Far-field Super-resolution Research Articles

Optical atompilz: Propagation-invariant strongly longitudinally polarized toroidal pulses

Recent advancements in optical, terahertz, and microwave systems have unveiled non-transverse optical toroidal pulses characterized by skyrmionic topologies, fractal-like singularities, space-time nonseparability, and anapole-exciting ability. Despite this, the longitudinally polarized fields of canonical toroidal pulses notably lag behind their transverse counterparts in magnitude. Interestingly, although mushroom-cloud-like toroidal vortices with strong longitudinal fields are common in nature, they remain unexplored in the realm of electromagnetics. Here, we present strongly longitudinally polarized toroidal pulses (SLPTPs), which boast a longitudinal component amplitude exceeding that of the transverse component by over tenfold. This unique polarization property endows SLPTPs with robust propagation characteristics, showcasing nondiffracting behavior. The propagation-invariant strongly longitudinally polarized field holds promise for pioneering light–matter interactions, far-field superresolution microscopy, and high-capacity wireless communication utilizing three polarizations.

Far-Field Super-Resolution Microscopy Using Evanescent Illumination: A Review

The resolution of conventional optical microscopy is restricted by the diffraction limit. Light waves containing higher-frequency information about the sample are bound to the sample surface and cannot be collected by far-field optical microscopy. To break the resolution limit, researchers have proposed various far-field super-resolution (SR) microscopy imaging methods using evanescent waves to transfer the high-frequency information of samples to the low-frequency passband of optical microscopy. Optimization algorithms are developed to reconstruct a SR image of the sample by utilizing the high-frequency information. These techniques can be collectively referred to as spatial-frequency-shift (SFS) SR microscopy. This review aims to summarize the basic principle of SR microscopy using evanescent illumination and introduce the advances in this research area. Some current challenges and possible directions are also discussed.

Far-field superresolution of thermal sources by double homodyne or double array homodyne detection

We propose two schemes for estimating the separation of two thermal sources via double homodyne and double array homodyne detection considering the joint measurement of conjugate quadratures of the image plane field.By using the Cramér–Rao bound, we demonstrate that the two schemes can estimate the separation well below the Rayleigh limit and have an advantage over direct imaging when the average photon number per source exceeds five.For arbitrary source strengths, double homodyne detection is superior to homodyne detection when the separation is above 25/4σ/N s , σ is the beam width, N s is the average photon number per source.A larger separation can be estimated better via double array homodyne detection with the superiority of flexible operation compared with other schemes. High-speed and high-efficiency detection enables the measurement schemes with potential practical applications in fluorescence microscopy, astronomy and quantum imaging.

Design and Analysis of the Dual-Band Far-Field Super-Resolution Metalens with Large Aperture.

The resolving power of metalens telescopes rely on their aperture size. Flat telescopes are advancing with the research on super-resolution confocal metalenses with large aperture. However, the aperture sizes of metalenses are usually bound within hundreds of micrometers due to computational and fabrication challenges, limiting their usage on practical optical devices like telescopes. In this work, we demonstrated a two-step designing method for the design of dual-band far-field super-resolution metalens with aperture sizes from the micro-scale to macro-scale. By utilizing two types of inserted unit cells, the phase profile of a dual-wavelength metalens with a small aperture of 100 μm was constructed. Through numerical simulation, the measured FWHM values of the focal spots of 5.81 μm and 6.81 μm at working wavelengths of 632.8 nm and 1265.6 nm were found to all be slightly smaller than the values of 0.61 λ/NA, demonstrating the super-resolution imaging of the designed metalens. By measuring the optical power ratio of the focal plane and the incident plane, the focusing efficiencies were 76% at 632.8 nm and 64% at 1265.6 nm. Based on the design method for small-aperture metalens, far-field imaging properties through the macro metalens with an aperture of 40 mm were simulated by using the Huygens-Fresnel principle. The simulation results demonstrate confocal far-field imaging behavior at the target wavelengths of 632.8 nm and 1265.6 nm, with a focal length of 200 mm. The design method for dual-band far-field super-resolution metalens with a large aperture opens a door towards the practical applications in the dual-band space telescope system.

