INVITED PAPER Terahertz Time-Domain Spectroscopy for Material Characterization The composition of glass and polymers, and the density and viscosity of lubricating oil, can be determined from the THz spectrographs of these materials. By Mira Naftaly and Robert E. Miles, Member IEEE ABSTRACT | Terahertz time-domain spectroscopy is used to study properties of nonpolar amorphous materials. Terahertz absorption spectra and refractive indices were measured in a number of glasses, lubricating oils, and polymers, and the results were correlated with material properties. KEYWORDS | Amorphous materials; terahertz spectroscopy I. INTRODUCTION Coherent time-domain spectroscopy (TDS) using terahertz radiation [1]–[3] has been employed to look at a wide variety of materials, including ceramics [4], semiconduc- tors [5], environmental pollutants [6], chemical mixtures [7], and gases [8]. Many of these materials were previously studied using far-infrared Fourier transform spectroscopy (FTS) in transmission and reflection modes. THz TDS differs from FTS in a number of important aspects [1]–[5] which give it some significant advantages. THz TDS uses coherent pulsed sources, typically of 1–2 ps duration; while FTS uses continuous-wave (CW) noncoherent sources. The TDS measurement is carried out in a pump–probe configuration, with the short probe pulse (G 0.1 ps) sweeping out the transient field of the THz pulse, giving a direct measurement of the field amplitude and phase. The THz spectrum is then obtained from the data by a Fourier transform. Since the absorption coefficient and the refractive index of the material studied are directly related to the amplitude and phase respec- tively of the transmitted field, both parts of the complex permittivity can be obtained by THz TDS. By contrast, FTS measures only the field intensity, and therefore provides only the absorption coefficient. Although the refractive index can be calculated from the FTS data by using Kramers–Kro ¨nig relations, the calculation is not straight- forward and has many potential sources of error. As a further consequence of its coherent pump–probe detec- tion scheme, TDS has a very high signal-to-noise ratio (SNR): typically > 10 6 in power, compared to 300 for FTS [5]. Moreover, the high-brightness, short-pulse sources used in THz TDS have peak optical intensities several orders of magnitude higher than the CW sources employed in FTS [5], allowing transmission studies to be carried out on materials with relatively high absorption coefficients. The frequency resolution of THz TDS can be as fine as 1 GHz, which is as good as the best of FTS instru- ments. However, the bandwidth of THz emitters is typically less than 5 THz, while that of FTS systems can extend into the near-infrared. Terahertz spectroscopy studies fall into two categories: those aiming to identify or differentiate substances on the basis of their THz transmission spectra, and those focusing on the optical and dielectric properties of materials at terahertz frequencies. In the present paper we attempt to relate the data obtained by terahertz spectroscopy to the material properties. Nonpolar materials were chosen for the study, because of their relatively good transmis- sion at THz frequencies enabling good quality data to be obtained. We describe the application of THz TDS to glasses, lubricating oils, and polymers. The measured terahertz absorption and refractive indices were corre- lated with material properties, and some relationships were identified. The results show the potential of Manuscript received October 31, 2005; revised March 17, 2007. This work was supported by the Research Councils U.K. Basic Technology Research Programme. The authors are with the School of Electronic and Electrical Engineering, University of Leeds, Leeds LS2 9JT, U.K. (e-mail: m.naftaly@leeds.ac.uk). Digital Object Identifier: 10.1109/JPROC.2007.898835 1658 Proceedings of the IEEE | Vol. 95, No. 8, August 2007 0018-9219/$25.00 Ó2007 IEEE