Nanophotonic Systems Research Articles

Multiresponsive Dielectric Metasurfaces Based on Dual Light‐ and Temperature‐Responsive Copolymers

AbstractTunability is essential for unlocking a range of practical applications of high‐efficiency metasurface‐based nanophotonic devices and systems. Increased research efforts in this area during recent years led to significant progress regarding tuning mechanisms, speed, and diverse active functionalities. However, so far almost all the demonstrated works are based on a single type of physical stimulus, thereby excluding important opportunities to enhance the modulation range of the metadevices, the available design options, as well as interaction channels between the metadevices and their environment. In this article, it is experimentally demonstrated that multi‐responsive metasurfaces can be realized by combining asymmetric, highly resonant metasurfaces with multi‐responsive polymeric materials. The respective copolymers combine light‐ and temperature‐responsive comonomers in an optimized ratio. This work demonstrates clearly reversible light‐responsive, temperature‐responsive, and co‐responsive tuning of the metasurface optical resonance positions at near‐infrared wavelengths, featuring maximum spectral resonance shifts of nearly twice the full‐width‐at‐half‐maximum and accompanied by more than 60% absolute modulation in transmittance. This work provides new design freedom for multifunctional metadevices and can potentially be expanded to other types of copolymers as well. Furthermore, the studied hybrid multiresponsive systems are promising candidates for multi‐dimensional sensing applications.

Bound states and photon emission in non-Hermitian nanophotonics

We establish a general framework for studying the bound states and the photon-emission dynamics of quantum emitters coupled to structured nanophotonic lattices with engineered dissipation (loss). In the single-excitation sector, the system can be described exactly by a non-Hermitian formalism. We have pointed out in the accompanying letter [Gong \emph{et al}., arXiv:2205.05479] that a single emitter coupled to a one-dimensional non-Hermitian lattice may already exhibit anomalous behaviors without Hermitian counterparts. Here we provide further detail on these observations. We also present several additional examples on the cases with multiple quantum emitters or in higher dimensions. Our work unveils the tip of the iceberg of the rich non-Hermitian phenomena in dissipative nanophotonic systems.

Resonance energy transfer near higher-order exceptional points of non-Hermitian Hamiltonians

Exceptional points (EPs) of both eigenvalue and eigenvector degeneracy offer remarkable properties of the non-Hermitian systems based on the Jordanian form of Hamiltonians at EPs. Here we propose the perturbation theory able to underpin the physics in the vicinity of the higher-order EPs. The perturbation theory unveils lifting of degeneracy and origin of the different phases merging at the EP. It allows us to analyze the photonic local density of states and resonance energy transfer, determining their spectral behaviors in a general form. Resonant energy transfer is investigated in analytical and numerical examples. We analytically find the resonance energy transfer rate near the third-order EP occurring in the system of three coupled cavities and reveal singularities caused by the interplay of the perturbation and frequency detuning from degenerate eigenfrequency. Numerical simulation of the coupled-resonator system reveals the vital role of a mirror for switching to the EP of the doubled order and corresponding enhancement of the resonance energy transfer rate. Our investigation sheds light on the behavior of nanophotonic systems in non-Hermitian environments.

A Reconfigurable Nanophotonic Heterostructure Engineered by a DNA Origami Switch.

DNA origami has been widely used to construct nanophotonic structures due to its unparallel capability in assembling photonic nanomaterials with nanometer precision. Nevertheless, the majority of fabricated fluorescence-based nanophotonic systems so far operate in the ultraviolet-visible (UV-Vis) region, which are accompanied by limitations of shallow penetration and potential light damage. Herein, we constructed a reconfigurable nanophotonic heterostructure by integrating an upconversion nanoparticle (UCNP) and a gold nanorod (AuNR) on a DNA origami switch (DOS) template. With the unique anti-Stokes luminescence of UCNP whose excitation wavelength is mostly located in the near-infrared (NIR) region, the obtained reconfigurable nanophotonic system can be excited by 980 laser light and generates visible light emission. In our nanophotonic system, the DNA origami template serves as a switch to reversibly tune the optical properties of UCNP. The construction of this nanophotonic heterostructure expands the toolbox of DNA origami-based nanophotonic systems that may hold great potential in optical biosensing and imaging.

