Roxana Rezvani Naraghi

Directional control of scattering by all-dielectric core-shell spheres

The optical size and intrinsic material properties of scattering particles introduce inherent restrictions on their scattering patterns. We show that large size, core-shell dielectric structures with spherical symmetry provide the necessary flexibility for exciting higher-order spherical modes and, consequently, allow us to control the directivity of the scattered radiation. Significant scattering can be generated over angular domains that were formerly believed to be accessible only to dipolar scattering.

Dynamic consequences of optical spin–orbit interaction

Scientists theoretically and experimentally demonstrate that the transformation of spin into orbital momentum can lead to a fundamentally new type of force acting transversally to the direction of propagation.

High precision refractometry based on Fresnel diffraction from phase plates

When a transparent plane-parallel plate is illuminated at a boundary region by a monochromatic parallel beam of light, Fresnel diffraction occurs because of the abrupt change in phase imposed by the finite change in refractive index at the plate boundary. The visibility of the diffraction fringes varies periodically with changes in incident angle. The visibility period depends on the plate thickness and the refractive indices of the plate and the surrounding medium. Plotting the phase change versus incident angle or counting the visibility repetition in an incident-angle interval provides, for a given plate thickness, the refractive index of the plate very accurately. It is shown here that the refractive index of a plate can be determined without knowing the plate thickness. Therefore, the technique can be utilized for measuring plate thickness with high precision. In addition, by installing a plate with known refractive index in a rectangular cell filled with a liquid and following the described procedures, the refractive index of the liquid is obtained. The technique is applied to measure the refractive indices of a glass slide, distilled water, and ethanol. The potential and merits of the technique are also discussed.

Applications of Fresnel diffraction from the edge of a transparent plate in transmission

When a transparent plane-parallel plate is illuminated at the edge region by a quasi-monochromatic parallel beam of light, diffraction fringes appear on a plane perpendicular to the transmitted beam direction. The sharp change in the refractive index at the plate boundary imposes an abrupt change on the phase of the illuminating beam that leads to the Fresnel diffraction. The visibility of the diffraction fringes depends on the plate thickness, refractive index, light wavelength, and angle of incidence. In this report we show that, by recording the visibility repetition versus incident angle, one can measure the plate refractive index, its thickness, and light wavelength very accurately. It is also shown that the technique is indispensable for specifying color dispersion in plate shape samples. The technique is applied to the measurement of dispersion in a fused silica plate and the refractive indices of soda lime slides.

Digital design of multimaterial photonic particles

Significance We present an approach for the facile fabrication of dielectric particles having the size of an optical wavelength yet endowed with a complex multimaterial internal nanoscale architecture. This methodology amounts to “digitally designing” the particle by precisely allocating the desired material at prescribed coordinates within the 3D volume of the particle. The digital design of such a photonic particle enables sophisticated strategies for controlling light scattering. As an example, without changing the size of a core–shell particle, its optical scattering strength can be tuned above or below that afforded by its constitutive materials by changing the core–shell diameter ratio. This work may lead to the development of new optical coatings and paints with exotic functionality. Scattering of light from dielectric particles whose size is on the order of an optical wavelength underlies a plethora of visual phenomena in nature and is a foundation for optical coatings and paints. Tailoring the internal nanoscale geometry of such “photonic particles” allows tuning their optical scattering characteristics beyond those afforded by their constitutive materials—however, flexible yet scalable processing approaches to produce such particles are lacking. Here, we show that a thermally induced in-fiber fluid instability permits the “digital design” of multimaterial photonic particles: the precise allocation of high refractive-index contrast materials at independently addressable radial and azimuthal coordinates within its 3D architecture. Exploiting this unique capability in all-dielectric systems, we tune the scattering cross-section of equisized particles via radial structuring and induce polarization-sensitive scattering from spherical particles with broken internal rotational symmetry. The scalability of this fabrication strategy promises a generation of optical coatings in which sophisticated functionality is realized at the level of the individual particles.

Passive near-field imaging with pseudo-thermal sources

We demonstrate experimentally that spurious effects caused by interference can be eliminated in passive near-field imaging by implementing a simple random illumination. We show that typical imaging artifacts are effectively eliminated when the radiation emitted by a pseudo-thermal source illuminates the sample and the scattered field is collected by an aperture probe over essentially all angles of incidence. This novel pseudo-thermal source can be easily implemented and significantly enhances the performance of passive near-field imaging.

Wide-field interferometric measurement of a nonstationary complex coherence function

We demonstrate an optimized two-step procedure for measuring the full complex coherence function. The measurement relies on a wavefront shearing interferometer that permits characterizing nonstationary fields over an extended angular domain. The accuracy of the coherence measurement was demonstrated by excellent agreement with theoretical predictions.

Babinet’s principle for mutual intensity

In classical diffraction theory, Babinets principle relates the electromagnetic fields produced by complementary sources. This theorem was always formulated for single-point quantities, both intensities or field amplitudes, in conditions where the full spatial coherence is implicitly assumed. However, electromagnetic fields are, in general, partially coherent, and their spatial properties are described in terms of two-point field-field correlation functions. In this case, a generalized Babinets principle can be derived that applies to the spatial coherence functions. We present both the derivation and the experimental demonstration of this generalized Babinet theorem.

Near-field imaging with pseudo-thermal sources

We provide a simple solution for a significant deficiency of near-field microscopy. We demonstrate experimentally that spurious effects caused by interference can be eliminated in passive near-field imaging by implementing a random illumination.

Diffusive scattering from single microspheres with well-dispersed dielectric nano-scale inclusions

By fabricating a new class of composite microspheres comprising a random distribution of well-dispersed high-refractive-index dielectric nanoparticles, we confirm that forward and backward multiply scattered fields in the visible are diffusive.