Mid-Infrared Serial Microring Resonator Array for Real-Time Detection of Vapor-Phase Volatile Organic Compounds.

Chip-scale infrared spectrometers consisting of a microring resonator array (MRA) were developed for volatile organic compound (VOC) detection. The MRA is serially positioned to serve as a wavelength sorting element that enables wavelength demultiplexing. Unlike conventional devices operated by a single microring, our MRA can perform multiwavelength mid-infrared (mid-IR) sensing by routing the resonant wavelength light from a broadband mid-IR source into different sensing channels. Miniaturized spectrometer devices were fabricated on mid-IR transparent silicon-rich silicon nitride (SiNx) thin films through complementary metal-oxide-semiconductor (CMOS) processes, thus enabling wafer-level manufacturing and packaging. The spectral distribution of the resonance lines and the optimization of the microring structures were designed using finite-difference time-domain (FDTD) modeling and then verified by laser spectrum scanning. Using small microring structures, the spectrum showed a large free spectral range (FSR) of 100 nm and held four spectral channels without crosstalk. Unlike near-infrared microrings using refractive index sensing, our MRA can detect hexane and ethanol vapor pulses by monitoring the intensity variation at their characteristic mid-IR absorption bands, thus providing high specificity. Applying multiwavelength detection, the sensor module can discriminate among various VOC vapors. Hence, our mid-IR MRA could be an essential component to achieve a compact spectroscopic sensing module that has the potential for applications such as remote environmental monitoring and portable health care devices.

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.

Efficient vortex beam generation using gradient refractive-index microphase plates.

Vortex beams were theoretically demonstrated by patterning a fiber facet with $N$-segment microphase plates. By changing the aluminum oxynitride material composition of each segment, gradient refractive-index phase plates (GRPs) were designed and introduced a ${{2}}\pi l$ azimuthal optical phase difference. The gradient index profile was able to convert a fiber Gaussian mode to a Laguerre-Gaussian mode with varieties of topological charge $l$. A three-dimensional finite-difference time-domain method was applied to calculate the near-field optical phase maps and the far-field beam profiles projected from the micro-GRPs. A uniform vortex beam with a symmetrical doughnut shape was obtained by optimizing the GRPs' radii and the number of segments. The micro-GRPs enabled flat optical components for efficient vortex beam generation.

Surface Functionalized Anodic Aluminum Oxide Membrane for Opto-Nanofluidic SARS-CoV-2 Genomic Target Detection.

An ultra-thin and highly sensitive SARS-CoV-2 detection platform was demonstrated using a nano-porous anodic aluminum oxide (AAO) membrane. The membrane surface was functionalized to enable efficient trapping and identification of SARS-CoV-2 genomic targets through DNA-DNA and DNA-RNA hybridization. To immobilize the probe oligonucleotides on the AAO membrane, the pore surface was first coated with the linking reagents, 3-aminopropyltrimethoxysilane (APTMS) and glutaraldehyde (GA), by a compact vacuum infiltration module. After that, complementary target oligos with fluorescent modifier was pulled and infiltrated into the nano-fluidic channels formed by the AAO pores. The fluorescent signal applying the AAO membrane sensors was two orders stronger than a flat glass template. In addition, the dependence between the nano-pore size and the fluorescent intensity was evaluated. The optimized pore diameter d is 200 nm, which can accommodate the assembled oligonucleotide and aminosilane layers without blocking the AAO nano-fluidic channels. Our DNA functionalized membrane sensor is an accurate and high throughput platform supporting rapid virus tests, which is critical for population-wide diagnostic applications result in a page being rejected by search engines.

Benzene Derivatives Analysis Using Aluminum Nitride Waveguide Raman Sensors.

Raman spectroscopy using aluminum nitride (AlN) optical waveguides was demonstrated for organic compound analysis. The AlN waveguide device was prepared by reactive sputtering deposition and complementary-metal-oxide semiconductor (CMOS) processes. A fundamental waveguide mode was observed over a broad visible spectrum and the waveguide evanescent wave was used to excite the Raman signals of the test analytes. The performance of the waveguide sensor was characterized by measuring the Raman spectra of the benzene derivative mixtures consisting of benzene, anisole, and toluene. The compositions and concentrations were resolved by correlating the obtained Raman spectrum with the characteristic Raman peaks associated with C-C, C-H, and C-O functional groups. With the advantages of real-time detection and enhanced Raman signal intensity, the AlN waveguides provided a sensor platform for nondestructive and online chemical compound monitoring.

