Taavi Repän

Dark-field hyperlens: Super-resolution imaging of weakly scattering objects

We propose a device for subwavelength optical imaging based on a metal-dielectric multilayer hyperlens designed in such a way that only large-wavevector (evanescent) waves are transmitted while all propagating (small-wavevector) waves from the object area are blocked by the hyper-lens. We numerically demonstrate that as the result of such filtering, the image plane only contains scattered light from subwavelength features of the objects and is completely free from background illumination. Similar in spirit to conventional dark-field microscopy, the proposed dark-field hyperlens is shown to enhance the subwavelength image contrast by more than two orders of magnitude. These findings are essential for optical imaging of weakly scattering subwavelength objects, such as real-time dynamic nanoscopy of label-free biological objects.

High aspect ratio titanium nitride trench structures as plasmonic biosensor

High aspect ratio titanium nitride (TiN) grating structures are fabricated by the combination of deep reactive ion etching (DRIE) and atomic layer deposition (ALD) techniques. TiN is deposited at 500 °C on a silicon trench template. Silicon between vertical TiN layers is selectively etched to fabricate the high aspect ratio TiN trenches with the pitch of 400 nm and height of around 2.7 μm. Dielectric functions of TiN films with different thicknesses of 18 – 105 nm and post-annealing temperatures of 700 – 900 °C are characterized by an ellipsometer. We found that the highest annealing temperature of 900 °C gives the most pronounced plasmonic behavior with the highest plasma frequency, ωp = 2.53 eV (λp = 490 nm). Such high aspect ratio trench structures function as a plasmonic grating sensor that supports the Rayleigh-Woods anomalies (RWAs), enabling the measurement of changes in the refractive index of the ambient medium in the wavelength range of 600 – 900 nm. We achieved the bulk refractive index sensitivity (BRIS) of approximately 430 nm/RIU relevant to biosensing liquids.

Optical reconfiguration and polarization control in semi-continuous gold films close to the percolation threshold

Controlling and confining light by exciting plasmons in resonant metallic nanostructures is an essential aspect of many new emerging optical technologies. Here we explore the possibility of controllably reconfiguring the intrinsic optical properties of semi-continuous gold films, by inducing permanent morphological changes with a femtosecond (fs)-pulsed laser above a critical power. Optical transmission spectroscopy measurements show a correlation between the spectra of the morphologically modified films and the wavelength, polarization, and the intensity of the laser used for alteration. In order to understand the modifications induced by the laser writing, we explore the near-field properties of these films with electron energy-loss spectroscopy (EELS). A comparison between our experimental data and full-wave simulations on the exact film morphologies hints toward a restructuring of the intrinsic plasmonic eigenmodes of the metallic film by photothermal effects. We explain these optical changes with a simple model and demonstrate experimentally that laser writing can be used to controllably modify the optical properties of these semi-continuous films. These metal films offer an easy-to-fabricate and scalable platform for technological applications such as molecular sensing and ultra-dense data storage.

Surface waves on metal-dielectric metamaterials

In this paper we analyze surface electromagnetic waves supported at an interface between an isotropic medium and an effective anisotropic material that can be realized by alternating conductive and dielectric layers with deep subwavelength thicknesses. This configuration can host various types of surface waves and, therefore, can serve as a platform allowing many applications for surface photonics. Most of these surface waves are directional and their propagation direction is sensitive to permittivities of the media forming the interface. Hence, their propagation can be effectively controlled by changing a wavelength or material parameters. We discover that two new types of surface waves with complex dispersion exist for a uniaxial medium with both negative ordinary and extraordinary permittivities. Such new surface wave solutions originate from the anisotropic permittivities of the uniaxial media, resulting in unique hyperbolic-like wavevector dependencies.

