Band 12, Heft 13 - Special Issue: Metamaterials and plasmonics in Asia, a tribute to Byoungho Lee

Non-Hermitian photonics and topological photonics, as new research fields in optics, have attracted much attention in recent years, accompanying by a great deal of new physical concepts and novel effects emerging. The two fields are gradually crossed during the development process and the non-Hermitian topological photonics was born. Non-Hermitian topological photonics not only constantly produces various novel physical effects, but also shows great potential in optical device applications. It becomes an important part of the modern physics and optics, penetrating into different research fields. On one hand, photonics system can introduce artificially-constructed gain and loss to study non-Hermitian physics. Photonics platform is an important methods and ways to verify novel physical phenomena and promote the development of non-Hermitian physics. On the other hand, the non-Hermitian topological photonics provides a new dimension for manipulating topological states. Active and dissipate materials are common in photonic systems; therefore, by using light pump and dissipation of photonic systems, it is expected to promote further development of topological photonics in device applications. In this review article, we focus on the recent advances and applications on non-Hermitian topological photonics, including the non-Hermitian topological phase transition and skin effect, as well as the applications emerging prosperously in reconfigurable, nonlinear and quantum optical systems. The possible future research directions of non-Hermitian topological photonics are also discussed at the end. Non-Hermitian topological photonics can have great potential in technological revolution and have the capacity of leading the development of both physics and technology industry.

In the past few decades, the discovery of topological matter states has ushered in a new era in topological physics, providing a robust framework for strategically controlling the transport of particles or waves. Topological photonics, in particular, has sparked considerable research due to its ability to construct and manipulate photonic topological states via photonic artificial microstructures. Although the concept of topology originates from condensed matter, topological photonics has given rise to new fundamental ideas and a range of potential applications that may lead to revolutionary technologies. Here, we review recent developments in topological photonics, with a focus on the realization and application of several emerging research areas in photonic artificial microstructures. We highlight the research trend, spanning from the photonic counterpart of topological insulator phases, through topological semimetal phases, to other emerging non-Hermitian topologies.

Unlike traditional optical components, which rely on the gradual accumulation of light along the optical path over a distance much larger than the wavelength to form a wavefront, metasurfaces manipulate light field properties on the wavelength thickness by specially arranging various meta-atoms. Due to the ease of integration and compact planar structure, metasurfaces play a key role in the light field manipulations. Here, we review the recent advances of metasurfaces in multi-dimensions, including light wavelength, polarization, orbital angular momentum(OAM), and angular response. Progress in these fields has brought new applications in areas such as imaging, display, communication, and information encryption, etc. Finally, we also discuss the challenges and prospects of metasurfaces applications.

Oblique plane microscopy (OPM) directly captures object information in a plane tilted from the focal plane of the objective lens without the need for slow z-stack acquisition. This unconventional widefield imaging approach is made possible by using a remote focusing principle that eliminates optical aberrations for object points beyond the focal plane. Together with oblique lightsheet illumination, OPM can make conventional lightsheet imaging fully compatible with standard biological specimens prepared on microscope slides. OPM is not only an excellent high-speed volumetric imaging platform by sweeping oblique lightsheet illumination without mechanically moving either the sample or objective lens in sample space, but also provides a solution for direct oblique plane imaging along any orientation of interest on the sample in a single shot. Since its first demonstration in 2008, OPM has continued to evolve into an advanced microscope platform for biological, medical, and materials science applications. In recent years, many technological advances have been made in OPM with the goal of super-resolution, fast volumetric imaging, and a large imaging field of view, etc. This review gives an overview of OPM’s working principle and imaging performance and introduces recent technical developments in OPM methods and applications. OPM has strong potential in a variety of research fields, including cellular and developmental biology, clinical diagnostics in histology and ophthalmology, flow cytometry, microfluidic devices, and soft materials.

