Far-field Super-resolution Research Articles

Far-field super-resolution scanning imaging based on fractal resonator

The resolution of traditional far-field imaging system is generally restricted by half of the wavelength of incident light due to the diffraction limit. The more specific reason is that evanescent waves carrying sub-wavelength information cannot propagate in the far field and make no contribution to the imaging. However, higher imaging resolution is required in practical applications. To realize the far-field super-resolution imaging, the imaging system should be able to collect both propagating waves and evanescent waves. Many designs have been proposed to solve this issue. In 2007, a far-field superlens was proposed by Liu et al. (Liu Z W, Durant S, Lee H, Pikus Y, Fang N, Xiong Y, Sun C, Zhang X 2007 <i>Nano Lett</i>. <b>7</b> 403) to realize far-field super-resolution in optical range, which consisted of a silver film and a nanoscale grating coupler. The silver film was used to amplify the evanescent waves, which were then converted into propagating waves by the sub-wavelength gratings. However, the special material properties limit the freedom of design. In microwave band, the incident components can be converted into Bloch modes by the resonant metalens, which consists of subwavelength resonators, and then be radiated to far field. Nevertheless, Green function between antenna and target is necessary, which is difficult to obtain due to the complex and even time-dependent imaging environment in practical applications, especially for super-resolution imaging system. It has been demonstrated in recent research that frequency information can be associated with spatial information of imaging target by localization resonant modes. Therefore, super-resolution imaging can be realized based on frequency information, without using Green function. Thus, a novel microstructure array is proposed to realize the far-field super-resolution scanning imaging based on a fractal resonator. The fractal resonator can work at several frequencies because of the self-similarity, which provides higher selectivity according to practical conditions. Several working statuses can be obtained for the resonator by adding photoconductive semiconductor switches, which are controlled by laser. On account of localization mode resonance, the array can realize the conversion between evanescent waves and propagating waves. Then with the help of antennas in the far-field to receive the frequency information, the location of imaging source can be confirmed according to the spectrum. Then by using the magnitude of resonant peak, sub-wavelength image can be reconstructed without using Green function. To verify the super-resolution scanning imaging characteristics of the array, an imaging simulation of “laugh face”-shaped target is performed. The image is reconstructed very well and the resolution determined by the period of the array is 20 mm, corresponding to <i>λ</i>/10. In view of the particularity of proposed fractal resonator, a novel scanning method is proposed. By combining the first and the third resonance, the imaging efficiency can be well improved compared with by the traditional point-by-point scanning method.

Far-field superresolution imaging using shaped acoustic vortices

The ability to overcome the limitations on resolution due to the effects of diffraction has attracted significant attention in recent years. Previously proposed methods to overcome this limit, and therefore achieve superresolution, have largely been restricted to operating within the near-field region of the aperture. In this work, we will describe how acoustic helicoidal waves can create acoustic vortices that are well below the resolution limit, and how this can enable far-field superresolution acoustic imaging. The acoustic vortices generated in this manner propagate from the near-field into the far-field through an arrangement of stable integer mode vortices, thereby enabling the generation of far-field superresolved features in the acoustic pressure field. In this paper, theoretical and numerical results will be presented for an acoustic aperture which is capable of generating superresolved far-field features in the radiated acoustic pressure, and results will be shown illustrating the superresolution capability of this novel technique. [Work supported by the Office of Naval Research.]

Retrieving the Size of Deep-Subwavelength Objects via Tunable Optical Spin-Orbit Coupling

We propose a scheme to retrieve the size parameters of a nanoparticle on a glass substrate at a scale much smaller than the wavelength. This is achieved by illuminating the particle using two plane waves to create rich and nontrivial local polarization distributions, and observing the far-field scattering pattern into the substrate. By using this illumination to control the induced complex dipole moment, the exponential decay of power radiated into the supercritical region, as well as directional scattering due to spin-orbit coupling can be exploited to retrieve the particle's shape, size, and position directly from the far-field scattering with high sensitivity and without the need for a complicated and time-consuming optimization algorithm. Our method brings about a far-field superresolution nanometrology scheme based on the interaction of vectorial light with nanoparticles.

