Tilted Bragg grating in a glycerol-infiltrated specialty optical fiber for temperature and strain measurements.

In this Letter, we propose an in-line tilted fiber Bragg grating sensor for temperature and strain measurements. The grating is inscribed in a specialty optical fiber using tightly focused femtosecond laser pulses and the line-by-line direct writing method. Beside the central core in which the grating is produced, a hollow channel filled with glycerol aqueous solution significantly improves the sensitivity of the fiber cladding modes due to its high thermo-optic coefficient. We show that the temperature sensitivity of the core mode is 9.8 pm/°C, while the one of the cladding modes is strongly altered and can reach -24.3 pm/°C, in the investigated range of 20-40°C. For the strain measurement, sensitivities of the core mode and the cladding modes are similar (∼0.60 pm/µÎµ) between 0 and 2400 µÎµ. The significative difference of temperature sensitivity between the two modes facilitates the discrimination of the dual parameters in simultaneous measurements.

Actively tunable linear and circular dichroic metamirrors based on single-layer graphene.

Aiming at the problems of low efficiency, single function and complex structure of the existing dichroic metamirrors, the actively tunable linear and circular dichroic metamirrors based on single-layer graphene are proposed in this study. The designed metamirrors are mainly composed of the ion-gel, patterned graphene, polyimide, polysilicon and gold substrates. The anisotropy of the achiral structures can be used to realize circular dichroism (0.8) and linear dichroism (0.9) in two directions at the same time without functional switching. Additionally, the incidence angle of electromagnetic waves, rather than the structural chirality, is used to create the exceptionally strong dichroism. The proposed metamirrors not only increase the integration, but also reduce the angular dispersion and complexity of the structure. What's more, by changing the Fermi level of graphene, the CD function of the metamirrors can be tuned in the range of 0 - 0.8, and the LD function can be tuned in the range of 0.22 - 0.9. The designed metamirrors can achieve dual functions under a wide range of incident angles, and can be widely used in various fields such as terahertz imaging, biological detection, optical sensing, and spectrometry.

Terahertz broadband tunable chiral metamirror based on VO2-metal hybrid structure.

Aiming at the problems of narrow working bandwidth, low efficiency, and complex structure of existing terahertz chiral absorption, we propose a chiral metamirror composed of C-shaped metal split ring and L-shaped vanadium dioxide (VO2). This chiral metamirror is composed of three layers of structure, a gold substrate at the bottom, the first polyethylene cyclic olefin copolymer (Topas) dielectric layer and VO2-metal hybrid structure as the top. Our theoretical results led us to show that this chiral metamirror has a circular dichroism (CD) value greater than 0.9 at 5.70 to 8.55 THz and has a maximum value of 0.942 at f = 7.18 THz. In addition, by adjusting the conductivity of VO2, the CD value can be continuously adjustable from 0 to 0.942, which means that the proposed chiral metamirror supports the free switching of the CD response between the on and off states, and the CD modulation depth exceeds 0.99 in the range of 3 to 10 THz. Moreover, we discuss the influence of structural parameters and the change of incident angle on the performance of the metamirror. Finally, we believe that the proposed chiral metamirror has important reference value in the terahertz range for constructing chiral light detectors, CD metamirrors, switchable chiral absorbers and spin-related systems. This work will provide a new idea for improving the terahertz chiral metamirror operating bandwidth and promote the development of terahertz broadband tunable chiral optical devices.

Multidimensional tunable graphene chiral metasurface based on coherent control.

The deep application of chiral metasurfaces requires higher flexibility. Herein, we propose a multidimensional tunable chiral graphene metasurface, which uses coherent control to obtain more than 0.8 circular conversion dichroism (CCD) at 2.4 THz as a transmission structure. Its operating frequency can be changed in the 1.3-2.4 THz range, while the amplitude has almost perfect modulation depth in the range of 0-0.8. The mechanism of differential absorption was analyzed through numerical simulation. The device designed is easy to obtain reverse CCD, which is used for unit layout and proves its advantages in near-field imaging. Our work has broadened the path for the development of chiral metasurfaces towards higher degrees of freedom.