Hybrid plasmonic–dielectric metal-nanowire coupler for high-efficiency broadband nonlinear frequency conversion

Pursuing nanometer-scale nonlinear converters based on second harmonic generation (SHG) is a stimulating strategy for bio-sensing, on-chip optical circuits, and quantum information processing, but the light-conversion efficiency is still poor in such ultra-small dimensional nanostructures. Herein, we demonstrate a highly enhanced broadband frequency converter through a hybrid plasmonic–dielectric coupler, a ZnTe/ZnO single core–shell nanowire (NW) integrated with silver (Ag) nanoparticles (NPs). The NW dimension has been optimized to allow the engineering of dielectric resonances at both fundamental wave and second harmonic frequencies. Meanwhile, the localized surface plasmon resonances are excited in the regime between the Ag NPs and ZnTe/ZnO dielectric NW, as evidenced by plasmon-enhanced Raman scattering and resonant absorption. These two contributors remarkably enhance local fields and consequently support the strong broadband SHG outputs in this hybrid nanostructure by releasing stringent phase-matching conditions. The proposed nanoscale nonlinear optical converter enables the manipulation of nonlinear light–matter interactions toward the development of on-chip nanophotonic systems.

Observation of Anderson phase in a topological photonic circuit

Disordered systems play a central role in condensed matter physics, quantum transport, and topological photonics. It is commonly believed that a topological nontrivial phase would turn into a trivial phase where the transport vanishes under the effect of Anderson localization. Recent studies predict a counterintuitive result, that adding disorder to the trivial band structure triggers the emergence of protected edge states, the so-called topological Anderson phase. Here, we experimentally observe such a topological Anderson phase in a CMOS-compatible nanophotonic circuit, which implements the Su-Schrieffer-Heeger (SSH) model with incommensurate disorder in the intercell coupling amplitudes. The existence of the Anderson phase is verified by the spectral method, based on the continuous detection of the nanoscale light dynamics at the edge. Our results demonstrate the inverse transition between distinct topological phases in the presence of disorder, as well as offering a single-shot measurement technique to study the light dynamics in nanophotonic systems.

Confining Light in Porous Perovskite Heterostructures for Light Amplification

Nanostructured porous media are capable of modulating light transport via diffusion or localization, while their combination with optical gain offers a solution to creating coherent light sources. Herein, we report on the development of a new class of optically active porous medium–porous perovskite heterostructures and demonstrate their potential in confining light via strong scattering for light amplification. Such heterostructures are chemically composed of 2D-networked CsPb2Br5 and 3D-networked CsPbBr3, which are attained upon direct water stimuli of 0D-networked Cs4PbBr6. Optical imaging reveals that light trajectories due to whispering gallery modes are formed in Cs4PbBr6 resonators to offer optical feedback for lasing. In contrast, light scattering due to structural inhomogeneity suppresses whispering gallery modes in porous heterostructures, while light is mainly confined in the pumping region. Despite the poor definition of microscopic cavities due to local disorder, the pump threshold of the porous heterostructure is only 1/3 to 1/5 that of Cs4PbBr6 and comparable to that of CsPbBr3 reported previously. Eigenvalue modeling was performed to analyze the eigenmodes, revealing that porous perovskite heterostructures support high-quality eigenmodes for lasing. The results suggest that perovskite heterostructures with interconnected porous scaffolds serve as promising complex nanophotonic systems for fundamental studies and potential applications.

Efficient Source of Shaped Single Photons Based on an Integrated Diamond Nanophotonic System.

An efficient, scalable source of shaped single photons that can be directly integrated with optical fiber networks and quantum memories is at the heart of many protocols in quantum information science. We demonstrate a deterministic source of arbitrarily temporally shaped single-photon pulses with high efficiency [detection efficiency=14.9%] and purity [g^{(2)}(0)=0.0168] and streams of up to 11 consecutively detected single photons using a silicon-vacancy center in a highly directional fiber-integrated diamond nanophotonic cavity. Combined with previously demonstrated spin-photon entangling gates, this system enables on-demand generation of streams of correlated photons such as cluster states and could be used as a resource for robust transmission and processing of quantum information.

Exciton Diffusion and Annihilation in Nanophotonic Purcell Landscapes

AbstractExcitons spread through diffusion and interact through exciton–exciton annihilation. Nanophotonics can counteract the resulting decrease in light emission. However, conventional enhancement treats emitters as immobile and non‐interacting. It neglects exciton redistribution between regions with different enhancements and the increase in non‐radiative decay at high exciton densities. Here, the authors went beyond the localized Purcell effect to exploit exciton dynamics and turn their typically detrimental impact into additional emission. As interacting excitons diffuse through optical hotspots, the balance of excitonic and nanophotonic properties leads to either enhanced or suppressed photoluminescence. The dominant enhancement mechanisms are identified in the limits of high and low diffusion and annihilation. Diffusion lifts the requirement of spatial overlap between excitation and emission enhancements, which are harnessed to maximize emission from highly diffusive excitons. In the presence of annihilation, improved enhancement is predicted at increasing powers in nanophotonic systems dominated by emission enhancement. The guidelines are relevant for efficient and high‐power light‐emitting diodes and lasers tailored to the rich dynamics of excitonic materials such as monolayer semiconductors, perovskites, or organic crystals.