Far-field superresolution shape reconstruction of objects using level-set-based geometrical full-waveform inversion

Far-field superresolution shape reconstruction of objects using level-set-based geometrical full-waveform inversion

Construction of Far-Field Super-Resolution Optical Field Based on Spatial Light Modulator

Construction of Far-Field Super-Resolution Optical Field Based on Spatial Light Modulator

Beyond superoscillation: general theory of approximation with bandlimited functions

We give a general strategy to construct superoscillating/supergrowing functions using an orthogonal polynomial expansion of a bandlimited function. The degree of superoscillation/supergrowth is controlled by an anomalous expectation value of a pseudodistribution that exceeds the band limit. The function is specified via the rest of its cumulants of the pseudodistribution. We give an explicit construction using Legendre polynomials in the Fourier space, which leads to an expansion in terms of spherical Bessel functions in the real space. The other expansion coefficients may be chosen to optimize other desirable features, such as the range of super behavior. We provide a prescription to generate bandlimited functions that mimic an arbitrary behavior in a finite interval. As target behaviors, we give examples of a superoscillating function, a supergrowing function, and even a discontinuous step function. We also look at the energy content in a superoscillating/supergrowing region and provide a bound that depends on the minimum value of the logarithmic derivative in that interval. Our work offers a new approach to analyzing superoscillations/supergrowth and is relevant to the optical field spot generation endeavors for far-field superresolution imaging.

Inverse design of sub-diffraction focusing metalens by adjoint-based topology optimization

Breaking the diffraction limit to realize imaging at the nanoscale is challenging in scientific research. Traditional sub-diffraction focusing metalens is obtained by arranging artificially selected unit cells, of which the design process is passive and complex. This paper brings up an inverse design idea of planar sub-diffraction focusing metalens based on super-oscillatory theory to solve these problems, starting from a desired focusing performance. The sub-diffraction focusing metalens is then obtained by iterative topology optimization with different initial structures. We introduce the adjoint-based topology inverse optimization into the structural design of sub-diffraction focusing metalens, which provides another way to design a sub-diffraction metalens for far-field unmarked super-resolution imaging. Based on this idea, we achieve a sub-diffraction focusing characterized by a focal radius of 0.75 times the Rayleigh diffraction limit, optimizing from a diffraction-limited focusing metalens. Moreover, focal radii between 0.63 and 0.73 times the Rayleigh diffraction limit are achieved by optimizing 11 sets of random initial metasurface structures with even no focusing performance. The results indicate that our method is independent of the initial structure distribution, which can be extended to the inverse design of other functional metasurfaces in imaging, lithography, and other fields.

Super-resolution multicolor fluorescence microscopy enabled by an apochromatic super-oscillatory lens with extended depth-of-focus

Planar super-oscillatory lens (SOL), a far-field subwavelength-focusing diffractive device, holds great potential for achieving sub-diffraction-limit imaging at multiple wavelengths. However, conventional SOL devices suffer from a numerical-aperture-related intrinsic tradeoff among the depth of focus (DoF), chromatic dispersion and focusing spot size. Here, we apply a multi-objective genetic algorithm (GA) optimization approach to design an apochromatic binary-phase SOL having a prolonged DoF, customized working distance (WD), minimized main-lobe size, and suppressed side-lobe intensity. Experimental implementation demonstrates simultaneous focusing of blue, green and red light beams into an optical needle of ~0.5λ in diameter and DOF > 10λ at WD = 428 μm. By integrating this SOL device with a commercial fluorescence microscope, we perform, for the first time, three-dimensional super-resolution multicolor fluorescence imaging of the “unseen” fine structures of neurons. The present study provides not only a practical route to far-field multicolor super-resolution imaging but also a viable approach for constructing imaging systems avoiding complex sample positioning and unfavorable photobleaching.

