Vasily N. Astratov

Optical coupling and transport phenomena in chains of spherical dielectric microresonators with size disorder

The optical transmission properties of chains or circuits of touching polystyrene microspheres with sizes in the 3–20μm range and a size dispersion of ∼1% are studied. The dye-doped spheres with fluorescent peaks due to whispering gallery modes were attached to one end of the chains. The effects of optical transport were detected using spatially resolved scattering spectroscopy. The attenuation was shown to be ∼3 to 4 dB per sphere for the modes with the best transport properties. A mechanism for the observed transport is suggested based on the formation of strongly coupled photonic modes in the systems of randomly detuned resonators with size disorder. It is shown that such circuits possess broad bandpass waveguiding characteristics essential for applications in integrated all-optical network devices.

Optical Super-Resolution by High-Index Liquid-Immersed Microspheres

It is experimentally shown that barium titanate glass microspheres with diameters (D) in the range 2–220 μm and with high refractive index (n ∼ 1.9–2.1) can be used for super-resolution imaging of liquid-immersed nanostructures. Using micron-scale microspheres, we demonstrate an ability to discern the shape of a pattern with a minimum feature size of ∼λ/7, where λ is the illumination wavelength. For spheres with D > 50 μm, the discernible feature sizes were found to increase to ∼λ/4. Detailed data on the resolution, magnification, and field-of-view are presented. This imaging technique can be used in biomedical microscopy, microfluidics, and nanophotonics applications.

Observation of nanojet-induced modes with small propagation losses in chains of coupled spherical cavities.

Nanojet-induced modes (NIMs) and their attenuation properties are studied in linear chains consisting of tens of touching polystyrene microspheres with sizes in the 2-10 micro m range. To couple light to NIMs we used locally excited sources of light formed by several dye-doped fluorescent microspheres from the same chain of cavities. We directly observed the formation and propagation of NIMs by means of the scattering imaging technique. By measuring attenuation at long distances from the source, we demonstrate propagation losses for NIMs as small as 0.5 dB per sphere.

Advantages of microsphere-assisted super-resolution imaging technique over solid immersion lens and confocal microscopies

We demonstrate a series of advantages of microsphere-assisted imaging over confocal and solid immersion lens microscopies including intrinsic flexibility, better resolution, higher magnification, and longer working distances. We discerned minimal feature sizes of ∼50-60 nm in nanoplasmonic arrays at the illumination wavelength λ = 405 nm. It is demonstrated that liquid-immersed, high-index (n ∼ 1.9-2.1) spheres provide a superior image quality compared to that obtained by spheres with the same index contrast in an air environment. We estimate that using transparent microspheres at deep UV wavelengths of ∼200 nm might make possible imaging of various nanostructures with extraordinary high ∼30 nm resolution.

Photonic nanojet-induced modes in chains of size-disordered microspheres with an attenuation of only 0.08dB per sphere

By using spatially resolved spectroscopy the authors demonstrate that the periodical focusing of light in straight chains of touching 5μm polystyrene microspheres is characterized with the periodicity of photonic nanojets corresponding to the size of two spheres. In transmission spectra of long (>20 spheres) chains they observe Fabry–Perot fringes with propagation losses of only 0.08dB per sphere in the maxima of transmission peaks. Due to mechanical robustness, tight focusing of the beam, high optical throughput, and broad spectral transmission properties such chains can be used in a variety of biomedical applications as optical microprobes with subwavelength spatial resolution.

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.

Periodically focused modes in chains of dielectric spheres

Recently we showed that light focusing and transport properties of chains of dielectric spheres with D>;>;10λ, where D is the diameter of the sphere and λ is the wavelength, are dominated by periodically focused modes (PFMs) which have extremely small propagation losses. In this work we show that along with a special case of PFMs in chains of spheres with index n = √3 which propagates in such structures without losses, similar periodic modes exist in a broad range of indices from 1.4 to 2.0. For each n such generalized PFMs have various radial extents in the regions between the neighbouring focused beams. We show that for 10-sphere long chains with 1.68 <; n <; 1.80 such modes have total propagation losses smaller than 1 dB. Using numerical ray tracing, we demonstrate that such chains filter radially polarized beams. Using collimated incident beams, a polarization degree in excess of 0.9 is demonstrated for 10-sphere long chains for 1.68 <; n <; 1.80 range. These properties make chains of microspheres instrumental for developing novel focusing and polarization components.

Optical coupling at a distance between detuned spherical cavities

Using numerical modeling, we observe surprisingly high coupling efficiencies (up to 0.1) between spatially separated spherical cavities with strongly detuned whispering gallery modes. We show that the coupling arises from resonance between a discrete energy eigenstate in the sphere containing the source of light and a continuum of “quasi”-whispering gallery modes with noncircular shape and reduced quality factors in the sphere receiving the electromagnetic energy. Such coupling effects may make possible broad spectral transmission effects in coupled resonator optical waveguides, previously thought to be excluded in a real system with significant size disorder.

Fundamentals and applications of microsphere resonator circuits

Microsphere resonator circuits have emerged as a technology allowing integration of cavities in all three spatial dimensions (3D) in chip-scale structures. This presentation is devoted to fabrication, optical properties and applications of mesoscale structures formed by microspheres. The “mesophotonics” properties of such structures stem from two properties of individual dielectric microspheres with dimensions comparable to the wavelength of light: (i) high quality factors of whispering gallery modes (WGMs) and (ii) subwavelength dimensions of the focused spots termed “photonic nanojets”. Traditionally, the resonant optical transport properties of such structures have been described by a tight binding approximation for photonic atoms. Recently we showed that the WGM-based transport in 2D and 3D systems with disorder can be understood on the basis of analogy with a percolation theory. Along with these resonant coupling effects very interesting properties of periodical focusing of light were observed in long chains of spheres. These properties were explained due to formation of nanojet-induced modes with small propagation losses. At the device level, these studies stimulate developing designs where coupled cavities are applied to developing narrow spectral filters, delay lines, arrayed-resonator light emitting devices, tight focusing micro-probes, sensors, and compact spectrometers.

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.