Chip-level mass detection for micro-LED displays based on regression analysis and deep learning.

Though micro-light-emitting diode (micro-LED) displays are regarded as the next-generation emerging display technology, challenges such as defects in LED's light output power and radiation patterns are critical to the commercialization success. Here we propose an electroluminescence mass detection method to examine the light output quality from the on-wafer LED arrays before they are transferred to the display substrate. The mass detection method consists of two stages. In the first stage, the luminescent image is captured by a camera by mounting an ITO (indium-tin oxide) transparent conducting glass on the LED wafer. Due to the resistance of the ITO contact pads and on-wafer n-type electrodes, we develop a calibration method based on the circuit model to predict the current flow on each LED. The light output power of each device is thus calibrated back by multi-variable regression analysis. The analysis results in an average variation as low as 6.89% for devices predicted from luminescent image capturing and actual optical power measurement. We also examine the defective or non-uniform micro-LED radiation profiles by constructing a 2-D convolutional neural network (CNN) model. The optimized model is determined among three different approaches. The CNN model can recognize 99.45% functioning LEDs, and show a precision of 96.29% for correctly predicting good devices.

A point-of-care electrochemical biosensor for the rapid and sensitive detection of biomarkers in murine models with LPS-induced sepsis.

Sepsis is a life-threatening condition, which is irreversible if diagnosis and intervention are delayed. The response of the immune cells towards an infection triggers widespread inflammation through the production of cytokines, which may result in multiple organ dysfunction and eventual death. Conventional detection techniques fail to provide a rapid diagnosis because of their limited sensitivity and tedious protocol. This study proposes a point-of-care (POC) electrochemical biosensor that overcomes the limitations of current biosensing technologies in the clinical setting by its integration with electrokinetics, enhancing the sensitivity to picogram level compared with the nanogram limit of current diagnostic technologies. This biosensor promotes the use of a microelectrode strip to address the limitations of conventional photolithographic fabrication methods. Tumor necrosis factor-alpha (TNF-α), interleukin-6 (IL-6), and microRNA-155 (miR-155) were monitored in a lipopolysaccharide (LPS)-induced septic mouse model. The optimum target hybridization time in a high conductivity medium was observed to be 60 s leading to the completion of the whole operation within 5 min compared with the 4-h detection time of the traditional enzyme-linked immunosorbent assay (ELISA). The limit of detection (LOD) was calculated to be 0.84, 0.18, and 0.0014 pg mL-1, respectively. This novel sensor may have potential for the early diagnosis of sepsis in the clinical setting.

Surface acoustic wave actuated plasmonic signal amplification in a plasmonic waveguide.

Enhancement of nanoscale confinement in the subwavelength waveguide is a concern for advancing future photonic interconnects. Rigorous innovation of plasmonic waveguide-based structure is crucial in designing a reliable on-chip optical waveguide beyond the diffraction limit. Despite several structural modifications and architectural improvements, the plasmonic waveguide technology is far from reaching its maximum potential for mass-scale applications due to persistence issues such as insufficient confined energy and short propagation length. This work proposes a new method to amplify the propagating plasmons through an external on-chip surface acoustic signal. The gold-silicon dioxide (Au-SiO2) interface, over Lithium Niobate (LN) substrate, is used to excite propagating surface plasmons. The voltage-varying surface acoustic wave (SAW) can tune the plasmonic confinement to a desired signal energy level, enhancing and modulating the plasmonic intensity. From our experimental results, we can increase the plasmonic intensity gain of 1.08 dB by providing an external excitation in the form of SAW at a peak-to-peak potential swing of 3 V, utilizing a single chip.

Monitoring levodopa oxidation and reduction reactions using surface plasmon resonance on a nanohole array electrode.