Direct Imaging of Weak‐to‐Strong‐Coupling Dynamics in Biological Plasmon–Exciton Systems

AbstractOptical coupling plays a pivotal role in nanophotonic systems, which can be divided into weak, intermediate, and strong‐coupling regimes. Monitoring optical coupling strength is, therefore, the key to understanding light–matter interactions. State‐of‐the‐art approaches based on spectral measurements offer the power to quantify and characterize optical coupling strength at a single cavity level. However, it remains challenging to dynamically characterize coupling strength during the transition from strong‐ to weak‐coupling regimes for many systems simultaneously. Here, a far‐field imaging technique is reported that can directly monitor optical coupling dynamics in plasmon–exciton systems, allowing multiple nanocavity emissions to be characterized from weak‐ to strong‐coupling regimes. Light‐harvesting biomolecules—chlorophyll‐a—is employed to study dynamic light–matter interactions in strongly coupled plasmonic nanocavities. Identification of coupling strength is achieved by extracting red, green, and blue (RGB) values from dark‐field images and an enhancement factor from fluorescence images. Lastly, the ability to monitor subtle changes of coupling dynamics in bioplasmonic nanocavity is demonstrated. These findings may deepen the understanding in light–matter interactions, paving new avenues toward applications in quantum‐based biosensing and imaging.

Intrinsic superflat bands in general twisted bilayer systems

Twisted bilayer systems with discrete magic angles, such as twisted bilayer graphene featuring moiré superlattices, provide a versatile platform for exploring novel physical properties. Here, we discover a class of superflat bands in general twisted bilayer systems beyond the low-energy physics of magic-angle twisted counterparts. By considering continuous lattice dislocation, we obtain intrinsic localized states, which are spectrally isolated at lowest and highest energies and spatially centered around the AA stacked region, governed by the macroscopic effective energy potential well. Such localized states exhibit negligible inter-cell coupling and support the formation of superflat bands in a wide and continuous parameter space, which can be mimicked using a twisted bilayer nanophotonic system. Our finding suggests that general twisted bilayer systems can realize continuously tunable superflat bands and the corresponding localized states for various photonic, phononic, and mechanical waves.

Functionalized hybridization of bismuth nanostructures for highly improved nanophotonics

Bismuth (Bi) has achieved remarkable progress due to its intriguing physicochemical properties, such as low toxicity, controllable stability, tunable bandgap, superior optical response, and strong diamagnetism. Bi-based hybrids have drawn increasing attention in recent years due to the integrated features of the Bi component and the synergistic effect on the separation and transfer of charges, holding great promises for versatile applications. In this Perspective, we systematically review the recent progress on the controlled synthesis of Bi-based heterostructures and their improved nanophotonic performances compared with those of mono-element Bi counterparts and present the existing challenges and future opportunities. It is anticipated that this Perspective can shed light on new designs of high-performance functional Bi-based heterostructures to meet the growing demand for next-generation nanophotonic systems.

Spin-decoupled omnidirectional anomalous refraction based on a single metasurface

Taking advantage of the flexible customization of dynamic and Pancharatnam–Berry phases on meta-atoms, spin-decoupled multifunctional metasurfaces have been realized for optical beams of orthogonal circularly polarized lights, which promotes the diverse development of nanophotonic devices. To date, spin-decoupled metasurfaces can only spatially split and deflect beams in coplanar directions not in non-coplanar, limiting further applications. Here, a single metasurface is proposed to experimentally as well as numerically demonstrate the spin-decoupled omnidirectional anomalous refraction. The results indicate that the three-dimensionally omnidirectional dual-beam refractions are attributed to arbitrary engineering of spin-independent phase gradients along any in-plane orientations of the single metasurface. It is believed that the proposed spin-decoupled omnidirectional metasurfaces are promising candidates for multifunctional applications in compact spin-based nanophotonic systems, such as polarized beam splitting, steering, and polarimeter.

