Evgeniy Shkondin

Fabrication of high aspect ratio TiO2 and Al2O3 nanogratings by atomic layer deposition

The authors report on the fabrication of TiO2 and Al2O3 nanostructured gratings with an aspect ratio of up to 50. The gratings were made by a combination of atomic layer deposition (ALD) and dry etch techniques. The workflow included fabrication of a Si template using deep reactive ion etching followed by ALD of TiO2 or Al2O3. Then, the template was etched away using SF6 in an inductively coupled plasma tool, which resulted in the formation of isolated ALD coatings, thereby achieving high aspect ratio grating structures. SF6 plasma removes silicon selectively without any observable influence on TiO2 or Al2O3, thus revealing high selectivity throughout the fabrication. Scanning electron microscopy was used to analyze every fabrication step. Due to nonreleased stress in the ALD coatings, the top parts of the gratings were observed to bend inward as the Si template was removed, thus resulting in a gradual change in the pitch value of the structures. The pitch on top of the gratings is 400 nm, and it graduall...

Large-scale high aspect ratio Al-doped ZnO nanopillars arrays as anisotropic metamaterials.

High aspect ratio free-standing Al-doped ZnO (AZO) nanopillars and nanotubes were fabricated using a combination of advanced reactive ion etching and atomic layer deposition (ALD) techniques. Prior to the pillar and tube fabrication, AZO layers were grown on flat silicon and glass substrates with different Al concentrations at 150-250 °C. For each temperature and Al concentration the ALD growth behavior, crystalline structure, physical, electrical and optical properties were investigated. It was found that AZO films deposited at 250 °C exhibit the most pronounced plasmonic behavior with the highest plasma frequency. During pillar fabrication, AZO conformally passivates the silicon template, which is characteristic of typical ALD growth conditions. The last step of fabrication is heavily dependent on the selective chemistry of the SF6 plasma. It was shown that silicon between AZO structures can be selectively removed with no observable influence on the ALD deposited coatings. The prepared free-standing AZO structures were characterized using Fourier transform infrared spectroscopy (FTIR). The restoration of the effective permittivities of the structures reveals that their anisotropy significantly deviates from the effective medium approximation (EMA) prognoses. It suggests that the permittivity of the AZO in tightly confined nanopillars is very different from that of flat AZO films.

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.

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)

Highly ordered transparent conductive oxide nanopillar metamaterials for mid-infrared plasmonics

Plasmonics for the mid-infrared wavelength regime offers unique applications such as molecular vibrational absorption spectroscopy for bio-sensing [1]. The quest for novel materials and structures has been continuing in the last decade [2]. Meanwhile, the fabrication of plasmonic metamaterial structures with large area in a reproducible manner is a tremendous challenge. In this report we show that highly ordered nanopillar structures made of aluminum-doped ZnO (AZO) can work as plasmonic components in the mid-infrared wavelength region.

Laguerre-Gauss beam generation in IR and UV by subwavelength surface-relief gratings

The angular momentum of light can be described by the states of spin angular momentum, associated with polarization, and orbital angular momentum, related to the helical structure of the wave front. Laguerre-Gaussian beams carry orbital angular momentum and their generation can be done by using an optical device known as q-plate. However, due to the usage of liquid crystals, these components may be restricted to operate in specific wavelengths and low power sources. Here we present the fabrication and characterization of q-plates made without liquid crystals, using processes of electron beam lithography, atomic layer deposition and dry etch techniques. We exploit the phenomenon of form birefringence to give rise to the spin-to-orbital angular momentum conversion. We demonstrate that these plates can generate beams with high quality for the UV and IR range, allowing them to interact with high power laser sources or inside laser cavities.

Advanced fabrication of hyperbolic metamaterials

Hyperbolic metamaterials can provide unprecedented properties in accommodation of high-k (high wave vector) waves and enhancement of the optical density of states. To reach such performance the metamaterials have to be fabricated with as small imperfections as possible. Here we report on our advances in two approaches in fabrication of optical metamaterials. We deposit ultrathin ultrasmooth gold layers with the assistance of organic material (APTMS) adhesion layer. The technology supports the stacking of such layers in a multiperiod construction with alumina spacers between gold films, which is expected to exhibit hyperbolic properties in the visible range. As the second approach we apply the atomic layer deposition technique to arrange vertical alignment of layers or pillars of heavily doped ZnO or TiN, which enables us to produce hyperbolic metamaterials for the near- and mid-infrared ranges.

Effective medium approximation for deeply subwavelength all-dielectric multilayers: when does it break down?

We report on theoretical analysis and experimental validation of the applicability of the effective medium approximation to deeply subwavelength (period ⩽λ/30) all-dielectric multilayer structures. Following the theoretical prediction of the anomalous breakdown of the effective medium approximation [H. H. Sheinfux et al., Phys. Rev. Lett. 113, 243901 (2014)] we thoroughly elaborate on regimes, when an actual multilayer stack exhibits significantly different properties compared to its homogenized model. Our findings are fully confirmed in the first direct experimental demonstration of the breakdown effect. Multilayer stacks are composed of alternating alumina and titania layers fabricated using atomic layer deposition. For light incident on such multilayers at angles near the total internal reflection, we observe pronounced differences in the reflectance spectra (up to 0.5) for structures with different layers ordering and different but still deeply subwavelength thicknesses. Such big reflectance difference values resulted from the special geometrical configuration with an additional resonator layer underneath the multilayers employed for the enhancement of the effect. Our results are important for the development of new homogenization approaches for metamaterials, high-precision multilayer ellipsometry methods and in a broad range of sensing applications.

Fabrication of deep-profile Al-doped ZnO one- and two-dimensional lattices as plasmonic elements

In this work, we report on fabrication of deep-profile one- and two-dimensional lattices made from Al-doped ZnO (AZO). AZO is considered as an alternative plasmonic material having the real part of the permittivity negative in the near infrared range. The exact position of the plasma frequency of AZO is doping concentration dependent, allowing for tuning possibilities. In addition, the thickness of the AZO film also affects its material properties. Physical vapor deposition techniques typically applied for AZO coating do not enable deep profiling of a plasmonic structure. Using the atomic layer deposition technique, a highly conformal deposition method, allows us to fabricate high-aspect ratio structures such as one-dimensional lattices with a period of 400 nm and size of the lamina of 200 nm in width and 3 μm in depth. Thus, our structures have an aspect ratio of 1:15 and are homogeneous on areas of 2×2 cm2 and more. We also produce two-dimensional arrays of circular nanopillars with similar dimensions. Instead of nanopillars hollow tubes with a wall thickness on demand from 20 nm up to a complete fill can be fabricated.