Optical sensors are widely used for refractive index measurement in chemical, biomedical and food processing industries. Due to specific field distribution of the resonances excited, optical sensors provide high sensitivity to ambient refractive index variations. The sensitivity of optical sensor is highly dependent on material and structure of the sensor. Here, we review six major categories of optical refractive index sensors using plasmonic and photonic structures: (i) metal-based propagating plasmonic eigenwave structures, (ii) metal-based localized plasmonic eigenmode structures, (iii) dielectric-based propagating photonic eigenwave structures, (iv) dielectric-based localized photonic eigenmode structures, (v) advanced hybrid structures, and (vi) 2D material integrated structures. Representative configurations working in the wavelength range of 400−2000 nm will be selected and compared in terms of bulk refractive index sensitivities, figure of merits and working wavelengths. A technology map is established in order to define the standard and development trend for optical refractive index sensors.

Rechargeable magnesium batteries have attracted increasing attention due to the high theoretical volumetric capacities, dendrite formation-free characteristic and low cost of Mg metal anodes. However, the development of magnesium batteries is seriously hindered by the lack of capable cathode materials with long cycling life and fast solid-state diffusion kinetics for highly-polarized divalent Mg ions. Herein, vanadium tetrasulfide (VS ) with special one-dimensional atomic-chain structure is reported to be able to serve as a favorable cathode material for high-performance magnesium batteries. Through a surfactant-assisted solution-phase process, sea-urchin-like VS nanodendrites are controllably prepared. Benefiting from the chain-like crystalline structure of VS , the S dimers in the VS nanodendrites provide abundant sites for Mg insertion. Moreover, the VS atomic-chains bonded by weak van der Waals forces are beneficial to the diffusion kinetics of Mg ions inside the open channels of VS . Through a series of systematic ex situ characterizations and density functional theory calculations, the magnesiation/demagnesiation mechanism of VS are elucidated. The VS nanodendrites present remarkable performance for Mg storage among existing cathode materials, exhibiting a remarkable initial discharge capacity of 251 mAh g at 100 mA g and an impressive long-term cyclability at large current density of 500 mA g (74 mAh g after 800 cycles).

Rechargeable magnesium (Mg) batteries assembled with dendrite-free, safe, and earth-abundant metal Mg anodes potentially have the advantages of high theoretical specific capacity and energy density. Nevertheless, owing to the large polarity of divalent Mg 2+ ions, the insertion of Mg 2+ into electrode materials suffers from sluggish kinetics, which seriously limit the performance of Mg batteries. Herein, we demonstrate an atomic substitution strategy for the controlled preparation of ultrathin black TiO 2−x (B-TiO 2−x ) nanoflakes with rich oxygen vacancies (OVs) and porosity by utilizing ultrathin 2D TiS 2 nanoflakes as precursors. We find out that the presence of OVs in B-TiO 2−x electrode material can greatly improve the electrochemical performances of rechargeable Mg batteries. Both experimental results and density functional theory simulations confirm that the introduction of OVs can remarkably enhance the electrical conductivity and increase the number of active sites for Mg 2+ ion storage. The vacancy-rich B-TiO 2−x nanoflakes exhibit high reversible capacity and good capacity retention after long-term cycling at large current densities. It is hoped that this work can provide valuable insights and inspirations on the defect engineering of electrode materials for rechargeable magnesium batteries.

MXene, a new class of two-dimensional nanomaterials, have drawn increasing attention as emerging materials for sensing applications. However, MXene-based surface plasmon resonance sensors remain largely unexplored. In this work, we theoretically show that the sensitivity of the surface plasmon resonance sensor can be significantly enhanced by combining two-dimensional Ti 3 C 2 T x MXene and transition metal dichalcogenides. A high sensitivity of 198 ∘ /RIU (refractive index unit) with a sensitivity enhancement of 41.43% was achieved in aqueous solutions (refractive index ∼1.33) with the employment of monolayer Ti 3 C 2 T x MXene and five layers of WS 2 at a 633 nm excitation wavelength. The integration of Ti 3 C 2 T x MXene with a conventional surface plasmon resonance sensor provides a promising approach for bio- and chemical sensing, thus opening up new opportunities for highly sensitive surface plasmon resonance sensors using two-dimensional nanomaterials.

A novel silicon on insulator (SOI) MEMS process has been designed and developed to realize a two axes thermally actuated single crystal silicon micromirror device, which consists of a mirror plate, four flexural springs and four thermal actuators. The mirror plate has the same thickness as a SOI device layer i.e. 4 µm. The SOI layer is selectively thinned down to 2 µm for fabricating flexural springs and thermal actuators. The thinning of the SOI layer is essential to lower (control) the flexural rigidity of the springs and the actuators and thus to achieve a higher tilt angle at low thermal power. The developed single wafer process is based on dry reactive ion etching CMOS compatible chemistries. The minimum chip size design of 1 mm × 1 mm has a 400 µm diameter mirror plate. Other chip designs include the mirror diameters in the range from 200 to 500 µm. This paper also presents a study on the mirror plate curvature, thermal actuation mechanism and the experimental results. The measured maximum angular deflection achieved was 17 • at an operating applied voltage of less than 2 V, and the radius of curvature of the mirror plate was in the range from 20 to 50 mm. The micromirror was developed for a miniature catheter optical probe for optical coherence tomography in vivo imaging. A low cross-sectional size of the probe and higher resolution are essential for investigating inaccessible pathologies in vivo. This required a compact micromirror chip and yet sufficiently large mirror plate (typically ∼500 µm or more), this trade-off was the key motivation for the research presented in this paper.

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A MEMS optical coherence tomography (OCT) probe prototype was developed using a unique assembly based on silicon optical bench (SiOB) methodology. The probe is formed by integrating a three-dimensional (3D) scanning micromirror, gradient refractive index (GRIN) lens and optical fiber on SiOB substrates having prefabricated self-aligned slots. The two-axis scanning micromirror is based on electrothermal actuation with required voltage less than 2 V for mechanical deflections up to 17°. The optical probe was enclosed within a biocompatible, transparent and waterproof polycarbonate tube with a view of in vivo diagnostic applications. The diameter of the miniature probe is less than 4 mm and the length of its rigid part is about 25 mm. The probe engineering and proof of concept of the probe were demonstrated by obtaining en face and three-dimensional OCT images of an IR card used as a standard sample.

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