Fano resonance in a microring resonator with a micro-reflective unit.

Fano resonance is considered to be a promising approach for integrated sensing. However, achieving and controlling Fano resonance lineshapes on ultra-compact chips remains a challenge. In this article, we propose a theoretic model based on the transfer matrix method (TMM) to quantitatively interpret the impact of a micro-reflective unit (MRU) etched in the straight waveguide of a microring resonator (MRR). Numerical calculations and FDTD simulations indicate that the size and position of the MRU can be used to control the Fano resonance lineshape. Since the MRU is etched in the coupling region, the reflection caused by the MRU will significantly enhance the intensity of the counter-clockwise (CCW) mode in the microring. When applied to a single nanoparticle sensing, clockwise (CW) and CCW modes will couple due to a single nanoparticles or rough cavity walls, resulting in a sharp shift and split of the Fano lineshape. The proposed model for single nanoparticle sensing is described by the scattering matrix, and the calculations show a well matches with FDTD simulations. The results show that the model proposed in this paper provides a new theoretical basis for controlling Fano resonance lineshape and presents a new approach for the integrated sensing of silicon photonic devices with high sensitivity.

Optically reconfigurable higher-order valley photonic crystals based on enhanced Kerr effect.

The reconfigurable higher-order topological states are realized in valley photonic crystals with enhanced optical Kerr nonlinearity. The inversion symmetry of the designed valley photonic crystal is broken due to the difference in optical responses between adjacent elements rather than their geometry structures. Therefore, by constructing photonic crystals with distinct topological phases, valley-dependent topological states can be realized, and their reconfigurability is demonstrated based on the Kerr effect. The investigated higher-order topological photonic crystals exhibit great robustness against the structural defects and inferior quality of pump introduced around the corner. Our work provides a new, to the best of our knowledge, platform for studying optical field manipulation and optical devices fabrication in the context of nonlinear higher-order topology.

High-performance lensless diffraction imaging from diverse holograms by three-dimensional scanning.

For lensless diffraction imaging, it is a challenging dilemma to achieve a large field of view (FOV) and high resolution with a small amount of data at the same time. Ptychography can reconstruct the high-resolution image and illumination light simultaneously. But the illumination is limited to a small size by a probe in typical ptychography. For large samples, it takes much time to collect abundant patterns and has strict requirements for the computing power of computers. Another widely applied method, multi-height measurement, can realize a wide FOV with several holograms. But, the recovered image is easily destroyed by the background noise. In this Letter, a lensless diffraction imaging method by three-dimensional scanning is proposed. All positions of the object are different in three directions instead of scanning schemes only on a plane or along the optic axis, so more diversity of diffraction information is obtained. We apply the illumination without the limit of a confined aperture, which means that the imaging FOV of a pattern is equal to the size of the utilized image sensor. In comparison with the multi-height method, our method can separate the illumination background noise from the retrieved object. Consequently, the proposed method realized high resolution and contrast, large FOV, and the removal of background noise simultaneously. Experimental validations and comparisons with other methods are presented.

Spectrum sampling optimization for quantitative phase imaging based on Kramers-Kronig relations.

Annular-illumination quantitative phase imaging based on space-domain Kramers-Kronig relations (AIKK) is a newly developed technique that is object-independent and non-iterative reconstructed inherently. Only capturing four low-resolution images, the AIKK system gains a resolution enhancement of nearly twofold. Under matching constraints between the illumination wave vector and pupil function aperture, we set a spectrum sampling criterion and establish a spectrum effective utilization model to search for the optimal solution of spectrum distribution for the specific annular structure. In view of the square spectrum structure, a diagonal-expanded sampling based AIKK method (DES-AIKK) is presented to get rid of the pixel aliasing problem. It is worth noting that the space-bandwidth-time product (SBP-T) further increases to 439.51 megapixels (1.8× of AIKK). Our work provides the guidelines and insights for designing the most suitable AIKK platform for high-throughput microscopic applications in pathology and real-time dynamic observation.

Simulation for multiwavelength large-aperture all-silicon metalenses in long-wave infrared.

