Band 10, Heft 11

Silicon photonics is playing a key role in areas as diverse as high-speed optical communications, neural networks, supercomputing, quantum photonics, and sensing, which demand the development of highly efficient and compact light-processing devices. The lithographic segmentation of silicon waveguides at the subwavelength scale enables the synthesis of artificial materials that significantly expand the design space in silicon photonics. The optical properties of these metamaterials can be controlled by a judicious design of the subwavelength grating geometry, enhancing the performance of nanostructured devices without jeopardizing ease of fabrication and dense integration. Recently, the anisotropic nature of subwavelength gratings has begun to be exploited, yielding unprecedented capabilities and performance such as ultrabroadband behavior, engineered modal confinement, and sophisticated polarization management. Here we provide a comprehensive review of the field of subwavelength metamaterials and their applications in silicon photonics. We first provide an in-depth analysis of how the subwavelength geometry synthesizes the metamaterial and give insight into how properties like refractive index or anisotropy can be tailored. The latest applications are then reviewed in detail, with a clear focus on how subwavelength structures improve device performance. Finally, we illustrate the design of two ground-breaking devices in more detail and discuss the prospects of subwavelength gratings as a tool for the advancement of silicon photonics.

Cavity optomechanical systems enable interactions between light and mechanical resonators, providing a platform both for fundamental physics of macroscopic quantum systems and for practical applications of precision sensing. The resonant enhancement of both mechanical and optical response in the cavity optomechanical systems has enabled precision sensing of multiple physical quantities, including displacements, masses, forces, accelerations, magnetic fields, and ultrasounds. In this article, we review the progress of precision sensing applications using cavity optomechanical systems. The review is organized in the following way: first we will introduce the physical principles of optomechanical sensing, including a discussion of the noises and sensitivity of the systems, and then review the progress in displacement sensing, mass sensing, force sensing, atomic force microscope (AFM) and magnetic resonance force microscope (MRFM), accelerometry, magnetometry, and ultrasound sensing, and introduce the progress of using quantum techniques especially squeezed light to enhance the performance of the optomechanical sensors. Finally, we give a summary and outlook.

The band gap of two-dimensional (2D) materials become a hot issue for photoelectric detection. Recently, public attention is thoroughly aroused as to the remarkable electrical transport characteristic and super photoresponse of 2D graphdiyne. The simulation results show that the photoresponse can be adjusted in various solutions based on the graphdiyne nanosheets with different sizes and thicknesses. Based on few-layered graphdiyne nanosheets prepared by a liquid-phase exfoliation method, a photoelectrochemical (PEC)-type few-layered graphdiyne photodetector is demonstrated in this paper. A group of PEC tests are carried out in neutral solution to verify the simulation results. The as-prepared graphdiyne photodetector possesses high photocurrent density, effective responsivity and excellent cycle stability in condition of KCl electrolyte and solar illuminance. The detectivity of the PEC-type graphdiyne photodetector can be easy to adjust by altering electrolyte concentration and other corresponding parameters, which indicates the proposed equipment can be a good candidate for photoelectric detection.

The strain technology is accelerating the progress on the CMOS compatible Ge-on-Si laser source. Here, we report a monolithically integrated microbridge-based emitting-detecting configuration, equipped with lateral p – i – n junctions, waveguide and gratings. The operating wavelength range of the emitting bridge and the detecting bridge are matched through the designed same dimensions of the two microbridges, as well as the strain. Strain-enhanced spontaneous emission and the effect of spectra red-shifting on low-loss transmission of on-chip light are discussed. Temperature dependence experiments reveal that in devices with highly strain-enhanced structure, the strain variation can offset the effect of electron thermalization, so that the performance of the device remains stable when temperature changes around room temperature.

Gradient metasurfaces have been extensively applied in the unprecedented control of light beams over thin optical components. However, these metasurfaces suffer from low efficiency when bending light through large angles and high fabrication demand when it requires fine discretion. In this work, we investigate all-dielectric metagratings based on the generalized Kerker effect induced by interference between Mie-type resonances. It allows extraordinary optical diffraction for beam steering through ultralarge angles. The coupling inside and between the lattices in the metagrating can be used to tune the excited states of the electric and magnetic resonances, including both the fundamental dipoles and high-order multipoles, leading to an ideal asymmetrical scattering pattern that redistributes the energy between the diffraction channels as required. The quadrupole and hexadecapole not only significantly enhance the working efficiency but also enable distinctive possibilities for wave manipulation that cannot be reached by dipoles. Utilizing a thin array of silicon rods, large-angle negative refraction and reflection are realized with almost unity efficiency under both transverse magnetic and transverse electric polarization. Compared with conventional metasurfaces, such an all-dielectric metagrating has the merits of high flexibility, high efficiency, and low fabrication requirements. The coupling and interactions among the multipoles may serve as a foundation for various forms of on-chip optical wave control.

