Tuning Smith-Purcell radiation by rotating a metallic nanodisk array.

Smith-Purcell radiation (SPR) refers to the far-field, strong, spike radiation generated by the interaction of the evanescent Coulomb field of the moving charged particles and the surrounding medium. In applying SPR for particle detection and nanoscale on-chip light sources, wavelength tunability is desired. Here we report on tunable SPR achieved by moving an electron beam parallel to a two-dimensional (2D) metallic nanodisk array. By in-plane rotating the nanodisk array, the emission spectrum of the SPR splits into two peaks, with the shorter-wavelength peak blueshifted and the longer-wavelength one redshifted by increasing the tuning angle. This effect originates from the fact that the electrons fly effectively over a one-dimensional (1D) quasicrystal projected from the surrounding 2D lattice, and the wavelength of SPR is modulated by quasiperiodic characteristic lengths. The experimental data are in agreement with the simulated ones. We suggest that this tunable radiation provides free-electron-driven tunable multiple photon sources at the nanoscale.

Improving Photoelectric Conversion with Broadband Perovskite Metasurface.

The miniaturization and integration of optoelectronic devices require progressive size reduction of active layers, resulting in less optical absorption and lower quantum efficiency. In this work, we demonstrate that introducing a metasurface made of hybrid organic-inorganic perovskite (HOIP) can significantly enhance broadband absorption and improve photon-to-electron conversion, which roots from exciting Mie resonances together with suppressing optical transmission. On the basis of the HOIP metasurface, a broadband photodetector has been fabricated where photocurrent boosts more than 10 times in the frequency ranging from ultraviolet to visible. The device response time is less than 5.1 µs at wavelengths 380, 532, and 710 nm, and the relevant 3 dB bandwidth is over 0.26 MHz. Moreover, this photodetector has been applied as a signal receiver for transmitting 2D color images in broadband optical communication. These results accentuate the practical applications of HOIP metasurfaces in novel optoelectronic devices for broadband optical communication.

Direct observation of terahertz topological valley transport.

Topological photonics offers the possibility of robust transport and efficiency enhancement of information processing. Terahertz (THz) devices, such as waveguides and beam splitters, are prone to reflection loss owing to their sensitivity to defects and lack of robustness against sharp corners. Thus, it is a challenge to reduce backscattering loss at THz frequencies. In this work, we constructed THz photonic topological insulators and experimentally demonstrated robust, topologically protected valley transport in THz photonic crystals. The THz valley photonic crystal (VPC) was composed of metallic cylinders situated in a triangular lattice. By tuning the relevant location of metallic cylinders in the unit cell, mirror symmetry was broken, and the degenerated states were lifted at the K and K' valleys in the band structure. Consequently, a bandgap of THz VPC was opened, and a nontrivial band structure was created. Based on the calculated band structure, THz field distributions, and valley Berry curvature, we verified the topological phase transition in such type of THz photonic crystals. Further, we showed the emergence of valley-polarized topological edge states between the topologically distinct VPCs. The angle-resolved transmittance measurements identified the bulk bandgap in the band structure of the VPC. The measured time-domain spectra demonstrated the topological transport of valley edge states between distinct VPCs and their robustness against bending and defects. Furthermore, experiments conducted on a topological multi-channel intersectional device revealed the valley-polarized characteristic of the topological edge states. This work provides a unique approach to reduce backscattering loss at the THz regime. It also demonstrates potential high-efficiency THz functional devices such as topologically protected beam splitters, low-loss waveguides, and robust delay lines.

Multiple-polarization-sensitive photodetector based on a perovskite metasurface.

Most polarization-sensitive photodetectors detect either linearly polarized (LP) or circularly polarized (CP) light. Here, we experimentally demonstrate a multiple-polarization photodetector based on a hybrid organic-inorganic perovskite (HOIP) metasurface, which is sensitive to both LP and CP light simultaneously. The perovskite metasurface is composed of a HOIP antenna array on a single-crystal HOIP film. Owing to the antenna anisotropy, the absorption of linearly polarized light at the metasurface depends on the polarization angle; also, due to the mirror asymmetry of the antenna elements, the metasurface is also sensitive to different circular polarizations. Polarization-dependent photocurrent responses to both LP and CP light are detected. Our results highlight the potential of perovskite metasurfaces for integrated photoelectric applications.

Multichannel Distribution and Transformation of Entangled Photons with Dielectric Metasurfaces.

