Norbert Palka

Military University of Technology | WAT · Institute of Optoelectronics

The interaction between condensed matter excitations and electromagnetic cavity fields serves as a rich playground for fundamental research and lies at the core of photonic and quantum technologies. Herein, the intriguing concept of composite states formed by distinct quasiparticles strongly coupled to the same optical cavity modes is experimentall...

We report on our experimental characterization of index-guiding photonic crystal fibers (PCF) from their far field intensity distribution. The algorithm presented be-low makes it possible to determine the geometrical pa-rameters of the PCF (core diameter, air hole spacing and air hole diameter) from its far field pattern. We obtained good agreement...

The main aim of this chapter is to present spectral characteristics of clothing textiles in the THz range. Such characteristics are important for understanding the interaction of the THz radiation with person wearing clothes as well as for design of modern THz body scanners. The proposed TDS-based setup provided automated textiles scanning both in...

The time domain spectroscopy (TDS) setup in a reflection configuration was exploited for non-destructive measurements of four samples with a 10-mm (approx.) leaf-shaped air void at the interface between the plastic and metal structures. We identified and measured the material parameters of the samples required for precise determination of the voids...

Recent advances in THz detection with the use of CMOS technology have shown that this option has the potential to be a leading method of producing low-cost THz sensors with integrated readout systems. This review paper, based on authors' years of experience, presents strengths and weaknesses of this solution. The article gives examples of some hint...

We report on the absorption properties of polarization-insensitive transmissive and reflective metamaterial absorbers based on two planar aluminium periodic structures and SU-8 epoxy resist. These absorbers were investigated using numerical simulation and experimental methods in the terahertz range (below 2 THz). SU-8 is a very promising organic ma...

In this work we present phase shifters with cholesteric liquid crystal (ChLC) for the terahertz (THz) range with fast modulation. The phase shift of presented phase shifters is equal ∼π at 2.5 THz. The characterization of the phase shifters was done by using terahertz time-domain spectroscopy from 0.3 THz up to 3 THz. Also electro-optical measureme...

ABSTRACTIn this paper, we present the results of the terahertz measurements of liquid crystal (LC) ferroelectric BaTiO3 nanoparticles (nps) suspensions in the range of frequency from 0.3 up to 3.0 THz. Two different sol-gel methods and the harvesting technique were used to fabricate the nanoparticles. Five LC materials served as hosts for the suspe...

Currently, artillery shells and grenades that are introduced into the market are based on melt-castable insensitive high explosives (IHEs), which do not explode while they run a risk of impact, heat or shrapnel. Particles of explosives (such as hexogen, nitroguanidine and nitrotriazolone) are suspended in different proportions in a matrix of 2.4-di...

Fast and intensive development of terahertz (THz) technology requires designing dedicated optical elements. Here, we generated complicated THz field distribution thanks to simple binary diffractive optical elements (DOEs) made from paper. The paper fabricated structures are lightweight, inexpensive, feasible for fast-prototyping purposes and they h...

We investigated six prospective explosive materials in the terahertz range using time-domain spectroscopy. A family of energetic azotetrazolate salts and two caged nitramines were studied. A number of distinct spectral features were observed in the 0.8–3.2 THz frequency range. In transmission configuration in ambient temperature, we determined the...

We report on the terahertz time-of-flight analysis of an internal structure of an ultra-high molecular weight polyethylene (UHMWPE) composite material, which is based on the BT10-tape from Dyneema®. This type of composite is very hard and resistant and therefore it is often used to manufacture personal armors such as bulletproof vests and helmets....

An ultra-high molecular weight polyethylene composite sample has been non-destructively imaged via X-ray and Terahertz waves. The sample stopped a projectile, which resulted in delaminations, a bulge, and a chamber. Although the X-ray computed tomography reliably showed all interesting internal features of the sample, the axial resolution was moder...

Results of a theoretical and experimental investigation of THz detection of randomly-oriented metal particles smaller than 1 mm embedded in pressboard are reported and critically compared with X-ray scanner results. A theoretical description based on Gaussian beam theory and a transfer function approach enabled simulation of signals transmitted aro...

