Sub-diffraction Volumes Research Articles

Deep subwavelength topological edge state in a hyperbolic medium.

Topological photonics offers the opportunity to control light propagation in a way that is robust from fabrication disorders and imperfections. However, experimental demonstrations have remained on the order of the vacuum wavelength. Theoretical proposals have shown topological edge states that can propagate robustly while embracing deep subwavelength confinement that defies diffraction limits. Here we show the experimental proof of these deep subwavelength topological edge states by implementing periodic modulation of hyperbolic phonon polaritons within a van der Waals heterostructure composed of isotopically pure hexagonal boron nitride flakes on patterned gold films. The topological edge state is confined in a subdiffraction volume of 0.021 µm3, which is four orders of magnitude smaller than the free-space excitation wavelength volume used to probe the system, while maintaining the resonance quality factor above 100. This finding can be directly extended to and hybridized with other van der Waals materials to broadened operational frequency ranges, streamline integration of diverse polaritonic materials, and compatibility with electronic and excitonic systems.

Plasmonic Enhanced Nanocrystal Infrared Photodetectors.

Low-dimensional nanomaterials are widely investigated in infrared photodetectors (PDs) due to their excellent optical and electrical properties. To further improve the PDs property like quantum efficiency, metallic microstructures are commonly used, which could squeeze light into sub-diffraction volumes for enhanced absorption through surface plasma exciton resonance effects. In recent years, plasmonic enhanced nanocrystal infrared PDs have shown excellent performance and attracted much research interest. In this paper, we summarize the progress in plasmonic enhanced nanocrystal infrared PDs based on different metallic structures. We also discuss challenges and prospects in this field.

Resonant Scattering Manipulation of Dielectric Nanoparticles

AbstractThe concentration and manipulation of light in the nanoscale range are fundamental to nanophotonic research. Plasmonic nanoparticles can localize electromagnetic waves within subdiffraction volumes, but they also undergo large Joule losses and inevitable thermal heating. Subwavelength dielectric nanoparticles have emerged as a new class of photonic building blocks that enhance light–matter interactions within nanometric volumes. These nanoparticles exhibit strong electric and magnetic responses with negligible energy dissipation. In recent decades, the design of efficient dielectric nanoresonators has seen tremendous progress. In this review, recent theoretical and experimental advances in characterizing the optical properties of dielectric nanoparticles, from resonant single‐particle scattering characteristics to multimodal interference in complex particle assemblies, are discussed. Specific attention is paid to novel strategies employed to manipulate far‐field Mie‐type scattering, enhance local electromagnetic field, and boost magnetic resonance, as well as ultimately achieve Fano‐like resonance, unidirectional scattering (Kerker conditions), and photon waveguide. A collection of emerging applications of dielectric nanoparticles is also highlighted and the fundamental prospects of designing all‐dielectric/metallic–dielectric photonic nanostructures are considered, particularly those of functional dielectric materials and all‐dielectric 3D assemblies.

Lower Exciton Number Strong Light Matter Interaction in Plasmonic TweezersSupported by the National Key Research and Development Program of China (Grant No. 2016YFA0301300), the Fundamental Research Funds for the Central Universities (Grant No. 2019XD-A09), and the National Natural Science Foundation of China (Grant No. 11574035).

The plasmonic nanocavity is an excellent platform for the study of light matter interaction within a sub-diffraction volume under ambient conditions. We design a structure of plasmonic tweezers, which can trap molecular J-aggregates and also serve as a plasmonic cavity with which to investigate strong light matter interaction. The optical response of the cavity is calculated via finite-difference time-domain methods, and the optical force is evaluated based on the Maxwell stress tensor method. With the help of the coupled oscillator model and virtual exciton theory, we investigate the strong coupling progress at the lower level of excitons, finding that a Rabi splitting of 230 meV can be obtained in a single exciton system. We further analyze the relationship between optical force and model volume in the coupling system. The proposed method offers a way to locate molecular J-aggregates in plasmonic tweezers for investigating optical force performance and strong light matter interaction.

Strategies to maximize performance in STimulated Emission Depletion (STED) nanoscopy of biological specimens.

Super-resolution fluorescence microscopy has become an important catalyst for discovery in the life sciences. In STimulated Emission Depletion (STED) microscopy, a pattern of light drives fluorophores from a signal-emitting on-state to a non-signalling off-state. Only emitters residing in a sub-diffraction volume around an intensity minimum are allowed to fluoresce, rendering them distinguishable from the nearby, but dark fluorophores. STED routinely achieves resolution in the few tens of nanometers range in biological samples and is suitable for live imaging. Here, we review the working principle of STED and provide general guidelines for successful STED imaging. The strive for ever higher resolution comes at the cost of increased light burden. We discuss techniques to reduce light exposure and mitigate its detrimental effects on the specimen. These include specialized illumination strategies as well as protecting fluorophores from photobleaching mediated by high-intensity STED light. This opens up the prospect of volumetric imaging in living cells and tissues with diffraction-unlimited resolution in all three spatial dimensions.

