Lab-on-a-chip optical biosensor platform: a micro-ring resonator integrated with a near-infrared Fourier transform spectrometer.

In this paper, we demonstrated the design and experimental results of the near-infrared lab-on-a-chip optical biosensor platform that monolithically integrates the MRR and the on-chip spectrometer on the silicon-on-insulator (SOI) wafer, which can eliminate the external optical spectrum analyzer for scanning the wavelength spectrum. The symmetric add-drop MRR biosensor is designed to have a free spectral range (FSR) of ∼19 nm and a bulk sensitivity of ∼73 nm/RIU; then the drop-port output resonance peaks are reconstructed from the integrated spatial-heterodyne Fourier transform spectrometer (SHFTS) with the spectral resolution of ∼3.1 nm and the bandwidth of ∼50 nm, which results in the limit of detection of 0.042 RIU.

A point-of-care biosensor for rapid detection and differentiation of COVID-19 virus (SARS-CoV-2) and influenza virus using subwavelength grating micro-ring resonator.

In the context of continued spread of coronavirus disease 2019 (COVID-19) caused by SARS-CoV-2 and the emergence of new variants, the demand for rapid, accurate, and frequent detection is increasing. Moreover, the new predominant strain, Omicron variant, manifests more similar clinical features to those of other common respiratory infections. The concurrent detection of multiple potential pathogens helps distinguish SARS-CoV-2 infection from other diseases with overlapping symptoms, which is significant for providing tailored treatment to patients and containing the outbreak. Here, we report a lab-on-a-chip biosensing platform for SARS-CoV-2 detection based on the subwavelength grating micro-ring resonator. The sensing surface is functionalized by specific antibody against SARS-CoV-2 spike protein, which could produce redshifts of resonant peaks by antigen-antibody combination, thus achieving quantitative detection. Additionally, the sensor chip is integrated with a microfluidic chip featuring an anti-backflow Y-shaped structure that enables the concurrent detection of two analytes. In this study, we realized the detection and differentiation of COVID-19 and influenza A H1N1. Experimental results indicate that the limit of detection of our device reaches 100 fg/ml (1.31 fM) within 15 min detecting time, and cross-reactivity tests manifest the specificity of the optical diagnostic assay. Furthermore, the integrated packaging and streamlined workflow facilitate its use for clinical applications. Thus, the biosensing platform presents a promising approach for attaining highly sensitive, selective, multiplexed, and quantitative point-of-care diagnosis and distinction between COVID-19 and influenza.

Dual-Polarization Bandwidth-Bridged Bandpass Sampling Fourier Transform Spectrometer from Visible to Near-Infrared on a Silicon Nitride Platform.

On-chip broadband optical spectrometers that cover the entire tissue transparency window (λ = 650-1050 nm) with high resolution are highly demanded for miniaturized biosensing and bioimaging applications. The standard spatial heterodyne Fourier transform spectrometer (SHFTS) requires a large number of Mach-Zehnder interferometer (MZI) arrays to obtain a broad spectral bandwidth while maintaining high resolution. Here, we propose a novel type of SHFTS integrated with a subwavelength grating coupler (SWGC) for the dual-polarization bandpass sampling on the Si3N4 platform to solve the intrinsic trade-off limitation between the bandwidth and resolution of the SHFTS without having an outrageous number of MZI arrays or adding additional active photonic components. By applying the bandpass sampling theorem, the continuous broadband input spectrum is divided into multiple narrow-band channels through tuning the phase-matching condition of the SWGC with different polarization and coupling angles. Thereby, it is able to reconstruct each band separately far beyond the Nyquist criterion without aliasing error or degrading the resolution. We experimentally demonstrated the broadband spectrum retrieval results with the overall bandwidth coverage of 400 nm, bridging the wavelengths from 650 to 1050 nm, with a resolution of 2-5 nm. The bandpass sampling SHFTS is designed to have 32 linearly unbalanced MZIs with the maximum optical path length difference of 93 µm within an overall footprint size of 4.7 mm × 0.65 mm, and the coupling angles of SWGC are varied from 0° to 32° to cover the entire tissue transparency window.

Fast, accurate, point-of-care COVID-19 pandemic diagnosis enabled through advanced lab-on-chip optical biosensors: Opportunities and challenges.

