Mode-order conversion in a Mach-Zehnder interferometer based on Chern insulators.

Mode-order conversion devices can provide a flexible platform to achieve mode coupling and optimizing in mode division multiplex (MDM) that can eliminate the restrain of capacity and density in photonic integration and communication. However, mode-order converters based on traditional photonic crystal (PC) waveguides are susceptible to defects, which always render device incapacitation in mode-order conversion. Herein, a mode converter designed by the Mach-Zehnder interferometer (MZI) structure is proposed to manipulate the conversion of topological edge states (TESs) based on Chern insulators consisting of gyromagnetic PCs. The back-and-forth conversion between fundamental and high-order modes is numerically demonstrated based on phase modulation in our proposed device, in which each mode can be immune to defects. This unique approach for converting the mode order of TES exploits a new perspective in MDM to design a high-performance multimode device, leading to potential applications in photonic integrated circuits (PIC), on-chip processors, and optical fiber communication.

Terahertz metalens for generating multi-polarized focal points and images with uniform intensity distributions.

Metasurfaces have provided a flexible platform for designing ultracompact metalenses with unusual functionalities. However, traditional multi-foci metalenses are limited to generating circularly polarized (CP) or linearly polarized (LP) focal points, and the intensity distributions are always inhomogeneous/chaotical between the multiple focal points. Here, an inverse design approach is proposed to optimize the in-plane orientation of each meta-atom in a terahertz (THz) multi-foci metalens that can generate multi-polarized focal points with nearly uniform intensity distributions. As a proof-of-principle example, we numerically and experimentally demonstrate an inversely designed metalens for simultaneously generating multiple CP- and LP-based focal points with homogeneous intensity distributions, leading to a multi-polarized image (rather than the holography). Furthermore, the multi-channel and multi-polarized images consisting of multiple focal points with homogeneous intensity distributions are also numerically demonstrated. The unique approach for inversely designing multi-foci metalens that can generate multi-polarized focal points and images with uniform intensity distributions will enable potential applications in imaging and sensing.

Manipulating electromagnetic waves in a cavity-waveguide system with nontrivial and trivial modes.

The coupled cavity-waveguide approach provides a flexible platform to design integrated photonic devices that are widely applied in optical communications and information processing. Topological photonic crystals that can excite the nontrivial edge state (ES) and corner state (CS) have an unprecedented capability to manipulate electromagnetic (EM) waves, leading to a variety of unusual functionalities that are impossible to achieve with conventional cavity-waveguide systems. In this Letter, two-dimensional photonic crystals consisting of an ES waveguide, a CS cavity, and a trivial cavity are proposed as a means to robustly control the transmission characteristics of electromagnetic waves. As a proof-of-principle example, the analog of electromagnetically induced transparency (EIT) that is tolerated in disorders due to the robustness of the CS is numerically demonstrated. In addition, the analog of multi-EIT is also verified by introducing a trivial cavity with two degenerate orthogonal modes. This unique approach for robustly manipulating EM waves may open an avenue to the design of high-performance filters, modulators, and on-chip processors.

Geometry phase for generating multiple focal points with different polarization states.

Conventional lenses are always large and bulky to achieve desired wave-manipulating functions, hindering the development of integrated and miniaturized optical systems. Metasurfaces, two-dimensional counterparts of metamaterials, can accurately tailor the wavefront of electromagnetic waves at subwavelength scale, providing a flexible platform for designing ultra-compact and ultra-flat lenses, namely as metalenses. However, the previous geometry-phase-based metalenses usually generate focal point(s) with only one special polarization state, i.e., either linearly-polarized (LP) state or circularly-polarized (CP) state, which inevitably degrades further applications. Here, we propose and experimentally demonstrate an approach for designing terahertz (THz) metalenses based on geometry phase that can generate multiple focal points with different polarization states. Under the illumination of LP THz waves, three focal points with left-hand CP (LCP), right-hand CP (RCP) and LP states are observed. Furthermore, the position of each focal point can be flexibly manipulated in free space. Geometry metasurfaces consisting of micro-rods with the same shape but different in-plane orientations are fabricated to demonstrate these properties. This unique approach may enable an unprecedented capability in designing multifunctional THz devices with potential applications in imaging, detecting and communications.

Terahertz-frequency temporal differentiator enabled by a high-Q resonator.

Terahertz (THz) fundamental "building blocks" equivalent to those used in multi-functional electronic circuits are very helpful for actual applications in THz data-processing technology and communication. Here, we theoretically and experimentally demonstrate a THz temporal differentiator based on an on-chip high-quality (Q) factor resonator. The resonator is made of low-loss high-resistivity silicon material in a monolithic, integrated platform, which is carefully designed to operate near the critical coupling region. The experiment demonstrates that the device can perform the first-order time derivative of the input signal electric field complex envelope at 214.72 GHz. Our investigation provides an effective approach for terahertz pulse re-shaping and real-time differential computing units.