Mid-infrared frequency doubling using strip-loaded silicon nitride on epitaxial barium titanate thin film waveguides.

Broadband mid-infrared (mid-IR) frequency doubling was demonstrated using nonlinear barium titanate (BTO) thin films. The device has a strip-loaded waveguide structure consisting of top silicon nitride (SiN) strips and an underneath BTO guiding layer. The epitaxial BTO was deposited on a strontium titanate (STO) substrate by pulsed-laser deposition. Through a SiN grating coupler, the pumping mid-IR light at wavelength λ=3.30-3.45µm was coupled into the nonlinear BTO layer, where the spectrum of the near-infrared (NIR) second-harmonic generation was characterized. The developed BTO waveguides provide a platform for mid-IR nonlinear integrated photonics and on-chip quantum optics.

Real-time isotopic methane detection using mid-infrared spectroscopy.

A real-time, nondestructive mid-infrared (mid-IR) platform was proposed for isotopic methane detection. The measurement system consisted of a tunable mid-IR laser, a miniaturized gas chamber, and a mid-IR signal receiver. The isotope ratio of the 12CH4/13CH4 was identified by measuring the mid-IR spectrum at λ=3.2-3.5µm.In-situ12CH4/13CH4 monitoring was then achieved by tracing the characteristic mid-IR absorption peaks assigned to the 12CH4 at λ=3.328µm and 13CH4 at λ=3.340µm. The real-time methane isotope analysis can be applied to environmental monitoring and petroleum industries.

Mid-Infrared Chalcogenide Waveguides for Real-Time and Nondestructive Volatile Organic Compound Detection.

A mid-infrared (mid-IR) sensor chip was demonstrated for volatile organic compound (VOC) detection. The sensor consisted of As2Se3 optical waveguides built by microelectronic fabrication processes. The VOC sensing performance was characterized by measuring acetone and ethanol vapors at their characteristic C-H absorption from λ = 3.40 to 3.50 µm. Continuous VOC detection with <5 s response time was achieved by measuring the intensity attenuation of the waveguide mode. The miniaturized noninvasive VOC sensor can be applied to breath analysis and environmental toxin monitoring.

Real-time and non-destructive gas mixture analysis using linear various filter enabled mid-infrared visualization.

Real-time gas mixture analysis has been demonstrated using various linear variable filter (LVF)-enabled mid-infrared (mid-IR) visualizations. Due to the characteristic absorptions of different gases, the algorithm-enabled sensing method has the ability to detect multi-component gas mixtures noninvasively. The proposed system consisted of a broadband light source, a gas mixing and delivery chamber made by polydimethylsiloxane (PDMS), a LVF, and a real-time monitoring mid-IR camera. The system performance was evaluated by detecting CH4 and C2H2 at their characteristic C-H absorptions from λ = 3.0 to 3.5 µm. A fast and accurate identification of gas samples was achieved. Therefore, our real-time and non-destructive gas analysis system enables a new visualization technology for environmental monitoring and industrial measurement.

Real-Time Gas Mixture Analysis Using Mid-Infrared Membrane Microcavities.

Real-time gas analysis on-a-chip was demonstrated using a mid-infrared (mid-IR) microcavity. Optical apertures for the microcavity were made of ultrathin silicate membranes embedded in a silicon chip using the complementary metal-oxide-semiconductor (CMOS) process. Fourier transform infrared spectroscopy (FTIR) shows that the silicate membrane is transparent in the range of 2.5-6.0 µm, a region that overlaps with multiple characteristic gas absorption lines and therefore enables gas detection applications. A test station integrating a mid-IR tunable laser, a microgas delivery system, and a mid-IR camera was assembled to evaluate the gas detection performance. CH4, CO2, and N2O were selected as analytes due to their strong absorption bands at λ = 3.25-3.50, 4.20-4.35, and 4.40-4.65 µm, which correspond to C-H, C-O, and O-N stretching, respectively. A short subsecond response time and high gas identification accuracy were achieved. Therefore, our chip-scale mid-IR sensor provides a new platform for an in situ, remote, and embedded gas monitoring system.

Mid-infrared spectrometer using opto-nanofluidic slot-waveguide for label-free on-chip chemical sensing.

