THz Air Photonics

Abstract

THz wave spectroscopy and imaging technologies are promising in security inspection applications. However, the following hurdles prevent THz technologies to be used in in situ applications, especially when standoff detection is required. First of all, the attenuation of THz waves in the atmosphere is higher than 100 dB/km, so it was previously thought impossible to perform long distance broadband THz wave sensing and spectroscopy, due to severe water vapor attenuation. Secondly, pulsed THz wave emitters using either real or virtual photocurrents, saturate when high excitation intensities are used. Further increase of the excitation power may even cause damage to the emitter. The saturation and damage of THz wave emitter limits the strength of the THz fields that can be generated from such emitters. Additionally, although pulsed THz wave generation and detection systems provide broadband spectral coverage, the spectrum does not generally cover the entire terahertz band continuously. Semiconductors or nonlinear crystals usually have phonon modes in the THz band. Absorption and dispersion due to photons result in dark areas in the measured THz spectrum. Finally, the reflection of THz waves, by both surfaces of the emitter or sensor, generates interference patterns in the THz spectrum. Confronted by those hurdles, using ambient air as the THz wave emitter and sensor becomes more and more interesting. By using ambient air as THz wave emitter and sensor, one can generate and detect THz waves close to the sample. Sending an optical beam instead a of THz wave, benefits long-distance standoff detection due to the relatively low attenuation experienced in the atmosphere. Since air or other gases are easily replaceable, damage is not a concern even if a strong laser field is used to generate the THz pulses. As a result, it is preferable in the generation of high intensity THz pulses. Finally, dry air has neither phonon bands nor boundary reflection surfaces, and thus provides continuous coverage along the entire bandwidth.