Polarized vortex Smith-Purcell radiation with cascaded metasurfaces.

We introduce the concept of polarized vortex Smith-Purcell radiation by the interaction of an electron beam and cascaded metasurfaces. The spin and orbital angular momenta of Smith-Purcell radiation are determined by the cascaded metasurface that consists of a grating and a phase gradient metasurface. The grating converts the electron beam radiation into the desired polarized light, while the phase gradient metasurface generates the vortex light. Furthermore, the vortex Smith-Purcell radiation with linear and circular polarizations can be achieved by the various cascaded metasurfaces. In particular, the conversion of chirality in the Smith-Purcell radiation carrying circular polarization is accompanied by the alteration of positive and negative topological charges. This work paves the way for generating polarized vortex electron radiation and is beneficial to promote the development of free-electron-driven devices.

Femtosecond Laser Inscribed Excessively Tilted Fiber Grating for Humidity Sensing.

We propose a humidity sensor using an excessively tilted fiber grating (Ex-TFG) coated with agarose fabricated using femtosecond laser processing. The processed grating showcases remarkable differentiation between TE and TM modes, achieving an exceptionally narrow bandwidth of approximately 1.5 nm and an impressive modulation depth of up to 15 dB for both modes. We exposed the agarose-coated TFG sensor to various relative humidity levels and monitored the resonance wavelength to test its humidity sensing capability. Our findings demonstrated that the sensor exhibited a rapid response time (2-4 s) and showed a high response sensitivity (18.5 pm/%RH) between the humidity changes and the resonant wavelength shifts. The high sensitivity, linearity, repeatability, low hysteresis, and excellent long-term stability of the TFG humidity sensor, as demonstrated in our experimental results, make it an attractive option for environmental monitoring or biomedical diagnosis.

All-fiber sensor for simultaneous measurement of refractive index and temperature based on hole-assisted three-core fiber.

A hole-assisted three-core fiber (HATCF) has been proposed as a sensor for simultaneous measurement of refractive index (RI) and temperature. An 8 mm long HATCF is fused between two single mode fibers (SMFs). One air hole of the HATCF is opened by femtosecond laser ablation technique to expose a suspended core to the external environment. Due to the same diameters of the two suspended cores, the resonance couplings between the center core and the two suspended cores occur at the same wavelength, which leads to a strong resonance dip. When the solution is filled into the open air hole, the resonance dip is split in two dips because the phase matching wavelength between center core and the suspended core in the open air hole is changed. Simultaneous measurement of RI and temperature can be achieved by monitoring the wavelengths of the two dips. The measured RI and temperature sensitivities are 1369 nm/RIU in the range of 1.333-1.388 and 83.48 pm/°C in the range of 25-70 °C. The proposed sensor has outstanding advantages such as simple structure, high integration and dual parameter measurement, making it a potential application in the field of biological detection.

Simultaneous measurement of temperature and salinity based on a hole-assisted dual-core fiber.

An optical fiber sensor based on a hole-assisted dual-core fiber (HADCF) has been proposed and experimentally demonstrated for dual-parameter measurements. The dual-mode interferometer created uses the LP01 mode and LP11 mode in the suspended core of a specialist optical fiber, combined with a directional coupler formed by using the suspended core and the center core in a 16 mm long HADCF. Using this, the simultaneous measurement of salinity (due to the presence of NaCl) and temperature has been achieved through monitoring the interference dip and resonance dip. The sensitivities of the measurement of salinity and temperature are 190.7 pm/‰ and -188.2 pm/°C, respectively. The sensor developed has the advantages of simplicity of fabrication, a high level of integration and the potential for measurement of dual parameters, supporting its potential applications in marine environment measurements.

Diverse ranking metamaterial inverse design based on contrastive and transfer learning.

Metamaterials, thoughtfully designed, have demonstrated remarkable success in the manipulation of electromagnetic waves. More recently, deep learning can advance the performance in the field of metamaterial inverse design. However, existing inverse design methods based on deep learning often overlook potential trade-offs of optimal design and outcome diversity. To address this issue, in this work we introduce contrastive learning to implement a simple but effective global ranking inverse design framework. Viewing inverse design as spectrum-guided ranking of the candidate structures, our method creates a resemblance relationship of the optical response and metamaterials, enabling the prediction of diverse structures of metamaterials based on the global ranking. Furthermore, we have combined transfer learning to enrich our framework, not limited in prediction of single metamaterial representation. Our work can offer inverse design evaluation and diverse outcomes. The proposed method may shrink the gap between flexibility and accuracy of on-demand design.

