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.
Resonant light pressure effects provide new degrees of freedom for optical manipulation of microparticles. In particular, they can be used for optical sorting of photonic atoms with extraordinary uniform resonant properties. These atoms can be used as building blocks of structures and devices with engineered photonic dispersions. To study the spectral shape of the force peaks, we developed a method to precisely control the wavelength detuning between the tunable laser emission line and central position of the whispering gallery mode (WGM) peaks in tapered fiber-to-microsphere water-immersed couplers. Our method is achieved by integrating optical tweezers to individually manipulate microspheres and based on preliminary spectral characterization of WGM peak positions followed by setting a precise amount of laser wavelength detuning for optical propulsion experiments. We demonstrated dramatic enhancement of the optical forces exerted on 20 μm polystyrene spheres under resonant conditions. Spectral properties of the resonant force enhancement were studies with controlled laser line detuning. In addition, we observed the dynamics of radial trapping and longitudinal propelling process and analyzed their temporal properties. Our studies also demonstrated a stable radial trapping of microspheres near the surface of tapered fiber for high speed resonant optical propulsion along the fiber.
We study the propulsion of polystyrene microspheres along water immersed silica tapered fibers. We observed a nearly
linear increase of the propulsion velocity with the sphere diameter increasing from 3 to 20 μm. By measuring the fiber
transmission spectra we demonstrate efficient evanescent coupling of light to whispering gallery modes (WGMs) in large
(>10 μm) polystyrene spheres. For 20 μm spheres we observed the depth of resonant dips ~ 3.5 dB in combination with
the Q-factors ~ 103. Due to small losses in the fiber ~1-2 dB we are able to determine the power in the tapered region and
to characterize quantitatively the optical propelling forces. The maximum value of the propelling velocity was 260 μm/s
and was observed for 15 μm spheres with guided power of only 43 mW. Such velocities are nearly an order of magnitude
higher than those observed for similar powers on waveguide structures. Using simple physical arguments we show that
for spheres with diameters larger than 10 μm the experimentally observed velocities of propelling are too high to be
explained by the conventional nonresonant scattering forces. We propose that these high velocities indicate that the
optical forces are enhanced in such cases due to resonant coupling effects.