By placing a single Au nanoparticle on the surface of a cadmium sulfide (CdS) nanowire, we demonstrate strong coupling of localized surface plasmon resonance (LSPR) modes in the nanoparticle and whispering gallery modes (WGMs) in the nanowire. For a 50-nm-diameter Au-nanosphere particle, strong coupling occurs when the nanowire diameter is between 300 and 600 nm, with a mode splitting up to 80 meV. Using a temperature-induced spectral shift of the resonance wavelength, we also observe the anticrossing behavior in the strongly coupled system. In addition, since the Au nanosphere has spherical symmetry, the supported LSPR mode can be selectively coupled with transverse electric (TE) and transverse magnetic (TM) WGMs in the nanowire. The ultracompact strong-coupling system shown here may provide a versatile platform for studying hybrid “photon–plasmon” nanolasers, nonlinear optical devices, and nanosensors.

The concept of topology is universally observed in various physical objects when the objects can be described by geometric structures. Although a representative example is the knotted geometry of wavefunctions in reciprocal space for quantum Hall family and topological insulators, topological states have also been defined for other physical quantities, such as topologically distinct Fermi surfaces and enhanced lattice degrees of freedom in hyperbolic geometry. Here, we investigate a different class of topological states – topological geometry of dynamical state trajectories – in non-Hermitian and nonlinear optical dynamics, revealing topologically protected oscillation quenching mechanisms determined by parity–time (PT) symmetry. For coupled systems composed of nonlinear gain and loss elements, we classify the topology of equilibria separately for unbroken and broken PT symmetry, which result in distinct oscillation quenching mechanisms: amplitude death and oscillation death. We then show that these PT-symmetric quenching mechanisms lead to immunity against temporal perturbations, enabling the applications of topologically protected laser modulation and rectification. The observed connection between the topological geometry of dynamical states, oscillation quenching phenomena in dynamical systems theory, and PT symmetry provides a powerful toolkit for noise-immune signal processing.

Optical bottle beams can be used to trap atoms and small low-index particles. We introduce a figure of merit (FoM) for optical bottle beams, specifically in the context of optical traps, and use it to compare optical bottle-beam traps obtained by three different methods. Using this FoM and an optimization algorithm, we identified the optical bottle-beam traps based on a Gaussian beam illuminating a metasurface that are superior in terms of power efficiency than existing approaches. We numerically demonstrate a silicon metasurface for creating an optical bottle-beam trap.

Interference usually occurs between two non-orthogonally polarized light beams. Hence, metasurface enabled polarization multiplexing is generally conducted under two orthogonal polarization states to realize independent intensity and/or phase modulations. Herein, we show that polarization multiplexed metasurfaces can work under three non-orthogonal polarization states to realize tri-channel image displays with independent information encoding. Specifically, enabled by orientation degeneracy, each nanostructure of the metasurface operates with triple-manipulations of light, i.e., two channels for independent intensity manipulation under π /4 and 3 π /8 linearly polarized (LP) light, respectively, and one channel for phase manipulation without polarization control. We experimentally demonstrate this concept by recording one continuous-brightness polychromatic image and one binary-brightness polychromatic image right at the metasurface plane, while a continuous-brightness polychromatic image is reconstructed in the far field, corresponding to three independent channels, respectively. More interestingly, in another design strategy with separated image encoding of two wavelengths, up to six independent image-display channels can be established and information delivery becomes safer by utilizing encryption algorithms. With the features of high information capacity and high security, the proposed meta-devices can empower advanced research and applications in multi-channel image displays, orbital angular momentum multiplexing communication, information encryption, anti-counterfeiting, multifunctional integrated nano-optoelectronics, etc.