Long-wave infrared imaging systems are widely used in the field of environmental monitoring and imaging guidance. As the core components, the long-wave infrared lenses suffer the conditions of less available materials, difficult processing, large volume and mass. Metalens composed of sub-wavelength structures is one of the most potential candidates to achieve a lightweight and planar optical imaging systems. Meanwhile, it is essential to obtain large-aperture infrared lenses with high power and high resolution. However, it is difficult to use the finite-difference time-domain method to simulate a large-aperture metalens with the diameter of 201 mm due to the large amount of computational memory and computational time required. Here, to solve the mentioned problem, we firstly propose a simulation method for designing a large-aperture metalens, which combines the finite-difference time-domain algorithm and diffraction integration. The finite-difference time-domain algorithm is used to simulate the meta-atom's transmitted complex amplitude and the one-dimensional simplification of the diffraction integral is to calculate the focused field distributions of the designed metalens. Furthermore, the meta-atom spatial multiplexing is applied to design the all-silicon metalenses with the aperture of 201 mm to realize dual-wavelength (10 and 11µm) achromatic focusing, super anomalous dispersion focusing and super normal dispersion focusing. The designed metalenses are numerically confirmed, which reveal the feasibility of all-silicon sub-wavelength structures to accomplish the multiwavelength dispersion control. The designed all-silicon metalenses have the advantage of lightweight and compact. The proposed method is effective for the development of large-aperture imaging systems in the long-wave infrared.

Channeled imaging spectropolarimeter reconstruction by neural networks.

Snapshot channeled imaging spectropolarimetry (SCISP), which can achieve spectral and polarization imaging without scanning (a single exposure), is a promising optical technique. As Fourier transform is used to reconstruct information, SCISP has its inherent limitations such as channel crosstalk, resolution and accuracy drop, the complex phase calibration, et al. To overcome these drawbacks, a nonlinear technique based on neural networks (NNs) is introduced to replace the role of Fourier reconstruction. Herein, abundant spectral and polarization datasets were built through specially designed generators. The established NNs can effectively learn the forward conversion procedure through minimizing a loss function, subsequently enabling a stable output containing spectral, polarization, and spatial information. The utility and reliability of the proposed technique is confirmed by experiments, which are proved to maintain high spectral and polarization accuracy.

Chiral and multiple one-way surface states on photonic gyroelectric metamaterials with small Chern number.

Topological one-way surface states allow light to pass through sharp corners without reflection. In order to enhance the capability of surface routing devices, multiple one-way surface modes are usually required. Different from previously reported multiple surface modes achieved with large Chern number photonic media, we realize multiple surface waves on a continuous medium with small Chern number, i.e., |C| = 1. The new topological phase is found when the hyperbolic and double semi-ellipsoid-like cone bands are simultaneously gapped by vacuum state. We also find the degeneracy of multiple one-way surface waves in the double semi-ellipsoid-like metamaterials. The propagation direction of the waves is determined by their own ellipticities. Our results may help to construct surface state devices with multiplexing capability and higher coupling efficiency.

Extraordinary transmission in an add-drop filter configuration driven by nonconservative coupling.

We proposed a nonconservative coupling scheme based on the add-drop filter configuration, in which a high Q factor passive microtoroid resonator is indirectly driven by an active unit, thus providing an additional coupling which might be comparable to a mode decay rate. Extraordinary scattering points are predicted when one of the supermodes becomes lossless. Specifically, when the inherent coupling strength is set at half of the mode's total decay rate, controllable transmission peaks can be realized by tuning the nonconservative coupling strength and phase delay. Our theoretic research might find potential application in tunable light steering based on non-Hermitian resonator systems.

Second harmonic generation enhancement and directional emission from topological corner state based on the quantum spin Hall effect.

Topological corner state has attracted much research interests since it does not obey the conventional bulk-edge correspondence and enables tightly confined light within small volumes. In this work, we demonstrate an enhanced second harmonic generation (SHG) from a topological corner state and its directional emission. To this end, we design an all-dielectric topological photonic crystal based on optical quantum spin Hall effect. In this framework, pseudospin states of photons, topological phase, and topological corner state are subsequently constructed by engineering the structures. It is shown that a high Q-factor of 3.66×1011 can be obtained at the corner state, showing strong confinement of light at the corner. Consequently, SHG is significantly boosted and manifests directional out-of-plane emission. More importantly, the enhanced SHG has robustness against a broad class of defects. These demonstrated properties offer practical advantages for integrated optical circuits.

Design, Synthesis and Characterization of a Novel Type of Thermo-Responsible Phospholipid Microcapsule-Alginate Composite Hydrogel for Drug Delivery.

