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.

High-accuracy wide-range refractive index demodulation based on under-sampled fiber F-P cavity length spectrum.

A scheme of fiber Fabry-Perot (F-P) cavity refractive index (RI) demodulation named under-sampled length spectrum retrieval (ULSR) is proposed. Unlike the wavelength spectrum method, ULSR can be used for physical quantity detection with just a monochromatic laser and photodetectors, avoiding the need for wideband lasers or expensive infrared spectrometers. Eight F-P cavities of different lengths were fabricated to sample the cavity length spectrum, and then the obtained under-sampled length spectrum was used to demodulate the RI of F-P cavity fillings. It was demonstrated that the ULSR system can achieve an index measurement accuracy of 1 × 10-4 in the glucose solution index range of 1.3294-1.3746 at wavelength λ = 1.55 µm. An index demodulation with higher accuracy and wider range is expected when more than 8 F-P cavities are used. The proposed scheme, with advantages of low system complexity, low cost, high reliability, high detecting accuracy, and wide detecting range, holds great promise for facilitating the wide application of F-P cavity sensors. Additionally, ULSR liberates wavelength freedom, making it a strong candidate for multiplexed sensing based on wavelength division multiplexing.

Optical fiber inclinometer with dynamically controllable excitation length of quantum dots liquid-core waveguide based on a photo-controlled bubble.

An ultracompact fiber inclinometer based on a bubble controlled by Marangoni force is proposed in this Letter. By coupling a 980-nm laser, the bubble can suspend in a quantum dots (QDs) liquid-core waveguide (LCW) due to the Marangoni effect. Under the excitation of a 405-nm laser, QDs LCW exhibit green emissions centered at 523 nm. When the tilt angle changes, the position of the bubble changes as well, which causes the variation of the 523-nm fluorescence intensity. The experimental results show that the sensitivity based on the peak intensity ratio (PIR) reaches 0.22/° with a linearity of 0.979 from 0° to 35°. Furthermore, the sensor has excellent stability and repeatability.

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.

Simultaneous Measurement of Refractive Index and Temperature Based on SMF-HCF-FCF-HCF-SMF Fiber Structure.

In this research, we proposed and experimentally verified a compact all-fiber sensor that can measure refractive index (RI) and temperature simultaneously. Two segments of hollow-core fiber (HCF) are connected to the two ends of the four-core fiber (FCF) as a beam splitter and a coupler, and then spliced with two sections of single-mode fibers (lead-in and lead-out SMF), respectively. The two hollow-core fibers can excite the higher-order modes of the four-core fiber and recouple the core modes and higher-order modes into the outgoing single-mode fiber, thereby forming inter-mode interference. The different response sensitivities of two interference dips to RI and temperature manifest that the proposed structure can achieve simultaneous measurement. From the experimental results, it can be seen that the maximum sensitivity of the sensor to RI and temperature is 275.30 nm/RIU and 94.4 pm/°C, respectively. When the wavelength resolution is 0.02 nm, the RI and temperature resolutions of the sensor are 7.74 × 10-5 RIU and 0.335 °C. The proposed dual-parameter optical sensor has the advantages of high sensitivities, good repeatability, simple fabrication, and structure. In addition, it has potential application value in multi-parameter simultaneous measurement.

Bovine serum albumin label-free concentration sensor based on silica corrosion quantitative monitoring system.

Bovine serum albumin (BSA) label-free concentration sensor based on silica corrosion quantitative monitoring system (SCQMS) has been proposed. Anti-resonance of hollow cylindrical waveguide (HCW) in SCQMS is simulated and investigated for monitoring corrosion rate quantitatively. Hydrofluoric acid (HF) samples with different concentrations are studied respectively, and the corrosion rate is obtained by demodulating the corresponding anti-resonance dips shift and free spectral range (FSR). Therefore, a high-precision SQCMS was prepared successfully. On this basis, a highly sensitive concentration sensor based on hole-assisted dual-core fiber (HADF) is prepared. The BSA samples with concentration from 0.2 mg/mL to 0.7 mg/mL are detected. The sensor has a high sensitivity of 30.04 nm/(mg/mL) and ultra-low limit of detection (LOD) of 0.05 mg/mL for the assisted core exposed to the target solution directly. We have demonstrated the SCQMS that can be a feasible tool for precise and quantitative corrosion of silicon structure safely. In addition, the concentration sensor structure has a wide application for ultra-low LOD, simple preparation process and high integration.

