THz-wave Spectroscopy Research Articles

Dimethyl ether gas sensing using rotational absorption spectroscopy in the THz frequency region from 220 to 330 GHz

Gas sensing for dimethyl ether (DME) using terahertz (THz) -wave absorption spectroscopy is demonstrated. Spectral absorption measurements for pure DME at room temperature and moderate pressures (1−8Torr, 1.33−10.67 mbar) are carried out in the frequency range of 220−330 GHz using a broadband microelectronics-based THz wave spectrometer. Measured transition frequencies and absorption strengths are in good agreement with literature and extend the prior work to conditions where considerable pressure broadening occurs, resulting in blended spectral features. Experimentally observed spectra are compared with simulated spectra, based on documented transition frequencies and absorption strengths. Collisional self-broadening half-widths for DME are derived by considering the pressure dependence of the linewidths of isolated transitions and were determined to be 0.41 ± 0.04 cm−1atm−1 (12.1 GHz bar−1). The results illustrate the potential for quantitative DME gas sensing in industrial and environmental applications using THz-wave spectroscopy.

Thin terahertz-wave phase shifter by flexible film metamaterial with high transmission

Thin terahertz (THz)-wave optical components are fundamentally important for integrated THz-wave spectroscopy and imaging systems, especially for phase manipulation devices. As described herein, a thin THz-wave phase shifter was developed using a flexible film metamaterial with high transmission and polarization independent properties. The metamaterial unit structure employs double-layer un-split ring resonators (USRRs) with a designed distance between the two layers to obtain phase retardance of π/2, thus constituting a THz-wave phase shifter. The metamaterial design keeps the transmission coefficient as high as 0.91. The phase shifter also has polarization independence due to the four-fold symmetry of the USRR structure. Because of the subwavelength feature size of the USRR, this shifter can offer benefits for manipulating the spatial profile for the THz-wave phase through design of a binary optics phase plate by arranging a USRR array. The thickness of 48 μm has benefits for developing integrated THz optics and other applications that demand compactness and flexibility. The developed film size of 5 cm × 5 cm from the device fabrication process is suitable for THz lenses or gratings of large optical components.

Compact Tunable Narrowband Terahertz-Wave Source Based on Difference Frequency Generation Pumped by Dual Fiber Lasers in MgO:LiNbO3

We demonstrate a high-average-power, single longitudinal-mode, and tunable terahertz (THz)-wave source based on difference frequency generation (DFG) in a MgO:LiNbO3 (MgO:LN) crystal. The waves for DFG are generated using a pair of Yb-doped pulsed fiber lasers with a master oscillator power fiber amplifier configuration. The average power of the THz-wave output reaches 450 μW at 1.07 THz (280 μm) at a linewidth of 7.2 GHz, and the tunability ranges from 0.35 to 1.07 THz under the pulse repetition frequency of 500 kHz. A short burn-in test of the THz wave is also carried out, and the output power stability is within ± 5% of the averaged power without any active stabilizing technique. The combination of MgO:LN-DFG and stable and robust fiber laser sources is highly promising for the development of high-average-power THz-wave sources, particularly in the high transmission sub-THz region. This approach may enable new applications of THz-wave spectroscopy in imaging and remote sensing.

Broadband (0.3∼11THz) reflection spectroscopy using terahertz air photonics

AbstractPulsed THz wave spectroscopy using air as the THz wave emitter with the excitation of femtosecond laser provides intense (>100 kV/cm) and broadband THz waves (usable bandwidth from 0.3 to 11 THz). Using the air-biased-coherent-detection (ABCD) method, air can also coherently detect pulsed THz waves over a broadband spectrum. By utilizing these two technologies, we developed a prototype THz air photonics time-domain reflection spectrometer, and applied it on many materials in normal reflection geometry. Optical properties of CaCO3 crystals and several other samples in the THz range were studied. The system provided a signal-to-noise ratio (SNR) over 1000:1, with 0.1 cm−1 frequency resolution. The results acquired from both transmission and reflection measurements were then compared.

Liquid-phase epitaxy of GaSe and potential application for wide frequency-tunable coherent terahertz-wave generation

GaSe is one of the most promising semiconductor crystals for wide frequency-tunable terahertz (THz) wave generation by photo mixing. The ε-type monocrystalline GaSe crystals were successfully grown by the liquid-phase epitaxy at constant and low (530–590 °C) growth temperatures under the controlled different selenium (Se) vapor pressures ( P Se ∼0–7.75 Torr). From the coherent THz-wave spectroscopy, the absorption spectra have shown different resonant frequencies and absorption coefficients due to the stoichiometry-dependent point defects which depend on the applied P Se . It is shown that the resonance in GaSe under P Se ∼0 Torr shifts towards the lower THz frequencies compared with those under high P Se , maybe due to the degraded intermolecular interactions caused by the introduction of Se vacancy-related defects. The absorption coefficients (1–5 THz) decreased according to the increase of Se vapor pressure, thus the transparency of GaSe under higher P Se is improved by an amount of 25% compared with that of Bridgman-grown crystals. By using Bridgman-grown GaSe crystals, coherent-THz wave was generated by the difference frequency method (DFM) in a wide frequency range of 0.1–70 THz. Coherent-THz spectroscopy is a revolutionary method for the evaluation of molecular structures and defects in organic molecule also could be analyzed.

THz-Wave Spectroscopy Applied to the Detection of Illicit Drugs in Mail

A series of experiments in the field of THz-wave spectroscopy is described. The measurement of absorption spectra of solid samples, powders, and liquids is demonstrated using various optical setups. One of the sources we used is a widely tunable coherent THz-wave generator relying on an optical parametric process: nanosecond Q-switched Nd : YAG laser light scattering from the polariton mode of a MgO-doped LiNbO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> crystal. For the measurement of the absorption of THz waves in liquids, a train of waves is allowed to oscillate inside a silicon prism. A liquid placed on a total internal reflection surface lowers the quality factor of the resonator, which allows determining the complex refraction index of the sample by frequency scanning. We have also developed a technique for THz chemical imaging, by introducing the component spatial pattern analysis. The spatial distribution of the chemicals, such as illicit drugs concealed in an envelope, is obtained from terahertz multispectral transillumination images and absorption spectra. To compensate for the low imaging speed, a prescreening step is proposed, by first detecting the presence of powders in the envelopes, which is achieved by measuring in real time the amount of scattering produced by the envelopes. Details and examples are provided.

Measurement of Hydrated Water in D-Glucose Powder Using THz-Wave Spectroscopy

Measurement of Hydrated Water in D-Glucose Powder Using THz-Wave Spectroscopy

All-solid-state, narrow linewidth, wavelength-agile terahertz-wave generator

An all-solid-state, narrow linewidth, wavelength-agile THz-wave parametric generator was developed. The THz-wave frequency can be set randomly or tuned rapidly and smoothly over the range from 0.6to2.4THz with a narrow linewidth of 50MHz. Using this THz-wave parametric generator, accurate and rapid measurements were made of the absorption spectrum of water vapor, with a wide frequency range from 0.8to2.1THz, from a single scan of the THz-wave frequencies. These results demonstrate that this THz-wave parametric generator is a powerful tool with practical applications in THz-wave spectroscopy.