Sub-diffraction Limit Focusing Research Articles

Designer graphene oxide ultrathin flat lens with versatile focusing property.

Graphene oxide (GO) flat lens has a thickness in nanoscale. They modulates the light field via both phase and amplitude modulation and hence possess excellent focusing property. In this paper, we develop a systematic design method to realize the ultrathin GO flat lens with various focusing properties. By using the Rayleigh-Sommerfield theory, the focusing property of ultrathin GO lenses is accurately calculated, then the genetic algorithm (GA) is employed to design the GO lenses. The lens works at visible frequency can have a large radius and long working distance. By setting different optimization objectives, extraordinary focusing property including sub-diffraction limit focusing with FWHM (∼1.96λ) and achromatic focusing with the wavelengths (450 nm, 550 nm, 650 nm) can be achieved. These innovative designs are fabricated and tested.

An entropy-controlled objective chip for reflective confocal microscopy with subdiffraction-limit resolution

Planar diffractive lenses (PDLs) with optimized but disordered structures can focus light beyond the diffraction limit. However, these disordered structures have inevitably destroyed wide-field imaging capability, limiting their applications in microscopy. Here, we introduce information entropy S to evaluate the disorder of an objective chip by using the probability of its structural deviation from standard Fresnel zone plates. Inspired by the theory of entropy change, we predict an equilibrium point {S}_{0}=0.5 to balance wide-field imaging (theoretically evaluated by the Strehl ratio) and subdiffraction-limit focusing. To verify this, a {NA}=0.9 objective chip with a record-long focal length of 1 mm is designed with S=0.535, which is the nearest to the equilibrium point among all reported PDLs. Consequently, our fabricated chip can focus light with subdiffraction-limit size of 0.44 λ and image fine details with spatial frequencies up to 4000 lp/mm experimentally. These unprecedented performances enable ultracompact reflective confocal microscopy for superresolution imaging.

Aberration-compensated supercritical lens for sub-diffractive focusing within 20° field of view.

The supercritical lens has shown a remarkable capability of achieving far-field sub-diffraction limited focusing through elaborating a modulated interference effect. Benefiting from the relative high energy utilization efficiency and weak sidelobe properties, the supercritical lens holds significant advantage in a series of application scenarios. However, all of the demonstrated supercritical lenses mainly work in the on-axis illumination condition, so the off-axis aberration effect will severely deteriorate its sub-diffraction limit focusing capability for the illuminating beam with an oblique angle. In this work, an aberration-compensated supercritical lens with single-layer configuration is proposed and experimentally demonstrated. Such a single-layer supercritical lens consists of multilevel phase configurations patterned with the two-photon polymerization lithography technique. The simulation and experimental recorded results show that the aberration-compensated supercritical lens with a numerical aperture value of 0.63 could achieve a far-field sub-diffraction limited focusing property within 20° field of view at a wavelength of λ = 633 nm. This monochromatic aberration-compensated supercritical lens with single-layer configuration indicates excellent potential in the development of laser scanning ultrahigh optical storage and label free super-resolution imaging.

Super-resolution imaging with large field of view for distant object through scattering media

Challenge of imaging through scattering media has been gradually solved by various approaches. The scattering media can be utilized as “scattering lens” and applied to sub-diffraction limit focusing and super-resolution imaging. But pioneering works of super-resolution imaging are mainly applied in the field of microscopy. In this work, we propose an effective method of super-resolution macroscopic imaging for distant objects based on the Fourier-domain shower-curtain effect. The scattering media are used as screens with larger aperture to record more spatial frequency information and super-resolution image reconstruction is achieved with phase retrieval algorithm. Experimental result shows a 9.93-fold resolution improvement by sacrificing the field of view (FOV). Ptychography is further introduced to increase the space-bandwidth product as well as the compressed FOV. The proposed method is promising in super-resolution imaging through scattering media for macroscopy.

