An ultra-compact fiber tip Michelson interferometer (MI), primarily aimed for a reproducible and stable high-temperature sensing probe, is developed and demonstrated. Both single-mode fiber (SMF) and polarization maintaining fiber (PMF) are considered and compared. The tip MI is fabricated by only using a one-step partial-polishing technique, which forms a half oblique and half vertical end face and functions as a beam splitter. A wide spectra analysis proved that the interferometer has an optical path difference (OPD) that is consistent across samples. When the lead-in fiber suffers from bending or twisting, the interference spectrum for the PMF case is more stable than that for the SMF case. Experimental results show a linear average temperature sensitivity of 15.15 pm/˚C in the range of 100˚C to 1000˚C for three tested PMF samples, and the difference between the sensitivities of the samples is less than 4.0%. The ease of fabrication, highly compact structure, reproducibility, and excellent resistance to mechanical disturbance performance suggest that the proposed PMF tip MI is highly promising as a high temperature sensing probe with high spatial resolution.
A transmissive single-layer Huygens unit cell based on induced magnetism is proposed to design low-profile and multi-focus metasurface. The Huygens unit cell consists of a pair of antisymmetric metal elements and a dielectric substrate with only 1.2 mm thickness (λ0/6.8 at 37 GHz). The surface currents flowing in the opposite directions form the circulating electric currents to induce the magnetic currents orthogonal to the electric currents. The full coverage of 2π phase is achieved through optimizing the parameters of the metal elements, which solves the problem of the incomplete phase coverage caused by layer number reduction. With Holographic theory, the compensating phase distribution on the metasurface is calculated. The incident plane wave can be converged to designated points in any desired fashion including focal number, location and intensity distribution, which exhibits outstanding manipulation capability. As the simulations and measured results show, the designed metasurface can achieve good multi-focus focusing characteristics. The focusing efficiency at the center frequency is 43.78%, and the relative bandwidth with 20% focusing efficiency exceeds 20%. The designed metasurface has the advantages of low profile, simple processing, and high efficiency, which has a wide range of application prospects in the fields of millimeter wave imaging, biomedical diagnosis and detection.
This airticle provides a review of transfer function-based (TF-based) surrogate optimization for electromagnetic (EM) design. Transfer functions (TF) represent the EM responses of passive microwave components versus frequency. With the assistance of TF, the nonlinearity of the model structure can be decreased. Parallel gradient-based EM optimization technique using TF in rational format and trust region algorithm is introduced first. Following that, we review the EM optimization using adjoint sensitivity-based neuro-TF surrogate, where the neuro-TF modeling method is in pole/residue format. The adjoint sensitivity-based neuro-TF surrogate technique can reach the optimal EM responses solution faster than the existing gradient-based surrogate optimization methods without sensitivity information. As a further advancement, we discuss the multifeature-assisted neuro-TF surrogate optimization technique. With the help of multiple feature parameters, the multifeature-assisted neuro-TF surrogate optimization has a better ability of avoiding local minima and can achive the optimal EM solution faster than the surrogate optimizations without feature assistance. Three examples are used to verify the above three methods.
Compared to natural materials, artificial subwavelength structures can enhance chiroptical effects in a stronger way, and the requirement of low material loss and wideband operation is desired in many situations. Here, we propose an all-dielectric chiral metasurface as a periodic array of centrosymmetric staggered silicon cuboid pairs to achieve strong circular dichroism in a wide band. As a demonstration, the designed chiral metasurface may strongly reflect the chosen circularly polarized light with the spin preserved in the operating wavelength range of 1.51~1.60 um while highly transmit (with an efficiency greater than 95%) the opposite circularly polarized light with the spin flipped. Then, two application cases are given for the designed chiral metasurface. A flat chiral meta-lens is constructed to produce wideband focusing in the transmission/reflection side while the disturbing from the opposite circular polarization is well blocked by high reflection/transmission. A chiral Fabry-Perot cavity is also constructed, which has an extremely high quality factor (2.1E4). The proposed method provides an efficient way to produce strong chiroptical effects and has a promising potential for various applications such as signal processing, sensing, radiation and detection.
In this work, we investigate the feasibility of applying deep learning to phase synthesis of reflectarray antenna. A deep convolutional neural network (ConvNet) based on the architecture of AlexNet is built to predict the continuous phase distribution on reflectarray elements given the beam pattern. The proposed ConvNet is sufficiently trained with data set generated by array-theory method. With radiation pattern and beam direction arrays as input, the ConvNet can make real-time and fairly accurate predictions in milliseconds with the average relative error below 0.7%. This paper shows that deep convolutional neural networks can ``learn'' the principle of reflectarray phase synthesis due to their inherent powerful learning capacity. The proposed approach may provide us a potential scheme for real-time phase synthesis of antenna arrays in electromagnetic engineering.