Non-Hermitian topological band structures such as symmetry-protected exceptional rings (SPERs) can emerge for systems described by the generalized eigenvalue problem (GEVP) with Hermitian matrices. In this paper, we numerically analyze a photonic crystal with negative index media, which is described by the GEVP with Hermitian matrices. Our analysis using COMSOL Multiphysics ® demonstrates that a SPER emerges for photonic crystals composed of split-ring resonators and metal-wire structures. We expect that the above SPER can be observed in experiments as it emerges at a finite frequency.

Optical metasurfaces have great potential to overcome the functional limitations of conventional optical devices. In addition to polarization- or wavelength-multiplexed metasurfaces, angle-multiplexed metasurfaces can provide new degrees of freedom, enabling previously unrealized complex functionality in diverse applications such as LiDAR, augmented reality glasses, and imaging. However, there have been fundamental trade-offs in transmission efficiency and angular sensitivity for practically important paraxial rays. In this paper, we overcome this limitation by breaking mirror symmetries of single-layer metasurface structures. Based on an effective medium theory, we intuitively explain which material parameters affect the sensitivity and efficiency and prove that high sensitivity and high efficiency can be achieved simultaneously by breaking the mirror symmetry. Based on this, we propose optimized metasurfaces for two applications: an angle-multiplexed beam-steering device with up to 93% relative efficiency and an angle-multiplexed metalens array that can break the fundamental resolution–density trade-off of microlens arrays with high efficiency. The proposed angle-selective designs could pave the way for the development of new classes of compact optical devices with novel functions.

Metasurface technology is revolutionizing the field of optics and pursuing expanded functions via technical developments, such as the integration of multiple metasurfaces with optical fibers. Despite several attempts to realize metasurface-on-fiber platforms, negligible fiber-facet areas pose a serious obstacle to efficient and precise fabrication. Herein, we demonstrate a novel sequential micro-punching process that enables rapid and precise stacking of multiple polymer metasurfaces on the end face of a single-mode optical fiber. Mesh-type nanohole metasurfaces are fabricated on a 1.8-μm-thick polymethyl methacrylate (PMMA) layer via e-beam lithography, and the PMMA layer is separated from the substrate and prepared in the form of a membrane using the external frame. Furthermore, the PMMA metasurfaces are sequentially punched through the fiber and stacked on top. Employing a micro-punching process, we demonstrate highly efficient all-polymer metalenses and orbital angular momentum (OAM) metasurfaces coupled with single-mode fibers operating in the telecommunication band. A 1550 nm laser beam passing through three metalens layers stacked on the fiber is focused at a distance of 135 μm with 83% efficiency. In addition, the 1550 nm beam passing through three OAM metasurfaces on the fiber is converted into a perfect vortex beam with a topological charge of 3. We believe that our proposed micro-punching process will cause a breakthrough in the fabrication of metasurface-integrated optical fibers that will be utilized in a wide range of applications.

We demonstrate an axisymmetric inverse-designed metalens to improve the performance of zone-plate-array lithography (ZPAL), one of the maskless lithography approaches, that offer a new paradigm for nanoscale research and industry. First, we derive a computational upper bound for a unit-cell-based axisymmetric metalens. Then, we demonstrate a fabrication-compatible inverse-designed metalens with 85.50% transmission normalized focusing efficiency at 0.6 numerical aperture at 405 nm wavelength; a higher efficiency than a theoretical gradient index lens design (79.98%). We also demonstrate experimental validation for our axisymmetric inverse-designed metalens via electron beam lithography. Metalens-based maskless lithography may open a new way of achieving low-cost, large-area nanofabrication.

Designing perfect anomalous reflectors is crucial for achieving many metasurface-based applications, but available design approaches for the cases of extremely large bending angles either require unrealistic gain–loss materials or rely on brute-force optimizations lacking physical guidance. Here, we propose a deterministic approach to design passive metasurfaces that can reflect impinging light to arbitrary nonspecular directions with almost 100% efficiencies. With both incident and out-going far-field waves given, we can retrieve the surface-impedance profile of the target metadevice by matching boundary conditions with all allowed near-field modes added self-consistently and then construct the metadevices deterministically based on passive meta-atoms exhibiting local responses. We design/fabricate two proof-of-concept microwave metadevices and experimentally demonstrate that the first one achieves anomalous reflection to a 70° angle with efficiency ∼98%, and the second one can generate multiple reflected beams with desired bending angles and power allocations. Our findings pave the way for realizing high-efficiency wave-control metadevices with desired functionalities.

