Volume 11, Issue 13

To meet the high demand of data transmission capacity, optical communications systems have been developed. In order to increase the channel numbers for larger communication bandwidth, multi-mode lasers and laser arrays can be used. As an alternative coherent light source, optical frequency comb (OFC) contains multi-wavelength signal, and hence enables communication with data stream of tens of terabit/s. Fully integrated electrically driven OFCs are expected as a compact, robust, and low-cost light source for data communication. In this review article, the recent development progress on fully integrated electrically driven OFC generators are reviewed, with focus on the demonstrations in the past five years. Based on comb generation approaches, the works are categorized into two main types: one is OFC generators based on four-wave mixing in high- Q resonator, and the other is OFC generators based on mode-locked laser. At the end, a summary and future outlook are provided.

The growing interest in transition metal dichalcogenides (TMDs) has encouraged researchers to focus on their nonlinear optical properties, such as harmonic generation (HG), which has potential for fundamental science and applications. HG is a nonlinear phenomenon used to study low-dimensional physics and has applications in bioimaging, optical signal processing, and novel coherent light sources. In this review, we present the state-of-the-art advances of HG in atomically-thin TMDs and their heterostructures. Different factors affecting the HG in TMDs such as strain, electric gating, excitonic resonance, phase and edge modulation, and valley-induced HG are discussed with a particular emphasis on the HG in heterostructure van der Waals TMDs. Moreover, we discuss the enhancement of HG in TMDs by incorporating cavities and nanostructures including the bound states in the continuum with extreme Q-factor. This work provides a concise summary of recent progress in engineering HG in atomically-thin TMDs and their heterostructures and a compact reference for researchers entering the field.

Solar-thermal technologies for converting chemicals using thermochemistry require extreme light concentration. Exploiting plasmonic nanostructures can dramatically increase the reaction rates by providing more efficient solar-to-heat conversion by broadband light absorption. Moreover, hot-carrier and local field enhancement effects can alter the reaction pathways. Such discoveries have boosted the field of photothermal catalysis, which aims at driving industrially-relevant chemical reactions using solar illumination rather than conventional heat sources. Nevertheless, only large arrays of plasmonic nano-units on a substrate, i.e., plasmonic metasurfaces, allow a quasi-unitary and broadband solar light absorption within a limited thickness (hundreds of nanometers) for practical applications. Through moderate light concentration (∼10 Suns), metasurfaces reach the same temperatures as conventional thermochemical reactors, or plasmonic nanoparticle bed reactors reach under ∼100 Suns. Plasmonic metasurfaces, however, have been mostly neglected so far for applications in the field of photothermal catalysis. In this Perspective, we discuss the potentialities of plasmonic metasurfaces in this emerging area of research. We present numerical simulations and experimental case studies illustrating how broadband absorption can be achieved within a limited thickness of these nanostructured materials. The approach highlights the synergy among different enhancement effects related to the ordered array of plasmonic units and the efficient heat transfer promoting faster dynamics than thicker structures (such as powdered catalysts). We foresee that plasmonic metasurfaces can play an important role in developing modular-like structures for the conversion of chemical feedstock into fuels without requiring extreme light concentrations. Customized metasurface-based systems could lead to small-scale and low-cost decentralized reactors instead of large-scale, infrastructure-intensive power plants.

The “one-to-many” problem is a typical challenge that faced by many machine learning aided inverse nanophotonics designs where one target optical response can be achieved by many solutions (designs). Although novel training approaches, such as tandem network, and network architecture, such as the mixture density model, have been proposed, the critical problem of solution degeneracy still exists where some possible solutions or solution spaces are discarded or unreachable during the network training process. Here, we report a solution to the “one-to-many” problem by employing a conditional generative adversarial network (cGAN) that enables generating sets of multiple solution groups to a design problem. Using the inverse design of a transmissive Fabry–Pérot-cavity-based color filter as an example, our model demonstrates the capability of generating an average number of 3.58 solution groups for each color. These multiple solutions allow the selection of the best design for each color which results in a record high accuracy with an average index color difference Δ E of 0.44. The capability of identifying multiple solution groups can benefit the design manufacturing to allow more viable designs for fabrication. The capability of our cGAN is verified experimentally by inversely designing the RGB color filters. We envisage this cGAN-based design methodology can be applied to other nanophotonic structures or physical science domains where the identification of multi-solution across a vast parameter space is required.