Liposomes are extensively used in drug delivery, while alginates are widely used in tissue engineering. However, liposomes are usually thermally unstable and drug-leaking when in liquids, while the drug carriers made of alginates show low loading capacities when used for drug delivery. Herein, we developed a type of thermo-responsible liposome-alginate composite hydrogel (TSPMAH) by grafting thermo-responsive liposomes onto alginates by using Ca2+ mediated bonding between the phosphatidic serine (PS) in the liposome membrane and the alginate. The temperature-sensitivity of the liposomes was actualized by using phospholipids comprising dipalmitoylphosphatidylcholine (DPPC) and PS and the liposomes were prepared by a thin-film dispersion method. The TSPMAH was then successfully prepared by bridge-linking the microcapsules onto the alginate hydrogel via PS-Ca2+-Carboxyl-alginate interaction. Characterizations of the TSPMAH were carried out using scanning electron microscopy, transform infrared spectroscopy, and laser scanning confocal microscopy, respectively. Their rheological property was also characterized by using a rheometer. Cytotoxicity evaluations of the TSPMAH showed that the composite hydrogel was biocompatible, safe, and non-toxic. Further, loading and thermos-inducible release of model drugs encapsulated by the TSPMAH as a drug carrier system was also studied by making protamine-siRNA complex-carrying TSPMAH drug carriers. Our results indicated that the TSPMAH described herein has great potentials to be further developed into an intelligent drug delivery system.

High-order acoustic vortex field generation based on a metasurface.

In this paper, we generalize the principle of a generated first-order acoustic vortex (AV) with the acoustic resonances (AR) of a metasurface, from which we have also proposed a method for generating the higher-order AVs by arranging the sequences of the AR metasurface properly. The usable frequency range of the designed AR metasurface has been investigated and is discussed in detail, which is about 4470-4600Hz and covers the range 0.977-1.006f (f=4573.31Hz is the preset working frequency) approximately. Meanwhile, the designed AR metasurface can produce a frequency-adjustable AV field efficiently by adjusting the structure size and parameters of the planar AR layer. By discretizing the continuous phase distribution, the multiplexed AV beam with multiple orbital angular momentum states can also be generated effectively by arranging AR metasurface properly. The generated single and multiplexed AV beams can also be detected with the aid of the acoustic reverse phase plates accordingly.

Photonic spin Hall effect on an ellipsoidal Rayleigh particle in scattering far-field.

We present the photonic spin Hall effect on an ellipsoidal Rayleigh particle, which amounts to a polarization-dependent shift in scattering far-field. Based on the dipole model, we demonstrate that such shift is unavoidable when the light incidence is inclined with respect to the main axis of the ellipsoidal Rayleigh particle. The result has general validity and can be applied to metal and dielectric materials. In addition, the photonic spin Hall effect also manifests itself in the optical force and torque exerted on the particle, which is promising for precision metrology, spin-optics devices and optical driven micro-machines. Due to wide existence of the Rayleigh particles in nature, we believe that our findings might provide a useful toolset for investigating polarization-dependent scattering of particles.

Dynamics of angular momentum-torque conversion in silicon waveguides.

We present a refined theoretical analysis on the relationship between the optical total angular momenta (TAM) and the optical torque (OT) in a birefringent silicon waveguide. By using the vector angular spectrum method, we demonstrate the dynamic evolutions of the OT, TAM, spin angular momentum (SAM), and orbital angular momentum (OAM). The SAM and OAM coexist and evolve simultaneously in the propagation. The ratio between the OAM and TAM is related to the incident wavelength and the size of waveguide. Moreover, we design a three-layer waveguide structure to convert the light chirality and generate high torque. The performance of such torque-generator is analyzed numerically in detail.

Tunable THz generalized Weyl points.

Weyl points, as linearly double degenerated point of band structures, have been extensively researched in electronic and classical wave systems. However, Weyl points' realization is always accompanied with delicate "lattice structures". In this work, frequency-tunable terahertz (THz) generalized Weyl points inside the parameter space have been investigated and displayed by a specially designed photonic crystal with polydimethylsiloxane (PDMS) immersed in 4-cyano'-pentylbipenyl (5CB) liquid crystals (LCs). The reflective phase vortices as a signature of the generalized Weyl points are observed through our numerically simulations. Besides, interface states between photonic crystals and any reflective substrates are fulfilled too. Meanwhile, we could also change the orientation of LC molecule by the external magnetic field so as to tune the frequency of the first two bands' Weyl point from 0.27698THz to 0.30013THz. This band lies in the short-range wireless communication. Thus, our proposal may be beneficial to the investigation and application of Weyl points' properties and strongly localized states.

Compact spoof surface plasmon polaritons waveguide drilled with L-shaped grooves.

