Echelon grating refractive index sensor.

There are few reports on optical refractive index sensors that have both high resonant-wavelength resolution (RWR) and high refractive index sensitivity (RIS). Herein, based on an echelon grating, we design a refractive index sensor that combines the two advantages together. The principal fringe of echelon grating has a small full width at half maximum and a good signal-to-noise ratio, leading to a high RWR. The wavefront splitting interference makes the sensor have high RIS. The large free spectral range (FSR) of the principal fringes expands the dynamic range of the sensor. The experimentally realized RWR, RIS, and FSR are 2 × 10-2 nm, 1.14 × 104 nm/RIU (RIU: refractive index unit), and 130 nm, respectively. The detection limit of refractive index is 1.59 × 10-6 RIU. The dynamic range of the sensor is 1.14 × 10-2 RIU. In addition, there are schemes to improve RWR and RIS, which can further reduce the detection limit of refractive index. The echelon grating refractive index sensor features low detection limit, low cost, high stability, and good robustness.

Hyperspectral measurement of dispersion in aqueous solution and its effect on ultra-sensitive interferometer.

Since the reported Sellmeier equation of water is fitted with sparse sampling points in the near-infrared region, the simulated refractive index sensitivity of dispersion enhanced interferometers deviates from the true value. Here, we measure the refractive index of aqueous sample based on hyperspectra, and research the effect of dispersion on ultra-sensitive interferometer. A piece of quartz plate is used to generate hyperspectra in the near-infrared region by building a wavefront splitting fiber Mach-Zehnder interferometer (WFSF-MZIs). The refractive index of saline water is tested after measuring the thickness of the quartz plate. By taking the wavelength of 1450 nm as break-point, the empirical dispersion equations of saline water are piecewise fitted. When the normal and abnormal dispersion are taken into account, the theoretical sensitivity of phase compensated WFSF-MZI is in good agreement with the experimental results. Our methodology provides a good reference in designing dispersion sensitized optical refractive index sensor for detecting aqueous samples.

Portable 3D-printed umbrella photoacoustic cell for trace gas detection.

We purpose a novel portable 3D-printed umbrella photoacoustic (PA) cell, to the best of our knowledge, for trace gas detection. Its simulation and structural optimization were performed via finite element analysis using COMSOL software. We investigate the factors affecting the PA signals using both experimentation and theory. By measuring methane, a minimum detection limit of 5.36 ppm (signal-to-noise ratio, 223.8) with a lock-in time of 3s was achieved. The proposed miniature umbrella PA system indicates the potential for a miniaturized and low-cost trace sensor.

Intensity-tunable terahertz bandpass filters based on liquid crystal integrated metamaterials.

In this paper, we demonstrate an intensity-tunable THz bandpass filter by introducing liquid crystal (LC) integrated with asymmetric frequency selective surface (FSS) and subwavelength metal gratings. Here, the tunable THz filter is derived from the inner polarization state conversion in composited devices, and the incident linear polarization can be converted into 90° orthogonal components. By controlling the LC orientation under the applied electric field with the metamaterial electrodes, the polarization conversion process can be actively modulated; thus, the polarization-dependent and tunable THz bandpass filter is achieved. Based on the multilayer design and the inner Fabry-Perot-like resonance mechanism, the LC-integrated metamaterials filter presents better filtering performance than the single FSS filter, and the Q-value is improved from 7.7 to 13.8 at the working frequency. Our simulated work paves the way for the design of new and efficient THz filters.

Alkali etched fiber Mach-Zehnder interferometer with compact sensor head.

We demonstrate a scheme for fabricating compact fiber Mach-Zehnder interferometer (MZI). A section of Ge-doped fiber (GDF) is sandwiched between two single-mode fibers. The sandwich structure is side polished to make the core of GDF exposed to the surroundings. Alkali solution is utilized to etch the core of GDF. A compact fiber MZI is achieved when about half of the core is etched. Compared with the traditional ones, our scheme for fabricating fiber MZI has the characteristics of low cost, environmentally friendly, and regular transmission spectrum. This fiber MZI not only reduces the consumption of the sample, but also brings forth a good potential for micro-scale detection of refractive index.

Femtosecond laser trapping nanoprinting of silver micro/nanostructures.

In this paper, silver micro/nanostructures composed of sintered nanoparticles were printed by capturing silver nanoparticles in water with 800 nm femtosecond laser trapping. Relationships of laser power, scanning speed, nanoparticle concentration, and the width and morphology of fabricated silver wire were systematically investigated. It is found that low scanning speed and high nanoparticle concentration favor the printing of silver wire with good morphology. A silver wire with width of 305 nm was printed. Electrical resistivities of printed wires are about 24 times that of bulk silver. Silver grid structures and dot arrays were printed by using this technology. Several three-dimensional silver cuboid structures were also printed. This work provides a protocol for printing of three-dimensional metallic micro/nanostructures using laser trapping. These printed structures have great application prospects in metamaterials, flexible electronics, and SERS.

Refractive index modulation in magnetophoresis of bioreaction induced self-assembled magnetic fluid.

We explore the refractive index (RI) modulation of bioreaction induced self-assembled magnetic fluid (SAMF) so as to facilitate the design of sensitive biosensors. Bovine serum albumin (BSA) is taken as a model protein to induce self-assembly of carboxylated magnetic nanoparticles (MNPs). A bidirectional magnetophoresis (BDMP) system is designed to generate uniform and large RI modulation to SAMF. A robust Mach-Zehnder interferometer compatible to the BDMP system is used for the RI detection. It is found that BDMP has greater ability in modulating the RI of magnetic fluid than the alignment of MNPs in a uniform magnetic field. The modulation depth of the RI increases with the increase of BSA concentration when applying a certain external magnetic field for a period of time. A larger magnetic field has greater ability to modulate the RI of SAMF. However, it may generate an over-modulation.

Photopatterning and Electrochemical Energy Storage Properties of an On-Chip Organic Radical Microbattery.

One potential way to fabricate battery-on-chip is photopatterning electrochemical energy storage materials directly on electronics through lithography, but applicable materials are primarily limited to transparent photocurable resins. The transparency of the photoresist would be sacrificed after extra addition of insoluble inorganic battery materials and conductors. Given the importance of radical polymers for their appropriate solubility, optical transparency, and radical robustness, they may have potential application in on-chip energy storage, transport, and conversion devices. Herein, an anodic photoresist is proposed by modifying the MicroChem SU8 resist with a radical polymer poly(2,2,6,6-tetramethyl-4-piperidinyl-N-oxyl methacrylate) and an ionic conductor lithium perchlorate. It can be photopatterned on silicon wafer with 10 µm scale resolution, and it exhibits charge/discharge potentials at ca. 0.68 V versus silver chloride electrode; the coulomb efficiency is regarded as nearly equaling 100%. Although the specific capacity of the photopatterned film electrode is found to be modest, 1 × 10-5 mA h·cm-2, it presents 1/8 of its theoretical electron storage ability. All-solid-state half-cells with circular features 30 µm in diameter are prepared by means of overlay exposure using the as-prepared photoresist and lithium perchlorate-modified SU8 as the anodic electrode and solid electrolyte, respectively. These results suggest a promising way of using radical polymers for the integration of electrochemical energy in microelectronics.