Optical skyrmion lattice in evanescent electromagnetic fields.

Topological defects play a key role in a variety of physical systems, ranging from high-energy to solid-state physics. A skyrmion is a type of topological defect that has shown promise for applications in the fields of magnetic storage and spintronics. We show that optical skyrmion lattices can be generated using evanescent electromagnetic fields and demonstrate this using surface plasmon polaritons, imaged by phase-resolved near-field optical microscopy. We show how the optical skyrmion lattice exhibits robustness to imperfections while the topological domain walls in the lattice can be continuously tuned, changing the spatial structure of the skyrmions from bubble type to Néel type. Extending the generation of skyrmions to photonic systems provides various possibilities for applications in optical information processing, transfer, and storage.

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.

Focusing and scanning microscopy with propagating surface plasmons.

Here we demonstrate a novel surface plasmon polariton (SPP) microscope which is capable of imaging below the optical diffraction limit. A plasmonic lens, generated through phase-structured illumination, focuses SPPs down to their diffraction limit and scans the focus with steps as small as 10 nm. This plasmonic lens is implemented on a metallic nanostructure consisting of alternating hole array gratings and bare metal arenas. We use subwavelength scattering holes placed within the bare metal arenas to determine the resolution of our microscope. The resolution depends on the size of the scanning SPP focus. This novel technique has the potential for biomedical imaging microscopy, surface biology, and functionalization chemistry.

Optical control of plasmonic Bloch modes on periodic nanostructures.

We study and actively control the coherent properties of surface plasmon polaritons (SPPs) optically excited on a nanohole array. Amplitude and phase of the optical excitation are externally controlled via a digital spatial light modulator (SLM) and SPP interference fringe patterns are designed and observed with high contrast. Our interferometric observations reveal SPPs dressed with the Bloch modes of the periodic nanostructure. The momentum associated with these dressed plasmons (DP) is highly dependent on the grating period and fully matches our theoretical predictions. We show that the momentum of DP waves can, in principle, exceed the SPP momentum. Actively controlling DP waves via programmable phase patterns offers the potential for high field confinement applicable in lithography, surface enhanced Raman scattering, and plasmonic structured illumination microscopy.