Ultrasensitive nanoscale optomechanical electrometer using photonic crystal cavities

Abstract High-precision detection of electric charge is critical for physical, chemical, and biological measurements. Nanophotonic optomechanical system confines the optical field at the nanoscale and enables a strong interaction between optical cavity and mechanical resonator. Its high optical quality factor cavity and strong optomechanical coupling are promising for precision sensing applications. Here an integrated optomechanical electrometer is proposed for the electric charge sensing using a zipper cavity with a suspended photonic crystal nanobeam (PCN) acting as a movable mechanical resonator. As the electrostatic force arising from the electric voltage to be measured interacts with the mechanical motion of the movable PCN and modulates its resonance through electrostatic stiffening effect, optomechanical coupling transduces the mechanical motion to the optical field with enhanced sensitivity. The resonance shift of the mechanical resonator can be monitored to detect the electric voltage with a sensitivity of 0.007 Hz / m V 2 $\mathrm{Hz}/\mathrm{m}{\mathrm{V}}^{2}$ . Moreover, the sensing performance can be further enhanced with the operation of the optomechanical electrometer in the self-sustained oscillation above threshold power. Owing to the narrow-linewidth of detector radio frequency (RF) spectrum with a large peak-to-noise floor ratio (up to 73.5 dB), the enhanced electrical sensitivity of 0.014 Hz / m V 2 $\mathrm{Hz}/\mathrm{m}{\mathrm{V}}^{2}$ is achieved with a high resolution of 1.37 m V 2 H z − 1 / 2 $1.37\,\mathrm{m}{\mathrm{V}}^{2}\mathrm{H}{\mathrm{z}}^{-1/2}$ . A theoretical minimal detectable electrostatic charge is calculated as 1.33 × 10 − 2 eH z − 1 / 2 $1.33{\times}{10}^{-2}\,\mathrm{eH}{\mathrm{z}}^{-1/2}$ by converting the measured electric voltage versus RF shift to an approximatively linear relationship. This on-chip optomechanical electrometry scheme provides a powerful solution to the ultrasensitive determination of charged nanoparticles in biological and chemical applications.

Nanophotonic resonance modes with the nanobem toolbox

nanobem is a matlab toolbox for the solution of Maxwell's equations for nanophotonic systems and the computation of resonance modes, sometimes also referred to as quasinormal modes or resonance states. It is based on a Galerkin scheme for the boundary element method, using Raviart-Thomas shape elements for the representation of the tangential electromagnetic fields at the particle boundary. The toolbox is written in an object-oriented manner with the focus on clarity rather than speed, and has been developed and tested for small to intermediate problems with a few thousand boundary elements. The computation of the resonance modes uses the contour integral method of Beyn. Program summaryProgram Title:nanobemCPC Library link to program files:https://doi.org/10.17632/63cwtv93ry.1Licensing provisions: GNU General Public License v3Programming language: MatlabNature of problem: Solve Maxwell's equations and compute resonance modes for optical resonators and nanophotonic systems with linear, homogeneous, and local materials separated by abrupt interfaces.Solution method: Galerkin implementation of boundary element method approach using Raviart-Thomas shape elements, and contour integral method for the computation of nanophotonic resonance modes.Additional comments including restrictions and unusual features: Toolbox has been developed and tested for small to intermediate problems with a few thousand boundary elements.

Collective Radiative Dynamics of an Ensemble of Cold Atoms Coupled to an Optical Waveguide.

We experimentally and theoretically investigate collective radiative effects in an ensemble of cold atoms coupled to a single-mode optical nanofiber. Our analysis unveils the microscopic dynamics of the system, showing that collective interactions between the atoms and a single guided photon gradually build up along the atomic array in the direction of propagation of light. These results are supported by time-resolved measurements of the light transmitted and reflected by the ensemble after excitation via nanofiber-guided laser pulses, whose rise and fall times are shorter than the atomic lifetime. Superradiant decays more than 1 order of magnitude faster than the single-atom free-space decay rate are observed for emission in the forward-propagating guided mode, while at the same time, no speed-up of the decay rate is measured in the backward direction. In addition, position-resolved measurements of the light that is transmitted past the atoms are performed by inserting the nanofiber-coupled atomic array in a 45-m-long fiber ring resonator, which allow us to experimentally reveal the progressive growth of the collective response of the atomic ensemble. Our results highlight the unique opportunities offered by nanophotonic cold atom systems for the experimental investigation of collective light-matter interaction.

Universal Theory of Light Scattering of Randomly Oriented Particles: A Fluctuational-Electrodynamics Approach for Light Transport Modeling in Disordered Nanostructures.

