Mode localization in plasmonic optomechanical resonators for ultrasensitive infrared sensing.

Uncooled infrared thermal detectors are gaining increasing attention owing to their ability to operate at room-temperature and their low cost. This study proposes a plasmonic optomechanical resonator for ultrasensitive long-wave infrared wave sensing based on mode localization mechanism. The mode-localized effect confines the plasmonic energy in the resonators and induces a significant modal amplitude shift through infrared irradiation, thus achieving highly sensitive detection. The results show that the detection sensitivity can reach 1.304 /mW, which is three-order improvement compared to the frequency-shift sensing metrics. The research provides a new approach to further improve the detection sensitivity of uncooled infrared sensors.

Research Progress on Intrinsically Conductive Polymers and Conductive Polymer-Based Composites for Electromagnetic Shielding.

Electromagnetic shielding materials are special materials that can effectively absorb and shield electromagnetic waves and protect electronic devices and electronic circuits from interference and damage by electromagnetic radiation. This paper presents the research progress of intrinsically conductive polymer materials and conductive polymer-based composites for electromagnetic shielding as well as an introduction to lightweight polymer composites with multicomponent systems. These materials have excellent electromagnetic interference shielding properties and have the advantages of electromagnetic wave absorption and higher electromagnetic shielding effectiveness compared with conventional electromagnetic shielding materials, but these materials still have their own shortcomings. Finally, the paper also discusses the future opportunities and challenges of intrinsically conductive polymers and composites containing a conductive polymer matrix for electromagnetic shielding applications.

Effect of Electrospinning Network Instead of Polymer Network on the Properties of PDLCs.

In this study, polymer-dispersed liquid crystal (PDLC) membranes were prepared by combining prepolymer, liquid crystal, and nanofiber mesh membranes under UV irradiation. EM, POM, and electro-optic curves were then used to examine the modified polymer network structure and the electro-optical properties of these samples. As a result, the PDLCs with a specific amount of reticular nanofiber films had considerably improved electro-optical characteristics and antiaging capabilities. The advancement of PDLC incorporated with reticulated nanofiber films, which exhibited a faster response time and superior electro-optical properties, would greatly enhance the technological application prospects of PDLC-based smart windows, displays, power storage, and flexible gadgets.

Fluorescent Dye-Doped Brightening Polymer-Stabilized Bistable Cholesteric Liquid Crystal Films.

Brightening polymer-stabilized bistable cholesteric liquid crystal (PSBCLC) films with doped fluorescent dyes were prepared using the polymerization-induced phase separation (PIPS) method. The transmittance performance behavior of these films in both states (focal conic and planar) and absorbance change in multiple dye concentrations were studied using a UV/VIS/NIR spectrophotometer. The change occurring in dye dispersion morphology with different concentrations was obtained by means of the polarizing optical microscope. The maximum fluorescence intensity of different dye-doped PSBCLC films was measured using a fluorescence spectrophotometer. Moreover, the contrast ratios and driving voltages of these films were calculated and recorded to demonstrate film performance. Finally, the optimal concentration of dye-doped PSBCLC films with a high contrast ratio and a relatively low drive voltage was found. This is expected to have great potential applications in cholesteric liquid crystal reflective displays.

Slot hybrid-core waveguides for temperature-independent integrated optical sensors.

We propose a novel type of waveguides, called the slot hybrid-core waveguides (HCWs), for temperature-independent integrated optical sensors. The HCWs are composed of different core materials having the opposite thermo-optic coefficients (TOCs) and, therefore, are immune to temperature variations. On this basis, slot HCWs are proposed for the microring resonator-based optical sensors, enabling the sensors to simultaneously present high sensitivities and temperature independence. The temperature-dependent wavelength shifts of the proposed sensors are calculated to be less than 1 pm/K while the sensitivities to the cladding refractive indices attain 468 nm/RIU and 536 nm/RIU, respectively, for the asymmetric and symmetric slot structures.

Sensors for simultaneous measurement of temperature and humidity based on all-dielectric metamaterials.

