Volume 11, Issue 7

With recent advances in nanofabrication technology, various metallic gap structures with gap widths reaching a few to sub-nanometer, and even ‘zero-nanometer’, have been realized. At such regime, metallic gaps not only exhibit strong electromagnetic field confinement and enhancement, but also incorporate various quantum phenomena in a macroscopic scale, finding applications in ultrasensitive detection using nanosystems, enhancement of light–matter interactions in low-dimensional materials, and ultralow-power manipulation of electromagnetic waves, etc. Therefore, moving beyond nanometer to ‘zero-nanometer’ can greatly diversify applications of metallic gaps and may open the field of dynamic ‘gaptronics.’ In this paper, an overview is given on wafer-scale metallic gap structures down to zero-nanometer gap width limit. Theoretical description of metallic gaps from sub-10 to zero-nanometer limit, various wafer-scale fabrication methods and their applications are presented. With such versatility and broadband applicability spanning visible to terahertz and even microwaves, the field of ‘gaptronics’ can be a central building block for photochemistry, quantum optical devices, and 5/6G communications.

Driven by new two-dimensional materials, great changes and progress have taken place in the field of ultrafast photonics in recent years. Among them, the emerging single element two-dimensional materials (Xenes) have also received much attention due to their special physical and photoelectric properties including tunable broadband nonlinear saturable absorption, ultrafast carrier recovery rate, and ultrashort recovery time. In this review, the preparation methods of Xenes and various integration strategies are detailedly introduced at first. Then, we summarize the outcomes achieved by Xenes-based (beyond graphene) fiber lasers and make classifications based on the characteristics of output pulses according to the materials characterization and nonlinear optical absorption properties. Finally, an outlook of the future opportunities and challenges of ultrafast photonics devices based on Xenes and other 2D materials are highlighted, and we hope this review will promote their extensive applications in ultrafast photonics technology.

Nonlocality is a fundamental concept in photonics. For instance, nonlocal wave-matter interactions in spatially modulated metamaterials enable novel effects, such as giant electromagnetic chirality, artificial magnetism, and negative refraction. Here, we investigate the effects induced by spatial nonlocality in temporal metamaterials, i.e., media with a dielectric permittivity rapidly modulated in time. Via a rigorous multiscale approach, we introduce a general and compact formalism for the nonlocal effective medium theory of temporally periodic metamaterials. In particular, we study two scenarios: (i) a periodic temporal modulation, and (ii) a temporal boundary where the permittivity is abruptly changed in time and subject to periodic modulation. We show that these configurations can give rise to peculiar nonlocal effects, and we highlight the similarities and differences with respect to the spatial-metamaterial counterparts. Interestingly, by tailoring the effective boundary wave-matter interactions, we also identify an intriguing configuration for which a temporal metamaterial can perform the first-order derivative of an incident wavepacket. Our theoretical results, backed by full-wave numerical simulations, introduce key physical ingredients that may pave the way for novel applications. By fully exploiting the time-reversal symmetry breaking, nonlocal temporal metamaterials promise a great potential for efficient, tunable optical computing devices.

We propose a straightforward and effective approach to design, by pattern-tunable strain-engineering, photonic topological insulators supporting high quality factors edge states. Chiral strain-engineering creates opposite synthetic gauge fields in two domains resulting in Landau levels with the same energy spacing but different topological numbers. The boundary of the two topological domains hosts robust time-reversal and spin-momentum-locked edge states, exhibiting high quality factors due to continuous strain modulation. By shaping the synthetic gauge field, we obtain remarkable field confinement and tunability, with the strain strongly affecting the degree of localization of the edge states. Notably, the two-domain design stabilizes the strain-induced topological edge state. The large potential bandwidth of the strain-engineering and the opportunity to induce the mechanical stress at the fabrication stage enables large scalability for many potential applications in photonics, such as tunable microcavities, new lasers, and information processing devices, including the quantum regime.

Opening a new door to tailoring electromagnetic (EM) waves, temporal boundaries have attracted the attention of researchers in recent years, which have led to many intriguing applications. However, the current theoretical approaches are far from enough to handle the complicated temporal systems. In this paper, we develop universal matrix formalism, paired with a unique coordinate transformation technique. The approach can effectively deal with temporally stratified structures with complicated material anisotropy and arbitrary incidence angles. This formulation is applied to various practical systems, enabling the solution of these temporal boundary related problems in a simple and elegant fashion, and also facilitating a deep insight into the fundamental physics.

The spatiotemporal dynamics of a surface plasmon polariton (SPP) wave packet (WP) that interacts with a plasmonic nanocavity on a metal surface are investigated via femtosecond time-resolved two-photon fluorescence microscopy and numerical calculations. The nanocavity, which consists of a metal–insulator–metal (MIM) laminar structure (longitudinal length: ∼100 nm), behaves as a subwavelength meta-atom possessing discretized eigenenergies. When a chirp-induced femto-second SPP WP is incident on the nanocavity, only the spectral component matching a particular eigenenergy is transmitted to continue propagation on the metal surface. This spectral clipping induces a spatial peak shift in the WP. The shift can be controlled by tuning the eigenenergy or chirp.