High-resolution compact spectrometer based on periodic tapered coreless fiber.

This Letter proposes a method that balances miniaturization and high performance of fiber optic speckle spectrometers. The periodically tapered coreless fiber is used as the scattering element to excite more higher-order modes in the coreless fiber. As a result, a remarkable spectral resolution of 0.03 nm in the near-infrared spectrum can be achieved with a 5-cm-long fiber. Narrow linewidth and broadband spectra in the wavelength of 1540-1560 nm are reconstructed separately, demonstrating the excellent performance of the designed all-fiber spectrometer. The spectral resolution of our proposed spectrometer is comparable to that of a 2-m multimode fiber spectrometer and has a significant improvement in miniaturization.

Annular and unidirectional transverse scattering with high directivity based on magnetoelectric coupling.

Transverse scattering is a special directional scattering perpendicular to the propagation direction, which has attracted great interest due to its potential applications from directional antennas, optical metrology to optical sensing. Here we reveal annular transverse scattering and unidirectional transverse scattering by magnetoelectric coupling of Omega particle. The annular transverse scattering can be achieved by the longitudinal dipole mode of the Omega particle. Furthermore, we demonstrate the highly asymmetric unidirectional transverse scattering by adjusting the transverse electric dipole (ED) and longitudinal magnetic dipole (MD) modes. Meanwhile, the forward scattering and backward scattering are suppressed by the interference of transverse ED and longitudinal MD modes. In particular, the lateral force exerted on the particle is accompanied by the transverse scattering. Our results provide a useful toolset for manipulating light scattered by the particle and broaden the application range of the particle with magnetoelectric coupling.

Bifunctional sensing based on an exceptional point with bilayer metasurfaces.

Exceptional points (EPs), the critical phase transition points of non-Hermitian parity-time (PT) systems, exhibit many novel physical properties and associated applications, such as ultra-sensitive detection of perturbations. Here, a bilayer metasurface with two orthogonally oriented split-ring resonators (SRRs) is proposed and a phase transition of the eigenpolarization states is introduced via changing the conductivity of vanadium dioxide (VO2) patch integrated into the gap of one SRR. The metasurface possesses a passive PT symmetry and an EP in polarization space is observed at a certain conductivity of the VO2. Two sensing schemes with the metasurface are proposed to achieve high-sensitivity sensing of temperature and refractive index in the terahertz (THz) range. The metasurface is promising for applications in THz biosensing and polarization manipulation.

Tunable circular polarization responses of twisted black phosphorus metamaterials.

As one of the most significant 2D materials, black phosphorus (BP) offers a promising way to manipulate the polarization state of light due to its in-plane anisotropy, however, reconfigurable polarization manipulation is still challenging in simple BP structure. Here, we propose a multilayer metamaterial with twisted BP nanostructures and numerically study its circular dichroism (CD) and circular birefringence (CB) responses. The dependences of the circular polarization responses in the twisted BP metamaterial have been fully investigated on geometrical and material parameters. The giant tunability enables the twisted BP nanostructure to be attractive for constructing BP-based metamaterials devices, such as polarizers, biosensors and modulators.

All-optical vector magnetic field sensor based on a side-polished two-core fiber Michelson interferometer.

A magnetic field sensor based on a side-polished two-core fiber (SPTCF)-based Michelson interferometer (MI) has been developed and demonstrated. The magnetic field sensor is composed of a standard single mode fiber (SMF) and a section of tapered TCF. By side-polishing a segment of the TCF, the effective index of the exposed core can be made sensitive to the environmental refractive index (RI). To evaluate its performance, a magnetic fluid is used to cover the polished region with a magnetic field sensitive material, where the sensor then measures the magnetic field intensity by sensing the RI change of the magnetic fluid through the evanescent field in the polished core. The SPTCF MI device developed allows for vector magnetic field sensing because of its asymmetric structure, with its highest directional sensitivity being 55.2 pm/degree. Experimental results obtained show that when the magnetic field is parallel to the side-polished plane, a sensitivity of 1.262 nm/mT can be achieved, operating over the magnetic flux density region of 0-5 mT and over a temperature range of 20∼85 °C, where the device is minimally affected by temperature changes. The sensor is well suited to a variety of potential applications given its low cost, strong anti-interference ability, simple structure and high stability.

Dual-channel refractive index sensor based on coupling between LP01 and LP11 modes in the tapered hole-assisted dual-core fiber.

