Mid-infrared laser pulse excitation for terahertz generation from organic liquids molecules

Strong laser field is able to rotate gas molecules and the rotated gas molecules would emit electromagnetic waves when these molecules transition to the lower rotational energy levels. However, related investigation on liquid molecule rotation has not been reported, let alone the radiation falls in terahertz (THz) range. Here, a theoretical model is proposed to study the mid-infrared femtosecond laser driving organic liquid molecules such as methanol, ethanol, and acetic acid molecules to emit THz radiation. The interaction between laser electric field and liquid molecules are analyzed based on quantum mechanics theory. It is found that the spectrum of the THz radiation from methanol molecules range from 0.1 to 5 THz, while the spectra from ethanol and acetic acid molecules show nearly the same frequency range of 0.1–2 THz. These results indicate that broadband THz radiation can be generated through the transitions of rotational energy levels of the rotational liquid molecules excited by a linear polarization mid-infrared laser.