Understanding the dissociation of excitons into long-lived free charge carriers is a crucial issue when considering the applications of transition metal dichalcogenides (excitonic semiconductors) oriented toward the use of solar energy (such as photovoltaics or photocatalysis). In our work, long-lived carriers have been observed by time-resolved microwave photoconductivity (TRMC) for the first time in both atomically thin and bulk MoS 2 , MoSe 2 , WS 2 , and WSe 2 crystals. The lifetime of majority carriers is close to microseconds and can even reach several microseconds due to different contribution of surface and defect states, as well as surface band bending (bulk). The three components depend on the material and vary from sample to sample, therefore determining the dynamics of the TRMC signal. The rise time of TRMC signal was found to be in the range of 0.1–0.2 μs and as it depends on the studied material it can be speculated that it is related to the dissociation time of excitons captured by traps.

Second-order photonic topological insulators (SPTIs) with topologically protected corner states provide a unique platform for realizing the robust manipulation of light in lower dimensions. Previous SPTIs proposed in  C 4 v -symmetric lattices are mainly based on the two-dimensional (2D) Su-Schrieffer–Heeger (SSH) model consisting of an even number of sites in the unit cell. Moreover, second-order topological phases within high-order band gaps are rarely explored. Here, we propose a new principle of SPTIs beyond the 2D SSH model, which is realized in C 4v -symmetric lattices consisting of an odd number of sites in the unit cell. The midgap-gap-ratios of these odd-order band gaps, from the first-order to the nineteenth-order with step of two-order, are maximized by the method of topology optimization. Second-order topological phases are successfully created within these sizeable band gaps and highly localized corner states are observed. Our work offers a new route for exploring high-order topological states in photonics and other classical systems.

Photoelectric properties of all-inorganic perovskite quantum dots (IPQDs) highly depend on their synthetic route. However, current synthetic processes of IPQDs are widely facing potential unsustainable issues of containing nonreusable and high-cost auxiliary materials. In this work, full-visible-spectrum IPQDs were successfully synthesized by an environmentally friendly ion-exchange approach with a renewable and low-cost anion exchange resin. Introducing anion exchange resin brings the improvement of both optical performance and surface morphology of the prepared IPQDs. The emission wavelength of IPQDs can be precisely controlled without changing their inherent crystal phase, and those IPQD’s single crystals with poor morphology and unstable structure are selectively removed. The photoluminescence quantum yield (PLQY) and the fluorescence lifetime of the three-primary-color IPQDs can be dramatically improved to 93.69, 89.99, and 65.03% and 71.3 ns, 22.2 ns, and 13.2 ns, respectively. Notably, the red-emitting PQDs at 622 nm exhibit a record high PLQY. By using the prepared IPQDs for photoluminescent color conversion, the three-primary-color light-emitting diodes (LEDs) provided high brightness and wide color gamut simultaneously. This study provides new ideas for the environmentally friendly and sustainable synthesis route of IPQDs, and it is expected to show great ambitions in the display field.

The ability to actively manipulate free-space optical signals by using tunable metasurfaces is extremely appealing for many device applications. However, integrating photoactive semiconductors into terahertz metamaterials still suffers from a limited functionality. The ultrafast switching in picosecond timescale can only be operated at a single frequency channel. In the hybrid metasurface proposed here, we experimentally demonstrate a dual-optically tunable metaphotonic device for ultrafast terahertz switching at frequency-agile channels. Picosecond ultrafast photoswitching with a 100% modulation depth is realized at a controllable operational frequency of either 0.55 THz or 0.86 THz. The broadband frequency agility and ultrafast amplitude modulation are independently controlled by continuous wave light and femtosecond laser pulse, respectively. The frequency-selective, temporally tunable, and multidimensionally-driven features can empower active metamaterials in advanced multiplexing of information, dual-channel wireless communication, and several other related fields.

Nonlinear nanophotonic devices have brought about great advances in the fields of nano-optics, quantum science, biomedical engineering, etc. However, in order to push these nanophotonic devices out of laboratory, it is still highly necessary to improve their efficiency. Since obtaining novel nanomaterials with large nonlinearity is of crucial importance for improving the efficiency of nonlinear nanodevices, we propose the two-dimensional (2D) perovskites. Different from most previous studies which focused on the 2D perovskites in large scale (such as the bulk materials or the thick flakes), herein we studied the 2D perovskites nanosheets with thickness of ∼50 nm. The high-order nonlinear processes including multi-photon photoluminescence and third-harmonic generation (THG) have been systematically investigated, and it is found the THG process can have a high conversion efficiency up to ∼8 × 10 −6 . Also, it is observed that the nonlinear responses of 2D perovskites have large optical anisotropy, i.e., the polarization ratio for the incident polarization dependence of nonlinear response can be as high as ∼0.99, which is an impressive record in the perovskite systems. Our findings reveal the properties of high efficiency and huge optical anisotropy in the nonlinear processes of 2D perovskite nanosheets, shedding light on the design of advanced integrated nonlinear nanodevices in future.