Nonlinear frequency mixings have shown an alternative way to create new electromagnetic sources in frequency ranges that are difficult to access with conventional techniques. To simultaneously use the fundamental frequency pump beam and multiple harmonic signals generated in the same channel, a device capable of separating each frequency component is required. Here, we propose and experimentally demonstrate metasurface-based spatial filters for the pump frequency and multiple harmonic frequencies. The metasurface was designed using eight different split ring resonator-based phase elements with 45° phase spacing, which allows wavefront shaping. The metasurface designed to have a one-dimensional gradient phase array produces cross-polarized reflection waves with different beam steering angles at the third- and fifth-harmonic frequencies (15 and 25 GHz) and operates as a metallic mirror at the fundamental frequency of 5 GHz. Our work suggests a new method to enable simultaneous use of broadband multi-frequency sources based on nonlinear frequency mixing.

Bound states in the continuum (BICs) have attracted significant interest in recent years due to their unique optical properties, such as infinite quality factor and wave localization. In order to improve the optical performance of BICs based devices, more degrees of freedom are required to tune BICs in high-dimension parameter space for practical applications. To effectively tune more BICs, we form a 2D synthetic parameter space based on a nanohole metasurface array. Multiple symmetry protected BIC modes with high Q factors can be achieved at high-order symmetry point. Through manipulating asymmetry parameters, BIC lines formed by a series of BIC modes can be found in the 2D synthetic parameter space. Moreover, the electric field distributions are investigated to demonstrate the generation and evolution of BICs. By measuring the absorption spectra, the tuning of multiple BICs with synthetic asymmetry parameters is experimentally explored, which agrees well with theoretical results. Therefore, our design can provide new insight for a variety of on-chip applications, such as nonlinear devices, integrated nanolasing array, and high-resolution sensors for infrared molecular detection.

We propose a metasurface antenna capable of real-time holographic beam steering. An array of reconfigurable dipoles can generate on-demand far-field patterns of radiation through the specific encoding of meta-atomic states i.e., the configuration of each dipole. Suitable states for the generation of the desired patterns can be identified using iteration, but this is very slow and needs to be done for each far-field pattern. Here, we present a deep-learning-based method for the control of a metasurface antenna with point dipole elements that vary in their state using dipole polarizability. Instead of iteration, we adopt a deep learning algorithm that combines an autoencoder with an electromagnetic scattering equation to determine the states required for a target far-field pattern in real-time. The scattering equation from Born approximation is used as the decoder in training the neural network, and analytic Green’s function calculation is used to check the validity of Born approximation. Our learning-based algorithm requires a computing time of within 200 μs to determine the meta-atomic states, thus enabling the real-time operation of a holographic antenna.

Optical phased array (OPA), as a promising beam steering technology, however, usually suffers from a narrow field of view (FOV) that limits its performances in applications. A miniaturized compact strategy to enlarge the beam steering angle is quite desirable for the solid-state OPA technique. Here an on-chip metasurface-doublet is proposed to offer angle magnification integrated with a port-selected optical phased array. It is implemented by combing convex and concave metalenses with the quadratic phase distribution, which is precisely integrated on the OPA chip by layer-by-layer fabrication process. Here, the OPA is fabricated in Lithium Niobate on Insulator (LNOI) platform. Our experiments show that the metasurface-doublet is able to achieve 1.54 times FOV amplification in a horizontal direction and with >41% working efficiency. Our results provide a feasible approach to achieve enlarged FOV for wide-angle beam steering and also imply a powerful platform in developing integrated multilayer metasurface devices.