Single-Molecule Nanoscopy Elucidates RNA Polymerase II Transcription at Single Genes in Live Cells

Transforming the vast knowledge from genetics, biochemistry, and structural biology into detailed molecular descriptions of biological processes inside cells remains a major challenge-one in sore need of better imaging technologies. For example, transcription involves the complex interplay between RNA polymerase II (Pol II), regulatory factors (RFs), and chromatin, but visualizing these dynamic molecular transactions in their native intracellular milieu remains elusive. Here, we zoom into single tagged genes using nanoscopy techniques, including an active target-locking, ultra-sensitive system that enables single-molecule detection in addressable sub-diffraction volumes, within crowded intracellular environments. We image, track, and quantify Pol II with single-molecule resolution, unveiling its dynamics during the transcription cycle. Further probing multiple functionally linked events-RF-chromatin interactions, Pol II dynamics, and nascent transcription kinetics-reveals detailed operational parameters of gene-regulatory mechanisms hitherto-unseen invivo. Our approach sets the stage for single-molecule studies of complex molecular processes in live cells.

Individual control and readout of qubits in a sub-diffraction volume

Medium-scale ensembles of coupled qubits offer a platform for near-term quantum technologies as well as studies of many-body physics. A central challenge for coherent control of such systems is the ability to measure individual quantum states without disturbing nearby qubits. Here, we demonstrate the measurement of individual qubit states in a sub-diffraction cluster by selectively exciting spectrally distinguishable nitrogen vacancy centers. We perform super-resolution localization of single centers with nanometer spatial resolution, as well as individual control and readout of spin populations. These measurements indicate a readout-induced crosstalk on non-addressed qubits below 4 × 10−2. This approach opens the door to high-speed control and measurement of qubit registers in mesoscopic spin clusters, with applications ranging from entanglement-enhanced sensors to error-corrected qubit registers to multiplexed quantum repeater nodes.

Comparative study of optical near-field transducers for heat-assisted magnetic recording

Heat-assisted magnetic recording (HAMR), widely considered to be the next generation technology for high-density data storage devices, uses a tiny plasmonic antenna called a near-field transducer (NFT) to focus light down to a subdiffraction volume. This results in a temporary and local rise in temperature of the recording medium thereby reducing its coercivity, allowing the external magnetic field to write data bits in the medium. The performance of any HAMR system strongly depends on the design of the NFT. The optical performance in terms of the optical coupling efficiency and the spot size for several different NFT designs, including the triangle antenna, E antenna, bowtie aperture, lollipop antenna, and C-aperture, are considered. Also, the corresponding temperature rise in the recording medium and the NFT is calculated and several figures of merit based on the temperature profile are compared for the different designs. This work gives a comparison of the relative performances of different types of NFT and can be a basis for choosing a suitable design for HAMR applications.

Nanomechanical motion transduction with a scalable localized gap plasmon architecture.

Plasmonic structures couple oscillating electromagnetic fields to conduction electrons in noble metals and thereby can confine optical-frequency excitations at nanometre scales. This confinement both facilitates miniaturization of nanophotonic devices and makes their response highly sensitive to mechanical motion. Mechanically coupled plasmonic devices thus hold great promise as building blocks for next-generation reconfigurable optics and metasurfaces. However, a flexible approach for accurately batch-fabricating high-performance plasmomechanical devices is currently lacking. Here we introduce an architecture integrating individual plasmonic structures with precise, nanometre features into tunable mechanical resonators. The localized gap plasmon resonators strongly couple light and mechanical motion within a three-dimensional, sub-diffraction volume, yielding large quality factors and record optomechanical coupling strength of 2 THz·nm−1. Utilizing these features, we demonstrate sensitive and spatially localized optical transduction of mechanical motion with a noise floor of 6 fm·Hz−1/2, representing a 1.5 orders of magnitude improvement over existing localized plasmomechanical systems.

Vacuum Rabi splitting in a plasmonic cavity at the single quantum emitter limit.

The strong interaction of individual quantum emitters with resonant cavities is of fundamental interest for understanding light–matter interactions. Plasmonic cavities hold the promise of attaining the strong coupling regime even under ambient conditions and within subdiffraction volumes. Recent experiments revealed strong coupling between individual plasmonic structures and multiple organic molecules; however, strong coupling at the limit of a single quantum emitter has not been reported so far. Here we demonstrate vacuum Rabi splitting, a manifestation of strong coupling, using silver bowtie plasmonic cavities loaded with semiconductor quantum dots (QDs). A transparency dip is observed in the scattering spectra of individual bowties with one to a few QDs, which are directly counted in their gaps. A coupling rate as high as 120 meV is registered even with a single QD, placing the bowtie-QD constructs close to the strong coupling regime. These observations are verified by polarization-dependent experiments and validated by electromagnetic calculations.

