This PDF file contains the front matter associated with SPIE Proceedings Volume 11164, including the Title Page, Copyright information, Table of Contents, Author and Conference Committee lists

Our recent studies in the human skin signatures indicate a strong correlation between the human skin emissivity and factors such as the body mass index, the gender, the age, and the ethnicities of the participants. The key innovation in this is in recognising that signatures from the human body enable regions of the body to be identified as skin. This will enable increased the detection probabilities of anomalies, and reduced the false alarm rates in security screening portals. This is a capability that is being demanded internationally by governments and in the UK by the Home Office Future Aviation Security Solutions (FASS) programme and the Joint Security and Resilience Centre (JSaRC).

In this paper, radiometric measurements conducted on human skin in the millimetre wave band region (80-100) GHz show variation in the human skin emissivity before and after conducting physical activity (jogging) subject to the same participant. The measurements were conducted on the palm of the hand and the back of the hand skin. The measurements reveal that the emissivity of the skin is significantly lower in the rest state of the body compared with the active state by mean values of 0.088 and 0.07 for the palm of the hand and the back of the hand skin respectively. The differences in the mean emissivity values were found to be linked to the length of time exercising and the hydration level of the skin i.e. (sweat). Radiometric measurements on palms of the hand and on the back of the hand skin before and after the application of an aqueous gel indicate a strong correlation between the human skin signature and the hydration level of the skin. The mean differences in emissivity values before and after the application of an aqueous gel indicate a scatter in the range of 0.02 to 0.26. These findings suggested trends in the human skin emissivity and indicate the potential of a new non-contact passive method for remote sensing of the physical state of human beings. Understanding these signatures and variations of the human skin emissivity are very important for both security screening (anomalies detection) and medical applications (non-invasive diagnosis of human body).

The characterization of dielectric materials is of great importance for many applications, being for instance quality control during product fabrication or status control of outside constructions over time. In many outside situations the objects of interest have limited accessibility, and the investigation has to be done without destruction of any part of the object. The use of microwaves, millimeter-waves or THz waves offers some penetration capability into matter, depending on its chemical and physical decomposition and of course frequency. Many objects of interest consist of a dielectric coating or enclosure, which can electromagnetically be treated as a dielectric layered structure or a dielectric slab surrounded by air. In the recent past an approach has been established using polarimetric millimeter-wave radiometry for extracting the real part of the permittivity of surfaces. Hereby an imaging radiometer scanner has been used to measure the first three components of the Stokes vector of a scene containing various targets of interest. Those are for instance plane surfaces being tilted such that cold sky radiation is reflected towards the radiometer. It was shown experimentally that reasonable values for the permittivity of such surfaces could be extracted. However, the estimation of possible error sources and their impact on the permittivity results are important to quantify the achievable quality of the method and its limits. This paper illustrates a first error analysis based on measurement results.

Terahertz technology exploits the so-called frequency gap between the infrared and microwaves, typically referred to as the frequency band from 100 GHz to 30 Terahertz. The use of techniques based on the Terahertz radiation has long been studied in fields such as astronomy and solid state physics. Being a non harmful radiation for human beings, the Terahertz radiation is very interesting for applications since it can be used without worrying for the safety of users and operators. Recent innovations in Terahertz technologies are bringing a wide variety of applications, from non-destructive evaluation, to homeland security, from quality control of food and agricultural products to the biomedical sector.

We report about an implementation of a sub-Terahertz investigation setup based on commercial solutions. The system is currently being used for non-invasive detection of metals inside parcels and boxes in security applications and in vivo studies of water leaf contents in smart agriculture.

THz TDS is one of tools, which can be effective for the security problem solution. As a rule, the detection and identification of substance is based on the spectral fingerprints: absorption frequencies of substance. To detect the substance, it is necessary to compare its absorption frequencies with those from database which contains the absorption frequencies of various substance. Therefore, a role of this data is one of the keys for the problem solution. As follows from analysis of papers, the measurements of the absorption frequencies were provided by using broadband THz pulse. Due to essential non-stationary response of a medium as well as broadband spectrum of the THz pulse, in the spectrum of pulse, transmitted through a substance, many additional frequencies appear. Among them maybe appear “false” absorption frequencies, appearance of which is caused by frequency conversion processes, as well as the frequencies, corresponding to a substance emission because of a high energy levels excitation due to the cascade mechanism. Both these mechanisms of the spectrum transformation strongly depend on the incident pulse duration.
Another additional factor resulting in observing of the false absorption frequencies is disordered (or ordered) packing for the substance. In THz range of frequencies, the most part of the packing is a disordered photonic structure and developing of the strategy to avoid its influence on the efficiency of detection and identification of substance is a key problem. We discuss once more approach for solving this problem in this paper.