A slanted-nanoaperture metal lens: subdiffraction-limited focusing of light in the intermediate field region

Diffraction of light limits the resolution of beam focusing with conventional lenses, as dictated by the Abbe limit, that is, approximately half the wavelength. Numerous techniques have been explored to overcome this limit. One of the most intensively explored approaches is to design a lens that operates in the near-field region, that is, with a focal length on the order of 10 nm, where evanescent fields can carry and project large in-plane wave-vectors (greater than free-space wave-vectors) to a focal plane. From a practical perspective, however, the requirement of such an ultra-short focal length puts too much constraint, since much longer focal length is commonly desired for intermediate or far-field operation. Here we report a method to beat the Abbe limit while operating with focal length greater than wavelength λ. Our approach is to tailor the radiation patterns of nanoaperture transmission by tilting aperture axes away from the surface of a metal film such that each slanted aperture transmits a highly directed, tilt-oriented beam onto a common focal point carrying maximal in-plane wave-vector components. The proposed nanoaperture array lens was fabricated by forming tilted nanoslits in a Ag, Al, or Cr film. We demonstrate minimal spot size of λ/3 (210-nm or 110-nm full-width half-maximum at λ = 633 nm or 325 nm, respectively) with 1–4λ focal length in air, beating the Abbe limit.

Planar Diffractive Lenses: Fundamentals, Functionalities, and Applications.

Traditional objective lenses in modern microscopy, based on the refraction of light, are restricted by the Rayleigh diffraction limit. The existing methods to overcome this limit can be categorized into near-field (e.g., scanning near-field optical microscopy, superlens, microsphere lens) and far-field (e.g., stimulated emission depletion microscopy, photoactivated localization microscopy, stochastic optical reconstruction microscopy) approaches. However, they either operate in the challenging near-field mode or there is the need to label samples in biology. Recently, through manipulation of the diffraction of light with binary masks or gradient metasurfaces, some miniaturized and planar lenses have been reported with intriguing functionalities such as ultrahigh numerical aperture, large depth of focus, and subdiffraction-limit focusing in far-field, which provides a viable solution for the label-free superresolution imaging. Here, the recent advances in planar diffractive lenses (PDLs) are reviewed from a united theoretical account on diffraction-based focusing optics, and the underlying physics of nanofocusing via constructive or destructive interference is revealed. Various approaches of realizing PDLs are introduced in terms of their unique performances and interpreted by using optical aberration theory. Furthermore, a detailed tutorial about applying these planar lenses in nanoimaging is provided, followed by an outlook regarding future development toward practical applications.

Advances in the far-field sub-diffraction limit focusing and super-resolution imaging by planar metalenses

Due to the fundamental laws of wave optics, the spatial resolution of traditional optical microscopy is limited by the Rayleigh criterion. Enormous efforts have been made in the past decades to break through the diffraction limit barrier and in depth understand the dynamic processes and static properties. A growing array of super-resolution techniques by distinct approaches have been invented, which can be assigned to two categories: near-field and far-field super-resolution techniques. The near-field techniques, including near-field scanning optical microscopy, superlens, hyperlens, etc., could break through the diffraction limit and realize super-resolution imaging by collecting and modulating the evanescent wave. However, near-field technique suffers a limitation of very short working distances because of the confined propagation distance of evanescent wave, and certainly produces a mechanical damage to the specimen. The super-resolution fluorescence microscopy methods, such as STED, STORM, PALM, etc., could successfully surpass the diffractive limit in far field by selectively activating or deactivating fluorophores rooted in the nonlinear response to excitation light. But those techniques heavily rely on the properties of the fluorophores, and the labelling process makes them only suitable for narrow class samples. Developing a novel approach which could break through the diffraction limit in far field without any near-field operation or labelling processes is of significance for not only scientific research but also industrial production. Recently, the planar metalenses emerge as a promising approach, owing to the theoretical innovation, flexible design, and merits of high efficiency, integratable and so forth. In this review, the most recent progress of planar metalenses is briefly summarized in the aspects of sub-diffractive limit focusing and super-resolution imaging. In addition, the challenge to transforming this academic concept into practical applications, and the future development in the field of planar metalenses are also discussed briefly.

A polychromatic approach to far-field superlensing at visible wavelengths

Breaking the diffraction barrier in the visible part of the electromagnetic spectrum is of fundamental importance. Far-field subwavelength focusing of light could, for instance, drastically broaden the possibilities available in nanolithography, light-matter interactions and sensing at the nanoscale. Similarly, imaging with a nanometric resolution could result in incredible breakthroughs in soft matter and biology. There have been numerous proposals in this regard based on metamaterials, structured illumination methods or diffractive optical components. The common denominator of all these approaches resides in their monochromatic nature. Here we show that using polychromatic light in dispersive metamaterials allows us to circumvent many limitations associated with previous monochromatic approaches. We design a plasmonic metalens based on metallic nanorods that, when used with broadband light fields, can beat the diffraction limit for imaging and focusing from the far field.