The digital and programmable metasurfaces, as opposed to conventional metasurfaces, offer a more sophisticated method of collaborating information and physics, showcasing several real-time controls to electromagnetic (EM) ways in succinct ways. In this work, we propose a multi-frequency amplitude-programmable (MFAP) metasurface with multiple frequency channels to enhance the presentation and manipulation of EM data. With this metasurface, the reflected amplitudes can be simultaneously and independently encoded between high (digit “1”) and low (digit “0”) levels. The amplitude code is unique, which exhibits both reflection coefficients and radiation patterns to allow for flexible multi-functional EM operations with frequency. For instance, the MFAP metasurface can be used to design innovative communication systems by transmitting various EM signals individually across the channels in time domain. It is also possible to carry out multi-bit transmissions by mixing these frequency channels. By introducing complex coding patterns in space domain, it is possible to manipulate EM powers with greater precision. A square-split ring meta-atom that can achieve stable single-frequency amplitude control and multi-frequency 1 bit amplitude-programmable features is described as a proof-of-concept. Varactors loaded on metallic structures of various sizes are switched between operating states to modify the amplitude codes at each frequency channel. The suggested MFAP metasurface’s validity is confirmed by simulations and measurements from a dual-channel MFAP metasurface prototype.

A broadband absorber that utilizes a dispersive metamaterial and covers the entire microwave X-band (8–12 GHz) is proposed in the present study. An ideal absorber attached to the surface of a perfect electric conductor requires the permittivity of the absorbing layer to be anomalously dispersive in the targeted broad frequency band. We show that anomalous dispersion of the permittivity for the X-band can be fitted to a two-pole Lorentz oscillator model and realized with the use of a double-layered, square-loop metamaterial. We explain the connection between the two-pole oscillator model and the double-layered, square-loop metamaterial using an equivalent circuit model and present explicit design rules for the metamaterial. We fabricate a 4-mm-thick metamaterial absorber with flexible silicon rubber, a resistor element, and conductive wire using carbon and silver conductive ink. Our metamaterial absorber achieves a reflectance of less than −20 dB over the entire X-band region.

Metamaterial absorbers over a broadband spectrum with high absorption, good angular tolerance, and easy configurations have essential importance for optical and optoelectronic devices. In this study, a hybrid metamaterial absorber comprising multilayered cermet thin films (multi-cermet) with tapered structure is designed and experimentally demonstrated. Combining optical interference of multi-cermet films and optical field localization of nano-cone structures, the average absorbance of both simulation and measurement are more than 98% in an ultrabroad bandwidth (300–3000 nm), and the proposed absorber shows a good angular tolerance as well. The composite process of two easy-operated and efficient methods, colloidal lithography, and magnetron sputtering, is employed for large-area fabrication. In addition, owing to flexible polyimide substrate, the proposed absorber also shows good bending and heating resistance, which reflects its potential in engineering application.

Perfect absorbers based on all-dielectric metasurfaces exhibit great potential in photodetection, photovoltaics, and imaging applications. This study proposes and demonstrates an all-dielectric broadband absorber comprising subwavelength-thick nanopillar Mie resonators in the visible light range. This nanopillar functions as a perfect absorber based on degenerate critical coupling with a characteristic “degenerate critical length.” At this length, the nanopillars are capable of achieving perfect absorption. Beyond this length, the peak of perfect absorption is not affected with further increases in the length of the nanopillars. Hence, this study realizes broadband absorption via the stacking of amorphous silicon and germanium nanopillars with the same width at different peak absorption wavelengths. The absorption spectra are almost independent of the order of the stacked structures; hence, the stacked nanopillars in the specific stacking order can behave as a vertical photon sorter, sorting photons based on the wavelength. This study provides a systematic route to the realization of broadband absorbers with vertical photon sorting capability via the vertical stacking of nanopillars.