Optimising superoscillatory spots for far-field super-resolution imaging.

Optical superoscillatory imaging, allowing unlabelled far-field super-resolution, has in recent years become reality. Instruments have been built and their super-resolution imaging capabilities demonstrated. The question is no longer whether this can be done, but how well: what resolution is practically achievable? Numerous works have optimised various particular features of superoscillatory spots, but in order to probe the limits of superoscillatory imaging we need to simultaneously optimise all the important spot features: those that define the resolution of the system. We simultaneously optimise spot size and its intensity relative to the sidebands for various fields of view, giving a set of best compromises for use in different imaging scenarios. Our technique uses the circular prolate spheroidal wave functions as a basis set on the field of view, and the optimal combination of these, representing the optimal spot, is found using a multi-objective genetic algorithm. We then introduce a less computationally demanding approach suitable for real-time use in the laboratory which, crucially, allows independent control of spot size and field of view. Imaging simulations demonstrate the resolution achievable with these spots. We show a three-order-of-magnitude improvement in the efficiency of focusing to achieve the same resolution as previously reported results, or a 26 % increase in resolution for the same efficiency of focusing.

Far-field super-resolution imaging based on wideband stereo-metalens

The resolution of traditional far-field imaging system is generally restricted by half of wavelength of incident light due to the diffraction limit. The reason is that evanescent waves carrying subwavelength information cannot propagate in the far-field and make no contribution to the imaging. To realize the far-field super-resolution imaging, the imaging system should be able to collect both propagation and evanescent waves. Many ideas were presented to provide feasible alternatives but with narrow frequency band. In this paper, a wideband metalens is proposed to realize far-field super-resolution based on stereometamaterials. A typical model of stereometamaterials is studied, which consist of a stack of two identical spiral resonators in each cell, with various twist angles. For each case, there are two observable resonances (-and +), obviously. The phenomenon can be explained as the plasmon hybridization between the two resonators due to their close proximity. The case with a twist angle of 90 is chosen as the basic cell to constitute the stereo-metalens (S-ML). The last S-ML can work in a frequency range from 1.06 to 1.53 GHz, which is much wider than the planar-metalens. Simulations of near-and far-field spectra are conducted to validate the conversion between evanescent waves and propagation waves. Then with the help of antennas in the far-field to receive the information, sub-wavelength image can be reconstructed. The simulations in frequency-and time-domain are performed to verify the super-resolution characteristics of the S-ML. In frequency-domain, an imaging simulation of L-shaped extended target is combined with multiple signal classification imaging method. The resolution defined by full width at half maximum is 19 mm, corresponding to /12. For comparison, a similar simulation without the S-ML is performed, indicating a resolution of 1.5. It shows the ability of the S-ML to enhance the imaging resolution. In time-domain, by using time reversal technique, the spatial super-resolution characteristic of the S-ML is validated. Compared with the planar-metalens, the S-ML has good spatial super-resolution characteristic. All results show that the S-ML has a good potential application in imaging.

Superresolution far-field imaging of complex objects using reduced superoscillating ripples

Superoscillation is a phenomenon where a wave oscillates locally faster than its highest Fourier component. While previous reports have shown attractive possibilities for a superoscillation-based far-field superresolution imaging device, it has also been recognized that a high-energy “sideband” region coexists with the superresolution features. This sideband causes strong restrictions and necessitates trade-offs in achievable resolution, viewing area, and sensitivity of the imaging device. In this work, we introduce a new class of superoscillation waveform—which consists of a diffraction-limited hotspot surrounded by low-energy superoscillating sidelobe ripples. This waveform alleviates the aforementioned trade-off and enables superresolution imaging for complex objects over a larger viewing area while maintaining a practical level of sensitivity. Using this waveform as the point spread function of an imaging system, we demonstrate the successful superresolution of Latin letters without performing scanning and/or post-processing operations.