Spin-independent metalens for helicity-multiplexing of converged vortices and cylindrical vector beams.

The converged vortex beam with a well-defined focal plane is an essential ingredient for trapping and rotating microparticles. Metasurfaces, two-dimensional metamaterials, provide an ultra-compact and flexible platform for designing a converged vortex by integrating the functions of a lens and vortex plate. A spin-defocused metasurface can further boost information capacity such as the multiplexing of helicity-dependent functionalities. Here we propose an approach to realize spin-defocused metalenses that can simultaneously focus terahertz (THz) waves with orthogonal spin states into helicity-dependent vortices based on pure geometric phases. Under the illumination of linearly polarized terahertz waves, all of the helicity-dependent vortices are observed, leading to helicity-multiplexing of converged vortices. Furthermore, the longitudinal multiplexing of converged cylindrical vector beams is demonstrated by superposition of helicity-dependent vortices. This unique approach for multiplexing converged vortices and cylindrical vector beams may open a window for designing future ultra-compact and multifunctional devices with potential applications in communications, optical trapping, and focusing.

Critical Factors for In Vivo Measurements of Human Skin by Terahertz Attenuated Total Reflection Spectroscopy.

Attenuated total reflection (ATR) geometry is a suitable choice for in vivo measurements of human skin due to the deep penetration of the field into the sample and since it makes it easy to measure the reference spectrum. On the other hand, there are several critical factors that may affect the terahertz (THz) response in these kinds of experiments. Here, we analyse in detail the influence of the following factors: the contact positions between the thumb and the prism, the contact pressure, the contact duration, and the materials of the prism. Furthermore, we use the THz-ATR technology to evaluate different types of handcream and also establish the theoretical model to investigate the reflectivity after interacting with the skin. The results agree well with experimental ones. Our analysis makes it clear the importance of controlling the above factors during measurements to enable reliable THz response and results which, in turn, may be used to monitor water motion in human skin and to predict possible diseases.

Flexibly designed spoof surface plasmon waveguide array for topological zero-mode realization.

We propose a flexibly designed photonic system based on ultrathin corrugated metallic "H-bar" waveguide that supports spoof surface plasmon polariton (SPP) at microwave frequencies. Five designs were presented, in order to demonstrate flexibility according to varying height, period, core width, rotation, and shifting on the "H-bar" unit of the waveguide. The propagation constant between two hybrid designs of period and height structure was then shown in order to study the coupling effect. Next, we constructed a coupled waveguide array that followed the Su-Schrieffer-Heeger (SSH) model. This model was constructed by a hybrid design with the identical propagation constant of each waveguide, except it had dimerized spacing. The propagation feature of topological zero mode was then observed as theoretically expected in the dimerized array. Our proposed spoof SPP waveguide array has great flexibility to be used as a powerful experiment platform, particularly in photonic simulation of the quantum or topological phenomena described by Schrödinger equation in condensed matters.

0.1-20 THz ultra-broadband perfect absorber via a flat multi-layer structure.

An ultra-broadband perfect absorber based on graded-index mechanism is designed and fabricated. The perfect absorber is comprised of a heavily-doped silicon absorption substrate and a flat six-layer antireflective structure. The refractive index of each layer was widely tuned by hollow polystyrene microsphere and TiO2 nanoparticle dopants, which can offer a gradually changed refractive index profile from 1.3 to 2.9. The experimental results show that 98% absorption can be achieved within the range of 0.1-20 THz. Moreover, the high absorption efficiency as well as the ultra-broad range can maintain for incident angle from 0 to 75° by the theoretical simulation.

Ultra-broadband terahertz perfect absorber by exciting multi-order diffractions in a double-layered grating structure.

Terahertz (THz) perfect absorber, as a useful functional device, has attracted considerable attention. Traditional metamaterial perfect absorbers are usually in response to single-frequency or multi-frequency owing to the resonance features of the metal-based sub-wavelength structure. In this paper, a simple double-layered doped-silicon grating structure was designed to realize an ultra-broadband and polarization-independent THz perfect absorber. Both theoretical and experimental results demonstrate that the incident THz waves ranging from 0.59 to 2.58 THz can be efficiently absorbed with an absorptivity of more than 95% and a bandwidth of about 2.0 THz. The excellent characteristic of this broad-bandwidth THz perfect absorber is mainly resulted from the air gap mode resonance together with the first-order and the second-order grating diffractions.

Illusion induced overlapped optics.

