Angela E. Klein

Generation and near-field imaging of Airy surface plasmons

We demonstrate experimentally the generation and near-field imaging of nondiffracting surface waves, plasmonic Airy beams, propagating on the surface of a gold metal film. The Airy plasmons are excited by an engineered nanoscale phase grating, and demonstrate significant beam bending over their propagation. We show that the observed Airy plasmons exhibit self-healing properties, suggesting novel applications in plasmonic circuitry and surface optical manipulation.

Controlling plasmonic hot spots by interfering Airy beams.

We predict and demonstrate the generation of a plasmonic hot spot on the surface of a metal film by the interference of two Airy surface plasmons. We show that the position of the hot spot can be controlled by the distance between the excitation gratings as well as by the phase front of the initial excitation. The observed effect constitutes a planar analogy to Airy beam autofocusing and offers new opportunities for spatially resolved surface plasmon sensing and optical surface tweezers.

Polarization-Resolved Near-Field Mapping of Plasmonic Aperture Emission by a Dual-SNOM System

We study the polarization characteristics of light emission and collection in the near field by the tips of a Dual-SNOM (two scanning near-field optical microscopes) setup. We find that cantilevered fiber probes can serve as emitters of polarized light, or as polarization-sensitive detectors. The polarization characteristics depend on the fiber type used for tip fabrication. In Dual-SNOM measurements, we demonstrate mapping of different field components of the plasmonic dipole pattern emitted by an aperture probe.

Image formation properties and inverse imaging problem in aperture based scanning near field optical microscopy

Aperture based scanning near field optical microscopes are important instruments to study light at the nanoscale and to understand the optical functionality of photonic nanostructures. In general, a detected image is affected by both the transverse electric and magnetic field components of light. The discrimination of the individual field components is challenging as these four field components are contained within two signals in the case of a polarization resolved measurement. Here, we develop a methodology to solve the inverse imaging problem and to retrieve the vectorial field components from polarization and phase resolved measurements. Our methodology relies on the discussion of the image formation process in aperture based scanning near field optical microscopes. On this basis, we are also able to explain how the relative contributions of the electric and magnetic field components within detected images depend on the chosen probe. We can therefore also describe the influence of geometrical and material parameters of individual probes within the image formation process. This allows probes to be designed that are primarily sensitive either to the electric or magnetic field components of light.

Enhancing resonances of optical nanoantennas by circular gratings.

Optical plasmonic antennas allow for localizing and enhancing light at the nanoscale. To enhance the application opportunities of optical antennas, their quality factor needs to be substantially improved. Here, we numerically and experimentally demonstrate that the resonance of a dipolar metallic disc antenna can be enhanced by a circular grating that obeys the Bragg condition. The supporting grating effectively collects energy from an extended spatial domain and guides it spectrally-selected into the central antenna, leading to a significantly enhanced field intensity at resonance. Accordingly, the quality factor of the antenna is enhanced by at least five times. The approach can be applied to other plasmonic systems, hence constituting an important ingredient to a future plasmonic tool box.

Characterization of a circular optical nanoantenna by nonlinear photoemission electron microscopy

AbstractWe report on the investigation of an advanced circular plasmonic nanoantenna under ultrafast excitation using nonlinear photoemission electron microscopy (PEEM) under near-normal incidence. The circular nanoantenna is enhanced in its performance by a supporting grating and milled out from a gold film. The considered antenna shows a sophisticated physical resonance behaviour that is ideal to demonstrate the possibilities of PEEM for the experimental investigations of plasmonic effects on the nanoscale. Field profiles of the antenna resonance for both possible linear polarizations of the incident field are measured with high spatial resolution. In addition, outward-propagating Hankel plasmons, which are also excited by the structure, are measured and analysed. We compare our findings to measurements of an isolated plasmonic nanodisc resonator and scanning near-field optical microscopy measurements of both structures. All results are in very good agreement with numerical simulations as well as analytical models that are also discussed in our paper.

Highly sensitive mode mapping of whispering-gallery modes by scanning thermocouple-probe microscopy

We propose a method for mapping optical near-fields with the help of a thermocouple scanning-probe microscope tip. As the tip scans the sample surface, its apex is heated by light absorption, generating a thermovoltage. The thermovoltage map represents the intensity distribution of light at the sample surface. The measurement technique has been employed to map optical whispering-gallery modes in fused silica microdisk resonators operating at near-infrared wavelengths. The method could potentially be employed for near-field imaging of a variety of systems in the near-infrared and visible spectral range.

Airy Plasmons: Bending Light on a Chip

Surface plasmons have the unique capacity to confine light to very small dimensions, but the losses that occur when they propagate have limited their utility.

Non-diffracting airy surface plasmons: generation, manipulations, and interference

We review our activities on the generation and manipulation of Airy surface plasmons. We demonstrate the excitation of surface Airy plasmon polaritons, study the interference of two Airy surface waves and methods for trajectory steering.

Near-field mapping of Airy plasmons

The field of plasmonics experiences an explosive growth with a number of developing applications in biosensing, particle manipulation and photonic circuitry. This development has motivated the emerging field of plasmon optics dealing with the manipulation and engineering of plasmon beams. Airy beams represent an important class of non-diffracting wavepackets [1] which evolution in space (or time) resembles curved trajectories. While known for decades in several fields of physics, the optical Airy beams have only recently been observed experimentally [2]. Importantly, for one-dimensional wavepackets, such as surface plasmon waves, the Airy beams represent the only possible class of non-diffracting beams. Very recently it was predicted theoretically [3] that one-dimensional self-accelerating beams can exist in the form of surface Airy plasmons, however such plasmons have never been demonstrated experimentally. Here we present the first experimental generation and direct observation of Airy plasmons on the surface of a gold metal film.