Photonic quantum information processing relies on operating the quantum state of photons, which usually involves bulky optical components unfavorable for system miniaturization and integration. Here, we report on the transformation and distribution of polarization-entangled photon pairs with multichannel dielectric metasurfaces. The entangled photon pairs interact with metasurface building blocks, where the geometrical-scaling-induced phase gradients are imposed, and are transformed into two-photon entangled states with the desired polarization. Two metasurfaces, each simultaneously distributing polarization-entangled photons to spatially separated multiple channels M (N), may accomplish M×N channels of entanglement distribution and transformation. Experimentally we demonstrate 2×2 and 4×4 distributed entanglement states, including Bell states and superposition of Bell states, with high fidelity and strong polarization correlation. We expect this approach paves the way for future integration of quantum information networks.

Bendable disordered metamaterials for broadband terahertz invisibility.

We experimentally demonstrate a bendable cloaking structure composed of obliquely stacked planar metallic shells that individually enclose the objects to be hidden. The ensemble of shells acts as a disordered oblique grating capable of bending along a curved structure and exhibits broadband invisibility from 0.2 to 1.0 THz. Hiding cloaked objects sized hundreds of microns could prevent the detection of certain powders that are sensitive to terahertz waves; such a cloaking structure can also be considered as a shape-changing passageway that transfers the electromagnetic waves without interfering with them. Our approach provides a unique way to achieve broadband electromagnetic invisibility.

Strong Localization of Surface Plasmon Polaritons with Engineered Disorder.

In this work, we experimentally demonstrate for the first time strong localization of surface plasmon polaritons (SPPs) at visible regime in metallic nanogratings with short-range correlated disorder. By increasing the degree of disorder, the confinement of SPPs is significantly enhanced, and the effective SPP propagation length dramatically shrinks. Strong localization of SPPs eventually emerges at visible regime, which is verified by the exponentially decayed fields and the vanishing autocorrelation function of the SPPs. Physically, the short-range correlated disorder induces strong interference among multiple scattered SPPs and provides an adequate fluctuation to effective permittivity, which leads to the localization effect. Our study demonstrates a unique opportunity for disorder engineering to manipulate light on nanoscale and may achieve various applications in random nanolasing, solar energy, and strong light-matter interactions.

Broadband integrated polarization rotator using three-layer metallic grating structures.

In this work, we demonstrate broadband integrated polarization rotator (IPR) with a series of three-layer rotating metallic grating structures. This transmissive optical IPR can conveniently rotate the polarization of linearly polarized light to any desired directions at different spatial locations with high conversion efficiency, which is nearly constant for different rotation angles. The linear polarization rotation originates from multi-wave interference in the three-layer grating structure. We anticipate that this type of IPR will find wide applications in analytical chemistry, biology, communication technology, imaging, etc.

Hybrid coupling enhances photoluminescence of monolayer MoS2 on plasmonic nanostructures.

The efficiency of photoluminescence (PL) of transition-metal dichalcogenides (TMDCs) significantly influences their practical applications in optoelectronic devices. In this work, we study multiple coupling among excitons, surface plasmons, and optical modes, and their effects on PL of monolayer MoS2 atop plasmonic nanohole arrays. Under the illumination of visible light, strong intensity enhancement of PL from monolayer MoS2 is observed in the system. We further demonstrate that there exist excitons induced from MoS2, localized and propagating surface plasmons excited from nanoholes, and optical modes related to the incident laser. And hybrid coupling of those modes significantly improves the PL signals and also lightens the PL images of monolayer MoS2. This work provides a unique way to improve the emission of monolayer TMDCs. The atomically thin TMDCs coupled to plasmonic metamaterials are also promising for advanced applications such as ultrathin integrated light-emission diodes, photodetection, and nanolasers.

Plasmonic band structures in doped graphene tubes.

We present theoretically the transport of plasmonic waves in doped graphene tube, which is made by rolling planar graphene sheet into a cylinder and periodic doping is applied on it. It is shown that periodic modulation of the Fermi level along the tube can open gaps in the dispersion relations of graphene plasmons and eventually create plasmonic band structures. The propagation of graphene plasmons is forbidden within the bandgaps; while within the band, the plasmonic waves present axially-extended field distributions and propagate along the tubes, yet well confined around the curved graphene surface. Furthermore, the bandgaps, propagation constants and propagation lengths of the modes in plasmonic band structures are significantly tuned by varying the Fermi level of graphene, which provides active controls over the plasmonic waves. Our proposed structures here may provide an approach to dynamically control the plasmonic waves in graphene-based subwavelength waveguides.