The traditional transformation-based cloak seems like it can only hide objects by bending the incident electromagnetic waves around the hidden region. In this paper, we prove that invisible cloaks can be applied to realize the overlapped optics. No matter how many in-phase point sources are located in the hidden region, all of them can overlap each other (this can be considered as illusion effect), leading to the perfect optical interference effect. In addition, a singular parameter-independent cloak is also designed to obtain quasi-overlapped optics. Even more amazing of overlapped optics is that if N identical separated in-phase point sources covered with the illusion media, the total power outside the transformation region is N2I0 (not NI0) (I0 is the power of just one point source, and N is the number point sources), which seems violating the law of conservation of energy. A theoretical model based on interference effect is proposed to interpret the total power of these two kinds of overlapped optics effects. Our investigation may have wide applications in high power coherent laser beams, and multiple laser diodes, and so on.

Controllable multiband terahertz notch filter based on a parallel plate waveguide with a single deep groove.

The influence of the air gap on the response of transmission for a transverse-electric mode parallel plate waveguide with a single deep groove has been experimentally studied. As the air gap is larger than the resonant wavelength of a high-order cavity mode in a single deep grooved waveguide, only the fundamental cavity mode can be excited and the single resonance (band) can be observed in a transmission spectrum. The decrease of the air gap can not only efficiently push the radiation of the fundamental cavity mode into the deep groove but also excite the high-order cavity modes, resulting in multiple resonances (multiband) in the corresponding spectrum. Based on the above observations, a tunable multiband terahertz notch filter has been proposed and the variation of the air gap has turned out to be an effective method to select band number. Experimental data and simulated results verify this band number tunability.

Accurate Assembly of Circular RNAs with TERRACE.

Circular RNA (circRNA) is a class of RNA molecules that forms a closed loop with its 5' and 3' ends covalently bonded. Due to this specific structure circRNAs are more stable than linear RNAs, admit distinct biological properties and functions, and have been proven to be promising biomarkers. Circular RNAs were severely overlooked previously owing to the biases in the RNA-seq protocols and in the detection algorithms, but recently gained tremendous attentions in both aspects. However, most existing methods for assembling circRNAs heavily rely on the annotated transcriptomes, and hence exhibit unsatisfactory accuracy when a high-quality transcriptome is unavailable. Here we present TERRACE, a new algorithm for full-length assembly of circRNAs from paired-end total RNA-seq data. TERRACE uses the splice graph as the underlying data structure to organize the splicing and coverage information. We transform the problem of assembling circRNAs into finding two paths that "bridge" the three fragments in the splice graph induced by back-spliced reads. To solve this formulation, we adopted a definition for optimal bridging paths and a dynamic programming algorithm to calculate such paths, an approach that was proven useful for assembling linear RNAs. TERRACE features an efficient algorithm to detect back-spliced reads that are missed by RNA-seq aligners, contributing to its much improved sensitivity. It also incorporates a new machine-learning approach that is trained to assign a confidence score to each assembled circRNA, which is shown superior to using abundance for scoring. TERRACE is compared with leading circRNA detection methods on both simulations and biological datasets. Our method consistently outperforms by a large margin in sensitivity while maintaining better or comparable precision. In particular, when the annotations are not provided, TERRACE can assemble 123%-412% more correct circRNAs than state-of-the-art methods on human tissues. TERRACE presents a major leap on assembling full-length circRNAs from RNA-seq data, and we expect it to be widely used in the downstream research on circRNAs.

Rotatable illusion media for manipulating terahertz electromagnetic waves.

Based on composite optical transformation, we propose a rotatable illusion media with positive permittivity and permeability to manipulate terahertz waves, and a new way to realize singular parameter-independent cloaks when the incident wave with a certain width propagates from specific incident directions. The fundamental mechanism of this kind of cloak is that the illusion media can be able to avoid the incident wave interacting with the objects. Comparing with traditional transformation-coordinate-based cloaks such as cylindrical-shaped cloaks, our cloaks are independent of singular material parameters. Furthermore, this type of rotatable illusion media can be applied to design tunable miniaturized high-directivity antenna (a small antenna array covered with the rotatable illusion media appears like a large one and meanwhile, the radiation directions of the small antenna array is tunable via this rotatable illusion media). Full wave simulations are performed to confirm these points.

Observation of electromagnetically induced transparency-like transmission in terahertz asymmetric waveguide-cavities systems.

Electromagnetically induced transparency (EIT)-like transmission was demonstrated in terahertz asymmetric parallel plate waveguides with two identical cavities. By shifting the position of the bottom cavity from the symmetric position in the propagation direction, both the phases of the propagating wave at resonances and the coupling strengths between two cavities are changed, resulting in exciting the additional asymmetric resonance and manipulating the detuning of two different resonant frequencies. The transparent peak between two resonances comes from the cancelation of symmetric and asymmetric resonances. We also use the physical picture of excitation of quasi-dark mode to explain this EIT-like transmission, which is similar to the metamaterial systems.