In this study, a new type of sensors based on all-dielectric metamaterials that can measure temperature and relative humidity simultaneously was designed and theoretically analyzed in detail. The proposed metamaterial sensor consists of a quartz substrate in the bottom layer, polydimethylsiloxane (PDMS) in the middle layer, and a periodic silicon structure on the top layer. CST Studio Suite was used to determine the transmission spectrum of the metamaterials in the near-infrared band using finite integration, and two transmission dips were observed. Then, polyvinyl alcohol (PVA) was used as the humidity-sensitive material to be coated on the surface of this metamaterial sensor, and these two transmission dips were used to measure the temperature and relative humidity simultaneously. Simulation results showed that the sensitivities of the two dips to the temperature are -0.224 and -0.069 nm/°C, and the sensitivities to the relative humidity are -0.618 and -0.521 nm/%, respectively. Based on the sensitivity matrix, the temperature and the relative humidity can be measured simultaneously. The proposed sensor has the advantages of polarization insensitivity, small size and low loss, which makes it have many application potentials in various research fields, including physics, biology and chemical sensing.

Flexible graphene-based metamaterial sensor for highly sensitive detection of bovine serum albumin.

A graphene-based metamaterial sensor working in the terahertz spectrum is proposed, simulated, and experimentally verified by measuring bovine serum albumin (BSA). Flexible, low-cost polyimide (PI) is used as the substrate, and aluminum with periodic square rings is chosen as the metal layer. Furthermore, the introduction of the graphene monolayer interacts with the molecules through π-π stacking, resulting in the highly sensitive detection of BSA by calculating the amplitude changes at the resonance frequency. The sensor, which is a biosensor platform that offers the advantages of a small size, high sensitivity, and easy fabrication, is a promising method for THz biological detection.

Broadband terahertz metamaterial absorber: design and fabrication.

We proposed and experimentally demonstrated a broadband terahertz (THz) metamaterial absorber based on a symmetrical L-shaped metallic resonator. The absorber structure produces two absorption peaks at 0.491 and 0.73 THz, with the absorption rates of 98.6% and 99.6%, respectively. Broadband absorption was obtained from 0.457 to 1 THz, achieving a >90% absorption bandwidth of 0.543 THz. By analyzing the distributions of the electric and magnetic field at the two resonance frequencies, electric and magnetic dipole resonances were proposed to explain the broadband absorption mechanism. Furthermore, various widths and lengths of the symmetrical L-shaped metallic resonator on the absorption characteristics were investigated. Moreover, the broadband absorption characteristic can be maintained with an incident angle of up to 45° for transverse-electric and 30° for transverse-magnetic polarization. Finally, we experimentally observed a >70% broadband absorption characteristic from 0.42 to 1 THz. This proposed absorber has the potential for bolometric imaging, modulating, and spectroscopy in the THz region.

High sensitivity temperature sensor based on a side-hole fiber.

This paper proposes a temperature sensor based on a side-hole fiber (SHF). The sensor is formed by single-mode fiber (SMF)-coreless fiber (CLF)-SHF-CLF-SMF fusion splicing. The SHF adopts the dislocation fusion splicing method to ensure that one air hole is exposed. Two different interferences form a superposition, making the response more sensitive. The experiment shows that the sensitivity during heating and cooling is 1.587 nm/°C and 1.681 nm/°C, respectively, in the temperature range of 25-45°C. The sensor has high temperature sensitivity, exhibits easy processing, is smaller in size, and has important research value for temperature monitoring in daily life and industrial production.

Optical Fiber Based Mach-Zehnder Interferometer for APES Detection.

A 3-aminopropyl-triethoxysilane (APES) fiber-optic sensor based on a Mach-Zehnder interferometer (MZI) was demonstrated. The MZI was constructed with a core-offset fusion single mode fiber (SMF) structure with a length of 3.0 cm. As APES gradually attaches to the MZI, the external environment of the MZI changes, which in turn causes change in the MZI's interference. That is the reason why we can obtain the relationships between the APES amount and resonance dip wavelength by measuring the transmission variations of the resonant dip wavelength of the MZI. The optimized amount of 1% APES for 3.0 cm MZI biosensors was 3 mL, whereas the optimized amount of 2% APES was 1.5 mL.

Microfluidic flow direction and rate vector sensor based on a partially gold-coated TFBG.

