Diffuse-Reflectance Spectroscopy Using a Frequency-Switchable Terahertz Quantum Cascade Laser

• Published in: IEEE transactions on terahertz science and technology Vol. 6; no. 2; pp. 341 - 347
• Main Authors: Valavanis, Alexander, Chowdhury, Siddhant, Burnett, Andrew D., Clarkson, Adam R., Bacon, David R., Khanna, Suraj P., Davies, Alexander Giles, Linfield, Edmund H., Dean, Paul
• Format: Journal Article
• Language: English
• Published: Piscataway IEEE 01.03.2016; The Institute of Electrical and Electronics Engineers, Inc. (IEEE)
• Subjects: Absorption; Absorption coefficient; Direct reduction; Frequency modulation; Infrared spectra; Powders; Quantum cascade lasers; Refractive index; Refractivity; Scattering; Signal to noise ratio; Signatures; Spectroscopy; submillimeter-wave technology
• ISSN: 2156-342X; 2156-3446
• DOI: 10.1109/TTHZ.2016.2515082

We demonstrate diffuse-reflectance (DR) spectroscopy of powders using a discretely-tunable terahertz-frequency quantum cascade laser (THz QCL) with a heterogeneous active region. DR signatures were obtained at frequencies of 3.06, 3.21, 3.28, and 3.35 THz, and the relative absorption coefficients were inferred at each frequency using a Kubelka-Munk (KM) scattering model. The spectral lineshapes reproduce the absolute Beer-Lambert (BL) absorption spectra of a range of materials, which were also measured using conventional transmission-mode THz time-domain spectroscopy. It is shown that the DR technique works reliably for materials that include pharmaceutical compounds and foodstuffs, with BL absorption coefficients in the range 2-10 mm -1 . This method is potentially suitable for automated material identification, without any requirement for a priori knowledge of the refractive index or scattering properties of the sampled material.