor.

A plasmon waveguide resonance (PWR) sensor is designed, fabricated, and tested for self-referenced biosensing. The PWR sensor is able to support two different polarizations, TM and TE. The TM polarization has a large sensitivity to variations in the background refractive index while the TE polarization is more sensitive to the surface properties. The ability of the PWR sensor to simultaneously operate in both TM and TE modes is used to decouple the background index variations (bulk effects) from the changes in adlayer thickness (surface effects) via multimode spectroscopy. To benchmark the performance of the PWR, a conventional surface plasmon resonance (SPR) sensor is fabricated and tested under the same conditions.

Multimode spectroscopy using dielectric grating coupled to a surface plasmon resonance sensor.

A new platform is proposed to solve one of the main shortcomings of surface plasmon resonance biosensors, namely, the cross sensitivity to surface and bulk effects. This approach is based on multimode spectroscopy in which three different modes are excited simultaneously. The proposed design consists of an SPR sensor loaded with a dielectric grating. The design parameters (dimensions and wavelength) are optimized with a genetic algorithm. The optimized design has two resonance modes excited with TM polarized light, each sensitive to surface effects, and one TE mode mostly sensitive to variations in the bulk fluid refractive index. Numerical and analytical methods are used to justify the simulation results, which are in good agreement. Finally, it is shown that, by applying three-mode spectroscopy, decoupling the properties of the attached biomaterial from the background index variations is possible with the proposed design.

Multiwell plate tools for controlling cellular alignment with grooved topography.

In many tissues, cells must be aligned for proper function. This alignment can occur at the cellular and/or subcellular (protein/molecular) level. The alignment of cytoskeletal components, in fact, precedes whole cell alignment. A variety of methods exist to manipulate cytoskeletal and whole cell alignment; one of the simplest and most predictable involves seeding adherent cells onto defined substrate topography. We present here two methods to create grooved multiwell plates: one involving microfabrication, which allows for custom design of substrate topography, and a simpler, inexpensive method using commercially available diffraction gratings. We also include methods for manual and automatic quantification of cell alignment.

Nonautonomous contact guidance signaling during collective cell migration.

Directed migration of groups of cells is a critical aspect of tissue morphogenesis that ensures proper tissue organization and, consequently, function. Cells moving in groups, unlike single cells, must coordinate their migratory behavior to maintain tissue integrity. During directed migration, cells are guided by a combination of mechanical and chemical cues presented by neighboring cells and the surrounding extracellular matrix. One important class of signals that guide cell migration includes topographic cues. Although the contact guidance response of individual cells to topographic cues has been extensively characterized, little is known about the response of groups of cells to topographic cues, the impact of such cues on cell-cell coordination within groups, and the transmission of nonautonomous contact guidance information between neighboring cells. Here, we explore these phenomena by quantifying the migratory response of confluent monolayers of epithelial and fibroblast cells to contact guidance cues provided by grooved topography. We show that, in both sparse clusters and confluent sheets, individual cells are contact-guided by grooves and show more coordinated behavior on grooved versus flat substrates. Furthermore, we demonstrate both in vitro and in silico that the guidance signal provided by a groove can propagate between neighboring cells in a confluent monolayer, and that the distance over which signal propagation occurs is not significantly influenced by the strength of cell-cell junctions but is an emergent property, similar to cellular streaming, triggered by mechanical exclusion interactions within the collective system.

Experimental demonstration of an integrated hybrid plasmonic polarization rotator.

We experimentally demonstrate an ultracompact (3.7 µm long) hybrid plasmonic polarization rotator operating around 1.55 µm for integrated silicon photonics circuits. The TM polarization of a silicon waveguide is rotated to the TE polarization with insertion losses as low as 1.5 dB and polarization extinction ratios larger than 13.5 dB.

An improved refractive index sensor based on genetic optimization of plasmon waveguide resonance.

Plasmon waveguide resonance (PWR) sensors are particularly useful for biosensing due to their unique ability to perform sensing with two different polarizations. In this paper we report a comprehensive performance comparison between the surface plasmon resonance (SPR) sensor and the PWR sensor in terms of the sensitivity and the refractive index resolution. Both sensors were optimized using a genetic algorithm to acquire their best performance for bulk sensing applications. The experimental results show that the PWR sensor has a refractive index resolution of 5 × 10(-7) RIU which is 6 times smaller than that of the optimized SPR sensor. The TE polarization in the PWR sensor has a resolution of 1.4 × 10(-6) RIU which is smaller than the SPR sensor. The polarization diversity in the PWR sensor is another advantage which can be used to improve the measurement reliability.

Pd-based integrated optical hydrogen sensor on a silicon-on-insulator platform.

We have experimentally demonstrated a compact, integrated optical hydrogen sensor on a silicon-on-insulator platform. The sensor consists of silicon waveguide coated with a thin palladium film. The sensitivity and response time of the sensor was tested for volumetric hydrogen concentrations ranging from 0% to 4%. The proposed hydrogen sensor shows great potential as a building block for an optical nose capable of simultaneous detection of multiple gases as well as environmental effects such as temperature and humidity.

Stress induced birefringence tuning in femtosecond laser fabricated waveguides in fused silica.

Femtosecond laser exposure produces form and stress birefringence in glasses, mainly controlled by laser polarization and pulse energy, which leads to challenges in certain applications where polarization mode dispersion or birefringence splitting is critical for the desired responses from optical devices. In this paper, parallel laser modification tracks with different geometries were applied to preferentially stress the laser-written waveguides and explore the possibility of tuning the waveguide birefringence in devices fabricated in bulk fused silica glass. Polarization splitting in Bragg grating waveguides showed the laser modification tracks to controllably add or subtract stress to the pre-existing waveguide birefringence, demonstrating independence from the nanograting induced form birefringence and the contributions from material stress. Stressing bars are shown that offer tunable birefringence in the range from ~0 up to 4.35 × 10(-4), possibly enabling great flexibility in designing polarization dependent devices, as well as making polarization independent devices.

Generalized exact dynamic localization in curved coupled optical waveguide arrays.

We observed exact dynamic localization in its general case in strongly coupled curved waveguide arrays with periodic, nonsquare-wave curvatures. We achieved a six times change in the dynamic localization bandwidth by changing a design parameter in our deviated-square-wave curvature. By analyzing the relation between the bandwidth and curvature profile, we offer a general method to control the localization bandwidth.