Enhanced directionality of photoluminescence emission has attracted attention due to its diverse application areas ranging from single-photon sources to fluorescence sensing and bio-imaging. Utilization of null phase advance in epsilon-near-zero (ENZ) medium is an important scheme to achieve the directive emission. Despite various designs proposed for ENZ-based directive emission, most of the ENZ mediums are restricted to subwavelength structures of metallic plasmonics or inorganic dielectrics. Here, we introduce an organic ENZ film placed on top of a polymeric fluorophore film to demonstrate a directive emission. By taking advantage of the structural coherence in the P3HT film and the ENZ response in the squaraine molecular film, 42 % increase in directive emission is achieved. Capability to control directive emission with organic ENZ films is highly useful in applications requiring bio-compatibility of a fluorophore-embedding medium.

The magic angle of Twistronics has attracted a lot of attention because of its peculiar electrical characteristics. Moiré patterns formed by the superlattice of a twisted bilayer change overall physical properties. Circular dichroism can also be manipulated through the generated moiré pattern. Here, we report a polymer-based twisted bilayer meta-device fabricated by multilayer nanoimprint technology and study the magic angle of chirality. The superlattice of the bilayer meta-device creates moiré patterns and brings unique chiral optical responses. The bilayer nanoimprint technology is developed for metasurfaces with relative twist angles. Via the twist angle control, polymer materials with a low refractive index can manipulate the electric field of the light and reveal the chiral magic angle. Moreover, the shape of the meta-atoms plays a key role in chiral magic angle tuning. The chirality engineering by the reported nanoimprint technology and chiral meta-devices may contribute to applications in chiral imaging, biomedical sensing, lasing, and tunable optical devices.

Single-molecule localization microscopies have gained much attention for their efficient realization of a sub-diffraction-limit imaging with the resolution down to the 10-nm range. In contrast to conventional localization microscopes, which rely on particular fluorescent probes in specific conditions, metamaterial-assisted super-resolution microscopies can be implemented with any fluorescent dye under general conditions. Here, we present a systematic study of fluorescence engineering in metamaterial assisted localization microscopy by using cyclic group metasurfaces coated with a fluorescent film. Tailored variations are clearly demonstrated in both the photoluminescence intensity and the photobleaching lifetime of fluorophores based on the spatially varied Purcell effect near the metasurfaces. The enhanced emissions and blinking dynamics of the fluorophores on these metasurfaces lead to an increased signal-to-noise ratio, and therefore give rise to a super-resolution localization image with 0.9-nm localization accuracy. Our results are not only beneficial for super-resolution localization imaging but also push the control of light–matter interactions beyond the diffraction limit.

Localized surface plasmon polaritons can confine the optical field to a single-nanometer-scale area, strongly enhancing the interaction between photons and molecules. Theoretically, the ultimate enhancement might be achieved by reducing the “photon size” to the molecular extinction cross-section. In addition, desired control of electronic transitions in molecules can be realized if the “photon shape” can be manipulated on a single-nanometer scale. By matching the photon shape with that of the molecular electron wavefunction, optically forbidden transitions can be induced efficiently and selectively, enabling various unconventional photoreactions. Here, we demonstrate the possibility of forming single-nanometer-scale, highly intense fields of optical vortices using designed plasmonic nanostructures. The orbital and spin angular momenta provided by a Laguerre–Gaussian beam are selectively transferred to the localized plasmons of a metal multimer structure and then confined into a nanogap. This plasmonic nano-vortex field is expected to fit the molecular electron orbital shape and spin with the corresponding angular momenta.

Electrons injected into one-dimensional (1D) metals are efficiently converted into infrared plasmons because the unique property of the Luttinger liquid, a strongly correlated electronic matter in one-dimensional (1D) metals, prohibits excitations of other quasiparticles. Using the Luttinger liquid behavior, the electrically driven 1D metals can be used as a feed for optical nanoantennas. Nanoantennas can couple the 1D Luttinger liquid plasmons in the feed to the radiating photons in free space. In this work, we suggest design principles for the 1D metallic Luttinger liquid feed and the nanoantennas to obtain high injection and radiation efficiencies, respectively. We expect that our work can promote experimental efforts to realize electrically driven Luttinger liquid-fed nanoantennas and efficient infrared light sources.