The traditional method of monitoring the oxidation and reduction of biomedical materials usually relies on electrochemical (EC) measurement techniques. Here, we demonstrate a surface plasmon resonance (SPR) method to monitor the oxidation process. Using levodopa L-dopa as the target analyte, a nanohole sensing plate is embedded in the EC electrode to enhance the oxidation signal and generate SPR. Cyclic voltammetry (CV) measurement was first conducted to understand the baseline of EC response of L-Dopa. Then, the redox reactions were simultaneously monitored through SPR measurements during the CV voltage scan. The results showed that the limit of detection using traditional CV reached 1.47 µM while using EC-SPR, the limit of detection improved to 1.23 µM. Most importantly, we found a strong correlation between CV current profiles and the SPR reflection spectra. Our results facilitate detecting electrochemical reactions using an optical probing method.

Effect of radiative and nonradiative energy transfer processes of light-emitting diodes combined with quantum dots for visible light communication.

Though light-emitting diodes (LEDs) combined with various color conversion techniques have been widely explored for VLC (visible light communication), E-O (electro-optical) frequency responses of devices with quantum dots (QDs) embedded within the nanoholes have rarely been addressed. Here we propose LEDs with embedded photonic crystal (PhC) nanohole patterns and green light QDs for studying small-signal E-O frequency bandwidths and large signal on-off keying E-O responses. We observe that the E-O modulation quality of PhC LEDs with QDs is better than a conventional LED with QDs when the overall blue mixed with green light output signal is considered. However, the optical response of only QD converted green light shows a contradictory result. The slower E-O conversion response is attributed to multi-path green light generation from both radiative and nonradiative energy transfer processes for QDs coated on the PhC LEDs.

GaN Vertical Transistors with Staircase Channels for High-Voltage Applications.

In this study, we propose and simulate the design of a non-regrowth staircase channel GaN vertical trench transistor, demonstrating an exceptional threshold and breakdown characteristic for high power and high frequency applications. The unique staircase design provides a variable capacitance through the gate-dielectric-semiconductor interface, which results in a high breakdown voltage of 1.52 kV and maintains a channel on-resistance of 2.61 mΩ∙cm2. Because of the variable length and doping profile in the channel region, this model offers greater flexibility to meet a wide range of device application requirements.

Voltage-modulated surface plasmon resonance biosensors integrated with gold nanohole arrays.

Surface plasmon resonance (SPR) has emerged as one of the most efficient and attractive techniques for optical sensors in biological applications. The traditional approach of an EC (electrochemical)-SPR biosensor to generate SPR is by adopting a prism underneath the sensing substrate, and an angular scan is performed to characterize the reflectivity of target analytes. In this paper, we designed and investigated a novel optical biosensor based on a hybrid plasmonic and electrochemical phenomenon. The SPR was generated from a thin layer of gold nanohole array on a glass substrate. Using C-Reactive Protein (CRP) as the target analyte, we tested our device for different concentrations and observed the optical response under various voltage bias conditions. We observed that SPR response is concentration-dependent and can be modulated by varying DC voltages or AC bias frequencies. For CRP concentrations ranging from 1 to 1000 µg/mL, at the applied voltage of -600 mV, we obtained a limit of detection for this device of 16.5 ng/mL at the resonance peak wavelength of 690 nm. The phenomenon is due to spatial re-distribution of electron concentration at the metal-solution interface. The results suggest that CRP concentration can be determined from the SPR peak wavelength shift by scanning the voltages. The proposed new sensor structure is permissible for various future optoelectronic integration for plasmonic and electrochemical sensing.

Luminescence efficiency improvement of small-size micro light-emitting diodes by a digital etching technology.

The decrease of light output efficiency with the reduction of LED (light-emitting diode) die size is one of the challenges of micro-LED displays. Here we propose a digital etching technology that employs multi-step etching and treatment to mitigate sidewall defects exposed after mesa dry etching. In this study, by two-step etching and N2 treatment, the electrical properties of the diodes show an increase of forward current and a decrease in reverse leakage due to suppressed sidewall defects. An increase of light output power by 92.6% is observed for 10 × 10-µm2 mesa size with digital etching, as compared with that with only one step etching and no treatment. We also demonstrated only 1.1% decrease in output power density for a 10 × 10-µm2 LED as compared with a 100 × 100-µm2 device without performing digital etching.

