Optical Diffraction Limit Research Articles

Anomalous Nernst Effect-Based Near-Field Imaging of Magnetic Nanostructures.

The anomalous Nernst effect (ANE) gives rise to an electrical response transverse to magnetization and an applied temperature gradient in a magnetic metal. A nanoscale temperature gradient can be generated by the use of a laser beam applied to the apex of an atomic force microscope tip, thereby allowing for spatially resolved ANE measurements beyond the optical diffraction limit. Such a method has been previously used to map in-plane magnetized magnetic textures. However, the spatial distribution of the out-of-plane temperature gradient, which is needed to fully interpret such ANE-based imaging, was not studied. We therefore use a well-known magnetic texture, a magnetic vortex core, to demonstrate the reliability of the ANE method for imaging of magnetic domains with nanoscale resolution. Moreover, since the ANE signal is directly proportional to the temperature gradient, we can also consider the inverse problem and deduce information about the nanoscale temperature distribution. Our results together with finite element modeling indicate that besides the out-of-plane temperature gradients there are even larger in-plane temperature gradients. Thus, we extend the ANE imaging to study the out-of-plane magnetization in a racetrack nanowire by detecting the ANE signal generated by in-plane temperature gradients. In all cases, a spatial resolution of ≈70 nm is obtained. These results are significant for the rapidly growing field of thermoelectric imaging of antiferromagnetic spintronic device structures.

Practical limits to spatial resolution of magnetic imaging with a quantum diamond microscope

Widefield quantum diamond microscopy is a powerful technique for imaging magnetic fields with high sensitivity and spatial resolution. However, current methods to approach the ultimate spatial resolution (<500 nm) are impractical for routine use as they require time-consuming fabrication or transfer techniques to precisely interface the diamond sensor with the sample to be imaged. To address this challenge, we have designed a co-axial sensor holder that enables simple, repeatable sensor–sample interfacing while being compatible with high numerical aperture (NA) optics. With our new design we demonstrate low standoffs <500 nm with a millimeter sized sensor. We also explore the relationship between spatial resolution and NA spanning from 0.13 to 1.3. The spatial resolution shows good agreement with the optical diffraction limit at low NA but deviates at high NA, which is shown to be due to optical aberrations. Future improvements to our design are discussed, which should enable magnetic imaging with <500 nm resolution in an accessible, easy-to-use instrument.

Singular dielectricnanolaser with atomic-scale field localization.

Compressing the optical field to the atomic scale opens up possibilities for directly observing individual molecules, offering innovative imaging and research tools for both physical and life sciences. However, the diffraction limit imposes a fundamental constraint on how much the optical field can be compressed, based on the achievable photon momentum1,2. In contrast to dielectric structures, plasmonics offer superior field confinement by coupling the light field with the oscillations of free electrons in metals3-6. Nevertheless, plasmonics suffer from inherent ohmic loss, leading to heat generation, increased power consumption and limitations on the coherence time of plasmonic devices7,8. Here we propose and demonstrate singular dielectric nanolasers showing a mode volume that breaks the optical diffraction limit. Derived from Maxwell's equations, we discover that the electric-field singularity sustained in a dielectric bowtie nanoantenna originates from divergence of momentum. The singular dielectric nanolaser is constructed by integrating a dielectric bowtie nanoantenna into the centre of a twisted lattice nanocavity. The synergistic integration surpasses the diffraction limit, enabling the singular dielectricnanolaser to achieve an ultrasmall mode volume of about 0.0005 λ3 (λ, free-space wavelength), along with an exceptionally small feature size at the 1-nanometre scale. To fabricate the required dielectric bowtie nanoantenna with a single-nanometre gap, we develop a two-step process involving etching and atomic deposition. Our research showcases the ability to achieve atomic-scale field localization in laser devices, paving the way for ultra-precise measurements, super-resolution imaging, ultra-efficient computing and communication, and the exploration of light-matter interactions within the realm of extreme optical field localization.

