Dragan Tanasković

Influence of impurity distribution on thermal coefficients of resistivity and piezoresistivity of diffused layers in silicon

This work presents a numerical investigation of impurity distribution changes caused by technological parameters variations and their influence on thermal sensitivity of piezoresistive pressure sensors. It is shown that the ratio of thermal coefficients of resistivity and piezoresistivity for a sensor may be optimized by a proper choice of process parameters. In this way the conditions are provided for simple passive temperature compensation. The goal of this work was to give a simplified and quick procedure for numerical prediction of the impurity profiles corresponding to optimum values of thermal coefficients of silicon pressure sensors. Contrary to the generally accepted theory, it was shown that the thermal coefficients depend not only on the impurity concentration on the surface, but on their distribution near the surface as well, which can be used in practical optimization of these coefficients.

A low-loss double-fishnet metamaterial based on transparent conductive oxide

In this work, we consider the applicability of transparent conductive oxide (TCO) materials such as indium tin oxide (ITO) for the fabrication of fishnet metamaterials. We utilized the finite element method to simulate the scattering parameters of the ITO-based fishnet metamaterial with rectangular aperture in a 300 nm-square unit cell. We further used effective extraction methods to retrieve the complex effective refractive index and the impedance of the metamaterial. We calculated the figure of merit defined as the ratio between the real and the imaginary parts of the effective dielectric permittivity of the ITO-based double-fishnet metamaterial and compared it with that of gold-based fishnets. We conclude that the use of TCO can decrease overall losses in fishnet metamaterial and increase the figure of merit. Our approach does not require the use of active media with external pumping and at the same time is applicable to any metamaterial geometry.

Scanning Probe-Shaped Nanohole Arrays with Extraordinary Optical Transmission as Platform for Enhanced Surface Plasmon-Based Biosensing

We analyzed the use of subwavelength ordered nanohole patterns for detection of miniscule amounts of biological analytes. Owing to their surface plasmon resonance (SPR) operation, such structures show an extraordinarily high optical transmission in visible spectrum, at the same time concentrating the illumination to the very small area of the nanoholes and being much more sensitive to refractive index changes than the conventional SPR sensors. We applied an approximate analytical approach to design our V-shaped, nanometer-sized hole arrays. We utilized scanning-probe nanolithography to fabricate the designed nanoholes in silver substrate. We investigated the topography of the obtained patterns by atomic force microscopy. Compared to conventional SPR optical sensors, the nanohole array-based structures allow for the detection of smaller quantities of analytes, enhance nonlinear effects and ensure improved sensitivities to different analytes

MEMS accelerometer with all-optical readout based on twin-defect photonic crystal waveguide

A novel design is proposed for a Si accelerometer sensor chip where a stress-tunable photonic crystal is used as the sensitive element to optically measure the deflection of a bulk-micromachined diaphragm with a boss. The photonic crystal may be 1D stack, 2D channel type, microbridge, etc. The photoelasticity effect is used instead of customarily utilized piezoresistivity and thus an all-optical readout is obtained. A feasibility analysis of the proposed structure is performed. 3D finite element modeling is used to determine the mechanical properties of the structure and the transfer matrix method to calculate its electromagnetic behavior. The advantages of the proposed concept are its simplicity, all-optical operation, monolithic integration with the sensor chip and compatibility with contemporary fiber-optic and integrated optics systems and devices.

Plasmonic metamaterial with fishnet superlattice for enhanced chemical sensing

In this work we consider a novel type of superlattice fishnet metamaterial for chemical sensing. By superimposing two fishnet metamaterial lattices we aim to achieve increased functionality compared to the standard fishnet metamaterials. We investigate spectral response of our structure to the changes in surrounding medium to be utilized in single wavelength readout chemical sensing. Additionally we compare the dispersion relation of plasmon modes in our superlattice fishnet metamaterial to the single lattice metamaterial and analyze the nature of coupling between plasmon modes. We present possibilities for multispectral sensing operation as well as for switching the resonant plasmon modes between sublattices by purely optical means.