A mid-infrared (mid-IR) spectrometer for label-free on-chip chemical sensing was developed using an engineered nanofluidic channel consisting of a Si-liquid-Si slot-structure. Utilizing the large refractive index contrast (Δn ∼ 2) between the liquid core of the waveguide and the Si cladding, a broadband mid-IR lightwave can be efficiently guided and confined within a nanofluidic capillary (≤100 nm wide). The optical-field enhancement, together with the direct interaction between the probe light and the analyte, increased the sensitivity for chemical detection by 50 times when compared to evanescent-wave sensing. This spectrometer distinguished several common organic liquids (e.g., n-bromohexane, toluene, isopropanol) accurately and could determine the ratio of chemical species (e.g., acetonitrile and ethanol) at low concentration (<5 µL/mL) in a mixture through spectral scanning over their characteristic absorption peaks in the mid-IR regime. The combination of CMOS-compatible planar mid-IR microphotonics, and a high-throughput nanofluidic sensor system, provides a unique platform for chemical detection.

Mid-infrared materials and devices on a Si platform for optical sensing.

In this article, we review our recent work on mid-infrared (mid-IR) photonic materials and devices fabricated on silicon for on-chip sensing applications. Pedestal waveguides based on silicon are demonstrated as broadband mid-IR sensors. Our low-loss mid-IR directional couplers demonstrated in SiN x waveguides are useful in differential sensing applications. Photonic crystal cavities and microdisk resonators based on chalcogenide glasses for high sensitivity are also demonstrated as effective mid-IR sensors. Polymer-based functionalization layers, to enhance the sensitivity and selectivity of our sensor devices, are also presented. We discuss the design of mid-IR chalcogenide waveguides integrated with polycrystalline PbTe detectors on a monolithic silicon platform for optical sensing, wherein the use of a low-index spacer layer enables the evanescent coupling of mid-IR light from the waveguides to the detector. Finally, we show the successful fabrication processing of our first prototype mid-IR waveguide-integrated detectors.

Air-clad silicon pedestal structures for broadband mid-infrared microphotonics.

Toward mid-infrared (mid-IR) silicon microphotonic circuits, we demonstrate broadband on-chip silicon structures, such as: (i) straight and bent waveguides and (ii) beam splitters, utilizing an air-clad pedestal configuration which eliminates the need for typical mid-IR-lossy oxide cladding. We illustrate a sophisticated fabrication process that can create high-quality pedestal structures in crystalline silicon, while preserving its mid-IR transparency. A fundamental waveguide mode is observed between λ=2.5 µm and λ=3.7 µm, and an optical loss of 2.7 dB/cm is obtained at λ=3.7 µm. Our pedestal silicon structures show 50:50 mid-IR power splitting enabling the further development of mid-IR silicon microphotonics.

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.

Fabrication, characterization and optical properties of Au-decorated Bi2Se3 nanoplatelets.

Au-decorated Bi2Se3 nanoplatelet heterostructures are fabricated by a two-step process of thermal CVD at 600 °C and magnetron sputtering at room-temperature. The crystal structures and binding energies of rhombohedral Bi2Se3 and FCC Au are determined by XRD, HRTEM, XPS, and Raman spectroscopy. XPS and Raman spectroscopy reveal the interaction between Au and Bi2Se3 by shifting in the binding energies of Au-Au, Au-Se and Bi-Se bonds and the wavenumber of A1g2 and Eg2 modes. Au-decorated Bi2Se3 nanoplatelet heterostructures are observed using FESEM, and confirmed by XPS, Raman spectroscopy, and HRTEM imaging. Their optical band gap of the Au-decorated Bi2Se3 nanoplatelet heterostructures increases with Au thickness about 1.92-fold as much as that of pristine Bi2Se3 (0.39 eV), owing to the Burstein-Moss effect. The optical absorptance of the Au-decorated Bi2Se3 nanoplatelet heterostructures revealed increment with wavelength from 200 to 500 nm and decrement with increasing wavelength from 500 to 800 nm.

Detection of volatile organic compounds using mid-infrared silicon nitride waveguide sensors.

Mid-infrared (mid-IR) sensors consisting of silicon nitride (SiN) waveguides were designed and tested to detect volatile organic compounds (VOCs). SiN thin films, prepared by low-pressure chemical vapor deposition (LPCVD), have a broad mid-IR transparent region and a lower refractive index (nSiN = 2.0) than conventional materials such as Si (nSi = 3.4), which leads to a stronger evanescent wave and therefore higher sensitivity, as confirmed by a finite-difference eigenmode (FDE) calculation. Further, in-situ monitoring of three VOCs (acetone, ethanol, and isoprene) was experimentally demonstrated through characteristic absorption measurements at wavelengths λ = 3.0-3.6 µm. The SiN waveguide showed a five-fold sensitivity improvement over the Si waveguide due to its stronger evanescent field. To our knowledge, this is the first time SiN waveguides are used to perform on-chip mid-IR spectral measurements for VOC detection. Thus, the developed waveguide sensor has the potential to be used as a compact device module capable of monitoring multiple gaseous analytes for health, agricultural and environmental applications.