Absorption spectra of explosives such as hexahydro-1,3,5-trinitro- 1,3,5-triazine (RDX), pentaerythritol tetranitrate (PETN), and mixtures of both were measured by terahertz time domain spectroscopy (THz-TDS). Chemometrics was applied to quantitative analysis of terahertz absorbance spectra obtained in transmission mode to predict the relative amou...

In this study common clothing and variety of textile materials were used in research on its influence on remote materials identification. Experimental setup was designed for the terahertz reflection spectroscopy of different materials located at a distance up to 5 m. The source of the radiation is a tunable solid-state optical parametric oscillator...

Detection of explosives has always been a priority for homeland security. Jointly, terahertz spectroscopy and imaging are emerging and promising candidates as contactless and safe systems. In this work, we treated data resulting from hyperspectral imaging obtained by THz-time domain spectroscopy, with chemometric tools. We found efficient identific...

THz-TDS techniques are applied to investigate selected pharmaceutical samples. Investigations were performed on selected pharmaceutical samples with active pharmaceutical ingredients (API) - famotidine, ranitidine, fenofibrate, lovastatin, simvastatin, aspirin, ketoconazole, acyclovir (hydrated and non-hydrated), on excipients - lactose, glucose (h...

In this work we study the properties of E7, Liquid Crystal (LC), doped with ferroelectric nanoparticles (NPs) in terahertz (THz) range. BaTiO3 is a ferroelectric material, which as a compound for a hybrid LC/NPs, has demonstrated increase of the dielectric anisotropy and the birefringence of a LC material in the optical regime [1]. The birefringenc...

An ultra-high molecular weight polyethylene composite sample totally punctured by a projectile was examined by THz TDS raster scanning method in reflection configuration. The scanning results correctly match the distribution of delaminations inside the sample, which was proven with cross-sectional and frontal views after waterjet cutting. For furth...

The multilayer structure of an ultra-high molecular weight polyethylene composite material was investigated in the terahertz (THz) spectral range by means of time domain spectroscopy (TDS) technique. Such structures consist of many alternating layers of fibres (∼150), each being perpendicular to the other and each having a thickness of about 50 μm....

Diffractive optical elements such as hyperbolic lens and quasi-spherical lens converting a divergent terahertz beam into the focal line segment perpendicular to the optical axis was designed. Due to the fact that the length of the line segment was longer than the aperture of the designed elements the non-paraxial approach was used. The structures w...

Chemometric analysis was applied on terahertz absorbance 3D images, in transmission. The goal is to automatically discriminate some explosives on images and quantify mixtures of RDX/PETN in the frequency range of 0.2–3 THz. Partial Least Square (PLS) was applied on THz absorbance multispectral images to quantify individual product inside pure sampl...

We show that a hybrid LC-ferroelectric nanoparticle suspension of liquid crystal E7 doped with BaTiO3 nanoparticles leads to 10% increase in birefringence in the THz region of spectrum as compared to pure E7. Doped liquid crystals can be used to increase performance of THz modulators and waveplates. BaTiO3 nanoparticles used in the mixture were syn...

The application of Glass Fibre Reinforced Polymers (GFRP) in many branches of industry has been increasing steadily. Many research works focus on damage identification for structures made out of such materials. However, not only delaminations, cracks or other damage can have a negative influence of GFRP parts performance. Previous research proved t...

The multilayer structure of an ultra-high molecular weight polyethylene (UHMWPE) composite material was investigated in the terahertz (THz) spectral range by means of time-domain spectroscopy (TDS) technique. Such structures consist of many alternating layers of fibers, each being perpendicular to the other and each having a thickness of about 50 μ...

In this paper we present an extensive theoretical and numerical analysis of monolithic high-index contrast grating, facilitating simple manufacture of compact mirrors for very broad spectrum of vertical-cavity surface-emitting lasers (VCSELs) emitting from ultraviolet to mid-infrared. We provide the theoretical background explaining the phenomenon...

