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1. Freestanding BaTiO ₃ ‐Au Vertically Aligned Nanocomposite toward Flexible Multi‐Sensing Platform

   https://doi.org/10.1002/adfm.202418004  

   Tsai, Benson_Kunhung; Huang, Jialong; Yu, Ya‐Ching; Lee, Meng_Hao; Stegman, Benjamin_Thomas; Flores, Emiliano_Joseph; Tong, Patrick_Zhang; Xu, Ke; Zhou, Shiyu; Shen, Jianan; et al (January 2025, Advanced Functional Materials)

   Abstract Flexible and wearable sensors show enormous potential for personalized healthcare devices by real‐time monitoring of an individual's health. Typically, a single functional material is selected for one sensor to sense a particular physical signal while multiple materials will be selected for multi‐mode sensing. Vertically aligned nanocomposites (VANs) have recently demonstrated various material combinations and novel coupled multifunctionalities that are hard to achieve in any single‐phase material alone, including multiphase multiferroics, magneto‐optic coupling, and strong magnetic and optical anisotropy. Integrating these novel VANs into wearable sensors shows enormous potential in multi‐mode sensing owing to their multifunctional nature. In this work, the transfer of VANs onto polydimethylsiloxane as a novel flexible chemical and pressure sensor is demonstrated. For this demonstration, the classical BaTiO₃‐Au VAN with combined plasmonic and piezoelectric properties is used to demonstrate a multi‐sensing mechanism. A thin water‐soluble buffer of Sr₃Al₂O₆serves as a buffer layer for the epitaxial growth and transfer process. The electrical output based on the piezoelectric responses and identifying 4‐mercaptobenzoic acid by surface‐enhanced Raman spectroscopy reveal great potential for free‐standing VANs in a wearable multifunctional sensing platform. 

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2. Integrated polarization-free Bragg filters with subwavelength gratings for photonic sensing

   https://doi.org/10.1364/OE.504043  

   Pimbi, Daniel; Mia, Md_Borhan; Jaidye, Nafiz; Ahmed, Ishtiaque; Hasan, Mehedi; Ahmed, Syed_Z; Kim, Sangsik (January 2024, Optics Express)

   We present polarization-free Bragg filters having subwavelength gratings (SWGs) in the lateral cladding region. This Bragg design expands modal fields toward upper cladding, resulting in enhanced light interaction with sensing analytes. Two device configurations are proposed and examined, one with index-matched coupling between transverse electric (TE) and transverse magnetic (TM) modes and the other one with hybrid-mode (HM) coupling. Both configurations introduce a strong coupling between two orthogonal modes (either TE-TM or HM₁-HM₂) and rotate the polarization of the input wave through Bragg reflection. The arrangements of SWGs help to achieve two configurations with different orthogonal modes, while expanding modal profiles toward the upper cladding region. Our proposed SWG-assisted Bragg gratings with polarization independency eliminate the need for a polarization controller and effectively tailor the modal properties, enhancing the potential of integrated photonic sensing applications. 

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3. Towards lab-on-chip ultrasensitive ethanol detection using photonic crystal waveguide operating in the mid-infrared

   https://doi.org/10.1515/nanoph-2020-0576  

   Rostamian, Ali; Madadi-Kandjani, Ehsan; Dalir, Hamed; Sorger, Volker J.; Chen, Ray T. (April 2021, Nanophotonics)