Comparison of atmospheric transmission in the bands ranging from terahertz (THz) to infrared (IR) bands using MODTRAN and available literature data show that THz radiation is not dramatically attenuated in adverse conditions when compared to IR bands especially at long distances. Favorable THz bands and mechanisms for detection of low IR signature target under adverse conditions in these bands are determined. In this study, a set of experiments are conducted to compare THz detection mechanisms, which are THz detectors and IR microbolometer cameras. Typically, IR Microbolometer cameras are not sensitive enough to detect THz waves and their sensitivity needs to be improved. While THz microbolometer cameras are being continuously improved the use of a THz to IR converter intermediary component allows a cost-effective solution to the problem. A THz to IR converter based on a metasurface absorber is studied in this regard. The effectiveness of the device is simulated and the temperature difference that can be detected with the IR microbolometer camera is modeled based on heat transfer equations.

In the THz range, high-T_C superconductor (HTS) hot electron bolometers (HEB) are offering a competitive alternative to moderately cooled (e.g., 60 to 80 K) Schottky mixers. This is due to HTS HEBs large expected bandwidth (tens of GHz), and low local oscillator power requirements (tens of microwatts, as compared to several milliwatts required for Schottky diode pumping). Indeed, the large instantaneous bandwidth is driven by the very short electron to phonon relaxation time in Y-Ba-Cu-O HTS oxide − 1 to 2 ps, typically, whereas it is about 20 ns in NbN, a low- T_C superconductor (LTS). Besides, as for the LTS counterparts, it is mandatory to grow ultra thin high quality HTS epitaxial films, in order to process micro or nano-bolometers (nano-constrictions) exhibiting good mixing performances. Early HEB models were based on the point bolometer approach, which describes the device in terms of thermal reservoirs only. We have extended the hot spot model (initially introduced for LTS HEBs) to Y-Ba-Cu-O HEBs, taking into account the spatial dependence of the electron and phonon temperatures along the nano-constriction. We have also introduced the THz frequency effects in the Y-Ba-Cu-O superconducting transition as well as the impedance matching between the nanoconstriction and the antenna. We have checked the feasibility of stand-off target detection operating in the passive mode with an Y-Ba-Cu-O HEB THz heterodyne mixer. For instance, detection at 5 m through cotton cloth in passive imaging mode could be readily achieved in standard humidity conditions with 10 K resolution at 2.5 THz.

Since the observation of pyroelectric properties in oxygen depleted semiconducting Y-Ba-Cu-O, the interest of its amorphous phase (a-YBCO) obtained at low deposition temperature (150°C) has been demonstrated for near-infrared (NIR) detection. In the first part of this paper, we investigate material aspects of a-YBCO thin films (surface morphology, electrical transport and optical properties) for a better understanding of the microstructure vs. conductivity relationship. In the second part, we report on the NIR characterization of planar and trilayer detector devices fabricated on silicon substrates. These detectors exhibit a very fast response (time constant τ = 1.9 μs for planar device; τ = 0.12 μs for trilayer device) as compared to commercially available pyroelectric sensors. The best noise equivalent power (NEP) and detectivity D* , which are at the state of art, were observed at 10 kHz modulation frequency: NEP = 2.0 pW/Hz1/2 and D* = 6.6×10⁹ cm·Hz^1/2/W for planar device; NEP = 2.6 pW/Hz^1/2 and D* = 5.7×10⁹ cm·Hz^1/2/W for trilayer device. We have interpreted this fast response by means of an analytical model without adjustable parameters. In the third part, the potential of THz detection is examined, in the case of a-YBCO coupling to a planar antenna. The general coupling conditions of the THz incident radiation to a-YBCO are examined first, with relation to the film THz absorption coefficient and conductivity measured by time-resolved spectroscopy. The coupling conditions of the film to the readout circuitry are then examined, with relation to the Schottky nature of the metal/a-YBCO contacts.

Today, the fact that non-ionizing THz and mm wave can be used in many areas reveals the necessity of devices that can efficiently detect these waves. Compared to existing commercial detectors, cost-effective neon indicator lamps acting as glow discharge detectors (GDDs) are known to be better in speed and responsivity. In GDDs, detection occurs as a result of the interaction between radiation and plasma. Although there are many studies in the literature trying to explain this interaction, these studies are limited to qualitative explanations made as a result of the experiments and the analytical models. Since GDDs are low pressure, non-local thermal equilibrium (non-LTE) plasma lamps, the plasma in these detectors has been simulated by using the parallel 1d3v Particle in Cell/Monte Carlo Collision (PIC/MCC) code, which can be used to study the kinetic behavior of particles. The interaction between plasma and THz/mm wave have also been included to the simulation code in order to examine the effect of this interaction. The results show that not only the electrical but also the optical detection mechanism that has been recently shown using GDDs could be analyzed with this code. Therefore, in this study, the reason for the brighter glow observed as a result of plasma-THz/mm wave interaction is investigated. In this context, the effect of THz/mm wave on one of excited states responsible for the light emitted by GDD is simulated and the results are discussed.

