Chip-scale optical airflow sensor

The study addresses the need for compact, fast, and wide-range airflow sensors for applications in environmental monitoring, aerodynamics, navigation control, and biomedical engineering. Conventional electrical sensing approaches (thermoresistive, piezoresistive, resistive, capacitive, magnetoelastic, mechanoluminescent) face trade-offs between response speed and measurement range. Fiber-optic airflow sensors offer high sensitivity and fast optical response but typically rely on bulky external optical coupling and alignment between light sources, fibers, and analyzers, limiting robustness and miniaturization; moreover, response time to airflow is often not reported. The authors propose eliminating external light-coupling by integrating light emission and detection on a GaN chip and using a flexible PDMS membrane as the airflow-responsive element. The research question is whether a chip-scale GaN optoelectronic platform, combined with a compliant PDMS membrane, can deliver a fast-response, wide-range optical airflow sensor without external coupling components.

Prior airflow sensors use diverse transduction principles: thermoresistive, piezoresistive, resistive, capacitive, magnetoelastic, and mechanoluminescent approaches. Optical fiber-based airflow sensors with architectures such as fiber Bragg gratings, Fabry–Pérot interferometers, microcantilevers, and other nanostructures have improved sensitivity but at the cost of complex, bulky systems that require precise optical alignment and external coupling between source and analyzer. For GaN-based implementations, only one previous report used a suspended GaN membrane with a detectable airflow up to 2.779 ms⁻¹ and a switch-on time of 1 s, but free-standing GaN films are fragile under high airflow and structural deformation can strain the GaN epitaxy, degrading device efficiency. GaN-based optoelectronics have shown promise in monolithic chip-scale integration for visible light communication, biosensing, and imaging, motivating their use here for compact airflow sensing with an alternative flexible sensing medium (PDMS) to avoid rigid-substrate limitations.

Device fabrication and packaging: GaN chips (on GaN/sapphire templates) were fabricated via wafer-scale microfabrication, including photolithography, dry/wet etching, and deposition of metal and oxide layers. The chip integrates an InGaN/GaN MQW LED (emitter) and an InGaN/GaN MQW photodiode (PD, detector). A bottom SiO2-based distributed Bragg reflector (DBR) redirects emitted light upward. Transparent sapphire aids light extraction; ITO serves as a transparent contact. A flexible PDMS membrane was molded using a droplet-based process: deionized water droplets placed in a plastic Petri dish were used as templates; after curing, an Al film was adhered to the PDMS membrane (leveraging PDMS surface tackiness). The PDMS block was fixed over the GaN chip by gluing the film edges with PDMS gel and curing, forming a suspended reflective membrane above the chip. The assembled device was mounted on an Al PCB package for electrical connections (LED driven by a current source; PD read by ammeter). Working principle: The LED emits green light (InGaN MQWs); the DBR promotes upward radiation. Light propagating through sapphire is reflected by the Al-coated PDMS membrane. Airflow deforms the PDMS membrane, modulating the spacing/geometry and thus the reflected light reaching the on-chip PD. The PD absorbs light in the InGaN layer, producing a photocurrent proportional to received intensity. Electrical/optical characterization: LED I–V measured with a Keithley 2450; forward voltage 2.44 V at 5 mA; series resistance ~33.2 Ω. EL spectra at 5 mA showed a peak wavelength of 521.5 nm and spectral width of 26.8 nm. PD spectral responsivity decreases with wavelength and overlaps the LED emission by ~30 nm (due to QCSE and band-tail effects). Under no-airflow, PD I–V at reverse bias showed dark/low-level currents ~10⁻⁹ A; with LED at 5 mA, PD photocurrent increased to ~10⁻⁵ A (>5 orders of magnitude). A linear relationship between LED drive current and PD photocurrent was observed. Airflow measurement setup: An airflow generator fed an air pipe mounted on a linear motorized stage to control outlet position relative to the sensor. Airflow rates were calibrated with a commercial meter (Taicang Huayu, WS200B). During airflow tests, LED current was fixed at 5 mA and PD was zero-biased; photocurrent was recorded while varying airflow rate and direction (using a rotation stage for angular dependence).