Fractal plasmonics: subdiffraction focusing and broadband spectral response by a Sierpinski nanocarpet

Plasmonic nanostructures offer a great potential to enhance light-matter interaction at the nanometer scale. The response upon illumination at a given wavelength and polarization is governed by the characteristic lengths associated to the shape and size of the nanostructure. Here, we propose the use of engineered fractal plasmonic structures to extend the degrees of freedom and the parameters available for their design. In particular, we focus on a paradigmatic fractal geometry, namely the Sierpinski carpet. We explore the possibility of using it to achieve a controlled broadband spectral response by controlling the degree of its fractal complexity. Furthermore, we investigate some other arising properties, such as subdiffraction limited focusing and its potential use for optical trapping of nano-objects. An attractive advantage of the focusing over more standard geometries, such as gap antennas, is that it occurs away from the metal surface (≈ 80 nm) at the center of the nanostructure, leaving an open space accessible to objects for enhanced light-matter interaction.

Experimental Investigation of Focusing of Gold Planar Plasmonic Lenses

To experimentally demonstrate the subwavelength focusing of depth-tuned or non-depth-tuned plasmonic lenses, we first designed this type of lens using diffraction-coupling-angle based method, then fabricated the structure in gold thin film with focused ion beam, and finally characterized its focusing behavior using near-field scanning optical microscope. It is found that this type of lens has a resolution limit on the focal plane due to the field represented by angular spectrum having a cut-off frequency, while at the near field the lens has sub-diffraction limit focusing capability due to the existence of high-angular-frequency components in the field.

Properties of sub-diffraction limited focusing by optical phase conjugation

Recent work has demonstrated sub-diffraction limited focusing using time-reversal mirrors and sources in scattering media at microwave frequencies. We numerically investigate the possibility of observing analogous effects in the optical domain using small cylindrical scatterers of realistic dielectric materials combined with an enclosing optical phase conjugate mirror in two-dimensional geometries. Such focusing is possible but appears not to significantly exceed the focusing available from an equivalent homogenized material, and is highly sensitive to precise scatterer configuration.

Proposal for Superfocusing at Visible Wavelengths Using Radiationless Interference of a Plasmonic Array

Radiationless electromagnetic interference has been proposed to focus light below the diffraction limit. This has been demonstrated for microwave wavelengths; however, those approaches cannot be extended directly to visible wavelengths due to the material response and limitations from nanofabrication. In this Letter, a different approach is proposed to enable subdiffraction-limit focusing in the visible. The approach is based on the highest-order mode of a metal-dielectric waveguide array. Comprehensive simulations of focusing are provided for two different extreme examples: a structure that gives a 0.21 wavelength focus spot at a relatively large distance of 0.5 wavelength, and a sharp 0.057 wavelength focus spot at distance of 0.1 wavelength. The proposed nanostructures are comparable to those found in recent experiments.

Nanoscale optical field localization by resonantly focused plasmons

We experimentally demonstrate use of plasmonic resonant phenomena combined with strong field localization to enhance efficiency of confining optical fields in a Si waveguide. Our approach utilizes a plasmonic resonant nano-focusing-antenna (RNFA), that simultaneously supports several focusing mechanisms in a single nanostructure, integrated with a lossless Si waveguide utilized with silicon-on-insulator (SOI) technology, to achieve a sub-diffraction limited focusing with a nanoscale (deeply subwavelength) spot size. The metallic RNFA effectively converts an incoming propagating waveguide mode to a localized resonant plasmon mode in an ultrasmall volume in all 3 dimensions. The near-field optical measurements of the fabricated RNFA using heterodyne near-field scanning optical microscope (H-NSOM) validate the theoretical predictions showing strong optical field localization.

Subdiffraction-limited focusing lens

We describe techniques for making a diffractive optical element that produces a subdiffraction-limited spot size. We provide experimental verification, using a diffraction optical element that is constructed on a magneto-optic spatial light modulator.