Single pixel imaging as an alternative to traditional imaging methods, has attracted extensive attention in various research fields. Metasurfaces with subwavelength unit cells and compact footprint can be used as a substitute for traditional optical elements. In this work, we propose a single pixel imaging scheme based on metasurface composed of photon sieves, where spatial modulation is realized through shifting. Spatial multiplexing capability is demonstrated by this shifting mode, which can obtain more patterns in limited space and greatly increase the mask capacity. Benefited from the simple structure and easy manufacture of photon sieves, large capacity metasurface can be manufactured. Meanwhile, metasurfaces can simplify the single pixel imaging system, leading to the system miniaturization and integration. In addition, numerical and optical experiments prove that our proposal can operate at the range from the entire visible light to near-infrared light. Such scheme provides a new way for single pixel imaging and would be applied in microscopic imaging, dynamic imaging, hyperspectral imaging, and so on.

To meet the growing demand for photonic integration and device miniaturization, planar diffractive Fresnel zone plates (FZPs) are widely applied in integrated optical systems. However, challenges remain in fabricating FZPs with high efficiency and satisfying the requirement for cross-scale fabrication. This paper details a high efficiency method for fabricating ultrathin FZPs of different scales on metal films by using holographic femtosecond lasers. The FZPs are split into a series of element patterns that are printed in order by using corresponding modulated femtosecond pulses. The fabricated FZPs are spliced by the printed element structures with no FZP size limitation in theory. FZPs with an area varying across three orders of magnitude are presented to demonstrate the capability of cross-scale fabrication. The fabricated FZPs possess an excellent broadband focusing and imaging ability in the visible spectrum. Furthermore, the fabrication of other functional ultrathin lenses, such as axial multifocal zone plates, petal-like zone plates, and FZP arrays, is described, revealing the wide potential for the flexible and scalable fabrication method in on-chip integrated optical systems.

Epitaxial semiconductor quantum dots (QDs) have been demonstrated as on-demand entangled photon sources through biexciton–exciton (XX-X) cascaded radiative processes. However, perfect entangled photon emitters at the specific wavelengths of 880 nm or 980 nm, that are important for heralded entanglement distribution by absorptive quantum memories, remain a significant challenge. We successfully extend the QD emission wavelength to 880 nm via capping Stranski–Krastanow grown In(Ga)As/GaAs QDs with an ultra-thin Al x Ga 1− x As layer. After carefully investigating the mechanisms governing the vanishing of wetting-layer (WL) states and the anisotropy of QDs, we optimize the growth conditions and achieve a strong suppression of the WL emission as well as a measured minor fine structure splitting of only ∼(3.2 ± 0.25) μeV for the exciton line. We further extend this method to fabricate In(Ga)As QDs emitted at 980 nm via introducing InGaAs capping layer, and demonstrate a two-photon resonant excitation of the biexciton without any additional optical or electrical stabilized source. These QDs with high symmetry and stability represent a highly promising platform for the generation of polarization entanglement and experiments on the interaction of photons from dissimilar sources, such as rare-earth-ion-doped crystals for solid quantum memory.