• The sensor detects airflow rates up to 53.5 ms⁻¹; beyond this, the photocurrent saturates (no further increase).
• Fast temporal response: 12 ms response time (as stated in the abstract).
• Linear sensitivity to airflow over the measured range: slopes of 0.2664 and 0.2686 µA/(ms⁻¹) for increasing and decreasing airflow, respectively; R² values > 0.98.
• Under a fixed LED current of 5 mA and zero PD bias, the PD photocurrent increases with airflow due to membrane deformation bringing the reflective Al film closer, increasing coupled light to the PD.
• A stable background photocurrent of ~20 µA is present, independent of airflow, originating from light guided within the sapphire (total internal reflection at the sapphire/air interface with a critical angle ~34.4°).
• Directional dependence: at an airflow rate of 38.7 ms⁻¹, photocurrent decreases from 9.44 µA to 7.88 µA as the sensor is rotated from 0° to 80°; with further angle increase, the photocurrent change drops rapidly to 1.95 µA.
• LED characteristics: forward voltage 2.44 V at 5 mA; series resistance ~33.2 Ω; light output increases with current. Emission peak 521.5 nm, FWHM 26.8 nm.
• PD characteristics: spectral responsivity overlaps the LED spectrum by ~30 nm; PD photocurrent at reverse bias increases from ~10⁻⁹ A (dark) to ~10⁻⁵ A under LED illumination at 5 mA; linear dependence of PD photocurrent on LED current.

Integrating an InGaN/GaN MQW LED and PD on a GaN/sapphire chip with a compliant PDMS membrane and reflective Al layer eliminates the need for external optical coupling, addressing bulkiness and alignment issues in fiber-based optical airflow sensors. Airflow-induced deformation of the PDMS membrane effectively transduces flow velocity into an optical reflectance change detectable by the on-chip PD, yielding a wide measurable range (up to 53.5 ms⁻¹) and a fast response (12 ms). The strong linearity (R² > 0.98) indicates reliable quantitative measurement across the operating range. The observed angle dependence reflects the geometric coupling efficiency variation with airflow direction, while the ~20 µA background photocurrent arises from internal light guiding in sapphire; both phenomena are consistent with the optical design and do not preclude accurate flow measurement after calibration. Compared to prior GaN membrane approaches with limited range (~2.779 ms⁻¹) and slow response (~1 s), the proposed architecture offers substantially improved dynamic range, speed, and robustness due to the flexible PDMS membrane and monolithic on-chip optoelectronics. The demonstrated concept also suggests feasibility for applications such as human breathing monitoring where compactness and fast response are essential.

The work demonstrates a chip-scale optical airflow sensor that monolithically integrates a GaN-based LED and PD with a flexible, Al-coated PDMS membrane, removing external light-coupling components. The device achieves a wide measurable airflow range up to 53.5 ms⁻¹ with linear response and a fast 12 ms temporal response, and shows potential for practical applications including breathing monitoring. Key device characteristics (green emission at 521.5 nm, strong LED–PD spectral overlap, and linear LED-to-PD current transfer) underpin its performance. Future research could focus on reducing background guided-light photocurrent (e.g., through optical isolation or surface texturing), extending the dynamic range before saturation via membrane geometry optimization, improving angular insensitivity, integrating on-chip readout/processing for compact systems, and exploring sensor arrays for spatial flow mapping.

• Background photocurrent (~20 µA) from internal light guiding in sapphire is insensitive to airflow and may require calibration or optical isolation for low-flow measurements.
• Photocurrent saturates beyond ~53.5 ms⁻¹ as the membrane approaches the chip surface, limiting the upper measurable range.
• Sensitivity depends on the incidence angle of airflow; photocurrent decreases with increased rotation angle, indicating directional dependence.
• Full affiliation details and some experimental parameters (e.g., comprehensive response-time characterization conditions) are not provided in the excerpt; long-term stability and environmental robustness were not detailed.