Ultra-thin size-controllable surface plasmon polariton laser by PDMS-assisted imprinting

Plasmonic laser has great potential to overcome the optical diffraction limit, playing a crucial role in advancing nanophotonics and nanoelectronics for on-chip integration. However, current plasmonic lasers face several challenges, such as the difficulty in controlling nanowire (NW) size, disordered arrangement, and complicated fabrication process. Herein, ultra-thin gain media for plasmonic lasers below the cutoff size of the photonic mode are prepared using the polydimethylsiloxane-assisted imprinting. This method enables precise control over the size of the perovskite NW, with the minimum size achievable being 60 nm. As a result, the plasmonic lasing is achieved from the CsPbBr3 NW-based device with a threshold as low as ∼49.13 μJ cm−2 and a Quality Factor (Q) of 1803 at room temperature, demonstrating its capability for achieving high-quality lasing. Meanwhile, a dual-pumping time-resolved fluorescence study suggests that the radiative recombination lifetime of CsPbBr3 NWs is shortened by a factor of 10 due to the Purcell effect, confirming the plasmonic effect exhibited by the device. Furthermore, a plasmonic laser array is developed using this method, demonstrating the applicability of the imprinting method in complex graphic fabrication. This breakthrough provides a solution for the application of plasmonic laser arrays in optoelectronic integration.

Identification of z-axis filopodia in growth cones using super-resolution microscopy.

A growth cone is a highly motile tip of an extending axon that is crucial for neural network formation. Three-dimensional-structured illumination microscopy, a type of super-resolution light microscopy with a resolution that overcomes the optical diffraction limitation (ca. 200 nm) of conventional light microscopy, is well suited for studying the molecular dynamics of intracellular events. Using this technique, we discovered a novel type of filopodia distributed along the z-axis ("z-filopodia") within the growth cone. Z-filopodia were typically oriented in the direction of axon growth, not attached to the substratum, protruded spontaneously without microtubule invasion, and had a lifetime that was considerably shorter than that of conventional filopodia. Z-filopodia formation and dynamics were regulated by actin-regulatory proteins, such as vasodilator-stimulated phosphoprotein, fascin, and cofilin. Chromophore-assisted laser inactivation of cofilin induced the rapid turnover of z-filopodia. An axon guidance receptor, neuropilin-1, was concentrated in z-filopodia and was transported together with them, whereas its ligand, semaphorin-3A, was selectively bound to them. Membrane domains associated with z-filopodia were also specialized and resembled those of lipid rafts, and their behaviors were closely related to those of neuropilin-1. The results suggest that z-filopodia have unique turnover properties, and unlike xy-filopodia, do not function as force-generating structures for axon extension.

Controlling dark solitons on the healing length scale

While usually the optical diffraction limit is setting a limit for the lengthscales on which a typical alkali Bose–Einstein condensate can be controlled, we show that in certain situations control via matter waves can achieve smaller resolutions. For this we consider a small number of impurity atoms which are trapped inside the density dip of a dark soliton state and show that any grey soliton state can be obtained by selectively driving the impurity atoms. This allows to fully control the position and velocity of the dark soliton, and also study controlled collisions between these non-linear objects.

Direct laser patterning of ruthenium below the optical diffraction limit

We describe a method that can be used to produce ruthenium/ruthenium oxide patterns starting from a ruthenium thin film. The method is based on highly localized oxidation of a small surface area of a ruthenium film by means of exposure to a pulsed laser under ambient conditions. Laser exposure is followed by dissolution of the un-exposed ruthenium in a NaClO solution, which leaves the conductive, partially oxidized ruthenium area on the substrate. Spatially selective oxidation, material removal, and, by implication, patterning, are, therefore, achieved without the need for a photoresist layer. Varying the exposure laser parameters, such as fluence, focus diameter, and repetition rate, allows us to optimize the process. In particular, it enables us to obtain circular Ru/RuO2 islands with a sub-diffraction-limited diameter of about 500 nm, for laser exposure times as short as 50 ms. The capability to obtain such small islands suggests that heat-diffusion is not a limiting factor to pattern Ru by laser heating on a (sub-)micron scale. In fact, heat diffusion helps in that it limits the area where a sufficiently high temperature is reached and maintained for a sufficiently long time for oxidation to occur. Our method provides an easy way to produce metallic Ru/RuO2 (sub-)micron structures and has possible applications in semiconductor manufacturing.

Untrained neural network enabling fast and universal structured-illumination microscopy.