We propose a refractive index (RI) sensor based on a tapered hole-assisted dual-core fiber (HADCF). The sensor is fabricated by splicing a tapered HADCF between two single-mode fibers and operates on the coupling between the fundamental mode and the low-order mode in two cores. The HADCF is tapered to meet the phase matching condition between the fundamental mode (LP01) in the central core and the low-order mode (LP11) in the eccentric core. The tapered waist of the fiber becomes thinner; the coupling wavelength has a blue shift. Glycerin solutions of different RIs were injected into the air hole. The RI sensitivity of 936.69 nm/RIU is obtained in the RI range of 1.335-1.360. The multi-channel RI sensor cascaded by HADCFs with different taper lengths is obtained and can simultaneously measure the RI of different solutions. The proposed device has the advantages of high sensitivity, simple structure, and stable performance. The special microfluidic channel in the HADCF can protect the tested solution from external environmental pollution.

VO2-enabled transmission-reflection switchable coding terahertz metamaterials.

Coding metamaterials have offered unprecedented degrees of freedom to manipulate electromagnetic waves in time and frequency domains in terms of various coding sequences, however, it is still challenging to realize dynamic coding metamaterials in the terahertz range. Here, we propose VO2-enabled transmission-reflection switchable coding terahertz metamaterials consisting of multilayered gold and VO2 patterns. The insulator-to-metal transition of VO2 leads to switch between the refractive and reflective scattering beams by changing the temperature. The four 2-bit elements are used to construct coding metasurface-based OAM generator with l = 1. Remarkably, the transmission-reflection switching functionality of the coding metasurface can be achieved at different frequencies. In addition, the novel designs in our work can achieve EM waves manipulation and provide a useful method to dynamically switch transmission-reflection response in the THz frequency regime.

Topologically Enabled Ultrahigh-Q Chiroptical Resonances by Merging Bound States in the Continuum.

Ultrahigh-Q chiroptical resonance metasurfaces based on merging bound states in the continuum (BICs) are investigated and numerically demonstrated. The destruction of C2 symmetry results in the leakage of BICs into quasi-BICs, and a chiral quasi-BIC is obtained by oblique incidence or continuous destruction of the mirror symmetry of the structure. Due to the significant topological properties of merging BICs, the Q factor (over 2 × 105) of the chiral resonance peak obtained is much higher than that of the previous work. Moreover, the proposed structure is easy to fabricate because no additional out-of-plane asymmetry is introduced. The proposed scheme is of importance in chiral biosensing applications.

Versatile polarization manipulation in vanadium dioxide-integrated terahertz metamaterial.

Broadband and switchable versatile polarization metamaterial is crucial in the applications of imaging, sensing and communication, especially in the terahertz frequency. Here, we investigated versatile polarization manipulation in a hybrid terahertz metamaterial with bilayer rectangular rods and a complementary vanadium dioxide (VO2) layer. The VO2 phase transition enables a flexible switching from dual-band asymmetric transmission to dual-band reflective half-wave plate. The full width half maximum (FWHM) bandwidths of dual-band asymmetric transmission are 0.77 and 0.21 THz, respectively. The polarization conversion ratio (PCR) of the reflective metamaterial is over 0.9 in the frequency ranges of 1.01-1.17 THz and 1.47-1.95 THz. Angular dependences of multiple polarization properties are studied. The proposed switchable polarization metamaterial is important to the development of multifunctional polarization devices and multichannel polarization detection.

Active beam manipulation and convolution operation in VO2-integrated coding terahertz metasurfaces.

Coding metasurfaces have received tremendous interest due to their unprecedented control of beams through the flexible design of coding sequences. However, realizing tunable coding metasurfaces with scattering-pattern shifts in the terahertz range is still challenging. Here, we propose a VO2-integrated coding metasurface to realize a thermally controlled scattering-pattern shift by convolution operation. The required phase profiles and high amplitudes of 1-bit and 2-bit coding metasurfaces are easily obtained only by changing the length of the VO2 cut-wires. The insulator-metal phase transition of the VO2 cut-wires leads to an ultrafast switching effect between multiple deflected scattering beams and one normally reflected beam. In particular, the VO2 phase transition contributes to dynamical convolution operations of the 2-bit coding metasurface. The proposed VO2-integrated coding metasurfaces are important for realizing tunable terahertz beam manipulation as well as arbitrary required scattering beams.

Two-dimensional vector bending sensor based on a hole-assisted three-core fiber coupler.