Polarization-dependent strong coupling between surface plasmon polaritons and excitons in an organic-dye-doped nanostructure.

In this work, we demonstrate polarization-dependent strong coupling between surface plasmon polaritons (SPPs) and excitons in the J-aggregates-attached aperture array. It is shown that the excitons strongly couple with the polarization-dependent SPPs, and Rabi splittings are consequently observed. As a result, the polarization-dependent polariton bands are generated in the system. Increasing the incident angle, the polaritons disperse to higher energies under transverse-electric illumination, while the polaritons disperse to lower energies under transverse-magnetic illumination. Therefore, at different polarization incidence, we experimentally achieve distinct polaritons with opposite dispersion directions. In this way, tuning the polarization of the incident light, we can excite different polaritons whose energy propagates to different directions. Furthermore, by retrieving the mixing fractions of the components in these polariton bands, we find that the dispersion properties of the polaritons are inherited from both the SPPs and the excitons. Our investigation may inspire related studies on tunable photon-exciton interactions and achieve some potential applications on active polariton devices.

Band modulation and in-plane propagation of surface plasmons in composite nanostructures.

In this work, we have experimentally and theoretically studied band modulation and in-plane propagation of surface plasmons (SPs) in composite nanostructures with aperture arrays and metallic gratings. It is shown that the plasmonic band structure of the composite system can be significantly modulated because of coupling between the aperture and grating. By changing the relative positions between these optical components, the resonant modes would shift or split. And the resonant SP modes launched on the structure surface can be effectively modified by the geometric parameters. Further, we provide an experimental observation to directly show the SP in-plane propagation by using far-field measurements, which agree with the simulated results. Our study offers a convenient way for observing the SP propagation in far field, and provides unique composite nanostructures for possible applications in subwavelength optodevices, such as optical sensors and detectors.

Broadband absorption and efficiency enhancement of an ultra-thin silicon solar cell with a plasmonic fractal.

We report in this work that quantum efficiency can be significantly enhanced in an ultra-thin silicon solar cell coated by a fractal-like pattern of silver nano cuboids. When sunlight shines this solar cell, multiple antireflection bands are achieved mainly due to the self-similarity in the fractal-like structure. Actually, several kinds of optical modes exist in the structure. One is cavity modes, which come from Fabry-Perot resonances at the longitudinal and transverse cavities, respectively; the other is surface plasmon (SP) modes, which propagate along the silicon-silver interface. Due to the fact that several feature sizes distribute in a fractal-like structure, both low-index and high-index SP modes are simultaneously excited. As a whole effect, broadband absorption is achieved in this solar cell. Further by considering the ideal process that the lifetime of carriers is infinite and the recombination loss is ignored, we demonstrate that external quantum efficiency of the solar cell under this ideal condition is significantly enhanced. This theoretical finding contributes to high-performance plasmonic solar cells and can be applied to designing miniaturized compact photovoltaic devices.

Coupling of surface plasmons and excited optical modes in metal/dielectric grating stacks.

In this work, we investigate the coupling of surface plasmons and excited optical modes in metal/dielectric grating stacks theoretically and experimentally. We have observed three kinds of modes in these structures: the cavity mode, the propagated surface plasmon (PSP) mode and the localized surface plasmon (LSP) mode, which can enhance the optical transmission. Firstly, it is shown that the cavity mode is excited in the grating stacks. And the cavity mode has redshift if we enhance the thickness of metal layers, while it has blueshift when we increase the thickness of dielectric layers. The redshift of the cavity mode also occurs when the number of repeating layers is increased. Secondly, the PSP mode is also excited, which can be described by the effective permittivity method. It is found that the PSP modes are coupled with each other, which leads to a modified dispersion relation of surface plasmon polaritons (SPP). The theoretical analysis is in good agreement with the observed transmission enhancement in the grating stacks. And the coupling of PSPs also leads to a blueshift when the number of metal layers is increased. Thirdly, the LSP mode, generated in single metal strip, can also enhance the optical transmission of the grating stacks. Yet the transmission intensity induced by LSP decreases rapidly with increasing the number of metal layers. The investigations here may have potential applications in designing plasmonic metamaterials and subwavelength optical devices.