Expanding NIR-II Lanthanide Toolboxes for Improved Biomedical Imaging and Detection

ConspectusOptical imaging with high spatiotemporal resolution has been developed as a vital technology to reveal biological mysteries. Researchers have made countless profound innovations to promote the investigations on cell biology, especially after the implementation of far-field optical super-resolution imaging technology. Intrinsically, in-depth understanding of physiological activities inevitably requires optical imaging in living organisms at tissue, cell, or even single molecule levels. However, such a requirement usually encounters a great bottleneck because traditional optical imaging has little ability to penetrate biological tissues. Recently, the newly emerged optical imaging technology based on the second near-infrared window (NIR-II, 1000∼1700 nm) has changed this predicament. With suppressed light-scattering and diminished tissue autofluorescence, NIR-II optical imaging can easily achieve micrometer scale resolution at sub-centimeter tissue depth. Therefore, researchers regard it as a new avenue for in vivo optical imaging research.The past decade has witnessed increased interest in the field of in vivo optical imaging in the NIR-II window, accompanied by the significant development of NIR-II luminescence probes including small organic molecules, single-walled carbon nanotubes (SWNTs), quantum dots (QDs), and lanthanide nanoprobes. Among these luminescent probes, lanthanide nanoprobes, including lanthanide nanoparticles and lanthanide complexes, have become one of the most promising contrast agents. Owing to their abundant emissions with a narrow-band, lanthanide nanoprobes are usually favored when performing in vivo multiplexing imaging. In addition, their inherently long luminescence lifetimes (>1 μs) offer them great opportunity to implement time-gated imaging that can efficiently avoid autofluorescence interference. Besides, the superior photostability of lanthanides probes satisfies the requirements of long-term in vivo imaging. Based on these advantages, researchers have carried out extensive exploration on optical properties and functionalities of lanthanide nanoprobes.In this Account, we highlight our contributions to developing the NIR-II lanthanide toolboxes for improved biomedical imaging and detection, with a special focus on the future development of lanthanide nanoprobes for biological applications. First, we summarize our efforts on optimizing the excitation and emission wavelengths of the NIR-II lanthanide nanoprobes and analyze how these wavelengths affect the spatial resolution and penetration depth of in vivo optical imaging. Then, we discuss the time domain relevant achievements that are realized by well-designed lanthanide nanoparticles and luminescence lifetime imaging system. After that, we present the attempts to achieve persistent luminescence lanthanide nanoparticles for excitation-free in vivo optical imaging. We also conclude the breakthroughs in the design and synthesis of lanthanide complexes as well as their unique biological imaging results. At the end, we emphasize the future challenges and developments of the NIR-II lanthanide nanoprobes for in vivo optical imaging. By taking our work as a guide, we hope that this Account will provide the rationale behind the development of the NIR-II lanthanide tools and reveal their future opportunities in bioapplications.

Water-immersion supercritical metalens

Far-field super-resolution lenses are attractive due to their great potential in the application of label-free far-field optical super-resolution. In past decades, there has been a fast development in superoscillatory optical lenses, which allows engineering of optical point-spread functions in the far-field beyond the traditional Abbe diffraction limit. With the decrease of focal spot size, the sidebands will inevitably increase, especially when the spot size is close to 0.358λ/NA, where NA is the numerical aperture. However, it is of critical importance to reduce the spot size while maintaining comparatively small sidelobes for many practical applications. Here, we demonstrate a water-immersion supercritical metalens with a high-numerical aperture of 1.15. Experimental results show the generation of a super-resolution hot spot as small as 0.33λ, which is very close to the superoscillation criterion, at a distance of 60λ from the lens. More importantly, there is no significant sideband on the focal plane, and the strength of the sidelobes near the hot spot is lower than 13.3 % of the central lobe intensity. The deep compression of hot spot and the absence of large sidelobes offer a promising way to further improve the resolution of far-field label-free super-resolution microscopy.

Superoscillation focusing with suppressed sidebands by destructive interference.

Optical superoscillation, a phenomenon that the local optical field can oscillate much faster than that allowed by its highest harmonic, can significantly overcome the Abbe diffraction limit. However, as the spot size is compressed below the superoscillation criteria of 0.38λ/NA, huge sidebands will inevitably appear around the central lobe with intensity hundreds of times higher than that of the central lobe. Here, we propose an approach to realize superoscillation by using destructive interference. The central lobe size can be compressed beyond the superoscillation criteria without formation of strong sidebands by destructive interference between focused fields. Such a super-resolution metalens can find its application in label-free far-field super-resolution microscopy.

