Volodymyr Tkachenko

• Ph.D.
• Researcher at Italian National Research Council

A set of protein biomarkers are largely recognized as responsible of neurodegeneration mechanisms and hence as potential targets to be detected in low abundant concentrations in body fluids for performing early diagnosis. As an example, the Tau protein experiences a transition phase from a native disorder conformation into a preaggregation state, w...

To meet the growing demand for early fatal disease screening among large populations, current fluorescence detection instruments aiming at point-of-care diagnosis have the tendency to be low cost and high sensitivity, with a high potential for the analysis of low-volume, multiplex analytes with easy operation. In this work, we present the developme...

The characterization of pyroelectric materials is essential for the design of pyroelectric-based devices. Pyroelectric current measurement is the commonly employed method, but can be complex and requires surface electrodes. Here, we present noncontact electrostatic voltmeter measurements as a simple but highly accurate alternative, by assessing the...

The detection of low abundant biomarkers is of great interest in different clinical cases, such as the early diagnosis of Alzheimer's disease. Here we present what we call pin-based pyro-electrohydrodynamic jet (p-jet) sensor that can linearly tune the accumulation rate of biomolecules in tiny droplets. In this newly revealed pin-based configuratio...

In digital holography (DH) modality for lab on chip application the cells passing through the field of view (FOV) of the microscope can be detected and analyzed even if they are flowing at different depths. In fact, in-focus imaging of each cell can be easily retrieved thanks to the ability of DH to obtain numerical focus ex-post the recording proc...

The appearance of the first laser approximately 12 years after the invention of holography by Gabor (1948) revolutionized the field of optical metrology. In fact, the invention of holographic interferometry enabled the exploitation of interferometry on non-mirror surfaces and full-scale objects. The holography-based measurement methods has been imp...

It is well known how the quantification of tiny amounts of molecular species in biological assays is a crucial issue. Herein, a deep investigation on a pyroelectrohydrodynamic jet system is presented in aim to demonstrate its capability to accumulate a large number of tiny aqueous droplets on the surface of a target slide with high efficiency. It m...

Hypothesis Thin liquid films are important in many scientific fields. In particular, films with both the surface layers exposed to a different fluid phase, known as freestanding films, are relevant in the ambit of foams and emulsions. Hence, there is a great interest in developing novel techniques allowing to form large and stable freestanding liqu...

The demand for sensors capable of measuring low-abundant collagen in human fluids has highly increased in recent years. Indeed, collagen is expected to be a biomarker for chronic diseases and could monitor their progression. Here we show detection of highly diluted samples of collagen at picogram level thanks to an innovative pyro-electrohydrodynam...

We investigate a hybrid re-configurable three dimensional metamaterial based on liquid crystal as tuning element in order to build novel devices operating in the terahertz range. The proposed metadevice is an array of meta-atoms consisting of split ring resonators having suspended conducting cantilevers in the gap region. Adding a “third dimensi...

We have studied the formation of topological defects in liquid crystal (LC) matrices induced by multiwalled carbon nanotubes (MWCNTs) and external electric fields. The defects are ascribable to a distortion of the LC molecular director in proximity of the MWCNT surface. The system is analyzed macroscopically using spectroscopic variable angle ellip...

We investigate new classes of artificial materials exhibiting unconventional properties in order to build novel devices operating in the Terahertz regime. The design, fabrication and electromagnetic characterization of tunable metamaterials with unit cells based on Split Ring Resonators are presented. By incorporation of a nematic liquid crystal in...

The aim of this work is to investigate new classes of artificial materials exhibiting unconventional properties in order to build novel devices operating in the Terahertz regime. We focus on the design, fabrication and characterization of tunable metamaterials with unit cells based on Split Ring Resonators. By incorporation of a nematic liquid crys...

We study the presence of dark and bright modes in a planar metamaterial with a double rod unit cell introducing geometric asymmetry in rod lengths. The dark mode displays a Fano-type resonance with a sharp asymmetric profile, rendering it far more sensitive than the bright mode to slight variations of the dielectric environment. This peculiar featu...

We study the sharp Fano-type resonance in a dimer metamaterial based on nanorods with different lengths. Breaking the length symmetry results in the excitation of a dark mode that weakly couples to the free space. Interference between the dark mode and the higher frequency bright mode gives rise to the peculiar asymmetric and sharp profile of the r...