Terahertz and infrared radiation have unique properties applicable to the field of surveillance and security systems. We investigated the possibility of detecting potentially dangerous objects covered by various types of clothing using passive imagers operating at 1.2 mm (250 GHz) and long-wavelength infrared at 6–15 μm (20–50 THz). We developed a...

We report research results with regard to AlGaAs/GaAs structure processing for THz quantum-cascade lasers (QCLs). We focus on the processes of Ti/Au cladding fabrication for metal–metal waveguides and wafer bonding with indium solder. Particular emphasis is placed on optimization of technological parameters for the said processes that result in wor...

THz beam shaping via a single diffractive optical element is used to convert a divergent beam into a focal line segment perpendicular to the optical axis. The novel structure was designed for narrowband applications as a kinoform element and we successfully applied it in active, high-speed, THz linear scanners. The theoretical approach and experime...

This work presents a terahertz system designed for the reflection spectroscopy of different materials located at a distance up to 5 m. The source of the radiation is a tunable solid-state optical parametric oscillator, which generates a narrow-band nanosecond pulses in the range of 0.7-2.5 THz. The signal is detected with relatively fast and having...

This paper presents results of computer simulation of 1D monolithic high refractive index contrast grating (MHCG) reflector also called surface grating reflector (SGR). We analyzed optical properties of the GaAs reflector designed for 980 nm wavelength with respect to the grating parameters variation. We also determined the electric field patterns...

We present extensive numerical analysis of a high-contrast grating VCSEL emitting at 980 nm. Using a three-dimensional, fully vectorial optical model, we investigate the influence of a non-uniform grating with a broad range of geometrical parameters on the modal behavior of the VCSEL. Properly designed and optimized, the high-contrast grating confi...

The polymorphism of resorcinol has been complementary studied by combining Raman, time-domain terahertz and inelastic neutron scattering spectroscopy with modern solid-state density functional theory (DFT) calculations. The spectral differences, emerging from the temperature-induced structural phase transition, have been successfully interpreted wi...

We report on the terahertz analysis of the internal structure of ultra-high molecular weight polyethylene composite materials. The visualisation of interfaces between the perpendicularly arranged plies and the identification of stacking faults is possible because the THz refractive index depends on the orientation of fibres in the plies. Thicknesse...

Terahertz radiation is within the frequency range from 100 GHz to 10THz. This radiation has specific characteristics in terms of imaging. The radiation is harmless to the human body because the energy transferred by electromagnetic waves in this range of frequencies are very small thus there is no ionization of matter. The development of imaging de...

We present an experimental and numerical analysis of a planar metamaterial that selectively absorbs terahertz radiation. The metamaterial is produced as a planar two-dimensional network of electromagnetic resonators and is characterized by electric and magnetic responses in a narrow frequency range. The unit cell of the metamaterial has fourfold sy...

We report our research on processing of AlGaAs/GaAs structures for THz quantum-cascade lasers (QCLs). We focus on the processes of fabrication of Ti/Au claddings for metal-metal waveguides and the wafer bonding with indium solder. We place special emphasis on the optimum technological conditions of these processes, leading to working devices. The w...

Risks to the safety of public zones (generally available for people) are related mainly to the presence of hidden dangerous objects (such as knives, guns, etc.) and their usage. Modern system for the monitoring of such zones attempt to detect dangerous tools using multispectral cameras working in different spectral ranges: the visible radiation, ne...

In this paper two different methods of the manufacture of metamaterial structures using a laser treatment were presented. One of them is related to direct metallization of the surface of aluminum nitride ceramics (AlN) by interaction of laser radiation. The second method involves a process of selective removal of the conductive layer (silver) in th...

Detection of concealed dangerous objects is a very demanding problem of public safety. So far, the problem of detecting objects hidden under clothing was considered only in the case of airports but it is becoming more and more important for public places like metro stations, and government buildings. The development of imaging devices and explorat...

We present results on the comparison of different THz technologies for the detection and identification of a variety of explosives from our laboratory tests that were carried out in the framework of NATO SET-193 “THz technology for stand-off detection of explosives: from laboratory spectroscopy to detection in the field” under the same controlled c...

