Farzaneh Abolmaali

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 (Du2009≥u200920λ, 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 Du2009≤u200910λ 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.

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 2–5u2009μm range by using photonic jets produced by sapphire, polystyrene, and soda-lime glass microspheres with diameters in the 90–300u2009μm 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.

Coupling properties and sensing applications of photonic molecules

Photonic molecules formed by microresonators with coupled whispering gallery modes were studied by finite-difference time-domain modeling. Mode splitting was observed due to coherent coupling. Spatial mode distribution was shown to be different for each coupled mode. The advantages of photonic molecules over single resonators for sensing applications were demonstrated.

Spectral finger-prints of photonic molecules

Clusters of circular resonators with coupled whispering gallery modes (WGMs) have interesting spectral and optical transport properties. We employ the finite-difference time-domain method to numerically study these properties. Using evanescent couplers, we show that various configurations of cavities display distinct spectral features (mode splitting) in the vicinity of their WGMs resonances. These features can be considered as spectral finger-prints of different clusters of cavities. We observed some of these spectral finger-prints experimentally using sorted polystyrene microspheres with uniform WGM resonances. The results can be used for understanding mechanisms of light transport in such complex optical networks and for developing spectral filters, delay lines, sensors, microspectrometers, and spectral markers.

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.

Observation of the influence of the gain on parity-time-symmetric properties of photonic molecules with coupled whispering gallery modes

Parity-time (PT) symmetry breaking in coupled whispering gallery mode type resonators is studied by finite-difference time-domain modeling. Normal mode splitting is studied as a function of the coupling strength. It is demonstrated that in bi-atomic molecules with distributed gain and loss, reduction of the coupling beyond a certain value leads to PT symmetry breaking.

Identification of whispering gallery modes in a fiber based sensor platform

Development of whispering gallery mode (WGM) sensors of nanoparticles deposited on a sidewall surface of circular resonators require identification of polarizations and modal numbers of WGMs involved. Using analytical modeling approach based on Mie scattering theory, we identified WGMs excited experimentally in a side-coupled configuration with controlled polarization.

Spectral signatures of photonic molecules with hybridized whispering gallery modes

Based on analogy between quantum mechanics and the classical electrodynamics, we sorted dielectric microspheres with almost identical positions of their whispering gallery mode (WGM) resonances as photonic atoms. Such microspheres were assembled in a wide range of structures including linear chains and planar photonic molecules. We studied WGM hybridization effects in such structures using side coupling by tapered microfibers as well as finite difference time domain modeling. We demonstrated that the patterns of WGM spectral splitting are representative of the symmetry, number of constituting atoms and topology of the photonic molecules. Such splitting patterns can be viewed as “spectral signatures” of corresponding molecules. Excellent agreement was found between measured and calculated fiber-transmission spectra for different molecules.

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.