Beyond GHz optical frequency up-converted modulation of LEDs with integrated acousto-optic transducer.

Traditional visible light communication (VLC) via light-emitting diodes (LEDs) employs the on-off keying (OOK) modulation scheme. Even though optical frequency modulation has many advantages, it is hardly used for LED VLC because a high carrier frequency cannot be applied to the LED cavity due to the resistance-capacitance limit. Here, by monolithically integrating an LED with an integrated digital transducer, we experimentally demonstrate the intermixing of gigahertz surface acoustic waves and electrical data signals in the LED cavity at room temperature. An optical transmitter was realized by in situ frequency up-conversion of the data signals from an LED, which has the advantages of improving transmission performance by up-shifting the data spectrum away from low-frequency noise. Our proposed integrated acousto-optic transducer opens a new developing scheme on the frequency up-mixed data encoding of an LED beyond its inherent modulation bandwidth for future VLC.

Analysis of the Biochemical Reaction Status by Real-Time Monitoring Molecular Diffusion Behaviors Using a Transistor Biosensor Integrated with a Microfluidic Channel.

Traditional methods of monitoring biochemical reactions measure certain detectable reagents or products while assuming that the undetectable species follow the stoichiometry of the reactions. Here, based upon the metal-oxide thin-film transistor (TFT) biosensor, we develop a real-time molecular diffusion model to benchmark the concentration of the reagents and products. Using the nicotinamide adenine dinucleotide (NADH)-oxaloacetic acid with the enzyme of malate dehydrogenase as an example, mixtures of different reagent concentrations were characterized to extract the ratio of remaining concentrations between NAD+ and NADH. We can thus obtain the apparent equilibrium constant of the reaction, (8.06 ± 0.61) × 104. Because the whole analysis was conducted using a TFT sensor fabricated using a semiconductor process, our approach has the advantages of exploring biochemical reaction kinetics in a massively parallel manner.

Observation of Acoustic Charge Transport in GaN-Based Quantum Well Structures Under Surface Acoustic Wave Excitations.

Though the acoustoelectric and acoustooptic interactions have been widely studied on III-V semiconductors, most studies were conducted at low temperature to avoid the influence of lattice scattering. Here, we demonstrate larger-than-1-GHz optical oscillation and acoustic charge transport at room temperature from a nitride-based LED (light-emitting diode) by acoustic wave excitation. The optical oscillation is generated by the harmonics of acoustic waves, while the acoustic transport of carriers was observed from the LED light output at different acoustic excitation frequencies. Furthermore, with the change of bias voltage, the frequency response of light output varies correspondingly due to the thermal disturbance that affects the lattice vibration and acoustic wave propagation. The results also suggest that we are able to achieve a tunable optical resonant frequency by varying bias voltages without changing the LED structure.

Observation of 990-MHz Optical Oscillation From Light Emitters Excited by High-Order Harmonics of Surface Acoustic Waves.

Optomechanical properties have been widely explored on the interactions between phonon, photon, and electrons. The applications range from acoustic filters for mobile handsets to quantum information science./However, up to date, the interaction between harmonic modes of surface acoustic waves (SAWs) and photons has not been studied in detail. Here, we develop radio frequency (RF) - modulated light emitters driven by the coupling between electrical and acoustic signals at room temperature. The light emitter demonstrates a 990-MHz oscillation behavior which cannot be solely achieved by electrical driving due to resistance-capacitance (RC) limit. Instead, the result is attributed to the excitation by the harmonics of SAWs in the light emitter. The ~gigahertz light oscillation enables a new architecture for information processing. In this work, we also demonstrate the coupling between acoustooptical and electrooptical interactions by simultaneously applying 990-MHz acoustic signals and 20-MHz modulated electrical inputs.