The sudden rise of the worldwide severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) pandemic in early 2020 has called into drastic action measures to perform instant detection and reduce the rate of spread. Common clinical and nonclinical diagnostic testing methods have been partially effective in satisfying the increasing demand for fast detection point-of-care (POC) methods to slow down further spread. However, accurate point-of-risk diagnosis of this emerging viral infection is paramount as the need for simultaneous standard operating procedures and symptom management of SARS-CoV-2 will be the norm for years to come. A sensitive, cost-effective biosensor with mass production capability is crucial until a universal vaccination becomes available. Optical biosensors can provide a noninvasive, extremely sensitive rapid detection platform with sensitivity down to ∼67 fg/ml (1 fM) concentration in a few minutes. These biosensors can be manufactured on a mass scale (millions) to detect the COVID-19 viral load in nasal, saliva, urine, and serological samples, even if the infected person is asymptotic. Methods investigated here are the most advanced available platforms for biosensing optical devices that have resulted from the integration of state-of-the-art designs and materials. These approaches include, but are not limited to, integrated optical devices, plasmonic resonance, and emerging nanomaterial biosensors. The lab-on-chip platforms examined here are suitable not only for SARS-CoV-2 spike protein detection but also for other contagious virions such as influenza and Middle East respiratory syndrome (MERS).

Heterogeneously integrated ITO plasmonic Mach-Zehnder interferometric modulator on SOI.

Densely integrated active photonics is key for next generation on-chip networks for addressing both footprint and energy budget concerns. However, the weak light-matter interaction in traditional active Silicon optoelectronics mandates rather sizable device lengths. The ideal active material choice should avail high index modulation while being easily integrated into Silicon photonics platforms. Indium tin oxide (ITO) offers such functionalities and has shown promising modulation capacity recently. Interestingly, the nanometer-thin unity-strong index modulation of ITO synergistically combines the high group-index in hybrid plasmonic with nanoscale optical modes. Following this design paradigm, here, we demonstrate a spectrally broadband, GHz-fast Mach-Zehnder interferometric modulator, exhibiting a high efficiency signified by a miniscule VπL of 95 V µm, deploying a one-micrometer compact electrostatically tunable plasmonic phase-shifter, based on heterogeneously integrated ITO thin films into silicon photonics. Furthermore we show, that this device paradigm enables spectrally broadband operation across the entire telecommunication near infrared C-band. Such sub-wavelength short efficient and fast modulators monolithically integrated into Silicon platform open up new possibilities for high-density photonic circuitry, which is critical for high interconnect density of photonic neural networks or applications in GHz-fast optical phased-arrays, for example.

Portable Automatic Microring Resonator System Using a Subwavelength Grating Metamaterial Waveguide for High-Sensitivity Real-Time Optical-Biosensing Applications.

The slow light sensor techniques have been applied to bio-related detection in the past decades. However, similar testing-systems are too large to carry to a remote area for diagnosis or point-of-care testing. This study demonstrated a fully automatic portable biosensing system based on the microring resonator. An optical-fiber array mounted on a controller based micro-positioning system, which can be interfaced with MATLAB to locate a tentative position for light source and waveguide coupling alignment. Chip adapter and microfluidic channel could be packaged as a product such that it is cheap to be manufactured and can be disposed of after every test conducted. Thus, the platform can be more easily operated via an ordinary user without expertise in photonics. It is designed based on conventional optical communication wavelength range. The C-band superluminescent-light-emitting-diode light source couples in/out the microring sensor to obtain quasi-TE mode by grating coupler techniques. For keeping a stable chemical binding reaction, the cost-effective microfluidic pump was developed to offer a specific flow rate of 20 µL/min by using a servo-motor, an Arduino board, and a motor driver. The subwavelength grating metamaterial ring resonator shows highly sensitive sensing performance via surface index changes due to biomarker adhered on the sensor. The real-time peak-shift monitoring shows 10 µg/mL streptavidin detection of limit based on the biotin-streptavidin binding reaction. Through the different specific receptors immobilized on the sensor surface, the system can be utilized on the open applications such as heavy metal detection, gas sensing, virus examination, and cancer marker diagnosis.

Inkjet-Printed Graphene-Based 1 × 2 Phased Array Antenna.

Low-cost and conformal phased array antennas (PAAs) on flexible substrates are of particular interest in many applications. The major deterrents to developing flexible PAA systems are the difficulty in integrating antenna and electronics circuits on the flexible surface, as well as the bendability and oxidation rate of radiating elements and electronics circuits. In this research, graphene ink was developed from graphene flakes and used to inkjet print the radiating element and the active channel of field effect transistors (FETs). Bending and oxidation tests were carried out to validate the application of printed flexible graphene thin films in flexible electronics. An inkjet-printed graphene-based 1 × 2 element phased array antenna was designed and fabricated. Graphene-based field effect transistors were used as switches in the true-time delay line of the phased array antenna. The graphene phased array antenna was 100% inkjet printed on top of a 5 mil flexible Kapton® substrate, at room temperature. Four possible azimuth steering angles were designed for -26.7°, 0°, 13°, and 42.4°. Measured far-field patterns show good agreement with simulation results.