Spectral characteristics of the interference optical filter based on a free-standing mesoporous silicon film containing nematic liquid crystal E7 are studied experimentally. The porous structure represents two distributed Bragg reflectors divided by a quarter-wave microcavity having resonance near 1600 nm. Transmission spectra of the filter are mea...

We report on the growth mode of N,N′-bis (n-octyl)-dicyanoperylenediimide (PDI-8CN2) on sexithiophene (T6) thin films, studied with different structural, morphological and optical techniques. We aim to individuate the most favorable conditions for the realization of heterostructure devices. The crystalline quality was established by X-ray patterns...

We simulate spectra of the optical microcavity based on a composite (mesoporous silicon)/(nematic liquid crystal). The incident angle-dependent calculations predict the transmission peak blueshift and splitting of transverse electric and transverse magnetic polarized modes due to birefringence of both porous host material and the anisotropic filler...

We present an outline on the use of ellipsometry for nematic liquid crystals characterization. Possibility of anisotropic refractive indices measurement with accuracy up to 10−4 is demonstrated in the visible and near infrared wavelength ranges for different temperatures. Liquid crystal director distributions were measured both in cells with and wi...

We describe an experimental procedure for accurate measurement of anchoring energy strength of liquid crystal cells. This technique is based on the possibility of gathering a large amount of very precise data about the linear optical response of the cell in different experimental conditions, using spectroscopic ellipsometry. Then, a careful data an...

We have investigated properties of the nematic liquid crystal mixture E7 confined in thin porous films fabricated by electrochemical etch of silicon wafers. The use of spectroscopic ellipsometry is proposed for deriving information about volume fraction, effective ordinary and extraordinary refractive indices and preferred director orientation of t...

We present the design of a thermally tunable interference filter for the NIR range. The filter is based on a free-standing porous silicon multilayer filled with the nematic liquid crystal mixture E7. The porous silicon stack is constituted by a quarter-wave microcavity sandwiched between two distributed Bragg reflectors. Reversible thermal tuning o...

The director tilt angle distribution in vertically aligned nematic liquid crystal displays has been investigated by means of variable angle spectroscopic ellipsometry. Liquid crystal vertical alignment has been realized by thermal evaporation of SiO _x . By changing the deposition angle, it is possible to control the pretilt angle. The di...

Multiple-beam holography has been widely used for the realization of photonic quasicrystals with high rotational symmetries not achievable by conventional periodic crystals. Accurate control of the properties of the interfering beams is necessary to provide photonic band-gap structures. Here we show, by finite difference time domain (FDTD) simulati...

A nonzero pretilt angle is generally used in vertical aligned (VA) mode liquid crystal (LC) displays to avoid domain formation during molecular reorientation by electric field, as well as to reduce both driving voltage and response time. The pretilt effect on electro-optical properties of a VA LC cell has been investigated. Pretilt was controlled c...

Free-standing multilayer porous silicon (PS) film containing half-wave microcavity is installed into a planar glass cell and infiltrated with a nematic liquid crystal (LC) mixture E7. Here we present the results of theoretical and experimental research of the LC orientational properties and spectral characteristics of the microcavity under variable...

We present the design of an interference filter containing a free-standing porous silicon (PS) multilayer film with a microcavity. The estimated pore diameter is 40 – 70 nm. The PS film is installed into a planar glass cell and infiltrated with a nematic liquid crystal (LC) mixture E7. The resonant wavelength of the structure is continuously tuned...

Recently important efforts have been dedicated to the realization of a new kind of photonic crystals, known as photonic quasicrystals, in which the lack of the translational symmetry is compensated by rotational symmetries not achievable by the conventional periodic crystals. Here we show a novel approach to their fabrication based on the use of a...

We present the simulated distribution of the local director of a nematic liquid crystal inside cylindrical macropores under the influence of an electric field. The Frank free energy approach is used to describe the nematic behavior. The influence of both molecular anchoring strength and pore radius is investigated. The results of this analysis are...

We present the simulation results for the local director distribution of a nematic liquid crystal (NLC) confined inside cylindrical macropores of porous silicon (PS). The Frank free energy approach is used to describe the equilibrium configuration of the NLC in porous media. The influence of the molecular anchoring strength and of an externally app...

We present the simulated distribution of the local director of nematic liquid crystal (NLC) which is confined inside cylindrical macropores of the porous silicon. The Frank free energy approach is used to describe the NLC in porous medium. The influence of molecular anchoring strength and external electric field applied is investigated. Bruggeman a...