Shifting media for carpet cloaks, antiobject independent illusion optics, and a restoring device.

In traditional cases, the basic principle of carpet cloak is that a two-dimensional bump above ground plane is compressed into a one-dimensional line which is close to the ground plane. As a result, the bump can be hidden. In this paper, different from the traditional carpet cloak, we propose a shifting media to achieve the carpet cloak. By covering with the shifting media, an object floating on a ground plane can be directly shifted beneath the ground plane (rather than compressing it into a line), resulting in a floating carpet cloak. Antiobject independent illusion optics, i.e., turning an object into another one without any antiobjects, can be realized based on this kind of shifting media. As an application of the shifting media, a restoring device, by which the broken objects (such as antiques) can be perfectly restored, is also investigated.

Spin-decoupled excitation and wavefront shaping of structured surface waves via on-chip terahertz metasurfaces.

Surface waves (SWs) are of great importance in terahertz (THz) photonics applications due to their subwavelength properties. Hence, it is crucial to develop surface wavefront shaping techniques, which is urgent in modern information technologies. In this paper, a new scheme is proposed to realize SW excitation and spin-decoupled wavefront shaping with an ultracompact planar meta-device working in the THz range. The meta-device is composed of two parts: meta-atoms (in the center) and plasmonic metals (on the left and right sides). By carefully setting the geometry size and rotation angle of each meta-atom, the encoded spin-decoupled phase distributions for both left circularly polarized (LCP) and right circularly polarized (RCP) incident THz waves are determined. In this way, circularly polarized (CP) incident THz waves can be converted to SWs propagating along plasmonic metals with unique wavefront profiles, i.e., Bessel and focusing profiles. Full-wave simulations and THz near-field scanning experiments were performed to verify the functionalities of the meta-device, both of which are in great agreement with theoretical predictions. Our findings may provide more solutions to design THz integrated photonic devices and systems.

A Review on Terahertz Technologies Accelerated by Silicon Photonics.

In the last couple of decades, terahertz (THz) technologies, which lie in the frequency gap between the infrared and microwaves, have been greatly enhanced and investigated due to possible opportunities in a plethora of THz applications, such as imaging, security, and wireless communications. Photonics has led the way to the generation, modulation, and detection of THz waves such as the photomixing technique. In tandem with these investigations, researchers have been exploring ways to use silicon photonics technologies for THz applications to leverage the cost-effective large-scale fabrication and integration opportunities that it would enable. Although silicon photonics has enabled the implementation of a large number of optical components for practical use, for THz integrated systems, we still face several challenges associated with high-quality hybrid silicon lasers, conversion efficiency, device integration, and fabrication. This paper provides an overview of recent progress in THz technologies based on silicon photonics or hybrid silicon photonics, including THz generation, detection, phase modulation, intensity modulation, and passive components. As silicon-based electronic and photonic circuits are further approaching THz frequencies, one single chip with electronics, photonics, and THz functions seems inevitable, resulting in the ultimate dream of a THz electronic-photonic integrated circuit.

Tunable Transmissive Terahertz Linear Polarizer for Arbitrary Linear Incidence Based on Low-Dimensional Metamaterials.

In this work, we propose a structure consisting of three metamaterial layers and a metallic grating layer to rotate the polarization of arbitrary linearly polarized incidence to the y-direction with high transmissivity by electrically tuning these metamaterials. The transfer matrix method together with a harmonic oscillator model is adopted to theoretically study the proposed structure. Numerical simulation based on the finite difference time-domain method is performed assuming that the metamaterial layers are constituted by graphene ribbon arrays. The calculation and simulation results show that the Drude absorption is responsible for the polarization rotation. Fermi level and scattering rate of graphene are important for the transmissivity. For a polarization rotation of around 90°, the thickness of either the upper or lower dielectric separations influences the transmission window. For a polarization rotation of around 45° and 135°, the lower dielectric separations decide the frequency of the transmission window, while the upper dielectric separations just slightly influence the transmissivity.

Manipulating the field distribution via optical transformation.

Using the coordinate transformation theory, we propose a way to control the field distribution of electromagnetic wave in a waveguide filling with properly designed transformation media. The results show that the field distribution of the electromagnetic wave can be compressed and amplified in both longitudinal and transverse direction. A realizable layered waveguide based on the discrete optical transformation is also designed to manipulate the field distribution of the electromagnetic wave. Potential applications, i.e., turn a narrow (wide) slit into a large (small) window, are investigated in detail.