Kenneth W. Allen

Super‐resolution microscopy by movable thin‐films with embedded microspheres: Resolution analysis

Microsphere-assisted imaging emerged as a surprisingly simple way of achieving optical super-resolution imaging. In this work, we use movable PDMS thin films with embedded high-index barium titanate glass microspheres a sample scanning capability was developed, thus removing the main limitation of this technology based on its small field-of-view.

Movable thin films with embedded high-index microspheres for super-resolution microscopy

Microsphere-assisted imaging emerged as a surprisingly simple way of achieving optical super-resolution imaging. In this work, we use movable PDMS thin films with embedded high-index barium titanate glass microspheres a sample scanning capability was developed, thus removing the main limitation of this technology based on its small field-of-view.

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.

Microsphere-chain waveguides: Focusing and transport properties

It is shown that the focusing properties of polystyrene microsphere-chain waveguides (MCWs) formed by sufficiently large spheres (D ≥ 20λ, where D is the sphere diameter and λ is the wavelength of light) scale with the sphere diameter as predicted by geometrical optics. However, this scaling behavior does not hold for mesoscale MCWs with D ≤ 10λ resulting in a periodical focusing with gradually reducing beam waists and in extremely small propagation losses. The observed effects are related to properties of nanojet-induced and periodically focused modes in such structures. The results can be used for developing focusing microprobes, laser scalpels, and polarization filters.

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.

Increasing sensitivity and angle-of-view of mid-wave infrared detectors by integration with dielectric microspheres

We observed up to 100 times enhancement of sensitivity of mid-wave infrared photodetectors in the 2–5 μm range by using photonic jets produced by sapphire, polystyrene, and soda-lime glass microspheres with diameters in the 90–300 μm range. By finite-difference time-domain (FDTD) method for modeling, we gain insight into the role of the microspheres refractive index, size, and alignment with respect to the detector mesa. A combination of enhanced sensitivity with angle-of-view (AOV) up to 20° is demonstrated for individual photodetectors. It is proposed that integration with microspheres can be scaled up for large focal plane arrays, which should provide maximal light collection efficiencies with wide AOVs, a combination of properties highly attractive for imaging applications.

Super-resolution imaging by arrays of high-index spheres embedded in transparent matrices

We fabricated thin-films made from polydimethylsiloxane (PDMS) with embedded high-index (n~1.9-2.2) microspheres for super-resolution imaging applications. To control the position of microspheres, such films can be translated along the surface of the nanoplasmonic structure to be imaged. Microsphere-assisted imaging, through these matrices, provided lateral resolution of ~λ/7 in nanoplasmonic dimer arrays with an illuminating wavelength λ=405 nm. Such thin films can be used as contact optical components to boost the resolution capability of conventional microscopes.

Integrated microsphere arrays: Light focusing and propagation effects

Integration of microspheres inside micro-capillaries or hollow waveguides may allow development of compact focusing tools for a variety of biomedical and photonics applications. However, problems associated with developing focusing microprobes involve the multimodal structure of noncollimated beams delivered by fibers and waveguides. By using numerical ray tracing, it is shown that serial spherical microlenses filter out spatially periodic modes which can be used for obtaining tightly focused beams. Experimental studies are performed for spheres with sizes from 10 to 300 μm with different indices of refraction ranging from 1.47 to 1.9. The chains were assembled inside plastic tubing with bore sizes matching the size of the spheres. By using high index spheres, it is demonstrated that these structures are capable of focusing light in contact with tissue. The beam attenuation properties of such chains are found to be in good agreement with numerical modeling results. Potential applications of integrated microsphere arrays include ultra-precise intraocular and neurosurgical laser procedures, photoporation of cells, and coupling of light into photonic microstructures.

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.

Photonic nanojet-induced modes: From physics to applications

The effects of periodical focusing of light are studied in chains of spheres with diameters varying from 2 µm to 300 µm and with index of refraction varying from 1.3 to 2.5. Experimentally, we show that the coupled focused beams decrease in size along the chain of polystyrene microspheres with index n = 1.59, reaching wavelength-scale dimensions in the case of small beads with 4 < D/λ < 10, where D is the spheres diameter and λ is the wavelength of light. We show that these effects are determined by the existence of so-called photonic nanojet-induced modes with the period approximately equal to the size of two spheres. By using numerical ray tracing we show that in the limit of geometrical optics such effect of “tapering” of optical beams does not exist for spheres with n = 1.59, however it should be very pronounced in a narrow range of indices around n = 1.75. The results can be used for developing various focusing devices for photonics and biomedical optics applications.