   Abstract Thanks to the unique molecular fingerprints in the mid-infrared spectral region, absorption spectroscopy in this regime has attracted widespread attention in recent years. Contrary to commercially available infrared spectrometers, which are limited by being bulky and cost-intensive, laboratory-on-chip infrared spectrometers can offer sensor advancements including raw sensing performance in addition to utilization such as enhanced portability. Several platforms have been proposed in the past for on-chip ethanol detection. However, selective sensing with high sensitivity at room temperature has remained a challenge. Here, we experimentally demonstrate an on-chip ethyl alcohol sensor based on a holey photonic crystal waveguide on silicon on insulator-based photonics sensing platform offering an enhanced photoabsorption thus improving sensitivity. This is achieved by designing and engineering an optical slow-light mode with a high group-index ofn_g = 73 and a strong localization of the modal power in analyte, enabled by the photonic crystal waveguide structure. This approach includes a codesign paradigm that uniquely features an increased effective path length traversed by the guided wave through the to-be-sensed gas analyte. This PIC-based lab-on-chip sensor is exemplary, spectrally designed to operate at the center wavelength of 3.4 μm to match the peak absorbance for ethanol. However, the slow-light enhancement concept is universal offering to cover a wide design-window and spectral ranges towards sensing a plurality of gas species. Using the holey photonic crystal waveguide, we demonstrate the capability of achieving parts per billion levels of gas detection precision. High sensitivity combined with tailorable spectral range along with a compact form-factor enables a new class of portable photonic sensor platforms when integrated with quantum cascade laser and detectors. 

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4. Large‐Scale Sub‐1‐nm Random Gaps Approaching the Quantum Upper Limit for Quantitative Chemical Sensing

   https://doi.org/10.1002/adom.202001634  

   Zhang, Nan; Hu, Haifeng; Singer, Matthew; Li, Kuang‐hui; Zhou, Lyu; Ooi, Boon_S; Gan, Qiaoqiang (October 2020, Advanced Optical Materials)

   Abstract Metallic nanostructures with nanogap features can confine electromagnetic fields into extremely small volumes. In particular, as the gap size is scaled down to sub‐nanometer regime, the quantum effects for localized field enhancement reveal the ultimate capability for light–matter interaction. Although the enhancement factor approaching the quantum upper limit has been reported, the grand challenge for surface‐enhanced vibrational spectroscopic sensing remains in the inherent randomness, preventing uniformly distributed localized fields over large areas. Herein, a strategy to fabricate high‐density random metallic nanopatterns with accurately controlled nanogaps, defined by atomic‐layer‐deposition and self‐assembled‐monolayer processes, is reported. As the gap size approaches the quantum regime of ≈0.78 nm, its potential for quantitative sensing, based on a record‐high uniformity with the relative standard deviation of 4.3% over a large area of 22 mm × 60 mm, is demonstrated. This superior feature paves the way towards more affordable and quantitative sensing using quantum‐limit‐approaching nanogap structures. 

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5. Deep-learning-assisted printed liquid metal sensory system for wearable applications and boxing training

   https://doi.org/10.1038/s41528-023-00272-1  

   Qiu, Ye; Zou, Zhihui; Zou, Zhanan; Setiawan, Nikolas Kurnia; Dikshit, Karan Vivek; Whiting, Gregory; Yang, Fan; Zhang, Wenan; Lu, Jiutian; Zhong, Bingqing; et al (August 2023, npj Flexible Electronics)

   Abstract Liquid metal (LM) exhibits a distinct combination of high electrical conductivity comparable to that of metals and exceptional deformability derived from its liquid state, thus it is considered a promising material for high-performance soft electronics. However, rapid patterning LM to achieve a sensory system with high sensitivity remains a challenge, mainly attributed to the poor rheological property and wettability. Here, we report a rheological modification strategy of LM and strain redistribution mechanics to simultaneously simplify the scalable manufacturing process and significantly enhance the sensitivity of LM sensors. By incorporating SiO₂particles into LM, the modulus, yield stress, and viscosity of the LM-SiO₂composite are drastically enhanced, enabling 3D printability on soft materials for stretchable electronics. The sensors based on printed LM-SiO₂composite show excellent mechanical flexibility, robustness, strain, and pressure sensing performances. Such sensors are integrated onto different locations of the human body for wearable applications. Furthermore, by integrating onto a tactile glove, the synergistic effect of strain and pressure sensing can decode the clenching posture and hitting strength in boxing training. When assisted by a deep-learning algorithm, this tactile glove can achieve recognition of the technical execution of boxing punches, such as jab, swing, uppercut, and combination punches, with 90.5% accuracy. This integrated multifunctional sensory system can find wide applications in smart sport-training, intelligent soft robotics, and human-machine interfaces. 

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