Influence of the interlayer on coupling of surface plasmons in a sandwiched structure with periodic array of nanoapertures.

In this work, we investigate the optical properties of a multilayer structure, where a SiO2 film is sandwiched by silver films with periodic array of sub-wavelength apertures. Due to the coupling of surface plasmons (SPs) between different layers, electric and magnetic resonances have been observed. By varying the thickness of the interlayer SiO2, we can modify relative phase of the SPs resonance and control the shifts of transmission peaks. Experimentally the multilayers are fabricated by magnetron sputtering and the array of apertures is milled by focused-ion-beam facility. The measured optical transmission spectra reasonably agree with our numerical calculation, which bases on three-dimensional finite-difference time-domain method. To understand the shifts of the peaks, we present a phenomenological explanation, considering the transmission peaks as energy levels, and the coupling of localized surface plasmons as perturbation. These results may have potential applications in designing plasmonic devices and tuning electromagnetic wave in nanophotonics.

Making metals transparent for white light by spoof surface plasmons.

From first-principles computations we reveal that metallic gratings consisting of narrow slits may become transparent for extremely broad bandwidths under oblique incidence. This phenomenon can be explained by a concrete picture in which the incident wave drives free electrons on the conducting surfaces and part of the slit walls to form spoof surface plasmons (SSPs). The SSPs then propagate on the slit walls but are abruptly discontinued by the bottom edges to form oscillating charges that emit the transmitted wave. This picture explicitly demonstrates the conversion between light and SSPs and indicates clear guidelines for enhancing SSP excitation and propagation. Making structured metals transparent may lead to a variety of applications.

Constructing Metastructures with Broadband Electromagnetic Functionality.

Electromagnetic metastructures stand for the artificial structures with a characteristic size smaller than the wavelength, which may efficiently manipulate the states of light. However, their applications are often restricted by the bandwidth of the electromagnetic response of the metastructures. It is therefore essential to reassert the principles in constructing broadband electromagnetic metastructures. Herein, after summarizing the conventional approaches for achieving broadband electromagnetic functionality, some recent developments in realizing broadband electromagnetic response by dispersion compensation, nonresonant effects, and several trade-off approaches are reviewed, followed by some perspectives for the future development of broadband metamaterials. It is anticipated that broadband metastructures will have even more substantial applications in optoelectronics, energy harvesting, and information technology.

Tunable multiband directional electromagnetic scattering from spoof Mie resonant structure.

We demonstrate that directional electromagnetic scattering can be realized in an artificial Mie resonant structure that supports electric and magnetic dipole modes simultaneously. The directivity of the far-field radiation pattern can be switched by changing wavelength of the incident light as well as tailoring the geometric parameters of the structure. In addition, we further design a quasiperiodic spoof Mie resonant structure by alternately inserting two materials into the slits. The results show that multi-band directional light scattering is realized by exciting multiple electric and magnetic dipole modes with different frequencies in the quasiperiodic structure. The presented design concept is suitable for microwave to terahertz region and can be applied to various advanced optical devices, such as antenna, metamaterial and metasurface.

Freely tunable broadband polarization rotator for terahertz waves.

A freely tunable polarization rotator for broadband terahertz waves is demonstrated using a three-rotating-layer metallic grating structure, which can conveniently rotate the polarization of a linearly polarized terahertz wave to any desired direction with nearly perfect conversion efficiency. This low-cost, high-efficiency, and freely tunable device has potential applications as material analysis, wireless communication, and THz imaging.

Position-sensitive spectral splitting with a plasmonic nanowire on silicon chip.

On-chip nanophotonics serves as the foundation for the new generation of information technology, but it is challenged by the diffraction limit of light. With the capabilities of confining light into (deep) subwavelength volumes, plasmonics makes it possible to dramatically miniaturize optical devices so as to integrate them into silicon chips. Here we demonstrate that by cascading nano-corrugation gratings with different periodicities on silver nanowires atop silicon, different colors can be spatially separated and chronologically released at different grating junctions. The released light frequency depends on the grating arrangement and corrugation periodicities. Hence the nanowire acts as a spectral splitter for sorting/demultiplexing photons at different nano-scale positions with a ten-femtosecond-level interval. Such nanowires can be constructed further into compact 2D networks or circuits. We believe that this study provides a new and promising approach for realizing spatiotemporal-sensitive spectral splitting and optical signal processing on nanoscales, and for general integration of nanophotonics with microelectronics.