Roadmap on Label-Free Super-Resolution Imaging.

Label-free super-resolution (LFSR) imaging relies on light-scattering processes in nanoscale objects without a need for fluorescent (FL) staining required in super-resolved FL microscopy. The objectives of this Roadmap are to present a comprehensive vision of the developments, the state-of-the-art in this field, and to discuss the resolution boundaries and hurdles which need to be overcome to break the classical diffraction limit of the LFSR imaging. The scope of this Roadmap spans from the advanced interference detection techniques, where the diffraction-limited lateral resolution is combined with unsurpassed axial and temporal resolution, to techniques with true lateral super-resolution capability which are based on understanding resolution as an information science problem, on using novel structured illumination, near-field scanning, and nonlinear optics approaches, and on designing superlenses based on nanoplasmonics, metamaterials, transformation optics, and microsphere-assisted approaches. To this end, this Roadmap brings under the same umbrella researchers from the physics and biomedical optics communities in which such studies have often been developing separately. The ultimate intent of this paper is to create a vision for the current and future developments of LFSR imaging based on its physical mechanisms and to create a great opening for the series of articles in this field.

Microconical silicon mid-IR concentrators: spectral, angular and polarization response.

It is widely discussed in the literature that a problem of reduction of thermal noise of mid-wave and long-wave infrared (MWIR and LWIR) cameras and focal plane arrays (FPAs) can be solved by using light-concentrating structures. The idea is to reduce the area and, consequently, the thermal noise of photodetectors, while still providing a good collection of photons on photodetector mesas that can help to increase the operating temperature of FPAs. It is shown that this approach can be realized using microconical Si light concentrators with (111) oriented sidewalls, which can be mass-produced by anisotropic wet etching of Si (100) wafers. The design is performed by numerical modeling in a mesoscale regime when the microcones are sufficiently large (several MWIR wavelengths) to resonantly trap photons, but still too small to apply geometrical optics or other simplified approaches. Three methods of integration Si microcone arrays with the focal plane arrays are proposed and studied: (i) inverted microcones fabricated in a Si slab, which can be heterogeneously integrated with the front illuminated FPA photodetectors made from high quantum efficiency materials to provide resonant power enhancement factors (PEF) up to 10 with angle-of-view (AOV) up to 10°; (ii) inverted microcones, which can be monolithically integrated with metal-Si Schottky barrier photodetectors to provide resonant PEFs up to 25 and AOVs up to 30° for both polarizations of incident plane waves; and iii) regular microcones, which can be monolithically integrated with near-surface photodetectors to provide a non-resonant power concentration on compact photodetectors with large AOVs. It is demonstrated that inverted microcones allow the realization of multispectral imaging with ∼100 nm bands and large AOVs for both polarizations. In contrast, the regular microcones operate similar to single-pass optical components (such as dielectric microspheres), producing sharply focused photonic nanojets.

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.

Overcoming the diffraction limit of imaging nanoplasmonic arrays by microspheres and microfibers.

Super-resolution microscopy by microspheres emerged as a simple and broadband imaging technique; however, the mechanisms of imaging are debated in the literature. Furthermore, the resolution values were estimated based on semi-quantitative criteria. The primary goals of this work are threefold: i) to quantify the spatial resolution provided by this method, ii) to compare the resolution of nanoplasmonic structures formed by different metals, and iii) to understand the imaging provided by microfibers. To this end, arrays of Au and Al nanoplasmonic dimers with very similar geometry were imaged using confocal laser scanning microscopy at λ = 405 nm through high-index (n~1.9-2.2) liquid-immersed BaTiO3 microspheres and through etched silica microfibers. We developed a treatment of super-resolved images in label-free microscopy based on using point-spread functions with subdiffraction-limited widths. It is applicable to objects with arbitrary shapes and can be viewed as an integral form of the super-resolution quantification widely accepted in fluorescent microscopy. In the case of imaging through microspheres, the resolution ~λ/6-λ/7 is demonstrated for Au and Al nanoplasmonic arrays. In the case of imaging through microfibers, the resolution ~λ/6 with magnification M~2.1 is demonstrated in the direction perpendicular to the fiber with hundreds of times larger field-of-view in comparison to microspheres.

Detachable microsphere scalpel tips for potential use in ophthalmic surgery with the erbium:YAG laser.