In microfluidic chips applications, the monitoring of the rate and the direction of a microfluidic flow is very important. Here, we demonstrate a liquid flow rate and a direction sensor using a partially gold-coated tilted fiber Bragg grating (TFBG) as the sensing element. Wavelength shifts and amplitude changes of the TFBG transmission resonances in the near infrared reveal the direction of the liquid flowing along the fiber axis in the vicinity of the TFBG due to a nanoscale gold layer over part of the TFBG. For a device length of 10 mm (and a diameter of 125 µm for easy insertion into microfluidic channels), the flow rates and the direction can be detectable unequivocally. The TFBG waveguiding properties allow such devices to function in liquids with refractive indices ranging from 1.33 to about 1.40. In addition, the proposed sensor can be made inherently temperature-insensitive by referencing all wavelengths to the wavelength of the core mode resonance of the grating, which is isolated from the fiber surroundings.

Tunable broadband all-silicon terahertz absorber based on a simple metamaterial structure.

We propose and demonstrate a modulatable all-silicon terahertz absorber based on a cylindrical metamaterial structure. Broadband absorption is obtained from 0.86 to 2.00 THz, with an average absorbance of 94%, indicating a wide absorption bandwidth of 1.14 THz. The maximum absorption, around 1.24 THz, is up to 98%. We employ simulation results to investigate the physical properties of the absorption, and we attribute the broadband absorption to a combination of electric dipole and magnetic dipole modes. Furthermore, the tunable response of the all-silicon terahertz absorber under the optical pump beam, with different fluences, is studied using a hierarchical model for simulating the carrier density of the gradient distribution. Moreover, different polarizations and oblique incidences of terahertz waves are used to verify the polarization and angle-of-incidence insensitivity of the device. The absorber provides a simple method to design a modulated broadband terahertz absorber, and the design scheme is scalable to develop various tunable broadband absorbers at other frequencies. This work holds great potential in modulator applications, imaging devices, and energy conversion.

Analogue of electromagnetically induced transparency with high-Q factor in metal-dielectric metamaterials based on bright-bright mode coupling.

Based on bright-bright mode coupling, we numerically demonstrated the analogue of electromagnetically induced transparency (EIT) with a high quality factor (Q) in a stacked metal-dielectric metamaterial (MM) in the near-infrared regime. The optical coupling between a high-Q toroidal dipole mode supported by a silicon rod array and a low-Q dipole mode supported by a silver strip array was investigated from the near-field to the far-field regimes. We realized and significantly enhanced the long-range coupling between the two resonance modes through the MM-induced Fabry-Pérot (FP) effect. EIT with a Q factor greater than 1×104 could be achieved even when the two resonant structures were approximately a wavelength apart. These findings may open new avenues for realizing high-Q EIT, which is useful for photonic devices and biosensing applications. The proposed method can be extended to microwaves and terahertz waves.

Uniform-loss cyclic arrayed waveguide grating router using a mode-field converter based on a slab coupler and auxiliary waveguides.

We designed and fabricated a silica-based 16×16 cyclic arrayed waveguide grating router (AWGR) with improved channel loss uniformity using a simple mode-field converter composed of a slab coupling region and auxiliary waveguides placed at the interface between the arrayed waveguides and the output star coupler of the AWGR. The mode-field converter transforms the fundamental Gaussian-shaped mode in the arrayed waveguides to a complex mode field which produces a flat-top far field at the image plane of the output star coupler. It does not change the overall construction of the AWGR and does not increase the device size. The experimental results show that loss non-uniformity for a 16×16 AWGR with 1.6 nm wavelength channel spacing is reduced from 3 to 0.5 dB after adapting the mode-field converter, and the crosstalk is improved by about 2 dB.

Spectrometer based on parallel-plate waveguides utilizing abnormal transmission.

We designed a new, simple device based on parallel-plate waveguides (PPWGs) utilizing abnormal transmission to achieve the analogous function of a terahertz spectrometer. The phase gradient of the PPWGs is π/mm, and the designed wavelength is 0.3 THz. When abnormal transmission occurs, the position of outgoing light changes with the wavelength of the incident light with a quadratic function relationship, which can realize the terahertz spectrometer. The effect of different polarization was verified to be negligible when the polarization angle was 0-10°. The PPWG-based spectrometer has the advantages of simple structure and easy fabrication and can further help the development of terahertz technology.

Reduction of non-uniformity for a 16 × 16 arrayed waveguide grating router based on silica waveguides.