It has been recently demonstrated that a metallic surface with periodic grooves can support a laterally-confined surface wave called spoof plasmon polaritons (SSPPs). Here we propose a SSPPs waveguide drilled with L-shaped grooves which can support SSPPs efficiently. Dispersion relations based on the modal expansion method (MEM) are derived and discussed. Under the deep subwavelength condition, a concise formula for the dispersion relations is obtained. Our results show that the dispersion relations are sensitive to the transversal depths. The L-shaped groove is equivalent to a deeper rectangular groove, but more compact than the straight one. As an example of the applications, the rainbow-trapping effect is realized by changing the transversal depths of the L-shaped grooves.

Investigation of mechanism: spoof SPPs on periodically textured metal surface with pyramidal grooves.

In microwave and terahertz frequency band, a textured metal surface can support spoof surface plasmon polaritons (SSPPs). In this paper, we explore a SSPPs waveguide composed of a metal block with pyramidal grooves. Under the deep subwavelength condition, theoretical formulas for calculation of dispersion relations are derived based on the modal expansion method (MEM). Using the obtained formulas, a general analysis is given about the properties of the SSPPs in the waveguides with upright and downward pyramidal grooves. It is demonstrated that the SSPPs waveguides with upright pyramidal grooves give better field-confinement. Numerical simulations are used to check the theoretical analysis and show good agreement with the analytical results. In addition, the group velocity of the SSPPs propagating along the waveguide is explored and two structures are designed to show how to trap the SSPPs on the metal surface. The calculation methodology provided in this paper can also be used to deal with the SSPPs waveguides with irregular grooves.

Propagation factors of multi-sinc Schell-model beams in non-Kolmogorov turbulence.

We derive several analytical expressions for the root-mean-square (rms) angular width and the M(2)-factor of the multi-sinc Schell-model (MSSM) beams propagating in non-Kolmogorov turbulence with the extended Huygens-Fresnel principle and the second-order moments of the Wigner distribution function. Numerical results show that a MSSM beam with dark-hollow far fields in free space has advantage over the one with flat-topped or multi-rings far fields for reducing the turbulence-induced degradation, which will become more obvious with larger dark-hollow size. Beam quality of MSSM beams can be further improved with longer wavelength and larger beam width, or under the condition of weaker turbulence. We also demonstrate that the non-Kolmogorov turbulence has significantly less effect on the MSSM beams than the Gaussian Schell-model beam.

Polarization-independent longitudinal multi-focusing metalens.

A novel multi-focusing metalens in the longitudinal direction has been proposed and investigated based on the equal optical path principle, which is independent on the incident polarizations and can be suitable for both of the linear and circular polarization incidences simultaneously. Here, three novel designing principles: partitioned mode, radial alternating mode and angular alternating mode, have been proposed firstly for constructing different types of the longitudinal multi-focusing metalenses. The performances of the designed metalenses based on the different designed methods have also been analyzed and investigated in detail, and the intensity ratio of the focusing spots can be tuned easily by modulating the numbers of the relative type of nanoantennas, which is significant for the micro-manipulating optics and the multi-imaging technology in the integrated optics.

Circular polarization analyzer based on an Archimedean nano-pinholes array.

A relative broadband circular polarization analyzer based on a single-turn Archimedean nano-pinholes array has been proposed and investigated systematically from visible spectrum to near infrared region. The spiral arrangement of circular nano-pinholes can implement spatially separated fields according to the relationship between the spiral direction of Archimedean structure and chirality of circularly polarized light (CPL). The enhanced-characteristics mechanisms of the single-turn spirally arranged Archimedean pinholes array have been deduced and investigated by the theoretical analysis and numerical simulation in detail. Different from the single operating wavelength of the spiral slit structure, this novel design also shows a relative wide range of the operating wavelengths in the focusing and defocusing effects. The new proposed circular polarization analyzer could find more extensive applications, such as analyzing the physiological properties of chiral molecules based on circular polarizations, full Stokes-parameter polarimetric imaging applications and so on.

Soliton dynamics in a PT-symmetric optical lattice with a longitudinal potential barrier.

We present dynamics of spatial solitons propagating through a PT symmetric optical lattice with a longitudinal potential barrier. We find that a spatial soliton evolves a transverse drift motion after transmitting through the lattice barrier. The gain/loss coefficient of the PT symmetric potential barrier plays an essential role on such soliton dynamics. The bending angle of solitons depends on the lattice parameters including the modulation frequency, incident position, potential depth and the barrier length. Besides, solitons tend to gain a certain amount of energy from the barrier, which can also be tuned by barrier parameters.