The reliable, high-sensitive, wireless, and affordable requirements for humidity sensors are needed in high-precision measurement fields. Quartz crystal microbalance (QCM) based on the piezoelectric effect can accurately detect the mass changes at the nanogram level. However, water-capture materials deposited on the surface of QCM generally show disadvantages in either cost, sensitivity, or recyclability. Herein, novel QCM-based humidity sensors (NQHSs) are developed by uniformly depositing green microspheres (GMs) of natural polymers prepared by the chemical synthesis of the emulsification/inner gel method on QCM as humidity-sensitive materials. The NQHSs demonstrate high accuracy and sensitivity (27.1 Hz/% RH) owing to the various hydrophilic groups and porous nano-3D deposition structure. Compared with the devices deposited with a smooth film, the frequency of the NQHSs shows almost no changes during the cyclic test and exhibits long-term stability. The NQHSs have been successfully applied to non-contact sensing human activities and remote real-time humidity monitoring via Bluetooth transmission. In addition, the deposited humidity-sensitive GMs and QCM substrate are fully recycled and reused (72% of the original value). This work has provided an innovative idea to construct environmental-friendly, high-sensitivity, and wireless humidity sensors.

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We present a high-absorption optical stack design for aluminum (Al) kinetic inductance detectors (KIDs). Aluminum can be easily processed in micro-fabrication and is the most conventional superconducting material for KIDs. However, it is challenging to achieve high absorption in the Al absorber because of its high reflection at optical wavelengths. By embedding the thin Al film between an anti-reflection (AR) coating layer and a dielectric-based distributed Bragg reflector, we show that close-to-unity absorption can be achieved around a single wavelength (e.g., ≈98.9% at 1518 nm). The reflection and transmission measurements agree well with the calculation based on the transmission matrix model. We also show our preliminary results of absorption ≥70% in a broader wavelength range (≈230nm) with multilayer AR coatings. The absorber design in a lumped-element KID is discussed. Our work paves the way to high-efficiency photon-counting and energy-resolving Al-based KIDs in the optical to NIR range.