We report on the terahertz analysis of an internal structure of an ultra-high molecular weight polyethylene (UHMWPE) composite material, which is based on the HB10-tape from Dyneema $^{\circledR }$ . This type of composite is very hard and resistant and therefore it is often used to manufacture personal armors such as bulletproof vests and helmet...

We report on the developed system for the detection of the objects hidden under people’s clothes. The system is based on a commercially available THz camera equipped with specially designed software for image processing and fusion. Additionally, a camera for visible light registers the image that is later processed and combined with the THz image b...

In this paper a method for producing resonant structures using laser micromachining is presented. In the spot of laser beam impact on AlN ceramics surface a conductive aluminum layer is formed. Compilation of process parameters allows for the fabrication of structures with resistance at Rs ~ 0.01Ω/Rs. It has been also found out that the maximum val...

We discuss thin optical structures that allow chromatic aberrations to be avoided in the THz domain. The paper contains the theoretical considerations, computer modeling and experimental evaluation of the high order kinoform diffractive elements in the THz range. According to the obtained results application of the high order kinoforms enables broa...

We propose a specially designed THz diffractive element for imaging purposes. The designing process was performed to obtain low attenuation and small weight. The element is optimized for broadband application, especially for remote detection of harmful materials. It provides low both chromatic and geometric aberrations.

Passive THz imaging devices have big potential for solution of the security problem. Nevertheless, one of the main problems, which take place on the way of using these devices, consists in modest image quality, produced by the majority of passive THz cameras. To change this situation, it is necessary to improve the engineering characteristics (reso...

Assuring the safety of crowded public places like e.g. airports is one of the most serious social issues. One of the aspects of the problem is the detection of potentially dangerous objects hidden under a person’s clothing. There are various solutions from simple metal detectors to X-ray scanners. Some of the available solutions are harmful to huma...

The terahertz (THz) band has lately become very interesting for scientists mainly because of its specific properties. One of the potential applications of THz devices is the security area. Imaging devices working in the THz range can be used to detect objects hidden under clothes. Since most of the commercially available THz cameras offer low quali...

Terahertz imaging, is the latest entry into the crowded field of imaging technologies. Many applications are emerging for the relatively new technology. THz radiation penetrates deep into nonpolar and nonmetallic materials such as paper, plastic, clothes, wood, and ceramics that are usually opaque at optical wavelengths. The T-rays have large poten...

We report on technical aspects connected with detection of the terahertz (THz) waves reflected from a small target which is situated at the distance of 5 meters. Details of experimental setup are presented. An optical parametric oscillator (OPO) was used as a THz nanosecond pulses radiation source and a hot-electron bolometer (HEB) was applied for...

Driver-PC (camera calibration image processing) Display (visualization) Pan-tilt-zoom IR camera Abstract — Terahertz imaging, is the latest entry into the crowded field of imaging technologies. Many applications are emerging for the relatively new technology. THz radiation penetrates deep into nonpolar and nonmetallic materials such as paper, plast...

Terahertz imaging makes it possible to acquire images of objects concealed underneath clothing by measuring the radiometric temperatures of dierent objects on a human subject. Finding objects hidden under clothes is one of key issues of public places security. A possible solution of this problem is a multispectral, terahertz-based screening system...

A growing interest in terahertz technology finds support in a large number of applications. One of the most interesting applications of terahertz waves is imaging. The terahertz range of electromagnetic radiation has large potential in the field of hidden objects detection because it is not harmful to humans [1, 2]. However, the main difficulty in...

The paper presents results of research on a reflection measurement of a Hexogen (RDX) sample from 5 m distance. An experimental setup consisted of a tuning optical parametric oscillator and a hot electron bolometer. The study was carried out with presence of water vapor in the air. The reflectivity of RDX is similar to the reflectivity measured by...

A numerical study of the electromagnetic properties of a liquid crystal-tunable metamaterial absorber (LCTMA) is presented. This artificial structure is characterized by an efficient absorption of terahertz electromagnetic radiation in a narrow frequency range. The selective absorption in the metamaterial is tuned by reconfiguration of a thin liqui...