In this paper, we report an approach for tuning the surface morphology and phase of Ge 2 Sb 2 Te 5 (GST) by using an ultrafast laser in a one-step process. Four surface micro/nanostructures with specific phase states were sequentially formed by changing the pulse energy: the modified ripple structure, the completely crystallized structure, the ablated nanodots, and the ablated ripple structure. A high correlation existed between the surface micro/nanostructures and their property. Through integrated property–structure modulation, multifunctional optical recording could be achieved by using modified ripples with specific crystallized phase states. The geometric grating morphology caused by the volume shrinkage effect during crystallization enabled modified ripples to exhibit a structural color based on the grating’s diffraction effect. Moreover, the considerable change in the reflectivity of the crystallized area enabled easy grayscale identification. On the basis of the spatially resolved phase-transition threshold effect, the integrated modulation of the geometric nanograting proportion and degree of crystallization was conducted in multilevel states. Notably, different from the fixed ablated surface structures, the printed modified surface structures could be erased and rewritten by controlling its phase state. This paper presents a promising method for producing dynamic tunable metasurfaces, conducting optical anticounterfeiting, and achieving information storage.

The optical all-pass filter (APF), which exhibits a constant amplitude response and a variable phase response, is a key to manipulating the optical phase without inducing signal amplitude distortion. High-order APFs are significantly demanded because they can afford large time delays and phase shifts. However, to date, only first-order APFs based on lossy waveguides have been reported. Although high-order APFs can be simply obtained by cascading multiple first-order APFs, the complexity and size are increased. To solve this problem, we propose and demonstrate a second-order APF using Mach–Zehnder interferometer-assisted microring resonators. The device is fabricated based on a silicon-on-insulator platform. Based on the second-order APF, an adjustable time delay between 553 and 948 ps is obtained, and the corresponding amplitude variation is less than 1.7 dB. Meanwhile, a microwave photonic phase shifter is also obtained based on the APF. The microwave phase shift can be adjusted from 0 to 3.27 π , with an RF power variation within 2.4 dB. Additionally, the second-order APF can be reconfigured to a first-order APF, which significantly enhances its flexibility. The reconfigured first-order APF can realize an adjustable time delay between 257 and 429 ps, and the amplitude variation is less than 0.9 dB. The proposed high-order APF provides a novel approach to manipulating optical signals.

Passively mode-locked lasers have been widely investigated as one of the effective methods to obtain ultrashort pulses. As an important part of passively mode-locked fiber lasers, the exploration of 2D material-based saturable absorber has become one of the hotspots in ultrafast photonics in recent years. Germanene, a novel 2D Dirac material, with ultrafast optical response and broadband optical absorption, is a promising alternative material for saturable absorber in mode-locked fiber lasers. In this paper, germanium nanosheets are prepared via liquid-phase exfoliated method, with the saturable absorption property systematically characterized in three major wavebands of the near-infrared region. The generation of ultrashort pulses based on germanene saturable absorber in fiber lasers is further realized, in a broad waveband (1000 nm) centered at 1061.1, 1559.3 and 1883.5 nm, respectively. In addition, noise-like pulses operation with central wavelength of 1558.6 nm is also obtained, and the formation of rogue waves is further demonstrated via statistical analysis. To the best of our knowledge, this is the first experimental verification of the broadband saturable absorption property of germanene-based devices, covering three major fiber laser wavelengths from 1.0 to 2.0 μm.

Dynamics of photoinduced species, as a key parameter for nanomaterials plays a significantly role in the performance of optoelectronic devices. In this work, the origin of broadband optical response for the emerging Ti 3 C 2 T x MXene is revealed by transient spectroscopic analysis. From ultraviolet to infrared, the steady-state and transient optical responses present wavelength-related features. The carrier lifetime is found to change from femtosecond to nanosecond time scale dominated by various photoinduced species, i.e., carrier and surface plasmon. The unique optoelectronic character enables photodetection. This fundamental study on carrier, plasmon dynamics, and application in photodetection is helpful for exploring MXene-based optoelectronic devices.