Structured-illumination microscopy (SIM) offers a twofold resolution enhancement beyond the optical diffraction limit. At present, SIM requires several raw structured-illumination (SI) frames to reconstruct a super-resolution (SR) image, especially the time-consuming reconstruction of speckle SIM, which requires hundreds of SI frames. Considering this, we herein propose an untrained structured-illumination reconstruction neural network (USRNN) with known illumination patterns to reduce the amount of raw data that is required for speckle SIM reconstruction by 20 times and thus improve its temporal resolution. Benefiting from the unsupervised optimizing strategy and CNNs' structure priors, the high-frequency information is obtained from the network without the requirement of datasets; as a result, a high-fidelity SR image with approximately twofold resolution enhancement can be reconstructed using five frames or less. Experiments on reconstructing non-biological and biological samples demonstrate the high-speed and high-universality capabilities of our method.

A Review of Microsphere Super-Resolution Imaging Techniques.

Conventional optical microscopes are only able to resolve objects down to a size of approximately 200 nm due to optical diffraction limits. The rapid development of nanotechnology has increased the demand for greater imaging resolution, with a need to break through those diffraction limits. Among super-resolution techniques, microsphere imaging has emerged as a strong contender, offering low cost, simple operation, and high resolution, especially in the fields of nanodevices, biomedicine, and semiconductors. However, this technology is still in its infancy, with an inadequate understanding of the underlying principles and the technology's limited field of view. This paper comprehensively summarizes the status of current research, the advantages and disadvantages of the basic principles and methods of microsphere imaging, the materials and preparation processes, microsphere manipulation methods, and applications. The paper also summarizes future development trends.

Subwavelength Photorefractive Grating in a Thin‐Film Lithium Niobate Microcavity

AbstractSubwavelength gratings play a fundamental and pivotal role in numerous science and applications for wave manipulation, exhibiting distinctive features such as filtering, phase manipulation, and anti‐reflection. However, conventional fabrication methods for ultrasmall periodic structures are constrained by the fundamental optical diffraction limit, making it challenging to produce subwavelength gratings for optics. Here, a novel technique is demonstrated to build a reconfigurable subwavelength photorefractive grating (SPG) in a thin‐film lithium niobate on the platform of an optical microcavity. Such SPGs are optically induced through the photorefractive effect and the subwavelength features originate from the spatial phase modulations of the pump's standing wave. The resulting SPGs lead to the mode splitting of two counter‐propagating modes inside the microcavity, exhibiting an Electromagnetically Induced Transparency (EIT)‐like transmission spectrum. Moreover, the unique subwavelength characteristic of SPGs enables first‐order quasi‐phase‐matching for backward second‐harmonic generation, a long‐standing problem in nonlinear optics. Also, free‐space‐to‐chip vertical nonlinear frequency conversion can be achieved in a similar manner. These results provide a flexible approach toward fabricating subwavelength gratings, which holds significant potential in various applications such as nonlinear frequency conversion, optical communication, sensing, and quantum technologies.

Direct observation of a crescent-shape chromosome in expanded Bacillus subtilis cells

Bacterial chromosomes are folded into tightly regulated three-dimensional structures to ensure proper transcription, replication, and segregation of the genetic information. Direct visualization of chromosomal shape within bacterial cells is hampered by cell-wall confinement and the optical diffraction limit. Here, we combine cell-shape manipulation strategies, high-resolution fluorescence microscopy techniques, and genetic engineering to visualize the shape of unconfined bacterial chromosome in real-time in live Bacillus subtilis cells that are expanded in volume. We show that the chromosomes predominantly exhibit crescent shapes with a non-uniform DNA density that is increased near the origin of replication (oriC). Additionally, we localized ParB and BsSMC proteins – the key drivers of chromosomal organization – along the contour of the crescent chromosome, showing the highest density near oriC. Opening of the BsSMC ring complex disrupted the crescent chromosome shape and instead yielded a torus shape. These findings help to understand the threedimensional organization of the chromosome and the main protein complexes that underlie its structure.

Pixelated non-volatile programmable photonic integrated circuits with 20-level intermediate states

Multi-level programmable photonic integrated circuits (PICs) and optical metasurfaces have gained widespread attention in many fields, such as neuromorphic photonics, optical communications, and quantum information. In this paper, we propose pixelated programmable Si3N4 PICs with record-high 20-level intermediate states at 785 nm wavelength. Such flexibility in phase or amplitude modulation is achieved by a programmable Sb2S3 matrix, the footprint of whose elements can be as small as 1.2 μm, limited only by the optical diffraction limit of an in-house developed pulsed laser writing system. We believe our work lays the foundation for laser-writing ultra-high-level (20 levels and even more) programmable photonic systems and metasurfaces based on phase change materials, which could catalyze diverse applications such as programmable neuromorphic photonics, biosensing, optical computing, photonic quantum computing, and reconfigurable metasurfaces.