We present a survey on organic composite and hybrid materials suitable for a new family of optical devices like thermo- and electro-tunable multilayer mirrors, microcavities and optical filters. The main component is a tunable and/or switchable Bragg grating. Two different classes of materials have been realized, characterized, and designed in form...

In this paper, we present our results for the anisotropic refractive index measurements of commonly used liquid crystal (LC) materials (E7 and 5CB) in the whole visible-near infrared range. In order to achieve a high accuracy in the obtained values, we have employed a combination of two techniques, namely, Mueller matrix spectroscopic ellipsometry...

Standard multiple-beam holography has been largely used to produce gratings in polymer-liquid crystal composites, like POLICRYPS, H-PDLC gratings and POLIPHEM [11. (a) Sutherland , R. L. , Natarajan , L. V. , Tondiglia , V. P. , Bunning , T. J. , & Adams , W. W. ( 1994 ). Appl. Phys. Lett. , 64 , 1074 . [CrossRef], [Web of Science ®]View all refere...

Porous silicon (PS) based devices are nowadays under an intense and widespread investigation in the optoelectronic and sensing fields. Recently, the range of possible applications has been widened by the use of the liquid crystals (LCs), which can be infiltrated in the PS sponge-like structure. The large changes of the optical properties, as exhibi...

The design and fabrication of active and passive hybrid photonic devices, with tunable optical properties, based on liquid crystals require a very accurate knowledge of their anisotropic refractive indices up to 100 ppm in the wavelength region of interest. At this aim, the authors have integrated two standard optical techniques commonly used in li...

A commercial variable angle spectroscopic ellipsometer from J. A. Woollam Company is modified in order to make possible measurements in guiding structures. The half leaky guided mode technique is realised for liquid crystal optical dispersion measurements. Features and problems related to experimental data interpretation and error sources in the re...

Soft materials exhibit very attractive mechanical, optical, and electro-optical properties and easy processing techniques. Combined with holographic techniques, they can be patterned quite easily creating many unique structures. We present here few examples, of electrically-switchable passive opto-electronic components based on composite materials...

Microcavity based on macroporous silicon infiltrated with liquid crystals is described theoretically using the proposed model. We use construction of electrical contacts, which allows apply electrical field perpendicularly to the pore axis. Various cases of the pore size and different pre-orientation of liquid crystal molecules are considered. Refl...

In this work microcavity model of tunable VCSEL is produced. The microcavity structure with active region limited by liquid crystal phase layers. Polarization independence is achieved by setting different preorientation of liquid crystal molecules in two layers. The resonance wavelength tuning possibility of such microcavity is investigated. Estima...

In this paper, the Bragg mirrors manufactured of the macroporous silicon infiltrated with liquid crystals are investigated. The reflectance spectrum of the structure is calculated using a transfer matrix method. The influence of the applied alternating field on reflectance spectrum of the structure is considered. The comparative analysis of the Bra...

We have studied the electro-optical and angular behavior of holographic-polymer-dispersed liquid crystal gratings at different wavelengths, in the visible and in the near-infrared range. As usual in these kinds of materials, a strong polarization dependent behavior was observed. Our samples showed very high diffraction efficiency for p-polarized ra...

In view of POLICRYPS (an acronym for polymer-liquid crystal-polymer slices) optimisation, this study analyses the dynamical response of POLICRYPS gratings and its relation with some geometrical and morphological factors. The final goal is to get overall response times in the sub-millisecond range. Results show that even for not-optimised grating sa...

We have studied the dynamical response of a new kind of holographic grating to a pulsed electric field. The structure, named POLICRYPS, consists of a sequence of homogeneous polymer slices separated by liquid-crystal films. We have found that both the rise and fall response times depend on the pulse duration and amplitude of the applied electric fi...

In this work, we propose spectroscopic ellipsometry as a suitable method for measuring optical properties in soft materials, polymers and liquid crystals, specially selected for use in photonics applications. We show the results of our measurements on some multilayered samples, in the range from visible to the near-IR region, of interest for teleco...

We have studied the dynamical response of a new polymer-liquid crystal composite, realised in a form of a thick grating. This structure, named POLICRYPS, consists in an alternate sequence of homogeneous polymer slides and liquid crystal stripes. Optimisation of response time is an important factor to propose POLICRYPS as basic element in different...