Several fundamental research and applications in biomedicine and microfluidics often require controlled manipulation of suspended micro- and nanoscale particles. Speckle tweezers (ST) by incorporating randomly distributed light fields have been used to control micro-particles with refractive indices higher than their medium and to perform manipulation tasks such as guiding and sorting. Indeed, compared to periodic potentials, ST represents a wider possibility to be operated for such tasks. Here, we extend the usefulness of ST into micro-particles of low index with respect to the surrounding. Repelling of such particles by high intensity regions into lower intensity regions makes them to be locally confined, and the confinement can be tuned by changing the average grain intensity and size of the speckle patterns. Experiments on polystyrenes and liposomes validate the procedure. Moreover, we show that ST can also manipulate the nano-particle (NP)-loaded liposomes. Interestingly, the different interactions of NP-loaded and empty liposomes with ST enable collective manipulation of their mixture using the same speckle pattern, which may be explained by inclusion of the photophoretic forces on NPs. Our results on the different behaviors between empty and non-empty vesicles may open a new window on controlling collective transportation of drug micro-containers along with its wide applications in soft matter.

Understanding and optimising the mechanisms of generation and extraction of hot carriers in plasmonic heterostructures is important for applications in new types of photodetectors, photochemistry and photocatalysis, as well as nonlinear optics. Here, we show using transient dynamic measurements that the relaxation of the excited hot-carriers in Au/Pt hetero-nanostructures is accelerated through the transfer pathway from Au, where they are generated, to Pt nanoparticles, which act as a hot-electron sink. The influence of the environment on the dynamics was also demonstrated. The time-resolved photoluminescence measurements confirm the modified hot-electron dynamics, revealing quenching of the photoluminescence signal from Au nanoparticles in the presence of Pt and an increased photoluminescence lifetime. These observations are signatures of the improved extraction efficiency of hot-carriers by the Au/Pt heterostructures. The results give insight into the time-dependent behaviour of excited compound nanoscale systems and provide a way of controlling the relaxation mechanisms involved, with important consequences for engineering nonlinear optical response and hot-carrier-assisted photochemistry.

Modern integrated photonic platforms should combine low-loss guiding, spectral flexibility, high light confinement, and close packing of optical components. One of the prominent platforms represents a one-dimensional photonic crystal combined with dielectric nanostructures that manipulate low-loss Bloch surface waves (BSWs). Proper design of nanostructures gives rise to a variety of optical resonances suitable for efficient capturing and controlling light. In this work, we achieve color-selective directional excitation of BSWs mediated by Mie resonances in a semiconductor nanoparticle. We show that a single silicon nanoparticle can be used as a subwavelength multiplexer switching the BSW excitation direction from forward to backward within the 30 nm spectral range with its central wavelength governed by the nanoparticle size. Our work opens a route for the on-demand fabrication of photonic nanocouplers with tailored optical properties and submicron footprint.

The capability to support large wave vector bulk plasmon polariton (BPP) waves enables the application of hyperbolic metamaterials (HMMs) in sensing. However, there is a challenge arising from the excitation of BPP, and the highly confined polarization waves are unable to meet the requirements of practical application. In this study, an HMM/bilayer silver nanoparticles (Ag NPs) platform is proposed that allows the excitation and utilization of BPP for use as a surface-enhanced Raman scattering (SERS) substrate. According to the research results, the bilayer Ag NPs provide stronger plasmonic property and act as a light-matter coupler, so as to generate a large wave vector of scattered light and excite the BPP within the HMM. Besides, Ag NPs provide the nano antenna structure, and decouple the BPP into localized surface plasmon (LSP) that can be used directly to excite the electric fields. In addition, HMM produces a modulating effect on the plasmon resonance peak, which makes it possible to overlap the spectrum of resonance peak with excitation wavelengths, thus leading to a strong absorption peak at the incident laser wavelength region. Experimentally, the platform was applied to achieve SERS detection for adenosine molecules with a concentration of 10 −6  M. It is believed that this plasmonic platform has a potential of application in surface-enhanced spectroscopy.

One compound metasurface with multiple functions and precise complex amplitude modulation is beneficial to photonic integration. Here, all-silicon bifunctional metasurfaces capable of independent amplitude and phase modulation in two circular polarized channels are proposed, which encode complex amplitude information by integrating propagation phase and Pancharatnam-Berry phase. A switchable power-controllable axial bifocal metalens directly illustrates the feasibility of the proposed modulation scheme. Another switchable power-controllable horizontal/vertical bifocal metalens characterizes the versatility and flexibility of this approach. The experimental results agree well with the simulations and theoretical expectations. In addition, we also discuss the broadband performance of the proposed metalens and the dynamic focusing behavior under optical pumping. The proposed approach can directly generate editable amplitude and phase profiles and can find applications in dynamic holography, dynamic display, and other fields.