Super-resolution reconstruction for subsurface defects of optical elements based on CdSe/ZnS quantum dots fluorescence dichroism

The subsurface defects (SSD) of optical elements have a significant impact on the service life and laser damage threshold of the inertial confinement nuclear fusion optical system. Due to the optical diffraction limit, fluorescence microscopy has limited image resolution for detecting SSD in optical elements, making it challenging to meet the requirements for precision detection. A super-resolution method is proposed in this paper for detecting SSD in optical elements based on the fluorescence polarization characteristics of CdSe/ZnS quantum dots fluorescence dichroism. By enhancing the imaging sparsity of adjacent fluorophores through a polarization modulated excitation laser and utilizing the fluorescence polarization modulation algorithm, the resolution is achieved beyond the optical diffraction limit. The results demonstrate that the proposed method in this paper can obtain super-resolution images of SSD in optical elements with at least a two-fold increase in lateral resolution. This approach effectively improves detection accuracy for SSD and holds significant guiding significance. In addition, it also has application value for assessing the quality of high-precision optical elements.

Wide-field Fourier magnetic imaging with electron spins in diamond

Wide-field magnetic imaging based on nitrogen-vacancy (NV) centers in diamond has been shown the applicability in material and biological science. However, the spatial resolution is limited by the optical diffraction limit (>200 nm) due to the optical real-space localization and readout of NV centers. Here, we report the wide-field Fourier magnetic imaging technique to improve spatial resolution beyond the optical diffraction limit while maintaining the large field of view. Our method relies on wide-field pulsed magnetic field gradient encoding of NV spins and Fourier transform under pixel-dependent spatial filters. We have improved spatial resolution by a factor of 20 compared to the optical resolution and demonstrated the wide-field super-resolution magnetic imaging of a gradient magnetic field. This technique paves a way for efficient magnetic imaging of large-scale fine structures at the nanoscale.

A 3D nanoscale optical disk memory with petabit capacity.

High-capacity storage technologies are needed to meet our ever-growing data demands1,2. However, data centres based on major storage technologies such as semiconductor flash devices and hard disk drives have high energy burdens, high operation costs and short lifespans2,3. Optical data storage (ODS) presents a promising solution for cost-effective long-term archival data storage. Nonetheless, ODS has been limited by its low capacity and the challenge of increasing its areal density4,5. Here, to address these issues, we increase the capacity of ODS to the petabit level by extending the planar recording architecture to three dimensions with hundreds of layers, meanwhile breaking the optical diffraction limit barrier of the recorded spots. We develop an optical recording medium based on a photoresist film doped with aggregation-induced emission dye, which can be optically stimulated by femtosecond laser beams. This film is highly transparent and uniform, and the aggregation-induced emission phenomenon provides the storage mechanism. It can also be inhibited by another deactivating beam, resulting in a recording spot with a super-resolution scale. This technology makes it possible to achieve exabit-level storage by stacking nanoscale disks into arrays, which is essential in big data centres with limited space.

Insight into plasmonics: resurrection of modern-day science (invited)

Plasmonics is a field of research and technology that focuses on the interaction between light and free electrons in a metal structure called plasmon. The study of plasmonics has gained significant attention in recent years due to its potential for several applications and its ability to manipulate light at nanoscale dimensions. Plasmonics enables the control of light at the nanoscale, far beyond the diffraction limit of conventional optics. This allows for the development of new devices and technologies with enhanced performance and functionality. In this paper, recent advances in plasmonics in medicine, agriculture, agriculture, environmental monitoring, lasers and solar energy harvesting are reviewed. Despite these promising prospects, plasmonic devices must overcome obstacles such as significant energy losses, complicated production processes, and the need for better material characteristics. Plasmonics will continue to advance because of ongoing work in nanotechnology, material science, and engineering, which will make it a more significant field with a wide range of usages in the future. In the end, the advantages and the limitations related to the realization of plasmonic devices in the real world are discussed.