Here, a newly designed 16×16 cyclic arrayed waveguide grating router, capable of achieving loss uniformity across the whole free spectral range (FSR), is fabricated and experimentally demonstrated. This device is based on commonly used silica waveguides, which are compatible with basic planar lightwave circuit technology. The design, simulation, and experimental verification of the proposed method are presented, with the experimental results showing that the loss non-uniformity of FSR reduced from 2.70 to 1.05 dB using this new design. This design brings in only tiny changes of mask and real estate.

Thermal stability of optical fiber metal organic framework based on graphene oxide and nickel and its hydrogen adsorption application.

The electrochemistry (EC) method was used to synthesize graphene oxide-nickel (GO-Ni) metal organic framework (MOF) that has the thickness of µm-level. The MOF's thermal stability and hydrogen adsorption and desorption capacity were measured by using an optical fiber Mach-Zehnder interferometer (MZI) sensor. This MZI was fabricated by core-offset fusion splicing one section of single mode fiber (SMF) between two SMFs. Experimental results showed that the GO-Ni MOF could be stabilized, even as the environmental temperature reached 125 °C. The MOF showed good hydrogen adsorption ability for the the MOF and hydrogen molecules's interactions.

Bessel-like beam generated by an axicon based on parallel-plate waveguides.

The axicon is the simplest and most effective optical element for generating the zero-order Bessel-like beam. The zero-order Bessel-like beam, which has the characteristics of small spot size, high brightness, good direction, and large collimation distance, can be applied to optical micromanipulation and power transmission. In this paper, we proposed and designed a structure for phase manipulation based on parallel-plate waveguides that can be used to realize the functionality of the axicon in the terahertz (THz) region. Meanwhile, we characterized the influence of the cone angle of the axicon and the waist radius of the incident Gaussian beam on the generated zero-order Bessel-like beam by simulation. The planar structure, consisting of a parallel stack of thin copper plates, can be easily fabricated to fulfill the phase requirement to realize the zero-order Bessel-like beam and also can be utilized in THz imaging systems, THz sensing systems, THz communication systems, etc.

Simultaneous measurement of refractive index and temperature based on all-dielectric metasurface.

In this paper, a novel kind of sensors for simultaneous measurement of refractive index and temperature based on all-dielectric metasurfaces is proposed. The metasurfaces are constructed by an array of silicon nanoblocks on top of the bulk fused silica substrate. We used three-dimensional full wave electromagnetic field simulation by finite integral method to accurately calculate the transmission spectrum of the metasurfaces. Two transmission dips corresponding to the electric and magnetic resonances are observed. Both dips shift as the ambient refractive index or the temperature changes. Simulation results show that the sensing sensitivities of two dips to the refractive index are 243.44 nm/RIU and 159.43 nm/RIU, respectively, while the sensitivities to the temperature are 50.47 pm/°C and 75.20 pm/°C, respectively. After introducing four holes into each silicon nanoblock, the electromagnetic field overlap in the surrounding medium can be further promoted, and the sensitivities to the refractive index increase to 306.71 nm/RIU and 204.27 nm/RIU, respectively. Our proposed sensors have advantages of polarization insensitive, small size, and low loss, which offer them high potential applications in physical, biological and chemical sensing fields.

Ultracompact silicon-on-insulator-based reflective arrayed waveguide gratings for spectroscopic applications.

Ultracompact reflective arrayed waveguide gratings (RAWGs) employing a half horseshoe-shaped waveguide layout and distributed Bragg reflector mirrors in the array region are designed and fabricated. Two sets of RAWGs with 400 and 200 GHz channel spacing are experimentally demonstrated for TE and TM polarizations, respectively. Because of the high-index contrast between the silicon core and the oxide cladding, these RAWGs have very compact sizes. With the off-centered light input, we obtained the minimal on-chip losses of 7 and 9 dB and cross talks of <-8 and <-5 dB for 9×400 GHz and 20×200 GHz RAWGs, respectively, for TE polarization. The measured minimal on-chip losses are 10 and 12.5 dB, and cross talks are <-11 and <-7 dB for 8×400 GHz and 10×200 GHz RAWGs, respectively, for TM polarization. These RAWGs can find applications for on-chip spectroscopic sensing.