The p-spectrum method is a time domain spectroscopy-based technique for reection analysis of materials (including explosives) that takes into account only part of the reflected signal, which is next analyzed via the Fourier transform. The method is limited to the frequencies of about 1.8 THz due to the pulse properties of our time domain spectrosco...

This paper presents an optical system for transmission spectral measurements in the terahertz range. The source of radiation is a tunable solid-state laser, which generates a narrow-band nanosecond pulses in the range 0.67- 2.68 THz and based on optical parametric oscillator phenomenon. The signal is detected with a fast and sensitive hot electron...

We report on measurements of transmission spectra of chosen materials (Hexogen, sugar, L-tartaric acid, 4-aminobenzoic acid) in the range 0.7-2.0 THz. The measurements were carried out by means of a setup, which bases on the optical parametric oscillator (OPO) combined with the hot electron bolometer (HEB). The setup consists of the commercially av...

We report on a new method for extracting the characteristic features of covered materials, including Hexogen, in the range 0.5–1.8 THz. This time domain spectroscopy-based technique takes into account only part of the signal reflected from a covered sample, and analyzes it by Fourier transform. The obtained power spectrum has distinctive peaks that...

The SDA (Spectral Dynamics Analysis) method is used for the detection and identification of the PWM C4 explosive with the surface having inhomogeneity, caused by action of the sandpaper with different grit on the explosive surface, or with the surface having various curvature of its surface. We show that the SDA-method is good tool for the detectio...

A growing interest in terahertz technology finds support in a large number of applications. One of the most interesting applications of terahertz waves is imaging. The terahertz range of electromagnetic radiation has large potential in the field of hidden objects detection because it is not harmful to humans. However, the main difficulty in the THz...

We report on a method for extracting the characteristic features of covered materials, including hexogen, in the range from 0.5 to 1.8 THz. This time-domain spectroscopy-based technique takes into account only part of the signal reflected from a covered sample and analyzes it by using the Fourier transform. The obtained power spectrum has distincti...

We report on construction and evaluation of a moving wirelessly-controlled thermal phantom for testing cameras working in the terahertz range (0.1-3.0THz). A terahertz camera operating at 0.25 THz and a standard thermal camera were used for tests. We compared the images taken for the phantom and a man and we obtained satisfactory similarities both...

In this paper, we report on measurements of absorption spectra of hexogen, penthrite, and octogen. The measurements were carried out by means of time domain spectroscopy and compared to spectra obtained from a setup, which bases on an optical parametric oscillator combined with a Golay cell. For time domain spectroscopy measurement, Teraview TPS 30...

In this paper, we report on measurements of reflection spectra of hexogen (RDX) by means of time domain spectroscopy. We analyzed both phase and amplitude of reflected impulses for three cases: detuning, samples with rough surfaces and samples with curved surfaces. It can be concluded that both spectral characteristics can be applied for identifica...

The application of pattern recognition methodology within chemistry, biology and other science domains, especially in security systems is becoming more and more important. Many classification algorithms are available in literature but decision trees are the most commonly exploited because of their ease of implementation and understanding in compari...

Detection of nanosecond pulses by fast and sensitive Hot Electron Bolometer (HEB) is reported. Pulses were generated by an Optical Parametric Oscillator (OPO)-based source. The laser can be tuned in the range 0.7-2.5 THz; its repetition rate equals to 53Hz, duration of the pulse is about 10-20ns, energy is 10nJ and spectral width 50GHz. HEB operate...

Detection of about 20 ns pulses by a fast and sensitive NbN-based Hot Electron Bolometer is reported. The pulses were generated by an Optical Parametric Oscillator operating in a non-collinear, intersecting cavity configuration. We present details of the oscillator and bolometer as well as the measurement setup. The linearity of the system was meas...

The influence of smoke on THz imaging was examined. We tested experimentally a passive THz camera - TS4 from ThruVision Systems Ltd. The camera detects natural human radiation at a frequency of 0.25THz. The distance camera-target could be 3-15 meters. Moreover, Time Domain Spectroscopy was used to investigate the influence of the amount of smoke on...