Article

Absorption-sensitive diffuse reflection imaging of concealed powders using a terahertz quantum cascade laser.

School of Electronic and Electrical Engineering, University of Leeds, Leeds, LS2 9JT, UK.
Optics Express (Impact Factor: 3.55). 05/2008; 16(9):5997-6007. DOI: 10.1364/OE.16.005997
Source: PubMed

ABSTRACT We report diffuse reflection imaging in air of concealed powdered samples using a terahertz quantum cascade laser. The sensitivity of the detection scheme to sub-surface absorption within samples is confirmed using fully-characterized powdered admixtures of polystyrene and polymethyl methacrylate (PMMA). Measurements of the backscattering intensity from these samples are then used in conjunction with Kubelka-Munk scattering theory, as well as several models based on the quasi-crystalline approximation, to extract the absorption coefficient of PMMA. Our research demonstrates the feasibility of high-resolution frequency-domain terahertz imaging for the detection and identification of concealed powders in a reflection geometry.

0 Bookmarks
 · 
76 Views
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We report on the realization of an active fully electronic THz camera operating at 645 GHz and room temperature. It currently acquires images with about 9000 pixels in 10 seconds. Each pixel contains the amplitude as well as the phase information of the reflected THz signal. Frequency modulation allows to select a specific working distance and to suppress spurious reflections. The typical image size is in the order of hundreds of cm<sup>2</sup>.
    Infrared, Millimeter and Terahertz Waves, 2008. IRMMW-THz 2008. 33rd International Conference on; 10/2008
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We report terahertz (THz) diffuse reflectance measurements of bulk powdered samples at a frequency of 2.83 THz using a narrowband quantum cascade laser. Samples studied comprise polydisperse powders with absorption coefficients extending over two orders of magnitude from ∼3 cm(-1) to >200 cm(-1). Diffuse reflectance measurements are used to obtain the effective absorption coefficient of these samples from the backscattering cross-section, predicted under the quasi-crystalline approximation (QCA) in the T-matrix formulation and in conjunction with the Percus-Yevick pair distribution function. Results are compared with effective absorption coefficients obtained from THz time-domain spectroscopy measurements on pressed pellet samples, and show good agreement over the range of effective absorption coefficients studied. We observe that the backscattering cross-section predicted under the QCA is strongly dependent on both the real and imaginary components of the complex permittivity of the sample, and we show that reliable determination of the absorption coefficient from diffuse reflectance measurements therefore requires knowledge of the sample's refractive index. This work demonstrates the applicability of diffuse reflectance measurements, using a THz frequency quantum cascade laser, to the high-resolution spectroscopic analysis of bulk powdered samples at THz frequencies.
    The Journal of Chemical Physics 04/2011; 134(13):134304. · 3.12 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: The terahertz (THz) frequency quantum cascade laser (QCL) is a compact source of high-power radiation with a narrow intrinsic linewidth. As such, THz QCLs are extremely promising sources for applications including high-resolution spectroscopy, heterodyne detection, and coherent imaging. We exploit the remarkable phase-stability of THz QCLs to create a coherent swept-frequency delayed self-homodyning method for both imaging and materials analysis, using laser feedback interferometry. Using our scheme we obtain amplitude-like and phase-like images with minimal signal processing. We determine the physical relationship between the operating parameters of the laser under feedback and the complex refractive index of the target and demonstrate that this coherent detection method enables extraction of complex refractive indices with high accuracy. This establishes an ultimately compact and easy-to-implement THz imaging and materials analysis system, in which the local oscillator, mixer, and detector are all combined into a single laser.
    Optics Express 09/2013; 21(19):22194-22205. · 3.55 Impact Factor

Full-text (2 Sources)

View
4 Downloads
Available from
Aug 22, 2014