Review of Integrated Optical Biosensors for Point-Of-Care Applications.

This article reviews optical biosensors and their integration with microfluidic channels. The integrated biosensors have the advantages of higher accuracy and sensitivity because they can simultaneously monitor two or more parameters. They can further incorporate many functionalities such as electrical control and signal readout monolithically in a single semiconductor chip, making them ideal candidates for point-of-care testing. In this article, we discuss the applications by specifically looking into point-of-care testing (POCT) using integrated optical sensors. The requirement and future perspective of integrated optical biosensors for POC is addressed.

Characterizations of protein-ligand reaction kinetics by transistor-microfluidic integrated sensors.

Understanding the binding affinities and kinetics of protein-ligand interactions using a label-free method is crucial for identifying therapeutic candidates in clinical diagnostics and drug development. In this work, the IGZO-TFT (thin-film transistor) biosensor integrated with a tailored microfluidic chip was developed to explore binding kinetics of protein-ligand biochemical interactions in the real-time manner. The IGZO-TFT sensor extracts the binding characteristics through sensing biomolecules by their electrical charges. Using lysozyme and tri-N-acetyl-D-glucosamine (NAG3) as an example, we established a procedure to obtain the parameters, such as the dissociation constant, Kd, and association rate constant, ka, that are critical to biochemical reactions. The correlation between the lysozyme concentration and TFT drain current signal was first constructed. Next, solutions of lysozyme and NAG3 of different mixing ratios were prepared. They were pre-mixed for various periods of reaction time before applying to the TFT sensor to extract signals of lysozyme molecules and the concentration remaining. With the knowledge of drain current changes at different reaction times, ka and Kd can be obtained. The values from our experiment are comparable to other methods, which suggests the proposed approach can be employed to explore protein-ligand interaction kinetics in the massively parallel manner if the TFT array is considered.

Evanescent Properties of Optical Diffraction from 2-Dimensional Hexagonal Photonic Crystals and Their Sensor Applications.

The sensitivity of traditional diffraction grating sensors is limited by the spatial resolution of the measurement setup. Thus, a large space is required to improve sensor performance. Here, we demonstrate a compact hexagonal photonic crystal (PhC) optical sensor with high sensitivity. PhCs are able to diffract optical beams to various angles in azimuthal space. The critical wavelength that satisfies the phase matching or becomes evanescent was used to benchmark the refractive index of a target analyte applied on a PhC sensor. Using a glucose solution as an example, our sensor demonstrated very high sensitivity and a low limit of detection. This shows that the diffraction mechanism of hexagonal photonic crystals can be used for sensors when compact size is a concern.

Comparison of single-/few-/multi-mode 850 nm VCSELs for optical OFDM transmission.

For high-speed optical OFDM transmission applications, a comprehensive comparison of the homemade multi-/few-/single-transverse mode (MM/FM/SM) vertical cavity surface emitting laser (VCSEL) chips is performed. With microwave probe, the direct encoding of pre-leveled 16-QAM OFDM data and transmission over 100-m-long OM4 multi-mode-fiber (MMF) are demonstrated for intra-datacenter applications. The MM VCSEL chip with the largest emission aperture of 11 µm reveals the highest differential quantum efficiency which provides the highest optical power of 8.67 mW but exhibits the lowest encodable bandwidth of 21 GHz. In contrast, the SM VCSEL chip fabricated with the smallest emission aperture of only 3 µm provides the highest 3-dB encoding bandwidth up to 23 GHz at a cost of slight heat accumulation. After optimization, with the trade-off set between the receiving signal-to-noise ratio (SNR) and bandwidth, the FM VCSEL chip guarantees the highest optical OFDM transmission bit rate of 96 Gbit/s under back-to-back case with its strongest throughput. Among three VCSEL chips, the SM VCSEL chip with nearly modal-dispersion free feature is treated as the best candidate for carrying the pre-leveled 16-QAM OFDM data over 100-m OM4-MMF with same material structure but exhibits different oxide-layer confined gain cross-sections with one another at 80-Gbit/s with the smallest receiving power penalty of 1.77 dB.