Strong-field terahertz (THz)–matter interaction permits the investigation of nonequilibrium behaviors in the nonperturbative zone. However, the unavailability of a high-field free-space THz source with high repetition rates, excellent beam quality, and high stability hinders its development. In this work, we obtain the nonlinear modulation dynamics of a “THz-nano” metasurface on silicon substrates using a time-resolved strong-field THz-pump THz-probe (TPTP) with a thousand orders local field enhancement through confining THz waves into nano-gaps (15 nm, λ /33,000). By switching the THz field strength, we successfully realize a self-modulation ∼50 GHz frequency shift, which is further verified via the TPTP ultrafast time-resolution technique. The phenomenon is attributed to the impact ionization (IMI) of the silicon substrate under the excitation of extremely confined strong THz fields in nano-gaps. Both strong-field induced intervalley scattering (IVS) and IMI effects of photodoped silicon occurring in nano-gaps and large-area substrates were also observed by 800 nm optical injection of carriers. These aforementioned findings provide a robust research platform for the realization of ultrafast time resolution nanoscale strong-field THz–matter interaction and new ideas for nonextreme laboratories to realize extreme THz science, applications, and THz nonlinear modulation device development.

We propose reconfigurable anomalous reflectors with stretchable elastic substrates. The proposed reflector dynamically controls the reflection direction by mechanically stretching the substrate to induce a physical change of the unit cell period. Owing to the simple and scalable tuning mechanism, the proposed approach is applicable in the millimeter-wave and terahertz bands for a wide reflection steering. To demonstrate the proposed approach, stretchable anomalous reflectors are designed at 140 GHz for normal incident waves. From full-wave simulations, we numerically confirm that highly efficient anomalous reflections with suppressed parasitic reflections in the undesired directions are achieved toward shallower angles as the substrate is stretched. We experimentally demonstrate that the proposed reflectors allow a dynamic control of the reflection direction with wide steering ranges of more than 20°. Moreover, we confirm that the measured efficiencies of the anomalous reflections hardly deteriorate when stretching and maintain practically acceptable performances of over 50 %. The proposed stretchable reflectors have a potential to be used for a reconfigurable intelligent surface (RIS) that realizes dynamic optimizations of the wireless environment in the 6G communication.

Metasurfaces substituted for naturally occurring materials make it possible to develop flat optics manipulating terahertz waves. However, the control of unprecedented material properties with metasurfaces frequently produces anisotropic material properties and has yet to be commonly adopted because of the limitation of functionalities as optical components. Here, we demonstrate an isotropic metasurface with polarization-independent material properties with the extremely high refractive index of 14.0 + j 0.49, low reflectance of 1.0 %, and high transmittance of 86.9 % at 0.31 THz. Measurements by terahertz time-domain spectroscopy (THz-TDS) verify that the fabricated metasurface with a high refractive index, low reflectance, and high transmittance works for terahertz waves with any polarization direction and results in the unprecedented material characteristics with polarization independence. The relative permittivity and relative permeability are 13.9 – j 1.4 and 13.8 + j 2.3, respectively. The sum of the dielectric and magnetic energy losses must also be considered to verify the conservation of energy for metasurfaces. The sum of the dielectric and magnetic energy losses is very close to positive values and the conservation of energy is largely satisfied. The proposed metasurface would offer optical components with attractive functionalities such as wavefront control, directivity enhancement, and optical vortices for 6G communications.

We propose a terahertz nanofuse through irreversible modulations in transmitted terahertz using nanowires-combined nanoantenna structures. Semiconductor and metal nanowires show irreversible reconfiguration in their geometry at an incident field of 20 kV/cm. The concept can be explained by terahertz-field-induced ionization or electromigration. A strongly localized field due to geometrical conditions causes a floated metal nanowire from one side of a metal nanoantenna to touch the opposite side, bridging two separate metal plates and creating a junction. For the bridged nanoantenna, the highly enhanced field induced the breaking of the connection across the metal sides of a nanoantenna. In the bridging and breaking cases, permanent transformation occurs in opposite structural forms. It encompasses a potential application as an optical fuse to protect sensitive terahertz devices under excessive field focus.