Disordered nanostructures are commonly encountered in many nanophotonic systems, from colloid dispersions for sensing to heterostructured photocatalysts. Randomness, however, imposes severe challenges for nanophotonics modeling, often constrained by the irregular geometry of the scatterers involved or the stochastic nature of the problem itself. In this Article, we resolve this conundrum by presenting a universal theory of averaged light scattering of randomly oriented objects. Specifically, we derive expansion-basis-independent formulas of the orientation-and-polarization-averaged absorption cross section, scattering cross section, and asymmetry parameter, for single or a collection of objects of arbitrary shape. These three parameters can be directly integrated into traditional unpolarized radiative energy transfer modeling, enabling a practical tool to predict multiple scattering and light transport in disordered nanostructured materials. Notably, the formulas of average light scattering can be derived under the principles of fluctuational electrodynamics, allowing the analogous mathematical treatment to the methods used in thermal radiation, nonequilibrium electromagnetic forces, and other associated phenomena. The proposed modeling framework is validated against optical measurements of polymer composite films with metal-oxide microcrystals. Our work may contribute to a better understanding of light–matter interactions in disordered systems, such as plasmonics for sensing and photothermal therapy, photocatalysts for water splitting and CO2 dissociation, photonic glasses for artificial structural colors, and diffuse reflectors for radiative cooling, to name just a few.

DNA origami enabled assembly of nanophotonic structures and their applications [Invited

Nanophotonics is an emerging hot area that finds applications in optics, sensing and energy harvesting. Conventional fabrication methods are generally limited by their low spatial resolution and patterning capability, which cannot meet the demands of developing advanced nanophotonic structures. DNA origami has enabled a number of novel bottom-up strategies to assemble nanophotonic systems with nanometer accuracy and high geometric freedom. In this review, we use several representative examples to demonstrate the great patterning capability of DNA origami and discuss about the promising applications of those systems. A brief perspective is provided at the end on potential future directions of DNA origami enabled self-assembly.

A learning based approach for designing extended unit cell metagratings

Abstract The possibility of arbitrary spatial control of incident wavefronts with the subwavelength resolution has driven research into dielectric optical metasurfaces in the last decade. The unit-cell based metasurface design approach that relies on a library of single element responses is known to result in reduced efficiency attributed to the inadequate accounting of the coupling effects between meta-atoms. Metasurfaces with extended unit-cells containing multiple resonators can improve design outcomes but their design requires extensive numerical computing and optimizations. We report a deep learning based design methodology for the inverse design of extended unit-cell metagratings. In contrast to previous reports, our approach learns the metagrating spectral response across its reflected and transmitted orders. Through systematic exploration, we discover network architectures and training dataset sampling strategies that allow such learning without requiring extensive ground-truth generation. The one-time investment of model creation can then be used to significantly accelerate numerical optimization of multiple functionalities as demonstrated by considering the inverse design of various spectral and polarization dependent splitters and filters. The proposed methodology is not limited to these proof-of-concept demonstrations and can be broadly applied to meta-atom-based nanophotonic system design and in realising the next generation of metasurface functionalities with improved performance.

Hybrid photonic-plasmonic cavities based on the nanoparticle-on-a-mirror configuration

Hybrid photonic-plasmonic cavities have emerged as a new platform to increase light–matter interaction capable to enhance the Purcell factor in a singular way not attainable with either photonic or plasmonic cavities separately. In the hybrid cavities proposed so far, the plasmonic element is usually a metallic bow-tie antenna, so the plasmonic gap—defined by lithography—is limited to minimum values of several nanometers. Nanoparticle-on-a-mirror (NPoM) cavities are far superior to achieve the smallest possible mode volumes, as plasmonic gaps smaller than 1 nm can be created. Here, we design a hybrid cavity that combines an NPoM plasmonic cavity and a dielectric-nanobeam photonic crystal cavity operating at transverse-magnetic polarization. The metallic nanoparticle can be placed very close ( < 1 nm ) to the upper surface of the dielectric cavity, which acts as a low-reflectivity mirror. We demonstrate through numerical calculations of the local density of states that this hybrid plasmonic-photonic cavity exhibits quality factors Q above 10 3 and normalized mode volumes V down to 10 − 3 , thus resulting in high Purcell factors ( F P ≈ 10 5 ), while being experimentally feasible with current technology. Our results suggest that hybrid cavities with sub-nanometer gaps should open new avenues for boosting light–matter interaction in nanophotonic systems.