Light Trapping Induced High Short-Circuit Current Density in III-Nitride Nanorods/Si (111) Heterojunction Solar Cells.

An effective-area photovoltaic efficiency of 1.27% in power conversion, excluding the grid metal contact area and under 1 sun, AM 1.5G conditions, has been obtained for the p-GaN/i-InGaN/n-GaN diode arrays epitaxially grown on (111)-Si. The short-circuit current density is 14.96 mA/cm2 and the open-circuit voltage is 0.28 V. Enhanced light trapping acquired via multiple reflections within the strain and defect free III-nitride nanorod array structures and the short-wavelength responses boosted by the wide bandgap III-nitride constituents are believed to contribute to the observed enhancements in device performance.

Electronic-photonic arithmetic logic unit for high-speed computing.

The past two decades have witnessed the stagnation of the clock speed of microprocessors followed by the recent faltering of Moore's law as nanofabrication technology approaches its unavoidable physical limit. Vigorous efforts from various research areas have been made to develop power-efficient and ultrafast computing machines in this post-Moore's law era. With its unique capacity to integrate complex electro-optic circuits on a single chip, integrated photonics has revolutionized the interconnects and has shown its striking potential in optical computing. Here, we propose an electronic-photonic computing architecture for a wavelength division multiplexing-based electronic-photonic arithmetic logic unit, which disentangles the exponential relationship between power and clock rate, leading to an enhancement in computation speed and power efficiency as compared to the state-of-the-art transistors-based circuits. We experimentally demonstrate its practicality by implementing a 4-bit arithmetic logic unit consisting of 8 high-speed microdisk modulators and operating at 20 GHz. This approach paves the way to future power-saving and high-speed electronic-photonic computing circuits.

Ultra Sensitivity Silicon-Based Photonic Crystal Microcavity Biosensors for Plasma Protein Detection in Patients with Pancreatic Cancer.

Defect-engineered photonic crystal (PC) microcavities were fabricated by UV photolithography and their corresponding sensitivities to biomarkers in patient plasma samples were compared for different resonant microcavity characteristics of quality factor Q and biomarker fill fraction. Three different biomarkers in plasma from pancreatic cancer patients were experimentally detected by conventional L13 defect-engineered microcavities without nanoholes and higher sensitivity L13 PC microcavities with nanoholes. 8.8 femto-molar (0.334 pg/mL) concentration of pancreatic cancer biomarker in patient plasma samples was experimentally detected which are 50 times dilution than ELISA in a PC microcavity with high quality factor and high analyte fill fraction.

InGaAs Membrane Waveguide: A Promising Platform for Monolithic Integrated Mid-Infrared Optical Gas Sensor.

Mid-infrared (mid-IR) absorption spectroscopy based on integrated photonic circuits has shown great promise in trace-gas sensing applications in which the mid-IR radiation directly interacts with the targeted analyte. In this paper, considering monolithic integrated circuits with quantum cascade lasers (QCLs) and quantum cascade detectors (QCDs), the InGaAs-InP platform is chosen to fabricate passive waveguide gas sensing devices. Fully suspended InGaAs waveguide devices with holey photonic crystal waveguides (HPCWs) and subwavelength grating cladding waveguides (SWWs) are designed and fabricated for mid-infrared sensing at λ = 6.15 µm in the low-index contrast InGaAs-InP platform. We experimentally detect 5 ppm ammonia with a 1 mm long suspended HPCW and separately with a 3 mm long suspended SWW, with propagation losses of 39.1 and 4.1 dB/cm, respectively. Furthermore, based on the Beer-Lambert infrared absorption law and the experimental results of discrete components, we estimated the minimum detectable gas concentration of 84 ppb from a QCL/QCD integrated SWW sensor. To the best of our knowledge, this is the first demonstration of suspended InGaAs membrane waveguides in the InGaAs-InP platform at such a long wavelength with gas sensing results. Also, this result emphasizes the advantage of SWWs to reduce the total transmission loss and the size of the fully integrated device's footprint by virtue of its low propagation loss and TM mode compatibility in comparison to HPCWs. This study enables the possibility of monolithic integration of quantum cascade devices with TM polarized characteristics and passive waveguide sensing devices for on-chip mid-IR absorption spectroscopy.

Pedestal subwavelength grating metamaterial waveguide ring resonator for ultra-sensitive label-free biosensing.