Vitreoretinal surgery is performed using mechanical dissection that sometimes results in iatrogenic complications, including vitreous hemorrhage, retinal breaks, incomplete membrane delamination, retinal distortion, microscopic damage, etc. An ultraprecise laser probe would be an ideal tool for cutting away pathologic membranes; however, the depth of surgery should be precisely controlled to protect the sensitive underlying retina. The ultraprecise surgical microprobe formed by chains of dielectric spheres for use with the erbium:YAG laser source (λ=2940 nm), with extremely short optical penetration depth in tissue, was optimized. Numerical modeling demonstrated a potential advantage of five-sphere focusing chains of sapphire spheres with index n=1.71 for ablating the tissue with self-limited depth around 10 to 20 µm. Novel detachable microsphere scalpel tips formed by chains of 300 µm sapphire (or ruby) spheres were tested on ophthalmic tissues, ex vivo. Detachable scalpel tips could allow for reusability of expensive mid-infrared trunk fibers between procedures, and offer more surgical customization by interchanging various scalpel tip configurations. An innovative method for aiming beam integration into the microsphere scalpel to improve the illumination of the surgical site was also shown. Single Er:YAG pulses of 0.2 mJ and 75-µs duration produced ablation craters in cornea epithelium for one, three, and five sphere structures with the latter generating the smallest crater depth (10 µm) with the least amount of thermal damage depth (30 µm). Detachable microsphere laser scalpel tips may allow surgeons better precision and safety compared to mechanical scalpels when operating on delicate or sensitive areas like the retina.

Formation of polarized beams in chains of dielectric spheres and cylinders.

Using numerical modeling, it is shown that chains of dielectric spheres and cylinders act as polarizers. The mechanism is based on gradual filtering of periodically focused modes with a certain polarization propagating with minimal losses due to Brewster angles conditions, whereas orthogonally polarized modes are strongly attenuated. It is shown that chains of cylinders filter linearly polarized beams, whereas chains of spheres filter radially polarized beams. In the geometrical optics limit, we show that in a range of sphere refractive indices 1.68-1.80 a degree of radial polarization in excess of 0.9 can be obtained in 10-sphere-long chains.

Characterization of novel microsphere chain fiber optic tips for potential use in ophthalmic laser surgery.

Ophthalmic surgery may benefit from use of more precise fiber delivery systems during laser surgery. Some current ophthalmic surgical techniques rely on tedious mechanical dissection of tissue layers. In this study, chains of sapphire microspheres integrated into a hollow waveguide distal tip are used for erbium:YAG laser ablation studies in contact mode with ophthalmic tissues, ex vivo. The laser's short optical penetration depth combined with the small spot diameters achieved with this fiber probe may provide more precise tissue removal. One-, three-, and five-microsphere chain structures were characterized, resulting in FWHM diameters of 67, 32, and 30 µm in air, respectively, with beam profiles comparable to simulations. Single Er:YAG pulses of 0.1 mJ and 75-µs duration produced ablation craters with average diameters of 44, 30, and 17 µm and depths of 26, 10, and 8 µm, for one-, three-, and five-sphere structures, respectively. Microsphere chains produced spatial filtering of the multimode Er:YAG laser beam and fiber, providing spot diameters not otherwise available with conventional fiber systems. Because of the extremely shallow treatment depth, compact focused beam, and contact mode operation, this probe may have potential for use in dissecting epiretinal membranes and other ophthalmic tissues without damaging adjacent retinal tissue.

Periodically focused modes in chains of dielectric spheres.

We show that, theoretically, Brewster angle conditions for transverse magnetic polarized rays can be periodically reproduced in chains of spheres with index n = [Formula: see text] giving rise to lossless periodically focused modes with 2D period, where D is the sphere diameter. Using ray tracing for a spherical emitter with the diameter D we show that chains of spheres work as filters of such modes at 1.72 < n < 1.85. This leads to tapering of the focused beams combined with the reduction of their attenuation along the chain. Experimentally, the "beam tapering" effect was observed in chains of 300 µm sapphire spheres with index ∼1.77 in visible.

Collective phenomena in photonic, plasmonic and hybrid structures.

Preface to a focus issue of invited articles that review recent progress in studying the fundamental physics of collective phenomena associated with coupling of confined photonic, plasmonic, electronic and phononic states and in exploiting these phenomena to engineer novel devices for light generation, optical sensing, and information processing.

Fiber coupling to BaTiO3 glass microspheres in an aqueous environment.

Compact microspheres with high-quality (Q) whispering gallery modes are required for many applications involving liquid immersion, such as sensing nanoparticles and studying resonant radiative pressure effects. We show that high-index (1.9 and 2.1) barium titanate glass (BTG) microspheres are perfect candidates for these applications due to their high-Q (∼10(4) in the 1100-1600 nm range) resonances evanescently excited in spheres with diameters of 4-15 µm. By reattaching the spheres at different positions along a tapered optical fiber, we show that the coupling constant exponentially increases with thinner fiber diameters. We demonstrate the close to critical coupling regime with intrinsic Q=3×10(4) for water immersed 14 µm BTG spheres.

