Aaron Brettin

Fundamental limits of super-resolution microscopy by dielectric microspheres and microfibers

In recent years, optical super-resolution by microspheres and microfibers emerged as a new paradigm in nanoscale label-free and fluorescence imaging. However, the mechanisms of such imaging are still not completely understood and the resolution values are debated. In this work, the fundamental limits of super-resolution imaging by high-index barium-titanate microspheres and silica microfibers are studied using nanoplasmonic arrays made from Au and Al. A rigorous resolution analysis is developed based on the object’s convolution with the point-spread function that has width well below the conventional (~λ/2) diffraction limit, where λ is the illumination wavelength. A resolution of ~λ/6-λ/7 is demonstrated for imaging nanoplasmonic arrays by microspheres. Similar resolution was demonstrated for microfibers in the direction perpendicular to the fiber axis with hundreds of times larger field-of-view in comparison to microspheres. Using numerical solution of Maxwell’s equations, it is shown that extraordinary close point objects can be resolved in the far field, if they oscillate out of phase. Possible super-resolution using resonant excitation of whispering gallery modes is also studied.

Contact microspherical nanoscopy: from fundamentals to biomedical applications

The mechanisms of super-resolution imaging by contact microspherical or microcylindrical nanoscopy remain an enigmatic question since these lenses neither have an ability to amplify the near-fields like in the case of far-field superlens, nor they have a hyperbolic dispersion similar to hyperlenses. In this work, we present results along two lines. First, we performed numerical modeling of super-resolution properties of two-dimensional (2-D) circular lens in the limit of wavelength-scale diameters, λ ≤ D ≤ 2λ, and relatively high indices of refraction, n=2. Our preliminary results on imaging point dipoles indicate that the resolution is generally close to λ/4; however on resonance with whispering gallery modes it may be slightly higher. Second, experimentally, we used actin protein filaments for the resolution quantification in microspherical nanoscopy. The critical feature of our approach is based on using arrayed cladding layer with strong localized surface plasmon resonances. This layer is used for enhancing plasmonic near-field illumination of our objects. In combination with the magnification of virtual image, this technique resulted in the lateral resolution of actin protein filaments on the order of λ/7.

Label-free nanoscopy with contact microlenses: Super-resolution mechanisms and limitations

Despite all the success with developing super-resolution imaging techniques, the Abbe limit poses a severe fundamental restriction on the resolution of far-field imaging systems based on diffraction of light. Imaging with contact microlenses, such as microspheres or microfibers, can increase the resolution by a factor of two beyond the Abbe limit. The theoretical mechanisms of these methods are debated in the literature. In this work, we focus on the recently expressed idea that optical coupling between closely spaced nanoscale objects can lead to the formation of the modes that drastically impact the imaging properties. These coupling effects emerge in nanoplasmonic or nanocavity clusters, photonic molecules, or various arrays under resonant excitation conditions. The coherent nature of imaging processes is key to understanding their physical mechanisms. We used a cluster of point dipoles, as a simple model system, to study and compare the consequences of coherent and incoherent imaging. Using finite difference time domain modeling, we show that the coherent images are full of artefacts. The out-of-phase oscillations produce zero-intensity points that can be observed with practically unlimited resolution (determined by the noise). We showed that depending on the phase distribution, the nanoplasmonic cluster can appear with the arbitrary shape, and such images were obtained experimentally.

Photonic jets for highly efficient mid-IR focal plane arrays with large angle‐of‐view

One of the trends in design of mid-wave infrared (MWIR) focal plane arrays (FPAs) consists in reduction of the pixel sizes which allows increasing the resolution and decreasing the dark currents of FPAs. To keep high light collection efficiency and to combine it with large angle-of-view (AOV) of FPAs, in this work we propose to use photonic jets produced by the dielectric microspheres for focusing and highly efficient coupling light into individual photodetector mesas. In this approach, each pixel of FPA is integrated with the appropriately designed, fixed and properly aligned microsphere. The tasks consist in developing technology of integration of microspheres with pixels on a massive scale and in developing designs of corresponding structures. We propose to use air suction through a microhole array for assembling ordered arrays of microspheres. We demonstrate that this technology allows obtaining large-scale arrays containing thousands of microspheres with ~1% defect rate which represents a clear advantage over the best results obtained by the techniques of directed self-assembly. We optimized the designs of such FPAs integrated with microspheres for achieving maximal angle of view (AOV) as a function of the index of refraction and diameter of the microspheres. Using simplified two-dimensional finite difference time domain (FDTD) modeling we designed structures where the microspheres are partly-immersed in a layer of photoresist or slightly truncated by using controllable temperature melting effects. Compared to the standard microlens arrays, our designs provide up to an order of magnitude higher AOVs reaching ~8° for back-illuminated and ~20° for front-illuminated structures.