Mode volume overlap factor is one of the parameters determining the sensitivity of a sensor. In past decades, many approaches have been proposed to increase the mode volume overlap. As the increased mode volume overlap factor results in reduced mode confinement, the maximum value is ultimately determined by the micro- and nano-structure of the refractive index distribution of the sensing devices. Due to the asymmetric index profile along the vertical direction on silicon-on-insulator platform, further increasing the sensitivity of subwavelength grating metamaterial (SGM) waveguide based sensors is challenging. In this paper, we propose and demonstrate pedestaled SGM which reduces the asymmetricity and thus allows further increasing the interaction between optical field and analytes. The pedestal structure can be readily formed by a controlled undercut etching. Both theoretical analysis and experimental demonstration show a significant improvement of sensitivity. The bulk sensitivity and surface sensitivity are improved by 28.8% and 1000 times, respectively. The detection of streptavidin at a low concentration of 0.1 ng/mL (∼1.67 pM) is also demonstrated through real-time monitoring of the resonance shift. A ∼400 fM streptavidin limit of detection is expected with a 0.01nm resolution spectrum analyzer based on the real-time measurement of streptavidin detection results from two-site binding model fitting.

Automated logic synthesis for electro-optic logic-based integrated optical computing.

Integrated optical computing attracts increasing interest recently as Moore's law approaches the physical limitation. Among all the approaches of integrated optical computing, directed logic that takes the full advantage of integrated photonics and electronics has received lots of investigation since its first introduction in 2007. Meanwhile, as integrated photonics matures, it has become critical to develop automated methods for synthesizing optical devices for large-scale optical designs. In this paper, we propose a general electro-optic (EO) logic in a higher level to explore its potential in integrated computing. Compared to the directed logic, the EO logic leads to a briefer design with shorter optical paths and fewer components. Then a comprehensive gate library based on EO logic is summarized. At last, an And-Inverter Graphs (AIGs) based automated logic synthesis algorithm is described as an example to implement the EO logic, which offers an instruction for the design automation of high-speed integrated optical computing circuits.

Silicon microdisk-based full adders for optical computing.

Due to the projected saturation of Moore's law, as well as the drastically increasing trend of bandwidth with lower power consumption, silicon photonics has emerged as one of the most promising alternatives that has attracted a lasting interest due to the accessibility and maturity of ultra-compact passive and active integrated photonic components. In this Letter, we demonstrate a ripple-carry electro-optic 2-bit full adder using microdisks, which replaces the core part of an electrical full adder by optical counterparts and uses light to carry signals from one bit to the next with high bandwidth and low power consumption per bit. All control signals of the operands are applied simultaneously within each clock cycle. Thus, the severe latency issue that accumulates as the size of the full adder increases can be circumvented, allowing for an improvement in computing speed and a reduction in power consumption. This approach paves the way for future high-speed optical computing systems in the post-Moore's law era.

Ultracompact Silicon-Conductive Oxide Nanocavity Modulator with 0.02 Lambda-Cubic Active Volume.

Silicon photonic modulators rely on the plasma dispersion effect by free-carrier injection or depletion, which can only induce moderate refractive index perturbation. Therefore, the size and energy efficiency of silicon photonic modulators are ultimately limited as they are also subject to the diffraction limit. Here we report an ultracompact electro-optic modulator with total device footprint of 0.6 × 8 µm2 by integrating voltage-switched transparent conductive oxide with one-dimensional silicon photonic crystal nanocavity. The active modulation volume is only 0.06 um3, which is less than 2% of the lambda-cubic volume. The device operates in the dual mode of cavity resonance and optical absorption by exploiting the refractive index modulation from both the conductive oxide and the silicon waveguide induced by the applied gate voltage. Such a metal-free, hybrid silicon-conductive oxide nanocavity modulator also demonstrates only 0.5 dB extra optical loss, moderate Q-factor above 1000, and high energy efficiency of 46 fJ/bit. The combined results achieved through the holistic design opened a new route for the development of next generation electro-optic modulators that can be used for future on-chip optical interconnects.

Improved TKM-TR methods for PAPR reduction of DCO-OFDM visible light communications.

