GaN integrated optical devices for glycerol viscosity measurement.

This Letter presents the fabrication and characterization of a chip-scale GaN optical device for measuring glycerol viscosity. The monolithically integrated GaN chip with a size of 1 × 1 mm2 comprises a light-emitting diode (LED) and a photodiode (PD) on a transparent sapphire substrate. The glycerol droplet applied to the device acts as a medium for coupling light from the LED to the PD. When a mechanical impulse is applied, the droplet undergoes a damped vibration that depends on its viscosity, causing a change in its shape and altering the path of light propagation. The viscosity of the glycerol sample can be determined by obtaining the rate of attenuation of the measured photocurrent signals. The proposed unit offers a fast time response in microseconds and requires only a small sample volume of 5 µl. The developed device is highly suitable for the practical measurement of glycerol viscosity due to its miniaturization, low cost, and ease of operation without the need for external optical components.

Fully Integrated Patch Based on Lamellar Porous Film Assisted GaN Optopairs for Wireless Intelligent Respiratory Monitoring.

Respiratory pattern is one of the most crucial indicators for accessing human health, but there has been limited success in implementing fast-responsive, affordable, and miniaturized platforms with the capability for smart recognition. Herein, a fully integrated and flexible patch for wireless intelligent respiratory monitoring based on a lamellar porous film functionalized GaN optoelectronic chip with a desirable response to relative humidity (RH) variation is reported. The submillimeter-sized GaN device exhibits a high sensitivity of 13.2 nA/%RH at 2-70%RH and 61.5 nA/%RH at 70-90%RH, and a fast response/recovery time of 12.5 s/6 s. With the integration of a wireless data transmission module and the assistance of machine learning based on 1-D convolutional neural networks, seven breathing patterns are identified with an overall classification accuracy of >96%. This integrated and flexible on-mask sensing platform successfully demonstrates real-time and intelligent respiratory monitoring capability, showing great promise for practical healthcare applications.

PDMS-assisted GaN optical hardness sensors.

In this Letter, an optical hardness sensor is fabricated based on a GaN-based device combined with finger-shaped PDMS. The chip-scale 1 mm × 1 mm GaN-based device is monolithically integrated with a light emitter and receiver responsible for light emission and photodetection, respectively. The micropatterned PDMS layer can effectively convert the hardness information of the measured object into an optical change detected by the receiver. Verified by experiment measurements, the sensor exhibits a linear response in a hardness range of 1-84 HA, a sensitivity of 0.24 µA/HA, a fast response time of 1.2 ms, and a high degree of repeatability and stability. The optical sensor has the characteristics of tiny size, high compactness, inexpensive fabrication cost, wide measurement range, and high stability, making it suitable for hardness measurement in practical applications.

Compact GaN-based optical inclinometer.

In this Letter, a compact optical inclinometer in sub-centimeter size is proposed and demonstrated. A 1×1 mm2 GaN-on-sapphire chip composed of a light-emitting diode and photodetector is fabricated through wafer-scale processes and integrated with a spherical glass cavity with a diameter of 5 mm, which contains ethanol as a liquid pendulum. When applying inclinations relative to the horizon, the extent to which the chip is immersed in ethanol changes, thereby altering the amount of light received by the on-chip detector. The underlying mechanisms of angle-dependent reflectance characteristics at the sapphire boundary are identified, and the measured photocurrent signal can be used as quantitative readouts for determining the angle of inclination from -60 to +60°. A linear response with a sensitivity of 19.4 nA/° and an estimated resolution of 0.003° is obtained over a wide linear range from -40 to +40°. Verified by a series of dynamic experiments, the developed inclinometer exhibits a high degree of repeatability and stability, which paves the way for its widespread usage and applications.

Compact integration of GaN-based photonic chip with microfluidics system.