Gradient-index infrared metamaterials based on metal-dielectric submicrometer pillar arrays

This paper presents the design and microfabrication of a two-dimensional metal-dielectric metamaterial structure based on an array of pillars with submicrometer diameters and heights. The diameters of pillars periodically vary along one axis in a sawtooth fashion and are constant along the other. The electromagnetic field distribution within this graded metamaterial was considered utilizing an accurate analytical approach. The pillar arrays were fabricated in photoresist and subsequently covered with a sputter-deposited aluminum film. Structures were defined by direct laser writing in photoresist film. Controlled overexposure has been applied in order to make pillar features smaller than the nominal resolution of the equipment. The structures were characterized by optical and atomic force microscopy and by angle-dependent Fourier Transform infrared spectroscopy. The produced graded frequency-selective surfaces may be used e.g. in sensing.

Enhancing performance of nanohole-based plasmonic sensors by transparent conductive oxides

We investigated a structure for surface plasmon-polariton-based chemical sensing consisting of a square 2D lattice of circular nanoapertures drilled in a plasmonic material on a dielectric substrate. Contrary to the usual approach, we did not consider opaque metal slabs, but instead investigated a thin film of transparent conductive oxide (TCO) as the plasmonic part. Thus we ensured a simultaneous use of activity of the TCO as chemical sensing material and of its Drude-type plasmonic behavior. The nanoaperture arrays in this case serve a dual function of a tunable plasmonic guide and of a grating-type coupler between surface plasmon plaritons and propagating modes. We performed FEM electromagnetic simulation of our structures used as sensors for the case of indium tin oxide (ITO). The proposed scheme exhibits a monolayer sensitivity to chemical or biological agents and is comparable to the previously proposed plasmonic sensors, including those based on extraordinary optical transmission nanoaperture arrays. It may seem surprising that the transmission difference caused by the presence of analyte was comparable to the values found in equivalent structures utilizing opaque metal. Lower absorption losses ensure higher overall transmission through the nanoaperture array and decreases the tunable transmission range, but simultaneosly opens a path toward the use of the proposed approach in alternative geometries, for instance for in-plane propagation utilizing endfire coupling.

Design of symmetric planar fishnet metamaterials for optical wavelength range

In this work we analyze fishnet-type optical metamaterials with full electromagnetic symmetry, fabricated in an ultrathin, freestanding film (self-supported nanomembrane). We utilized the finite element method to simulate the response of various laminar fishnet geometries, including metal-dielectric-metal and dielectric-metal-dielectric-metal-dielectric multilayers. We varied metal and dielectric thickness, nanoaperture radius, metal and dielectric materials. We considered the tunability of the spectral characteristics of the fishnet by bringing a dielectric analyte in contact with its exposed surfaces. Such tunability is of importance for all-optical sensors of chemical or biological analytes, but also for the performance of general fishnet nanoaperture arrays in real operating conditions, where the introduction of the ambient gases and fluids will modify their plasmonic properties.

Metal nanowire arrays with ultralow or negative effective permittivity for adsorption-based chemical sensing

We investigated wire-mesh media with plasmalike dispersion of dielectric permittivity as a potential medium for nanoplasmonic sensors utilizing adsorption of chemical, biochemical or biological analytes. Such structures belong to the ultra-low refractive index metamaterials and are typically used as one of the building blocks for zero- or negative index metamaterials. We analyzed a 2D array or conductive arrays with either a monomolecular/monatomic adlayer on the wire surface or fully immersed into analyte. We calculated the spectra reflection dip caused by the tuning of the effective electromagnetic properties of the wire mesh structure due to the adsorption of the analyte. We investigated the influence of various wire mesh media parameters, including conductivity, mesh geometry, etc., but also of the adsorbed analyte itself (effective optical properties, adlayer thickness). We considered the applicability of the structure in a reflection-based readout scheme without any special configurations for signal extraction.

Simple quasi-3D photonic crystal planar optical waveguides

Reports work on simple photonic crystal-based planar optical waveguides with enhanced characteristics. Leaky modes are suppressed and the overall performance improved by a combination of 2D planar photonic bandgap waveguides and 1D omnidirectional stacks, furnishing quasi-3D behavior. Only conventional planar technology and materials are used for fabrication.