The dc-biased optical orthogonal frequency division multiplexing (DCO-OFDM) system is experimentally demonstrated as an appealing candidate in future visible light communication (VLC) system. However, the intrinsic high PAPR drawback that the DCO-OFDM system suffers from still needs to be addressed and few effective approach has been found so far. To deal with this problem, in this paper, the tone reservation (TR) technique based the time domain kernel matrix (TKM-TR) schemes for reducing the PAPR are studied and applied to DCO-OFDM system. Aiming at the drawback of its severe tailing in previous TKM-TR schemes, first an improved TKM-TR scheme is proposed, in which the peak regrowth caused by severe tailing is eliminated by optimizing the scaling factors. In addition, considering the clipping ratio (CR) value in TKM-TR scheme is greatly related to the PAPR reduction performance, an extensively used heuristic global optimization algorithm, the particle swarm optimization (PSO) method is employed in TKM-TR to obtain a better CR for more PAPR reduction. Simulation results show that the improved TKM-TR scheme can efficiently address the tailing problem in previous TKM-TR schemes and achieve better PAPR reduction. Moreover, due to the powerful searching ability, PSO based TKM-TR scheme achieves more PAPR reduction and lower bit error rate (BER).

Observation of Third-order Nonlinearities in Graphene Oxide Film at Telecommunication Wavelengths.

All-optical switches have been considered as a promising solution to overcome the fundamental speed limit of the current electronic switches. However, the lack of a suitable third-order nonlinear material greatly hinders the development of this technology. Here we report the observation of ultrahigh third-order nonlinearity about 0.45 cm2/GW in graphene oxide thin films at the telecommunication wavelength region, which is four orders of magnitude higher than that of single crystalline silicon. Besides, graphene oxide is water soluble and thus easy to process due to the existence of oxygen containing groups. These unique properties can potentially significantly advance the performance of all-optical switches.

Electrokinetic Manipulation Integrated Plasmonic-Photonic Hybrid Raman Nanosensors with Dually Enhanced Sensitivity.

To detect biochemicals with ultrahigh sensitivity, efficiency, reproducibility, and specificity has been the holy grail in the development of nanosensors. In this work, we report an innovative type of photonic-plasmonic hybrid Raman nanosensor integrated with electrokinetic manipulation by rational design, which offers dual mechanisms that enhance the sensitivity for molecule detection directly in solution. For the first time, we integrate large arrays of synthesized plasmonic nanocapsules with densely surface distributed silver (Ag) nanoparticles (NPs) on lithographically patterned photonic crystal slabs via electric-field assembling. With the interdigital microelectrodes, the applied electric fields not only assemble the hybrid plasmonic nanocapsules on photonic crystal slabs, but also generate electrokinetic flows that focus analyte molecules to the Ag hot spots on the nanocapsules for surface-enhanced Raman scattering (SERS) detection. The synergistic effects of plasmonic-photonic resonance and the electrokinetic molecular focusing can promote the SERS enhancement factor (EF) robustly to ∼2 × 109. Various molecules including SERS probing molecules, nucleobases, and unsafe food additives can be detected directly from suspension. The innovative mechanism, design, and fabrication reported in this work can inspire a new paradigm for achieving high-performance Raman nanosensors, which is pivotal for lab-on-chip disease diagnosis and environmental protection.

Improving the detection limit for on-chip photonic sensors based on subwavelength grating racetrack resonators.

Compared to the conventional strip waveguide microring resonators, subwavelength grating (SWG) waveguide microring resonators have better sensitivity and lower detection limit due to the enhanced photon-analyte interaction. As sensors, especially biosensors, are usually used in absorptive ambient environment, it is very challenging to further improve the detection limit of the SWG ring resonator by simply increasing the sensitivity. The high sensitivity resulted from larger mode-analyte overlap also brings significant absorption loss, which deteriorates the quality factor of the resonator. To explore the potential of the SWG ring resonator, we theoretically and experimentally optimize an ultrasensitive transverse magnetic mode SWG racetrack resonator to obtain maximum quality factor and thus lowest detection limit. A quality factor of 9800 around 1550 nm and sensitivity of 429.7 ± 0.4nm/RIU in water environment are achieved. It corresponds to a detection limit (λ/S·Q) of 3.71 × 10-4 RIU, which marks a reduction of 32.5% compared to the best value reported for SWG microring sensors.

Inkjet Printing of High Performance Transistors with Micron Order Chemically Set Gaps.

This paper reports a 100% inkjet printed transistor with a short channel of approximately 1 µm with an operating speed up to 18.21 GHz. Printed electronics are a burgeoning area in electronics development, but are often stymied by the large minimum feature size. To combat this, techniques were developed to allow for the printings of much shorter transistor channels. The small gap size is achieved through the use of silver inks with different chemical properties to prevent mixing. The combination of the short channel and semiconducting carbon nanotubes (CNT) allows for an exceptional experimentally measured on/off ratio of 106. This all inkjet printed transistor allows for the fabrication of devices using roll-to-roll methodologies with no additional overhead compared to current state of the art production methods.