This Letter reports a demonstration of integrating a tiny GaN-based photonic chip with a PDMS microfluidics system. The photonic chip containing InGaN/GaN quantum wells is responsible for light emission and photodetection and fabricated through standard microfabrication techniques. The PDMS-enclosed chip is formed adjacent to the fluidic channel and operates in reflection mode, enabling the optical signals coupled into and out of the fluidic channel without the aid of external optics. The luminescence and photo-detecting properties are thoroughly characterized, confirming that the chip is capable of tracking the continuously flowing microdroplets with the changes of absorbance, length, and flow rate. The novel, to the best of our knowledge, photonic integration presented in this Letter is a significant step forward in the development of compact, miniature, and self-contained on-chip sensing systems, which are of great value in portable lab-on-a-chip applications.

GaN microdisk with direct coupled waveguide for unidirectional whispering-gallery mode emission.

Microdisks are excellent whispering-gallery mode (WGM) optical resonators, but their emissions are invariably in-plane isotropic due to their circularities and thus difficult to be extracted efficiently. In this work, a waveguide with a width of 0.16 µm directly coupled to a microdisk with a diameter of 10 µm is fabricated on a 0.77 µm thick GaN thin film containing InGaN/GaN multi-quantum wells. This eliminates the need for precision patterning required by evanescent coupling schemes in which coupling gaps of the order of tens of nanometers must be maintained. The fabrication was carried out using nanosphere and nanowire lithography. Non-evanescent coupling of WGMs to the waveguide from the microdisk is successfully demonstrated.

Multimodal dynamic and unclonable anti-counterfeiting using robust diamond microparticles on heterogeneous substrate.

The growing prevalence of counterfeit products worldwide poses serious threats to economic security and human health. Developing advanced anti-counterfeiting materials with physical unclonable functions offers an attractive defense strategy. Here, we report multimodal, dynamic and unclonable anti-counterfeiting labels based on diamond microparticles containing silicon-vacancy centers. These chaotic microparticles are heterogeneously grown on silicon substrate by chemical vapor deposition, facilitating low-cost scalable fabrication. The intrinsically unclonable functions are introduced by the randomized features of each particle. The highly stable signals of photoluminescence from silicon-vacancy centers and light scattering from diamond microparticles can enable high-capacity optical encoding. Moreover, time-dependent encoding is achieved by modulating photoluminescence signals of silicon-vacancy centers via air oxidation. Exploiting the robustness of diamond, the developed labels exhibit ultrahigh stability in extreme application scenarios, including harsh chemical environments, high temperature, mechanical abrasion, and ultraviolet irradiation. Hence, our proposed system can be practically applied immediately as anti-counterfeiting labels in diverse fields.

Chip-scale optical airflow sensor.

Airflow sensors are an essential component in a wide range of industrial, biomedical, and environmental applications. The development of compact devices with a fast response and wide measurement range capable of in situ airflow monitoring is highly desirable. Herein, we report a miniaturized optical airflow sensor based on a GaN chip with a flexible PDMS membrane. The compact GaN chip is responsible for light emission and photodetection. The PDMS membrane fabricated using a droplet-based molding process can effectively transform the airflow stimuli into optical reflectance changes that can be monitored by an on-chip photodetector. Without the use of external components for light coupling, the proposed sensor adopting the novel integration scheme is capable of detecting airflow rates of up to 53.5 ms-1 and exhibits a fast response time of 12 ms, holding great promise for diverse practical applications. The potential use in monitoring human breathing is also demonstrated.

Interface Engineering in Chip-Scale GaN Optical Devices for Near-Hysteresis-Free Hydraulic Pressure Sensing.

In this work, a compact, near-hysteresis-free hydraulic pressure sensor is presented through interface engineering in a GaN chip-scale optical device. The sensor consists of a monolithic GaN-on-sapphire device responsible for light emission and detection and a multilevel microstructured polydimethylsiloxane (PDMS) film prepared through a low-cost molding process using sandpaper as a template. The micro-patterned PDMS film functions as a pressure-sensing medium to effectively modulate the reflectance properties at the sapphire interface during pressure loading and unloading. The interface engineering endows the GaN optical device with near-hysteresis-free performance over a wide pressure range of up to 0-800 kPa. Verified by a series of experimental measurements on its dynamic responses, the tiny hydraulic sensor exhibits superior performance in hysteresis, stability, repeatability, and response time, indicating its considerable potential for a broad range of practical applications.