Compensating losses in polariton propagation with synthesized complex frequency excitation.

Surface plasmon polaritons and phonon polaritons offer a means of surpassing the diffraction limit of conventional optics and facilitate efficient energy storage, local field enhancement and highsensitivity sensing, benefiting from their subwavelength confinement of light. Unfortunately, losses severely limit the propagation decay length, thus restricting the practical use of polaritons. While optimizing the fabrication technique can help circumvent the scattering loss of imperfect structures, the intrinsic absorption channel leading to heat production cannot be eliminated. Here, we utilize synthetic optical excitation of complex frequency with virtual gain, synthesized by combining the measurements made at multiple real frequencies, to compensate losses in the propagations of phonon polaritons with dramatically enhanced propagation distance. The concept of synthetic complex frequency excitation represents a viable solution to the loss problem for various applications including photonic circuits, waveguiding and plasmonic/phononic structured illumination microscopy.

Laser manufacturing of spatial resolution approaching quantum limit

Atomic and close-to-atom scale manufacturing is a promising avenue toward single-photon emitters, single-electron transistors, single-atom memory, and quantum-bit devices for future communication, computation, and sensing applications. Laser manufacturing is outstanding to this end for ease of beam manipulation, batch production, and no requirement for photomasks. It is, however, suffering from optical diffraction limits. Herein, we report a spatial resolution improved to the quantum limit by exploiting a threshold tracing and lock-in method, whereby the two-order gap between atomic point defect complexes and optical diffraction limit is surpassed, and a feature size of <5 nm is realized. The underlying physics is that the uncertainty of local atom thermal motion dominates electron excitation, rather than the power density slope of the incident laser. We show that the colour centre yield in hexagonal boron nitride is transformed from stochastic to deterministic, and the emission from individual sites becomes polychromatic to monochromatic. As a result, single colour centres in the regular array are deterministically created with a unity yield and high positional accuracy, serving as a step forward for integrated quantum technological applications.

Quantification of membrane geometry and protein sorting on cell membrane protrusions using fluorescence microscopy.

Plasma membranes are flexible and can exhibit numerous shapes below the optical diffraction limit. The shape of cell periphery can either induce or be a product of local protein density changes, encoding numerous cellular functions. However, quantifying membrane curvature and the ensuing sorting of proteins in live cells remains technically demanding. Here, we demonstrate the use of simple widefield fluorescence microscopy to study the geometrical properties (i.e., radius, length, and number) of thin membrane protrusions. Importantly, the quantification of protrusion radius establishes a platform for studying the curvature preferences of membrane proteins.

Super-Resolution Exciton Imaging of Nanobubbles in 2D Semiconductors with Near-Field Nanophotoluminescence Microscopy.

Two-dimensional (2D) semiconductors, such as transition metal dichalcogenides, have emerged as important candidate materials for next-generation chip-scale optoelectronic devices with the development of large-scale production techniques, such as chemical vapor deposition (CVD). However, 2D materials need to be transferred to other target substrates after growth, during which various micro- and nanoscale defects, such as nanobubbles, are inevitably generated. These nanodefects not only influence the uniformity of 2D semiconductors but also may significantly alter the local optoelectronic properties of the composed devices. Hence, super-resolution discrimination and characterization of nanodefects are highly demanded. Here, we report a near-field nanophotoluminescence (nano-PL) microscope that can quickly screen nanobubbles and investigate their impact on local excitonic properties of 2D semiconductors by directly visualize the PL emission distribution with a very high spatial resolution of ∼10 nm, far below the optical diffraction limit, and a high speed of 10 ms/point under ambient conditions. By using nano-PL microscopy to map the exciton and trion emission intensity distributions in transferred CVD-grown monolayer tungsten disulfide (1L-WS2) flakes, it is found that the PL intensity decreases by 13.4% as the height of the nanobubble increases by every nanometer, which is mainly caused by the suppression of trion emission due to the strong doping effect from the substrate. In addition to the nanobubbles, other types of nanodefects, such as cracks, stacks, and grain boundaries, can also be characterized. The nano-PL method is proven to be a powerful tool for the nondestructive quality inspection of nanodefects as well as the super-resolution exploration of local optoelectronic properties of 2D materials.