Norik Janunts

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.

Light propagation in a free-standing lithium niobate photonic crystal waveguide

We report on the light propagation in a one-line-defect photonic crystal waveguide (W1 PhC WG) patterned into a 450 nm thick free-standing lithium niobate membrane by ion-beam enhanced etching. The Bloch wave vectors and transmission spectrum of this PhC WG were retrieved from optical near-field images. The experimental data show good agreement with simulations performed with the three-dimensional (3D) finite-element method and the 3D finite-difference time-domain method. Those results are promising for the development of integrated optics devices operating at telecom wavelengths and based on free-standing lithium niobate PhC membranes.

Doubly resonant optical nanoantenna arrays for polarization resolved

We report that rhomb-shaped metal nanoantenna arrays support multiple plasmonic resonances, making them favorable bio-sensing substrates. Besides the two localized plasmonic dipole modes associated with the two principle axes of the rhombi, the sample supports an additional grating-induced surface plasmon polariton resonance. The plasmonic properties of all modes are carefully studied by far-field measurements together with numerical and analytical calculations. The sample is then applied to surface-enhanced Raman scattering measurements. It is shown to be highly efficient since two plasmonic resonances of the structure were simultaneously tuned to coincide with the excitation and the emission wavelength in the SERS experiment. The analysis is completed by measuring the impact of the polarization angle on the SERS signal.

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.

Blistering during the atomic layer deposition of iridium

The authors report on the formation of blisters during the atomic layer deposition of iridium using iridium acetylacetonate and oxygen precursors. Films deposited on fused silica substrates led to sparsely distributed large blisters while in the case of silicon with native oxide additional small blisters with a high density was observed. It is found that the formation of blisters is favored by a higher deposition temperature and a larger layer thickness. Postdeposition annealing did not have a significant effect on the formation of blisters. Finally, changing purge duration during the film growth allowed us to avoid blistering and evidenced that impurities released from the film in gas phase were responsible for the formation of blisters.

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.

Controlling the excitation of radially polarized conical plasmons in plasmonic tips in liquids

Having virtues from plasmons and scanning probe microscopy (SPM), plasmonic tips employ radially polarized conical plasmons and create hot-spots at their apexes. Plasmonic tips are tapered and fully metal-coated vortex fibers that have M-shaped refractive index profiles. Vortex fibers allow the radially polarized mode to propagate over a long distance with high modal purity. When the fiber mode reaches the tapered region, it resonantly excites the plasmon mode at a metal-dielectric outer interface. In this paper, we study the plasmonic tips behavior in liquids both theoretically and experimentally. By adiabatically tapering the vortex fiber, the radially polarized mode gets confined from the fiber with a diameter of 115 μm down to the tapered part with a diameter of 1.42 μm as a waveguide mode. In this region, the plasmon mode gets excited thus reaches the apex with a diameter of 200 nm. Our calculations show that the plasmon coupling efficiency increases in liquids due to two competing processes: a significant increase of the interaction region and slight decrease of the penetration depth of fields in metal. By choosing a liquid that either allows or forbids the phase-matching, we demonstrate that the plasmon coupling efficiency can increase or vanish. Due to the wetting effect, a tapered liquid-layer forms over the tip like an additional waveguide and allows resonant coupling of fiber modes to the liquid layer.

Plasmonic tip based on excitation and superfocusing of the radially polarized surface plasmon polaritons(Conference Presentation)

Scanning Near-field Optical Microscopy (SNOM) is an essential tool in nano-optics and plasmonics. Among many variants of SNOM, a plasmonic tip is a new type of SNOM tip that is based on a resonant excitation and a superfocusing of a radially polarized conical surface plasmon polariton (SPP). The plasmonic tip is made of a tapered and fully metal-coated M-profiler fiber tips. An M-profile fiber guides the radially polarized fiber mode securely to the tapered region of the tip where it resonantly excites the radially polarized plasmon mode. This resonant excitation process allows us to have higher energy conversion efficiency that is up to 70% for 50 nm gold coating thickness from far-field to near-field than other SNOM tips like aperture tips (0.01% for 100 nm aperture). As the radially polarized plasmon mode further propagates towards the apex, its’ intensity increases anomalously, and its’ phase velocity decreases. Thus, the plasmon gets localized longitudinally as well as transversally due to the SPP nature. This phenomenon is known as a superfocusing of SPP, and in conical structure, it happens only for the fundamental radially polarized mode in the region where the tip radius is smaller than 50 nm. In this study, we introduce the plasmonic tip and explore the plasmon excitation process on a planar gold surface by plasmonic tips and circular aperture SNOM tips to understand the tip emission behavior in near-field. In the experiment, we use ring gratings that are milled on a planar gold surface and place a tip at the center of the structure to excite a planar SPP that propagates toward the grating and gets scattered. By imaging the scattered light through the grating, we study the plasmon excitation pattern and deduce the near-field at the apex. An emission through a small metal aperture (~10 nm) is well explained by Bethe theory that states that the near-field emission resembles that of a dipole. However, for an aperture tip with an aperture as large as 100 nm, we demonstrated that the dipole approximation well describes the excited SPP as long as a linearly polarized single mode is guided within the tip. When the aperture gets larger, the guided light within the tip becomes multimode; thus, the dipole approximation is no longer valid although the tip far-field emission looks like a Gaussian mode. For the plasmonic tip, we showed the emission can be approximated that of an out-of-plane dipole (oscillating perpendicular to the surface) despite the size of the apex. This method allows us also estimate the tilt of a tip with respect to the sample surface and purity of guided mode within the tip, and these information are essential for interpreting the detected signal from the sample. In conclusion, we introduce the plasmonic tip as an efficient SNOM tip due to its resonant excitation of SPP and superfocusing processes, and studied the near-field excitation characteristics in comparison with the conventional aperture tips.