Anisotropic materials with chirality or birefringence can be used to manipulate the polarization states of electromagnetic waves. However, the comparatively low anisotropy of natural materials hinders the miniaturization of optical components and devices at terahertz frequencies. In this study, we experimentally demonstrate that the relative phase retardation of a THz wave can be electrically controlled by integrating patterned mono- and bilayer graphene onto an otherwise isotropic metasurface. Specifically, we show that a refractive index for one of the orthogonal polarization states can be electrically controlled by modulating graphene’s conductivity, thereby weakening the capacitive coupling between adjacent meta-atoms in an anisotropic manner. With monolayer graphene, phase retardation of 15° to 81° between two orthogonal polarization states can be achieved. Maximum phase retardation of 90° through a metasurface with bilayer graphene suggests its use as a tunable quarter-wave plate. Continuous control from linear- to circular-polarization states may provide a wide range of opportunities for the development of compact THz polarization devices and polarization-sensitive THz technology.

An ultra-thin transition metal dichalcogenide (TMDC) layer can support guided exciton-polariton modes due to the strong coupling between excitons and photons. Herein, we report the guided mode resonance in an ultra-thin TMDC grating structure. Owing to the strong exciton resonances in TMDCs, a TMDC grating structure shows guided-mode resonance even at a thickness limit of ∼10 nm and is capable of realizing polaritonic dispersion in a monolithic grating structure. We investigated the polarization and thickness dependence of the optical dispersion relations of the tungsten disulfide (WS 2 ) grating structure. In addition, we confirmed that the monolithic WS 2 grating coupler can be used to couple the near-field guided exciton-polariton out into the far field. We believe that ultra-thin TMDC layers can facilitate sub-wavelength nanophotonic applications.

Owning to their unusual optical properties, such as electrical tunability and strong spatial confinement, two-dimensional surface polaritons (2DSPs) hold great promise for deep sub-wavelength manipulation of light in a reduced low-dimensional space. Control of 2DSPs is possible by using their interaction with a boundary between two media, similar to how light behaves in three-dimensional (3D) space. The understanding of the interaction in the 2D case is still in its early stages, unlike the 3D case, as in-depth investigations are only available in a few cases including the interaction of 2DSPs with structured 2D crystals. Here, we extend the scope of our understanding to the interaction of 2DSPs with metallic nano-plates on 2D crystals, focusing on the reflection of 2DSPs. Through our rigorous model, we reveal that, for strongly confined 2DSPs having much larger momentum than free space photons, the interaction results in almost total internal reflection of 2DSPs as the radiative coupling of the 2DSPs to free space is negligible. We also find that the reflection involves an anomalous phase shift dependent on the thickness of the nano-plate, due to the temporary storing of electromagnetic energy in the evanescent waves induced near the edge of the nano-plate. Our theory predicts that the phase shift saturates to an anomalous value, 0.885 π , as the nano-plate becomes thicker. Our work provides a detailed understanding of how to manipulate the 2DSPs by using one of the simplest nanostructures, essential for the further development of nanostructure-integrated low-dimensional devices for polariton optics.

The scattering matrix is the mathematical representation of the scattering characteristics of any scatterer. Nevertheless, except for scatterers with high symmetry like spheres or cylinders, the scattering matrix does not have any analytical forms and thus can only be calculated numerically, which requires heavy computation. Here, we have developed a well-trained deep neural network (DNN) that can calculate the scattering matrix of scatterers without symmetry at a speed thousands of times faster than that of finite element solvers. Interestingly, the scattering matrix obtained from the DNN inherently satisfies the fundamental physical principles, including energy conservation, time reversal and reciprocity. Moreover, inverse design based on the DNN is made possible by applying the gradient descent algorithm. Finally, we demonstrate an application of the DNN, which is to design scatterers with desired scattering properties under special conditions. Our work proposes a convenient solution of deep learning for scattering problems.