Contact focusing multimodal microprobes for ultraprecise laser tissue surgery.

Focusing of multimodal beams by chains of dielectric microspheres assembled directly inside the cores of hollow waveguides is studied by using numerical ray tracing. The device designs are optimized for laser surgery in contact mode with strongly absorbing tissue. By analyzing a broad range of parameters it is demonstrated that chains formed by three or five spheres with a refractive index of 1.65-1.75 provide a two-fold improvement in spatial resolution over single spheres at the cost of 0.2-0.4 attenuation in peak intensity of the central focused beam. Potential applications include ultra precise laser ablation or coagulation in the eye and brain, cellular surgery, and the coupling of light into photonic nanostructures.

Spectroscopy of coherently coupled whispering-gallery modes in size-matched bispheres assembled on a substrate.

Coupling between whispering-gallery modes is studied using imaging spectroscopy in pairs of size-selected (size deviation of <0.03%) spheres in a touching position. In a special geometrical configuration, peculiar shapes resembling kites are observed in the spectral images of bispheres by coupling of multiple pairs of azimuthal modes. The maximal WGM coupling is observed in the equatorial plane of spheres determined by the substrate. The WGM coupling constant is quantified for spheres from 2.9 to 6 microm for such modes.

Perturbations of whispering gallery modes by nanoparticles embedded in microcavities.

The effects of perturbations of whispering gallery modes (WGMs) in cylindrical microcavities by embedded particles are studied by FDTD modeling. The principal effects are: i) spectral shift of the WGM-related peaks caused by the variation of the average index, ii) broadening of the WGM peaks introduced by the scattering, and iii) splitting of the WGM peaks due to formation of symmetric (SSW) and antisymmetric (ASW) standing waves. The focus of this work is on the last effect. We show that it can be maximized by placing the nanoparticle inside the cavity at a position corresponding to the antinode of the radial distribution of intensity of WGM. It is demonstrated that in this case the magnitude of splitting reaches several angstroms for cavities with moderately high quality (Q approximately = 10(5)) WGMs. We show that for relatively small particles with radius <70 nm and index contrasts <0.2 the magnitude of SSW/ASW splitting is linearly dependent on the size and index of the nanoparticle. This allows developing biomolecular sensors based on measuring this splitting in porous cavities. It is predicted that a similar effect of splitting can occur in semiconductor microdisks and pillars where the role of embedded dielectric nanoparticles can be played by self-assembled quantum dots.

Introduction: physics and applications of microresonators.

An introduction to the Focus Issue devoted Physics and Applications of Microresonators is presented. An introduction includes a description of the area of microresontors and a description of the background and structure of the Focus Issue.

Percolation of light through whispering gallery modes in 3D lattices of coupled microspheres.

Using techniques of flow-assisted self-assembly we synthesized three-dimensional (3D) lattices of dye-doped fluorescent (FL) 5 mum polystyrene spheres with 3% size dispersion with well controlled thickness from one monolayer up to 43 monolayers. In FL transmission spectra of such lattices we observed signatures of coupling between multiple spheres with nearly resonant whispering gallery modes (WGMs). These include (i) splitting of the WGM-related peaks with the magnitude 4.0-5.3 nm at the average wavelength 535 nm, (ii) pump dependence of FL transmission showing that the splitting is seen only above the threshold for lasing WGMs, and (iii) anomalously high transmission at the WGM peak wavelengths compared to the background for samples with thickness around 25 mum. We propose a qualitative interpretation of the observed WGM transport based on an analogy with percolation theory where the sites of the lattice (spheres) are connected with optical "bonds" which are present with probability depending on the spheres' size dispersion. We predict that the WGM percolation threshold should be achievable in close packed 3D lattices formed by cavities with ~10(3) quality factors of WGMs and with ~1% size dispersion. Such systems can be used for developing next generation of resonant sensors and arrayed-resonator light emitting devices.

The effects of inter-cavity separation on optical coupling in dielectric bispheres.

The optical coupling between two size-mismatched spheres was studied by using one sphere as a local source of light with whispering gallery modes (WGMs) and detecting the intensity of the light scattered by a second sphere playing the part of a receiver of electromagnetic energy. We developed techniques to control inter-cavity gap sizes between microspheres with ~30nm accuracy. We demonstrate high efficiencies (up to 0.2-0.3) of coupling between two separated cavities with strongly detuned eigenstates. At small separations (<1 microm) between the spheres, the mechanism of coupling is interpreted in terms of the Fano resonance between discrete level (true WGMs excited in a source sphere) and a continuum of "quasi"-WGMs with distorted shape which can be induced in the receiving sphere. At larger separations the spectra detected from the receiving sphere originate from scattering of the radiative modes.