Highly Confined and Switchable Mid-Infrared Surface Phonon Polariton Resonances of Planar Circular Cavities with a Phase Change Material.

Mid-infrared (MIR) photonics demands highly confined optical fields to obtain efficient interaction between long-wavelength light and nanomaterials. Surface polaritons excited on polar semiconductor and metallic material interfaces exhibit near-fields localized on subwavelength scales. However, realizing a stronger field concentration in a cavity with a high quality ( Q) factor and a small mode volume is still challenging in the MIR region. This study reports MIR field concentration of surface phonon polaritons (SPhPs) using planar circular cavities with a high Q factor of ∼150. The cavities are fabricated on a thin film of the phase change material Ge3Sb2Te6 (GST) deposited on a silicon carbide (SiC) substrate. Scattering-type scanning near-field optical microscopy visualizes the near-field distribution on the samples and confirms directly that the SPhP field is strongly concentrated at the center of the centrosymmetric cavities. The smallest concentrated field size is 220 nm in diameter which corresponds to 1/50 of the wavelength of the incident light that is far below the diffraction limit. The thin GST film enhances the SPhP confinement, and it is used to switch the confinement off by tuning the cavity resonance induced by the phase change from the amorphous to the crystalline phase. This subwavelength and switchable field concentration within a high- Q polariton cavity has the potential to greatly enhance the light-matter interaction for molecular sensing and emission enhancement in MIR systems.

Exploring the detection limits of infrared near-field microscopy regarding small buried structures and pushing them by exploiting superlens-related effects.

We present a study on subsurface imaging with an infrared scattering-type scanning near-field optical microscope (s-SNOM). The depth-limitation for the visibility of gold nanoparticles with a diameter of 50 nm under Si3N4 is determined to about 50 nm. We first investigate spot size and signal strength concerning their particle-size dependence for a dielectric cover layer with positive permittivity. The experimental results are confirmed by model calculations and a comparison to TEM images. In the next step, we investigate spectroscopically also the regime of negative permittivity of the capping layer and its influence on lateral resolution and signal strength in experiment and simulations. The explanation of this observation combines subsurface imaging and superlensing, and shows up limitations of the latter regarding small structure sizes.

Visual search as a tool for a quick and reliable assessment of cognitive functions in patients with multiple sclerosis.

BACKGROUND: Despite the high frequency of cognitive impairment in multiple sclerosis, its assessment has not gained entrance into clinical routine yet, due to lack of time-saving and suitable tests for patients with multiple sclerosis. OBJECTIVE: The aim of the study was to compare the paradigm of visual search with neuropsychological standard tests, in order to identify the test that discriminates best between patients with multiple sclerosis and healthy individuals concerning cognitive functions, without being susceptible to practice effects. METHODS: Patients with relapsing remitting multiple sclerosis (n = 38) and age-and gender-matched healthy individuals (n = 40) were tested with common neuropsychological tests and a computer-based visual search task, whereby a target stimulus has to be detected amongst distracting stimuli on a touch screen. Twenty-eight of the healthy individuals were re-tested in order to determine potential practice effects. RESULTS: Mean reaction time reflecting visual attention and movement time indicating motor execution in the visual search task discriminated best between healthy individuals and patients with multiple sclerosis, without practice effects. CONCLUSIONS: Visual search is a promising instrument for the assessment of cognitive functions and potentially cognitive changes in patients with multiple sclerosis thanks to its good discriminatory power and insusceptibility to practice effects.