Far-Field Super-Resolution Imaging of Scatterers With a Time-Reversal System Aided by a Grating Plate

A time-reversal imaging system for far-field super-resolution imaging of scatterers is proposed. First, a grating plate, similar to the optical far-field super lens, is designed to convert near-field evanescent waves to propagative waves with wavevector modulation. Then, the scattered signals are processed by a time-reversal (TR) technique and retransmitted back. Finally, the images of the targets are reconstructed by doing demodulation operations in the k -domain to the TR field at the refocusing time. Imaging results show that two scatterers separated by 0.1 λ can be distinguished clearly.

Near-Field Periodic Subwavelength Holey Metallic Plate for Far-Field Superresolution Focusing

Far-field focusing with superresolution aided by a proposed metallic plate is investigated in this paper. The proposed near-field structure can be employed to distinguish point-like sources with subwavelength distance and enhance the field strength based on the spoof surface plasmon polaritons theory, and by the simulations of near- and far-field spectrums, it can be found that some near-field evanescent waves carried the superresolution information can be converted to propagation waves and propagate to the far-field. Therefore, with the help of time reversal imaging process, the far-field superresolution focusing is validated by the experiments in laboratory environment. A resolution of $\sim$ $\lambda$ /13 is achieved from the last results, which is well beyond the diffraction limit.

Superresolution using an acoustic vortex wave antenna

Acoustic waves, like their electromagnetic counterpart, exhibit a diffraction limit, which restricts the ability to resolve or generate fine structures in the pressure field. Previously proposed methods to overcome this limit, and therefore achieve superresolution, have mainly been limited to operating within the near-field region of the aperture. In this work, we describe how far-field superresolution can be achieved using shaped acoustic vortices, created by utilizing the topological diversity of an acoustic vortex wave antenna. Theoretical and numerical results will be presented for the design of an acoustic vortex wave antenna with arbitrary planar arrangement which is capable of generating superresolved far-field features in the radiated acoustic pressure. Variation of the antenna geometry enables different shaped acoustic vortex patterns to be achieved, which propagate from the near-field into the far-field through an arrangement of stable integer mode vortices. Despite the total aperture size being less than a wavelength in diameter, the proposed acoustic vortex wave antenna is shown to achieve far-field superresolution with feature sizes 4-9 times smaller than the resolution limit. Several examples will be presented and discussed in detail. [Work supported by the Office of Naval Research.]

Optical far-field super-resolution microscopy using nitrogen vacancy center ensemble in bulk diamond

We demonstrate optical far-field super-resolution microscopy using an array of nitrogen vacancy centers in bulk diamond as near-field optical probes. The local optical field, which transmits through the nanostructures on the diamond surface, is measured by detecting the charge state conversion of the nitrogen vacancy center. Locating the nitrogen vacancy center with a spatial resolution of 6.1 nm is realized with charge state depletion nanoscopy. The nanostructures on the surface of a diamond are then imaged with a resolution below the optical diffraction limit. The results offer an approach to build a general-purpose optical super-resolution microscopy technique and a convenient platform for high spatial resolution quantum sensing with nitrogen vacancy centers.

Quantum Theory of Superresolution for Two Incoherent Optical Point Sources

Rayleigh's criterion for resolving two incoherent point sources has been the most influential measure of optical imaging resolution for over a century. In the context of statistical image processing, violation of the criterion is especially detrimental to the estimation of the separation between the sources, and modern farfield superresolution techniques rely on suppressing the emission of close sources to enhance the localization precision. Using quantum optics, quantum metrology, and statistical analysis, here we show that, even if two close incoherent sources emit simultaneously, measurements with linear optics and photon counting can estimate their separation from the far field almost as precisely as conventional methods do for isolated sources, rendering Rayleigh's criterion irrelevant to the problem. Our results demonstrate that superresolution can be achieved not only for fluorophores but also for stars.