The two-phonon scheme of generation of terahertz (THz) photons by gold nanobars (GNBs) is considered. It is shown that in GNBs, by choosing their sizes, it is possible to provide conditions for converting the energy of longitudinal phonons with THz frequencies into the energy of THz photons. The prospects of designing GNBsbased soft THz radiation sources (frequencies: 0.14; 0.24; 0.41 and 0.70 THz) with a large flow cross-section (diameter ∼ 40 cm) intended for detection of hidden objects under clothing to ensure security in public places (airports, railway stations, stadiums, etc.) are assessed. The choice of the above frequencies is a compromise between the requirements of low absorption of THz radiation by water vapor in air, good penetration through the fabric of clothing, favoring a sufficient resolution of the imaging system, and an abundance of corresponding longitudinal phonons, capable of exciting Fermi electrons in GNBs. Estimates of the characteristics of the terahertz-to-infrared converter based on gold nanospheres (GNSs), which could work in tandem with these sources of THz radiation – as a means of visualization of hidden objects – are also given.

The measurements of the human torso for two individuals are presented via the generation of the Huynen polarisation fork technique and plotted on the Poincaré sphere, to ascertain characteristics that could be used to remove the effects of the torso when concealed weapons are placed against it. Measurements are taken with a frequency modulated continuous wave (FMCW) mono-static millimetre wave full polarimetric radar, operating at k-band (18 to 26 GHz). The system has been designed to explore the feasibility of using full polarimetry for the detection of concealed weapons, and person borne improvised explosive devices (PBIED). The philosophy of this scheme is a means to extract the maximum information content from a target which is in the nominally single spatial pixel (sometimes sub-pixel) configuration of stand-off (tens of metres) and crowd surveillance scenarios. The radar comprises a vector network analyser (VNA) and an orthomode transducer.

This paper describes algorithms for detection and evaluation of trajectory parameters of small, spatially moving objects by passive optical and radio thermal vision system. The algorithms are based on spatial and temporal image processing. During spatial processing, a system of equations representing a sufficient condition for the conjugation of direction vectors to objects in stereo pairs is solved. During temporal processing vectors of the directions on accessory to objects in a sequence of the periods of supervision are distributed. The results of the algorithms theoretical and experimental examination are given. They are showing the advantage of the joint application of the two approaches.

In this paper, we present the step-index sapphire fiber, applied as a THz probe. The low THz attenuation of sapphire makes it attractive for fabrication of THz optical components. Moreover, it has a high refractive index in THz range, which guarantees a strong modal confinement in a fiber core. The advantages of the edge-defined film-fed growth (EFG) technique allow for fabrication of fibers with close-to-cylindrical shape, the length of 1 m and longer, and the subwavelength diameter of 150 − 400 μm. In order to improve the coupling efficiency, the fiber has polished flat ends. We apply the fabricated 300-μm-diameter sapphire fiber for the THz near-field scanning-probe microscopy. The spatial resolution of our experimental setup is defined by the fiber diameter, thus, it reaches ~ λ/4 for the radiation wavelength λ = 1200 μm. The obtained images of the test objects demonstrate the advanced resolution, which is close to the theoretical limit and beyond the Abbe diffraction limit.

Terahertz (THz) solid immersion microscopy is a novel THz imaging modality, which provides both a sub- wavelength spatial resolution and a high energy efficiency, thanks to the absence of sub-wavelength apertures and probes in an optical scheme. In this work, we apply the finite-difference time-domain technique for solving the Maxwell's equations in order to analyze the performance of our original THz SIL arrangement. Namely, we estimate the theoretical limits for the spatial resolution and the depth of field of our optical system, as well as specify the confidential intervals for the alignment of optical elements. The observed results demonstrate the resolution of 0:15λ and the depth of field of 0:12λ(λ is an electromagnetic wavelength), justifying advanced performance of our THz SIL.

Development of the integration variable selection method for the numerical solution of the Cauchy problem is demonstrated. This method is applicable for the simulation of electromagnetic wave propagation in inhomogeneous media by geometric optics approximation. Usually, in the methods of the numerical solution of the Cauchy problem, the integration is carried out according to one pre-selected variable. This approach does not seem to be the most cost-efficient in terms of computing resources.

The equations of rays and eikonal in finite differences are considered, taking into account the anisotropy of the refractive index. The paper presents a block diagram of the algorithm for choosing the variable of integration. The integration is carried out on the variable selected at the current step, which is assigned the specified step value. The increments of the remaining variables are calculated by expressions depending on the selected integration variable so that the increments on the remaining variables do not exceed the value of the integration variable. The integration variable is selected again and the increments are calculated. This method saves computational resources and minimizes the risk of transition to adjacent phase trajectories. The paper presents a general flowchart of the selection algorithm and expressions for calculating the increments of other variables at each step. The algorithm for calculating increments for each variable is demonstrated. The variable selection algorithm is developed for the case of a 7-dimensional phase space. It includes the projection of the pulse on the three axes of the Cartesian coordinate system, the projection of the coordinate and the phase component. The phase component describes the phase of the wave at the selected point and is analogous to the time dependence.