Aluminum-doped Zinc Oxide Trench Hyperbolic Metamaterial as a Mid-infrared Sensing Platform

We demonstrate enhancement of infrared absorption of 5 nm thick silica layer in nanotrench structures that function as hyperbolic metamaterials. Such structures can serve as a highly sensitive platform for mid-infrared absorption spectroscopy. OCIS codes: (250.5403) Plasmonics; (160.4760) Optical properties; (160.3918) Metamaterials; (220.4241) Nanostructure fabrication; (280.4788) Optical sensing and sensors The mid-infrared (IR) spectroscopy in the wavelengths region between 2.5 25 μm (4000 – 400 cm −1 ) is a powerful tool to detect molecules and chemical bonds due to their particular absorption bands in this range [1-3]. It enables a variety of potential applications from gas sensing for environmental monitoring to medical and clinical diagnoses. However, due to the huge mismatch between the mid-infrared light wavelength and analyte molecules dimensions, which are typically on the order of several nanometers, it is challenging to detect very trace amounts of molecules by detecting their absorption signatures. Fig. 1. (a) Cross-sectional SEM images of AZO-based HMM structures and bird’s eye view of the structures. (b) A schematic illustrations of AZO trench structures with 5 nm thick SiO2. Hereby we report the use of hyperbolic metamaterials (HMMs) based on aluminum-doped zinc oxide (AZO) nanotrench structures as a sensing platform for the enhancement of molecular absorption of mid-IR light. HMMs are artificially designed structures that possess unusual indefinite dispersion in a certain region of frequencies and exhibit the hyperbolic shape of the isofrequency contours in the wavevector space [4]. AZO exhibits a plasmonic response, possessing a negative real part of the permittivity in the nearand mid-IR wavelength regions [5-7]. The structures are composed of multiple high aspect ratio (1:6.7) sub-wavelength AZO trenches on a Si substrate, Fig.1(a), providing 14.5 times more surface area for residing of analyte molecules than the flat surface. The whole structure is fabricated by the combination of deep UV lithography, dry etching, and ALD technique for AZO deposition, resulting in the uniform formation of deep trenches on a large scale area (2 × 2 cm 2 ). The fabrication process for the trench structures is fully compatible with the large-scale CMOS technology. Such AZO trench HMM structures support both surface waves and bulk plasmon waves in the broad wavelength range in midIR [7]. SeW2E.5.pdf Advanced Photonics Congress (BGPP, IPR, NP, Networks, NOMA, Sensors, SOF, SPPCom)

Numerical simulations of nanostructured gold films

We present an approach to analyse near-field effects on nanostructured gold films by finite element simulations. The studied samples are formed by fabricating gold films near the percolation threshold and then applying laser damage. Resulting samples have complicated structures, which then was captured using scanning transmission electron microscopy (STEM) and the obtained dark field images are used to set up COMSOL simulations corresponding to actual structures.

Gold micro- and nano-particles for surface enhanced vibrational spectroscopy of pyridostigmine bromide

Abstract Triangular gold microprisms and spherical silica nanoparticles with attached gold nano-islands were examined as an active nanostructures for the surface enhanced Raman and infrared spectroscopy. These particles were probed for the detection of pyridostigmine bromide as a safe analog of military compound sarin. Raman and infrared spectral bands of the pyridostigmine bromide were measured. Detailed correlation of obtained spectral bands with specific vibrations in pyridostigmine bromide was done. Silica nanoparticles with attached gold nano-islands showed more essential enhancement of the Raman signal than gold microcrystals. The reasons of such behaviour are discussed.

Dark-field hyperlens for high-contrast sub-wavelength imaging

By now superresolution imaging using hyperbolic metamaterial (HMM) structures – hyperlenses – has been demonstrated both theoretically and experimentally. The hyperlens operation relies on the fact that HMM allows propagation of waves with very large transverse wavevectors, which would be evanescent in common isotropic media (thus giving rise to the diffraction limit). However, nearly all hyperlenses proposed so far have been suitable only for very strong scatterers – such as holes in a metal film. When weaker scatterers, dielectric objects for example, are imaged then incident light forms a very strong background, and weak scatterers are not visible due to a poor contrast. We propose a so-called dark-field hyperlens, which would be suitable for imaging of weakly scattering objects. By designing parameters of the HMM, we managed to obtain its response in such way that the hyperlens structure exhibits a cut-off for waves with small transverse wavevectors (low-k waves). This allows the structure to filter out the background illumination, which is contained in low-k waves. We numerically demonstrate that our device achieves superresolution imaging while providing the strong contrast for weak dielectric scatterers. These findings hold a great promise for dark-field superresolution, which could be important in real-time dynamic nanoscopy of label-free biological objects for example.