Adjustable strong circular dichroism based on a tricircular arc metasurface.

Circular dichroism has promising applications in biology, molecular chemistry, and other fields. The key to obtaining strong circular dichroism is to introduce symmetry breaking into the structure, which leads to a great difference in the response to different circularly polarized waves. Here, we propose a metasurface structure based on three circular arcs, which produces strong circular dichroism. The metasurface structure combines the split ring with the three circular arcs and increases the structural asymmetry by changing the relative torsional angle. The causes of the strong circular dichroism are analyzed in this paper, and the influence of metasurface parameters on it is discussed. According to the simulation data, the response of the proposed metasurface to different circularly polarized waves varies greatly, with absorption of up to 0.99 at 5.095 THz for a left-handed circularly polarized wave and a maximum circular dichroism of over 0.93. In addition, the incorporation of the phase change material vanadium dioxide on the structure allows flexible modulation of circular dichroism and modulation depths of up to 98.6%. The change of angle within a certain range has little effect on the structural performance. We believe that this flexible and angle robust chiral metasurface structure is suitable for complex reality, and large modulation depth is more practical.

Ultra-compact on-chip meta-waveguide phase modulator based on split ring magnetic resonance.

With the development of photonic integration technology, meta-waveguides have become a new research hotspot. They have broken through the theoretical diffraction limit by virtue of the strong electromagnetic manipulation ability of the metasurface and the strong electromagnetic field limitation and guidance ability of the waveguide. However, the reported meta-waveguides lack research on dynamic modulation. Therefore, we analyze the modulation effect of the metasurface on the optical field in the waveguide and design an ultra-compact on-chip meta-waveguide phase modulator using split ring magnetic resonance. It has a very short modulation length of only 3.65 µm, wide modulation bandwidth of 116.8 GHz, and low energy consumption of 263.49 fJ/bit. By optimizing the structure, the energy consumption can be further reduced to 90.69 fJ/bit. Meta-waveguides provide a promising method for the design of integrated photonic devices.

Nested micro-ring refractive index sensor based on a subwavelength grating waveguide and the Vernier effect.

In this paper, a nested micro-ring refractive index sensor based on a subwavelength grating waveguide and the Vernier effect is proposed. In this scheme, the nested micro-ring structure is combined with a subwavelength grating structure to enhance the contact area between the optical field and the analyte, and the wavelength offset is doubled through the Vernier effect. The proposed sensor can effectively increase sensing sensitivity, taking into account the improvement of the free spectral range. This structure enables the device to reach a sensitivity of 8030 nm/RIU near 1550 nm wavelength in a deionized water environment, with a detection limit of 5.659×10-5 RIU and free spectral range of 41.956 nm. The device suggested in this study has a greater reduced footprint than the conventional micro-ring resonant sensor, measuring just 35µm×25µm. Due to its high integration, high sensitivity, and large free spectral range compared to conventional micro-ring resonant sensors, such structures are of great value in biosensing and environmental monitoring.

Semi-embedded slot waveguide electro-optic modulator.

Electro-optic modulators are essential devices on silicon photonic chips in modern optical communication networks. This paper presents a compact, low-loss electro-optic modulator. The modulation efficiency is greatly improved by embedding the lower half of the slot waveguide into the buried oxide layer and inserting graphene at the junction. The interaction of graphene with an optical field in a waveguide is studied using the finite element method. The functions of phase modulation and absorption modulation are realized by changing the gate voltage to change the chemical potential of graphene. The semi-embedded slot waveguide optical modulator has a length of 50 µm. After simulation verification, it can be used as an electro-absorption modulator and can achieve a modulation depth of 26.38 dB and an insertion loss of 0.60 dB. When used as an electro-refractive modulator, it can be realized with a linear change of phase from zero to π; the total insertion loss is only 0.59 dB. The modulator has a modulation bandwidth of 79.6 GHz, and the energy consumption as electro-absorption and electro-refraction modulation are 0.51 and 1.92 pj/bit, respectively. Compared with common electro-optic modulators, the electro-optic modulator designed in this paper has a higher modulation effect and also takes into account the advantages of low insertion loss and low energy consumption. This research is helpful for the design of higher-performance optical communication network devices.

