Bergin Gjonaj

Metafocusing by a Metaspiral Plasmonic Lens

We designed and realized a metasurface (manipulating the local geometry) spiral (manipulating the global geometry) plasmonic lens, which fundamentally overcomes the multiple efficiency and functionality challenges of conventional in-plane plasmonic lenses. The combination of spirality and metasurface achieves much more efficient and uniform linear-polarization-independent plasmonic focusing. As for functionality, under matched circularly polarized illumination the lens directs all of the power coupled to surface plasmon polaritons (SPPs) into the focal spot, while the orthogonal polarization excites only diverging SPPs that do not penetrate the interior of the lens, achieving 2 orders of magnitude intensity contrast throughout the entire area of the lens. This optimal functional focusing is clearly demonstrated by near-field optical microscopy measurements that are in excellent agreement with simulations and are supported by a detailed theoretical interpretation of the underlying mechanisms. Our results advance the field of plasmonics toward functional detection and the employment of SPPs in smart pixels, near-field microscopy, lithography, and particle manipulation.

Sub-100 nm focusing of short wavelength plasmons in homogeneous 2D space.

We present a direct measurement of short-wavelength plasmons focused into a sub-100 nm spot in homogeneous (translation invariant) 2D space. The short-wavelength (SW) surface plasmon polaritons (SPP) are achieved in metal-insulator-insulator (MII) platform consisting of silver, silicon nitride, and air. This platform is homogeneous in two spatial directions and supports SPP at wavelength more than two times shorter than that in free space yet interacts with the outer world through the evanescent tail in air. We use an apertureless (scattering) near-field scanning optical microscope (NSOM) to map directly the amplitude and phase of these SW-SPP and show they can be focused to under 70 nm without structurally assisted confinement such as nanoantennas or nanofocusing. This, along with the use of visible light at 532 nm which is suitable for optical microscopy, can open new directions in direct biological and medical imaging at the sub-100 nm resolution regime.

Spatiotemporal focusing in opaque scattering media by wave front shaping with nonlinear feedback

We experimentally demonstrate spatiotemporal focusing of light on single nanocrystals embedded inside a strongly scattering medium. Our approach is based on spatial wave front shaping of short pulses, using second harmonic generation inside the target nanocrystals as the feedback signal. We successfully develop a model both for the achieved pulse duration as well as the observed enhancement of the feedback signal. The approach enables exciting opportunities for studies of light propagation in the presence of strong scattering as well as for applications in imaging, micro- and nanomanipulation, coherent control and spectroscopy in complex media.

Nanoscale shaping and focusing of visible light in planar metal–oxide–silicon waveguides

Focusing light at the nanoscale has become a key factor in super-resolution applications. Dynamic control of this focusing can open new avenues in nanoelectronics and bioimaging, but it requires a platform that merges electronics with super-resolution capabilities. We present a planar metal–oxide–silicon (MOS) platform that allows us to shape, tune, and focus visible light at the nanoscale by compressing the wavelength of light fourfold, resulting in a scaled diffraction limit of 65xa0nm. We exemplify the control and flexibility by demonstrating nanovortex beams and short-wavelength super-oscillations of light that further enhance resolution toward 35xa0nm. Our platform achieves focusing strength similar to nanoantennas but without structural hotspots; hence it is possible to scan the focus via optical wavefront-shaping techniques. Super-resolution scanning without mechanical translations in a MOS platform can provide a building block for bioimaging, nanolithography, and lab-on-a-chip applications.

Linearly dichroic plasmonic lens and hetero-chiral structures.

We present an experimental study of Hetero-Chiral (HC) plasmonic lenses, comprised of constituents with opposite chirality, demonstrating linearly dichroic focusing. The lenses focus only light with a specific linear polarization and result in a dark focal spot for the orthogonal polarization state. We introduce the design concepts and quantitatively compare several members of the HC family, deriving necessary conditions for linear dichroism and several comparative engineering parameters. The HC lenses were experimentally investigated using aperture-less near field scanning microscope collecting the amplitude of the plasmonic near-field. Our results exhibit an excellent match to the simulation predictions. The demonstrated ability for linearly dichroic functional focusing could lead to novel sensing applications.

Double moiré structured illumination microscopy with high-index materials.

Structured illumination microscopy utilizes illumination of periodic light patterns to allow reconstruction of high spatial frequencies, conventionally doubling the microscopes resolving power. This Letter presents a structured illumination microscopy scheme with the ability to achieve 60 nm resolution by using total internal reflection of a double moiré pattern in high-index materials. We propose a realization that provides dynamic control over relative amplitudes and phases of four coherently interfering beams in gallium phosphide and numerically demonstrate its capability.

Far-field measurements of short-wavelength surface plasmons

We present direct far-field measurements of short-wavelength surface plasmon polaritons (SPP) by conventional optics means. Plasmonic wavelength as short as 231u2009nm was observed for 532u2009nm illumination on a Ag−Si3N4 platform, demonstrating the capability to characterize SPPs well below the optical diffraction limit. This is done by scaling a sub-wavelength interferometric pattern to a far-field resolvable periodicity. These subwavelength patterns are obtained by coupling light into counter-propagating SPP waves to create a standing-wave pattern of half the SPP wavelength periodicity. Such patterns are mapped by a scattering slit, tilted at an angle so as to increase the periodicity of the intensity pattern along it to more than the free-space wavelength, making it resolvable by diffraction limited optics. The simplicity of the method as well as its large dynamic range of measurable wavelengths make it an optimal technique to characterize the properties of plasmonic devices and high-index dielectric wavegui...

Silicon super-resolution in the visible

Planar Silicon waveguides provide up to 4 times wavelength compression, yielding sub-100nm focusing for visible light. Our near-field measurements for red light (671nm) show both wavelength compression (280nm) and super-focusing (80nm) to both bright and dark spots.