Spotlight on microspherical nanoscopy: Experimental quantification of super-resolution

A classification of label-free super-resolution imaging mechanisms is given based on the nonlinear reduction of the point-spread function (PSF), near-field scanning, image magnification and gain, structured and sparse illumination, and information approaches. We argue that the super-resolution capability of contact microspheres stems from an image magnification effect taking place in close proximity to the object with contributions of its optical near-fields. We discuss several conditions for quantifying the super-resolution in a label-free microscopy: i) use of standalone objects or long-period arrays as opposed to subwavelength periodic structures, ii) use a convolution with two-dimensional PSF for calculating images, and iii) avoidance of coherent imaging which can lead to dramatic artifacts. We demonstrate a resolution of ∼λ/7 for imaging nanoplasmonic structures and propose a combination of microspherical nanoscopy with nanoplasmonic illumination for imaging biomedical samples. We applied these techniques for imaging actin protein filaments and yeast cells and observed a resolution advantage over standard microscopy.

Use of photonic jets produced by dielectric microspheres for increasing sensitivity and angle-of-view of MWIR detectors

Recently, it was experimentally demonstrated (K.W. Allen et al., APL 108, 241108 (2016)) that microspheres can be used as contact microlenses to enhance the efficiency of collection of light by individual pixels in mid wave infrared (MWIR) focal plane arrays (FPAs). In this work, using finite difference time domain (FDTD) modeling, we optimized the designs of such FPAs integrated with microspheres for achieving maximal angle of view (AOV) as a function of the index of refraction and diameter of the spheres. We also designed structures where the spheres are partly immersed in a layer of photoresist. Our designs are developed for both front-side and back-side illuminated structures. Compared to standard microlens arrays, our designs provide much larger angle of view reaching ~15 degrees for front-illuminated and ~4 degrees for back-illuminated structures. Our designs allow decreasing the sizes of photosensitive mesas down to wavelength-scale dimensions determined by the minimal waists of the focused beams produced by the dielectric microspheres, so-called photonic jets. This opens a principle possibility to reduce the dark current and increase the operating temperature of MWIR FPAs. We also discuss the techniques of fabrication of such FPAs integrated with a large number of microspheres and show that suction assembly of microspheres is a promising method of obtaining massive-scale integration of microspheres onto the individual pixels with very small concentration of defects.

Microsphere nanoscopy for imaging of actin proteins

The use of contact microlenses first started being used as a simple method which allows resolving nanometer sized objects using visible light. The resolution beyond the classical diffraction limit has been reported for nanoplasmonic structures. However the mechanism is not fully understood whether it is from photonic nanojets, plasmon-polaritons, localized surface plasmon resonances, coherent modal excitation in metallic objects, or optical resonances in the microspherical lens itself is still debated in the literature. In this work, we applied contact high-index microspheres for fluorescence imaging of actin protein filaments. The microspheres were embedded in elastopolymer slabs. The fact that the filaments have nanometer-scale width, much smaller than the diffraction limit, significantly simplified the resolution analysis. Using microscope objective with a limited numerical aperture (NA=0.6), we achieve a resolution of ∼λ/1.5 for proteins through the microsphere and a resolution of ∼λ/0.5 without a microsphere.

Imaging of two-dimensional nanoplasmonic structures by nanoscopy with contact microlenses and various microscope objectives

The use of contact microlenses attracted a great deal of attention recently because in the case of imaging nanoplasmonic structures it showed the resolution values exceeding the classical diffraction limit. The mechanisms that allow for this type of imaging are presently still debated in the literature; these range from photonic nanojets, plasmon-polaritons, localized surface plasmon resonances and coherent modal excitation in metallic objects, as well as optical resonances in microsphereical lenses. The system of interest is metallic nanoobjects coupled to a dielectric microsphere and is considered as a directional antenna. In this work, we performed preliminary studies of how the resolution of this method depends on the numerical aperture of the microscope objective. We used 2D periodic nanoplasmonic arrays. Based on assumption of incoherent imaging, we demonstrated that the resolution is reduced from ∼λ/7 to ∼λ/4 for a reduction of NA from 0.95 to 0.60. In future work, we plan to test the main conclusions of this work using standalone and long-period structures.

Superresolution imaging with contact microspheres: Importance of numerical aperture

Optical nanoscopy with contact microlenses emerged as a simple method which allows overcoming the classical diffraction limit represented by the Abbes formula. The mechanisms of such imaging are debated in the literature including photonic nanojets, plasmon-polaritons, localized surface plasmon resonances and coherent modal excitations in metallic objects, as well as optical resonances in microspherical lenses. The system containing metallic nano-object coupled to a dielectric microsphere can be considered as a directional antenna. This means that the dependence of the resolution on the numerical aperture (NA) of the microscope objective can be different from that predicted by the Abbe formula. In this work, we tested this hypothesis experimentally and showed that reduction of NA from 0.95 to 0.6 leads to a decrease in resolution from ~ λ/7 to ~ λ/4, where λ is the illumination wavelength.