SPP excitation and coupling mechanism based on micro/nano fibers.

A hot trend in the development of optoelectronic devices is how to use the principle of surface plasmon resonance to enhance the performance of integrated photonics devices and achieve miniaturization. This paper proposes an accompanying waveguide coupling structure of micro/nano fibers, which consists of two parallel-placed micro/nano fibers (MNFs) coated with a silver film in the waist region and infused with a refractive index matching oil. In the overlapping region, there exists a segment of surface plasmon polaritons (SPPs) coupling area. The excitation and coupling characteristics of SPPs are studied through numerical simulation. Optimal coupling enhancement configuration is obtained by studying variables such as spacing distance, coupling length, and metal film thickness. A comparison is made with the SPP intensity of a single MNF, showing a 220% increase in electric field intensity, demonstrating its excellent coupling effect. By using this coupling structure, exploration of SPPs excitation and coupling mechanisms is enhanced, and structures resembling interferometric devices can be designed, providing new insights for high-performance miniaturized devices.

Influence of Annealing on Polymer Optical Fiber Bragg Grating Inscription, Stability and Sensing: A Review.

This article reviews recent research progress on the annealing effects on polymer optical fibers (POFs), which are of great importance for inscription, stability and sensing applications of fiber Bragg gratings (FBGs) in POFs due to their unique properties related to polymer molecular chains. In this review, the principle of annealing to reduce frozen-in stress in POFs drawing and different annealing timings are firstly summarized. Then, the annealing methods for POFs are introduced under several different conditions (temperature, humidity, strain, stress and solution). Afterwards, the principle of FBGs and several inscription techniques are reported. Subsequently, the annealing effects on the properties of POFs and polymer optical fiber Bragg gratings (POFBGs) quality are discussed. Finally, the influence of annealing on POFBG sensitivity is summarized. Overall, this paper provides a comprehensive overview of annealing techniques and their impact on both POFs and POFBGs. We hope that it will highlight the important progress made in this field.

Simulation study of a temperature-calibrated double-sided polished optical fiber SPR refractive index sensor.

The temperature of the environment directly affects the accuracy of refractive index (RI) measurement. Therefore, we propose a double-sided polished surface plasmon resonance (SPR) RI fiber sensor, which is available for simultaneous measurement of the RI and temperature in real time. The proposed sensor uses single-mode fiber as a special double-sided polishing structure. The double-sided polishing regions are coated with a gold-silver hybrid film; one side is additionally coated with graphene layers to increase detection sensitivity, and the other side is coated with polydimethylsiloxane on the metal layer for temperature sensing. The simulation result shows that in the range from 1.33 to 1.35, RI sensitivity reaches as high as 2600 nm/RIU. In the range from 15°C to 85°C, temperature sensitivity reaches as high as -3.5n m/∘ C. The full width at half maximum is 65 nm. Compared with previous studies, the sensitivity is slightly improved, and an excellent temperature compensation effect can be achieved. It is suitable for high-precision measurement of the environment and biochemical aspects.

Simulation Study of High Sensitivity Fiber SPR Temperature Sensor with Liquid Filling.