Far-Field Super-resolution Detection of Plasmonic Near-Fields.

We demonstrate a far-field single molecule super-resolution method that maps plasmonic near-fields. The method is largely invariant to fluorescence quenching (arising from probe proximity to a metal), has reduced point-spread-function distortion compared to fluorescent dyes (arising from strong coupling to nanoscopic metallic features), and has a large dynamic range (of 2 orders of magnitude) allowing mapping of plasmonic field-enhancements regions. The method takes advantage of the sensitivity of quantum dot (QD) stochastic blinking to plasmonic near-fields. The modulation of the blinking characteristics thus provides an indirect measure of the local field strength. Since QD blinking can be monitored in the far-field, the method can measure localized plasmonic near-fields at high throughput using a simple far-field optical setup. Using this method, propagation lengths and penetration depths were mapped-out for silver nanowires of different diameters and for different dielectric environments, with a spatial accuracy of ∼15 nm. We initially use sparse sampling to ensure single molecule localization for accurate characterization of the plasmonic near-field with plans to increase density of emitters in further studies. The measured propagation lengths and penetration depths values agree well with Maxwell finite-difference time-domain calculations and with published literature values. This method offers advantages such as low cost, high throughput, and superresolved mapping of localized plasmonic fields at high sensitivity and fidelity.

Electromagnetic exploration of far-field super-focusing nanostructured metasurfaces

Planar multi-annular nanostructured metasurfaces have provided a new way to realize far-field optical super-resolution focusing and nanoscopic imaging, due to the delicate interference of propagating waves diffracted from the metasurface mask. However, so far there are no proper methods that can be used to essentially interpret the super-focusing and nano-imaging mechanisms. This research proposes an electromagnetic methodology for the super-resolution investigation of nanostructured metasurfaces. We have physically modeled the polarization-dependent transmission effect of the subwavelength nanostructure and the vectorial imaging process of a high-numerical-aperture microscopic system. We have found theoretically and experimentally that the current design theories may produce imprecise results; the microscopic imaging experimental method can only detect transversely polarized electric field component and cannot map out three-dimensional total electric energy density distribution behind metasurfaces. This method will potentially be used in far-field nanoscopy, nanolithography, high-density optical storage, etc.

Terahertz far-field superresolution imaging through spoof surface plasmons illumination.

The applications of terahertz (THz) imaging are always restricted by the low resolution. We introduce here a new way to realize far-field superresolution imaging at THz frequency. Assisted by a new spoof surface plasmons (SSP) probe illumination, the images of two subwavelength separated slits can be reconstructed by a single shot. Although only 0.06λ resolution is numerically demonstrated at 0.3THz, the resolution potentially can be further enhanced by scaling down the size of the SSP probe. Deep subwavelength focusing is also achieved by the SSP probe. Our work may open a new avenue for SSP-based superresolution at longer wavelengths.

Far-Field Super-Resolution Imaging With Compact and Multifrequency Planar Resonant Lens Based on Time Reversal

Planar resonant lenses (PRLs) consisting of periodically distributed strip resonators with different lengths are proposed for far-field super-resolution electromagnetic imaging. With the help of PRLs, evanescent waves can be converted into propagation waves. So, based on time-reversal (TR) technique, time-reversal mirror in the far-field can receive the electromagnetic signals carrying the subwavelength information. Experiments are performed to validate the proposed method in laboratory environment at microwave band. Far-field super-resolution imaging results with resolutions of ${\lambda }/{20}$ and ${\lambda }/{10}$ , respectively, are obtained, well beyond the diffraction limit. The compact size and multifrequency property of the proposed PRLs will make them easily be integrated to new super-resolution imaging systems.

