We demonstrate a plasmonic fiber tip for relative humidity (RH) detection by integrating a gold nanomembrane onto the end-face of a multimode optical fiber via a flexible and high-efficiency transfer method. Fast water condensation/evaporation is responsible for the high performance of the fiber tip in response to RH. A high sensitivity of 279 pm/%RH is obtained in the range of 11 % ∼ 92 % R H . Taking advantage of the fast dynamics (response and recovery times of 156 ms and 277 ms), the plasmonic fiber tip offers an excellent detection capability to human breaths at varied frequencies and depths. The compact, easy-fabrication, and fast-dynamics plasmonic platform has versatile potential for practical applications, including environmental and healthcare monitoring, as well as biochemical sensing.

Breath is one of the most important physiological features of human life. In particular, it is significant to monitor the physical characteristics of breath, such as breath frequency and tidal volume. Breath sensors play an important role in the field of human health monitoring. However, an electronic breath sensor is not stable or even safe when the patient is in a Magnetic Resonance Imaging (MRI) system or during any oncology treatment that requires radiation and other high electric/magnetic fields. Fiber-optic-based sensors have attracted a considerable amount of attention from researchers since they are immune to electromagnetic interference. Here, we propose and demonstrate a fiber-optic-based relative-humidity (RH)-sensing strategy by depositing Ti₃C₂T_x nanosheets onto an etched single-mode fiber (ESMF). The humidity sensor function is realized by modulating the transmitted light in the ESMF using the excellent hydrophilic properties of Ti₃C₂T_x. Experiments show that the coated Ti₃C₂T_x nanosheets can effectively modulate the transmitted light in the ESMF in the relative humidity range of 30~80% RH. The sensor’s fast response time of 0.176 s and recovery time of 0.521 s allow it to be suitable for real-time human breath monitoring. The effective recognition of different breath rhythms, including fast, normal, deep, and strong breathing patterns, has been realized. This work demonstrates an all-optical Ti₃C₂T_x-based sensing platform that combines Ti₃C₂T_x with an optical fiber for humidity sensing for the first time, which has great promise for breath monitoring and presents novel options for gas-monitoring applications in the biomedical and chemical fields.

Efficient phase modulation plays a key role in optical and electro-optic (EO) devices. The rising metasurfaces provide a versatile platform for flexible phase control within subwavelength scale. Lithium niobate (LN) with excellent optical properties is one of the most outstanding EO materials. Here, we numerically design a LN-based metasurface which consists of two identical nanobars in each unit cell. By slightly breaking the structure symmetry, a quasi-bound state in the continuum (quasi-BIC) is excited with a high quality-factor ( $Q$ -factor), which also holds a strong local field enhancement. Significantly, by taking advantage of the EO effect of LN, the reflection phase delay can be modulated in the range of 0- $\pi $ using an external voltage as low as 14 V. Simultaneously, the reflectivity remains higher than 30% as voltage varies. Our design suggests a promising scheme for metasurface-based spatial light modulators.

We present a novel kind of optical sensor based on the electromagnetically induced transparency (EIT)-like effect in a Tamm plasmon multilayer structure, which consists of a metal film on a dielectric Bragg grating with alternatively stacked ${\mathrm{TiO}}_2$ and ${\mathrm{SiO}}_2$ layers and a defect layer. The defect layer can induce a refractive-index-sensitive ultranarrow peak in the broad Tamm plasmon reflection dip. This nonintuitive phenomenon in analogy to the EIT effect in atomic systems originates from the coupling and destructive interference between the defect and Tamm plasmon modes in the multilayer structure. Taking advantage of this EIT-like effect, we achieve an ultrahigh sensing performance with a sensitivity of 416 nm/RIU and a figure of merit (FOM) of $682{\mathrm{RIU}}^{-1}$. The numerical simulations agree well with the theoretical calculations. Additionally, the spectral line shape can be effectively tailored by changing the defect layer thickness, significantly promoting the dimensionless FOM from $0.76\times {10}^4$ to more than $2.4\times {10}^4$. Our findings will facilitate the achievement of ultrasensitive optical sensors in multilayer structures and open up perspectives for practical applications, especially in gas, biochemical, and optofluidic sensing.

Plasmonic devices using periodic metallic nanostructures have recently gained tremendous interest for color filters, sensing, surface enhanced spectroscopy, and enhanced photoluminescence, etc. However, the performance of such plasmonic devices is severely hampered by the solid substrates supporting the metallic nanostructures. Here, a strategy for freestanding metallic nanomembranes is introduced by taking advantages of hollow substrate structures. Large-area and highly uniform gold nanomembranes with nanohole array are fabricated via a flexible and simple replication-releasing method. The hollow structures include a hollow core fiber with 30 μm core diameter and two ferrules with their hole diameter as 125 and 500 μm, respectively. As a proof-of-concept demonstration, 2 times higher sensitivity of the bulk refractive index is obtained with this platform compared to that of a counterpart on a solid silica substrate. Such a portable and compact configuration provides unique opportunities to explore the intrinsic properties of the metal nanomembranes and paves a new way to fabricate high-performance plasmonic devices for biomolecule sensing and color filter.