A scanning near-field microscope provides nano-scale imaging capability of field induced THz wave emission spectra from semiconductor surfaces and interfaces. Combined with a scanning probe tip and femtosecond optical pulse excitation, THz wave emission with sub-100 nm spatial resolution has been demonstrated. The scanning probe tip modulates semiconductor surface field with nano-scale accuracy through the imaging charge dipole, the tunneling current, or the contact current. The modulated THz wave from the highly localized area under the scanning tip is detected in time-domain. This aperture-less imaging method leads the way to study nano-scale to atomic level emission spectroscopy at THz frequency range.
Gas sensing and identification in far infrared or THz band is useful because many polar molecules have unique spectral fingerprints in this range, which are from the rotational transitions of the molecules. We have investigated the potential of THz time-domain spectroscopy (THz TDS) as a quantitative analysis technique for gas sensing. Ammonia vapor has been chosen as a sample gas. The absorption cross section at 0.572 THz of ammonia in the pressure range of 0.2-20 Torr was extracted to be (5.7±0.3)×10-20 cm2/molecule. In addition, a pressure calibration curve based on pure ammonia measurements was obtained. Using this calibration curve, we made quantitative analysis on the mixture of ammonia and air at 100 Torr. The result shows that THz TDS is an appropriate technique for quantitative analysis of polar gas and gas mixture. We measured the THz spectra of ammonia at different partial pressures in ~590 Torr nitrogen (78% nitrogen in atmosphere), and obtained a pressure calibration curve. THz spectra of ammonia at different partial pressures in 760 Torr atmosphere were measured. Based on the principle of differential optical absorption spectroscopy (DOAS) and the pressure calibration curve got in ~590 Torr nitrogen, we obtained the ammonia partial pressures. The result is compared with the value measured by vacuum gauge and the maximum error is 30%. This indicates that THz TDS based on principle of DOAS is an applicable quantitative technique for sensing ammonia or other polar gases in atmosphere.
Water, at both the liquid and gas phase, maintains a high absorption coefficient in the terahertz (THz) frequency range. As a result, a major limitation of THz time-domain spectroscopy (THz-TDS) for real-world applications is water attenuation. The humidity in the atmosphere affects THz waves (T-ray) for long distance measurement and tracing materials, such as explosive materials. We measure air at various humidity and we report how humidity affects THz-TDS measurement. We also report the changes to spectrum amplitudes by measuring water vapor absorption in a vacuum chamber.
Terahertz (THz) time-domain spectroscopy (TDS) is a powerful measurement tool for characterizing materials with potential fingerprint capability. Due to its pulsed nature, the spectral resolution of THz-TDS is limited by its temporal scanning measurement and its dynamic range. A novel THz-TDS system with a large signal-to-noise ratio (SNR) improves the spectral resolution. Techniques that will enhance the performance of THz-TDS are demonstrated.