Vibration–rotation absorption spectrum of water vapor molecular in frequency selector at 0.5–2.5 THz range

Water vapor in the atmosphere is the main absorber for terahertz electromagnetic waves. The power absorption of water vapor had been measured in 0.5–2.5 THz by using terahertz time-domain spectroscopy. In time-domain waveforms, the main-peak of water vapor delayed 0.3 ps with very large attenuation compared with reference. In the transmittance spectra, seven atmospheric windows obtained indicate that terahertz wave will be able to propagate in quite a long distance at atmosphere. Experiment date and first principles calculation results described gallium phosphide crystal could be useful for terahertz frequency selector based on acousto-optic interaction. We obtained 30 absorption lines in the absorption spectra and the four locations of radical groups are characteristic absorption peaks.