Chip-Scale In Situ Salinity Sensing Based on a Monolithic Optoelectronic Chip.

Salinity is an indispensable parameter for various applications such as biomedical diagnostics, environmental chemical analysis, marine monitoring, etc. Miniaturized salinity sensors have significant potential in portable applications in various scenarios and designs with highly desirable features of convenience, reliability, economy, and high sensitivity and also the capability of real-time measurements. Herein, we demonstrate a highly refractive index-sensitive sensor based on a microscale III-nitride chip that consists of a light emitter and a photodetector. This highly monolithically integrated chip shows an excellent sensitivity of salinity of 2606 nA/(mol/L) (or 446 nA/%) and a response time of 0.243 s. In addition, wireless communication technologies can be easily integrated with the sensing device, which enables automatic remote control for data collection and postprocessing. Remarkably, a polymer-based antifouling coating on the surface of the sensing chip has been established to significantly improve its long-term stability in mimicked marine water. The demonstrated ultrasensitive, ultracompact, cost-effective, fast response, wireless-compatible, and easy-to-use features endow the current device with a huge potential for in situ salinity sensing under varying environmental conditions.

A Versatile, Incubator-Compatible, Monolithic GaN Photonic Chipscope for Label-Free Monitoring of Live Cell Activities.

The ability to quantitatively monitor various cellular activities is critical for understanding their biological functions and the therapeutic response of cells to drugs. Unfortunately, existing approaches such as fluorescent staining and impedance-based methods are often hindered by their multiple time-consuming preparation steps, sophisticated labeling procedures, and complicated apparatus. The cost-effective, monolithic gallium nitride (GaN) photonic chip has been demonstrated as an ultrasensitive and ultracompact optical refractometer in a previous work, but it has never been applied to cell studies. Here, for the first time, the so-called GaN chipscope is proposed to quantitatively monitor the progression of different intracellular processes in a label-free manner. Specifically, the GaN-based monolithic chip enables not only a photoelectric readout of cellular/subcellular refractive index changes but also the direct imaging of cellular/subcellular ultrastructural features using a customized differential interference contrast (DIC) microscope. The miniaturized chipscope adopts an ultracompact design, which can be readily mounted with conventional cell culture dishes and placed inside standard cell incubators for real-time observation of cell activities. As a proof-of-concept demonstration, its applications are explored in 1) cell adhesion dynamics monitoring, 2) drug screening, and 3) cell differentiation studies, highlighting its potential in broad fundamental cell biology studies as well as in clinical applications.

Ultracompact Chip-Scale Refractometer Based on an InGaN-Based Monolithic Photonic Chip.

Optical refractometer constitutes the core element for many applications, from determining the purity and concentration of pharmaceutical ingredients to measuring the sugar content in food and beverages, and the analysis of petroleum. Here, we demonstrated the monolithic integration of light-emitting diodes (LEDs) and photodetectors (PDs) to fabricate ultracompact refractometers with a chip size of 475 × 320 µm2. The light emission and photodetection properties of the devices containing the same InGaN/GaN multi-quantum wells have been characterized, confirming that the PD can respond to the emission of the LED. The flip-chip assembly of the chip enables the exposed sapphire substrate to be in direct contact with the solution, and the refractive index sensing capability governed by the change of critical angle and Fresnel reflection at the sapphire/solution interface has been investigated. The processing of the optically smooth surface of sapphire and the integration of high-reflectance distributed Bragg reflector beneath the devices facilitate the amount of light received by the PD. The monolithic chip is capable of detecting solutions with a refractive index ranging from 1.3325 to 1.5148 RIU and exhibits a sensitivity of 7.77 µA/RIU and a resolution of 6.4 × 10-6 RIU at the LED current of 10 mA. Rapid real-time responses of 33.9 ms for rise time and 34.7 ms for fall time are obtained in the detected photocurrent, thereby verifying the feasibility of the chip-scale refractometer.