Superresolution using synthetic shaped acoustic vortex waves in water

Acoustic vortex waves have previously been shown to be able to overcome the diffraction limit, through the use of subdiffraction-limited pressure nulls that propagate well into the farfield. Acoustic vortex waves, therefore, present a novel means to achieve farfield superresolution imaging. However, generating acoustic vortex waves in an experimental setting typically requires a complicated phased array consisting of multiple active elements in a fixed geometrical configuration. In this work, we describe how an acoustic vortex wave can be generated using a synthetized vortex wave array applied during post-processing to in-water 2D plane measurements obtained with a single moving acoustic source and a fixed receiver. The geometric versatility of the synthetic vortex array enables different shaped acoustic vortex patterns to be achieved using the same in-water measurements. Experimental and theoretical results of the magnitude and phase of the nearfield and farfield pressure fields will be presented for a variety of geometric configurations and vortex integer wave modes of the synthesized vortex wave array. The data show excellent agreement with expected results and demonstrates shaped acoustic vortices with superresolved features in both the nearfield and farfield. [Work supported by the Office of Naval Research.]

Subdiffraction Focusing Metalens Based on the Depletion of Bessel Beams

Superresolution microscopy is of great interests in modern optics and photonics applications. Subdiffraction focusing lens, that has superresolution focal spot with local spatial frequency greater than the maximum Fourier component, has attracted intensive attentions in past decade, because of the significant capability of unlabeled far-field superresolution. Here, we propose a dielectric subdiffraction focusing metalens based on the depletion of Bessel beams. Different from traditional binary wavefront modulation elements, this metalens is designed according to the reverse engineering of subdiffraction focal field, which is constructed from the depletion of a fundamental Bessel beam. With the independent amplitude and phase control ability of dielectric metasurface, this metalens enables breaking the diffraction limit and can produce a focal spot with full width at half maximum of 0.3<i>&#x03BB;</i>&#x002F;NA. This work may provide a platform for the design of integrated and compact photonic devices to control the light propagation dynamics and spin angular momentum associated with circular polarization.

Far-field unlabeled super-resolution imaging with superoscillatory illumination

Unlabeled super-resolution is the next grand challenge in imaging. Stimulated emission depletion and single-molecule microscopies have revolutionized the life sciences but are still limited by the need for reporters (labels) embedded within the sample. While the Veselago–Pendry “super-lens,” using a negative-index metamaterial, is a promising idea for imaging beyond the diffraction limit, there are substantial technological challenges to its realization. Another route to far-field subwavelength focusing is using optical superoscillations: engineered interference of multiple coherent waves creating an, in principle, arbitrarily small hotspot. Here, we demonstrate microscopy with superoscillatory illumination of the object and describe its underlying principles. We show that far-field images taken with superoscillatory illumination are themselves superoscillatory and, hence, can reveal fine structural details of the object that are lost in conventional far-field imaging. We show that the resolution of a superoscillatory microscope is determined by the size of the hotspot, rather than the bandwidth of the optical instrument. We demonstrate high-frame-rate polarization-contrast imaging of unmodified living cells with a resolution significantly exceeding that achievable with conventional instruments. This non-algorithmic, low-phototoxicity imaging technology is a powerful tool both for biological research and for super-resolution imaging of samples that do not allow labeling, such as the interior of silicon chips.

High‐Numerical‐Aperture Dielectric Metalens for Super‐Resolution Focusing of Oblique Incident Light

AbstractRecently, developments in superoscillatory optical devices have allowed for label‐free far‐field optical super‐resolution technology by allowing engineering of optical point‐spread functions beyond the traditional Abbe diffraction limit. However, such superoscillatory optical devices are optimized for the normal‐incident operation, which inevitably leads to a comparatively slow imaging acquisition rate in the application of super‐resolution microscopy. Here, a super‐resolution metalens is demonstrated with a high numerical aperture that can focus oblique incident light into a hot spot with a size smaller than the diffraction limit at the visible wavelength. This super‐resolution metalens has a numerical aperture of as large as 0.97, a focal length of 38.0 µm, a radius of 151.9 µm, and a field of view of 4° that enables super‐resolution focusing on the focal plane at a wavelength of λ = 632.8 nm. In a 5.6λ × 5.6λ field of view on the focal plane, the size of the focal spot is smaller than 0.45λ, which is only 0.874 times the corresponding Abbe diffraction limit. The super‐resolution metalens offers a promising way toward fast‐scanning label‐free far‐field super‐resolution microscopy.