The transition metal dichalcogenides (TMDs) have drawn great research attention, motivated by the derived remarkable optoelectronic properties and the potentials for high-efficient excitonic devices. The plasmonic nanocavity, integrating deep-sub wavelength confinement of optical mode with dramatic localized field enhancement, provides a practical platform to manipulate light–matter interaction. In order to obtain strong exciton–plasmon coupling effects, it’s crucial to match the vibration direction of exciton to the available strong localized in-plane electric field. Herein, we demonstrate the coupling effect of in-plane exciton in monolayer tungsten diselenide (WSe 2 ) to deterministic gap-plasmon field which is produced by nanometrically gapped collapsed nanofingers. The gap-plasmon field which is completely parallel to the in-plane excitons in WSe 2 will drive a strong exciton–plasmon coupling at room temperature. More interestingly, it is experimentally observed that the luminescence of exciton–polariton cannot be influenced by the temperature in the range from 77 K to 300 K due to the presence of nanofingers. According to the theoretical analysis results, we attribute this finding to the dielectric screening effect arising from the extremely strong localized electric field of plasmonic nanofingers. This work proposes a feasible way to harness and manipulate the exciton of low-dimensional semiconductor, which might be potential for quantum optoelectronics.

Metallic nanogaps are being widely used for sensing applications, owing to their ability to confine and enhance electromagnetic field within the hot spots. Since the enhanced field does not confine itself perfectly within the gap, however, fringe fields well away from the gap are of potential use as well in real systems. Here, we extend the concept of near field absorption enhancement by quantitatively analyzing terahertz absorption behavior of water molecules outside the hot spots of sub-20 nm-wide, ∼100 μm-long nanotrenches. Contrary to point-gaps which show negligible field enhancement at distances larger than the gap width, our extended nanogap act as a line source, incorporating significant amount of absorption enhancement at much longer distances. We observe absorption enhancement factors of up to 3600 on top of a 5 nm-wide gap, and still well over 300 at 15 nm away. The finding is well supported by theoretical analyses including modal expansion calculations, Kirchhoff integral formalism and antenna theory. Our results provide means to quantitatively analyze light-matter interactions beyond the hot spot picture and enable application of nanogaps for sensitive surface analyses of various material systems.

In this work, we tackle the problem of the spatially selective optical excitation of spin dynamics in structures with multiple magnetic layers. The 120 fs circularly polarized laser pulses were used to launch magnetization precession in an all-dielectric magneto-photonic crystals (MPC) formed by magnetic layers sandwiched between and inside two magnetic Bragg mirrors. Optical pump-probe experiments reveal magnetization precession triggered via ultrafast inverse Faraday effect with an amplitude strongly dependent on the pump central wavelength: maxima of the amplitude are achieved for the wavelength tuned at the cavity resonance and at the edge of the photonic bandgap. The optical impact on the spins caused by the inverse Faraday effect and spectrum of this effect are found to correlate mostly to the direct Faraday effect. We show that even though the pump laser pulses propagate along the whole structure tuning their wavelength allows localization of a larger spin precession either in the cavity layer or in the Bragg mirror layers selectively. The results pave the way to the ultrafast optical control of magnetization dynamics at a sub-wavelength scale that is vital for modern magneto-photonics and magnonics.

Halide perovskite quantum dots (PQDs) have exhibited significantly superior nonlinear optical properties compared to traditional semiconductor materials thanks to their peculiar physical and electronic structures. By further improving the nonlinear optical properties of PQDs, it is expected to adapt to ultrafast photonics applications. This work reported the nonlinear optical properties of methylammonium lead bromide-graphene (CH 3 NH 3 PbBr 3 -G) composites synthesized by growing CH 3 NH 3 PbBr 3 quantum dots directly from a graphene oxide lattice. Our experiments indicate that the combined advantages of the ultrafast charge transport properties from graphene and the strong charge generation efficiency of perovskite can be integrated together. The CH 3 NH 3 PbBr 3 -G composite exhibited enhanced saturable absorption properties with large modulation depth and very low saturation intensity. The transient absorption spectra and carrier dynamics analysis revealed that the enhancement of the saturated absorption properties of the composites mainly arose from the ultrafast charge transfer between G and CH 3 NH 3 PbBr 3 which promoted the coupling between different states. The results pave the way for the design of optical switches or mode lockers based on saturable absorbers with good performance.