We demonstrate a two-dimensional vector bending sensor based on a hole-assisted three-core fiber (HATCF) coupler. The sensor is built by splicing a section of HATCF between two single-mode fibers (SMFs). The resonance couplings between the center core and the two suspended cores of the HATCF occur at different wavelengths. Two completely discrete resonance dips are observed. The bending response of the proposed sensor is investigated over a 360° range. The bending curvature and direction can be identified by interrogating the wavelengths of the two resonance dips, and a maximum curvature sensitivity of -50.62 nm/m-1 is achieved at 0° direction. Moreover temperature sensitivity of the sensor is less than -34.9 pm/°C.

Highly sensitive vector bending sensor based on an embedded multimode D-shaped LPFG.

In this letter, a highly sensitive bending sensor based on an embedded multimode D-shaped long period fiber grating (EMD-LPFG) is proposed. The novel sensor is applied to carry out vector bending measurement. The proposed LPFG is fabricated by polishing on the prepared structure which is formed by periodically splicing between single mode fiber (SMF) and multimode fiber (MMF). Since the cross section of the embedded MMF is D-shaped, we named it EMD-LPFG. Due to the asymmetric modulation of the refractive index on the fiber by the CO2 laser, the sensor has the ability to distinguish the bending directions, and the MMFs provide higher bending response. The experimental transmission spectrum can match the simulation results well. The experimental results show that the average bending sensitivities in three orthogonal directions are 70.21 nm/m-1 (0°), 9.75 nm/m-1 (90°), -12.04 nm/m-1 (180°) and 9.98 nm/m-1 (270°), respectively. Meanwhile, the temperature sensitivity is 30 pm/°C in the range of 25 °C to 75 °C. According to the ultra-compact structure with the total length of 2.5 mm, high bending sensitivity and ability to distinguish the bending direction, the novel sensor has potential in bending measurement.

Multicore fiber integrated beam shaping devices for long-range plasmonic trapping.

The multicore fiber beam shaping devices based on surface plasmon polaritons (SPPs) have been proposed and demonstrated. The gold film is covered on the end face of the optical fiber. An air slit is perforated in the center of each core and the gratings with a fixed period are designed on the gold film on one side of the slit to obtain a deflected beam. Multiple deflected beams based on the multicore fiber interfere and form a periodic field, where the period of the interference field is determined by the deflection angle of the beams and the spacing between the cores. The interference field of the multiple deflected beams can be used to trap the nanosphere. The Maxwell stress tensor method is used to calculate the transverse and longitudinal trapping forces on a nanosphere. The nanosphere can be stably trapped at 45 µm away from the end face of the fiber. Such an all-fiber trapping system is compact and flexible integration, and is promising for long-working-distance and multiple-particle trapping.

Large asymmetric anomalous reflection in bilayer gradient metasurfaces.

Gradient metasurfaces have attracted much attention due to intriguing wavefront and polarization manipulation. Here, a bilayer gradient metasurface is constructed by use of a rectangular nanorod layer and its complementary nanoaperture. It reveals asymmetric anomalous reflection and symmetric anomalous transmission for two counter-propagating directions. The dependence of the anomalous reflection and transmission phenomena on nanostructure thickness are numerically studied in optical frequencies. The increasing metallic layer thickness of the gradient metasurface greatly enhances anomalous reflection of the left-handed circularly polarized wave (LCP) for the nanorod side and suppresses anomalous reflection for the other side. Both resonant frequencies of anomalous reflection and transmission linearly shift with the refractive index. The bilayer gradient metasurface is important for realizing wavefront modulation and optical sensing.

Switchable dual-band to broadband terahertz metamaterial absorber incorporating a VO2 phase transition.

We design and demonstrate a thermally switchable terahertz metamaterial absorber consisting of an array of orthogonal coupled split-ring metal resonators involving a VO2 phase transition. Numerical results indicate that the active metamaterial always absorbs the TE wave in dual-band regardless of insulating and metallic VO2, while the insulator-to-metal phase transition enables a switchable effect between dual-band and broadband absorption of the TM wave with the resonant frequency tunability of 33%. Especially under the metallic VO2 state, the absorption properties are polarization-dependent and exhibit a switching effect between dual-band and broadband absorption with the increase of the polarization angle. The tunable absorption mechanism can be explained by effective impedance theory and electric energy density distributions. The proposed dual-band to broadband metamaterial switching absorber may have broad applications in sensors, imaging and emitters.