Optical fiber sensor based on upconversion luminescence for synchronous temperature and curvature sensing.

A multifunctional optical fiber sensor based upconversion luminescence (UCL) for synchronous temperature and curvature sensing was proposed. The sensor was fabricated by assembling UCL nanoparticles doped by rare earth (RE) ions in polydimethylsiloxane (PDMS) materials. Temperature measurements were achieved through the fluorescent intensity ratio (FIR) technique with the dual green UC luminescence generated. The fabricated sensor provided the temperature sensitivity of 714.82 K-1 with excellent linearity (R2=0.997) at a temperature range of 303 to 423 K. In the lower temperature region, temperature measurement based on the FIR technology is almost independent on the fluorescence intensity of a 525 nm emission peak. Hence, deformation produced by the sensor through bending results in detectable and reversible changes in its reflected light, allowing the curvature to be simultaneously measured. The sensor can monitor temperature and curvature simultaneously, providing a new optical alternative for multi-parameters monitoring in the future.

Detection limit analysis of optical fiber sensors based on interferometers with the Vernier-effect.

Vernier-effect has been widely employed in interferometer-based optical fiber sensors to improve the sensitivities greatly. However, the influence of the Vernier-effect on detection limit (DL) that is more important for evaluating the actual performance of the sensor has not been discussed. Two gas pressure fiber sensors (a typical Fabry-Perot interferometer-based sensor and a Vernier sensor) are used to compare the DL of them by experiments. Both the theoretical analysis and the experimental results show that, though the Vernier-effect magnifies the spectrum shift sensitivity, it also magnifies the value of the smallest detectable wavelength shift. As a result, the actual DL of the sensor is not improved by employing the Vernier-effect. If the contrast ratio of the Vernier envelope is not optimized enough for most of the reported sensors, the DL can even degenerate greatly.

Multifunctional analysis and verification of lightning-type electromagnetic metasurfaces.

Aiming at the problems that most of the existing electromagnetic metasurfaces have single function and narrow application scope, a highly integrated lightning-type metasurface is proposed in this study. It can realize the functions of circular dichroism (CD), absorption of electromagnetic waves, broadband x-to-y cross polarization conversion (CPC) function, linear-to-circular polarization conversion (LTC-PC) function and asymmetric transmission (AT), and its functions are also analyzed and verified. The designed metasurface consists of the bottom grating structure, the lower SiO2, the middle lightning-type graphene, the upper SiO2, the top graphene and photosensitive silicon. Through numerical calculations, the CD of design can reach more than 85% at 4.22 THz. The function of bimodal absorption is achieved at 4.09 and 8.69 THz. At 7.41∼8.21 THz, the polarization conversion ratio (PCR) of the metasurface reaches more than 99%. Simultaneously, the function of LTC-PC can be formed when PCR is 50%. Finally, when the designed metasurface is in the transmissive state, the AT of design is close to 60% at 7.84 THz. This design provides a new design idea and method for biomedical detection, image processing, modulators, smart switches, optical diodes and other fields.

Femtosecond laser direct writing of a 3D microcantilever on the tip of an optical fiber sensor for on-chip optofluidic sensing.