Enhanced Photocurrent of the Ag Interfaced Topological Insulator Bi2Se3 under UV- and Visible-Light Radiations.

Bi2Se3 is a topological quantum material that is used in photodetectors, owing to its narrow bandgap, conductive surface, and insulating bulk. In this work, Ag@Bi2Se3 nanoplatelets were synthesized on Al2O3(100) substrates in a two-step process of thermal evaporation and magnetron sputtering. X-ray diffractometer (XRD), high-resolution transmission electron microscopy (HRTEM), Raman spectroscopy, and x-ray photoelectron spectroscopy (XPS) revealed that all samples had the typical rhombohedral Bi2Se3. Field-emission scanning electron microscopy (FESEM)-energy dispersive x-ray spectroscopy (EDS), XPS, and HRTEM confirmed the presence of the precipitated Ag. The optical absorptance of Bi2Se3 nanoplatelets in UV-visible range decreased with the Ag contents. Results of photocurrent measurements under zero-bias conditions revealed that the deposited Ag affected photosensitivity. A total of 7.1 at.% Ag was associated with approximately 4.25 and 4.57 times higher photocurrents under UV and visible light, respectively, than 0 at.% Ag. The photocurrent in Bi2Se3 at 7.1 at.% Ag under visible light was 1.72-folds of that under UV light. This enhanced photocurrent is attributable to the narrow bandgap (~0.35 eV) of Bi2Se3 nanoplatelets, the Schottky field at the interface between Ag and Bi2Se3, the surface plasmon resonance that is caused by Ag, and the highly conductive surface that is formed from Ag and Bi2Se3. This work suggests that the appropriate Ag deposition enhances the photocurrent in, and increases the photosensitivity of, Bi2Se3 nanoplatelets under UV and visible light.

Real-time and non-destructive hydrocarbon gas sensing using mid-infrared integrated photonic circuits.

A chip-scale mid-infrared (mid-IR) sensor was developed for hydrocarbon gas detection. The sensor consisted of amorphous Si (a-Si) optical ridge waveguides that were fabricated by complementary metal-oxide-semiconductor (CMOS) processes. The waveguide exhibited a sharp fundamental mode through λ = 2.70 to 3.50 µm. Its sensing performance was characterized by measuring methane and acetylene. From the spectral mode attenuation, the characteristic C-H absorption bands associated with methane and acetylene were found at λ = 3.29-3.33 µm and λ = 3.00-3.06 µm, respectively. In addition, real-time methane and acetylene concentration monitoring was demonstrated at λ = 3.02 and 3.32 µm. Hence, the mid-IR waveguide sensor enabled an accurate and instantaneous analysis of hydrocarbon gas mixtures.

All-nanoparticle layer-by-layer coatings for Mid-IR on-chip gas sensing.

Functionalization of optical waveguides with submicron coatings of zinc peroxide (ZnO2) and silica (SiO2) nanoparticles (NPs) is reported that enabled selective concentration of acetone vapors in the vicinity of the waveguide, boosting the sensitivity of a mid infrared (MIR) on-chip detector. Controlled thickness was achieved by introducing precise control of the substrate withdrawal speed to the layer-by-layer (LbL) deposition technique.

Room-Temperature Ferroelectric LiNb6Ba5Ti4O30 Spinel Phase in a Nanocomposite Thin Film Form for Nonlinear Photonics.

Tetragonal tungsten bronze (TTB) materials are one of the most promising classes of materials for ferroelectric and nonlinear optical devices, owing to their very unique noncentrosymmetric crystal structure. In this work, a new TTB phase of LiNb6Ba5Ti4O30 (LNBTO) has been discovered and studied. A small amount of a secondary phase, LiTiO2 (LTO), has been incorporated as nanopillars that are vertically embedded in the LNBTO matrix. The new multifunctional nanocomposite thin film presents exotic highly anisotropic microstructure and properties, e.g., strong ferroelectricity, high optical transparency, anisotropic dielectric function, and strong optical nonlinearity evidenced by the second harmonic generation results. An optical waveguide structure based on the stacks of α-Si on SiO2/LNBTO-LTO has been fabricated, exhibiting low optical dispersion with an optimized evanescent field staying in the LNBTO-LTO active layer. This work highlights the combination of new TTB material designs and vertically aligned nanocomposite structures for further enhanced anisotropic and nonlinear properties.