Non-Hermitian skin effect denotes the exponential localization of a large number of eigen-states at boundaries in a non-Hermitian lattice under open boundary conditions. Such a non-Hermiticity-induced skin effect can offset the penetration depth of in-gap edge states, leading to counterintuitive delocalized edge modes, which have not been studied in a realistic photonic system such as photonic crystals. Here, we analytically reveal the non-Hermitian skin effect and the delocalized edge states in Maxwell's equations for non-Hermitian chiral photonic crystals with anomalous parity-time symmetry. Remarkably, we rigorously prove that the penetration depth of the edge states is inversely proportional to the frequency and the real part of the chirality. Our findings pave a way towards exploring novel non-Hermitian phenomena and applications in continuous Maxwell's equations.
Micro-nano opto-electronic devices are demanded to be highly efficient and capable of multiple working wavelengths in several light-matter interaction applications, which is a challenge to surface plasmonics owing to the relatively higher intrinsic loss and larger dispersion. To cross the barriers, a plasmonic metasurface combining both high Q-factors (highest Q > 800) and multiple resonant wavelengths is proposed by arranging step-staged pyramid units in lattice modes. Different numerical relations for nonlinear frequency conversions have been constructed because of its strong tunability. Also, characteristics of high radiation efficiency (> 50%) and largelocalized optical density of state (> 104) have been proved through the numerical simulation. Such tunable high-Q metasurface can be implemented to quantum nonlinear process and enable the strong light-matter interaction devices into reality.
Topological refractions created by valley sonic crystals (VSCs) have attracted great attentions in the communities of physics and engineering owing to the advantage of zero reflection of sound and the potential for designing advanced acoustic devices. In previous works, topological refractions of valley edge states are demonstrated to be determined by the projections of the valleys K and K′, and two types of topological refractions generally exist at opposite terminals or different frequency bands. However, the realization of tunable topological refractions at the fixed frequency band and terminal still poses great challenge. To overcome this, we report the realization of tunable topological refractions by VSCs with triple valley Hall phase transitions. By simply rotating rods, we realize 3 types of topological waveguides (T1, T2 and T3) composed of two VSCs, in which the projections of the observed valley edge states can be modulated between K and K′. Additionally, based on the measured transmittance spectra, we experimentally demonstrate that these valleyedge states are almost immune to backscattering against sharp bends. More importantly, we realize tunable topological refractions at the fixed frequency band and terminal, and experimentally observe the coexistence of positive and negative refractions for T1 and T3, and negative refractions for T2. The proposed tunable topological refractions have potential applications in designing multi-functional sound antennas and advanced communication devices.
Integrated time delays are important for self-forced oscillation techniques in opto-electronic oscillators (OEO). Add-drop filters (ADFs) resonators using optical waveguide coupled to micro-ring resonators (MRR) are suitable for integrated optical time delays but suffer from a limited expected delay. 2-dimensional (2-D) photonic crystals (PhCs) with line defect are employed as confined optical waveguide to realize ADF resonators where longer optical delays than standard homogenous resonators are achieved by leveraging the slow-light effect. Moreover, achieving time delay up to microseconds (μs) is envisioned by cascading multiple identical ADF based on dispersive 2-D PhC micro-resonators. The focus of this paper is to devise a hybrid modeling procedure for accurate calculations of achieved time delays in various complex structures, while a combined electromagnetic modeling and analytical calculation technique overcomes a substantial computational resources and long computation times for a brute forced full-wave design and modeling. This innovative hybrid modeling for time delay estimation of cascaded ADFs is proposed for the first time to optimize physical design within short time period. First, transfer function performance of a homogenous ADF resonator is simulated using finite-difference-time-domain (FDTD) for both the full structure and structures with bi-fold symmetry and compared against proven analytical solutions to demonstrate accuracy of bi-fold symmetry while the computational resources are economized. The same modeling procedure is then extended to predicting performance of 2-D PhC based ADF resonator by quantifying key physical parameters of coupling factor, complex optical propagation constant, and optical transfer function for ADF resonator for the ring radius of curvature about 1.5 μm with various coupling gaps between feed waveguide and resonator guide. These parameters and the effective group index calculated by OptiFDTD software are applied to the analytical expressions to estimate single 2-D PhC ADF and attain a simulated time delay of 200 ps. The estimated time delay of 70 cascaded 2-D PhC based ADF resonators with R of 100 μm is estimated to be about 925 ns for the on-resonance frequency of 1534 nm.