Far-Field Super-Resolution Imaging of Nano-transparent Objects by Hyperlens with Plasmonic Resonant Cavity

A plasmonic resonant cavity-based hyperlens is theoretically proposed and demonstrated to achieve far-field phase contrast images of nano-transparent objects. The phase contrast super-resolution imaging is mainly contributed to the excited surface plasmons inside hyperlens and cavity structure surrounding nano-objects, which help to greatly enhance evanescent waves generated by nano-transparent objects and convert weak phase information to light intensity distribution with high contrast at the zoomed imaging plane of hyperlens. As examples, nano-dielectric object imaging is numerically demonstrated with half-pitch resolution about lambda/10 and a minimum distinguishable refractive index difference down to 0.15.

New modes in label-free super resolution based on photo-modulated reflectivity.

The recent advances in far-field super-resolution (SR) microscopy rely on, and therefore are limited by the ability to control the fluorescence of label molecules. We demonstrated a far field label-free SR methodology that relies on the nonlinear response of the reflectance to photo-modulation by a pump laser. Here we extend our approach in two directions. We show that the method can be further simplified and improved by using a single beam rather than a pump and probe or by adding spatial probe modulation to improve resolution. Additionally, we demonstrate SR in sectioning and further investigate the dynamics of non-linearity in photo-modulated reflectance. These new modalities of nonlinear photo-modulated reflectivity (NPMR) enhance its applicability using lower orders of nonlinear response.

Super resolution methodology based on temperature dependent Raman scattering.

The recent advances in far-field super-resolution (SR) microscopy rely on, and therefore are limited by the ability to control the fluorescence of label molecules. We suggest a new, label-free, far-field SR microscopy based on temperature dependence of Raman scattering. Here, we present simulation and experimental characterization of the method. In an ultrafast pump-probe scheme, a spatial temperature profile is optically excited throughout the diffraction-limited spot; the Raman spectrum is probed with an overlapping laser. Thermally induced shifts, recorded in a specific spectral region of interest (ROI), enable spatial discrimination between areas of different temperature. Our simulations show spatial resolution that surpasses the diffraction limit by more than a factor of 2. Our method is compatible with material characterization in ambient, vacuum and liquid, thin and thick samples alike.

Far-field superresolution by imaging of resonant multiples

We show that superresolution imaging in the far-field region of the sources and receivers is theoretically possible if migration of resonant multiples is employed. A resonant multiple is one that bounces back and forth between two scattering points or two neighboring reflectors. For a source with frequency f , N roundtrips in propagating between two scatterers increases the effective frequency to 2N × f and decreases the effective wavelength λ to λ/2N. Thus, interbed multiples can, in principle, be used as high-frequency probes to estimate detailed properties of layers. This is not only applicable to crustal reflections, but also to mantle and core reverberations of interest to earthquake seismologists.

Stimulated Emission Depletion Microscopy Resolves Individual Nitrogen Vacancy Centers in Diamond Nanocrystals

Nitrogen-vacancy (NV) color centers in nanodiamonds are highly promising for bioimaging and sensing. However, resolving individual NV centers within nanodiamond particles and the controlled addressing and readout of their spin state has remained a major challenge. Spatially stochastic super-resolution techniques cannot provide this capability in principle, whereas coordinate-controlled super-resolution imaging methods, like stimulated emission depletion (STED) microscopy, have been predicted to fail in nanodiamonds. Here we show that, contrary to these predictions, STED can resolve single NV centers in 40-250 nm sized nanodiamonds with a resolution of ≈10 nm. Even multiple adjacent NVs located in single nanodiamonds can be imaged individually down to relative distances of ≈15 nm. Far-field optical super-resolution of NVs inside nanodiamonds is highly relevant for bioimaging applications of these fluorescent nanolabels. The targeted addressing and readout of individual NV(-) spins inside nanodiamonds by STED should also be of high significance for quantum sensing and information applications.