Far-field midinfrared superresolution imaging and spectroscopy of single high aspect ratio gold nanowires

Limited approaches exist for imaging and recording spectra of individual nanostructures in the midinfrared region. Here we use infrared photothermal heterodyne imaging (IR-PHI) to interrogate single, high aspect ratio Au nanowires (NWs). Spectra recorded between 2,800 and 4,000 cm-1 for 2.5-3.9-μm-long NWs reveal a series of resonances due to the Fabry-Pérot modes of the NWs. Crucially, IR-PHI images show structure that reflects the spatial distribution of the NW absorption, and allow the resonances to be assigned to the m = 3 and m = 4 Fabry-Pérot modes. This far-field optical measurement has been used to image the mode structure of plasmon resonances in metal nanostructures, and is made possible by the superresolution capabilities of IR-PHI. The linewidths in the NW spectra range from 35 to 75 meV and, in several cases, are significantly below the limiting values predicted by the bulk Au Drude damping parameter. These linewidths imply long dephasing times, and are attributed to reduction in both radiation damping and resistive heating effects in the NWs. Compared to previous imaging studies of NW Fabry-Pérot modes using electron microscopy or near-field optical scanning techniques, IR-PHI experiments are performed under ambient conditions, enabling detailed studies of how the environment affects mid-IR plasmons.

Image scanning difference microscopy.

Here, we propose a novel imaging method, which is called image scanning difference microscopy (ISDM), for superresolution imaging. In ISDM, we implement a detector array composed of 19 avalanche photodiodes (APD) rather than single-point detector in standard confocal microscopy for reconstructing superresolved images with higher signal-to-noise ratio (SNR). Combining with our former proposed fluorescence emission difference (FED) method, we have achieved a lateral resolution of 111 nm (∼λ/6) without the damage of image quality, the highest FED resolution to the best of our knowledge. With its simple setup and remarkable performance, we believe that ISDM can become a versatile observation tool in biology and other fundamental studies. LAY DESCRIPTION: Fluorescence emission difference (FED) microscopy is a really simple and generalisable superresolved fluorescence microscopy method based on PSF engineering and difference algorithm recently. Compared to stimulated-emission-depletion fluorescence microscopy (STED), FED don't need complicated system or precise alignment and polarisation, available for wide variety of dyes and has low photobleaching and phototoxicity for living cells. However, the distortion caused by negative value is one of the biggest obstacles to the further development of FED. In light of this, we propose a novel superresolution imaging method based on the FED method with parallel detection system, which is called image scanning difference microscopy (ISDM). Our method has achieved a significant breakthrough in FED, increasing the resolution further while reducing artefacts generated by negative values, which cannot be accomplished through combining other methods. In addition, ISDM does not require complex setup and optical alignment, long time imaging and imposing no constraint on dyes. Importantly, we realised a transverse resolution of ∼λ/6 (triple diffraction limit) with single wavelength, single incident path and low light intensity, which has never been achieved in any other far-field superresolution microscopy.

Superoscillation: from physics to optical applications

The resolution of conventional optical elements and systems has long been perceived to satisfy the classic Rayleigh criterion. Paramount efforts have been made to develop different types of superresolution techniques to achieve optical resolution down to several nanometres, such as by using evanescent waves, fluorescence labelling, and postprocessing. Superresolution imaging techniques, which are noncontact, far field and label free, are highly desirable but challenging to implement. The concept of superoscillation offers an alternative route to optical superresolution and enables the engineering of focal spots and point-spread functions of arbitrarily small size without theoretical limitations. This paper reviews recent developments in optical superoscillation technologies, design approaches, methods of characterizing superoscillatory optical fields, and applications in noncontact, far-field and label-free superresolution microscopy. This work may promote the wider adoption and application of optical superresolution across different wave types and application domains.

Saturated Stimulated-Raman-Scattering Microscopy for Far-Field Superresolution Vibrational Imaging

Superresolution microscopy based on stimulated Raman scattering (SRS) is appealing for label-free quantitative imaging of biological and biomedical systems with higher resolution than conventional SRS microscopy, which is diffraction-limited. This study experimentally demonstrates a saturated SRS (SSRS) technique to break the diffraction limit for vibrational imaging of biological samples, such as chloroplasts and Hela cells. The technique holds great promise for studying living cells.