Efficient Heat Dissipation of Uncooled 400-Gbps (16×25-Gbps) Optical Transceiver Employing Multimode VCSEL and PD Arrays.

An effective heat dissipation of uncooled 400-Gbps (16×25-Gbps) form-factor pluggable (CDFP) optical transceiver module employing chip-on-board multimode 25-Gbps vertical-surface-emitting-laser (VCSEL) and 25-Gbps photodiode (PD) arrays mounted on a brass metal core embedded within a printed circuit board (PCB) is proposed and demonstrated. This new scheme of the hollow PCB filling with thermally-dissipated brass metal core was simulated and used for high temperature and long term stability operation of the proposed 400-Gbps CDFP transceiver. During one-hour testing, a red-shift of central wavelength by 0.4-nm corresponding temperature increment of 6.7 °C was observed with the brass core assisted cooler module. Such a temperature change was significantly lower than that of 28.3 °C for the optical transceiver driven with conventional circuit board. After 100-m distance transmission over a multimode fiber (OM4), the 400-Gbps CDFP transceiver exhibited dispersion penalty of 2.6-dB, power budget of ≧ 3-dB, link loss of ≦ 0.63-dB, mask margin of 20%, and bit error rate (BER) of <10-12 with maintained stability more than one hour. The developed 400-Gbps CDFP transceiver module employing low-power consumption VCSEL and PD arrays, effective coupling lens arrays, and well thermal-dissipation brass metal core is suitable for use in the low-cost and high-performance data center applications.

Cancer cell identification by bi-color ZnO and TiO2 nanowires.

Semiconductor nanocomposites provide advantages beyond the capability of typical fluorescent materials for cancer detection. In this work, nanowire-based probes with dual color channels are employed to demonstrate the capacity of cancer cell detection. Purple emitting ZnO/antibody probes are applied to detect cancer cells and meanwhile TiO2 /antibody probes with green light emission are applied to identify normal fibroblast cells. A series of quantitative analyses are conducted to verify the correlation between the concentrations of ZnO and TiO2 probes, cell numbers, and peak intensities of the PL spectra. The results provide a quantitative reference for developing nanowire-based cancel cell probes.

Thin-Film Transistor-Based Biosensors for Determining Stoichiometry of Biochemical Reactions.

The enzyme kinetic in a biochemical reaction is critical to scientific research and drug discovery but can hardly be determined experimentally from enzyme assays. In this work, a charge-current transducer (a transistor) is proposed to evaluate the status of biochemical reaction by monitoring the electrical charge changes. Using the malate-aspartate shuttle as an example, a thin-film transistor (TFT)-based biosensor with an extended gold pad is demonstrated to detect the biochemical reaction between NADH and NAD+. The drain current change indicates the status of chemical equilibrium and stoichiometry.

Transient Induced Molecular Electronic Spectroscopy (TIMES) for study of protein-ligand interactions.

We present a method, Transient Induced Molecular Electronic Spectroscopy (TIMES), to detect protein-ligand interactions without any protein engineering or chemical modification. We developed a physics model for the TIMES signal and mathematically formulated the problem to attain physical insight of protein-ligand interactions without any disturbances by molecular probes, fluorescent labels, or immobilization of molecules. To demonstrate the functionality of this method, we have used the TIMES signals to find the dissociation constants for the affinity of reactions, the shear-stress dependent adsorption time of molecules on surface, and other interesting features of protein-ligand interaction in native conditions. As a unique tool, TIMES offers a simple and effective method to investigate fundamental protein chemistry and drug discoveries.