Nano-Confined Polymer Structures for Protein Binding

Stimulated emission depletion lithography (STED-lithography), a further development of multiphoton polymerization (MPP) lithography, belongs to the most promising methods for 2D and 3D structuring of polymer scaffolds with structure sizes in the nanometer range. Already in the early developmental stages of STED-microscopy, it was pointed out that a nano-confined effective point-spread function (PSF) can be applied to spatially confine photo-chemical reactions to sub-diffraction volumes. In optical STED-lithography, one laser pulse excites photo-initiators for radical polymerization and a second donut-shaped laser beam locally inhibits these starter molecules in the outer rim of the PSF. This confinement of the excitation volume leads to a restriction of the size of the polymerized structures. Such a STED-based approach facilitates the creation of structures with sizes below the limits of conventional MPP. Currently, feature sizes as small as 55 nm and a resolution of 120 nm of adjacent lines can be achieved.We use stimulated emission depletion (STED) lithography for the assembly of polymeric structures down to several nanometers in any desired geometry. Protein adhesive photoresists allow manufacturing of nano-confined, protein functionalized structures. The structures show good biocompatibility and allow an easy biofunctionalization with proteins down to a single protein level. Furthermore, we use STED-lithography to create 3D compound structures of different acrylate polymers with distinct properties. A µm-sized protein repellent scaffold consists of multi-photon lithography (MPP) fabricated features, carrying STED-lithography written sub-100 nm protein-binding sites.Combining STED lithography with fluorescence microscopy allows us to produce well characterized, biocompatible structures, applicable to biological assays.

Subdiffraction, Luminescence-Depletion Imaging of Isolated, Giant, CdSe/CdS Nanocrystal Quantum Dots

Subdiffraction spatial resolution luminescence depletion imaging was performed with giant CdSe/14CdS nanocrystal quantum dots (g-NQDs) dispersed on a glass slide. Luminescence depletion imaging used a Gaussian shaped excitation laser pulse overlapped with a depletion pulse, shaped into a doughnut profile, with zero intensity in the center. Luminescence from a subdiffraction volume is collected from the central portion of the excitation spot, where no depletion takes place. Up to 92% depletion of the luminescence signal was achieved. An average full width at half-maximum of 40 ± 10 nm was measured in the lateral direction for isolated g-NQDs at an air interface using luminescence depletion imaging, whereas the average full width at half-maximum was 450 ± 90 nm using diffraction-limited, confocal luminescence imaging. Time-gating of the luminescence depletion data was required to achieve the stated spatial resolution. No observable photobleaching of the g-NQDs was present in the measurements, which allowed imaging with a dwell time of 250 ms per pixel to obtain images with a high signal-to-noise ratio. The mechanism for luminescence depletion is likely stimulated emission, stimulated absorption, or a combination of the two. The g-NQDs fulfill a need for versatile, photostable tags for subdiffraction imaging schemes where high laser powers or long exposure times are used.

Photoactivatable Synthetic Dyes for Fluorescence Imaging at the Nanoscale.

The transition from conventional to photoactivatable fluorophores can bring the resolution of fluorescence images from the micrometer to the nanometer level. Indeed, fluorescence photoactivation can overcome the limitations that diffraction imposes on the resolution of optical microscopes. Specifically, distinct fluorophores positioned within the same subdiffraction volume can be resolved only if their emissions are activated independently at different intervals of time. Under these conditions, the sequential localization of multiple probes permits the reconstruction of images with a spatial resolution that is otherwise impossible to achieve with conventional fluorophores. The irreversible photolysis of protecting groups or the reversible transformations of photochromic compounds can be employed to control the emission of appropriate fluorescent chromophores and allow the implementation of these ingenious operating principles for superresolution imaging. Such molecular constructs enable the spatiotemporal control that is required to avoid diffraction and can become invaluable analytical tools for the optical visualization of biological specimens and nanostructured materials with unprecedented resolution.

Cell investigation of nanostructures: zero-mode waveguides for plasma membrane studieswith single molecule resolution

Plasma membranes are highly dynamic structures, with key molecular interactionsunderlying their functionality occurring at nanometre scales. A fundamental challenge inbiology is to observe these interactions in living cells. Although fluorescence microscopy hasenabled advances in characterizing molecular distributions in cells, optical techniques arerestricted by the diffraction limit. We address this limitation with an approach based onzero-mode waveguides (ZMWs), which are optical nanostructures that confinefluorescence excitation to sub-diffraction volumes. Successful use of ZMWs with cellmembranes is reported in this paper. We demonstrate that plasma membranes from livecells penetrate these nanostructures. Cellular exploration of the nanoaperturesdepends heavily on actin filaments but not on microtubules. Thus, membranesenter the confined excitation volume, and diffusion of individual fluorescent lipidscan be monitored. Through fluorescence correlation spectroscopy, we comparedDiIC12 and DiIC16 fluorescent labels incorporated into plasma membranes and found distinctive diffusionbehaviours. These results show that the use of optical nanostructures enables themeasurement of membrane events with single molecule resolution in sub-diffractionvolumes.