Millimeter-wave (mmWave) and orbital angular momentum (OAM) multiplexing are two key technologies for modern wireless communications, where significant efforts have been devoted to combining these two technologies for extremely high channel capacities. Recently, programmable metasurfaces have been extensively studied for stimulating dynamic multi-mode OAM beams, owing to their ability of subtle dynamic modulation over electromagnetic waves in a digital manner. However, programmable metasurfaces for mmWave OAM stimulation are rarely mentioned, due to the requirement of extremely high processing precision for mmWave applications. In this paper, a programmable metasurface is presented to stimulate dynamic multi-mode mmWave vortex beams. The proposed metasurface is composed of electronically reconfigurable units, which is obtained through configuration integration of a PIN diode within each radiation patch for modulating the unit resonant property. Both low reflection losses and stabilized inverse phase states are obtained for the binary unit coding states within the operation band. Through modulating the real-time coding distribution on the metasurface by programmable bias circuit, the generation of mmWave OAM beams with mode numbers l  = 0, l  = +1, l  = +2, and l  = +3 are numerically designed and experimentally verified. Our study paves a new perspective for the cross amalgamation of both mmWave and multi-mode OAM technologies.

Due to the strong ability of recognizing electromagnetic (EM) environment and adaptively control of EM waves, the intelligent metasurfaces have received great attention recently. However, the intelligent metasurface with frequency recognition for adaptive manipulation of the EM waves has not been studied. Here, we propose a frequency-recognition intelligent metasurface to precisely control the spatial EM waves under the agile frequencies with the help of a real-time radio-frequency sensor and an adaptive feedback control system. An active meta-atom is presented to reach 2 bit phase coding and 1 bit amplitude coding capacities to control the amplitude and phase independently. Experimental results demonstrate that the metasurface can recognize different frequency of the incoming wave with very high resolution, and can adaptively realize the self-defined multiple frequency agilities to manipulate the reflected EM waves without any human participation. As example, the intelligent metasurface with frequency recognition can adaptively operate wave absorption at 5.36 GHz, reflection to normal direction at 5.38 GHz, deflection to −30° at 5.40 GHz, random diffusion at 5.42 GHz, and deflection to +33° at 5.44 GHz by detecting the incoming frequency at the resolution of 0.02 GHz.

It is the basic characteristic of pure vortex light that there is a phase singularity at the origin. Such a singularity may be multiple degenerate, which determines the order of vortex light. Singularities splitting phenomenon means that singularities no longer concentrate at the origin but distribute around the space, usually occurring in impure vortex light. In this paper, we demonstrate the singularities splitting phenomenon and propose an analysis method, based on which one may rapidly estimate the modal components of impure vortex light. As two common singularity discrimination methods, the spiral and fork wire interference patterns are compared in distinguishing splitting singularities. The most widely used spiral interference pattern is revealed to be the worst form because of the low resolution. Instead, the fork wire interference pattern is with higher and easily adjusted resolution. 1‰ impurity is still able to be distinguished through fork wire interference patterns in the experiment.

Integrated silicon photonic waveguide biosensors have shown great potential for detecting bio-molecules because they enable efficient device functionalization via a well-developed surface chemistry, as well as simple scalable manufacturing, which makes them particularly suitable for low-cost point-of-care diagnostic. The on-chip integrated biosensors can be broadly classified into two types: (i) high-quality factor resonator sensors and (ii) interferometric sensors relying on non-resonant optical elements such as e.g. integrated waveguides. The former type usually requires a broadband or a tunable light source as well as complicated signal post-processing to measure a shift of the resonance frequency, while the latter exhibits a relatively low sensitivity due to the lack of efficient light recycling and phase accumulation mechanism in low quality factor elements. Additionally, high quality factor resonant photonic structures can be very sensitive to the presence of other non-target molecules in the water solution, causing sensor vulnerability to any noise. In this work, we combine a computational “inverse design” technique and a recently introduced high-contrast probe cleavage detection (HCCD) technique to design and optimize waveguide-based biosensors that demonstrate high sensitivity to the target molecule while being less sensitive to noise. The proposed biosensors only require a single frequency (or narrow-band) source and an intensity detector, which greatly simplifies the detection system, making it suitable for point-of-care applications. The optimal integrated sensor design that we demonstrate shows 98.3% transmission for the positive (target detected, probes cleaved) state and 4.9% transmission for the negative (probes are still attached) state at 1550 nm wavelength. The signal intensity contrast (20.06-fold transmission increase) shown in this work is much greater than the shift of the resonance frequency (less than 1% wavelength shift) observed in conventional ring-resonator-based biosensors. The new design may pave the way for realizing a single-frequency highly sensitive and selective optical biosensor system with a small physical footprint and a simple optical readout on a silicon chip.