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 10's to 100's 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.

The existence of unique absorption spectrum patterns for many chemicals at terahertz frequencies has opened an exciting avenue in non-contact safe detection of such materials by terahertz spectroscopy. However, scattering of THz waves, which have wavelengths on the order of material grain sizes, by surface roughness challenges the sensitivity of this detection scheme in practice. In this work, we present terahertz time domain spectroscopy results for materials with rough surfaces. Both reflection from and transmission through lactose, which has sharp absorption peaks in the terahertz regime, are studied and the effect of increasing scattering through controlled surface roughness is investigated. Such electromagnetic scattering can alter the terahertz absorption spectrum and thus obscure the detection of chemicals. Furthermore we examine electro-optic detection of terahertz signals reflected from randomly rough targets with a theoretical electromagnetic system perspective and provide a method to retrieve coherent reflection responses from rough surface targets.

The effects of surface scattering on terahertz reflection spectrum for explosive detection are studied by measuring terahertz reflection pulses from sandpapers with different roughness coated with gold. The experimental results show that the amplitude decrease and pulse broadening of the detected signal caused by the surface scattering result in the width reduction of Gaussian distribution of the specular scattering coefficient spectrum. A simple analytical model is applied to the analysis of experimental results and good agreements are obtained.

Terahertz (THz) imaging is emerging as a potentially powerful method of detecting explosive devices, even in the presence of occluding materials. However, the characteristic spectral signatures of pure explosive materials may be altered or obscured by electromagnetic scattering caused by their granular nature. This paper presents THz transmission measurements of granular systems representative of explosives and presents results from dense media theory that accurately explain the observed scattering response.