In this paper, a high sensitivity fiber temperature sensor based on surface plasmon resonance is designed and studied. In the simulation, the single mode fiber is polished to remove most of the cladding, and then gold and silver films are added. Finally, it is embedded in the heat shrinkable tube filled with a thermo-optic coefficient liquid for curing. The numerical simulation results show that the sensing characteristics are sensitive to the remaining cladding thickness of the fiber, the thickness of the gold film and the thickness of the silver film. When the thermo-optic coefficient of the filling liquid is -2.8 × 10-4/°C, the thickness of the gold film, the thickness of the silver film and the thickness of the remaining cladding of the fiber are 30 nm, 20 nm and 1 µm, respectively. The sensitivity of the sensor designed in this paper can reach -6 nm/°C; this result is slightly higher than that of similar research in recent years. It will have a promising application prospect in flexible wearable temperature sensors, smart cities and other fields.

Side-Polish Plastic Optical Fiber Based SPR Sensor for Refractive Index and Liquid-Level Sensing.

In this work, a simple side-polish plastic optical fiber (POF)-based surface plasmon resonance (SPR) sensor is proposed and demonstrated for simultaneous measurement of refractive index (RI) and liquid level. The effects of side-polish depths on the sensing performance were studied. The experimental results show that the SPR peak wavelength will be changed as the RI changes, and the SPR peak intensity will be changed with the liquid level variation. By monitoring the changes in peak wavelength and intensity, the RI and liquid level can be detected simultaneously. Experimental results show that an RI sensitivity of 2008.58 nm/RIU can be reached at an RI of 1.39. This sensor has the advantages of simple structure and low cost, which has a good prospect in the field of biochemical sensing.

Tunable circular dichroism based on graphene-metal split ring resonators.

The chiroptical response of the chiral metasurface can be characterized by circular dichroism, which is defined as the absorption difference between left-handed circularly polarized incidence and right-handed circularly incidence. It can be applied in biology, chemistry, optoelectronics, etc. Here, we propose a dynamically tunable chiral metasurface structure, which is composed of two metal split-ring resonators and a graphene layer embedded in dielectric. The structure reflects right-handed circularly polarized waves and absorbs left-handed circularly polarized waves under normal incidence. The overall unit structural parameters of the chiral metasurface were discussed and analyzed, and the circular dichroism was 0.85 at 1.181 THz. Additionally, the digital imaging function can be realized based on the chiral metasurface structure, and the resolution of terahertz digital imaging can be dynamically tuned by changing the Fermi level of graphene. The proposed structure has potential applications in realizing tunable dynamic imaging and other communication fields.

Dual-color meta-image display with a silver nanopolarizer based metasurface.

Plasmonic metallic nanostructures with anisotropic design have unusual polarization-selective characteristic which can be utilized to build nanopolarizers at the nanoscale. Herein, we propose a dual-color image display platform by reconfiguring two types of silver nanoblocks in a single-celled metasurface. Governed by Malus's law, the two types of silver nanoblocks both acting as nanopolarizers with different orientations can continuously modulate the intensity of incident linearly polarized red and green light pixel-by-pixel, respectively. As a result, an ultra-compact, high-resolution, and continuous-greyscale dual-color image can be recorded right at the surface of the meta-device. We demonstrate the dual-color Malus metasurface by successfully encoding and decoding a red-green continuously-grayscale image into a metasurface sample. The experimentally captured meta-image with high-fidelity and resolution as high as 63500 dots per inch (dpi) has verified our proposal. With the advantages such as continuous grayscale modulation, ultrathin, high stability and high density, the proposed dual-color encoded metasurfaces can be readily used in ultra-compact image displays, high-end anti-counterfeiting, high-density optical information storage and information encryption, etc.

An Enhanced Plastic Optical Fiber-Based Surface Plasmon Resonance Sensor with a Double-Sided Polished Structure.