Selective manipulation of energy levels plays an essential role in realizing multichannel wave devices. One of the representative examples is to utilize the concept of quasi-isospectrality: a family of wave systems with an almost identical spectrum except for a part of energy levels. Most approaches toward quasi-isospectrality have employed analytical methods based on symmetry or tridiagonalization, such as supersymmetry, Householder, or Lanczos transformations. Although such analytical approaches provide deterministic and stable designs based on operator factorizations, the mathematical strictness in the factorizations, at the same time, hinders isospectral engineering in a given multidimension. Here we develop the semi-analytical method for engineering isospectrality in multidimensional photonic systems. The method provides the systematic perturbation for the target energy level shifts by decomposing the allowed form of system changes into the perturbation basis. We demonstrate the isospectrality of lower-, higher-, and random-order states while imposing the designed shifts on the other states. The stability analysis shows that the accuracy of the method is determined by the ranges of isospectral state numbers and perturbation strength. The systematic, free-form, and multidimensional natures of the proposed method show great potential for the platform-transparent design of multichannel devices.

We demonstrate heterogeneous integration of active semiconductor materials into the conventional passive metal-insulator-metal (MIM) waveguides to provide compact on-chip light generation and detection capabilities for chip-scale active nanophotonic platforms. Depending on its bias conditions, a metal-semiconductor-metal section can function as either a light emitting diode or a photodetector directly connected to the MIM waveguides. We experimentally verify the independent and combined operations of electrically-driven on-chip light sources and photodetectors.

Moulding the flow of phononic waves in three-dimensional (3D) space plays a critical role in controlling the sound and thermal properties of matter. To this end, 3D phononic crystals (PnCs) have been considered the gold standard because their complete phononic bandgap (PnBG) enables omnidirectional inhibition of phononic wave propagation. Nevertheless, achieving a complete PnBG in the high-frequency regime is still challenging, as attaining the correspondingly demanded mesoscale 3D crystals consisting of continuous frame networks with conventional fabrications is difficult. Here, we report that a DNA origami-designed-3D crystal can serve as a hypersonic 3D PnC exhibiting the widest complete PnBG. DNA origami crystallization can unprecedentedly provide 3D crystals such that continuous frame 3D crystals at the mesoscale are realizable. Furthermore, their lattice symmetry can be molecularly programmed to be at the highest level in a hierarchy of symmetry groups and numbers, which can facilitate the widening of the PnBG. More importantly, conformal silicification can render DNA origami-3D crystals rigid. Overall, we predict that the widest hypersonic PnBG can be achieved with DNA origami-designed 3D crystals with optimal lattice geometry and silica fraction; our work can provide a blueprint for the design and fabrication of mesoscale 3D PnCs with a champion PnBG.

In this article, we propose all-dielectric carpet cloaks composed of jungle gym shaped dielectric unit cells and present a design strategy for three-dimensional (3-D) anisotropy control based on the transformation optics. The carpet cloaks are 3-D printable and operate with polarization independent incident waves in arbitrary incident angles due to the 3-D anisotropy control. Realizable anisotropic permittivities of cubic and rectangular unit cells are numerically studied based on the relative permittivity and loss tangent of ɛ r = 2.9 and tan  δ = 0.02 of ultra-violet curing resin measured at the microwave frequency. It is shown that the unit cell has little frequency dependence even with the anisotropy in the low frequency range where the effective medium approximation is valid. A carpet cloak is designed based on the design method with a quasi-conformal coordinate transformation and implemented with the unit cells taking into account its realizable anisotropy. Polarization independent 3-D cloaking operations of the designed cloak are confirmed numerically. The designed cloak is fabricated by stereolithography 3-D printing technology and its cloaking performances are evaluated experimentally at 10 GHz. It is shown that non-specular reflections are well suppressed by the carpet cloak for both TE and TM incident waves with different incident angles of 30, 45, and 60°. Frequency independent cloaking operations are also shown experimentally in the X-band. The measured near-field distributions and bistatic radar cross sections are in good agreement with simulated predictions and the validity of the design method is confirmed.