Scott Schecklman

Terahertz material detection from diffuse surface scattering

The potential for terahertz (THz) spectroscopy to detect explosives and other materials of interest is complicated by rough surface scattering. Our previous work has demonstrated that by averaging over diffuse observation angles and surfaces, spectral features could be recovered from laboratory measurements and numerical computer simulations. In addition to averaging, a low-pass cepstrum filter was used to reduce noise due to the random rough surface. This paper expands on these concepts by using the cepstrum of both the random rough surface and the material properties of the target material to choose an optimal cutoff frequency for the filter. The utility of these techniques is evaluated using laboratory measurements and Monte Carlo simulations for many sets of random surface realizations. The Kirchhoff Approximation is used to quickly model diffuse scattering from dielectric materials with gradually undulating rough surfaces when the incident and diffuse scattering angles are near the surface normal. Th...

Modeling Rough-Surface and Granular Scattering at Terahertz Frequencies Using the Finite-Difference Time-Domain Method

Exploration of the terahertz (THz) portion of the electromagnetic spectrum has recently expanded due to advances in ultrafast optical laser systems. The application of THz imaging to detect explosive materials (and other chemical agents) is a promising potential application because of the unique spectral signatures found for many explosives in the THz band. However, since the wavelength of THz radiation is on the order of tens to hundreds of micrometers, the rough interface between materials and the granular nature of material mixtures (such as explosives) may cause frequency-dependent scattering, which could mask the spectral signature. Thus, to evaluate the effectiveness of THz imaging systems, it is necessary to characterize the combined volume and rough-surface scattering effects. Because of the complexity of the media and the requirement for broadband modeling, the finite-difference time-domain (FDTD) formulation is an ideal tool. In this paper, transmission measurements through granular media and reflection measurements from controlled rough surfaces are shown to be in good agreement with FDTD results. Methods for extracting the material spectral peaks from a limited number of measurements are presented and discussed.

Scattering effects in terahertz reflection spectroscopy

Recent advances in ultrafast optical laser technology have improved generation and detection of energy within the terahertz (THz) portion of the electromagnetic (EM) spectrum. One promising application of THz spectroscopy is the detection of explosive materials and chemical or biological agents. This application has been motivated by initial measurements that indicate that explosives may have unique spectral characteristics in the THz region thus providing a discernible fingerprint. However, since THz wavelengths are 10s to 100s of microns in scale, rough interfaces between materials as well as the granular nature of explosives can cause frequency-dependent scattering that has the potential to alter or obscure these signatures. For reflection spectroscopy in particular the measured response may be dominated by rough surface scattering, which is in turn influenced by a number of factors including the dielectric contrast, the angle of incidence and scattering, and the operating frequency. In this paper, we present measurements of THz scattering from rough surfaces and compare these measurements with analytical and numerical scattering models. These models are then used to predict the distortion of explosive signatures due to rough surface interfaces with varying surface height deviations and correlation lengths. Implications of scattering effects on the performance of THz sensing of explosive materials are presented and discussed.

Three-dimensional broadband terahertz synthetic aperture imaging

Abstract. Terahertz (THz) technology holds great promise for applications such as explosives detection and nondestructive evaluation. In recent years, three-dimensional (3-D) THz imaging has been considered as a potential method to detect concealed explosives due to the transparent properties of packaging materials in the THz range. Another important advantage of THz systems is they measure the electric field directly. They are also phase coherent, supporting synthetic aperture (SA) imaging. In this paper, a near-field synthetic aperture THz imaging system is investigated for its potential use in detecting hidden objects. Frequency averaging techniques are used to reduce noise side-lobe artifacts, and improve depth resolution. System depth resolution is tested and characterized for performance. It will be shown that, depending on system bandwidth, depth resolution on the order of a few hundred microns can be achieved. A sample consisting of high-density polyethylene and three ball-bearings embedded inside is imaged at multiple depths. 3-D images of familiar objects are generated to demonstrate this capability.

Terahertz Spectral Imaging Using Correlation Processing

Terahertz (THz) reflection imaging has the potential to extract unique spectral signatures from various materials of interest, but these signatures can be distorted by rough surface scattering, low signal strength, and alignment errors. One method of addressing these challenges and increasing performance is to coherently combine reflection information across frequency bands. This method, called correlation processing, uses both phase and magnitude information to provide concurrent imaging and chemical mapping. This paper first shows imaging results from pressed pellets made of both lactose and high density polyethylene. It will be shown using measured THz scattered data and Monte Carlo simulations that correlation processing can correctly distinguish between materials with spectral signatures and other compounds. THz spectral surface profiling results from the pressed pellet are provided and discussed.

