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.

Terahertz Time-Domain Spectroscopy for Material Characterization

Terahertz (THz) radiation occupies part of the electromagnetic spectrum between the infrared and microwave bands. Until recently, technology at THz frequencies was under-developed compared to the rest of the electromagnetic spectrum, leaving a gap between millimeter waves and the far-infrared (FIR). In the past decade, interest in the THz gap has been increased by the development of ultrafast laser-based T-ray systems and their demonstration of diffraction-limited spatial resolution, picosecond temporal resolution, DC-THz spectral bandwidth and signal-to-noise ratios above 104. This chapter reviews the development, the state of the art and the applications of T-ray spectrometers. Continuous-wave (CW) THz-frequency sources and detectors are briefly introduced in comparison to ultrafast pulsed THz systems. An emphasis is placed on experimental applications of T-rays to sensing and imaging, with a view to the continuing advance of technologies and applications in the THz band.

T-Ray Sensing and Imaging

Continuous-wave terahertz spectroscopy by photomixing is applied to the analysis of mainstream cigarette smoke. Using the wide tunability of the source, spectral signatures of hydrogen cyanide (HCN), carbon monoxide (CO), formaldehyde (H2CO), and water (H2O) have been observed from 500 to 2400 GHz. The fine spectral purity allows direct concentration measurement from the pure rotational transitions of HCN and CO. The quantification of the measurement was validated by the means of a calibration gas containing CO. The potential of this technique for trace gas detection is demonstrated with an estimated detection limit of HCN equal to 9 parts in 10(6).

Detection and quantification of multiple molecular species in mainstream cigarette smoke by continuous-wave terahertz spectroscopy.

Generation and control of pulsed terahertz-frequency radiation have received extensive attention, with applications in terahertz spectroscopy, imaging and ultrahigh-bandwidth electro-optic signal processing. Terahertz 'polaritonics', in which terahertz lattice waves called phonon-polaritons are generated, manipulated and visualized with femtosecond optical pulses, offers prospects for an integrated solid-state platform for terahertz signal generation and guidance. Here, we extend terahertz polaritonics methods to patterned structures. We demonstrate femtosecond laser fabrication of polaritonic waveguide structures in lithium tantalate and lithium niobate crystals, and illustrate polariton focusing into, and propagation within, the fabricated waveguide structures. We also demonstrate a 90 degrees turn within a structure consisting of two waveguides and a reflecting face, as well as a structure consisting of splitting and recombining elements that can be used as a terahertz Mach-Zehnder interferometer. The structures permit integrated terahertz signal generation, propagation through waveguide-based devices, and readout within a single solid-state platform.

Terahertz polariton propagation in patterned materials.

Although heretofore unrealized, it has long been known the rotational fingerprints of gases that occupy the submillimeter/terahertz (SMM/THz) spectral region can provide a basis for analytical systems with unique gas detection, identification, and quantification capabilities. Among these capabilities are near absolute specificity, even in complex mixtures, quantitative analysis, and excellent sensitivity to small samples. This paper describes such a system, self-contained in a 1 cu. ft. package. This system combines modern SMM/THz technology, sorbents to capitalize on the small sample requirements of the spectroscopic technique, and modern computational power to use the information contained in the complex rotational fingerprints. Moreover, the system and approach described show a clear path to future sensor systems that can be even smaller and more robust, as well as very inexpensive.

Compact Submillimeter/Terahertz Gas Sensor With Efficient Gas Collection, Preconcentration, and ppt Sensitivity

We use the technique of terahertz time-domain spectroscopy to experimentally and theoretically study the absorption and the dispersion of ammonia vapor. Our measurements show small deviations from the rotationinversion spectrum which originate from transitions in the O2 hot band and from the (NH3)2 dimer. We investigate the molecular tunneling in ammonia and apply our new molecular response theory to analyze the inversion spectrum of this molecule. The theory includes the molecular response of polar molecules to an external electric field over the duration of a collision with the molecular response time as a control parameter, and it unifies the basic collision theories of Lorentz, van Vleck -Weisskopf, and Debye. The calculated absorption and dispersion based on this theory fits the measurements over the full spectral range of the THz spectrum and allows to determine a response time of the order of 100 fs.

Stillwater Oklahoma

Papers

THz Time-Domain Spectroscopy on Ammonia