Real-time detection of the concentration of input fluid is essential for optofluidic sensing, especially in the case of biochips and organ-on-a-chip systems. In this paper, a microcantilever structure that enables temperature and liquid concentration sensing was fabricated on the tip of the optical fiber by femtosecond laser direct writing (two-photon polymerization, TPP) technology. An open Fabry-Pérot interferometer (F-P) structure was formed between the end of the optical fiber and the cantilever, so the sensor becomes quite sensitive to the localized temperature, concentration and refractive index of the target liquids. The reasonable size parameters of the cantilever were determined by structural stress analysis and interference spectrum analysis. By integrating the fiber sensor with a microfluidic chip, an on-chip optofluidic sensing platform is developed, which shows high sensitivities of the temperature (92.7 pm °C-1), concentration (0.3287 nm (g L-1)-1), and refractive index (1385.819 nm RIU-1). The reported optofluidic sensing platform demonstrates reasonably high stability and satisfactory sensing effect, holding great promise for applications in lab-on-a-chip systems.

High-temperature-sensitive and spectrum-contrast-enhanced sensor using a bullet-shaped fiber cavity filled with PDMS.

Low temperature sensitivity and low spectral contrast are serious but common issues for most Fabry Perot (FP) sensors with an air cavity. In this paper, a high-temperature-sensitive and spectrum-contrast-enhanced Fabry Perot interferometer (FPI) is proposed and experimentally demonstrated. The device is composed of a hollow cylindrical waveguide (HCW) filled with polydimethylsiloxane (PDMS) and a semi-elliptic PDMS end face. The semi-elliptic PDMS end face increases the spectral contrast significantly due to the focusing effect. Experimentally, the spectral contrast is 11.97 dB, which is two times higher than the sensor without semi-elliptic PDMS end face. Ultra-high temperature sensitivity of 3.1501 nm/°C was demonstrated. The proposed sensor exhibits excellent structural stability, high spectral contrast and high temperature sensitivity, showing great potential in biomedicine, industrial manufacturing, agricultural production and other applications.

Micro-newton strain force and temperature synchronous fiber sensor with a high Q-factor based on the quartz microbubble integrated in the capillary-taper structure.

A micro-newton strain force and temperature synchronous fiber sensor with a high Q-factor is proposed. The sensor is based on a commercial quartz microbubble (QMB, the diameter is less than 80 µm) that is attached to the end surface of the suspending taper integrated in the hollow core fiber. The multi-beam interference and long-active-length make the sensor show both high sensitivity (0.150 nm/mN) and Q-factor (1470 based on the 3dB-bandwidth). The actual detection limit of the strain force reaches about 50 µN. The UV-cured polymer between the QMB and taper improves the temperature sensitivity. The strain force and temperature can be demodulated synchronously by using band-pass filtering and sensing matrix. The sensor can have actual application in micro-newton strain force detection as its low cost and flexible structure.

Ultrafast metamaterial all-optical switching based on coherent modulation.

We report a demonstration of an ultrafast all-optical switching with unique light control effects. The all-optical switching consists of a gold film with asymmetric split rings and a silica substrate. The device effectively controls the transmission and absorption of continuous pulses in the communication band (1200-1800 nm) and short pulses with a pulse duration of 80 fs by using the interaction of two coherent beams on nano-metamaterials with a thickness of only 50 nm. The metamaterial can achieve more than 90 % output control under continuous light irradiation. When the pulse duration is 80 fs, the switching contrast ratio is greater than 3 : 1 and the modulation bandwidth is greater than 12.5 THz. Switching time can be on the order of femtosecond. This paper provides a new structure for ultra-high speed optical data processing components in coherent networks.

Dual-parameter demodulated torsion sensor based on the Lyot filter with a twisted polarization-maintaining fiber.

We proposed a novel torsion sensor based on the Lyot filter with the twisted polarization-maintaining fiber (PMF) acting as the birefringence medium. Lyot filter is formed by two linear polarizers and a piece of PMF. Based on the high birefringence of the PMF, the output polarization rotates with a rate equal to the twisting rate applied on the PMF, and the sensor realizes a high sensitivity of 90.072 dB/rad. The proposed sensor also demonstrated a low strain sensitivity of 2.32 ×10 - 6 rad/µÉ›. On the other hand, based on the phase hits of the polarization interference, the wavelength sensitivity reaches 15.477 nm/rad. The monitoring range of the wavelength demodulation is complementary with the intensity demodulation in one cycle, making the valid sensing range of the proposed sensor expand. The proposed highly sensitive compact torsion sensor, with large sensing range and low crosstalk, has potential applications in many fields such as manufacturing industry, civil engineering, aerospace industry and modern smart structure monitoring.