An enhanced plastic optical fiber (POF)-based surface plasmon resonance (SPR) sensor is proposed by employing a double-sided polished structure. The sensor is fabricated by polishing two sides of the POF symmetrically along with the fiber axis, and a layer of Au film is deposited on each side of the polished region. The SPR can be excited on both polished surfaces with Au film coating, and the number of light reflections will be increased by using this structure. The simulation and experimental results show that the proposed sensor has an enhanced SPR effect. The visibility and full width at half maximum (FWHM) of spectrum can be improved for the high measured refractive index (RI). A sensitivity of 4284.8 nm/RIU is obtained for the double-sided POF-based SPR sensor when the measured liquid RI is 1.42. The proposed SPR sensor is easy fabrication and low cost, which can provide a larger measurement range and action area to the measured samples, and it has potential application prospects in the oil industry and biochemical sensing fields.

Intensity-Modulated Polymer Optical Fiber-Based Refractive Index Sensor: A Review.

The simple and highly sensitive measurement of the refractive index (RI) of liquids is critical for designing the optical instruments and important in biochemical sensing applications. Intensity modulation-based polymer optical fiber (POF) RI sensors have a lot of advantages including low cost, easy fabrication and operation, good flexibility, and working in the visible wavelength. In this review, recent developments of the intensity modulation POF-based RI sensors are summarized. The materials of the POF and the working principle of intensity modulation are introduced briefly. Moreover, the RI sensing performance of POF sensors with different structures including tapered, bent, and side-polished structures, among others, are presented in detail. Finally, the sensing performance for different structures of POF-based RI sensors are compared and discussed.

Spin-orbital coupling of quadratic-power-exponent-phase vortex beam propagating in a uniaxial crystal.

Recent studies have shown that quadratic-power-exponent-phase (QPEP) vortex and modified QPEP vortex have some novel properties and potential applications in optical manipulation, orbital angular momentum (OAM) communication, OAM multicasting and so on. In these applications, there may be potential need of processing these kinds of beams by using uniaxial crystals. In this paper, the analytical propagation equations of Gaussian QPEP vortex and modified QPEP vortex propagating in uniaxial crystals are derived and the evolution of the angular momentum via spin-orbital coupling during the propagation is investigated. This may be meaningful for guiding and promoting the applications of the QPEP vortex and modified QPEP vortex.

Polarization controllable generation of flat superimposed OAM states based on metasurface.

In this paper, two kind of metasurface-based flat orbital angular momentum (OAM) superposition-state generators are proposed, which can generate OAM superposition states possessing tens of OAM modes being evenly spaced by topological charge number 1. The power spectra of the generated OAM superposition states are flat. The power variation of the OAM modes of the OAM superposition states from one generator is less than 3 dB, and the power variation of the OAM modes of the OAM superposition states from the other one is less than 0.3 dB. By controlling the left-handed and right-handed circular polarization states of the incident light, the OAM spectra of the OAM superposition states generated in the two polarization cases are separated from each other, therefore, the proposed generators are light polarization controllable. In addition, the two generators can operate efficiently on a wide wavelength region ranging from 635nm to 730nm. Our work may have some potential applications, such as used for OAM multicasting, OAM based optical manipulation, or manufacturing integrated OAM-superposition-state generators and OAM modulation devices.

Displacement Sensor Based on a Small U-Shaped Single-Mode Fiber.

A simple structure and easily fabricated displacement sensor was proposed and demonstrated based on a bending-induced fiber interferometer. In the design, the fiber interferometer was formed only by bending the single-mode fiber into a small U-shape without splicing, tapering, or heating pre-processing, which effectively reduces the complexity of the fabrication process, greatly enhances the mechanical strength of the sensor, and lowers the cost in the displacement sensing applications. The displacement sensing performances for the sensor with different bending radii of 3.3 mm, 4.4 mm, 5.0 mm, and 6.3 mm were investigated. Experimental results showed that the sensor had a good linear response, and for the bending radii of 3.3, 4.4, 5.0, and 6.3 mm, the proposed sensors showed high sensitivities of 134.3, 105.1, 120.9, and 144.1 pm/µm, respectively.