Measurement and modeling of rough surface effects on terahertz spectroscopy

Recent improvements in sensing technology have driven new research areas within the terahertz (THz) portion of the electromagnetic (EM) spectrum. While there are several promising THz applications, several outstanding technical challenges need to be addressed before robust systems can be deployed. A particularly compelling application is the potential use of THz reflection spectroscopy for stand-off detection of drugs and explosives. A primary challenge for this application is to have sufficient signal-to-noise ratio (SNR) to allow spectroscopic identification of the target material, and surface roughness can have an impact on identification. However, scattering from a rough surface may be observed at all angles, suggesting diffuse returns can be used in robust imaging of non-cooperative targets. Furthermore, the scattering physics can also distort the reflection spectra, complicating classification algorithms. In this work, rough surface scattering effects were first isolated by measuring diffuse scattering for gold-coated sandpaper of varying roughness. Secondly, we measured scattering returns from a rough sample with a spectral signature, namely α-lactose monohydrate mixed with Teflon and pressed with sandpaper to introduce controlled roughness. For both the specular and diffuse reflection measurements, the application of traditional spectroscopy techniques provided the ability to resolve the 0.54 THz absorption peak. These results are compared with results from a smooth surface. Implications of the results on the ability to detect explosives with THz reflection spectroscopy are presented and discussed. In addition, the Small Perturbation Method (SPM) is employed to predict backscatter from lactose with a small amount of roughness.

Modeling Terahertz Diffuse Scattering from Granular Media Using Radiative Transfer Theory

Terahertz (THz) spectroscopy can potentially be used to probe and characterize inhomogeneous materials. However, identiflca- tion of spectral features from difiuse scattering by inhomogeneous ma- terials has not received much attention until now. In this paper, THz difiuse scattering from granular media is modeled by applying radiative transfer (RT) theory for the flrst time in THz sensing. The difiuse scat- tered fleld from compressed polyethylene (PE) pellets containing steel spheres was measured in both transmission and re∞ection modes using a THz time domain spectroscopy (THz-TDS) system. The RT model was validated by successfully reproducing qualitative features observed in experimental results. Difiuse intensity from granular media contain- ing lactose was then simulated using RT theory. In the results, spectral features of lactose were observed in the difiuse intensity spectra from the granular media.

A Computational Method to Predict and Study Underwater Noise due to Pile Driving

A hybrid modeling approach that uses the parabolic equation (PE) with an empirical source model is presented to study and predict the underwater noise due to pile driving in shallow, inhomogeneous environments over long propagation ranges. The empirical source model uses a phased point source array to simulate the time-dependent pile source. The pile source is coupled with a broadband application of a PE wave propagation model that includes range dependent geoacoustic properties and bathymetry. Simulation results are shown to be in good agreement with several acoustic observations of pile driving in the Columbia River between Portland, OR and Vancouver, WA. The model is further applied to predict sound levels in the Columbia River and study the effects of variable bathymetry and sediment configurations on underwater sound levels.

Physics-based processing for terahertz reflection spectroscopy and imaging

The spectra obtained from Terahertz (THz) reflection imaging can be distorted by scattering from rough interfaces, layers, and granular inclusions. Since the facets of the object being imaged are not generally aligned normal to the THz beam, the received signal is produced from diffuse scattering, which can be appreciably lower in signal strength than specular returns. These challenges can be addressed with advanced signal processing approaches based upon the coherent and incoherent combination of returns from multiple sensors and frequencies. This paper presents two examples of physics-based processing strategies applied to THz imaging spectroscopy. The first method is based on synthetic aperture processing of a 2D sensor array to provide variable depth focused images of buried inclusions (a ball bearing embedded in polyethylene sample). The second method uses correlation processing to coherently combine multiple sensors and multiple frequencies to extract material signatures from measurements of THz scattering from rough interfaces. Results for both methods show an increase in performance relative to conventional imaging or spectroscopy approaches.

Measurement and modeling of terahertz spectral signatures from layered material

Many materials such as drugs and explosives have characteristic spectral signatures in the terahertz (THz) band. These unique signatures hold great promise for potential detection utilizing THz radiation. While such spectral features are most easily observed in transmission,real life imaging systems will need to identify materials of interest from reflection measurements,often in non-ideal geometries. In this work we investigate the interference effects introduced by layered materials,whic h are commonly encountered in realistic sensing geometries. A model for reflection from a layer of material is presented,along with reflection measurements of single layers of sample material. Reflection measurements were made to compare the response of two materials; α-lactose monohydrate which has sharp absorption features,and polyethylene which does not. Finally,the model is inverted numerically to extract material parameters from the measured data as well as simulated reflection responses from the explosive C4.