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.

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.

Laser-induced suspension of a microbubble in a liquid-filled fiber microcavity for large-range tilt sensing.

In this Letter, we propose a compact fiber tilt sensor based on a microbubble suspended in a liquid-filled microcavity at the end of a single-mode fiber. By coupling a single-frequency laser with enough power, the microbubble could suspend in the microcavity due to the Marangoni effect, which constitutes a Fabry-Perot interferometer. When the tilt angle changes, the position of the microbubble changes as well, which causes the variation of the dominant frequency of the interference fringes in the spectrum. The experimental results show that the tilt angle sensitivity of the sensor reaches ${3.64} \times {{10}^{ - 4}}\;{{\rm nm}^{ - 1}}/{\rm Deg}$3.64×10-4nm-1/Deg at a wide sensing range from ${-}{45}^\circ $-45∘ to 45° with a good repeatability.

Vortex chirality-dependent filtering in helically twisted single-ring photonic crystal fibers.

Helically twisted microstructured optical fibers have a wide application prospect in the field of optical vortex communications. In this paper, a helically twisted single-ring photonic crystal fiber (HS-PCF) is proposed for orbital angular momentum (OAM) vortex modes selective filtering. And the theoretical framework of OAMs filtering is also constructed. Positive and negative OAM vortexes have different transmission losses in HS-PCF, and the loss difference between them increases significantly after the twist rate reaches a certain value. Such fibers can filter out OAMs with a certain sign of the topological charge (depending on the handedness of the thread), while dissipating oppositely charged OAMs. In addition to the general OAMs, i.e., zero-order radial vortex modes, the helically twisted fiber also performs a good selective filtering for the first-order radial vortex modes. Remarkably, the filtering bandwidth of HS-PCF is very broad, covering four communication bands from O- to C-band. This kind of fiber can be used as a broadband OAMs filter.

Simple fiber-optic sensor for simultaneous and sensitive measurement of high pressure and high temperature based on the silica capillary tube.

A simple fiber-optic sensor for simultaneous measurement of high pressure and high temperature was proposed. The sensor was simply fabricated by splicing two sections of silica capillary tubes (SCTs) with different inner diameters to the single-mode fiber. The thick core SCT functions as a Fabry-Perot (FP) micro-cavity and an anti-resonant reflecting waveguide at the same time. The two different sensing mechanisms lead to the high contrast sensitivity values of pressure and temperature (‒3.76 nm/MPa, 27.7 pm/°C and 4.24 nm/MPa, 0.82 pm/°C). We also proposed a simple and effective method to evaluate the actual sensitivities of two-parameter sensors by using linear programming, which shows that our sensor is more sensitive than others in high pressure and high temperature simultaneous detection. Besides, low cost, good mechanical property and convenient reflective probe make the sensor more competitive in actual application.

Compact optical fiber temperature sensor with high sensitivity based on liquid-filled silica capillary tube.

We propose a highly sensitive fiber temperature sensor based on a section of liquid-sealed silica capillary tube inserted in a single-multi-single-mode fiber structure. The liquid polymer is filled into the silica capillary tube through two micro-holes drilled by a femtosecond laser. Then the micro-holes are blocked by UV curable adhesive with ultra-small volume. Obvious Mach-Zehnder interference peaks were shown in its transmission spectrum. The proposed fiber temperature sensor can be reliably used for actual point detection owing to its high sensitivity (8.09 nm/°C), good linearity (99.93%), compact size, good mechanical property, high fabrication efficiency, and good repeatability and stability.